Sintering and Consolidation of Ceramics
Course KGP003
High Temperature Materials
By
Docent. N. Menad
Dept. of Chemical Engineering
and Geosciences
Div. Of process metallurgy
Luleå University of Technology
( Sweden )
Sintering and Consolidation of Ceramics
Solid State Sintering
The Phenomenon of Sintering What is Sintering?
Sintering commonly refers to processes involved in
the heat treatment of powder compacts at elevated
temperatures, usually at T > 0.5Tm [K], in the
temperature range where diffusional mass transport
is appreciable.
Successful sintering usually results in a dense
polycrystalline solid. However, sintering can
proceed only locally (i.e. at contact point of
grains), without any appreciable change in the
average overall density of a powder compact.
Sintering is thermal treatment of fine-grained material at a temperature below the melting point of the main
constituent, for the purpose of increasing its grain size and strength by bonding together the particles. Theories about
what happens exactly during sintering have provided the subject matter of innumerable conferences and learned
scientific papers. Suffice to say that atomic diffusion takes place and the welded areas formed during compaction
grow until eventually may be lost completely. Re-crystallization and grain growth may follow, and the pores tend to
become rounded and the total porosity, as a percentage of the whole volume, tends to decrease.
Reconstructions (virtual slices)
perpendicular to the cylindrical
axis showing Cu particles at
different stages of the sintering
process: (a) before sintering, (b)
after sintering at 1000°C, and (c)
after sintering at 1050°C.
Identical regions (inside the
rectangle of (a)) are shown at a
higher magnification below.
Reconstructed slices of a
compacted Distaloy sample
before (a) and after (b)
sintering at 1130°C. The
direction of prior
compaction is along the
vertical direction in the
paper plane
Developmentof
Development ofsinter
sintertexture
texture
< 1300 °C
Residual hematite
Coke Melting Point
Hematite
Secondary hematite
> 1300 °C
Clay
Fine mineral
N. Menad, H. Tayibi, Fernando Garcia Carcedo and A. Hernández
Minimization methods for emissions generated from sinter strands: a review
J. of clean production, Pages 740-747, 2005
BasicThermodynamics
Basic Thermodynamicsof
ofSintering
Sintering
The driving force for sintering is a decrease in the surface free energy of
powdered compacts, by replacing solid-vapor interfaces (of surface
energy ΓSV) with solid-solid (ΓSS) interfaces, where ΓSS < ΓSV.
Thermodynamically, then, sintering is an irreversible process in which a
free energy decrease is brought about by a decrease in surface area.
BasicThermodynamics
Basic Thermodynamicsof
ofSintering
Sintering
At a typical specific surface S of ceramic powders S = 1-10 m2/g and ΓSV = 1 -
2 J/m2. The resulting excess of surface energy of ΓSV = 1-20 J/g is small
compared with the heat of chemical reactions (> 1 kJ/g) but still sufficient to
drive the sintering processes. Some mechanisms can lead to a waste of
sintering driving force without assisting densification. These are the grain
coarsening mechanisms, which are driven by the same force as sintering.
The change of system energy dE due to sintering is therefore composed of the
increase due to the creation of new grain boundary areas, dASS > 0, and due
to the annihilation of vapor-solid interfaces, dASV < 0. The necessary global
thermodynamic condition for the sintering to proceed is
dE = ΓSS dASS + ΓSV dASV < 0
The sintering process will stop when dE = 0
ΓSS dASS + ΓSV dASV = 0 or ΓSS / ΓSV = - dASV / dASS
BasicThermodynamics
Basic Thermodynamicsof
ofSintering
Sintering
The sintering progress can be represented by plotting the total free surface
area ASV, vs the total surface area of the grain boundaries ASS. At the start
of sintering, all surface area equals the free surface area, since no grain
boundaries exist yet, ASV = ASV0 and ASS = 0. As sintering proceeds, ASV
decreases and ASS increases, in such a way that a monotonically
decreasing curve is obtained with the slope - dASV / dASS. If, at any point
during sintering, the value of the slope reaches ΓSS / ΓSV, the sintering must
stop due to the above equilibrium condition. The principal objective of
sintering is the elimination of porosity, i.e. a minimum ASV, possibly ASV =0
and ASS large, meaning little grain growth. It is thus desirable that the
sintering stop condition is reached when the slope dASV / dASS close to
zero, i.e. ΓSS << ΓSV. Through this type of thermodynamic considerations it
is suggested that sintering can be encouraged through manipulation of the
doping and/or the environment, so the surface energy is maximized.
