TRANSPARENT ALUMINA CERAMICS FABRICATED USING 3D

PRINTING AND VACUUM SINTERING

BY

DAVID M. CARLONI

A THESIS
SUBMITTED TO THE FACULTY OF

ALFRED UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

IN

MATERIALS SCIENCE AND ENGINEERING

ALFRED, NEW YORK

APRIL, 2020
 TRANSPARENT ALUMINA CERAMICS FABRICATED USING 3D
PRINTING AND VACUUM SINTERING

BY

DAVID M. CARLONI

B.S. ALFRED UNIVERSITY (2018)

SIGNATURE OF AUTHOR

APPROVED BY
YIQUAN WU, ADVISOR

STEVEN TIDROW, ADVISORY COMMITTEE

ANTHONY WREN, ADVISORY COMMITTEE

WILLIAM CARLSON, CHAIR, ORAL THESIS DEFENSE

ACCEPTED BY
GABRIELLE GAUSTAD, DEAN
KAZUO INAMORI SCHOOL OF ENGINEERING
Alfred University theses are copyright protected and
may be used for education or personal research only.
Reproduction or distribution in part or whole is
prohibited without written permission from the author.

Signature page may be viewed at Scholes Library,

New York State College of Ceramics, Alfred University,
Alfred, New York.
 ACKNOWLEDGMENTS

I would like to acknowledge my parents and my brother Joe for their support in my
academic career. Furthermore, I would like to acknowledge my advisor, Dr. Yiquan Wu
for the opportunity to perform this research investigation and for his guidance throughout,
as well as my other colleagues in our research group for their help, including Elo Chen,
Xuan Chen, Fang Guan, Yiyu Li, Ting Liu, Iva Milisavljevic, Shengquan Yu, and
Guangran Zhang. Finally, I would like to acknowledge the faculty and staff who helped in
this work, including my advisory committee of Dr. Steven Tidrow and Dr. Anthony Wren
as well as Dr. Darren Stohr and Francis Williams. I am very thankful for everyone and
their contributions.

iii
 TABLE OF CONTENTS
Page
Acknowledgments ............................................................................................................................... iii
List of Tables ........................................................................................................................................vi
List of Figures ..................................................................................................................................... vii
Abstract .................................................................................................................................................ix

I. INTRODUCTION ............................................................................................................................... 1

A. TRANSPARENT CERAMICS..................................................................................................................1
1. Background ..................................................................................................................................1
2. Materials for Transparent Ceramics ............................................................................................. 3
3. Material Preparation and Synthesis Methods ...............................................................................7
4. Transparent Ceramic Processing Methods ...................................................................................8
B. 3D PRINTING OF CERAMICS ............................................................................................................. 14
1. Background ................................................................................................................................ 14
2. Methods and Technologies ......................................................................................................... 15
3. Current State and Future of Ceramic 3D Printing ...................................................................... 25
C. AIMS OF THIS WORK ........................................................................................................................ 27

II. STARTING MATERIALS AND SLURRY PREPARATION ...................................................... 28

A. INTRODUCTION ................................................................................................................................ 28
B. EXPERIMENTAL PROCEDURE ........................................................................................................... 29
C. RESULTS AND DISCUSSION .............................................................................................................. 31
D. CONCLUSIONS .................................................................................................................................. 36

III. 3D PRINTING AND CIP PROCESSING ....................................................................................... 37

A. INTRODUCTION ................................................................................................................................ 37
B. EXPERIMENTAL PROCEDURE ........................................................................................................... 39
1. 3D Printing ................................................................................................................................. 39
2. CIP Processing ........................................................................................................................... 40
C. RESULTS AND DISCUSSION .............................................................................................................. 41
1. 3D Printed Samples .................................................................................................................... 41
2. CIP Samples ............................................................................................................................... 47
D. CONCLUSIONS .................................................................................................................................. 48

IV. POST-PROCESSING ........................................................................................................................ 48

A. INTRODUCTION ................................................................................................................................ 48
B. EXPERIMENTAL PROCEDURE ........................................................................................................... 49

iv
 C. RESULTS AND DISCUSSION .............................................................................................................. 50
D. CONCLUSIONS .................................................................................................................................. 60

V. SUMMARY AND CONCLUSIONS ................................................................................................ 60

VI. FUTURE WORK ............................................................................................................................... 61

VII. REFERENCES .............................................................................................................................. 62

VIII. APPENDIX .................................................................................................................................... 80

1. Table of Relative Density and Grain Size Data Among Sintered Samples. ............................... 80

v
 LIST OF TABLES

Page
Table 1. Overview of Commonly Used Transparent Ceramic Materials. ......................... 6

Table 2. Overview of Popular AM Technologies. ........................................................... 24

vi
 LIST OF FIGURES

Page
Figure 1. Scattering sources in polycrystalline ceramics, adapted from Ikesue et al.12 ..... 2

Figure 2. Schematic of a dry-press process. .................................................................... 10

Figure 3. Schematic of a CIP process. ............................................................................. 10

Figure 4. Schematics of popular AM techniques, adapted from Zocca, Colombo,

Gomes, and Günster .96 ................................................................................... 16

Figure 5. XRD analysis of prepared Al2O3 powder. ........................................................ 32

Figure 6. Particle size and distribution of prepared Al2O3 powder.................................. 32

Figure 7. SEM image of prepared Al2O3 powder. ........................................................... 33

Figure 8. Rheology of slurries with varying alumina content. ........................................ 35

Figure 9. Rheology of slurries with varying isobam per mixed powder. ........................ 35

Figure 10. Diagram of Hyrel System 30M 3D printer. .................................................... 38

Figure 11. 3D printed alumina ceramics. ......................................................................... 41

Figure 12. Printing time as a function of print move speed. ............................................ 42

Figure 13. Printing time as a function of nozzle size. ...................................................... 43

Figure 14. Printing time as a function of layer height. .................................................... 45

Figure 15. Green density as a function of layer height. ................................................... 45

Figure 16. Alumina ceramics 3D printed with varying extrusion widths. ....................... 46

Figure 17. CIP processed alumina ceramic...................................................................... 47

Figure 18. 3D printed transparent alumina ceramics. ...................................................... 51

vii
Figure 19. Progression of a representative sample through the process. ......................... 51

Figure 20. XRD analysis of sintered alumina ceramics. .................................................. 52

Figure 21. Relative density as a function of sintering temperature for 6-hour hold time.53

Figure 22. Relative density as a function of sintering time at 1850°C. ........................... 54

Figure 23. Relative sintered density at 1850°C for 6 hours as a function of layer
height............................................................................................................... 54

Figure 24. Average grain size as a function of SSS time at 1850°C. .............................. 55

Figure 25. SEM microstructure images of SSS ceramics at 1850°C for varying times. . 56

Figure 26. Average grain size as a function of sintering temperature for SSS and TSS
ceramics. ......................................................................................................... 56

Figure 27. SEM microstructure images of SSS ceramics sintered at varying

temperatures for 6 hours. ................................................................................ 57

Figure 28. SEM microstructure images of TSS ceramics sintered at varying

temperatures. ................................................................................................... 57

Figure 29. Total transmittance measurements for alumina ceramics from 3D printing
and CIP............................................................................................................ 59

viii
 ABSTRACT

Transparent alumina ceramics were fabricated using an extrusion-based 3D printer

and post-processing steps, including debinding, vacuum sintering, and polishing. Printable
slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies
of varying shapes and sizes. Post-process two-step vacuum sintering was found to increase
density while reducing grain size and thus improving the transparency of the sintered
alumina ceramics in comparison to single-step sintering. The 3D printed and two-step
vacuum sintered alumina ceramics achieved greater than 99% relative density and total
transmittance values of approximately 61% at 400 nm and 70% at 800 nm and above, which
was comparable to that of conventional CIP processed alumina ceramics. This
demonstrates the capability of 3D printing to compete with conventional transparent
ceramic forming methods along with the additional benefits of freedom of design,
production of complex shapes, and others.

ix
 I. INTRODUCTION

A. Transparent Ceramics

1. Background
Transparent polycrystalline ceramics have received significant attention and
development around the world in recent years for optical applications over more
conventional single crystal and glass materials. The technology behind transparent
polycrystalline ceramics can be considered to have been invented by Robert L. Coble and
General Electric (GE) in the early 1960’s with their patent of “Transparent alumina and
method of preparation”1 and sale of the Lucalox™ sodium vapor lamp. This lamp was a
result of research conducted at GE on sintering of Al2O3 (alumina) ceramics that yielded a
pore-free polycrystalline material which, at the time, exhibited novel transmittance and
corrosion resistance properties and was significantly more cost-effective to produce than
single crystal alumina (sapphire). Lucalox has been considered to be a revolution in
lighting technology and led to further research and development in the area of transparent
polycrystalline ceramics.2-3
In the many years that have followed, significant progress has continued to be made
in developing transparent polycrystalline ceramics. Currently, transparent polycrystalline
ceramics have been well-demonstrated in areas such as the aerospace and defense
industries for windows and domes,4-5 laser materials,6-7 armor4, 8 and other applications
which require high performance even under potentially extreme environments.9 Along with
their wide range of relatively inexpensive and efficient processing options, transparent
polycrystalline ceramics have shown the ability to achieve thermal, mechanical, and optical
properties which are comparable to or even surpass that of transparent single crystals and
glasses.4-5, 7-8, 10-12 Single crystal fabrication, despite typically yielding excellent properties,
requires long and expensive growth and machining processes to obtain a usable product.
This somewhat limits the production of single crystal materials unless they are absolutely
needed for the application. On the other hand, glasses typically do not exhibit the robust
mechanical and thermal properties of ceramics. Hence, polycrystalline ceramics are well-

1
balanced and versatile materials that combine both desirable properties and processing
options.9
Polycrystalline ceramic materials still pose many processing challenges towards
achieving transparency, though, due to their inherent scattering sources that are not
typically exhibited by single crystals and glasses. These scattering sources include grain
boundaries, pores, secondary phases and impurities, and surface roughness. Furthermore,
materials which possess asymmetric crystal structures present additional birefringence or
double-refraction effects (optical anisotropy). These scattering sources are represented
schematically in Figure 1 which was adapted from Ikesue et al.13 All of these combined
attributes limit the amount of light which can be transmitted through the material. Through
modern technology, careful design, and proper processing, however, it has been possible
to reduce the effect of, or even remove, the scattering sources in polycrystalline materials,
making them transparent.9, 12, 14-15 The passing years have brought and continue to bring
additional research and technological developments that provide improvements and new
opportunities and within this field. The below section provides information on the current
state of transparent polycrystalline ceramics in terms of materials and processing.

Figure 1. Scattering sources in polycrystalline ceramics, adapted from Ikesue et al.12

2
 2. Materials for Transparent Ceramics
Proper starting materials are necessary to make transparent ceramics and there are
currently a variety to choose from. To maximize optical quality, it is typically preferred
that the material possess an optically isotropic crystal structure so as to eliminate the
birefringence (double-refraction) effect which can be experienced across the grain
boundaries of optically anisotropic materials. Additionally, dopants and additives are often
included in small quantities to achieve more desirable properties or for specific
applications. This section will provide information on commonly used materials for the
fabrication of transparent polycrystalline ceramics as well as summarize some of the
research that has been performed and applications for these materials. Table 1 is presented
at the end of this section and displays a summary of the materials covered along with some
of their typical properties and the reports and references used throughout this paper.
Aluminum oxide (α-Al2O3), also referred to as alumina, is a well-known and
versatile ceramic which is notable for its high strength and hardness as well as being the
first polycrystalline transparent ceramic material as invented by Coble and GE.1-2, 16-21
Alumina possesses an optically anisotropic crystal structure which somewhat limits it’s
optical properties in comparison to other materials. Therefore fast consolidation processes
and sintering aids, like MgO, are typically used to reduce the grain size in relation to the
wavelength of light, thus mitigating the birefringence effect.22 Alternatively, oriented
grains can be achieved through the application of a strong magnetic field during
processing.23 Despite these processing hurdles, transparent alumina ceramics have still
been widely reported for a variety of fabrication methods and applications, most notably
for use as transparent armor and protective windows in extreme environments due to the
rugged properties.8, 24 Peelen et al. studied transparent alumina extensively and produced
two comprehensive papers on the light scattering and optical properties of alumina.14, 17
Krell et al. reported on using sintering and hot isostatic pressing (HIP) to achieve near full
density and submicron grain size transparent alumina ceramics with 55-65% in-line
transmission as well as high microhardness and bending strength.25 In a different study
Grasso et al. used high-pressure spark plasma sintering (SPS) to fabricate pure alumina
ceramics which achieved 65.4% transmission and less than 200 nm grain sizes.26 Mata-

