Processing of Ceramic Matrix Composites

Question 1

what are some of the main features of ceramic matrix?

Answer 1

• Melting points for ceramics are very high
  Often similar fibres
  Ceramics glasses lower
• Reaction between matrix and fibres possible.
• Low ductility / high hardness.
• Important to have a few voids.

Question 2

What is does powder processing involve?

Answer 2

• Ceramic particles, short fibres/ whiskers, binder mixer.

Question 3

How does powder processing work?

Answer 3

Either:
- Pressed into component shape (low temp), then sintered (up to 1000 deg).
- Pressed at high temperatures.
- Use “Hot isostatic pressing”

• Binder burns off during pressing

Question 4

What happens in powder processing, during heating?

Answer 4

• Nothing melts.
• Diffusion at high temperatures forms bonds between particles and to fibres.
• Hot pressing and HIP also leads to compaction - eliminates voids.

Question 5

What are the disadvantage of powder processing?

Answer 5

• Limited to short fibres.
• Limited to low reinforcement volume fraction.
• Whiskers difficult as these small fibres make it difficult to compact.
• Whiskers often break during pressing.

Question 6

What does slurry processing use?

Answer 6

_ Use of aqueous slurries helps mixing and initial shaping.
- Ceramic particles and fibres mixed in aqueous solution.
- Dispersed by ultrasonic mixing.

Question 7

How does slurry processing work?

Answer 7

When shaped, mixed then dispersed.

Then they can either be:
- Dried and formed / presses, then sintered.
- Dried then hot -pressed / HIP
- Slip cast (slurry cast into shape and dried and sintered)
Can also be used for continuous fibre composites.
- Similar method to filament winding.

Question 8

What does the liquid state processing involves?

Answer 8

• Only possible with glass - ceramic matrices.
• Glasses ten to have much lower melting points than crystalline ceramics.
• Glass- ceramic process like glasses, but then heat - treatment allows crystals to form (devitrification)
• Improves strength.

Question 9

What is sol - gel processing?

Answer 9

• Sol-gel is a thick dispersion of very small (~10nm) particles.
• Normally produced by chemical precipitation.
• Sol-gel can be simply poured through preform of fibres and then heated.
• Often needs several cycles to get good density.
• Or can be used with short fibres in a similar manner to slurry methods.
• Small particles give better bonding on heating (larger surface area).

Question 10

What is chemical vapour infiltration?

Answer 10

• Like CVD.
• Fibre preform used.
• GAs mixture (carrier gas + reacting gases) passed through preform.
• Ceramic deposits on preform.
• Complex temperature control can be used to deposit.
• Complex temperature control can be used to deposit ceramic progressively through preform.