Classes(Categories)
Classes (Categories)of
ofSintering
Sintering
The majority of non-silicate ceramics are processed through high-temperature
treatment and sintering of powder compacts with little (<2 vol%) or no liquid
phases. This is defined as Solid State Sintering, with predominant mass
transport (i.e. densification mechanism) through solid-state diffusion.
However, most silica-containing ceramics, including traditional porcelains as
well as advanced silicon nitride, sinter in the presence of viscous glass-type
liquids, with predominant mass transport (i.e. densification mechanism)
through viscous flow. This is defined as Viscous Sintering.
If the liquid component of the sintering system has low viscosity (e.g. molten
cobalt in the “classical” system of WC/Co) the process is defined as Liquid
Sintering. In this system the predominant densification mechanism is through
rearrangement of the solid particles “submerged” in and wetted by the low
viscosity liquid, and through dissolution and re-precipitation of the solid.
WhyCeramics
Why Ceramicshave
haveto
tobe
besintered?
sintered?
Ceramic processing is based on the sintering of powder compacts
rather than melting/solidification/cold working (characteristic for
metals), because:
ceramics melt at high temperatures
solidified microstructures can not be modified through additional plastic
deformation and re-crystallization due to brittleness of ceramics.
the resulting coarse grains would act as fracture initiation sites.
low thermal conductivities of ceramics (<30-50 W/mK), in contrast to
high thermal conductivity of metals (in the range 50-300 W/mK) cause
large temperature gradients, and thus thermal stress and shock in
melting-solidification of ceramics.
However, some ceramic refractories, such as Al2O3, Al2O3/ZrO2, are
manufactured through the melting / casting / solidification process. If
made properly, they are superior to the sintered versions, and the
processing costs of large, ~ 1 m tall blocks is lower.
•Whydo
•Why dowe
weneed
needSintering?
Sintering?
The principal goal of sintering is the reduction of compact porosity.
Sometimes the initial spaces between compacted grains of ceramics are
called “voids”, to differentiate them from the isolated spaces = pores,
which occur in the final stages of sintering. The sintering process is
usually accompanied by other changes within the material, some
desirable and some undesirable. The largest changes occur in
- strength, elastic modulus
- hardness, fracture toughness
- electrical and thermal conductivity
- permeability to gases and liquids
- average grain number, size and shape
- distribution of grain size and shape
- average pore size and shape
- distribution of pore size and shape
- chemical composition and crystal structure
•Whydo
•Why dowe
weneed
needSintering?
Sintering?
Sintering is a widely used but very complex phenomenon. The
fundamental mechanisms of sintering are still a matter of controversy.
Experimental quantification of changes in pore fraction and geometry
during sintering can be attempted by several techniques, such as:
- Dilatometry
- Buoyancy
- Gas absorption
- Porosimetry
- Indirect methods (e.g. hardness)
- Quantitative microscopy
The description of the sintering process has been derived from model experiments (e.g.
sintering of a few spheres) and by observing powdered compact behavior at elevated
temperatures. The following phenomena were observed and modeling was attempted:
- increase of inter-particle contact area with time
- rounding-off of sharp angles and points of contact
- in most cases, the approach of particle centers
and overall densification
- decrease in volume of interconnected pores
- continuing isolation of pores
- grain growth and decrease in volume of isolated pores
The development of microstructure and densification during sintering is a direct
consequence of mass transport through several possible paths (one of these paths is
usually predominant at any given stage of sintering):
- GAS phase (evaporation/condensation)
- LIQUID phase (solution/precipitation)
- SOLID phase (lattice diffusion)
- INTERFACES (surface diffusion, grain boundary diffusion)
- VISCOUS or PLASTIC FLOW, under capillary pressure (internal)
or externally applied pressure (pressure-sintering, hot-pressing,
hot-isostatic-pressing)
Since certain mechanisms of mass transport can be dominant in some systems, two
broad categories of sintering are recognized:
where all densification is achieved through changes in
particle shape, without particle rearrangement or the
presence of liquid
where some liquid that is present at sintering temperatures
aids compaction. Grain rearrangement occurs in the initial
stage followed by a solution-re-precipitation stage. Usually,
the liquid amount is not sufficient to fill the green-state
porosity in normal liquid-assisted sintering of ceramics. In
many instances, supposedly Solid State Sintering proceeds
in the presence of previously undetected (or transient) small
amounts of liquid (perhaps introduced as impurities during
the powder preparation stage, such as silicates in oxide
ceramics Al2O3, ZrO2).
