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MIME 465 Midterm 2 > Solid State Sintering > Flashcards

Solid State Sintering Flashcards

1
Q

Solid State Sintering Definition

A

Elimination of pores or void space between particles
Shrinkage and densification
Development of strength in a green compact
Optimization of properties

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2
Q

Sintering Mechanism

A

Material transport by diffusion or viscous flow (liquid)

heat energy: temperature 0.6 - 0.8T

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3
Q

Phases of sintering

A

Initial stage
Intermediate stage
Final stage

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4
Q

Initial stage

A

Particle-particle bonding and neck formation
Creation of a grain boundary
pores at triple points

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5
Q

Intermediate stage

A

Pore volume decreases

Pores become more rounded

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6
Q

Final stage

A

pore spherodize
pore isolation within grains
grain boundary becomes fully developed

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7
Q

Driving force for sintering

A

Lowering of total free energy of the system

Most important is replacement of particle solid/vapour surfaces by solid/solid boundaries

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8
Q

Effect of particle size on solid state sintering

A

Densification as a function of temperature

Finer particles sinter more readily than coarse ones

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9
Q

Solid state sintering kinetics

A

Shrinkage behaviour is partly time dependent.
Temperature also has a strong influence on kinetics
n = 0.4 usually indicates solid state sintering
Volume shrinkage assumed to be isotropic

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10
Q

Variables that solid state sintering kinetics depend on

A 
Self diffusion coefficient
particle size
atomic volume of diffusing vacancies
particle surface energy
temperature
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11
Q

Sintering transport mechanisms

A

No shrinkage:
Evaporation condensation
Surface diffusion

Shrinkage:
Volume diffusion
Grain boundary diffusion
Plastic flow

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12
Q

Plastic flow

A

Radius of curvature of a solid surface can generate stresses below the surface, which can induce plastic deformation of particle
Pressure within solid particle, or around a spherical closed pore
Variation of pressure for any radius of curvature

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13
Q

Pressure of solid particle when there is no surface tension

A

Pressure inside particle = pressure outside particle

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14
Q

Compression in particle when surface tension exists

A

Pressure inside particle is greater than pressure outside particle.
Reduction of surface due to surface tension leads to compression in particle
Volume tends to decrease, and vacancies diffuse outside

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15
Q

What happens when a particle is at equilibrium?

A

Work done by surface energy = work done by difference of pressure

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16
Q

What does it mean when particle pressure is negative?

A

Pressure around a spherical closed pore

Surface tension decreases volume of pore

17
Q

Role of plastic flow during sintering

A

During beginning of sintering, rho is really small and the stress at the neck can cause widening of the neck through plastic deformation

18
Q

Volume diffusion (at high temperature)

A

Concentration of vacancies below the surface changes with radius of curvature
gradient of vacancies contribute to sintering

19
Q

Concentration of vacancies below a curved surface and role during sintering

A

For a curved surface with negative radius of curvature (solid in tension) formation of vacancies is easier than for a flat surface

20
Q

Mechanisms of volume diffusion and how to increase it

A

More contact points per particle

21
Q

Grain boundary diffusion

A

Very fast at low temperature

Dependent on grain boundary diffusion coefficient, temperature and time

22
Q

Surface diffusion

A

prominent at low temperature
used by difference in curvature between the surface of the neck and the surface of the particle - difference in chemical potential between the surfaces

Dependent on surface diffusion coefficient and interatomic distance

23
Q

Evaporation condensation

A

Vapour pressure at interface greater than at neck

24
Q

Intermediate and Final stage microstructure development

A

Shrinkage creates pores at grain triple junctions and some isolated pores develop within grains (alumina)
No shrinkage creates pores within grains due to pore breakaway and isolation (silicon)

25
Q

Final stage gas behaviour

A

Gas trapped in the pores inhibit densification independently on sintering time
Corresponds to balance between capillary pressure from curved pore surface and internal gas pressure

26
Q

When is the only time it is possible to get a fully dense component?

A

When sintering is done in vacuum

27
Q

Grain growth

A

Controlled by boundary curvature
Concave boundaries grow, and opposite for convex boundaries
Elimination of convex boundaries and fine grains

28
Q

Grain boundary pore interaction

A

Under equilibrium, pores form stable dihedral angle along boundary or grains triple junctions
As grain grows, pores initially pointed a boundary
as boundary advances, breakaway situation occur and boundary breaks free, leaving an isolated pore

29
Q

Factors affecting solid state sintering

A 
Temperature
Time
Particle size distribution
Green density of compact
Shape and surface of particles
Microstructure of particles
Chemical composition of particles
Chemical reactions during sintering
Sintering atmosphere
30
Q

Temperature and solid state sintering

A

Increasing sintering temperature increases sintering speed

31
Q

Time and solid state sintering

A

Degree of sintering increases with increasing time, effect is small compared to temperature dependence
Loss of driving force with increasing time at any temperature is one of the reasons why it is difficult to remove all porosity by sintering

32
Q

Particle size distribution and solid state sintering

A

Speed of sintering increases when size fo particle decreases

  • greater pore/solid interfacial area providing greater driving force
  • promotes all types of diffusion transport: surface, grain boundary, and volume diffusion
33
Q

Green density and solid state sintering

A

When green density decreases, sintering rate increases
Final density increases when green density increases
Decreasing green density leads to greater shrinkage

34
Q

Shape of particles and solid state sintering

A

Better contact increases sintering level

Decreasing sphericity and increasing macro or microroughness

35
Q

Microstructure of powder and solid state sintering

A

Grain boundaries increase diffusion

36
Q

Chemical composition and solid state sintering

A

Impurities and contamination reduce sintering

37
Q

Chemical reaction and solid state sintering

A

Can accelerate sintering

38
Q

Sintering atmosphere and solid state sintering

A

If reduction of surface oxide scale occurs, increase in sintering is observed

MIME 465 Midterm 2 (5 decks)

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