3
Osoro and coworkers demonstrated slip cast processed transparent alumina ceramics
prepared using vacuum sintering as well as conventional sintering in air.27
Similar to alumina, aluminum oxynitride (AlON) also exhibits great mechanical
properties, but possesses the optically isotropic cubic spinel crystal structure which enables
it to typically achieve improved transmission properties. ALON™ is the marketed name
for AlON as patented by Raytheon Company in 1985 and currently assigned to Surmet
Corporation.28 AlON is primarily manufactured using proprietary processing methods for
applications such as transparent armor as well as domes and windows for missiles and
aircraft. AlON sometimes incorporates an additional non-reflective coating to achieve
improved transmission.29-31 Jiang et al. reported on transparent AlON ceramic pellets
fabricated using cold isostatic pressing (CIP) followed by pressureless sintering and hot
isostatic pressing (HIP) post-treatment which achieved transmittance values as high as
84.8% at 600 nm (visible range) and 86.1% at 2000 nm (near-infrared range).32 Kumar
and Johnson have also reported transparent AlON ceramics fabricated using slip casting
and vacuum sintering with in-line transmittance values of approximately 80% in the
wavelength region from 0.22 – 6 µm.33 A study on properties including, but not limited to,
optical scatter, refractive index, and absorption of AlON transparent ceramics of varying
composition was conducted by Hartnett et al. as well.34
Magnesium aluminate spinel (MgAl2O4), often referred to as just “spinel”, is a well-
rounded material with good optical, mechanical, and thermal properties which also
possesses the cubic spinel crystal structure. Unlike AlON, magnesium aluminate spinel
powders and processing methods are not proprietary, enabling relatively inexpensive and
easier routes to utilize this material.35 Transparent polycrystalline spinel ceramics have
also been primarily of interest in applications such as transparent armor and protective
windows for sensors and high energy lasers in harsh environments.8, 36 Tokariev and
coworkers conducted a study on the mechanical properties and grain size effects of
transparent polycrystalline spinel ceramics fabricated via uniaxial pressing followed by
vacuum sintering and post HIP treatment.37 Khasanov et al. reported on transparent
polycrystalline spinel ceramics formed under varying SPS parameters which achieved
72.6% transmission at a wavelength of 555 nm as well as a high microhardness of 18.52
GPa and Young’s modulus of 212 GPa.38

4
 Yttrium Oxide (Y2O3), also known as yttria, was originally invented in 1966 by
GE and Richard C. Anderson as a ThO2-doped yttria product under the trade name
Yttralox™ which was later patented in 1970.39 Yttria possesses the cubic bixbyite crystal
structure and exhibits excellent optical and thermal properties as well as a wide
transmission range, but is relatively lacking in mechanical properties. The mechanical
properties somewhat limited yttria to applications in environments which are less extreme
and do not require a high level of durability. Efforts have been made, however, to achieve
more desirable mechanical properties (as well as optical and thermal properties) through
doping, enabling yttria to be used in applications such as IR windows and domes for
missiles as well as solid-state lasers. Hogan et al. conducted a fairly comprehensive review
and study on transparent polycrystalline yttria ceramics including a comparison of studies
on undoped and La2O3-doped yttria showing significantly improved optical, thermal, and
mechanical properties with doping due to controlled grain growth.40-41 Hou et al. reported
on and investigated the mechanisms behind the use of another dopant, zirconia (ZrO2), to
fabricate isostatically pressed yttria disks which were sintered under vacuum and achieved
transmittance values as high as 83.1% when the zirconia content was optimized.42 Yttria
was also one of the first polycrystalline materials to be reported for application in solid-
state lasers by Greskovich and Chernoch in 1973 with their study on Nd-doped Yttralox .43
In a more recent study, Kong et al. reported on a high power 9.2 W diode-end-pumped
polycrystalline Yb-doped yttria ceramic (Yb:Y2O3) which demonstrated higher laser
efficiency than single crystal yttrium aluminum garnet (YAG) with the same dopant.44
Yttria Alumina Garnet (Y3Al5O12), commonly abbreviated as YAG, combines the
useful properties of both yttria and alumina and is highly studied in the field of transparent
ceramics for solid-state laser applications. YAG is commonly used in conjunction with
dopants of rare-earth ions because they can easily substitute for the yttrium ions of the host
lattice due to their similar size and same valence state (+3) as well as their useful range of
emission wavelengths.45 Although YAG is more conventionally used as a single crystal,
Ikesue and coworkers have conducted multiple studies on polycrystalline Nd:YAG and
have demonstrated that the polycrystalline ceramics can exhibit optical and laser properties
that match, or even surpass, that of single crystal Nd:YAG ceramics grown by the
Czochralski method.46-48 In more recent studies, Zhou et al. fabricated highly transparent

5
polycrystalline YAG ceramics with varying levels of MgO dopant via isostatic pressing
and subsequent vacuum sintering which achieved transmittance values as high as 84.5% at
a wavelength of 1064 nm.49 Wagner et al. also conducted a study on transparent Ce:YAG
ceramic phosphors fabricated via SPS and post-sintering HIP treatment which achieved
about 82% in-line transmission at a wavelength of 530 nm and exhibited sufficient
photoluminescence intensity, making it a good candidate for high-powered light emitting
diode (LED) applications.50

Table 1. Overview of Commonly Used Transparent Ceramic Materials.

Material Density Transmission Melting Young’s Knoop Typical References
(g/cm3) Range (μm) Point (°C) Modulus Hardness Applications & Reports
(GPa) (kg/mm2)
Al2O3 3.98 0.2-6.0 2072 345-400 1500-2200 Armor, 1-3, 8, 9,
windows/domes 13, 15-25,
71
AlON 3.70 0.2-6.0 2150 321-323 1850-1950 Armor, 4, 5, 8, 9,
windows/domes 26-32
MgAl2O4 3.58 0.3-6.5 2135 270-277 1400-1650 Armor, 4, 8, 9, 33-
windows/domes, 36, 56, 73,
lasers 79, 85, 183
Y2O3 5.01 0.2-8.5 2157 165-174 650-720 Windows/domes, 7, 9, 37-42,
lasers, 55, 57, 80,
scintillators 84,
YAG 4.55 0.2-5.5 1940 200-300 1215 Lasers, 6, 7, 9, 20,
scintillators 43-48, 54,
60, 68, 69,
72, 78, 81,
86, 87

6
 3. Material Preparation and Synthesis Methods
In addition to the type of the material chosen, the process of preparing synthetic
materials is another important consideration in transparent polycrystalline ceramic
fabrication. Impurities and inhomogeneities can cause undesirable scattering, thus
reducing the final optical quality which can be achieved by the ceramic. For good optical
quality, raw powder materials of at least 99% purity are required and it is important to work
in a clean environment. Additionally, the quality, homogeneity, and sintering of the final
material is typically a function of the method used to prepare or synthesize the powder.
Some of the most commonly used methods along with examples of their use in literature
are discussed next.
Solid-state reaction (SSR) is probably the most widely used synthesis method. In
SSR, powders are mixed in stoichiometric ratios for the desired final product, formed, and
then heated to high temperatures. The powders react at the high temperatures to form a
solid phase prior to densification.51 This method is relatively simple and enables the use
of commercial powder products, but it is necessary to understand the reaction kinetics and
select the proper heating profile to ensure that the reaction achieves the desired composition
and phase structure. Typically, powders are milled prior to SSR to achieve a homogeneous
mixture as well as a finer particle size and higher surface area which maximizes the number
of contact points and area of contact between particles. There have been many
demonstrations of SSR in the fabrication of transparent ceramics, such as the works of
Ikesue et al.,52 Lee et al.,53 and others.
Chemical co-precipitation is another popular synthesis technique. This is a wet
chemical method which has advantages such as lower processing temperatures and higher
level mixing of precursors when compared to SSR.54 Co-precipitation involves combining
solutions of precursor powders slowly to form a precipitation solution which is agitated,
filtered, and aged over time to eventually form a cake of material that is crushed and
calcined to yield the final material. This method enables the formation of highly
homogeneous and pure phase materials with good sinterability at relatively low
temperatures. This is a lengthy process, however, which can take from several hours to
days to perform. Li et al.55 and Zhang et al.56 report on the use of the co-precipitation
method to fabricate different transparent garnet ceramics.

7
 The sol-gel technique is another wet-chemical method with similar positive and
negative attributes as chemical co-precipitation. In this process, a colloidal suspension
with the ceramic powder is formed along with the slow addition of organic content and
heat over time to form a gel-like substance which is subsequently calcined to yield the final
material. Dupont and coworkers studied different parameters in the sol-gel process for the
preparation of yttria nanopowders.57 Additionally, Balabanov et al.58 and Hajizadeh-Oghaz
et al.59 both report on using the sol-gel process to synthesize MgAl2O4 and Y2O3 powders,
respectively, for transparent ceramic applications.
In flame spray pyrolysis (FSP), a solution of the precursor powders is sprayed into
a mist which is heated using a flame causing the solvent to burn off and the resulting
material is collected on a substrate. This is a somewhat new and less-utilized process, but
enables the relatively rapid and inexpensive formation of highly homogeneous
nanopowders.60 Teoh et al. provides a good review of the FSP technique and
applications.61 Jones et al.62 and Katz et al.63 have used FSP synthesis for the fabrication
of transparent Nd:YAG and Er:YAG, respectively.

4. Transparent Ceramic Processing Methods

A variety of techniques currently exist for the processing of transparent ceramics
from forming to sintering and post-processing. The ultimate goal of processing is to
achieve near full-density ceramic structures with limited scattering sources. There is no
one correct way to process transparent ceramics and every method possesses advantages,
disadvantages, and applications. The discussion below provides a review on the most
commonly utilized processing methods along with examples of research and reports in
literature.
Dry-pressing has been a very widely used ceramic forming method for a long time
due to its simplicity. Dry-pressing involves filling a die with powder and then uniaxially
pressing it with either one or two rams in order to form a relatively dense compacted green
structure, as shown schematically in Figure 2. Often times, a small amount of binder is
added to the powder to aid in the pressing process and provide lubrication between
particles, thus enabling improved compaction behavior as well as green mechanical
properties.22 Additionally, powders with an optimized particle size and distribution can
improve packing efficiency during pressing.64-67 For transparent ceramic fabrication, dry-
8
pressing is typically used in conjunction with cold isostatic pressing (CIP). In CIP, the
green ceramic structure is placed into a sealed flexible mold or pouch within an enclosed
chamber where uniform (isostatic) pressure is applied via an immersion fluid at room
temperature to compact the structure, as depicted in Figure 3. This additional process
further improves the green body density. CIP can also be used independent of dry-pressing
with an unformed powder being used directly.22 After dry-pressing and CIP, debinding
and sintering processes are required. Debinding simply involves heating the ceramic in a
furnace up to a temperature which burns off the binder. Pressureless vacuum sintering is
the commonly utilized sintering process which involves the use of a heating element in a
vacuum chamber. Sintering without vacuum in other atmospheres has been shown to
adversely affect grain growth and densification behavior as well as introduce impurities,
which is why a vacuum furnace is typically used.68-70 The temperatures for debinding and
sintering depend upon what materials are being used. Zhang et al. and Ge et al. report on
highly transparent Ho:YAG and Nd:YAG ceramic pellets, respectively, which were both
fabricated using dry-pressing and CIP followed by vacuum sintering.71-72 Additionally,
many of the reports from the previous materials section include dry-pressing, CIP, and
vacuum sintering.32, 42, 49, 73 Overall, dry-pressing and CIP followed by vacuum sintering
are simple, consistent, and efficient processes, but do not offer much flexibility in terms of
part design since not all shapes can handle being made by dry-pressing and CIP.
Furthermore, dies need to be made specifically for individual part designs and cannot be
easily modified without additional processes and tooling costs.

9
Figure 2. Schematic of a dry-press process.

Figure 3. Schematic of a CIP process.

