SINTERING OF CERAMICS
BY: Mohammad Ali
DEFINITION
• Sintering commonly refers to processes involved
in the heat treatment of powder compacts at
elevated temperatures, 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
A MODEL SKETCH
BASIC THERMODYNAMICS OF
SINTERING
• Sintering is an irreversible process in which a
free energy decrease is brought about by a
decrease in surface area.
• The driving force for sintering is a decrease in
the surface free energy of powdered compacts,
by replacing solid-vapour interfaces (of surface
energy sv) with solid-solid (ss) interfaces,
where ss < sv.
BASIC THERMODYNAMICS OF
SINTERING (contd.)
• 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 vapoursolid interfaces, dAsv < 0.
• The necessary thermodynamic condition for the
sintering to proceed is:
dE = ss dAss + sv dAsv < 0
WHY CERAMICS HAVE TO BE
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.
• As-solidified microstructures can not be modified
through additional plastic deformation and
recrystallisation due to brittleness of ceramics.
WHY CERAMICS HAVE TO BE
SINTERED? (contd.)
• 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.
WHAT HAPPENS DURING
SINTERING
• Increase of interparticle contact area with time
• Rounding-off of sharp angles and points of
contact
• In most cases, the approach of particle centres
and overall densification
• Decrease in volume of interconnected pores
• Continuing isolation of pores
• Grain growth and decrease in volume of isolated
pores
SINTERING STAGES
Three sintering stages are discussed here
with major changes taking place against
each stage.
INITIAL STAGE OF SINTERING
• Local point of contact formation or
"fusion", without shrinkage of compact.
This is accompanied by smoothing of the
free surface of particles.
• Neck formation at the contact point, with
the resulting concave curvature at the
neck, in contrast to the convex curvature
on the particle surface.
INITIAL STAGE (contd.)
• 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).
INTERMEDIATE STAGE OF
SINTERING
• Neck growth,
• Pores forming arrays of interconnected
cylindrical channels
• Particle centres approaching one another, with
the resulting compact shrinkage.
INTERMEDIATE STAGE (contd.)
• 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.
• During sintering, if the only material transport
mechanism originates on the surface of
particles, no compact shrinkage takes place.
• In such case, a change of the shape and size of
pores and particles is observed and commonly
termed as grain growth or coarsening.
FINAL STAGE OF SINTERING
• Isolation of pores, i.e. relative density exceeding
~93%
• Elimination of porosity
• Grain growth
FINAL STAGE (contd.)
• 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 if the grain
boundaries remain attached to the pores.
FINAL STAGE (contd.)
• This favourable situation happens only if the
pores follow the movement of the grain
boundaries and are not trapped within grains.
• This means that discontinuous grain growth (i.e.
few grains growing at a very large rate at the
expense of all other grains, trapping porosity on
its path) must be stopped.
• It is suppressed through grain growth limiting
additives, such as secondary phase particles at
grain boundaries, and/or appropriate time and
temperature control of the sintering process.
CHANGES OCCURING DURING
SINTERING OF A
WHITEWARE TRIAXIAL (silica-mullite-leucite)
AT APPROX. TEMP. C
Up to 100
Loss of moisture
100-200
Removal of adsorbed water
500
Oxidation of organic matter
575
Little overall volume damage
980
Start of shrinkage
1050-1100
Glass forms from feldspar, mullite
grows, shrinkage continues
1200
More glass, isolation of pores
1250
Max. densification, pores at
min.(60%glass,21%mullite,19%quartz)
SINTERING CATEGORIES
• Solid state sintering occurs when the powder
compact is densified wholly in a solid state at the
sintering temperature.
• Whereas liquid phase sintering occurs when a
liquid phase is present in the powder compact
during sintering.
• Transient liquid phase sintering is a combination
of liquid phase sintering and solid state sintering.
In this sintering technique a liquid phase forms
in the compact at an early stage of sintering, but
the liquid disappears as sintering proceeds and
densification is completed in the solid state.
