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Bioceramic composites for orthopaedic applications: A comprehensive review

of mechanical, biological, and microstructural properties

Article in Ceramics International · September 2020

DOI: 10.1016/j.ceramint.2020.09.214

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 Ceramics International 47 (2021) 3013–3030

Contents lists available at ScienceDirect

Ceramics International
journal homepage: www.elsevier.com/locate/ceramint

Review article

Bioceramic composites for orthopaedic applications: A comprehensive

review of mechanical, biological, and microstructural properties
Deepika Shekhawat a, Amit Singh a, M.K. Banerjee b, Tej Singh c, Amar Patnaik a, *
a
Mechanical Engineering Department, M.N.I.T, Jaipur, 302017, India
b
Department of Metallurgical and Materials Engineering, M.N.I.T, Jaipur, 302017, India
c
Savaria Institute of Technology, Eötvös Loránd University, Szombathely, 9700, Hungary

A R T I C L E I N F O A B S T R A C T

Keywords: Bioceramics have been widely utilized for orthopaedic applications in which the biocompatibility and me­
Bioceramics chanical properties of the materials are vital characteristics to be considered for their clinical use. Till date,
Orthopaedic implants extensive studies have been devoted to developing a range of scientific ways for tailoring the microstructure of
Microstructure
bioceramics in order to attain the trade-off of mechanical properties and biocompatibility of the final product.
Mechanical properties
Owing to low reactivity, earlier stabilization and longer functional life of bioceramic, the developed implants are
Biocompatibility
capable of replicating the mechanical behaviour of original bone. As the safety of the patient and its ultimate
functionality are the ultimate goal of the selected implant material hence, the present literature survey in­
vestigates and brings forth the important aspects associated to the mechanical, biological and microstructural
characteristics of bioceramics employed in orthopaedic applications. The review paper majorly focuses on
effective utilization of various materials as an additive in bioceramics and processing techniques used for
enhancement of properties, enabling the use of material in orthopaedic applications. The influence of various
additives on the microstructure, mechanical properties and biological performance of developed bioceramics
orthopaedic implants has been elaborately discussed. Furthermore, future prospects are proposed to promote
further innovations in bioceramics research.

cardiovascular stents. Some implant materials fail early during their

functional period owing to certain shortcomings like wear, biological
1. Introduction
factors like infection, loosening of implants, and mismatch of elastic
modulus with the bone or other body parts and low strength. On those
Orthopaedic medical procedure relies significantly upon the
grounds, need for second surgery becomes imperative. Great challenge
advancement of biomaterials utilized for the reclamation and replace­
is faced in revision surgery in terms of cost of operation, post-surgery
ment of damaged parts of the human body. The human bodies are
pain, and the rate of success of the implanted part. Medical implant
vulnerable to various disabling and painful injuries for example dislo­
encounters various sets of challenges in terms of mechanical, biological
cation, strains and fractures. The bone breakage is termed as fracture
and thermal characteristics of the selected material [1,2]. These implant
and generally caused by the forces that surpass the strength of bone
devices must be capable of enduring large torques, forces acting due to
tissue. In order to heal the bone fracture, a surgery is carried out to
compression and shear in normal loading conditions for excellent me­
implant the additional material which may carry the whole-body load.
chanical force transfer. Hence, it is vitally important to manufacture
The implanted materials are generally termed as biomaterials and
implant materials that are hard, corrosion resistant, aging resistant,
should exhibit excellent mechanical properties namely elastic modulus,
tough, wear resistant, bioactive, biocompatible and is capable of deliv­
yield strength and ultimate tensile strength to withstand various
ering good service life in terms of durability (survivorship). Generally,
biomechanical forces. Besides, low density, good biocompatibility along
polymers, metals, ceramic and their composites are used as implant
with higher corrosion and wear resistance are the other desired prop­
biomaterials. Owing to their similarity with that of the mineral com­
erties of the implanted materials. These properties may satisfy the or­
ponents of the bones, their significant biocompatibility as well as
thopaedic fixation devices and load bearing applications such as bone
osseointegration with the host tissues makes ceramic materials the
plates, rods, screws, wires, joint replacements, dental implants and also

* Corresponding author. National Institute of Technology, Jaipur Mechanical Engineering, Jaipur, Rajasthan, 302017, India.
E-mail address: apatnaik.mech@mnit.ac.in (A. Patnaik).

https://doi.org/10.1016/j.ceramint.2020.09.214
Received 9 August 2020; Received in revised form 19 September 2020; Accepted 21 September 2020
Available online 22 September 2020
0272-8842/© 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
D. Shekhawat et al. Ceramics International 47 (2021) 3013–3030

Abbreviations MnO2 Manganese oxide

MWCNT Multi-wall carbon nanotubes
Al2O3 Alumina Si3N4 Silicon nitride
Al–Y-PSZ Alumina-yttria-partially stabilized zirconia SPS Spark plasma sintering
APTZNA Alumina Platelet Toughened Zirconia Non-Aging SrO Strontium oxide
ATZ Alumina toughened zirconia THA Total hip arthroplasty
CaF2 Calcium fluoride; TiN Titanium nitride
Ca-TZP Calcia-stabilized tetragonal zirconia TiO2 Titanium oxide
CeO2 Cerium oxide or Ceria TZP Tetragonal zirconia polycrystal
Ce–Y-TZP Ceria-yttria stabilized tetragonal zirconia Y2O3 or Y Yttrium oxide or Yttria
Cr2O3 Chromium oxide YSZ Yttria-stabilized zirconia
CS/rGO Calcium silicate-reduced graphene oxide Y-TZP Yttria-stabilized tetragonal zirconia polycrystal
Er2O3 Erbium oxide Y-ZTA Yttria-stabilized zirconia toughened alumina
HAp Hydroxyapatite ZA8Sr8–Ce11 Zirconia-alumina-aluminate composite
HIP Hot isostatic pressing ZrO2 Zirconia
MgO Magnesium oxide ZTA Zirconia toughened alumina
Mg-PSZ Magnesia-partially stabilized zirconia

essential player in today’s orthopaedic market [3,4]. Bioceramics are of bioceramic implants, different techniques are available but for
ceramic materials utilized in making the implants and orthopaedic choosing an appropriate one depends on the desirable properties ex­
application devices for the ultimate purpose of repairing and replacing pected from the resulting bioceramic. Ultimate use of the bioceramic as
the diseased and injured parts inside the human body (e.g. teeth, bone, a biomaterial device decides its fabrication steps and post-fabrication
skeleton, and joints, Fig. 1) [4–6]. treatment. Also understanding of the implant-tissue relationship
In more general terms, implants are labeled as soft tissue and hard makes it important factor as it sternly affects the patient’s life, it also
tissue implants. Soft tissue implants are employed in restoring func­ concerns with the selection, design and fabrication of the implant ma­
tioning related to liver, skin, heart tissue, blood vessels, kidney, cartilage terial [8]. On the basis of tissue-implant interface reaction, there are
and ligaments therefore they must possess adequate elastic modulus, three classes in which bioceramics are generally classified; (a) bioactive,
tensile strength and flexural strength suggesting utilization of polymers, (b) bioresorbable, and (c) bioinert as briefed in Table 1 [9]. Bioinert
and novel metals. Whereas, hard tissue implant materials are exercised ceramics mainly includes alumina (Al2O3) and zirconia (ZrO2) ceramics,
for restoring dental, shoulder, bones, knee and hip joints to get rid of that are also entitled as biotolerant materials that doesn’t encourage any
immobility and pain for which these materials must hold high hardness, interfacial bonding amongst bone and implants [10]. The selection of
higher fracture toughness, adequate elastic modulus, wear resistance, proper biomaterial depends upon several important aspects and prop­
corrosion resistance, and durable implying utilization of bioceramics erties related to that particular material that are briefed in detail in
and composites as the most suitable candidates [7]. For the fabrication Table 1. In more general terms, implants are labeled as soft and hard

Fig. 1. Implants and orthopaedic application devices for human body. Idea was taken from Ref. [6].

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Table 1
Characteristic attributes of bioceramics used in biomedical applications [6,7,11–22].
Types of Material Property Applications
bioceramics
Density Hardness Tensile Compressive Fracture
(g/cm3) (HV) strength strength (MPa) toughness (MPa.
(GPa) m1/2)

Bioinert Al2O3 >3.97 1800–2000 250–300 4000 4–5 Femoral head implants
ZrO2 6.53 1400 330 2000 10 Endosseous implants
Y-TZP 6.05 1200 300–400 3000–4000 6–12 Replacement of diseased or fractured
ZTA 4.40 1600–1800 414 4000 6–10 part of knee, hip, shoulder, wrist, elbow,
Mg-PSZ 5.75 1200 300–400 2000–3000 6–10 tooth etc.
ZrO2–3Y-TZP 6.06 1200 _ 2200 8
ZrO2–Mg-PSZ 5.74 1200 _ 1600 8
ATZ 80% ZrO2–20% 5.40 1400 _ 2500 5
Al2O3
APTZNA 75% TZP- 5.4 1300 _ _ _
25% Al2O3
Zircalon 5 (YSZ) 6.13 1350 _ >2000 8
Bioactive Hydroxyapatite 3.1 350 _ 600 0.6–1 Coating in implants
(HAp)
Bioactive glasses 2.6 600 0.12–0.25 40–60 0.5–1 Bone grafts, bone defect fillers
Biodegradable Calcium phosphate 3.05 _ 0.03–0.2 20–900 <1.0 Replacement of the surrounding tissue,
bone grafts

tissue implants. Soft tissue implants are employed in restoring func­ higher fracture toughness, adequate elastic modulus, wear resistance,
tioning related to liver, skin, heart tissue, blood vessels, kidney, cartilage corrosion resistance, and durable implying utilization of ceramics and
and ligaments therefore they must possess adequate elastic modulus, composites as the most suitable candidates [23]. Selection of bio­
tensile strength and flexural strength suggesting utilization of polymers, materials for orthopaedic implant application depends on several lead­
and novel metals. Whereas, hard tissue implant materials are exercised ing factors which are depicted in Fig. 2 [8].
for restoring dental, shoulder, bones, knee and hip joints to get rid of It was reported in literature that polymer and metallic implants
immobility and pain for which these materials must hold high hardness, release ions which proved toxic and hazardous to living tissues.

Fig. 2. Selection characteristics of biomaterials employed in orthopaedic devices [10,12,13,19].