10
 Hot-pressing (HP) is another uniaxial pressing method, but involves the application
of heat and pressure to simultaneously form and sinter the ceramic in one step. The HP
process typically utilizes a graphite heating element to heat a vacuum chamber which
contains a set-up of equipment similar to that in the aforementioned dry-pressing process.
As a related process, spark plasma sintering (SPS), also known as field assisted sintering
technique (FAST), applies an electrical current directly through a graphite mold as opposed
to using a heating element. In this manner, SPS/FAST can enable faster and more efficient
processing.74 Schlup et al. conducted a study on hot-pressing parameters for alumina
ceramics and achieved 65.3% in-line transmission in the visible region.75 Zhang et al.
reported on Gd:YAG ceramics fabricated via SPS which achieved in-line transmission
values of 77.1% in the visible range and exhibited ultraviolet emission via
photoluminescence excitation.76 Talimian and coworkers also recently conducted a study
on SPS parameters and their effect on contamination and discoloration in transparent
MgAl2O4 ceramics.77 In addition to these studies, some reports in the previous materials
section also include SPS.26, 38, 50 Similar to dry-pressing and CIP, HP and SPS/FAST are
limited in the size and shapes of the structures that they can produce. Furthermore, the
facilities and technology involved in HP and SPS/FAST are relatively complex, expensive,
and require a good understanding of many interconnected parameters. These fabrication
methods find advantages in improved efficiency however, due to their shorter processing
times which are completed in a single-step as well as a high degree of densification as
demonstrated in the reports.
Slip/mold casting, gel casting, and tape casting offer a much different approach to
the aforementioned dry processing methods. In general, wet ceramic processing methods
can offer improved control of particle-particle interactions, resulting in increased
homogeneity and other more desirable properties over dry ceramic processing routes.78-79
In slip/mold casting, an aqueous ceramic suspension is prepared with additional binders
and dispersants and formed into a mold of the desired shape which is allowed to dry over
time. This method enables the fabrication of structures of any shape which can be made
into a mold. Alternatively, in gel casting, water-soluble monomers are added to the
suspension and the mixture is polymerized within a mold. Gel casting has been known to
exhibit advantages over slip/mold casting through the production of near-net shape parts

11
with improved homogeneity and green strength.78, 80
In tape casting, the ceramic
suspension is cast through a “doctor blade” to form a thin sheet or tape. These thin tapes
can then be cut, layered, laminated, or pressed together as needed to form the desired
structure.81 These casting processes can also be combined with CIP processing to further
increase green density. Once the casted parts are sufficiently dry, they are debinded and
then sintered. As with the aforementioned dry-pressing and CIP processes, vacuum
sintering is typically employed as the densification method for casted transparent ceramics.
Appiagyei et al. reported on slip casted YAG ceramics using an aqueous suspension which
was vacuum sintered and achieved transmittance values greater than 80% in the
wavelength range from 340 – 800 nm.82 In a very recent study, Liu et al. reported on using
a combination of gel casting, CIP, and vacuum sintering to fabricate transparent MgAl2O4
ceramics which achieves good mechanical properties as well as in-line transmittance values
of 79.8% at 400 nm and 86.4% at 1064 nm.83 Jin et al. reported on zirconia-doped yttria
ceramics fabricated using tape casting and vacuum sintering which achieves a high in-line
transmittance value of 81.7% at 1000 nm.84 A couple of reports from the previous section
also included casting methods to form transparent ceramics.27, 33
Despite the many
advantages of wet processing, there also some disadvantages. Wet processing methods can
be complex in terms of proper slurry preparation and require a great understanding of
rheological behavior and interactions between particles. Furthermore, the wet processing
methods are typically less efficient and take longer due to the additional steps and processes
required, such as slurry preparation and drying.
Similar to CIP, hot isostatic pressing (HIP) is a post-sintering treatment method for
transparent ceramics; however, HIP is conducted at an elevated temperature with an inert
gas as the fluid pressure medium.22 In this method, the ceramic is initially sintered
(sometimes known as pre-sintering), followed by HIP treatment at high temperatures to
complete the densification. This combination of processes has been shown to improve the
density, microstructure, and optical properties of ceramics.85-87 Furthermore, since the
ceramic is previously sintered, it should have sufficient strength to undergo the HIP
treatment without breaking, thus enabling a wide variety of initial forming methods and
structures to be compatible with this post-treatment method. Wang et al. reported highly
transparent yttria ceramics fabricated using low temperature pre-sintering followed by HIP

12
treatment which achieves fine grain sizes and 81.7% in-line transmission without the use
of sintering additives.88 Luo et al. also reports on the use of pre-sintering and post HIP
treatment to fabricate transparent Co:MgAl2O4 ceramics for eye-safe solid-state laser
applications.89 Some of the aforementioned reports also utilized post HIP treatment.25, 32,
73

Following sintering, some transparent ceramic materials may require an annealing

process. Sintering processes can introduce defects and residual stresses as well the
formation of oxygen vacancies and color centers which produce undesirable results
towards achieving high optical quality. Oxygen vacancies are primarily formed due to the
reducing atmosphere found within most vacuum furnaces with graphite heating elements.
Annealing in air at a high temperature can help fill these oxygen vacancies and relieve
residual stresses to improve transmission in optical ceramics. Fu et al. and Zhang et al.
both demonstrate improved optical properties through annealing in air in their reports on
transparent Nd:YAG.90-92
As the final step in nearly all transparent ceramic fabrication processes, thorough
polishing is required. The polishing process is necessary to reduce scattering losses due to
surface roughness. Achieving a mirror-like finish is the ideal end goal and there are
different methods to accomplish quality surface finishes. Most commonly, polyurethane
polishing plates and cloth pads are used along with a polishing suspension composed of
fine abrasive particles from a material of high hardness, such as diamond. Additionally,
abrasive materials like alumina and silica can be used in an alkaline solution to provide an
additional chemical polishing effect which can reduce subsurface damage as well; this is
known as chem-mechanical polishing (CMP).93-95 Magnetic field-assisted finishing
(MAF) is another alternative technique which utilizes an applied magnetic field and
abrasive magnetic particles to polish. The MAF process offers more precise and controlled
polishing over the more conventional methods for optics.95-96

13
B. 3D Printing of Ceramics

1. Background
Additive manufacturing (AM), also known as 3D printing, is a general term used
to describe the fabrication of a physical part from a digital 3D model by the successive
addition of material.97 In this process, a 3D computer-aided design (CAD) model is
transformed into a “stereolithography” (.stl) file where it is approximated by small triangles
and then “sliced” into a series of 2D cross-sections that are built upon one another at
specified thicknesses to form the final part.98-99 3D printing enables the fabrication of a
wide array of controllably complex structures that might otherwise be difficult or even
impossible to make.97, 100 Parts can be formed in either a point-by-point, line-by-line, or
layer-by-layer manner and post-processing may or may not be necessary depending upon
the materials and techniques chosen.100
AM is considered to be a relatively new process and is significantly different than
more traditional subtractive or equivalent manufacturing processes such as machining or
casting, respectively, which have been used for hundreds of years.98, 100-101 Some of the
first AM technology was developed in the 1980’s in the form of stereolithography by
Charles Hull and 3D Systems, Inc. and has been regarded as a manufacturing revolution,
garnering significant attention around the world.97, 99-100 Originally, AM technology was
developed with the focus being on flexibility of design, prototyping, and reducing the time-
to-market for new products.97-98, 101
Due to significant research and technological
advancements, including 3D modelling, computer numeric control (CNC), and others, AM
has become a method that is used worldwide to produce highly complex and customizable
products with excellent properties that are unable to be made using any other method.98,
101-102

Currently, 3D printing is integrated into many different industries such as

biomaterials, pharmaceutical, aerospace, automotive, architecture, and others and has
become so accessible that simple instruments can even be purchased in the low thousand
or even hundred US dollar range for personal use in homes.102 3D printing has become
widely used because of the many attractive attributes it possesses. In the past, it would be
necessary to have experienced workers with a great understanding of different machinery,
tools, and materials in order to fabricate customized parts.102 AM technology reduces the

14
tooling and labor costs associated with making customized parts through its flexible and
autonomous nature while also making it possible to quickly and easily modify the design
of a part using modeling software. Furthermore, it is possible to fabricate parts of very
small geometries to a fine degree of precision due to the high resolution of modern AM
technology. Finally, the additive nature of 3D printing enables materials, energy, and costs
savings since there is little to no waste material and structures can be intelligently designed
with controlled porosity to limit material use while maintaining desired properties.97 With
all of these benefits possible from AM technology, the strategies of production and
business can be changed to create entirely new products with different functionalities or
improve upon existing ones.101
Thus far, the primary materials that have seen success when used for 3D printing
have been polymers and metals.100-101 However, in recent years, ceramic materials have
made their way into the 3D printing world. The first reports of AM with ceramic materials
were published in the 1990’s by Marcus100, 103 and Sachs.100, 104 Since then, there has been
some steady development in ceramic AM, including techniques specific to ceramics.
Ceramics are of interest for applications in a variety of fields and can be characterized by
their excellent thermal and chemical stability, hardness, high mechanical strength, as well
as optical and electrical properties. The combination of 3D printing technology with
ceramic materials opens a door for many potential opportunities.100-101 However, AM of
ceramics poses many challenges due to inherent processing requirements and the fact not
every AM technology can be utilized to produce ceramics.101 So, despite the progress that
has been made so far, AM of ceramics remains relatively underdeveloped, leaving
significant research and development efforts to fully enable the potential of ceramic AM.

2. Methods and Technologies

A variety of AM technologies currently exist and can be classified by feedstock
among dry powders, slurries, and bulk-solids. Despite being relatively new, ceramic AM
technology is still largely based upon traditional ceramic processing practice and
techniques.100-101 Below is a review of the most commonly used AM technologies today
along with their attributes and applications. Schematics of some of the AM technologies
are presented in Figure 4 (adapted from Zocca et al.101) and a brief overview of the
different AM technologies is presented at the end of this section in Table 2.
15
Figure 4. Schematics of popular AM techniques, adapted from Zocca, Colombo, Gomes, and Günster .96
16
 a. Slurry Methods
In slurry-based AM techniques, ceramic particles are typically suspended in an
aqueous medium which may include other additives such as dispersants, binders,
plasticizers, and photopolymers. Depending on the application and specific technique, the
slurry can vary greatly in both composition and rheology. In general, slurries will vary
between approximately 30-60 vol% solids-loading and have viscosities on the order of
mPa·s to Pa·s. In order to obtain the best results, it is necessary to develop a well-balanced
and stable slurry which fits the needs of the technique chosen and the final application.
The mechanical properties of parts fabricated using slurry-based techniques are typically
superior to powder-based techniques due to improved particle-particle interactions and
packing.100
First developed in 1997 by Cesarano and coworkers105-106, direct ink writing (DIW),
also referred to by other names such as robocasting (RC), is a general term used to describe
techniques that involve a highly viscous ink or paste being extruded as a filament through
a moving nozzle at room temperature. This enables the fabrication of large as well as
micron-scale complex parts that experience great shape retention without typically utilizing
any sort of built-in support structures.107-109 The main parameters of interest in this process
are the rheological properties and behavior of the suspension and the nozzle size.97 DIW
commonly exhibits issues with porosity, poor surface quality, and poor adhesion between
layers, however.97, 100 Therefore, this technology is not always implemented for dense
engineering ceramics, but more commonly for porous ceramic structures that do not require
high surface quality or resolution. Currently, this technology is being used for applications
such as medical implants and tissue engineering,110-116 filters,117-118 energy devices,119-120
and others.100
As related technologies to DIW, inkjet printing (IJP) and contour crafting (CC)
utilize a similar process. IJP was first developed for ceramics by Blazdell and coworkers
in 1995 and utilizes small computer controlled printheads to deposit a slurry onto a
substrate in either a continuous or drop on demand (DOD) mode.121-122 The resulting parts
are highly dependent on the rheological behavior of the slurry and the printing
parameters.97, 100 Parts are typically limited to small and simple shapes due to the droplets
being mere nanoliters in size and the method being incapable of producing overhanging or

17
hollow structures. IJP finds most of its applications in thin functional layers and
microelectronics.97, 100, 102 On the other hand, CC resembles an automated version of a very
traditional technique used in pottery. The process uses a viscous clay blend that is deposited
through a nozzle onto a surface that is subsequently flattened to size using a spatula.102
This technique utilizes very large nozzle sizes and high pressures which is what separates
it from DIW and IJP. For these reasons, CC is typically applied for large-scale parts in
construction. Although mainly used with cement, there have been some uses of CC for
ceramics. In construction, CC offers the ability for unique design options not typically
attainable due to equipment constraints as well as on-site construction with local
materials.97
Stereolithography (SL, STL, or SLA) is one of the oldest as well as widely used
AM techniques and was first developed in the 1980s by Charles Hull and 3D Systems
Inc.97, 99-100, 123 SL and related technologies are unique in their use of pre-ceramic polymers
(PCPs), which are suspensions composed of photocurable polymers and ceramic particles.
In SL, a build plate is submerged the distance of one layer into a vat containing the PCP
which is selectively cured (solidified) point-by-point using a UV laser through a process
known as photopolymerization. Once an entire layer is cured, the build plate is again
moved the distance of one layer and the process repeats until finished. After the part is
done printing, any excess material is removed. Afterwards, the part may undergo
additional curing, pyrolysis, and other post-processing steps. The finished part is referred
to as a polymer derived ceramic (PDC).97, 99-100, 124-126 Parts fabricated by SL exhibit high
surface quality along with fine features down to a micron scale. To achieve such surface
quality and micron scale features, many parameters must be taken into consideration. The
suspension must possess well-dispersed ceramic particles that do not have greater light
absorption than the polymeric medium as well as exhibit long-term stability and suitable
viscosity for printing. Furthermore, the exposure period of the laser must be fine-tuned to
prevent over- or under-curing. For these reasons, SL is a relatively complicated, expensive,
and slow technique with very limited materials selection. Due to their high performance,
quality, and fine resolution, however, PDC parts created via SL are widely used for
complex, small-scale structures, such as in microelectronic components,127-128 bone tissue
engineering,129-130 and many other technologies.97, 99-100