Simple model experiments with spheres or rods illustrate
sintering phenomena, classified into three stages of
sintering, as discussed below.
1. Initial stage of sintering, involving
1A. Local point of contact formation or "fusion",
without shrinkage of compact. This is accompanied by
smoothing of the free surface of particles.
1B. Neck formation at the contact point, with the resulting
concave curvature δn (where δn =1/rn) at the neck, in
contrast to the convex curvature on the particle surface of
radius r, where r >> r n. The two radiuses of the neck
curvature, r n and y, represent an experimental justification
for the two-sphere model of sintering.
•The processes 1A and 1B result in densification of the sintering component by
~10%. That is, if the relative green density after forming of the particle compact
was 60%, the density after initial stage would be about 70% of the theoretical
density TD. However, the 10% densification in the initial stage is reached very
quickly (seconds or minutes) after exposing powder to high temperature,
because of the large surface area and the high driving force for sintering.
2. Intermediate stage of sintering, involving
2A. neck growth,
2B. pores forming arrays of interconnected
cylindrical channels,
2C. particle centers approaching one another,
with the resulting compact shrinkage.
The shrinkage in the intermediate stage can result in additional densification by as
much as 25%, or to a total of about 95% of the TD (theoretical density), compare.
However, shrinkage does not necessarily have to take place during the
intermediate stage of sintering. For example, shrinkage would not occur if matter
was transported FROM the particle SURFACE, and proceeded through either gas,
solid or along interface as surface diffusion.
3. Final stage of sintering, involving:
3A. Isolation of pores, i.e. relative density
exceeding ~93%
3B. Elimination of porosity
3C. Grain growth
The final sintering stage begins at about 93-95% of
theoretical density, when porosity is already
isolated. Ideally, at the end of this stage all porosity
is eliminated. The complete elimination of porosity
in the final stage of sintering can only happen when
all pores are connected to fast, short diffusion paths
along grain boundaries (or, equivalently, if the grain
boundaries remain attached to the pores).
Sintering process
Before sintering After sintering
Sintering
A(s) + B(s) AB(l)
A+L B+L
A+B
A 1 2 B
EmpiricalModeling
Empirical Modelingof
ofSintering
Sintering
A large number of equations describing the
variation of porosity P/P0 (or shrinkage ΔL/L0)
versus time t have been proposed. All of
these equations use simplified assumptions
of idealized models, or are based purely on
empirical observations. These equations
usually have forms of:
P/P0 = ( a + bt )c or P - P0 = dln(t/t0)
or (ΔL/L0 )n = at, where:
a,b,c,n = constants.
•A remarkable feature of most of these equations is that, using curve-fitting procedures, a
nearly perfect fit can be obtained with experimental data on sintering. However, little or no
insight into sintering mechanisms is obtained and incorrect conclusions are drawn when
physical meanings are attributed to the fitted constants. This is because real systems
deviate significantly from idealized models:
*particles are non-uniformly distributed in space and rearrange during sintering
*necks grow asymmetrically
*different sintering stages (neck formation, neck growth, rearrangement, grain growth
with or without shrinkage, closed porosity sintering) are controlled by different
mechanisms which interact and overlap in a complex way
Themass
The masstransport
transportfrom
fromsurfaces:
surfaces:evaporation-condensation
evaporation-condensation
The mass transport can proceed through a gas
phase, driven by a differential in vapor pressure.