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Whereas, bioceramics are reported to form interfacial bonding with ZTA (zirconia toughened alumina) ceramics via two different methods i.
human bones and stimulate the composition of bones. Bioactive glass, e. first, milling and hot pressing; second, by electrochemical dispersing,
Al2O3 and ZrO2, calcium phosphate and hydroxyapatite (HAp) are bio­ slip casting and pressure-less sintering. Authors examined the perfor­
ceramics which have been widely used for the orthopaedic applications mance of fabricated composites after the addition of ZrO2 and Y2O3
nowadays owing to their excellent osteo-conductivity, biocompatibility content. They also examined the effect of processing parameters upon
and similarity to the inorganic component of bone. Apart from various the material’s mechanical properties. Results of the study indicated that
beneficial properties, bioceramics faced limitations load-bearing by varying composition and processing methods toughening of Al2O3
implant applications because they are brittle, weak and inferior work­ can be employed in distinguished ways to boost the strength and
ability [9,24]. These limitations in bioceramics were reported to over­ toughness of the final composite. Trabelsi et al. [30] made an attempt to
come by introducing various filler as reinforcement and surface examine the relationship between mechanical properties and wear
modification techniques. Literature was full on studies focused on the resistance of Al2O3–ZrO2 composites. In result they found that
inclusion of different fillers and surface modification/coating techniques dispersing ZrO2 particles in the matrix of Al2O3 elevated the fracture
to improve the performance of bioceramic implants. The bioceramic behaviour whereas wear resistance was reduced. This toughness was
applications have made great advances in the last century, suggesting found to increase owing to toughening mechanism that is also respon­
that the clinical applications of bioceramics mainly focused on the me­ sible for counter balancing the implications of hardness reduction on the
chanical properties of the materials and biocompatibility was a primary wear resistance. Orange et al. [31] examined the effect of microstructure
requirement for clinical applications. Hence, the aim of this review is to and temperature (room and high) on the mechanical behaviour (fracture
provide a comprehensive description on the research status of bio­ strength, toughness, and slow crack growth resistance) of the fabricated
ceramics and their composites targeted for orthopaedic applications. ZTA ceramics as compared to pure Al2O3. In result they revealed that
The effect of filler/reinforcing materials and processing techniques on combined processing of Al2O3–ZrO2 composites exhibited enhanced
the microstructural, mechanical and biological performance of bio­ mechanical properties as compared to pure Al2O3. Also wear and ther­
ceramic composites are extensively discussed and reported. Finally, the mal fatigue values attained presented improved behaviour.
review concluded with future recommendations to obtain long-lasting Patil and Mutsuddy [32] in their study emphasized on the careful
bioceramic implants. processing of raw ceramic powders followed by two-stage sintering
process that can help in avoiding high-priced techniques like hot
2. Mechanical properties of bioceramic composites pressing or hot isostatic pressing that are generally opted for attaining
higher densification in the final ceramic composites. Pores were elimi­
The real problem behind the failure of hip joint implant is that under nated thereby giving full densification by sintering the samples in air
circumstances of sudden impact the material owing low mechanical and in presence of gas. The room temperature hardness value attained
characteristics (young’s modulus, hardness, and fracture toughness) was 17.8 GPa and fracture toughness was 4.8 MPa√m. These improved
faces premature failure. As it is well known to all that human hip joint is values of hardness and toughness were observed due to the trans­
the largest load bearing joint next to knee joint it confronts many re­ formation toughening mechanism caused owing to the presence of fine
petitive forces while performing various physical activities such as sized ZrO2 particles in the final composites. Another study was carried
walking, running, exercising and jumping [25]. Apart from these out by Piconi et al. [33] in which stability of Y-TZP (yttria-stabilized
rigorous activities there are some common diseases also related to joints tetragonal zirconia polycrystal) was evaluated by in-vitro and in-vivo (in
like osteoarthritis, avascular necrosis, rheumatoid arthritis, tumour, rabbits) aging the yttria coated powders (140 ◦ C, 0.2 MPa water pres­
femoral neck fracture, whereas other reason includes sports injuries and sure). Yttria coated powders illustrated lower strength degradation in
road accidents thereby necessitating the hip surgery [25,26]. During comparison to co-precipitated powders. Repeated sterilization in steam
normal activities normal bone experiences stress level of 40 MPa, did not indicate any effect on the wear of yttria coated powder. Kong
whereas hip joint faces 3 times the body weight. On contrary during et al. [34] proposed another method of fabricating ceramic composite by
rigorous activities like jumping, running, etc. hip joint experiences stress mixing HAp with TZP (tetragonal zirconia polycrystal) powder coated
10 times the body weight. The most interesting characteristics of stress with Al2O3. The coating was executed by dispersion of TZP powder in a
here is its recurrent and varying nature that relies on numerous activities boehmite suspension followed by regulating the pH so as to induce
like climbing, running, standing, stretching, and sitting. For this reason, hetero-coagulation. About three times higher values for the strength
the selected biomaterial must possess enough fatigue strength, high (300 MPa) and fracture toughness (3 MPa√m) were attained owing to
wear and corrosion resistance, elastic modulus and biocompatibility. All incorporation of ZrO2 (15 vol%) and Al2O3 (30 vol%) as compared to
these precise requirements from the ultimate implant material necessi­ pure HAp. Coating aided in eliminating the deleterious response
tate for breaking new grounds and advancements in the domain of amongst HAp and ZrO2.
biomedical science, material science and manufacturing science [27]. Again, another group Riu et al. [35] evaluated the implications of
Inert ceramics like Al2O3 and YSZ (yttria-stabilized zirconia) have adding Cr2O3 into Al2O3 matrix by investigating the changes in the
been used as implant materials for hip joint prostheses owing to their microstructure and their influence on the mechanical properties of the
bioinert nature, enhanced mechanical properties and wear resistive Al2O3. The observed results indicated that changes in the microstruc­
nature. Inopportunely, Al2O3 illustrates mediocre strength and fracture tures of Al2O3 as a consequence of diffusion of Cr ions resulted in
toughness which acts as its major drawback as well as a leading reason enhanced mechanical properties (hardness and elastic modulus) of the
for its ultimate failure in orthopaedics. In this regard, many authors have final specimens. Whereas, fracture strength deteriorated above 2 mol.%
studied the influence of adding different ceramic stabilizers like yttrium addition of Cr2O3 particles. Another approach was chosen by Hodgson
oxide or yttria (Y2O3), magnesium oxide (MgO), chromium oxide and Cawley [36] in which Y-TZP were doped with TiO2 particles. Based
(Cr2O3), cerium oxide (CeO2) etc. in the base matrix of Al2O3, ZrO2 and on their findings, it was observed that by keeping titanium oxide (TiO2)
silicon nitride (Si3N4) for evaluating their performance in biomedical content below 0.25%, Y-TZP material with higher density that can be
application. sintered at lower temperature could be attained. TiO2 additions also
Kosmac et al. [28] examined the involvement of t-ZrO2 (tetragonal caused increase of transformation toughening that could be observed in
zirconia) and m-ZrO2 (monoclinic zirconia) particles in the fracture the microstructure post high temperature treatment. Their study also
toughness of fabricated Al2O3–ZrO2 composites. Results indicated that suggested that reduction in mobility due to TiO2 addition may act as
toughness was improved for the fabricated composites in the presence of favorable to ZrO2 based composites where porosity retainment and/or
micro-cracking and transformation toughening mechanisms, where surface activity are of prime importance. Similarly, Zhigachev et al. [37]
micro-cracking being the dominant one. Homerin et al. [29] fabricated examined the influence of incorporating TiO2 upon mechanical

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properties and phase composition of Ca-TZP (calcia stabilized tetragonal % demonstrated decrease in the mechanical properties (hardness, frac­
zirconia) ceramic. Outcomes of the study indicated reduction in the ture toughness, modulus of elasticity, and modulus of rupture) of the
hardness value of TiO2 doped Ca-TZP from 12.2 GPa (undoped Ca-TZP) final composites. Drop in mechanical properties were attributed to
to 11.3 GPa (0.75 mol.% TiO2 doped Ca-TZP). Whereas, in comparison higher apparent porosity (induced due to solid state mixing of powders)
to undoped Ca-TZP (~5 MPa√m) the highest value of fracture tough­ owing to ceria addition. Celli et al. [45] conducted a study to analyze the
ness (9.1 MPa√m) was observed for the ceramics having TiO2 in the fractal geometry of the cracks generated in Al2O3–ZrO2composites. A
range of 0.5–0.65 mol.%. Silva et al. [38] fabricated two series of working correlation was observed amongst fractal dimension and frac­
ZrO2-HAp composites (40 and 60 vol% of ZrO2) and evaluated the sin­ ture toughness that suggested intergranular fracture takes place for pure
tering behavior, changes in microstructural characteristics, and modi­ ZrO2, pure Al2O3 and for higher percentage of Al2O3-based composites.
fications in the mechanical properties of the composites. Raw powders On the other side, composites with higher ZrO2 content resulted in
were uniaxially pressed at 700 MPa, sintered at 1200–1500 ◦ C in air for transgranular fracture mechanism and all these results were validated
3 h. Relative densities ranged in between 89 and 91% of the theoretical by analyzing the fractal energies evaluated via both the classical and
values. Mechanical tests were conducted for evaluating density, fractal methods. This study also addressed the potential of finding
micro-Vickers hardness, Young’s modulus, poisson’s ratio and ultimate fracture behaviour of brittle materials by explaining their complex
compression strength and were compared to the mechanical properties fracture mechanisms. Understanding transformation toughening mech­
of the human tissues and biomaterials. Results revealed that majority of anism has made impressive development so far whereas customizing
enhanced properties were observed for composites having 60 vol% ZrO2 and tailoring the toughness of ceramics remained another major chal­
content. Higher ZrO2 phase content was the prompt reason in the series lenge in today’s scenario. In the view of this, Basu et al. [46] made an
leading to enhanced properties. On the other hand, excellent mechanical attempt to tailor the toughness of ZrO2–Al2O3 composites prepared by
properties were attained for the series sintered at 1300 ◦ C and/or careful engineering of the ZrO2 matrix by opting mixing route followed
1400 ◦ C whereas worst properties were accomplished for all the com­ by similar sintering conditions. This proposed methodology pointed out
posites sintered at 1500 ◦ C due to large scale micro-cracking that toughness of up to ~10 MPa√m was achieved for the fabricated
phenomenon. TZP-Al2O3 composite that was doubled than the commercially available
Tanaka et al. [39] developed a new nanocomposite, Ce-TZP/Al2O3 ZrO2-20 wt% Al2O3 ceramics. Same group did another study for evalu­
and compared its biocompatibility, phase stability, and wear charac­ ating the changes in the microstructure and mechanical properties of
teristics with the existing conventional ceramics (Al2O3 and Y-TZP) that Y-TZP ceramics with the varying yttria content. Y-TZP ceramics filled
are employed in total joint prostheses. Taking mechanical properties with 20 wt% Al2O3 were prepared via powder mixing and hot pressing
into consideration, sintered samples were examined for elastic modulus route (vacuum, 1450 ◦ C, 1 h). Fracture toughness of the fabricated ce­
(resonance vibration method, 247 GPa), flexural strength (3-point ramics was able to customize the value in between the range of 2
bending, 941 ± 34 MPa), hardness (Vicker indentation, 11.71 ± 0.03 MPa√m to 10 MPa√m owing to monitored accretion of m-ZrO2 parti­
GPa) and fracture toughness (indentation-fracture method, 20.05 ± cles via proposed methodology. Superior toughness value (10 MPa√m)
0.22 MPa√m). The proposed nanocomposites illustrated good me­ along with the enhanced flexural strength (~1250 MPa) was attained for
chanical properties in comparison to Al2O3 and Y-TZP ceramics and mixed grade 2 mol.% yttria prepared via hot pressing. Microstructural
suggested that it could potentially be consumed for hip joint replace­ study done using electron probe microanalysis uncovered the existence
ment material. Maneshian and Banerjee [40] mentioned that prolonged of purposely added yttria-free ZrO2 particles during initial powder
ball milling (mechanical alloying) and its subsequent sintering could mixtures that probably yielded redistribution of the yttria during sin­
help in achieving Al2O3/ZrO2 composites with improved fracture tering process that ultimately increased the transformability. This study
toughness, hardness, and densification. A. H. De Aza et al. [41] suggested that by tuning the composition of the starting powder in
emphasized on evaluating the role of processing conditions, crack yttria-doped ZrO2 resulted in higher toughness values in the range from
propagation, microstructures and their impact on the mechanical 2 to 10 MPa√m by controlled addition of ZrO2 particles to 3 mol.% Y2O3
properties of new generation Al2O3, ZrO2 and ZTA bioceramics pro­ [47].
cessed via colloidal processing technique. On the basis of slow Silva et al. [48] made an attempt to examine the toxicity and potency
crack-growth behaviour studies for the selected three ceramics, results of using Si3N4based ceramics for load bearing applications. Ceramics
presented greater reliability for orthopaedic applications by bearing were prepared in two different compositions (sintering aids: ytterbium,
loads twice the monolithic Al2O3 without differed failure. Lin and Duh yttrium and Al2O3) fabricated via sintering in carbon resistance furnace
[42] evaluated fracture toughness and hardness of Ce–Y-TZP (ceria-yt­ (nitrogen atmosphere). Mechanical properties evaluated for fabricated
tria stabilized tetragonal zirconia) ceramics fabricated by precipitating, ceramics were fracture toughness (5 MPa√m) and hardness (13 GPa).
milling, iso-statically pressing, and sintering. Hardness of ceramics With these excellent mechanical characteristics and non-toxic behaviour
presented direct relationship with the composition of Ce–Y-TZP ce­ of Si3N4 based ceramics proved to be a potential candidate for high load
ramics. Study also revealed that fracture toughness increased with the bearing applications in human body like knee joint, hip balls etc. Moraes
increase in the starting temperature of m→t transformation and tetra­ et al. [49] proposed another Y-TZP composites fabricated by altering the
gonality. In the same year, Kim et al. [43] examined the effect of ZrO2 content from 5 to 80 wt%. Result of their study showed that by
incorporating calcium fluoride (CaF2) upon densification, sinterability, adding ZrO2 into composites several desirable mechanical properties
and mechanical properties of HAp-10, 20, 30, 40 ZrO2 (0, 5, and 10 vol% like densities, flexural strength and fracture toughness were increased.
CaF2) composites for biomedical applications. This study suggested that They also quoted that these composites were able to achieve flexural
for the composites prepared at temperature 1350 ◦ C treated for 1 h, strength of 93% and fracture toughness of 29% when compared to pure
flexural strength of composites having 2.5 vol% CaF2presented the Al2O3 ceramics. Similarly, Ahn et al. [50] conducted a study to evaluate
highest strength value (~180 MPa) in comparison to the other fabri­ the effect of reinforcing ZrO2 on the resulting microstructure and me­
cated non-incorporated CaF2 composites (~40 MPa). On further in­ chanical properties of HAp based nanocomposites. 1.5 wt% ZrO2 illus­
crease in CaF2 after 2.5 vol% the flexural strength value dramatically trated the optimized results for the Vickers hardness. Similarly, 1.5 wt%
decreased. Similarly, the fracture toughness for the composites incor­ of ZrO2particles resulted in the increased bending strength of HAp-ZrO2
porating 2.5 vol% CaF2 showed the maximum toughness value (~2.3 nanocomposites from 183 to 243 MPa. The dominating phase of HAp
MPa√m). Mangalaraja et al. [44] discussed the correlation with adding grains were of size ≤100 nm that indicated its potency for providing
ceria content (1–5 vol%) on the physical, mechanical, and thermal higher bioactivity.
properties of Y-ZTA (yttria stabilized zirconia toughened alumina) Magnani and Brillante [51] also carried out an experimental work to
composites. The results with the changes in ceria content from 0 to 5 vol analyze the impact of varying the composition and sintering procedure