18
 As related methods to SL, digital light processing (DLP) and two-photon
polymerization (TPP) offer slight variations to the photopolymerization process. DLP uses
thousands of digital micromirror devices (DMDs) to form a patterned mask or projection
screen which exposes the light source to only certain areas on the build plate. This allows
for a single layer to be cured all at once and therefore is a significant improvement in terms
of process efficiency when compared to SL while still maintaining micron-scale resolution.
This technique has been demonstrated for fabricating high density ceramics and PDCs with
mechanical properties comparable to that of conventionally prepared ceramics.131 DLP has
been primarily used in the fabrication of fine-featured structures, such as metamaterials,
heat exchangers,132 piezoelectric devices,133 and other technologies.100 Alternatively, in
TPP, polymerization is initiated by the absorption of 2 photons from a high-intensity near-
infrared or green laser source in a submicron volume. In SL and DLP, only one photon is
used, and polymerization only takes place on the surface. In this way, TPP process enables
parts to be fabricated that would be impossible via SL or DLP.134 Currently, TPP is limited
to transparent PCP slurries as feedstock, and has been demonstrated in the fabrication of
high-quality ceramic structures at scales as small as nanometers.135-142 TPP suffers from
longer processing times as well as the inability to produce large-scale parts.100

b. Powder Methods
In powder-based techniques, a loose ceramic powder is typically laid out as a bed
which acts as a support structure to build parts upon. The powder can be bonded together
and formed into a layer in different ways, depending upon the technique chosen. The
bonding step in this process is the main source of cost and complexity and there are many
considerations to make. After a layer is formed, a fresh bed of powder is added for the
next layer, and the process continues until the part is finished followed by the removal of
excess powder.100 Due to the presence of the powder bed, it is not necessary to design
support structures for the part. In general, parts fabricated via dry powder techniques tend
to be more porous and have weaker mechanical properties than those fabricated via slurry
techniques, especially when a binder is used as the fusion source.97, 100 The porosity and
weak mechanical properties can be reduced or even alleviated, however, with proper
processing to achieve a good particle size and distribution and maximize packing which
can also lead to greater resolution.97, 99, 143

19
 Patented in 1986 by Deckard and Beaman,144 selective laser sintering (SLS) is a
highly complex and challenging process due to the different mechanisms and interactions
that must be considered. The first reports for the use of SLS for ceramics were made by
Lakshminarayan and coworkers in 1990 at the University of Texas at Austin.100, 145-146 SLS
utilizes a carbon dioxide laser to selectively heat the surface of the powder bed to the point
of sintering at specific locations, thus joining the particles together and forming a layer.99-
100
Due to the refractory nature of ceramics, a very high-powered laser with sufficient
exposure time is necessary to properly sinter the particles. This can be difficult and time
consuming to achieve, therefore it is more common to mix in other materials which act as
sintering aids, fluxes, and binders that form a glassy phase within the part.100 SLS
accommodates a broad range of ceramic materials and is capable of producing high-quality
parts with good mechanical properties when the material and process parameters are finely
tuned and supporting processes are included.99 Using SLS alone, ceramic parts are
fabricated with less than 50% of theoretical density due to significant porosity and exhibit
high shrinkage. By performing additional infiltration and isostatic pressing to compliment
the SLS process, it is possible to achieve densities as high as 94% of theoretical, as
demonstrated by Shahzad et al. 100, 147-152 Processing and material parameters highly effect
the properties of the resulting part as well. In general, it is preferred to use micron-sized
powders of spherical shape that are homogeneously coated with a small amount of binder
and contained in an inert atmosphere. SLS suffers intrinsically from poor resolution and
surface quality and is incapable of producing full density parts. Due to all of this, SLS is
a relatively costly and time-consuming process but has found application in the aerospace
and medical industries to fabricate controllably porous and complex structures.99-100
Selective laser melting (SLM) is similar to SLS and was first developed by the
Fraunhofer Institute of Laser Technology in 1996.153 SLM is a single-step process where
the powder bed is selectively irradiated by a laser and is fully melted without the use of a
secondary phase, such as a binder. As mentioned previously, ceramics have very high
melting points which makes this process difficult and expensive due to the high-energy
laser requirements. For this reason, SLM has not seen much development in the field of
ceramics.100 The benefits of this technique are that it is currently the only AM method
which is theoretically capable of producing near full density and homogeneous parts that

20
are ready to use in a single step. SLM not only allows for time savings due to the lack of
additional processing, but also results in highly pure parts due to the lack of additional
materials.97, 100, 154
In practice however, SLM alone has not been very successful in
achieving fully-dense and high quality parts in a single step due to challenges in properly
tuning the process and laser parameters to ensure sufficient melting, and, due to thermal
stresses associated with the rapid heating and cooling, results in structural problems in the
fabricated parts.100, 155-158 Some additional techniques have been proposed to improve upon
SLM, such as preheating the powder to help reduce thermal stresses159-160 as well as
different technologies such as laser micro sintering (LMS)161-162, LENS™ (covered next in
this review), and slurry-SLM which are all directly based on SLM. For the aforementioned
reasons, SLM is not necessarily a method of choice for manufacturing ceramic materials.
More research and development are required to solve the problems associated with SLM
before this technology will be more commonly used in the ceramic industry. However,
SLM technology has found some use for producing ceramic medical implants.99-100, 160, 163
Directed energy deposition (DED) is a technique which goes by many names, such
as laser engineered net shaping (LENS™) and others. DED/LENS™ is essentially an
extension of the SLM technique described earlier that was developed to help increase the
density of parts and reduce defects.100 In DED a powdered feedstock material as well as
the substrate are melted by a high-power laser and injected onto a substrate.97, 99 The
injected material fuses with the substrate and solidifies upon cooling to form the part.97, 100,
124, 164
This is slightly different than SLM in that no powder bed is utilized and the
feedstock material is melted prior to being injected.97 Although this technology is mostly
used for metals and alloys, there have been a number of successful demonstrations with
ceramics too.99-100, 165-167 DED technology has the advantages of relatively high printing
speeds and the capability of fabricating large (meter-scale) parts with strong mechanical
properties. Furthermore, DED can print with multiple materials at the same time as well as
across multiple axes and can be combined easily with subtractive machining to have a more
complete process. However, DED typically suffers from its inability to create parts of high
complexity or surface quality. DED is primariy applied in the aerospace and automotive
industries for repairing parts and other applications.97

21
 Not to be confused with the general term “3D Printing”, three-dimensional printing
(3DP™), sometimes referred to as binder-jetting, is a specific technique which was
licensed by MIT in 1989.99-100 3DP was first developed by Sachs et al. at MIT, but was
not developed for ceramics until later in the 1990’s.100-101 In 3DP, an aqueous organic
binder solution is sprayed through print heads onto specific regions of the powder bed.99
The powder is bound together by the solution and then solidifies to form a layer.99-101, 124
3DP is more simple and affordable than the other powder methods and is a good alternative
for when the powder material being used has a very high melting point or using high
powered lasers is not an option.97 3DP offers the ability to use a wide variety of ceramic
materials to produce a range of both large and small structures without requiring integrated
supports.99-100 To achieve a high-quality part, it is necessary to optimize the rheology of
the binder, process/printing parameters, and properties of the ceramic powder.97, 143 In
general, one should use a binder with molecular weight less than 15,000 and a fine ceramic
powder with spherical-shaped particles for optimum performance.97, 100
The main
drawbacks of 3DP are that it tends to produce parts with low resolution and poor surface
quality.100 Furthermore, 3DP suffers from high porosity and weak mechanical properties.97
For these reasons, 3DP is not typically employed for high-performance or advanced
structural ceramics, but has found use primarily in the medical industry for applications
that require parts which are not fully-dense.100, 168-169

c. Bulk-Solid Methods
Bulk-solid technologies include solid feedstocks that are not in powder form. Due
to the brittle nature of ceramics, it may be necessary to use composite feedstocks which
possess properties that enable printing. In general, bulk-solid techniques tend to produce
parts of large size and increased mechanical properties, but suffer from limited material
selection and the inability to produce fine and complex structures.
Fused deposition modeling (FDM) is the most widely used AM technique and has
the largest shipment of units around the world. Although it was originally developed for
polymers by Crump et al. in 1992,170 it was first reported for the use of ceramics in 1995
by Yardimci et al. at Rutgers University.171 The FDM process uses composite filaments
composed of thermoplastic binders which are highly-loaded with ceramic particles.100 This
filament material is then heated to a semi-liquid state until it is able to be extruded through

22
a nozzle and deposited onto a platform where it solidifies to form a layer. For smooth and
quality printing, the ceramic-to-binder ratio, dispersity, and particle size and distribution
should be optimized to achieve a viscosity in the range of 10 to 100 Pa·s. Furthermore, the
layer thickness as well as the width and orientation of the filament highly affects the
resulting mechanical properties.97, 99-100, 172 The main appeal with this technology comes
with its flexibility of machine/unit size, the low cost of equipment and materials, and the
overall fast and simple building process.97, 99-100, 124 The FDM process suffers from the
limited selection of thermoplastic materials as well as poor surface quality and resolution,
interlayer distortion, and weak mechanical properties.97, 99-100, 172-173
Laminated Object Manufacturing (LOM) was originally developed for paper,
plastics, and metals, yet was first reported on for the fabrication of ceramic parts by Griffin
et al. in 1994.174 Technically speaking, LOM is a combination of subtractive and additive
manufacturing processes, but will be considered as a 3D printing technique for the purpose
of this review.100 In LOM, a laser or mechanical cutter is used to cut previously prepared
thin sheets of materials which are coated with thermal adhesive agents. The layers are
stacked on top of each other and bound together (laminated) by the application of heat and
pressure.99-100 Excess material is typically left as support throughout the process and is
removed at the end.97, 164 LOM has been shown to be very successful in creating high
density parts with little to no deformation with a microstructure comparable to that of
ceramics prepared by conventional methods.100 This process is not very costly and works
well for fabricating large and simple structures.99 LOM is limited, by the inability to create
small features or complex shapes and can suffer from interlayer delamination, interfacial
anisotropy, and low dimensional accuracy and surface quality.97, 100 Furthermore, parts
fabricated via LOM have been demonstrated to have highly anisotropic properties
depending on the orientation of the part.175 Also, due to the intrinsic subtractive nature of
this process, some material and time is wasted in cutting the layers and excess material. 97,
99, 124
Due to the recent demand of complex ceramic structures, development and use of
LOM has been relatively limited.100

23
 Table 2. Overview of Popular AM Technologies.
Method Feedstock Mechanisms Resolution Applications Notes References
& Reports
DIW Slurry Extrusion 10μm-mm Structural -Simple, cheap, quick 20, 92, 95,
components, -Low quality and 100-115
porous poor properties
ceramics
IJP Slurry Extrusion 10μm-mm Thin layers, -Slow, limited in 92, 95, 97,
(droplets) small size/shape of parts 116, 117
electronics
CC Slurry Extrusion mm-cm Large -Good mechanical 92, 97
structures properties
SL Slurry Polymerization 1-100μm Small, complex -High quality parts 92, 94, 95,
(photo- structures -Complex, slow, 118-125
active) expensive
DLP Slurry Polymerization 1-100μm Small, complex -More efficient and 95, 126-
(photo- (photo mask) structures similar quality to SL 128
active)
TPP Slurry Polymerization nm-μm Small, -Very high resolution 95, 129-
(photo- (two photons) functional -Long process, 137
active) ceramics limited to small parts
SLS Powder Sintering 100μm-mm Porous -Complex and 94, 95,
ceramics expensive 139-147
-Needs supporting
processes
SLM Powder Melting/fusion 100μm-mm Structural parts -Complex and 92, 95,
expensive 148-158
-Not desirable for
refractory materials
DED Powder Melting, 250μm-mm Large -Improved from SLM 92, 94, 95,
extrusion structures 119, 159-
162, 183
3DP Powder Binder jetting 50μm-mm Lightweight -Simple and versatile 92, 94-96,
and porous -Lacking mechanical 119, 138,
parts properties 163, 164
FDM Bulk-solid Extrusion 100μm-mm Functional -Cheap and quick 92, 94, 95,
(composite composites -Limited materials, 119, 165-
filaments) poor quality parts 168
LOM Bulk-solid Cutting, 100μm-mm Large -Anisotropic 92, 94, 95,
lamination structures properties, limited 119, 159,
complexity 169, 170

