Ceramics- Basics of Sintering

Question 1

What is sintering?

Answer 1

A heat treatment of the compacted powder after compaction (green component)

Question 2

What does sintering result in?

Answer 2

Removal of pores between starting particles. Formation of strong bonds between neighbouring particles. Shrinkage of the component

Question 3

Driving force for sintering

Answer 3

The reduction in the total free energy of the system. Major driving force is free energy associated with surface energy

Question 4

When does calcination occur?

Answer 4

Before compaction

Question 5

What is calcination?

Answer 5

Heat treatment to obtain materials in the chemically desired form. Often involves thermal decomposition, evaporation, phase transition. Physical characteristics like shapes and porosity not the prime concern.

Question 6

What is sintering for?

Answer 6

Heat treatment to obtain materials in the physically desired form from powders. Physical and mechanical characteristics like shape, porosity, density are of interest.

Question 7

What is sintering based on?

Answer 7

The slow diffusion of atoms at high temperatures

Question 8

What must happen before sintering?

Answer 8

Powdered materials must be compacted or pre-shaped in a mould

Question 9

Reduction of surface area as driving force

Answer 9

Loose powder replaced with a bonded solid. Loose powder has high surface energy in solid-vapour surfaces γsv. Bonded solid has lower energy in solid-solid grain boundaries γGB.

Question 10

Attractive force in early stage of sintering

Answer 10

P=-γsv/ρ
Produced at neck between particles.
As sintering proceeds ρ get larger so P smaller

Question 11

What is ρ?

Answer 11

If two neighbouring particles are spheres, as they combine ρ is half the width of the overlap
Formula is
ρ=X^2/4R
Where R is radius of spheres.
Put a circle radius ρ in gap at top then X is vertical distance from centre of spheres to bottom of this circle’s circumference

Question 12

Graph of log(shrinkage) vs log(time)

Answer 12

Shrinkage=densification
Starts low and increases slowly in initial stage. Curves up to steeper straight line then gradient decreases in intermediate stage. Continues at ever decreasing gradient in final stage

Question 13

Initial stage of sintering

Answer 13

Rearrangement of particles and formation of strong bond or neck at the contact points between particles. Relative density of a compact may increase from 0.5 to 0.6 due mostly to increased packing of particles.

Question 14

Intermediate stage of sintering

Answer 14

Size of necks grow and amount of porosity decreases and particles move closer leading to shrinkage of component. This stage is over when pores are isolated (closed porosity). Majority of shrinkage occurs during this stage and relative density at the end may be about 0.9.

Question 15

Final stage of sintering

Answer 15

Involves slow elimination of closed porosity generally by diffusion of pores along grain boundaries with only a little densification of the component. Grain boundaries are regions of more open crystal structure than the grains themselves so diffusion along them is more rapid.

Question 16

Surface transport mechanism of mass transport

Answer 16

Not involved in sintering. Transport path is surface diffusion or evaporation-condensation. There is no densification in either. Surface diffusion example is for covalent solids. Evap-condens example is low-stability compounds like NaCl.

Question 17

Bulk (volume) transport as mechanism for mass transport

Answer 17

Transport paths are grain boundary diffusion, volume diffusion, plastic flow, viscous flow. All result in densification. GB for most crystalline materials. Volume diffusion restricted by defect structure of ceramics. Plastic flow not significant in crystalline ceramics. Viscous flow when a glass is present.