Processing of ceramics
Processing of ceramics
Sintering or
densification or
firing
Forming
T 2Tm/3
powder
compact or
“green”
ceramic
Processing of ceramics
Starting powders
Milling
Mix with binder and other additives
Solvents:
Water
Organic solvents
(Tb= 100-140°C)
EtOH-MEK
Toluene-EtOH
Slurry/Slip/Paste
Shape forming:
Slip casting
Tape casting
Extrusion
Dry/Granulate
Powder compact:
green
Powder compaction/shaping:
Dry pressing
Injection moulding
Dry
Binder burnout
Binder burnout
Conventional
Hot-pressing
Hot isostatic pressing
Spark plasma sintering
Sintering
Finishing
Dense
polycrystalline
body: ceramic
Sintering
Machining and finishing
Particle packing and granulation
“perfect” powder
Maximum packing
In
practice
powders
are
composed of particles with a
size distribution and often the
particle shape is not spherical.
Random packing of spheres with
a log-normal distribution 67%.
Particle packing and granulation
Particles with sharp edges
or formed by irregular
aggregates do not flow and
do not pack efficiently (r.d.
<50%).
Granulation
is
essential.
Angular shape of milled alumina
Sketch of an agglomerate
with uncontrolled shape
Granules can be obtained by forcing a
rigid ceramic paste through the mesh of
a sieve or, better, using a spray-drier
Schematic diagram of a spray-drier
Powder consolidation and shaping
Uniaxial pressing
Viscous slip
(50% solid)
Plaster of
Paris mould
Dry powder, very
simple shapes.
Die-wall friction
introduces density
gradients which lead
to differential
densification and
distortions during
sintering
Uniform pressure gives
uniform green density and
limits lamination. Used for
mass production of spark
plugs and high-voltage
insulators
Isostatic pressing
Traditional pottery industry
and technical ceramics
(zirconia, Si3N4, SiC)
Moving
band
Water
Slip casting
Extrusion
Slip surplus
A ceramic paste containing
binders and lubricants is forced
through the orifices of a die.
Components with uniform
section and high
length/diameter ratio, such as
rods and tubes. Used also for
thick dielectric substrates.
Tape casting
Injection moulding
Up to 30% of organic
additives (deflocculant,
binder and plasticizer).
Water or organic solvents.
Used for electronic
substrates, multilayer
ceramic capacitors and
actuators.
Ceramic powder + 40%
thermoplastic, need
careful burnout.
Complex shapes, high
shrinkage (15-20%).
Powder consolidation and shaping
Powder consolidation and shaping
•Binder: gives the dry shape (green) sufficient strength for handling before sintering (starch, cellulose
ethers, polyvinyl alcohol, polymethacrylates, polyvinylbutyral).
•Deflocculant/dispersant: gives the suspension a high stability (electrostatic and electrosteric stabilization)
against sedimentation/flocculation required for casting (ammonium polyacrylate, citric acid).
•Plasticizer: gives flexibility to tapes and deformability to granules by lowering the Tg (glass transition
temperature) of binders (glicerine, butyl benzyl phthalate, poly(ethylen) glycol)
•Lubricant: decreases die-powder and granule-granule friction (salts of stearic acid)
Powder consolidation and shaping
The stages of dry pressing
Dry-bag isostatic pressing
Extrusion
Double-gated injection moulding device
Powder consolidation and shaping
Tape
Drying chamber
Casting head
Schematic of a doctor blade casting unit
Compact tape casting unit
IR lamp
Casting head
Non-continuously working laboratory casting unit
Continuously-working (20 cm/min)
industrial casting units
Sintering and grain growth
Sintering: removal of pores between particles accompanied by shrinkage (densification) and
grain growth.
Types of sintering
Solid-state sintering (SSS)
only in high-purity compounds
Liquid phase sintering (LPS)
<20% liquid; impurities or specific additives
Viscous glass sintering or viscous flow (VGS)
Densification of glass powders
Viscous composite sintering or vitrification (VCS)
>20% liquid: whitewares, porcelains
Driving force for sintering: reduction of surface area and lowering of surface energy. High
energy solid-gas surfaces are replaced by low energy solid-solid interfaces (grain boundaries).
At microscopic level, the driving force is related to the difference in surface curvature and
consequently of partial pressure and chemical potential between different parts of the system.
Neck formation
Pore removal
and shrinkage
Effect of particle size: the smaller the particles, the higher the radius of curvature and the
chemical potential  higher sintering rate.
Sintering and grain growth
Effect of curvature on thermodynamic properties
Laplace equation for a spherical droplet
r
2
P 
r
Pressure difference across a curved
interface. For a planar surface, ΔP = 0
Effect of curvature on vapour pressure (Thomson’s equation)
ln
P
V  2  V: molar volume