24
 3. Current State and Future of Ceramic 3D Printing
AM technology of the present day has some clear advantages as well as
disadvantages when compared to conventional manufacturing methods. The recent
demand for highly complex ceramic parts for various applications has led to research and
development in the field of AM technologies with the goal of optimizing the process to
maximize the many benefits that AM presents. AM technology has grown and changed
rapidly in the past few decades, which makes it difficult to predict what state it will
eventually attain in the future. With current trends in development, however, it is predicted
that AM technology will continue to merge with conventional subtractive and equivalent
manufacturing and eventually even become more valuable to industry than either of the
two.98 In recent years, AM technology has become more widely accepted, affordable, and
user-friendly to the point where the general public can have success operating a 3D printer
without extensive training or experience and possibly even own one in their own home.97-
98, 102
This assimilation of AM technology into industry as well as society is important as
the exposure will hopefully spark interest and attention towards future progress. Although
it is not clear where AM technology will be years from now, there are certainly predictions
and considerations that can and should be made.
The aerospace and medical industries are currently some of the heaviest users of
ceramic AM technology and illustrate significant promise for the future of this field.97
Aircraft and other vehicles require parts with high strength-to-weight ratios and complex
cross-sections that can also withstand high temperatures and are resistant to corrosion,
which makes 3D printing as well as ceramics a great fit.97, 99, 176 Ultra-high temperature
ceramic and ceramic composite components are already commonly used in the aerospace
industry and the implementation of AM technology has and can continue to advance the
aerospace field. Another area that is of specific interest within the aerospace industry is
spare parts and repair. Repairing aircraft and vehicles can be very inefficient in terms of
cost and time, particularly if the required part is very old or complex and only a small
number of the part is needed.99 Using modeling software, it is possible to design spare
parts that are outdated or no longer available which could drastically affect the spare parts
supply chain and repair markets. Furthermore, it is costly for a company to keep spare parts

25
and equipment in their inventory just in case there is a need; with AM technology, the part
can be produced as needed.100, 176
In the medical industry, ceramic AM technology has been nearly essential for bone
scaffolds, tissue engineering, implants, and other patient-specific custom products.97, 177
Through scanning and modeling, nearly perfect prosthetics and replacement parts can be
made which improves the insertion process as well as the patients adjustment to the part,
recovery time, and overall functionality.99, 178 AM can even be useful for surgeries where
doctors can create physical models which are used to prepare and practice ahead of time.97,
99, 179-180
These models can be shared among different doctors and researchers relatively
quickly and easily to ensure that the design is proper and make any adjustments that are
necessary.97 Finally, the controlled porosity of ceramics fabricated via AM technology is
essential for bone ingrowth and proper stiffness and strength in implants.99, 181 All of the
aforementioned attributes explain how AM technology yields parts which enable faster and
easier operations with better results both functionally and cosmetically than those produced
99, 182-184
via other methods. There are some drawbacks that limit progress in this field,
such as difficulties in adhering to the FDA.97, 185-186 Furthermore, there are material issues
from a bio- and machine-compatibility perspective. Since not all ceramics are appropriate
as biomaterials, and not necessarily all biomaterials can be printable, there are fairly limited
options for material selection.97, 187
From a production standpoint, AM is not currently comparable to conventional
methods for mass production and may not be for some years without changes in perspective
or technological development. Mass production is different from mass customization
(large amount of parts unique from each other), in which 3D printing has a significant
advantage over conventional technology.97 According to a study conducted on producing
identical parts using conventional and 3D printing technology, the 3D printed parts were
approximately 10 times more expensive per part. Furthermore, when incorporating the
tooling and set-up costs to fabricate the parts with conventional technology, the 3D printed
parts were only more cost-effective in smaller volumes (27 parts or less) after which the
conventional technologies became cheaper to maintain production.102 So, in order to reap
the benefits of AM technology, it is necessary to step back and reconsider how to design,
create, and use parts from a different point of view without concern about conventional

26
limitations or how things have always been done.98 Maximizing materials and energy
savings in the future through innovative design will not only improve individual businesses
but also have positive environmental and societal impacts as well. Realistically, it is
unreasonable to assume that 3D printing will dominate all manufacturing, but ideally,
companies of the future will find ways to implement AM together with conventional
manufacturing where they are best suited for optimal efficiency. Such strategy is even
being implemented at some companies currently.98 Ultimately, the significant economic
benefits of AM are dependent on innovative design and implementation strategies.

C. Aims of this Work

As has been discussed within the literature review of this thesis, both transparent
ceramics and 3D printing technology are exciting fields of research which have received
significant interest in recent years and have great potential for the future. The combination
of these technologies has the potential to yield very interesting and useful results in terms
of producing customizable transparent ceramic structures. The combination of these
technologies can be used for a variety of applications, such as in the military, defense, and
aerospace industries for armor, windows, domes, laser devices, and more. Due to ceramic
AM technology being relatively under researched and developed, it has been inherently
difficult to produce high quality and fully-dense structural ceramics; however, AM
technology currently has significant use for manufacturing porous and/or hollow
structures. For the fabrication of transparent ceramics, it is absolutely necessary that the
structure be as dense as possible with minimal defects, pores, impurities, secondary phases,
and surface roughness. These requirements create challenges towards realizing the benefits
of combining 3D printing as a forming method for transparent ceramics. In the literature,
there have been some, but very few, reports on 3D printing with optical ceramic materials
such as Jones et al.62 who fabricated transparent YAG laser ceramics via DIW as well as
Pappas et al.188 who used DED to fabricate transparent MgAl2O4 ceramics. These reports
on 3D printing of transparent ceramics do not demonstrate the formation of complex
shapes, and utilize either complex and expensive processes (DED) or require significant
treatment process like CIP and post-HIP to achieve the reported results.

27
 The goal of the research effort reported in this thesis is two-fold. First, these efforts
are undertaken to demonstrate that 3D printing technology can be used to fabricate
transparent ceramics with additional characteristic benefits of simple and efficient
production of complex structures of varying shapes and sizes. Second, this work will
compare the 3D printed transparent ceramics to more conventional transparent ceramic
forming methods, such as CIP, processed in a similar way to determine if there is a
significant difference in quality and properties. To achieve these goals, green ceramic
bodies will be formed of varying shapes and sizes using an alumina slurry along with the
Hyrel System 30M 3D printer and EMO-25 extruder, which is a DIW technology. Slurry
and 3D printing parameters will be optimized to yield the best quality parts and post-
processing steps will be established to achieve transparency in the fabricated ceramics.
Additionally, CIP ceramics will be formed and processed under nearly identical conditions
as a comparison. Finally, testing and characterization will be performed on all samples to
evaluate quality and properties.

II. STARTING MATERIALS AND SLURRY PREPARATION

A. Introduction
Fabricating ceramics which can achieve transparency begins with high quality
starting materials. Impurities and improperly processed powders can inhibit densification
as well as introduce undesirable scattering sources. Therefore, it is critical to select highly
pure powders for starting materials. For these reasons, Baikalox High Purity CR 10D
Al2O3 with 625 ppm MgO was the powder of choice. In this case, the additional MgO is a
sintering aid that is known to enhance densification and suppress grain growth, which is
desirable for transparent ceramic processing.22
Using the alumina powder, a slurry is prepared that is suitable for 3D printing with
the Hyrel System 30M and EMO-25 extruder. Slurry preparation requires the use of a
medium, in this case deionized (DI) water, as well as additives, such as binders and
dispersants, to achieve desirable rheological behavior. In this work, Kuraray ISOBAM™-
104 was used, which is an alkaline hydrosoluble copolymer of isobutylene and maleic
anhydride that has previously demonstrated success in transparent ceramic processing and

28
is notable for its multifunctional use as both a binder and dispersant in a variety of
applications.189-194 Similarly, CIP processing also typically requires the use of a binder. In
this case, PVA was chosen, since it is a commonly used binder for pressing applications
due to its ability to improve compaction and mechanical properties in formed parts.64-67
After forming, it is necessary that these organic materials be removed prior to sintering
through a “debinding” process which is discussed later in the post-processing sections.
These investigations have been conducted to characterize the raw alumina powder
for slurry and CIP processing as well as establish “printable” slurry parameters. The
powder is evaluated for phase purity, specific surface area (SSA), and particle size and
distribution with slurries evaluated for rheological properties both quantitatively and
qualitatively with relation to the desired printing behavior.

B. Experimental Procedure
The raw Baikalox High Purity CR 10D Al2O3 powder was ball-milled in ethanol
using high purity zirconia milling media for 24 hours to achieve a fine particle size. After
24 hours of milling, the powder was dried for 24 hours to from a solid cake which was
crushed via a mortar and pestle to obtain a usable powder. The obtained powder was
characterized via X-ray diffraction (XRD) using a Bruker D2 Phaser X-Ray
Diffractometer. The measurement was conducted using Cu k-alpha radiation (30 kV, 10
mA) with a start and end angle of 10° and 75° 2Ɵ, respectively, step size of 0.02° 2Ɵ, and
count time of 0.3s. The results were then analyzed using the MDI Jade 9 and PDF-4+
software as well as the ICDD database for phase identification. The powder was further
characterized for specific surface area (SSA) using the Brunaer-Emmet-Teller (BET)
method and a Micromeretics FlowPrep 060 to first de-gas the sample for 10 minutes at
room temperature, 1 hour at 150°C, and another 10 minutes at room temperature followed
by performing the measurement using a Micromeretics Gemini VII. The powder was
characterized for particle size (diameter) and distribution via preparation of an
approximately 4 vol% solids-loading slurry dispersed with Darvan C which was measured
using a Micromeretics SediGraph III PLUS. To accompany the particle size and
distribution data, a representative image of the powder was taken using an FEI Quanta

29
200F Scanning Electron Microscope with secondary electrons to show the particle size and
morphology.
For evaluating 3D printing rheology, multiple ceramic slurries were prepared with
varying proportions of batch ingredients according to the following recipe: 68-74 weight
percent (wt.%) Baikalox High Purity CR 10D Al2O3 powder with 625 ppm MgO, 26-32
wt% DI H2O, and 0.4-1.0 wt% ISOBAM™-104 per mixed Al2O3 powder. The
aforementioned range of batch ingredients was based upon preliminary research efforts as
well as review of similar work in literature.62, 195-196 The alumina/water and isobam content
were incrementally varied to experiment with the effect of each on the resulting rheological
properties of the slurry. In one set of batch recipes, the isobam content was held constant
at 0.7 wt% per mixed alumina powder, while the alumina content was varied in increments
of 2 wt% between 68-74 wt%. In another set of batch recipes, the alumina content was
held constant at 72 wt%, while the isobam content was varied in increments of 0.3 wt%
per mixed alumina powder from 0.4-1.0 wt%. The slurries were prepared by combining
the batch ingredients in a shear mixer until a viscous paste was formed. Zirconia milling
media of about 1 cm diameter was then added to the milling jar and the jar was placed in
an MTI Corporation SFM-1 Desk-Top Planetary Ball Miller for 1 - 1.5 hours. Once milled,
the slurry was loaded into a shear mixer within a customized vacuum chamber to de-gas
for at least 15 minutes, or until the air bubbles were removed.
Following de-gassing, the prepared slurry was loaded onto the measurement plate
of a TA Instruments Discovery HR-2 Hybrid Rheometer. Observations regarding the flow
behavior and shape retention of the slurry were made during the loading process.
Measurements on the rheometer were conducted at room temperature under the flow sweep
procedure at shear rates from 0.01 – 100 s-1 using a 40 mm parallel plate geometry to obtain
curves of viscosity as a function of shear rate. Flow behavior was calculated directly from
the curve of viscosity as a function of shear rate using the power law function:
ƞ = 𝐾𝑦̇ (𝑛−1) (1)
where ƞ is the viscosity, 𝑦̇ is the shear rate, K is the consistency index (viscosity at 𝑦̇ =1),
and n is the power law index (also known as flow index). Different flow index values
correspond to different flow behaviors: n equal to 1 is Newtonian, n greater than 1 is shear
thickening, and n less than 1 is shear thinning.197-198

30
C. Results and Discussion
The XRD measurement for the prepared Baikalox CR 10D Al2O3 powder is shown
in Figure 5 along with the pattern for PDF #04-015-8996 which was obtained through the
search and match tools using the MDI Jade 9 and PDF-4+ software/database. This PDF
card corresponds to a synthetic corundum (α-Al2O3). It can be seen from the figure that
the two diffraction patterns are nearly identical with no detectable second phases,
indicating that the prepared powder is of a single α-Al2O3 phase. Since the amount of MgO
in the powder (625 ppm) is well below the detection limits of XRD, no secondary MgO
XRD lines were observed. BET measurements reported that the specific surface area of
the prepared powder was 7.9571 ± 0.1174 m2/g. The particle size and distribution data are
presented in Figure 6. The D10, D50, and D90 represent the mass-percentage of particles
with diameters less than or equal to the indicated values. For the prepared alumina powder,
the D10, D50, and D90 were 0.384, 0.731, and 2.238 µm, respectively (e.g. 10% of
particles had a diameter less than or equal to 0.384 µm, and so on). The particle size with
the highest mass-frequency was 0.596 µm. Additionally, an SEM image of the powder
taken with secondary electrons is shown in Figure 7 and is consistent with the particle size
and distribution data as well as shows that the particles are relatively spherical (although
not perfectly spherical) in shape. For maximum packing efficiency and contact between
particles, it is ideal to have particles which are as close to perfectly spherical in shape as
possible. These combined data demonstrate that the prepared powder exhibited pure phase
and a narrow particle size and distribution centered on a fine particle size. These traits are
all desirable for the processing of transparent ceramics in terms of reducing potential
scattering sources and improving sinter ability through the high degree of contact between
fine particles with high surface areas.

