Liquid Phase Sintering Flashcards
Definition of liquid phase sintering
Addition of a second powder which has lower melting temperature (Cu in Fe for example)
Sintering rates are much faster when a liquid is formed during sintering
What is the key requirement of liquid phase sintering?
Wetting
Brazing of particles
Occurs when solid is soluble in liquid
Diffusion coefficient is many times higher than in solid state
What equation has to be satisfied for spreading to occur?
energy of liquid vapour - energy of solid vapour plus energy of solid liquid must be greater than zero
Advantages of liquid phase sintering
More homogeneous density
Faster
Occurs at lower temperatures the solid state
Lower cost - a benefit for manufacturers
Disadvantages of liquid phase sintering
Presence of grain boundary phase means poorer high temperature properties (strength, creep resistance..)
Large variation in dimensions
Liquid phase sequence
- Mixed powders
- Rearrangement
- Solution precipitation
- Solid State
Sintering time vs. densification parameters
Shorter times and lower parameters: liquid flow
Longer times and greater parameters: solid state
In between: solution and precipitation
Capillarity developed during liquid phase sintering
Liquid wets particles and penetrates grain boundaries
Capillary pressure aids sintering and draws particles into closer contact
When will capillary pressure be higher?
For smaller volumes of liquid
Liquid formation and particle rearrangement:
Rapid densification, beginning of dissolution of solid
Solution precipitation
Small particles dissolve into liquid, diffuse through liquid and precipitate onto the larger particles
Pore elimination and grain growth
Solid state sintering
Stage 1: Liquid flow
rapid sintering
Negligible role of diffusion
Important in systems with low or no liquid solubility
Can reach 100% dens. if volume of liquid can fill all voids and if it wets all solid particles
Stage 2: Solution precipitation
Slower densification than in stage 1
Happens only if solid has limited solubility in liquid
The two mechanisms in solution precipitation
- Small particles dissolve and precipitate onto larger ones, lowering chemical potential
- Pressure at particles contact point (higher chem potential) and diffusion of atoms in liquid towards regions with lower chemical potential
Stage 3: Pore elimination
Formation of neck between particles
Necks stop liquid flow between particles
Almost no densification during this stage
Factors affecting shrinkage during liquid phase
Solid solubility in liquid Surface energy Sintering time Particle size activation energy for solid diffusion in liquid Diffusive frequency factor Temperature
Shrinkage during liquid phase sintering
Full density obtained in minutes
In low solubility systems, densification is low
Increase in volume fraction of liquid facilitates sintering, but can cause slumping if too high
Volume fraction needed for small dihedral angle of grain boundary
less than 20%
Particle coarsening
Particle enlarges with either one half (prismatic of flat-faced) or one third (rounded) power of time
Factors affecting particle coarsening
solid liquid surface energy atomic volume solid solubility in liquid solid diffusivity through liquid absolute temperature dihedral angle volume fraction of liquid
F(L)
Cumulative fraction of particle size
L
G/G50
G50
median particle size
How can alloying during sintering affect the final product?
Volume change will occur
How can chemical reactions during sintering affect the final product
Chemical reactions can remove impurities and improve component quality
ex: C+ FeO = Fe + Co
How can you control volatilization of species in sintering
Control compaction pressure and sintering temperature to control dimension variation