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upon the mechanical properties as well as residual stresses in ZrO2 Toughening mechanism was the primary reason behind transformation
–Al2O3 composites. ZTA was reinforced with small amount of Cr2O3 and toughening. Hardness and strength got reduced owing to increased TiN
magnetoplumbite fabricated under different conditions. The addition of grain size along with the increasing TiN content. Toughness got reduced
Cr2O3 contributed to enhanced fracture toughness (highest attained for owing to decreased transformation toughening from t→m (tetragonal to
0.5 wt% Cr2O3 and 2 mol.% Y2O3) owing to stress induced trans­ monoclinic) ZrO2 phase transformation, whereas elastic modulus
formation toughening mechanism whereas the t-ZrO2 to m-ZrO2 trans­ demonstrated direct relationship with the increasing titanium nitride
formation was suppressed with the incorporation of platelets (TiN) content. Mazzocchi et al. [59] made an effort to investigate three
reinforcements (magnetoplumbite). This study also indicated that sta­ different series of Si3N4 based ceramic material for orthopaedic implants
bilizer content strongly affected the transformability. During the same that was comprised of its processing, microstructure, mechanical prop­
year, Zhang and his co-workers [52] prepared HAp-ZrO2 based com­ erties and its cytotoxicity. Results revealed that these ceramics with
posites by dispersing them in aqueous media of polyacrylic acid and proper sintering aids or secondary phases would help in providing
glutamic acid. After attaining slurries in well-stabilized state the green enhanced mechanical properties and absence of intergranular phases
samples possessing higher density were slip casted. Pressure-less sin­ post development illustrating excellent bonding between the elements
tering (1450 ◦ C, 2 h) was performed for green samples to attain higher ensuring good stress transfer. Nath et al. [60] investigated the resulting
density. The outcomes of the study demonstrated that proposed method mechanical, microstructural and tribological performance of microwave
of colloidal processing and pressure-less sintering proved to be well sintered calcia-doped ZrO2ceramics. Outcomes of the study indicated
suited methodology to prepare HAp-ZrO2 composites with higher me­ 97.5% theoretical density for 8 mol.% calcia-doped ZrO2and 91.6% for
chanical and biological properties desirable for orthopaedic applica­ calcia-doped ZrO2. Hardness increased with the increasing sintering
tions. Sort of similar work was carried out by Sung et al. [53] a year later temperature and values attained were 10 GPa for 8 mol.% calcia-doped
in which they prepared HAp and YSZ nano-ceramic by synthesizing ZrO2 whereas 9 GPa for 16 mol.% calcia-doped ZrO2. Toughness value
homogeneous mixture of the two by chemical co-precipitation, drying, was highest for 8 mol.% calcia-doped ZrO2 ceramics (6 MPa√m) in
calcination followed by hot pressing (1100 ◦ C, 1 h in vacuum atmo­ comparison to other ceramics.
sphere). When evaluated for mechanical properties of the fabricated Bartuli et al. [61] made an attempt to fabricate dense and cellular
nanocomposites, improved properties were attained owing to involve­ Y-TZP ceramics via sol gel casting route and examined its resulting
ment of YSZ into HAp matrix, i.e. fracture toughness (~2.1 MPa√m) mechanical properties and characterization. Sintering of Y-TZP ceramic
and flexural strength (155 MPa). Successfully developed HAp/YSZ powders post gel casting resulted in 170 GPa of elastic modulus and 400
nanocomposites proved to be potential material for load bearing or­ MPa of modulus of rupture. The total amount of pores generated directly
thopaedic applications by illustrating improved properties like flexural affected the mechanical properties of the fabricated ceramics. Rascon
strength (155 ± 16 MPa), elastic modulus (135 ± 15 GPa), hardness (7.2 et al. [62] made an attempt to evaluate mechanical properties of ATZ
± 0.6 GPa), and fracture toughness (2.1 ± 0.2 MPa√m) for 25 wt% YSZ (alumina toughened zirconia) and ZTA based ceramic composites
nanocomposites. Whereas, Gain et al. [54] fabricated porous HAp-ZrO2 fabricated via varying Al2O3 and ZrO2 content. The main aim of this
nanocomposites by incorporating polymethyl methacrylate powders as study was to assess the effect of sintering on hardness and fracture
a source of pore forming agent during the processing. The polymethyl toughness of ATZ and ZTA. Relative density attained was about 94%,
methacrylate powders helped in forming random distribution of pores in hardness attained was in range 9.5 and 21.9 GPa, and fracture toughness
the final composite. Better results were attained for HAp-ZrO2 nano­ was 3.6 MPa√m. This study showed that ATZ and ZTA composites can
composites as compared to monolithic HAp. Monolithic HAp nano­ be visualized as more promising material for fabrication of implants
composites attained compressive strength of 31.4 MPa, whereas for instead of pure oxides. Bernal et al. [63] studied the mechanical prop­
HAp-ZrO2 nanocomposites it was 34.9 MPa. Elastic moduli were found erties of Al2O3 ceramics under sinter and sinter-HIP conditions. Relative
to be 19.6 GPa (HAp) and 23.4 GPa (HAp-ZrO2) that was observed to be densities of the samples attained were above 98%, hardness achieved
equivalent to human cortical bones. was 19 GPa, and fracture toughness reached 5.2 MPa√m for the samples
Bal et al. [55] made an attempt to study and test the Si3N4 based prepared via hot isostatic pressing (HIP) sintering process.
ceramic bearing (femoral heads and acetabular cups) that were fabri­ Later on Oungkulsolmongkol et al. [64] made an attempt to study the
cated via sintering and hot isostatic pressing the acquired powders for effect of incorporating ZrO2 and SrO (strontium oxide) additives upon
the final application in bearing materials for hip joint prosthesis. Me­ the mechanical properties (hardness and toughness) and characteristics
chanical testing was done on the fabricated bearings and properties of Al2O3-based composites. Results revealed that simultaneous addition
evaluated included flexural strength (920 ± 70 MPa), elastic modulus of SrO and ZrO2 in the same Al2O3 matrix was not able to produce better
(306 GPa), Vickers hardness (15 ± 1 GPa), and fracture toughness (10 ± hardness whereas it furnished enhanced fracture toughness in compar­
1 MPa√m) and these properties were compared with that of CoCr, ison to SrO–Al2O3 and ZrO2–Al2O3 composites. Whereas, Maiti and Sil
Al2O3, Y-TZP, and ZTA based orthopaedic bearing materials and results [65] approached towards generating a relationship between fracture
showed that Si3N4 presented a favorable blend of mechanical properties toughness characteristics and morphology of developed Al2O3 ceramics
and proved itself to be a efficacious material for the THA (total hip via sintering process. Their main objective was to investigate the
arthroplasty) bearing. Dudnik et al. [56] made an attempt to prepare domination of sintering temperature (1500, 1600 and 1700 ◦ C) as well
nanocrystalline ZrO2 rich ZrO2–Y2O3–CeO2–Al2O3 powder. Fracture as soaking time upon the fracture toughness of Al2O3 ceramics. Fracture
toughness value attained was in the range 6.4–16.8 MPa√m for the toughness was found to be lower (4.6 and 5.0 MPa√m) for the samples
powders sintered (400–1300 ◦ C) under various conditions. Another sintered at 1700 ◦ C in which major mode is of transgranular fracture
group of researchers [57] intended to evaluate the fracture toughness, type. Crack deflection is entitled as the leading phenomenon causing
strength and slow crack growth behaviour of fabricated ceria stabilized high fracture toughness in samples sintered at 1500 and 1600 ◦ C.
ZrO2–Al2O3 nanocomposite for biomedical applications. Outcomes of Echeberria et al. [66] evaluated the resulting microstructure and frac­
the study revealed values for threshold (~4.5 MPa√m) and fracture ture toughness values for the sintered and hot isostatic pressed
toughness (8.8 MPa√m) that were greater than the ceramics (i.e. Al2O3 multi-wall carbon nanotubes (MWCNT)-reinforced ZTA nano­
and Y-TZP) that are being used in the current time. The cyclic threshold composites. Hardness values were found to reduce with the incorpora­
value (4.5. MPa√m) for the current material was higher than the con­ tion of MWCNT in both cases. Whereas fracture toughness got increased
ventional biomedical grade material being used currently (i.e. Al2O3 and with the incorporation of MWCNT content for sintered composites on
ZrO2). Arin et al. [58] prepared and characterized ZrO2 ceramics and contrary for hot isostatic pressed composites slight decrease was
composites with varying Y2O3, CeO2 (2–3 mol.%) and Al2O3 content and observed. Similar work was carried out by Kalmodia et al. [67] to
also synthesized 35–60 vol% TiN reinforced ZrO2 composites. evaluate the mechanical, characterization, and biocompatibility of SPS