31
 Figure 5. XRD analysis of prepared Al2O3 powder.

Figure 6. Particle size and distribution of prepared Al2O3 powder.

32
 Figure 7. SEM image of prepared Al2O3 powder.

In terms of slurry rheology, the proportions of the batch ingredients were found to
significantly affect the viscosity and therefore the behavior of the slurry both qualitatively
and quantitatively. The rheology as a function of alumina content is presented in Figure
8 and the rheology as a function of isobam per mixed powder is presented in Figure 9
along with the calculated flow index (n) values, using Equation 1, in parentheses for the
slurries in both figures. The data shows that incremental increases in alumina and isobam
per mixed powder as small 2 wt% and 0.3 wt%, respectively, can cause as large as an order
of magnitude increase in the viscosity (Pa-s). In practice, higher viscosity slurries tend to
exhibit greater control and shape retention than the lower viscosity slurries. On the other
hand, the higher viscosity slurries tended to be more difficult to prepare in terms of
achieving a homogeneous mixture and thorough de-gassing. Therefore, maintaining a
balance between these parameters is necessary for printing. In this case, slurries with 68 –
74 wt% alumina and 0.4 – 1.0 wt% isobam per mixed powder are considered “printable”,
with the slurry containing 72 wt% alumina and 0.7 wt% isobam per mixed powder being
ideal for printing. Below this range, slurries are not sufficiently viscous for printing and
above this range, slurries are too viscous to be properly prepared for printing. Furthermore,
it can be seen from both Figure 8 and Figure 9 that all the slurries possessed flow index
values less than 1, indicating shear-thinning behavior. Shear-thinning describes a decrease

33
in viscosity with increasing shear rate, which can also be seen and confirmed by the data.197
In comparison to other similar works in literature,195-196 the slurries in this thesis exhibited
significantly lower flow index values, indicating very strong shear-thinning behavior.
Practically speaking, shear-thinning behavior is necessary for 3D printing with a DIW
printer and indicates that there will be a transitional period of rheological behavior before,
during, and after extrusion with the 3D printer. Shear-thinning behavior enables a high
viscosity slurry to become fluid enough to be extruded through a small nozzle and then
return to a higher viscosity state, allowing for proper flow and workability during printing
without sacrificing control and final shape retention. Shear-thinning behavior will also be
taken into consideration and discussed in subsequent sections of this thesis with regard to
3D printing parameters and achieving a balance between factors such as throughput,
printing time, and quality of the printed parts.

34
 Figure 8. Rheology of slurries with varying alumina content.

Figure 9. Rheology of slurries with varying isobam per mixed powder.

35
D. Conclusions
In summary, this section discusses the starting materials used for 3D printing and
how they were used to prepare slurries of suitable behavior for 3D printing. The alumina
powder used contained a small fraction of MgO as a sintering aid, but was otherwise of
single α-Al2O3 phase according to the results of XRD analysis. Additionally, the powder
was also characterized for SSA and particle size and distribution. It was determined that
the particles had a relatively high SSA of 7.9571 ± 0.1174 m2/g, were fine in size (50% of
the particles less than 0.731 µm), and narrowly distributed in size. Furthermore, SEM
images show that the particles were not perfectly spherical in shape, yet were relatively
spherical. This combined data is desirable for transparent ceramic processing in terms of
reducing potential for scattering sources due to impurities as well as indicating good
potential densification behavior due to the small particle sizes with high surface areas and
nearly spherical shapes for increased packing efficiency and contact between particles.
Using this alumina powder, along with DI water and isobam, multiple slurries were
prepared to evaluate rheological behavior both quantitatively and qualitatively. The
rheology measurements show that small incremental increases in alumina content and
isobam per mixed powder causes significant increases in the viscosity. Higher viscosity
slurries are found to exhibit more desirable behavior in terms of potential for 3D printing,
but are less process-friendly and consistent than the slurries of lower viscosity. For these
reasons, it has been determined that slurries used for 3D printing should contain between
68 – 74 wt% alumina and 0.4 – 1.0 wt% isobam per mixed powder, with an ideal slurry
containing 72 wt% alumina and 0.7 wt% isobam per mixed powder. Outside of this range,
slurries are either too viscous to prepare properly, or too fluid for printing. Additionally,
it has been determined that each slurry exhibited shear-thinning behavior, which is
necessary for extruding a viscous slurry through a small nozzle during 3D printing.

36
 III. 3D PRINTING AND CIP PROCESSING

A. Introduction
Conventionally, transparent ceramics have been fabricated using methods such as
dry pressing and cold isostatic pressing (CIP), casting, hot-pressing (HP), and others as
mentioned previously in Section I part A.199 These fabrication methods are capable of
producing quality transparent ceramics, but are either incapable of fabricating complex
structures of varying shapes and sizes or require additional tooling costs and processes to
complete such structures. It was not until relatively recent advances in technology occurred
that 3D printing has emerged as a viable option for the fabrication of transparent ceramics
with the additional benefit of freedom of design. Despite the progress that has been made,
3D printing of quality ceramic structures which possess minimal defects and can be
sintered to near-full density requires careful consideration and fine-tuning of the many
interconnected parameters in the printing process.
The 3D printer used in this work, the Hyrel System 30M, utilizes a fixed extruder
outfitted with different sized nozzles and a computer-controlled build platform which can
move in the x-, y-, and z-direction as shown in Figure 10. Within the 3D printer’s Repetrel
software, various part files can be directly uploaded and edited in terms of size and
orientation. Furthermore, different printing parameters can be edited within the Slic3r
software to change the printing behavior and achieve various outcomes. The most directly
applicable 3D printing parameters for the research conducted in this thesis were print move
speed, nozzle size, extrusion width, and layer height. Fill density is another important 3D
printing parameter and needs to be held at 100% in order to achieve a full density part; for
this reason, the fill-density parameter was not experimented with. Print move speed
controls the speed of the movements when the extruder is actively building the part. Nozzle
size is the inside diameter of the chosen nozzle. Extrusion width is how wide of a path is
extruded by the printer and affects how many paths comprise a layer. Layer height is how
tall each layer is and therefore how many layers make up a part. The majority of these
parameters can be further broken down into subsections for a higher level of control, such
as initial layer height, external perimeter print speed, and others. It should be noted that,
due to the shear-thinning slurry behavior, 3D printing parameters which effect throughput

37
will also change the viscosity of the slurry. Since transparent ceramics require near full
theoretical density, the parameters must be optimized to achieve a nearly defect free and
dense structure. Once each parameter is set, the “print recipe” is saved and “sliced” into
G-code (programming language for 3D printers), and the print job is ready to begin.
Following printing, it is necessary for the part to dry to complete the forming process.

Figure 10. Diagram of Hyrel System 30M 3D printer.

Compared to the 3D printing process, CIP processing is relatively simple. In CIP

processing, a dry-press is first used to compact the powder into a disc with the assistance
of a binder. Polyvinyl alcohol (PVA) is a commonly used binder for these purposes and
was therefore employed for this study.64-67 The formed disc is then isostatically-pressed
by applying pressure via an immersion fluid to compact the disc further and create a
relatively dense green body. The dry-pressing and isostatic pressing processes possesses
few parameters and can only produce limited shapes.
These investigations have been conducted for the fabrication of many samples
while optimizing the 3D printing parameters to consistently obtain high quality and dense
green ceramic structures of varying shapes and sizes in an efficient manner. Furthermore,
the work that has been completed in this section has a direct impact on the results of
subsequent post-processing and the final quality of the parts. The 3D printing process and

38
parameters have been evaluated for printing time, using visual observations during and
after printing, and green body density measurements.

B. Experimental Procedure

1. 3D Printing
Different part files were directly uploaded to the Repetrel software in the .stl file
format and edited as desired for both orientation and scale in the x-, y-, and z-directions.
Printing parameters were selected in the Slic3r software to create a print recipe. Each of
the following printing parameters were experimented with to create a wide variety of print
recipes: print move speed, nozzle size, layer height, and extrusion width. The following
print recipe was used as a “default” from which the previously mentioned parameters were
varied from and compared with: print move speed of 1.5 mm/s, nozzle size of 0.84 mm
(18-gauge), layer height of 0.4 mm (~47.6% of nozzle size), and extrusion width of 1.26
mm (~150% of nozzle size). Although there were subsections which could be controlled
to a higher degree of fidelity, most parameter sets were held constant for a given run. For
example, if the print move speed was set to 1.5 mm/s, every print move speed within the
run was performed at 1.5 mm/s, rather than making individual adjustments for specific
print moves such as external perimeters, infill, and others. Once all the printing parameters
were set, the part file was sliced into G-code according to the print recipe using the Slic3r
software.
An identical process from Section II part B of this work was used to prepare the
“ideal” printable slurry containing 72 wt% Al2O3 and 0.7 wt% isobam per mixed powder.
Following preparation of the slurry, the EMO-25 extruder was loaded with the slurry using
a plastic spatula and fitted with the nozzle that corresponded with the printing parameters.
The nozzle sizes available with their inside diameters in parentheses were: 16-gauge (1.20
mm), 18-gauge (0.84 mm), 20-gauge (0.60 mm), and 22-gauge (0.41 mm). The extruder
was then secured and screwed tightly into the extruder assembly on the 3D printer. Next,
a sheet of mylar was taped onto the glass build plate for the part to be built upon. The
extruder was calibrated to the build plate with the mylar on it using the z-calibrate feature
and a piece of paper placed on top of the build plate and mylar by raising the build platform
until the nozzle barely contacted the paper. Once calibrated, the extruder was primed until

39
there was a consistent flow of material, after which the print job was started and the
calculated time to print provided by the software was recorded.
During printing, observations were recorded to evaluate slurry performance,
adhesion and formation of layers, infill characteristics, dimensional accuracy, and visual
quality. Small adjustments were made mid-print using the “z-fine adjustment” tool which
moves the build platform vertically by 25 µm to maintain a proper distance between the
nozzle and build platform or part as needed. Following a successful print, the mylar sheet
with the part built upon it was removed from the build plate and set aside for visual
inspection prior to being covered for drying at room temperature for 24 hours. This process
was repeated for all printed parts. After 24 hours of drying, the parts which were of
acceptable quality were kept for additional processing. Furthermore, the green body
density was recorded of numerous parts that could be determined from physical
measurements using a caliper and mass balance. The green body density was reported as
a percentage relative to the theoretic density of α-Al2O3 reported to be 3.98 g/cm3 within
the ICDD database as matched from the previous XRD analysis in Section II part C. It
should be noted that more precise density measurements, such as the Archimedes method,
could not be used since an immersion fluid would destroy the green body.

2. CIP Processing
The powder was prepared for dry-pressing by combining approximately 2 g of
alumina powder with 0.25 mL of Sigma-Aldrich 98-99% hydrolyzed PVA (molecular
weight 31,000- 50,000) and thoroughly grinding and mixing the two with a mortar and
pestle until homogenized. The mixture was then loaded into a circular die of approximately
2 cm in diameter and dry-pressed using a Carver, Inc. manual uniaxial press with a force
of 1 metric-ton for 5 seconds. The resulting pressed disc was wrapped using pieces of a
nitrile glove and placed within a flexible plastic pouch. The plastic pouch was vacuum
sealed and loaded into an Autoclave Engineers CP 360 Cold Isostatic Press. The CIP
process was performed at a pressure of 250 MPa for 1 minute. Following CIP, the disc
was removed and measured for its volume and mass to calculate the green body density.
This process was repeated to produce many nearly identical samples.