Evaporation-condensation takes place because the
vapor pressure p1 on a curved surface of radius r1 is
different from that on a flat surface (p0 , r0 = ∞) or
any other surface at r2. This is expressed by the
Kelvin equation:
ln (p1/p2) = (Ω ΓSV / RT) (1/r1 + 1/r2)
where Ω = the molar volume of the species. For
the simple case of r2 = r0 = ∞ and p2 = p0 (that is,
for a flat surface), the above equation simplifies to:
ln (p1/p2) ≈ (p1-p0)/p0 = Δp/p0 so Δp/p0 = (Ω ΓSV)/(R T r1)
The
Themass
masstransport
transportfrom
fromsurfaces:
surfaces:evaporation-condensation
evaporation-condensation
The most important case of evaporation-
condensation mass transport is from the surface
of convex (spherical) particles (r > 0) to the
surface of concave necks (rn < 0) where |rn| << |r|
at the contact region. No shrinkage occurs in this
process and y3 is a linear function of time:
y3 = α2 t and h/r = ΔL/L = 0
This type of transport dominates at the relatively high vapor pressures
of 10-4 atmospheres, found for halides (e.g. NaCl at 700°C). The log-
log plot of the neck size versus time has a slope close to 1/3. The
evaporation-condensation process is suspected to stop the
sinterability of some non-oxide ceramics (such as SiC, B4C) at very
high temperatures > 2100°C
Themodel
The modelfor
formass
masstransport
transportfrom
fromsurfaces:
surfaces:surface
surface
orvolume
or volumediffusion
diffusionfrom
fromsurface,
surface,DS
DS
Similar to evaporation-condensation, no shrinkage results from
the other two mechanisms of mass transport from particle
surfaces to necks through the two possible diffusion paths.
(I) volume diffusion from particle surfaces to necks:
y4 = α3 t and h/r = ΔL/L = 0
(II) surface diffusion, along particle surfaces to necks:
y7 = α4 t and h/r = ΔL/L = 0
Themodel
The modelfor
formass
masstransport
transportfrom
fromwithin
withinthe
theparticle
particle
volumeor
volume orfrom
fromthe
thegrain
grainboundary,
boundary,DL
DLororDB
DB
For mass transfer originating in the particle volume or at the grain
boundary, the particle centres approach
(h > 0) and shrinkage takes place.
(I) for volume diffusion from grain boundary:
y5 = α5 t and thus (h/r)2.5 = (ΔL/L)2.5 = β5 t
(II) for grain boundary diffusion from grain boundary:
y6 = α6 t and thus (h/r)3 = (ΔL/L)3 = β6 t
. Examples of Solid State Sintering
Since ceramics are composed of at least two elements (typically
oxygen or nitrogen and metal ions) Both ionic species must diffuse
together to maintain the electrical neutrality of the system.
Therefore, it is the diffusion coefficient of the slowest moving ion
along its fastest path that controls mass transfer, and therefore
densification during solid state sintering. To enhance sintering,
the slowest moving ion must be identified and its diffusion along
the fastest path should be "encouraged" through:
Chemical doping,
Atmospheric control
An appropriate time/temperature cycle
Sintering of MgO
MgO sintering
MgO sintering proceeds
proceeds through
through lattice
lattice diffusion.
diffusion. Diffusion
Diffusion of
of
magnesiumisisfaster
magnesium fasterthan
thanoxygen,
oxygen,DDMg >>DDO. . The Thesintering
sinteringrate
rate
Mg O
isisincreased
increasedififthe
thediffusivity
diffusivityof
ofoxygen
oxygenisisincreased
increasedthrough
throughthe
the
creation of
creation of additional
additional oxygen
oxygen vacancies
vacancies through
through doping
doping with
with
M2O(for
M2O (forexample
exampleNa2O).
Na2O).
Anotherexample
Another exampleof ofsolid
solidstate
statesintering
sinteringisisthe
thesintering
sinteringofofAl2O3.
Al2O3.
Initially,ititwas
Initially, wasobserved,
observed,that thatsmall
smalladditives
additivesof ofMgO
MgO(0.25
(0.25wt%)
wt%)inin
Al2O3 allowallow the
the achievement
achievement of of fine-grained
fine-grained material
material atat full
full
Sintering of Al2O3
Al2O3
density (which
density (which isis the
the ceramist’s
ceramist’s dream).
dream). Additional
Additional micro-
micro-
structural observations
structural observations revealed
revealed that that MgO
MgO eliminates
eliminates thethe
discontinuous grain
discontinuous grain growth
growth ofof Al2O3.