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sintered Al2O3 and MWCNT reinforced HAp composites. Results studied the evolution and interaction of microstructure as well as effect
revealed that fracture toughness got increased from 1.18 MPa√m to on the mechanical properties of ZTA due to incorporation of ceria hex­
2.07 MPa√m for HAp matrix post the incorporation of Al2O3 and 2.3 aaluminate (CA6). Results showed that materials exhibited higher
times enhancement was observed for the fracture toughness after rein­ strength and toughness values with the increase in sintering tempera­
forcement of MWCNT. tures from 1475 to 1500 ◦ C. Dehestani and Adolfsson [77] examined the
Akin et al. [68] developed Al2O3 and Al2O3-YSZ composites effect of low temperature aging and mechanical properties of monolithic
employing spark plasma sintering technique at 1350 and 1400 ◦ C for 3Y-TZP, 10Ce-TZP, and 12Ce-TZP and their composites (30 vol% Al2O3).
300 s (40 MPa pressure). Relative density attained was ~99% for Amongst all, 12Ce-TZP containing composites exhibited the highest
monolithic Al2O3 and Al2O3-YSZ composites and the hardness values resistance to low temperature aging and 3Y-TZP induced composites
attained for the monolithic Al2O3 was less than those of Al2O3-YSZ presented highest bending strength.
composites. 10 vol% ZrO2 restricted the grain growth of Al2O3. Hardness Bian et al. [78] made an attempt to analyze the effect of micro­
value decreased with increasing content of YSZ. 30 vol% YSZ composites structure of composite powders upon the microstructure as well as on
presented the highest value for fracture toughness (i.e. 6.2 MPa√m) and the properties of Al2O3 matrix ceramics prepared via microwave sin­
this relationship showed a linear relationship with that of YSZ content in tering. Composites were prepared with the micro-sized and nano-sized
the Al2O3matrix. Sommer et al. [69] adopted a high performance powder mixture. Microstructure obtained for the nano-sized compos­
manufacturing technology to develop Al2O3 based ceramic composites ites were denser, finer and homogeneous in nature. Nano-sized com­
by incorporating Cr2O3 and yttria. Injection parameters were carefully posites presented enhanced mechanical properties (flexural strength,
selected so as to get high dimensional accuracy and defect free samples 85.3%; Vickers hardness, 130.3%; fracture toughness, 25.7%) in com­
without significant loss in mechanical properties. To do so proper parison to micro-sized composites. Nath et al. [79] studied the effect of
debonding parameters were selected so that destroying of injection incorporating TiO2 on the densification and mechanical properties of
molded samples could be prevented. Bending strength value obtained sintered Al2O3–Cr2O3. Amongst all the compositions, addition of 1 wt%
was 570 MPa and fracture toughness attained was 3.52 MPa√m. TiO2 in Al2O3–Cr2O3 enhanced the densification without deterioration
Whereas Ramesh et al. [70] undertook a study to examine the effect of of flexural strength. The Al2O3–Cr2O3without TiO2 sintered in reducing
short sintering holding time (12 min) on the characteristics of undoped condition, exhibited higher bulk density than the sample sintered under
and MnO2 (manganese oxide; 1 wt%) doped Y-TZP ceramics. Sintered air atmosphere. Rittidech et al. [80] attempted a study to find out the
samples at temperature of 1150–1600 ◦ C were further examined for implication of adding 1–8 wt% yttria in Al2O3–ZrO2 ceramics on
physical and mechanical properties (density, hardness, and fracture resulting phase formation and fracture toughness. Grain shapes of the
toughness). When samples of MnO2 (1 wt%) doped Y-TZP ceramics were fabricated ceramics did not get affected due to yttria content whereas
sintered at 1250 ◦ C relative density of about 95% was achieved. When smaller sized grains presented higher fracture toughness values
sintered at 1450 ◦ C with the holding time of 12 min, doped and undoped (maximum value attained was 4.827 MPa√m for 4 wt% yttria). Naglieri
ceramics presented alike elastic modulus values (~200–205 GPa). In­ et al. [81] made an attempt to evaluate the implication of ZrO2 (5–20 vol
crease in fracture toughness was observed for the ceramic samples sin­ %) content on Al2O3–ZrO2 composites on the resulting microstructure
tered above 1400 ◦ C with 12 min holding time. Goyos et al. [71] in their and mechanical properties. Outcomes of the study revealed that
investigation reported a comparative study in terms of mechanical maximum hardness values and fracture toughness values were attained
characteristics, ageing and phase transformation of Ce-TZP/Al2O3 for 10 vol% ZrO2 incurred as a result of t→m transformation at the crack
nanocomposites fabricated via two different methods i.e. the colloidal tip. Whereas Kern [82] discussed the impact of adding cerium hex­
technique and the simple powder mixing route. Ce (10 and 12 mol. aaluminate precipitates (in-situ formed) on the resulting properties of
%)-TZP/Al2O3 nanocomposites exhibited distinguished fracture mech­ Al2O3-24 vol% ZrO2 nanocomposites. Fabricated ATZ nanocomposites
anisms whereas flexural strength and fracture toughness values were revealed highest intrinsic toughness (8 MPa√m), and highest bending
identical. They emphasized that conventional powder mixing route was strength (825 MPa). Hardness observed for the resulting compositions
also able to yield similar results as achieved via colloidal processing were in the order of 12Ce-30 A > 12Ce-30LA6>12Ce-30SA6 (Table S1).
route. Although TiO2-doped Al2O3–ZrO2 (3Y) ceramics presents superior On the other hand high value of crack tip toughness was observed for
mechanical properties but its superplastic (compression deformation) 12Ce-30 A and 12Ce-30LA6. Rejab et al. [83] addressed the changes
behaviour and related mechanism was investigated by Guoqing and occurring in the microstructure and fracture toughness of ZTA incor­
co-workers [72]. TiO2 (0, 1, 4 and 8 wt%) incorporated Al2O3–ZrO2 (3Y) porated with MgO and CeO2. Keeping constant MgO at 0.7 wt% and
ceramics presented enhanced strain rate with the increase in TiO2 con­ varying the CeO2 content (0.3–7 wt%) hardness increased from 14.51 to
centration. With the incorporation of TiO2 superplasticity of Al2O3–ZrO2 14.99 GPa and fracture toughness values increased from 5.78 ± 0.16 to
(3Y) ceramic was improved. Kern [73] reported a study in which he 6.59 ± 0.23 MPa√m. Meybodi et al. [84] prepared Al2O3-20 wt%
investigated the microstructure, phase and correlation between Al2TiO5 (aluminium titanate) composites by mixing nanopowders of
strength-toughness of hot pressed 1 Yb-2Nd-ATZ (ytterbia-neodymia Al2O3 and TiO2 by sintering at different sintering temperatures (1300,
costabilized ATZ) composites. Results of the phase composition showed 1400, and 1500 ◦ C) for 2 h. The research findings indicated enlargement
phase separation with respect to the increasing sintering temperature. of the grain sizes of Al2O3–Al2TiO5 ceramic composites with the
Higher fracture resistance, stress intensity and bending strength were consequent increase in the sintering temperature resulting in the
attained for the composites owing to a highly transformable t-ZrO2 improved hardness values (for 1300 ◦ C, 4.8; 1400 ◦ C, 6.2; and 1500 ◦ C,
matrix. Kern and Gadow [74] made an attempt to enhance the me­ 8.5 GPa) of the fabricated samples.
chanical properties of Y-TZP/Al2O3 composites by incorporating Following year in 2014, Pulgarin and Albano [85] investigated the
tougher and transformable 2.5 Y-TZP fabricated via powder coating. sintering, resulting microstructure and hardness of the fabricated
Mechanical properties evaluated were bending strength (1900 MPa) and Al2O3–ZrO2 composites. For the composites sintered above 1400 and
fracture toughness (4.8–5.5 MPa√m). Results also revealed that with 1500 ◦ C (Al2O3) and 10.5 vol% Al–Y-PSZ (alumina-yttria-partially sta­
the increasing Al2O3 content the transformability apparently declined bilized zirconia), the grain size of Al2O3 after sintering were greater than
for TZP. Garcỉa and Hotza [75] made efforts to study the effect of Al2O3 1 μm that attributed for decreased hardness value. Ragurajan et al. [86]
particles and/or platelet size (4.7, 5.3 and 11 μm) upon the mechanical examined the influence of incorporating cerium oxide (CeO2; 0.3–1.0 wt
properties of Al2O3–ZrO2 composites. Outcomes of this study suggested %) on the mechanical properties and sintering behaviour of Y-TZP.
that strength and toughness of the resulting composites could be opti­ Mechanical properties were enhanced with the incorporation of CeO2
mized by proper platelet size distribution. Fracture toughness of 3Y-TZP and sintering temperature also played significant role. Amongst all, 0.5
was increased owing to Al2O3 broken platelets addition. Kern [76] wt% CeO2 resulted in optimal performance by offering bulk density (5.9