40
C. Results and Discussion

1. 3D Printed Samples
Various ceramic parts were successfully fabricated via 3D printing using the Hyrel
System 30M, EMO-25 extruder, a variety of print recipes, and printable slurries.
Representative images of the fabricated ceramics are shown in Figure 11. These images
demonstrate the capacity of 3D printing to fabricate many different parts which vary in
both complexity of shape as well as size utilizing the same materials, equipment, and
processes. Printing parameters were observed to significantly affect the efficiency of the
printing process, throughput and slurry behavior (due to shear-thinning), and quality of the
output part. Although many printing defects are inherent to the 3D printing process, such
as inconsistent printing of paths/layers, formation of “blobs”, insufficient initial and
interlayer adhesion, and others, experimenting with and optimizing the printing parameters
made it possible to mitigate or even remove some defects. If a part possessed printing
defects which were deemed too significant to be usable, however, the part was not carried
through the remainder of the process. The average relative green body density across the
measured 3D printed parts was 1.68 ± 0.05 g/cm3.

Figure 11. 3D printed alumina ceramics.

41
 a. Print Move Speed
The print move speed was found to have a significant effect on the continuity of
the print as well as initial and interlayer adhesion. When the print move speed was set to
5 mm/s or higher, the build platform was moving too fast for the slurry to handle. This
mismatch resulted in improper adhesion and print lines became inconsistent or broken. In
other words, the slurry was unable to sufficiently flow out of the extruder and form onto
the surface (build plate or previous layer) causing the extruded slurry to tend to be pulled
into a line which was thinner than expected and would often break. This problem was
mitigated by decreasing the print move speed. However, when the print move speed was
set to 1 mm/s or slower, the print time became excessively long. The slow print speed and
long print time would cause additional problems such as print lines and layers drying mid-
print and the nozzle becoming clogged due to the low throughput. Therefore, a print move
speed between 1.5 – 4.0 mm/s was slow enough to enable proper adhesion and forming,
yet fast enough to prevent premature drying and clogging of the nozzle. Figure 12 shows
the print time (calculated by the Repetrel software) as a function of print move speed for a
rectangular prism part of 15 mm x 15 mm x 3 mm printed under the default parameters. It
can be seen that small changes in the print move speed, such as 1 mm/s, can affect the print
time of the parts in this study significantly. Within the 1.5 – 4.0 mm/s range, there was no
significant difference observed in the quality/properties of the printed parts.

Figure 12. Printing time as a function of print move speed.

42
 b. Nozzle Size, Layer Height, and Extrusion Width
The nozzle size, layer height, and extrusion width are all somewhat interconnected
and are typically expressed as percentages of one another. It was crucial to fine-tune these
parameters to achieve good quality parts. In general, smaller values of these parameters
results in improved quality and accuracy of the printed parts at the cost of longer printing
times/lower throughput. However, as with the previous parameters, it was determined that
is necessary to find a well-balanced set of values for each to achieve the best quality parts.
Depending upon the size of the part, certain nozzle sizes are more appropriate. For
example, if a relatively large nozzle size is chosen for a small part with fine details, the
nozzle will be inherently incapable of producing it well. On the other hand, if a small
nozzle is chosen for a large part, the part will take excessively long to print, causing issues
with premature drying and adhesion between layers. Figure 13 presents the printing time
(calculated using the Repetrel software) as a function of nozzle size for the same 15 mm x
15 mm x 3 mm rectangular prism part printed under the default parameters. It can be seen
that as the nozzle size decreases, there is a significant increase in printing time. As a
general guideline and recommendation based on observations, the nozzle size should not
exceed about 20% of the smallest feature size nor be smaller than about 4% of the longest
dimension of the structure to maximize both quality and efficiency.

Figure 13. Printing time as a function of nozzle size.

43
 For layer height, it has been determined that a value between approximately 25 –
70% of the nozzle diameter is suitable for printing. When the layer height is larger than
70%, despite the print time being fast, there is an excessively high throughput which leads
to poor control and low dimensional accuracy. However, when the layer height was less
than 25% of the nozzle size, the layers were too thin and the print time was too long,
resulting in pre-mature drying and poor interlayer adhesion/formation. In between this
range, adequate parts could be produced. Figure 14 shows the calculated printing time as
a function of layer height for the same aforementioned rectangular prism part being printed
with a 0.84 mm nozzle. It can be seen that as the layer height increases, the printing time
decreases, until around 0.4 mm (~48% of nozzle size), after which there is not a significant
decrease in printing time. This indicates that there is no significant additional benefit in
increasing the layer height past thus point, making a value of ~48% of the nozzle size an
ideal balance between quality and time efficiency. It was also found that the layer height
does more than effect the part visually. When five identical parts were all printed under
the default parameters but with different layer heights, there was a measurable difference
in their green densities. Figure 15 shows the relative green density of the five parts as a
function of their layer height along with inset images of the green parts with 0.3 mm and
0.5 mm layer heights as an example for visual comparison. It can be seen that as the layer
height increases, the relative green density decreases. This is likely due to two observable
tendencies. First, parts with small layer heights tend to have smaller defects that are
“corrected for” or “covered up” by the many layers which are printed on top of it as
opposed to parts with large layer heights which typically have larger defects and fewer
subsequent layers to fix the problem. Second, parts with large layer heights tend to form
more loosely packed edges and layers as opposed to the small layer height parts which
form more densely packed layers and edges as supported by the inset photographs shown
in Figure 15.

44
Figure 14. Printing time as a function of layer height.

Figure 15. Green density as a function of layer height.

45
 For extrusion width, the parameter needs to be just large enough to sufficiently infill
the part and create a dense green body. A value of approximately 150% of the nozzle size
is sufficient for creating an overlap between each printing path, thus ensuring complete
infill without noticeable changes in surface/edge finish or dimensional accuracy. Figure
16 shows the results of using vastly different extrusion width values to clearly demonstrate
its effect. When the extrusion width is too small, there are gaps in the structure (insufficient
infill/under-extrusion), and when the extrusion width is too large, there is a loss in accuracy
and fine finish on the surface/edge (over-extrusion). When the parameters are set properly,
however, it can be seen that a filled-in structure with a fine finish along the surface and
edges is achieved.

Figure 16. Alumina ceramics 3D printed with varying extrusion widths.

46
 2. CIP Samples
The CIP process does not possess many variables to fine-tune and the parts
produced were of sufficient quality under the set processing parameters. An example
picture of a CIP processed sample is shown in Figure 17. It can be seen that the part is of
a simple disk-shape and does not possess any significant defects. The average relative
green body density of the CIP samples was 1.71 ± 0.04 g/cm3. This density value is higher
and has a smaller standard deviation than the 3D printed samples, indicating that CIP
processing can produce denser and more consistent green bodies. A possible explanation
for this is the combination of a lack of significant inherent defects introduced in the dry-
pressing process as well as the subsequent CIP process enables a higher formed density
through additional compaction.

Figure 17. CIP processed alumina ceramic.

47
D. Conclusions
In this section, it has been demonstrated that 3D printing can produce alumina
ceramic samples of varying shapes/sizes without making significant changes to the
materials and processes. Furthermore, quality printed parts and efficient processing can be
achieved through the optimization of printing parameters such as: print move speed, nozzle
size, layer height, and extrusion width. Additionally, CIP alumina samples of nearly
identical disk-shapes were fabricated as a conventional forming method to compare to.
The CIP samples achieved slightly higher green body densities and less variability in the
density values than the 3D printed samples, which is likely due to inherent defects
introduced through 3D printing that are not experienced in dry-pressing as well as the
additional compaction of the body through the subsequent CIP process.

IV. POST-PROCESSING

A. Introduction
Following fabrication of a ceramic part, post-processing steps are critical to
maintaining the quality established in the green bodies and achieving the desired final
properties. Drying and debinding processes involve heating the ceramic to a temperature
at which the water and organic content is fully removed to prepare the green body for
sintering. Vacuum sintering processes are then used to densify the green body. Lastly,
polishing is performed to reduce the surface roughness. In these processes, it is important
to handle the samples carefully and use slow heating rates to avoid cracking.
As mentioned previously, transparent ceramics are able to achieve the greatest
optical quality when they simultaneously possess high density, minimal defects and
secondary phases, low surface roughness, and small grain sizes. Traditionally, single-step
sintering (SSS) profiles are used to densify ceramics, through simply heating the ceramic
from room temperature to a chosen temperature, holding at that temperature for a chosen
time, and cooling down to room temperature. It is well-known, and has been widely
demonstrated, that as sintering temperature and sintering time increase, the density and
grain size will increase as well. This means that at the high temperatures required for full-
densification, significant grain growth will also be present. For optical ceramics, it is

48
important to have small grain sizes to reduce scattering. Furthermore, from an application
stand point, large grain sizes are also undesirable in achieving good mechanical
properties.200-202 As an alternative, two-step sintering (TSS) profiles offer a potential
improvement for transparent ceramic processing. Using a TSS profile, the ceramic is
heated from room temperature to the sintering temperature and then immediately dropped
to a lower temperature where it is held for a chosen time before cooling back down to room
temperature. For a TSS profile, the high starting temperature initiates grain boundary
diffusion, while the lower holding temperature limits grain boundary migration, thus
enabling the suppression of grain growth during densification.203-205
In this section, the drying, debinding, sintering, and polishing parameters are
studied for the 3D printed and CIP processed ceramics. These investigations include using
both SSS and TSS profiles. The goal of this study has been to establish a set of parameters
which is capable of achieving high-quality transparent ceramics as well as determine the
merit of 3D printing as a method of transparent ceramic fabrication when compared to
more conventional methods. The fabricated ceramics have been evaluated for their quality
and transparency using their phase purity, density, average grain size, and total
transmittance as well as their visual appearance.

B. Experimental Procedure
The previously prepared 3D printed and CIP samples, as described in Section III,
were transferred to a Quincy Lab, Inc. Model 30 GC Lab Oven held at approximately 70°C
and dried for 24 hours. The oven dried samples were then debinded in a Neytech Vulcan
Benchtop Furnace Model 3-550 under the following profile: heat from room temperature
to 500°C at a rate of 0.5°C/min, hold at 500°C for 4 hours, heat from 500°C to 1000°C at
a rate of 1°C/min, hold at 1000°C for 3 hours, auto-cool to room temperature. The debinded
samples were placed into a Thermal Technology Inc. High Vacuum Graphite Furnace to
sinter using either an SSS or TSS profile. The SSS profile was as follows with the final
sintering temperature ranging from 1650 – 1875°C: heat from room temperature to 1300°C
at a rate of 10°C/min, hold at 1300°C for 1 hour, heat from 1300°C to the final sintering
temperature at a rate of 2°C/min, hold at the final temperature for 6 hours, and cool to room
temperature at a rate of 10°C/min. Alternatively, the TSS profile was as follows: heat from

49
room temperature to 1300°C at a rate of 10°C/min, hold at 1300°C for 1 hour, heat from
1300°C to either 1850°C or 1875°C at a rate of 2°C/min, cool to 1800°C at a rate of
2°C/min, hold at 1800°C for 8 hours, and cool to room temperature at a rate of 10°C/min.
Following sintering, XRD measurements were conducted using a Bruker D2 Phaser
and Cu k-alpha radiation (30 kV, 10 mA) with a start and end angle of 10° and 75° 2Ɵ,
respectively, step size of 0.02° 2Ɵ, and count time of 0.3 s. The XRD data were analyzed
using MDI Jade 9 and PDF-4+ software as well as the ICDD database for phase
identification. Density measurements were conducted using the Archimedes method at
room temperature with DI H2O as the immersion liquid. The sintered samples which were
visibly transparent were grinded down to a flat surface with a thickness of approximately
1.5 mm and polished using a Buehler EcoMet 3000 Variable Speed Grinder-Polisher and
a MetLab Corporation 6 µm diamond suspension. Total transmittance measurements were
conducted on the polished sample using a Thermo Scientific Evolution 220 UV-Visible
Spectrophotometer in the wavelength range from 190-1100 nm. Any opaque, translucent,
or poorly transparent samples were not grinded or measured for total transmittance but still
received polishing for microstructure evaluation. The polished samples were thermally-
etched using the following profile to expose the grain boundaries: heat from room
temperature to 1450°C at a rate of 5°C/min, hold at 1450°C for 5 hours, cool to room
temperature at a rate of 10°C/min. The microstructure was documented by first gold-
coating the sample using a Cressington Sputter Coater 108 and then viewing and capturing
images of the sample on an FEI Quanta 200F Scanning electron microscope using a
combination of both secondary and backscattered electrons. The average grain size was
measured using the line-intercept method and Nano Measurer 1.2 software.

