Liquid Phase Sintering

Question 1

Definition of liquid phase sintering

Answer 1

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

Question 2

What is the key requirement of liquid phase sintering?

Answer 2

Wetting
Brazing of particles
Occurs when solid is soluble in liquid
Diffusion coefficient is many times higher than in solid state

Question 3

What equation has to be satisfied for spreading to occur?

Answer 3

energy of liquid vapour - energy of solid vapour plus energy of solid liquid must be greater than zero

Question 4

Advantages of liquid phase sintering

Answer 4

More homogeneous density
Faster
Occurs at lower temperatures the solid state
Lower cost - a benefit for manufacturers

Question 5

Disadvantages of liquid phase sintering

Answer 5

Presence of grain boundary phase means poorer high temperature properties (strength, creep resistance..)
Large variation in dimensions

Question 6

Liquid phase sequence

Answer 6

1. Mixed powders
2. Rearrangement
3. Solution precipitation
4. Solid State

Question 7

Sintering time vs. densification parameters

Answer 7

Shorter times and lower parameters: liquid flow
Longer times and greater parameters: solid state
In between: solution and precipitation

Question 8

Capillarity developed during liquid phase sintering

Answer 8

Liquid wets particles and penetrates grain boundaries

Capillary pressure aids sintering and draws particles into closer contact

Question 9

When will capillary pressure be higher?

Answer 9

For smaller volumes of liquid

Question 10

Liquid formation and particle rearrangement:

Answer 10

Rapid densification, beginning of dissolution of solid

Question 11

Solution precipitation

Answer 11

Small particles dissolve into liquid, diffuse through liquid and precipitate onto the larger particles

Question 12

Pore elimination and grain growth

Answer 12

Solid state sintering

Question 13

Stage 1: Liquid flow

Answer 13

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

Question 14

Stage 2: Solution precipitation

Answer 14

Slower densification than in stage 1

Happens only if solid has limited solubility in liquid

Question 15

The two mechanisms in solution precipitation

Answer 15

1. Small particles dissolve and precipitate onto larger ones, lowering chemical potential
2. Pressure at particles contact point (higher chem potential) and diffusion of atoms in liquid towards regions with lower chemical potential

Question 16

Stage 3: Pore elimination

Answer 16

Formation of neck between particles
Necks stop liquid flow between particles
Almost no densification during this stage

Question 17

Factors affecting shrinkage during liquid phase

Answer 17

Solid solubility in liquid
Surface energy
Sintering time
Particle size
activation energy for solid diffusion in liquid
Diffusive frequency factor
Temperature

Question 18

Shrinkage during liquid phase sintering

Answer 18

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

Question 19

Volume fraction needed for small dihedral angle of grain boundary

Answer 19

less than 20%

Question 20

Particle coarsening

Answer 20

Particle enlarges with either one half (prismatic of flat-faced) or one third (rounded) power of time

Question 21

Factors affecting particle coarsening

Answer 21

solid liquid surface energy
atomic volume
solid solubility in liquid
solid diffusivity through liquid
absolute temperature
dihedral angle
volume fraction of liquid

Question 22

F(L)

Answer 22

Cumulative fraction of particle size

Question 23

L

Answer 23

G/G50

Question 24

G50

Answer 24

median particle size

Question 25

How can alloying during sintering affect the final product?

Answer 25

Volume change will occur

Question 26

How can chemical reactions during sintering affect the final product

Answer 26

Chemical reactions can remove impurities and improve component quality
ex: C+ FeO = Fe + Co

Question 27

How can you control volatilization of species in sintering

Answer 27

Control compaction pressure and sintering temperature to control dimension variation