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g/cc), hardness (13.2 GPa), Young’s modulus (211 GPa), fracture Consequent year Ojha et al. [97] developed different compositions of
toughness (6.4 MPa√m) and best sintering temperature was found to be c-CeO2/α-Al2O3 in situ and obtained results were compared with that of
in range 1400–1450 ◦ C. Rejab et al. [87] discussed the changes occur­ the pure forms of c-CeO2 and α-Al2O3. They also investigated the
ring in phases, mechanical properties, and microstructures of ZTA-5 wt. changes observed in the morphological and mechanical properties of the
% CeO2 ceramics reinforced with varying content of MgO (0–2 wt%). synthesized composites. The higher c-CeO2 concentration in composites
The fabricated ceramic composites with 0.5 wt% MgO content presented resulted in decreased mechanical strength of the composites. The
the most optimal properties, i.e. fracture toughness value attained was optimal combination of mechanical properties was achieved with lower
9.14 MPa√m, and hardness attained was 1591 HV. Benavente et al. [88] c-CeO2 concentration. The optimal combination of elements giving
made an attempt to study the mechanical and microstructural changes in higher young’s modulus of 235 GPa and hardness of 13 GPa that were
the developed Al2O3–ZrO2 (5, 10 and 15 vol%) nanocomposites by mi­ found to be similar to those of values for the pure Al2O3. Gafur et al. [98]
crowave sintering procedure and results were compared with that of developed Al2O3–ZrO2 composites by varying 3 mol.% yttria (0–20 vol
samples fabricated via conventional heating. Mechanical properties like %) along with MgO (in smaller amount as a sintering aid) via powder
density, Young’s modulus (367 GPa for 10 vol% of ZrO2), hardness (20 compaction method followed by sintering route. It was discovered that
GPa for 10 vol% of ZrO2), fracture toughness (6 MPa√m for 15 vol% of 5 vol% and 2 vol% of 3Y–ZrO2 presented maximum values for micro­
ZrO2) were increased and samples revealed homogeneous microstruc­ hardness and elastic modulus. Whereas, 20 vol% of 3Y–ZrO2 illustrated
ture with microwave sintering as compared to conventional heating. maximum values for fracture toughness and flexural strength. However,
Whereas, Dadkhah et al. [89] examined the physical properties precisely same group of researchers [99] investigated the effect of MgO (0–1.5 wt
density and wear of nano sized MgO-doped-Al2O3 composites prepared %)addition along with varying sintering temperatures upon physical,
via sintering. Owing to pinning effect the grain boundaries got reduced mechanical and microstructural characteristics of ZTA-TiO2composites
thereby causing the physical properties of sintered MgO-doped-Al2O3 to keeping the weight percentages of ZrO2 (15 wt%) and TiO2 (0.5 wt%)
improve. constant. Sintered ceramic composites were analyzed for density,
Subsequent year, Manshor et al. [90] discussed the effect of adding porosity, hardness and fracture toughness. Results indicated that up to
TiO2 in ZTA ceramics and its consequential changes in the phase 1% MgO addition; physical and mechanical properties were enhanced
development, mechanical characteristics and microstructure evolution whereas upon exceeding MgO content greater than 1 wt% no fruitful
were observed. Results indicated that great influence was observed by effect was observed. Gottwik et al. [100] reinforced strontium hex­
adding TiO2 and optimum properties were achieved for 3 wt% TiO2 aaluminate into ZrO2 based materials and resulting residual strength
which resulted in improved and enhanced values for density (4.10 g/cc), was compared post indentation and grinding process. Newly developed
hardness (1615.8 HV), and fracture toughness (6.56 MPa√m) and this strontium hexaaluminate-toughened ZrO2 illustrated superior mechan­
composition proved to be the best one amongst all. Rittidech et al. [91] ical properties post indentation and grinding in comparison to Y-TZP
studied the impact of adding yttrium oxide (Y2O3; 2, 4, 6 and 8 wt %) on and ATZ (Table S1). Mechanical properties attained were elastic
structural, microstructural and mechanical characteristics of modulus (213 GPa), strength (>1700 MPa), and fracture toughness (8.6
Al2O3–ZrO2 ceramic composites fabricated via two-stage sintering pro­ MPa√m) that pointed out its utility in medical implants owing to its
cess. Amongst all the compositions, 6 wt% Y2O3 added composites supplementary strengthening mechanism. Rejab et al. [101] experi­
illustrated the best results for the densification, micro-hardness and mented with the addition of varying content of CeO2 (0–7 wt%) into ZTA
fracture toughness. Also two stage sintering aided in achieving proper ceramics and evaluated consequent modifications in the hardness and
densification and grain growth inhibition. Abbas et al. [92] utilized a toughness of the final composites fabricated via hot isostatic technique.
relatively low-cost production process known as gelcasting to synthesize Vickers hardness and toughness was affected owing to the formation of a
ZTA ceramics. ZTA ceramic samples prepared via gelcasting and secondary phase and hot isostatic pressing also affected these properties.
pressure-less sintering were able to achieve maximum values for density The highest density (4.48 g/cc), toughness (8.92 MPa√m) and hardness
(99.1%), hardness (1902 HV), fracture toughness (5.43 MPa√m) and (1838.3 HV) were obtained from 5 wt% CeO2 incorporation whereas
flexural strength (618 MPa). Naga et al. [93] reinforced tantalum generated lowest porosity of 0.37%. Sequeira et al. [102] developed a
pentoxide (Ta2O5) into ZTA and the resulting composites were tested for ZrO2–Al2O3 composites and sintered composites attained were of higher
physical and mechanical properties. Optimal results were attained for density (>97%) with enhanced mechanical properties for ATZ com­
0.36 vol% Ta2O5 doped ZTA in comparison to pure ZTA ceramics i.e. posites i.e. fracture toughness (5 MPa√m) and flexural strength (1394
Vickers hardness (1867 HV for 0.36 Ta2O5-ZTA), flexural strength (314 MPa) and revealed higher hardness for ZTA composites (1846 HV).
MPa 0.36 Ta2O5-ZTA), fracture toughness (7.19 MPa√m 0.36 Marta et al. [103] adopted a distinguished technique to fabricate the
Ta2O5-ZTA). Kern et al. [94] processed Al2O3–ZrO2 composites via composites via surface coating the selected powders followed by milling
surface modification route to get enhanced hardness and wear resistance and slip casting. Both the materials illustrated enhanced mechanical
in the resulting composites. Surface modification included a wet properties along with negligible sensitivity to ageing thereby exhibiting
chemical process in which Al2O3 particles were coated with zirconium possibilities for reliable implant devices for biomedical applications.
chloride (ZrCl4) followed by thermal treatment. With the increasing Manshor et al. [104] developed ZTA-TiO2-Cr2O3 ceramic composites
temperature, microstructural coarsening was seen, ultimately present­ keeping in mind the accomplishment of improved microstructure and
ing high hardness (1900 HV) and higher wear resistance ((6.9 ± 0.1) x mechanical properties at lower sintering temperature as compared to
10− 9 mm3/Nm) in the final composites sintered at 1475 ◦ C. Manshor conventional heating route. Investigation revealed optimal results for
et al. [95] evaluated the changes in phase, mechanical properties and microwave sintered samples at 1350 ◦ C imparting greater values for
microstructure of ZTA-TiO2 composites by reinforcing chromium oxide density (theoretical density, 95.74%), enhanced hardness (1803 HV),
(Cr2O3). Amongst all the compositions, the optimal results were found and superior fracture toughness (9.61 MPa√m) and thus proving mi­
with 0.6 wt% Cr2O3-ZTA-TiO2 composites accomplishing augmented crowave sintering as a useful method for achieving improved results in
mechanical properties like density (4.063 g/cc), hardness (1681 HV), terms of microstructure and other mechanical properties of resultant
and fracture toughness (7.15 MPa√m). Bartolomé et al. [96] fabricated samples. Zhang et al. [105] investigated the stability of ATZ composites
ZrO2/Al2O3 nanocomposites using hybrid nanoparticles prepared via fabricated from 0.4 mol.% lanthanum oxide doped 2 mol. Y-TZP matrix
CO2 laser co-vaporization followed by spark plasma sintering. These on the resulting phase transformation, mechanical properties and slow
ceramic composites proved to be promising candidates for hip replace­ crack growth. Results indicated that these composites were able to
ment implant materials with outstanding mechanical properties having provide better outcomes in terms of threshold (4 MPa√m) and fracture
flexural strength of 1500 MPa, fracture toughness of 6.8 MPa√m in toughness (7.1 MPa/m) as compared to 3 mol. Y-TZP and undoped ATZ
addition to high resistance to low temperature degradation. composites. In an another study, Khaskhoussi et al. [106] examined the

