Solid State Sintering Flashcards
Solid State Sintering Definition
Elimination of pores or void space between particles
Shrinkage and densification
Development of strength in a green compact
Optimization of properties
Sintering Mechanism
Material transport by diffusion or viscous flow (liquid)
heat energy: temperature 0.6 - 0.8T
Phases of sintering
Initial stage
Intermediate stage
Final stage
Initial stage
Particle-particle bonding and neck formation
Creation of a grain boundary
pores at triple points
Intermediate stage
Pore volume decreases
Pores become more rounded
Final stage
pore spherodize
pore isolation within grains
grain boundary becomes fully developed
Driving force for sintering
Lowering of total free energy of the system
Most important is replacement of particle solid/vapour surfaces by solid/solid boundaries
Effect of particle size on solid state sintering
Densification as a function of temperature
Finer particles sinter more readily than coarse ones
Solid state sintering kinetics
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
Variables that solid state sintering kinetics depend on
Self diffusion coefficient particle size atomic volume of diffusing vacancies particle surface energy temperature
Sintering transport mechanisms
No shrinkage:
Evaporation condensation
Surface diffusion
Shrinkage:
Volume diffusion
Grain boundary diffusion
Plastic flow
Plastic flow
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
Pressure of solid particle when there is no surface tension
Pressure inside particle = pressure outside particle
Compression in particle when surface tension exists
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
What happens when a particle is at equilibrium?
Work done by surface energy = work done by difference of pressure
What does it mean when particle pressure is negative?
Pressure around a spherical closed pore
Surface tension decreases volume of pore
Role of plastic flow during sintering
During beginning of sintering, rho is really small and the stress at the neck can cause widening of the neck through plastic deformation
Volume diffusion (at high temperature)
Concentration of vacancies below the surface changes with radius of curvature
gradient of vacancies contribute to sintering
Concentration of vacancies below a curved surface and role during sintering
For a curved surface with negative radius of curvature (solid in tension) formation of vacancies is easier than for a flat surface
Mechanisms of volume diffusion and how to increase it
More contact points per particle
Grain boundary diffusion
Very fast at low temperature
Dependent on grain boundary diffusion coefficient, temperature and time
Surface diffusion
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
Evaporation condensation
Vapour pressure at interface greater than at neck
Intermediate and Final stage microstructure development
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)
Final stage gas behaviour
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
When is the only time it is possible to get a fully dense component?
When sintering is done in vacuum
Grain growth
Controlled by boundary curvature
Concave boundaries grow, and opposite for convex boundaries
Elimination of convex boundaries and fine grains
Grain boundary pore interaction
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
Factors affecting solid state sintering
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
Temperature and solid state sintering
Increasing sintering temperature increases sintering speed
Time and solid state sintering
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
Particle size distribution and solid state sintering
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
Green density and solid state sintering
When green density decreases, sintering rate increases
Final density increases when green density increases
Decreasing green density leads to greater shrinkage
Shape of particles and solid state sintering
Better contact increases sintering level
Decreasing sphericity and increasing macro or microroughness
Microstructure of powder and solid state sintering
Grain boundaries increase diffusion
Chemical composition and solid state sintering
Impurities and contamination reduce sintering
Chemical reaction and solid state sintering
Can accelerate sintering
Sintering atmosphere and solid state sintering
If reduction of surface oxide scale occurs, increase in sintering is observed
