Ceramics- Compaction

Question 1

General shape of pressed density vs punch pressure graph

Answer 1

Starts at origin and rises very steeply but then curves to level off quite soon

Question 2

Details about pressed density vs punch pressure graph

Answer 2

Initial rate of densification is high but decreases rapidly for pressures above 5-10MPa. Little densification occurs above 50MPa but die wear is high. Different ceramics show different curves.

Question 3

Pressures used in industry for compaction

Answer 3

Commonly less than 100MPa for high-performance technical ceramics

Question 4

What are the 3 stages of compaction?

Answer 4

Flow and rearrangement
Deformation
Densification (in this order)

Question 5

Granule flow and rearrangement

Answer 5

The first stage of compaction. Particle rearrangement occurs involving the relative movement and rotation of the particles

Question 6

Granule deformation

Answer 6

Second stage of compaction. Elastic deformation of particles occurs initially. Further compaction produces non-recoverable deformation (plastic flow) at the contact points between particles

Question 7

Granule densification

Answer 7

Stage 3 of compaction. Localised fragmentation of the particles in the highly stressed regions. With greater compaction pressures general plastic flow and fragmentation occurs

Question 8

Intragranular and intergranular pores

Answer 8

Intragranular pores are pores inside a particle. Intergranular pores are the empty spaces between particles

Question 9

Intrusion volume Vs pore size graph for densification stage

Answer 9

Intrusion volume (amount of pore) in cm^3/g. Pore size log scale in μm and increases towards left. As filled in die: starts flat near origin, then curves up steeply to new flat level, then curves up to another flat level as pore size decreases (going right). When pressed: whole line is shifted down when greater pressure used. At 70MPa: the first curve up to a new flat level doesn’t happen and only the final one does

Question 10

Where are the different types of porosity for the intrusion volume Vs pore size graph?

Answer 10

Intergranular is for region around first curve up to a new level (spread around 10μm). Intragranular is for region around second curve up to highest level (between 1 and 0.1μm)

Question 11

Density of pressed powder formula

Answer 11

Dcompact=Dfill+mLog(Pa/PY)
Dfill is density of powder as filled in die
m is compaction constant
Pa is applied pressure
P subscript Y is apparent yield pressure

Question 12

Pressure transmission ratio

Answer 12

PH/Pa=exp(-fK(h/v)Afriction/Apressing)
Or
PH/Pa=exp(-4fK(h/v)H/D)
Where P subscript H is transmitted axial pressure at depth H
Pa is applied pressure at depth 0
f is wall friction
K subscript h/v is horizontal/vertical pressure ratio
Af is friction area
Ap is pressing area
D is width

Question 13

For the pressure transmission ratio what is H?

Answer 13

The hight of the material being compressed if single action cylindrical compaction. Half of this if double action cylindrical compaction.

Question 14

What does friction between particles and particles and die walls cause?

Answer 14

Uneven density distribution where green density is higher at the top and in the pellet centre. Powder near the die wall and a moving plunger is sheared more than elsewhere. Compaction pressure decreases with distance from moving plunger. For single ended pressing the height/diameter ratio may have to be less than 1. For double ended the ratio can be doubled.

Question 15

Pressure profile at beginning of stage 2

Answer 15

Pressure greatest at top corners. Uniform through top middle and centre. Lowest across bottom.

Question 16

Pressure profile at end of stage 2

Answer 16

Pressure still greatest at top corners. High in rest of top half apart from small region at top middle where is medium. Lowest in bottom corners. Medium rest of bottom half

Question 17

Common defects in dry-pressed body

Answer 17

Laminations- lines of separation within body
End cap- top chunk becomes separated
Ring cap- top corners become separated

Question 18

What causes defects in dry-pressed body?

Answer 18

Stresses produced by differential springback

Question 19

Solving problem of laminationd

Answer 19

Better die-wall lubrication and reformulating the additives to increase granules plasticity

Question 20

Solving end capping

Answer 20

Use highly polished and slightly tapered die

Question 21

Solving ring capping

Answer 21

Maintaining equipment to keep the gap between punch and die wall smaller than granule size