 
Pr    RT  r  : surface tension
r
If r > 0, P > P(r=)
r (micron)
P/P(r=)
1
1.002
0.1
1.02
0.01
1.21
0.001
7.03
For a cavity (r < 0),
P < P(r=)
Pore
Grain
Positive curvature
Negative curvature
Effect of curvature on chemical potential
 2 

 r 
i  i r     Vi 
Grain
Nul curvature
Ostwald ripening
1 1
i r1   i r2   Vi 2   
 r1 r2 
r1
r2
Particles with different curvature have different vapour
pressure and chemical potential. Therefore they are
not in equilibrium and the larger one will grow at the
expense of the smaller one.
Sintering and grain growth
Stages of sintering
(a, b) Initial stage sintering. Formation of strong bonds and necks
between particles at the contact points. Moderate decrease of porosity
(initial 40-50%) from particle rearrangement.
(c) Intermediate stage sintering. The size of the necks increases and
the amount of porosity decreases. The sample shrinks (the centers of
the grains move towards each other. The grains transforms from
spheres to truncated octahedra (tetrakaidecahedra). This stage
continues until pores are closed (r.d. 90%).
tetrakaidecahedron
(d) Final stage sintering. Pores are slowly eliminated and major grain
6s+8h faces
growth can occur.
In hot-pressing and hot isostatic pressing an additional driving force is provided by the external
stress/pressure.
Initial stage
Intermediate stage
Final stage
Sintering and grain growth
Sintering mechanisms
Mechanism
Source
Sink
Densification
1
Surface diff.
Surface
Neck
No
2
Evaporationcondensation
Surface
Neck
No
4
Volume diff.
Grain
boundary
Neck
Yes
6
Grain boundary
diffusion
Grain
boundary
Neck
Yes
Grain boundary
In ionic materials, the mobility of the slowest moving species dominates the
diffusion process and sintering rates. This explain the strong dependence of
sintering kinetics on nature and amount of uncontrolled impurities, dopants
and sintering aids. Grain boundary diffusion is the most important
densification mechanisms in many oxides.
Surface diffusion &
evaporation-condensation
Positive curvature
Negative curvature
Volume and grain
boundary diffusion
Nul curvature
Sintering and grain growth
Grain growth
Grain boundary
Driving force for grain growth: difference of chemical
potential (Gibbs’ free energy) across a curved
interface
G  Vi
 gb
r
The grain boundaries with mobility Mgb migrate
towards their centre of curvature at a velocity
Atoms
v  M gb gb
1
r
Concave boundaries
Grains with 6 sides: no grain boundary migration
Grains with <6 sides: the grains grow smaller
Grains with >6 sides: the grains grow larger
gb
120°
gb
Convex boundaries
gb
Sintering and grain growth
Grain growth
General relationship:
d  d0  k t
m
m
m = 2-3;
m = 2 if
d (d ) k

dt
d
Grain growth in undoped and Mg-doped alumina
Dopants in solid solution affect depress grain
growth because they segregate at grain boundaries
reducing:
- the interfacial energy
- the grain boundary mobility
Grain growth is inhibited by pores, second phase inclusions and solid solution impurities. Pores and solid
inclusions act as pinning centres for weakly curved grains. The critical grain size at which grain growth
stops is given by (Zener):
Df 
di
Vi
Df: limiting grain size
di: diameter of inclusions
Vi: volume fraction of inclusions
Grain boundary pinned
by a pore
Dragging and agglomeration of
pores determined by grain boundary
migration
Liquid phase sintering
• Enough liquid phase must be present (1-5 vol.%).
• The liquid must wet the solid (contact angle θ<<90°).
• The solid must be partially soluble in the liquid.
•Driving forces are higher for small particles (stronger
capillary forces) with high surface energy and high
solubility
 > 90°: nonwetting
 < 90°: wetting
 = 0°: spreading
Particle rearrangement: the liquid spread on the
particles which rotate and slip. Significant
densification occurs, up to 70%, without modification
of particle and pore morphology.
Solution-precipitation:
(1) Ostwald ripening. Small particles dissolve in the
liquid and the material precipitates on bigger particles
because solubility depends on the radius of curvature.
(2) Dissolution occurs in the neck region because of
Laplacian compressive force and material redeposit
away of the neck region.
(3) Sharp corners dissolve and material precipitates
on regions of lower curvature.
Coalescence. When enough grain growth has
occurred, a solid skeleton is formed and the pores
becomes closed (at 90% re. dens.). Pore elimination
can proceed by solid-state diffusion.