Processing of ceramics

Processing of ceramics

powder compact or“green”

ceramic

Forming

Sintering ordensification or

firing

T 2Tm/3

ConventionalHot-pressing

Hot isostatic pressingSpark plasma sintering

Starting powders

Milling

Mix with binder and other additives

Dry/Granulate

Powder compaction/shaping:Dry pressing

Injection moulding

Binder burnout

Sintering

Machining and finishing

Slurry/Slip/Paste

Shape forming:Slip casting

Tape castingExtrusion

Dry

Binder burnout

Sintering

Finishing

Powder compact:green

Dense polycrystalline body: ceramic

Processing of ceramics

Solvents:WaterOrganic solvents (Tb= 100-140°C)EtOH-MEKToluene-EtOH

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%.

Schematic diagram of a spray-drier

Particle packing and granulation

Angular shape of milled alumina

Sketch of an agglomerate with uncontrolled shape

Particles with sharp edges or formed by irregular aggregates do not flow and do not pack efficiently (r.d. <50%). Granulation is essential.

Granules can be obtained by forcing a rigid ceramic paste through the mesh of a sieve or, better, using a spray-drier

Powder consolidation and shaping

Uniaxial pressing Isostatic pressing

Slip castingTape casting

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

Viscous slip (50% solid)

Plaster of Paris mould

Slip surplus

Water

Extrusion

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.

Traditional pottery industry and technical ceramics (zirconia, Si3N4, SiC)

Injection moulding

Ceramic powder + 40% thermoplastic, need careful burnout. Complex shapes, high shrinkage (15-20%).

Moving band

Up to 30% of organic additives (deflocculant, binder and plasticizer). Water or organic solvents. Used for electronic substrates, multilayer ceramic capacitors and actuators.

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 pressingDry-bag isostatic pressing

Extrusion Double-gated injection moulding device

Powder consolidation and shaping

Compact tape casting unit

Drying chamber

Casting head

Tape

Non-continuously working laboratory casting unit

Casting head

IR lamp

Continuously-working (20 cm/min) industrial casting units

Schematic of a doctor blade casting unit

Sintering: removal of pores between particles accompanied by shrinkage (densification) and grain growth.

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.

Sintering 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

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.

Laplace equation for a spherical droplet

rP

2r Pressure difference across a curved

interface. For a planar surface, ΔP = 0

Sintering and grain growth

Effect of curvature on vapour pressure (Thomson’s equation)

rRT

V

rP

P 2ln V: molar volume

: surface tension

r (micron) P/P(r=)

1 1.002

0.1 1.02

0.01 1.21

0.001 7.03

Effect of curvature on chemical potential

r

Vr iii

2

2121

112

rrVrr iii 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.

Ostwald ripening

For a cavity (r < 0), P < P(r=)

r

If r > 0, P > P(r=)

Effect of curvature on thermodynamic properties

Negative curvature

Positive curvature

Nul curvature

Pore

Grain

Grain

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%).(d) Final stage sintering. Pores are slowly eliminated and major grain 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

tetrakaidecahedron6s+8h faces

Sintering and grain growth

Mechanism Source Sink Densification

1 Surface diff. Surface Neck No

2 Evaporation-condensation

Surface Neck No

4 Volume diff. Grain boundary

Neck Yes

6 Grain boundary diffusion

Grain boundary

Neck Yes

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.

Sintering mechanisms

Surface diffusion &evaporation-condensation

Volume and grain boundary diffusion

Negative curvature

Positive curvature

Nul curvature

Grain boundary

Sintering and grain growth

Driving force for grain growth: difference of chemical potential (Gibbs’ free energy) across a curved interface

rVG gbi

The grain boundaries with mobility Mgb migrate towards their centre of curvature at a velocity

Grains with 6 sides: no grain boundary migrationGrains with <6 sides: the grains grow smallerGrains with >6 sides: the grains grow larger

Grain boundary

Atoms

r

Grain growth

gb

gbgb

120°

Convex boundaries

Concave boundaries

rMv gbgb

1

Sintering and grain growth

Grain growth

General relationship: tkdd mm 0 m = 2-3; m = 2 if

Grain growth in undoped and Mg-doped alumina

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):

i

if V

dD

Df: limiting grain sizedi: diameter of inclusionsVi: volume fraction of inclusions

Dopants in solid solution affect depress grain growth because they segregate at grain boundaries reducing: - the interfacial energy - the grain boundary mobility

Grain boundary pinned by a pore

Dragging and agglomeration of pores determined by grain boundary migration

d

k

dt

dd

)(

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.