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effect of adding TiO2 content upon properties and microstructural densities observed were greater than 93% for both pressing techniques.
prospects of 50 wt% Al2O3-50 wt% ZrO2 doped with 12 mol.% CeO2. As compared to uniaxially pressed composites cold isostatically pressed
The optimal results were obtained for this ternary composite and composites attained hardness 26% higher and a reduction in fracture
enhanced mechanical properties were attained with 5 wt% TiO2 doped toughness of about 8% was observed for 0.01 wt% MWCNT. Results
Al2O3–ZrO2 composites in addition to this exaggerated grain growth showed that better mechanical characteristics were observed with
were also observed in the fabricated ceramic owing to the presence of MWCNT doped ZTA as compared to single wall carbon nanotubes doped
TiO2. Mishra et al. [107] reported that 20 wt% of nano mica in hy­ ZTA composites.
droxyapatite matrix exhibited the optimal combination of mechanical Sabree and Mahdi [119] fabricated YSZ nanocomposites with vary­
properties like hardness 2.8 GPa, fracture toughness 1.51 MPa√m and ing weight percentages of HAp (5 and 10 wt%) via cold pressing fol­
~98% densification. Shon et al. [108] studied the changes occurring in lowed by sintering. Experimental outcomes showed reduction in
the microstructure and mechanical properties of nanostructures of 4 hardness and bending strength with the incorporation of HAp to YSZ,
mol.% Y2O3–ZrO2 and graphene-doped 4 mol.% Y2O3–ZrO2 composites whereas with the increasing sintering temperature from 1300 ◦ C to
fabricated via rapid consolidation and sintering. Results indicated 1400 ◦ C caused hardness and bending strength value to increase for all
enhanced fracture toughness values with the incorporation of graphene the tested nanocomposites. Another attempt was made by Leonov [120]
without affecting the hardness of the composites. Vasanthavel et al. in which he evaluated the resulting microstructure and properties of
[109] evaluated the impact of adding CeO2 upon structural and me­ Al2O3 nanofibers (1, 5 and 10 wt%)-ATZ composites synthesized via SPS
chanical behaviour of ZrO2–SiO2 glass ceramic composites synthesized route. Improved fracture toughness and hardness values were achieved
via sol-gel method and heat treatment. Ternary oxide system presented with the addition of Al2O3 nanofibers. Amongst all 1 wt% Al2O3 nano­
superior mechanical properties owing to dense and crystalline grains as fibers presented the highest fracture toughness which was 24% higher
compared to binary system (ZrO2–SiO2). than monolithic ZrO2 whereas microhardness got increased by 14% with
This past year, Goswami et al. [110] conducted a research work to the incorporation of 5 wt% Al2O3 nanofibers. Another group of re­
examine the structural, mechanical and wear characteristics of fabri­ searchers evaluated the variation in the hardness and compression
cated Al2O3 and varying Cr2O3 content based ceramic composites. strength of hydroxyapatite reinforced with varying amount (0.05–0.5 wt
Amongst all the prepared compositions, 1.5 wt% Cr2O3 with 70.5 wt% %) of MWCNT [121]. Results indicated for the improved ceramic me­
Al2O3 exhibited tremendous mechanical as well as wear properties chanical properties (hardness and compression strength) with the
presenting better alternative for the implant material. Whereas Das et al. incorporation of MWCNT. Compressive strength obtained was in the
[111] examined the biocompatibility, wear and mechanical character­ range of 100–230 MPa which was close to the compressive strength of
istics of pressure-less sintered Si2N4 ceramic composites. Samples sin­ cortical bone. Sui et al. [122] investigated the influence of adding
tered at 1700 ◦ C (2 h) exhibited optimal results in terms of density erbium oxide (Er2O3; 0, 1, 3, 5, 10 and 15 wt%) into ZTA-3 wt.% TiO2
(~93%), hardness (16.7 ± 0.1 GPa) and young’s modulus (252 GPa). composites and evaluated resultant phase changes and mechanical
Again, Kuntz and Krüger [112] evaluated the influence of varying Cr2O3 characteristics of the sintered composites. Amongst all the fabricated
(0, 0.16, 0.33 and 0.50 wt%) content upon the resultant properties samples composites with 5 wt% Er2O3 presented highest density
(hardness, toughness, Young’s modulus and scratch resistance) of ZTA. (99.93%), fracture toughness values (7.92 MPa√m), and hardness
Results indicated that up to 0.5 wt% Cr2O3 addition no effect was values (1752 HV). Arab et al. [123] evaluated the impact of adding MgO
observed in the resultant mechanical properties. While, varying sinter­ upon the mechanical and dynamic properties of ZTA ceramic compos­
ing temperatures increased hardness values while negligible effect was ites. The findings of the study showed increased hardness for ZTA
observed for the fracture toughness. Whereas, Kern and Gommeringer composites having ≤0.7 wt% MgO whereas porosity got reduced owing
[113] for the first time published a work comprising of 1 mol.% yttria-6 to pinning effect of MgO. ZTA ceramics with 0.2 wt% MgO provided
mol % ceria-costabilized 10 vol% Al2O3-10 vol% strontium hex­ improved compressive strength whereas beyond 0.2 wt% MgO
aaluminate ceramic composites fabricated via slip casting and sintering. compressive strength deteriorated owing to Al2O3 grain size reduction.
The composites presented full density and higher fracture resistance Most recently a new method known as direct energy deposition
(10.4 MPa√m) whereas maximum strength was attained (900 MPa) method developed by Hu et al. [124] was very different from other
owing to different mechanisms like transformation toughening, conventional manufacturing processes for customized and complex
micro-cracking, crack deflection and crack bridging. Maji and Choubey shaped ZTA components that used to be costly and time consuming as
[114] developed ATZ composites fabricated vial uniaxial pressing and well. They studied the influence of incorporating ZrO2 content (0–41.5
treatment through different sintering route. Results from the study wt%) upon microstructures and mechanical properties of ZTA parts
showed decrease in porosity and fracture toughness whereas increase in fabricated via direct energy deposition. Results indicated that 5, 10, and
density and hardness was found owing to increase in weight percentage 20 wt% ZrO2 content tailored the new 3D quasi-continuous network
(0%–30%) of Al2O3 content in ATZ. Caravac et al. [115] investigated the microstructure whereas 30, 35, and 41.5 wt% ZrO2 content presented
effect of sandblasting treatment upon the mechanical properties as well eutectic microstructure which is beneficial for toughening mechanism in
as aging resistance of Al2O3, ZrO2 and ZTA based ceramics. All the three resultant ZTA. Manuel et al. [125] prepared a dense Al2O3–ZrO2sub­
selected materials demonstrated different reactions against sandblasting strates doped with lanthanum oxide (La2O3). These fabricated ceramic
treatment amongst which Al2O3 faced weakening owing to residual composites presented density of 94–99% theoretical density, hardness of
stresses whereas ZrO2 and ZTA substantially reinforced it. Also, resis­ 985–1036 HV, fracture strength of 218–254 MPa, and fracture tough­
tance to aging was increased in the sandblasted ZrO2 owing to mono­ ness ≥2.6 MPa√m. All these porous substrates exhibited lower chemical
clinic to tetragonal recrystallization. Whereas, Meena and Karunakar solubility and non-cytotoxic behaviour. Whereas, Sktani et al. [126]
[116] developed ATZ nanocomposites by utilizing spark plasma sinter­ examined the combined addition of calcium oxide and calcium car­
ing (SPS) process. The improved properties were attained like density bonate and its influence on the microstructure and mechanical proper­
(99.33%), hardness (19.86 GPa) and grain size (385 nm) owing to the ties of ZTA. Porosity in the samples got reduced in ZTA samples owing to
suppressed grain growth. Zhou et al. [117] attempted to evaluate the which resulting hardness (1627 HV) improved with the incorporation of
influence of adding mullite fiber content in varying weight (5–15) per­ calcium oxide. Whilst the fracture toughness of the ZTA samples was
centages in hydroxyapatite composites. Densification (2.153 g/cc) of observed to be stable and was found to be affected with the amount and
composites increased resulting in improved hardness (191.87 ± 12.11 shape of hibonite grains. Table S1 indicates a summary of the mechan­
HV) and bending strength. Rojas et al. [118] studied the effect of uni­ ical properties accomplished bioceramic composites.
axial pressing, cold isostatic pressing and sintering on the mechanical Safinajafabadi et al. [127] made an attempt to analyze the effect of
properties of carbon nanotubes-reinforced ZTA composites. Relative adding organic dispersants on tailoring the structural, physical and

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mechanical properties of Al2O3–ZrO2 ceramic composites. Nanopowders mispositioning, periprosthetic fracture, surgical approach, polyethylene
were prepared from both wet and dry milling routes and amongst both wear, patients age, component sizing, and minor trauma [143–149].
dry milling turned out to be the better choice. Glucose, maltose, and Hence, there is imperative need for developing and manufacturing safe
dextrin were used as organic dispersants and amongst them dextrin biocompatible implant materials. Rapid osseointegration over ortho­
proved to be the ideal dispersing agent that aided in preventing powder paedic implant depends upon tissue/cells forming capability amongst
agglomeration resulting in smaller particle size, increased hardness the implants and bones [150]. Metals and metal alloys like Co, Ti, Cr, Sr
(~20 GPa) and greater fracture toughness (~11 GPa). Dhar et al. [128] etc. acts as bioinert with the biological structures inside the human body
attempted to examine the changes in physical, mechanical properties of and research related to metals have been carried out in past [151–155].
ZTA composites doped with 0–8 wt% of TiO2. This study revealed In this context, metal alloy based implants lack in osseointegration
gradual increase in flexural strength, hardness and fracture toughness properties that restricts their biological attachment with the nearby
with the incorporation of TiO2, attaining its highest value at 4 wt% and bone tissues and also affects the durability of the implant. Moreover,
then reduced beyond this upon further addition of TiO2. Agac and these metal implants releases metal ions owing to their recurrent
co-workers [129] pointed out that incorporation of 0.5 wt% TiO2 and 1 interaction with the nearby environment that causes adverse effects on
wt% Al2O3 co-doped 3 mol. Y-TZP sintered at 1450 ◦ C for 2 h had a the patients’ health.
considerable influence on the morphology, mechanical and biological A study was conducted recently by Sorrentino et al. [156] in which
characteristics of the fabricated 3Y-TZP ceramic. Tuan et al. [130] bacterial adhesion was compared on different bearing materials that are
examined the mechanical properties of dense Al2O3/(t-ZrO2+m-ZrO2) used in total hip arthroplasty i.e. metals, polymers, and advanced ce­
composites fabricated by sintering at 1600 ◦ C. Strength enhancement is ramics. Metals and polymers after 24 or 48 h faced infection due to
encountered owing to refinement of microstructures, surface stresses Staphylococcus aureus and Staphylococcus epidermidis. While, ceramics
induced by surface grinding thereby reaching the value of 940 MPa (3 confronted reduced bacterial adhesion as compared to the metals and
times that of Al2O3). Incorporating t-ZrO2 and m-ZrO2 leads to increase polymers. In general, four types of implant-tissue interaction responses
of toughness values as high as twice the toughness of Al2O3 alone owing take place post-surgery inside the host body and those responses have
to t→m phase transformation of ZrO2. Santos and coworkers [131] made been summarized in Fig. 3. As an alternative to metal based implants,
an attempt to evaluate the properties of Y-TZP/Al2O3 nanocomposites ceramic materials has received increased attention owing to excellent
prepared by high-energy ball milling (milling time: 1, 5, 10, 30, and 60 properties like high mechanical characteristics, corrosion resistance,
h) and sintering (temperature: 1400 ◦ C and 1600 ◦ C for 120 min). Re­ lower thermal conductivity, greater fracture toughness, superior ther­
sults revealed that nanocomposites obtained after powder milling for 60 mal stability, outstanding biocompatibility, excellent thermal shock
h and sintering at 1400 ◦ C exhibited higher toughness (8 MPa√m) and resistance, lower wear characteristics and higher thermal expansion
hardness (15 GPa) values as compared to other samples. Similarly for the [155,157–163].
same sintering temperature condition (at 1400 ◦ C), Saldaña et al. [132] Piconi et al. [33] made an attempt to evaluate the stability and
intended to assess the mechanical properties of t-ZrO2 polycrystals in-vivo ageing of co-precipitated and yttria coated powders in Y-TZP
fabricated via hot isostatic pressing technique. Best mechanical prop­ composites. Samples were implanted in quadriceps muscles, femur
erties i.e. modulus of rupture or fracture resistance (1600 MPa), and notch and proximal tibia methaphysis. Results revealed that animals
Weibull modulus (10.6)were attained at sintering and HIP temperature were sacrificed at 1 and 6 months without showing any sign of corro­
of 1400 ◦ C. Moreover, the fracture resistance showed indirect relation­ sion. In NZW rabbit femur new formed bone from cortex towards the
ship with the HIP temperature. Miao et al. [133] investigated the effect implant was seen therefore suggesting potential use of yttria stabilized
of incorporating 0–40 vol% TiO2 on Y-TZP ceramics sintered at 1300 ◦ C, oxides in the form of coating of grains for biomedical applications.
1400 ◦ C, and 1500 ◦ C. High hardness values of 860–1000 kg/mm2 and Tanaka et al. [39] evaluated the biocompatibility of Ce-TZP/Al2O3 and
toughness values of 4–4.5 MPa√m were attained for Y-TZP-TiO2 com­ monolithic Al2O3 nanocomposites by implanting in paraspinal muscles
posites sintered at 1500 ◦ C in comparison to the values attained for the of 36 Wister rats. Post 24 weeks of implantation, no inflammatory re­
composites sintered at 1300 ◦ C. Table S1 indicates a summary of the action was observed and similar tissue reactions were observed having
mechanical properties accomplished by ceramic composites. thin fibrous capsules around both the nanocomposites. These results
suggested the promising behaviour of Ce-TZP/Al2O3 nanocomposites for
3. Biological properties of bioceramic composites total joint replacement applications. Guzman et al. [164] assessed the
cytocompatibility of 25 vol% Ti-doped Al2O3 materials fabricated via
Total hip replacement surgery was declared as the operation of the SPS route. This novel material presented good cytotoxic properties along
century [134] as it was the most frequently performed surgery world­ with the advancement in the stages of osteoblast cells (proliferation and
wide that helped millions in overcoming their pain of osteoarthritis, hip early differentiation) on the composite surface. Another group of re­
fractures, or tumors, rheumatoid arthritis [135] and this number is ex­ searchers Kim et al. [43] studied the variation in densification, me­
pected to escalate to double in the coming years [136]. The domain of chanical and biological characteristics by reinforcing CaF2into
biomaterials has dramatically made advances in recent decades and has HAp-ZrO2. In-vitro proliferation test of these composites revealed
evolved into more improved and advanced medical devices produced osteoblast-like cell response that indicated comparable cell viability.
from metals to ceramic. It is acknowledged that once a biomaterial is Zhang et al. [52] prepared HAp-ZrO2 composites via powder
implanted inside the body, it induces several reactions in the biological dispersion and slip casting. Biological tests revealed attachment, scat­
media. For example, as soon as an implant is inserted there are cases tering and spreading of mesenchymal stem cells upon HAp-ZrO2 com­
when bacterial infections and total disinfections are confronted along posites. Silva et al. [165] examined the potential of utilizing Si3N4 for
the bone/material interface owing to impoverish biocompatibility of clinical applications. Samples of Si3N4 were implanted into rabbits’
that implant material leading to re-surgeries. Traditional antibiotic tibias for about 8 weeks and results showed the growth of adjacent bone
therapies seem to fail in curing these biomaterial related infections tissues around the implants.
[137]. It is indicated that early detection of the injury that might harm Wang et al. [166] examined the biocompatibility and sintering
the implant later on is more effectual rather than examination of the characteristics of ZrO2 nanoparticles produced via vapor-phase hydro­
patients when they experience pain or face loss of function [138,139]. lysis. In vitro cytotoxicity and cell culture by making use of human
The most common causes of revision surgery for THA and its ultimate umbilical vein endothelial cells lines was utilized in order to evaluate
failure are instability and dislocations whose frequency ranges between the biocompatibility of ZrO2 nanoparticles. ZrO2 nanoparticles exhibi­
0.3% and 10% for THA and 28% for revision THA [140–142]. Besides, ted good biocompatibility when evaluated it’s in vitro cytotoxicity and
there are several other complications too like component hemolysis tests. Rascon et al. [62] examined the impact on biological