Al2O3. ItIt was
was found
found that
that grain
grain
boundariesdo
boundaries donot
notbreak
breakaway
awayfromfromthe
thepores,
pores,which
whichprevents
preventsthe
the
inclusionof
inclusion ofpores
porestrapped
trappedinside
insidenew
newlarge
largegrains,
grains,with
withslow/long
slow/long
diffusion paths
diffusion paths for
for densification.
densification. The The mechanism
mechanism by by which
which MgO
MgO
slows down
slows down grain
grain boundary
boundary movement
movement inin alumina
alumina could
could be
be as
as
follows
follows
- The majority of MgO doped into Al2O3 resides at the grain
boundaries, because the dissolution of MgO in Al2O3 is small, ~300
ppm. This is due to the relatively large difference in ionic radius: 0.72A
for Mg2+ and 0.53A for Al3+
- Any fast migration of the grain boundary would have to incorporate
Mg2+ ions into the Al2O3 lattice, with the resulting increase in internal
energy, unless a NEW compound, spinel, forms.
Arguments on the role of MgO in Al2O3 sintering continue at the present tim
Note that other grain-growth inhibitors can be added to Al2O3. For
example, submicron ZrO2 particles (10-20 vol%) residing at the triple
points of grain boundaries of alumina effectively inhibit grain growth
according to the equation: r4 - r04 = a t, where: r = the average particle size
after time t, r0 = the average initial particle size at t=0, and a = constant.
Sintering of SiC
Pressureless sintered SiC requires B and C as additives, which
affect the relative diffusion coefficients. Presence of silica
impurities in SiC can enhance evaporation-condensation
mechanisms through a volatilization reaction
2SiO2 (solid) + SiC (solid) → 3SiO(gas) +CO (gas)
Carbon removes SiO2 and Si from the surface of SiC according
to the carbothermal reduction reaction:
SiO2 + 3C → SiC + 2CO; C+Si → SiC
Thus, fewer defects are present at the surface and diffusion along the
surface decreases. This slows down the coarsening mechanisms.
Simultaneously, boron has been found to selectively segregate toward
grain boundary regions, where its role is unclear, i.e. it is suspected that:
Pressure Assisted Sintering
Pressure Assisted Sintering - Technology
Pressure assisted sintering increases the contact pressure of
particles and thus the driving force for sintering, compared to
pressureless solid state sintering
by ~ one order of magnitude in hot pressing (HP) at 20-
40 MPa pressure
by ~ two orders of magnitude in hot isostatic pressing
(HIP) at 200 - 300 MPa pressure
Example of Viscous Sintering: Porcelains
Sintered nano barium titanate
NETZSCH Dilatometer 402 C Equipment
Materials Powdered metal and
composite materials
Thermal expansion and
Measurement
sintering behaviour
figure shows the thermal expansion and rate of
expansion of a silicon nitride green body. The
sintering step starting at 1201°C is due to the
influence of the sintering additives. The main
shrinkage step occurred at 1424°C (extrapolated
onset). The effect above 1760°C is most probably
due to evaporation of additives
The Laser Sintering Process
Certain proportion of the laser energy
is transformed into heat, this depends
upon the powder material and its
characteristics. Laser output can be
minimised through maintaining the
powder at a temperature just below its
melting point
Selective Laser Sintering (SLS)
This process is a well known procedure for rapid tooling/prototyping.
Its principle is the following: A powder made from synthetic materials,
coated metal powder or mixture of metal powders is deposited in a thin
layer. In a next step a Laser beam heats and melts the powder either
partially or fully. Repeated application of this procedure creates rapidly
full scale objects in one manufacturing step
This Figure shows a cut through a sintered aluminium-
bronze part. The part has a dense structure with fine
Surface treated by with SLS grains. Therefore, manufacturing metal foring tools by
selective Laser-Sintering seems to be a valuable
alternative to conventional manufacturing procedures
Sintering Practice Exercises
1. Name 5 different changes that can occur during heating of a ceramic material.
2. What are the 3 principal sintering processes?
3. What does liquid sintering mean?
4. What does viscous sintering (vitrification) mean?
5. What are the main driving forces during sintering?
6. What are the main driving forces during actual “solid state” sintering?
7. Which transport mechanisms occur during solid state sintering?
8. What is the effect of increased temperature on solid state sintering?
9. Name at least 4 parameters that are crucial during solid state sintering?
10. Describe the different stages of solid state sintering.
11. What requirements must be fulfilled in order to attain total pore
elimination during solid state sintering?
12. Why is sintering easier in the presence of a liquid phase?
13. Describe the reaction mechanisms during liquid sintering.
14. How does vitrification differ form “normal” liquid sintering?
15. During the study of a solid sintering process at constant temperature
it was discovered that the neck radius, y, is proportional to t0.2.
What is the dominating transport mechanism in this sintering process?