ISOTHERMAL SINTERING
• Pure ZrO2 and ZrO2+14wt%Al2O3 were
subjected to pressure-less sintering in vacuum
at 1100 C (0.4Tm) for different periods of time.
• Nearly full densities have been achieved in all
cases, with average grain sizes not exceeding
100nm.
• Very small grain size(<30nm) found in case-2,
due to homogeneous distribution of Al2O3 phase
particles hindering grain coarsening by pinning
of grain boundaries.
TWO STAGE SINTERING
• 2SS is able to refine microstructure and in turn it
improves grain size dependent material
properties.
• 2SS simply modifies sintering route by firing
sample at hi-temp follow by rapid cooling down
and dwelling at lower temp.
PROCESS CONDITION for Zirconia, heating at
1350 C, follow by 900 C is found to be able to
achieve comparable hardness as iso-thermal
sintering at 1500 C.
SINTERING VARIABLES
The major variables which determine
sinterability and the sintered
microstructure of a powder compact
may be divided into two categories:
Material variables,
&
Process variables.
MATERIAL VARIABLES
• The variables related to raw materials are said
as material variables.
• These include chemical composition of powder
compact, powder size, powder shape, powder
size distribution, degree of powder
agglomeration, etc.
• These variables influence the powder
compressibility and sinterability (densification
and grain growth).
PROCESS VARIABLES
• Process variables involved in sintering are
mostly thermodynamic variables.
• These variables include temperature, time,
atmosphere, pressure, heating and cooling rate.
SINTERING TEMP FOR SOME
COMMON CERAMICS
SINTERING ADDITIVES
• Sintering additives are usually added to
powders to enhance the sinterability and
to control the microstructure.
• Addition of Ni to W for improving
sinterability.
• Addition of MgO to Al2O3 for suppressing
abnormal grain growth (as pinning agent)
and improving densification.
EFFECT OF MgO DOPING
• The greater the amount of MgO added, the
greater the linear shrinkage, and as a result the
greater the density as well.
• In the sintering process, both densification and
grain growth are in a competition. i.e. the
densification process is limited if mass transport
occurs for grain growth, and vice versa.
• Since the presence of MgO in Al2O3 reduces the
grain growth, the mass transport is mainly for
densification. Therefore, to some extent, denser
ceramics can be expected for higher MgO
dopings.
The changes of linear shrinkage for various
MgO doping concentrations
with various sintering time (in log scale)
BINDERS/LUBRICANTS
• Sometimes organic binders such as polyvinyl
alcohol are added to hold the green body
together.
• These burn out during the firing (at 200-350°C).
• Sometimes organic lubricants are added during
pressing to increase densification.
• It is not uncommon to combine these, and add
binders and lubricants to a powder, then press.
• Improved densification reduces the sintering
time needed.
SINTERING KILNS
• Tunnel kilns and periodic kilns are commonly
used for ceramics sintering (firing).
• In periodic kilns heating and cooling sintering
stages are conducted according to a prescribed
procedure.
• In tunnel kilns the sintered parts are conveyed
through different temperature zones.
• Typical tunnel kiln has three zones:
1. Preheat zone for removing lubricant and other
organic materials;
2. Sintering zone where the diffusion occurs;
3. Cooling zone where the sintered parts cool
TUNNEL KILN
ADVANTAGES OF SINTERING
•
•
•
•
The parts produced have an excellent surface
finish, and good dimensional accuracy.
The porosity inherent in sintered components
is useful for specialized application such as
filters and bearings.
Refractory materials which are impossible to
shape using other methods can be fabricated
by sintering with metals of lower melting
points.
A wide range of parts with special electrical
and magnetic properties can be produced.
CURRENT TRENDS
Selective laser sintering (SLS) is a rapid
process that allows to generate complex parts
by solidifying successive layers of powder
material on top of each other.
• Solidification is obtained by fusing or sintering
selected areas of the successive powder layers
using thermal energy supplied through a laser
beam.
SPS (spark plasma sintering)
THE END