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Fig. 3. Different implant-tissue interactions and their consequent responses inside the host body.

properties of ZrO2–Al2O3 composites by testing fabricated samples in Mehrali et al. [172] fabricated calcium silicate-reduced graphene
mouse fibroblast cells and experimenting for 24, 48 and 72 h in a methyl oxide (CS/rGO) and evaluated their mechanical, in vitro biocompati­
thiazole tetrazoliumassay test. Results showed great biocompatibility bility by soaking in simulated body fluid and cell culture. Results illus­
without any cytotoxic reaction over cell viability tests that depended trated development of human osteoblast cells on CS/rGO composites.
upon the distinguished factors like grain shape, grain size as well as Improvement in the proliferation rate and alkaline phosphatase activity
homogeneous/heterogeneous grain spreading in the ceramic compos­ of cells was observed on CS/rGO composites as compared to pure cal­
ites. Hadjicharalambous et al. [167] fabricated three ceramicswith the cium silicate ceramics thereby suggesting its promising nature to be used
varying porosity content and studied its resulting influence upon in biomedical implants. Matsumoto et al. [173] fabricated ZrO2-HAp
pre-osteoblastic cell attachment and proliferation on the surface of ZrO2 composites by mixing, compacting and sintering the powder mixture at
and Al2O3. The biological response on ZrO2 was superior to Al2O3 1500 ◦ C for 5 h. Mixing ratio 70/30 (ZrO2-HAp) showed strength similar
whereas beneficial cell growth was observed for pore size of approxi­ to cortical bone. Biocompatibility tests illustrated higher protein
mately 150 μm and for 50% porosity content. Albeit, Salari et al. [168] adsorption with cellular affinity. Animal experimentation confirmed its
evaluated the biocompatibility of hybrid nanocomposites fabricated by higher osteo-conductive behaviour. Another group of authors [111]
reinforcing nano ZrO2 into the matrix of polymer-HAp. ZrO2 incorpo­ evaluated in-vitro cytotoxicity of Si3N4 fabricated via sintering and cell
ration escalated osteoblast activities along with the cell adhesion that morphologies revealed promising cell-material interactions with the
illustrated its improved biological properties for the implant liner. increasing cell culture duration. Whereas, Lu et al. [174] examined the
Altmann et al. [169] assessed the influence of newly fabricated role of surface roughness and surface texture upon bacterial adhesion
ZrO2-based ceria-stabilized implant material upon particular cell func­ behaviour occurring between implant-cell interactions. In-vitro study
tions tested in-vivo, in-vitro and rat model. In vitro, in vivo and animal evaluated the adhesion between Staphylococcus aureusbacteria (implant
study revealed that porous novel ceria-stabilized ZA8Sr8–Ce11 (zirco­ related bacterial infection) and YSZ bioceramic surface. It was
nia-alumina-aluminate composite) ceramics is most favorable material concluded that as the surface roughness was reduced from micron to
for clinical use exhibiting highest bone-to-implant contact values and nano scale level the consequent change in the surface state was observed
presented more implant stability post implantation as compared to other (hydrophobic to hydrophilic) thereby gradually reducing the bonding
groups. Ce-stabilized ZrO2 presented biocompatible nature when tested strength of the bacterial-surface. Thus, it was suggested that for
in-vivo and in-vitro. In addition to this, preclinical investigations of this obstructing the primary adhesion and proliferation of bacteria on the
novel material in comparison to other groups revealed presence of implant interface, the need for nano-scale surface roughness is highly
osteoblast on porous ZA8Sr8–Ce11P confirming its favorability as desirable along with the eradication of unidirectional surface textures
implant material with a greater bone-to-implant interaction as well as that will help in reducing the implant related infections. Flamant et al.
implant stability after implantation. On the other hand, Liu et al. [170] [175] assessed the osseointegration and antibacterial properties of in­
fabricated graphene-Al2O3 nanocomposites for biomedical applications jection molded ceramic implants (ZTA). A novel process called selective
by ball milling the initial powders followed by pressure-less sintering. etching was adopted to generate nano-roughness and nano-porosity that
Cell adhesion tests revealed enhanced spread of stem cells upon gra­ was used to load and deliver the antibiotic drug. Micro-topography was
phene doped-Al2O3 material indicated that this material could be uti­ controlled due to injection molding as well as selective etching process
lized in a variety of biomedical applications. Whilst Ormanci et al. [171] on ZTA samples. In addition to this, etching caused formation of
developed Al2O3-YSZ-TiO2 composites via SPS and evaluated for their nano-porosity inside ZTA samples that acted as carrier for drug delivery.
obtained mechanical and biological (in-vivo) behaviour. Samples were Sequeira et al. [102] developed ZrO2–Al2O3 composites for orthopedic
implanted in the femur of Wistar rats that after 6 weeks of implantation applications and evaluated for its mechanical and osteoblastic cyto­
illustrated the development of osseointegration process that aided in compatibility tests. Results revealed existence of osteoblastic cell ac­
forming the soft tissue layers in place of bone defected area. Amongst all tivities in terms of adhesion, proliferation and functional biological
the compositions, 80 vol% - 20 vol% YSZ-5 mass % TiO2 proved to be the activities. Agac et al. [129] illustrated faster cell growth and adhesion
best integration confirming higher osseointegration and during in-vitro experimentation on the TiO2 and Al3O3 co-doped 3Y-TZP
biocompatibility. ceramic surface.

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Most recently, Wimmer [176] and team performed an experimental suppression of aging induced tetragonal to monoclinic transformation
work to evaluate the articulation of Al2O3–ZrO2 ceramic composites with the incorporation of metal oxides into it [204–207]. More pre­
held as a counterface against live cartilage and compared the results cisely, some of the common metal oxides include CuO, Al2O3, and MnO2
with the survivorships of CoCrMo-based implants. Results of the study into Y-TZP ceramics. Inclusion of these metal oxides into Y-TZP ceramics
indicated less cell and tissue damage with Al2O3–ZrO2 composites than resulted in improved sinterability, along with the finer grain sizes, and
CoCrMo. Although, the interaction between different cells plays crucial enhanced toughness strength. Sigulinski et al. [208] examined the
role in the physiological functioning of cells. Henceforth, for in-vitro changes in phase composition and fracture toughness of Si3N4–ZrO2 by
evaluation, several co-cultured models representing the similar poten­ incorporating CeO2 prepared via hot pressing technique. Stress induced
tial interaction of the tissues with the implant inside the host body, have micro-cracking reported to be the dominating toughening mechanism in
been designed and developed so far like endothelial cell-osteoblast [177, hot pressed (1700 ◦ C) samples having 3.81 mol.% CeO2. Similarly,
178], osteoclast-osteoblast [179], chondrocytes-osteoblast [180] that Verma et al. [157] studied the effect of adding CeO2in the sintering of
depicts the efficiency of 3D co-culture system in monitoring cell sur­ Al2O3–ZrO2 and its impact on the resulting microstructural evolution
vival, cell adhesion, osteoblastic differentiation etc. Table S2 indicates a and resulting mechanical properties. Abnormal grain growth of Al2O3
summary of the biological properties accomplished by bioceramic was restricted owing to ZrO2 addition as it connects with the Al2O3
composites. particles that resulted in strong bonding as well as grain refinement.
High toughness was attained in Al2O3 composites as transformation of
4. Microstructural characteristics of bioceramic composites t→m ZrO2 was suppressed by Al2O3. Kern and Gadow [209] evaluated
the mechanical properties and resistance to low temperature degrada­
The fundamental problem with ceramic materials is their brittle tion of 2.5Y-TZP-Al2O3 composites. Al2O3 addition lowered the sintering
nature distinguished by low fracture toughness. A lot of work and efforts temperature along with exaggerated grain growth. Furthermore, for
have been executed in the past to enhance the fracture toughness of the greater sintering temperatures higher than 1450 ◦ C, the sintered density
ceramics [181–183]. In light of this one potential approach is to fabri­ got reduced. Tanaka et al. [39] developed a new nanocomposite,
cate ceramic composites [184,185]. In cases of this nature, the ceramic Ce-TZP/Al2O3 and compared its phase stability with the existing con­
powders that are being utilized and incorporated could be in micro-size ventional ceramics (Al2O3 and Y-TZP) that are employed in total joint
or nano-size out of which nano-sized particles attracts more attention of prostheses. This novel composite illustrated resistance to aging degra­
the researchers and scientists owing to their unique properties [186, dation post hydrothermal treatment for 18 h in 121 ◦ C vapors. Phase
187]. Pure Al2O3 is one of the most widely explored ceramic material transformation to monoclinic phase (25.3 vol%) was observed in the
having high hardness and enough strength on contrary possesses poor final Ce-TZP/Al2O3 nanocomposites. Whereas, Rittidech and Tunkasiri
toughness [187]. Whereas, ZrO2 is another ceramic material that is [210] co-precipitated Al2O3–ZrO2 with 25 mol.% ZrO2 content. Majority
known for higher strength and higher toughness is known promote the of phases contributed to m-ZrO2 and α-Al2O3 whereas phase t-ZrO2 was
toughness of Al2O3 [188], but on another note, it encounters t→m phase observed as minor phase that dropped with the rising temperature. The
transformation and this process has been investigated by various re­ micrograph revealed normal grain growth along with the uniform
searchers across the globe [189,190]. Some other good characteristics of microstructure. Rittidech and Suekwamsue [91] studied the effect of
ZrO2 includes wear resistant, higher compressive strength, and is bio­ incorporating dopants (2, 4, 6, and 8 wt% Y2O3) upon structural, me­
logically non-reactive. ZrO2 based ceramics like Y-TZP are capable of chanical and microstructural characteristics of 0.50Al2O3-0.50ZrO2
attaining superior strength and fracture toughness at room tempera­ composites fabricated by two stage sintering. Resulting composites
tures. For this reason, they have been widely explored in the field of revealed adequate densification and obstruction of grain growth
biomedical science. This higher strength and greater resistance to frac­ amongst all 6 wt% yttria optimal results for densification, and me­
ture were due to the stress induced phase transformation of tetragonal to chanical properties. Composition in samples presented t-ZrO2 phase in
stable monoclinic form of ZrO2 [191,192]. In general, transformation higher fraction. Mahmood et al. [99] observed secondary phase for more
toughening is held responsible for producing high crack resistance. than 0.5% MgO incorporation into ZTA-TiO2 ceramics. At 1520 ◦ C with
Phase transformation owing to stress generation at the crack tip evokes 1% MgO provided closed packed and denser structure while observed by
transformation of metastable t-phase to m-phase that instigates microscope.
compressive stresses owing to volume expansion [193]. Leriche et al. Basu et al. [46] examined the generated tensile residual stresses
[194] prepared ZTA composites by varying ZrO2 in a percentage from 5 developed in ZrO2 matrix composites, and results revealed higher
to 46 vol% and yttria from 0 to 3 mol.% by adopting different techniques toughness of TZP matrix albeit of reduced t-ZrO2phase. The inhomoge­
of mechanical method of mixing, pressing and ultimate sintering. Their neous distribution of Y2O3 obtained via mixing route contributed to
prime goal was to evaluate and compare the final ceramic composites enhanced transformation toughening in the composites. Chevalier et al.
prepared via different techniques for physical, crystallographical and [211] assessed the impact of cubic phase upon the isothermal aging
microstructural characteristics. The microstructural characteristics behaviour of Y-TZP ceramics fabricated by sintering at 1450 ◦ C and
revealed that fabricated composites owe all required attributes that are 1550 ◦ C for hip replacement prosthesis. Materials sintered at 1550 ◦ C
responsible for higher mechanical performances. As it is a well-known revealed presence of cubic grains enriched by Y2O3 which causes
fact that there are significant atoms that resides on the surface or in decrease of Y2O3 content in the adjacent tetragonal grains. These grains
the region of grain boundaries in any nanostructured material, therefore tend to promote tetragonal to monoclinic transformation thereby having
in result it is expected to attain certain distinguished behaviour and detrimental consequences on aging resistance, suggesting processing at
properties from such an arrangement which has been examined and low temperature sintering (1450 ◦ C and 1450 ◦ C) to avoid dual
confirmed by several authors [195–199]. Meanwhile, Kobayashi et al. cubic-tetragonal microstructure. Whereas, Wei and Gremillard [212]
[200] dealt with the issue of low-temperature degradation of Y-TZP. He examined the influence of sintering conditions on the microstructure
was the one who observed that at lower operating temperatures i.e. near and ageing behaviour of Y-TZP ceramics. This study showed convincing
250 ◦ C, the strength as well as toughness decreased especially in water relationship amongst the microstructural features and ageing parame­
comprising environments. Studies demonstrate that the phase trans­ ters and for the same sintering cycle, different powders results in
formation from t→m phase is the leading reason for aging, which further distinguished microstructure. Lóh et al. [213] evaluated the effect of
leads to the formation of micro-cracks which gets accelerated subject to temperatures and holding time for Al2O3 prepared by two-step sintering.
the presence of water vapors [201,202]. This phase transformation Ultimately, the effect on the resulting densification and grain size was
further promotes grain pull out, surface roughening [203], and micro estimated. Al2O3 with 99% purity sintered at 1550 ◦ C for 5 min and at
cracking [160]. There are various researches that have claimed 1500 ◦ C for 4 h illustrated superior relationship amongst increased