C. Results and Discussion

Transparent alumina ceramics have been successfully fabricated via 3D printing
and post-processing steps including debinding, vacuum sintering, and polishing. Figure
18 shows examples of transparent ceramic samples formed via 3D printing. Figure 19
presents photographs of a representative sample through each step of the process.
Combined, Figures 18 and 19 demonstrate the progression through the process and
transparency of samples visually. When the sintered samples were analyzed via XRD, it

50
was determined that the diffraction pattern matches PDF #04-015-8996 (α-Al2O3) as shown
in Figure 20. As compared with the diffraction patterns from the PDF card (lower),
prepared alumina powder from Section II (center), and a representative sintered alumina
ceramic (upper), it can be seen that the patterns match nearly perfectly with no other phases
detected. The XRD results and analysis indicate that no additional impurities or secondary
phases were accumulated due to the 3D printing and post-processing. The slight variations
in the relative intensities of the peaks among the three diffraction patterns could be due to
differences in the orientation and size of crystallites in a powder as opposed to a
formed/sintered sample. The properties of the fabricated ceramics such as density, grain
size, and transmittance were found to be highly dependent on the sintering conditions.
Sintering parameters were varied in terms of both holding temperature and holding time.
Furthermore, single-step sintering (SSS) and two-step sintering (TSS) profiles were
investigated.

Figure 18. 3D printed transparent alumina ceramics.

Figure 19. Progression of a representative sample through the process.

51
 Figure 20. XRD analysis of sintered alumina ceramics.

Figure 21 presents the relative density as a function of sintering temperature for

3D printed samples sintered using SSS profiles with a hold time of 6 hours and TSS profiles
along with data from CIP samples to compare with. Among all samples, it can be seen that
the density increases significantly with sintering temperature and reaches near-full density
values at the highest tested temperature of 1875°C. Furthermore, it can be seen that all the
samples achieved nearly identical density values at this peak point, with the CIP TSS
samples achieving only slightly higher densities than the rest. The peak density values
achieved by 3D printing were 99.91 ± 0.08% for SSS and 99.92 ± 0.03% for TSS while the
highest CIP density achieved was 99.93 ± 0.05% using a TSS profile. Additionally, the
relative density as a function of sintering time for the 3D printed samples sintered using
SSS profiles at a holding temperature of 1850°C is presented in Figure 22 along with data
from CIP samples to compare with. This data demonstrates that the ceramic does not reach
the highest density attainable until 4 hours of sintering, after which the density remains
relatively constant. Furthermore, even when sintered under identical profiles, the CIP
samples achieved higher densities. Referring back to Section III Part C, the CIP samples
were able to achieve higher green body densities due to fewer processing defects and
additional compaction during forming. The lower green body density of the 3D printed

52
samples limits the density which can be achieved through sintering, which is an explanation
for why the CIP samples were denser after sintering even under identical profiles.
Additionally, also in Section III Part C, the 3D printed ceramics show an increase in green
density with decreasing layer height. When these same samples with varying layer heights
were sintered using identical SSS profiles at 1850°C for 6 hours, the trend of increasing
density with decreasing layer height was observed again, as shown in Figure 23. This
information further supports the claims/results from earlier where it was noted that parts
with large layer heights tend to have larger defects and fewer densely-packed layers and
edges than parts with smaller layer heights.

Figure 21. Relative density as a function of sintering temperature for 6-hour hold time.

53
 Figure 22. Relative density as a function of sintering time at 1850°C.

Figure 23. Relative sintered density at 1850°C for 6 hours as a function of layer height.

54
 The average grain size as a function of sintering time for 3D printed samples
sintered using an SSS profile at 1850°C is presented in Figure 24 along with representative
images of the microstructure from SEM in Figure 25. These data show that both the
average grain size and variation in grain size increases significantly with sintering time.
Furthermore, the SEM images demonstrate this grain size-sintering time trend visually as
well as show that the microstructure is dense and free of significant pores and secondary
phases. Additionally, the average grain size as a function of sintering temperature for 3D
printed samples for both SSS and TSS profiles is presented in Figure 26 along with the
representative images of the microstructure from SEM for the SSS and TSS samples in
Figure 27 and Figure 28, respectively. In a similar trend, the average grain size and
variation in grain size increases significantly with sintering temperature. Furthermore, it
is demonstrated that TSS profiles can lead to a reduction in grain size at identical sintering
temperatures. For example, at 1875°C, the SSS samples have an average grain size of
113.27 ± 17.48 µm while the TSS samples have an average grain size of 87.69 ± 13.53 µm.
Again, the SEM images also demonstrate the grain size-sintering temperature trend
visually and show a dense microstructure free of pores and additional phases. These data,
combined with the previous density data, show that TSS profiles can help to achieve full-
density while limiting grain growth, which is beneficial for transparent ceramic processing.

Figure 24. Average grain size as a function of SSS time at 1850°C.

55
 Figure 25. SEM microstructure images of SSS ceramics at 1850°C for varying times.

Figure 26. Average grain size as a function of sintering temperature for SSS and TSS ceramics.

56
Figure 27. SEM microstructure images of SSS ceramics sintered at varying temperatures for 6 hours.

Figure 28. SEM microstructure images of TSS ceramics sintered at varying temperatures.

57
 The most visually transparent of the sintered ceramics were grinded and polished
down to an approximately 1.5 mm thickness and measured for total transmittance. Samples
which were sintered below 1850°C were found to be either opaque or only somewhat
translucent, so they were not measured for total transmittance. Figure 29 presents the
measured total transmittance data along with inset photographs of some of the best samples
between SSS and TSS as well as 3D printing and CIP processing. It can be seen from both
the data and the photographs that the best 3D printed and CIP processed ceramics achieved
similar levels of transparency. The total transmittance measurements show that the 3D
printed and CIP processed TSS ceramics reached nearly identical maximum values of
approximately 70% at wavelengths above 800 nm (near IR range). In the visible
wavelength range (~400 - 700 nm), however, there was a more significant measurable
difference between the samples. For example, at 400 nm, the CIP TSS sample reached
about 65% transmission while the 3D printed TSS samples reached only about 61% and
59% at 1875°C and 1850°C, respectively. This slight drop-off in transparency from CIP
to 3D printing is likely due to defects introduced in the printing process causing scattering
as well as limiting the density which can be achieved through sintering. The 3D printed
SSS samples were significantly less transparent than the rest and reached at most
approximately 63% transmission at 800 nm and above and even lower in the visible range
when sintered at 1850°C. The reason for lower transmission is because the SSS samples
have both lower densities and larger grain sizes than the other samples. These results
demonstrate that 3D printed ceramics can achieve similar optical properties to CIP
processed ceramics and that TSS profiles can yield improved transparency over SSS
profiles.

58
Figure 29. Total transmittance measurements for alumina ceramics from 3D printing and CIP.

In comparison to the results of other works in literature which used conventional

processing methods, the 3D printed samples in this report achieve similar results. Kim et
al. reported on SPS processed alumina ceramics of 1 mm thickness which achieved a
maximum total transmittance of approximately 80% above 1000 nm, but about 70% at 800
nm.206 Yang et al. also reported on 1 mm thick transparent alumina ceramics fabricated by
isostatic pressing and sintering in H2 atmosphere which achieved a total transmittance as
high as 86% between about 300 – 800 nm.207 These values from literature are higher than
what was achieved in this report, which can be attributed to the significant difference in
thickness between the samples (1 mm vs 1.5 mm), since transmittance decreases with
increasing sample thickness. When taking into account the difference in thickness, though,
the results become more comparable.

59
D. Conclusions
The data in this section indicates a few major conclusions for this work. First,
transparent ceramics of varying shapes and sizes can be made successfully through 3D
printing and post-processing steps including debinding, vacuum sintering, and polishing.
Furthermore, the sintering parameters, such as sintering temperature, hold time, and use of
SSS and TSS profiles, significantly affect the density, grain size, and total transmittance of
the fabricated ceramics. When the sintering parameters are optimized, it is possible to
achieve density and total transmittance values through 3D printing which are similar to
those of more conventional CIP processed structures. The best 3D printed sample was
achieved through a TSS profile up to 1875°C and then held at 1800°C for 8 hours. This
sample reached 99.92 ± 0.03% relative density, an average grain size of 87.69 ± 13.53 µm,
and total transmittance values of about 70% above 800 nm and about 61% at 400 nm. In
comparison, a CIP sample sintered using the same profile achieved 99.93 ± 0.05% relative
density and total transmittance values of about 70% above 800 nm and about 65% at 400
nm.

V. SUMMARY AND CONCLUSIONS

Using an extrusion-based 3D printer and post-processing steps, including

debinding, vacuum sintering, and polishing, it has been demonstrated that near full-density
transparent alumina ceramics of varying shapes and sizes can be fabricated which are of
optical quality comparable to that achieved using more conventional processing methods
for transparent ceramics, such as CIP. The best quality 3D printed transparent ceramics
have been achieved through the use of optimized slurry and printing process parameters as
well as a two-step sintering profile. With conventional methods, fabricating transparent
ceramics of varying shapes and sizes would require additional tooling costs and processes.
In this study, the same 3D printer and process have been used throughout, with only slight
adjustments made to specific parameters. Furthermore, the simplistic nature of the
extrusion-based 3D printer, as compared to other 3D printing methods, along with the
aqueous slurry and use of a vacuum furnace, allows for relatively conventional ceramic
processing principles and know-how to be applied, making the entire process more

60
accessible to a larger population. Using these methods, customizable transparent ceramic
structures can be fabricated with greater relative ease and efficiency via 3D printing and
vacuum sintering when compared with conventional methods without a significant
difference in quality

VI. FUTURE WORK

Additional work in this area could aim towards experimenting with and optimizing
the processing parameters further to see if better results can be achieved. As mentioned
previously in Section II, each 3D printing parameter is divided into additional subgroups,
but these subgroups were held constant for consistency. By experimenting with each
subgroup parameter, higher quality parts can potentially be produced in a more efficient
manner. Furthermore, for the sintering parameters, different combinations of temperatures
and hold times can be investigated in the two-step sintering profiles to achieve higher
densities and smaller grain sizes. Although post-HIP treatment was not used in this work,
it has been widely demonstrated in literature to improve the properties of optical ceramics
and can also be implemented in this process following vacuum sintering.
Due to the versatile nature of the 3D printer used in this work, nearly any ceramic
which can be incorporated into a printable slurry may be compatible with this process. So,
although only Al2O3 was used in this work, other transparent ceramic materials, such as
Y2O3, YAG, and others along with different dopants could be explored as well. Such
materials may yield more functional ceramic parts to be produced via 3D printing, such as
solid-state lasers and scintillating devices.
In this thesis, the main focus was on the optical properties of the ceramics, with no
testing or measurements made regarding the mechanical, thermal, and electrical properties.
Such testing may be an additional area of interest for many researchers to evaluate and see
if 3D printing is still comparable to conventional processing methods in that regard. So,
although the main goals of this research were accomplished, there is still much work in this
area that may be accomplished in the future.

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 VIII. APPENDIX

1. Table of Relative Density and Grain Size Data Among Sintered Samples.
Process Sintering Temperature Hold time Average Relative Density (%) Average Grain
type Size (µm)
3D SSS 1650 6 98.24 ± 0.32 7.36 ± 1.21
Printed
3D SSS 1750 6 98.46 ± 0.35 25.74 ± 2.84
Printed
3D SSS 1800 6 99.06 ± 0.36 45.25 ± 7.23
Printed
3D SSS 1850 2 99.26 ± 0.26 43.56 ± 17.42
Printed
3D SSS 1850 4 99.45 ± 0.19 57.89 ± 9.77
Printed
3D SSS 1850 6 99.41 ± 0.33 94.01 ± 9.47
Printed
3D SSS 1850 8 99.36 ± 0.34 108.11 ± 17.36
Printed
3D SSS 1850 10 99.37 ± 0.30 114.89 ± 18.604
Printed
3D SSS 1875 6 99.91 ± 0.08 113.27 ± 17.48
Printed
3D TSS 1850, 1800 8 99.61 ± 0.17 57.90 ± 9.91
Printed
3D TSS 1875, 1800 8 99.92 ± 0.03 87.69 ± 13.53
Printed
CIP SSS 1850 2 99.42 ± 0.09 n/a
CIP SSS 1850 4 99.72 ± 0.17 n/a
CIP SSS 1850 6 99.79 ± 0.14 n/a
CIP SSS 1850 8 99.74 ± 0.14 n/a
CIP SSS 1850 10 99.67 ± 0.21 n/a
CIP TSS 1850, 1800 8 99.89 ± 0.08 n/a
CIP TSS 1875, 1800 8 99.93 ± 0.05 n/a

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