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densification and reduced grain size. Lóh and co-workers [214] evalu­ vol% ZrO2 nanocomposite synthesized via colloidal processing route
ated the influence of the microstructural properties of Al2O3 prepared by presented highest crack resistance. Lastly a best composition of new
two-step sintering on its mechanical properties. Improved mechanical nanocomposite (Al2O3-10 vol% ZrO2) was suggested in providing
properties were attained post two-step treating the Al2O3 samples. improved lifetime and reliability of ceramic based joint prosthesis. Li
Factorial analysis was adopted to reveal the individual effect of both et al. [221] conducted research work on fabricating 3 mol.% Y-TZP-20
temperature and holding time. From this study temperature was found wt% Al2O3 composites with HAp in volume fraction 10–50% via SPS
to be the most influential parameter. However, the most prominent technique. At 1400 ◦ C theoretical density reached 97.8% and caused the
impact on the average grain size occurred as a consequence of the formation of tricalcium phosphates in the fabricated ceramic compos­
interaction transpiring amongst holding time and temperature in the ites, ZrO2 and HAp showed no direct reaction amongst them leading to
second step. In comparison to the other sintering techniques, two step reduced grain growth of ZrO2 and Al2O3. Valdez et al. [222] carried out
sintering assisted in reducing the grain size of micro Al2O3. On the other experimental study to find out the effect of adding Al2O3 and La2O3 to
hand, Zhigachev et al. [37] witnessed the changes occurring in the phase Y-TZP ceramics and to see the impact on hydrothermal degradation
composition of calcia-TZP with the incorporation of TiO2 (0–1 mol.%). without losing its mechanical properties (especially fracture toughness).
Results indicated that owing to chemical reaction amongst calcia and Without sacrificing the density, grain size or fracture toughness char­
TiO2 till 0.75 mol.% only slight increment of t-ZrO2 was observed, acteristics, hydrothermal degradation was decelerated for 3 mol.%
whereas doping with 1 mol.% TiO2 caused t→m phase transformation of Y-TZP by doping Al2O3 and La2O3.
ZrO2 and ended up in tremendous surface cracking of the material.
Similarly, Dhar et al. [128] made an attempt to assess the impact of 5. Future scope
adding TiO2 particles in 80 wt% Al2O3-20 wt% ZrO2. The micrographs
revealed the hindrance of grain growth to be the maximum at 4 wt% In recent years, different types of ceramic biomaterials have been
TiO2 beyond which grain size keeps on increasing owing to secondary developed so far in order to attain improved bioactivity and biocom­
phase formation. Hossen et al. [215] examined the impact on structural patibility, and this rate of current research indicates that tremendous
and mechanical properties of ZTA upon dispersing different weight progress will be made in this field. Future research in the area of bone
percentages of ZrO2 prepared via slurry method and sintering. The t→m repair/bone replacement material must emphasis on the evolution of
phase transformation of ZrO2 contributed in the overall toughening ef­ more efficient processing techniques (e.g. hot press sintering, spark
fect. Increasing ZrO2 content leads to decrease in the grain size of ZrO2 plasma sintering, additive manufacturing etc.) that are capable of
which on the other hand increases with the rising sintering temperature. fabricating the implant with optimized structure that is bioinspired and
Chintapalli et al. [216] investigated the stability of SPS Y-TZP samples bone like structure. In either material, the desirable mechanical prop­
with respect to hydrothermal ageing and grinding. Samples possessing erties and other outstanding characteristics (bioactivity and biocom­
grain sizes less than 300 nm were resistive to hydrothermal degradation, patibility) emerge not only from the coalition of the two or more ceramic
whereas materials with greater than or equals to 300 nm grain sizes powders, but also from the intricate structural arrangement. Both these
confronted severe degradation. Smaller grain size caused higher acti­ two aspects must be kept in mind while fabricating the ceramic com­
vation barrier and residual stress levels resulting in the high resistance to posites with the properties matching with the bones. Apart from
degradation. Marchi et al. [217] assessed the physico-chemical (crys­ numerous attempts made in this direction, complete success is still
talline phases, functional groups, and crystalline phases) characteristics lacking in terms of clinical achievements. The future of bioceramic
of ZrO2–TiO2 composites before and after the application of a bone-like materials fabricated via different routes to improve the mechanical
apatite layer that helps in improving the osteointegration. The results properties and cellular response in implantation is continually ongoing
revealed crystalline behaviour for composites with 30% TiO2 and but encouraging. Successful implant is one that not only requires hard
amorphous behaviour for 0–10% TiO2 containing samples. The coated tissue and soft tissue compatibility, but in addition also demands anti­
calcium phosphate layer revealed presence of carbonate apatite bacterial properties to restrict the emergence of biofilm. Efforts must be
contributing for good bioactivity. Ngashangua et al. [218] did a sys­ taken to overcome the concern of initial microbial adhesion and pro­
tematic investigation to assess the addition of varying level of MgO in duction of biofilm, that usually takes place due to occurrence of re­
ZrO2–Al2O3 composites. During heat treatment, the consequent changes actions amongst the implant surface and the surrounding biological
encountered in the structure and phase behaviour of ZrO2–Al2O3 com­ environment. So, for this purpose, many researchers are trying to
posites processed via citrate-nitrate sol-gel process were also investi­ fabricate implants via different modified techniques along with more in
gated. Pure t-ZrO2 phase was formed as confirmed by characterization depth and comprehensive research to discover the fundamental mech­
results at 900 ◦ C. Formation of magnesium aluminate started from the anism behind surface properties and tissue response. It is important to
reaction between ZrO2 and Al2O3 at around 1000 ◦ C and its amount understand the relationship between structure-property-mechanism of
depended upon two leading factors i.e. the MgO content, secondly the selected implant materials for which following key points for future
heat treatment temperatures. Saldaña et al. [132] assessed the low research are summarized:
temperature degradation resistance of Y-TZP in air and in pressurized
water and in result found that samples with grain sizes less than 0.36 μm 1. Besides investigating the connections between structure-property-
were capable of resisting the degradation in vigorous and pressurized mechanism, it is of utmost importance to look after precise regula­
hot-water conditions. A study by Maneshian and Banerjee [219] found tion of nanostructures to build a systematic constructing relation
that high energy ball milling could act as driving force for provoking between structural characteristics and fabrication conditions. This
reverse martensitic transformation (m→t phase transformation) in the will be helpful in monitorable adjustment of mechanical properties.
Al2O3-15 vol% ZrO2 nanocomposites. Formation of tetragonal meta­ 2. A set of key parameters are anticipated to be found and utilized to
stable phase is identified if somehow monoclinic phase formation is identify/tailor the formation capability of nano-ceramic powders,
suppressed by applying certain milling conditions. Aza et al. [220] expediting the evolvement of high performance nanostructural
studied the crack growth behaviour of ZTA ceramics prepared in the ceramic biomaterials.
range 0–15 vol% ZrO2 via powder mixing and colloidal route. Double 3. Further exploration is required for the newly developed nano­
torsion technique was employed for the crack velocities 10− 12 to 10− 2 structures in terms of strength and ductility improvement mecha­
m/s to evaluate the crack propagation and observed the effect of pro­ nisms and to analyze the fracture mechanism via experimental
cessing conditions, selected compositions on the resulting microstruc­ characterizations, theoretical modelling and simulations.
tures. Ultimately the effect of microstructure on the slow-crack-growth 4. Various nanostructures can be introduced into one material with the
deportment was carried out. Results of the study revealed that Al2O3-10 help of different advanced techniques these days. The exhaustive

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study of behaviour and interplay of these nanostructures might help Appendix A. Supplementary data
in the formation of multi-structure optimization approach, further
helping in optimizing the mechanical properties of the material. Supplementary data to this article can be found online at https://doi.
5. It is firmly believed that in addition to in-depth knowledge of org/10.1016/j.ceramint.2020.09.214.
fabrication-structure-property-mechanism relationship, the
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