Melt-infiltrated ceramic matrix compositeds

Question 1

What is the structure of MI-CMC’s?

Answer 1

~ hexagonal boron nitride has covalently-bonded sheets eld together by van der waals forces
–> referred to as white graphite, and in powder forms, has similar lubricating properties

~ BN coatings on tows of SiC fibers (not woven fabric) are applied by chemical vapor deposition, CVD
–> the coating prevents chemical attack of the fibers during processingand provides a weak mechanical interface between the fiber and matrix for enhanced toughness and graceful failure

Question 2

How are MI-CMC’s made?

Answer 2

~ using a wet-drum winding process, SiC and carbon particles, mixed with a polymer binder, are infiltrated between the fibers to produce uni-directional pre-preg tapes

~ these tapes are laid up with an alternating pattern of orientation and autoclaved (lower pressure clip with a gas) to consolidate the stacked tapes to the desired geometric configuration

~ the binder is converted to additonal carbon by pyrolysis to form a porous preform

~ molten silicon is infiltrated and reacts with free carbon to form additional SiC. Some free silicon invariably remains (5-15%) there is a good CTE match between Si and SiC

Question 3

What happens to BN fiber during the melt infiltration process of molten silicon?

Answer 3

~ BN fiber tow coatings can be degraded by contact with molten silicon during the melt infiltration process
–> an overcoating of SiC or Si3N4 is commonly used to protect it

Question 4

What is the composition of the composite?

Answer 4

~ vol% 20-25% fiber, 8-10% fiber coating, 63-70% matrix, and <2% porosity

Question 5

What is the upper use temp?

Answer 5

~ limited by the melting point (1414C) and low creep resistance of silicon)

Question 6

What happens when steam products from hydrocarbon combustion react with silicon containing compounds?

Answer 6

~ they form gaseous hydroxide species

~ the problem can be mitigated using environmental barrier coatings (EBC’s)
–> EBC’s are rare earth silicates (ex: Y2Si2O7 or Yb2Si2O7) which have good CTE match to SiC
–> they serve as a barrier to H2O vapor ingress

Question 7

What happens when a crack forms in CMC’s?

Answer 7

~ when a growing crack encounters a fiber, the weak interface between the matrix and fiber causes the crack to divert around the fiber; this redirection can often arrest the crack

~ if the crack passes around the fiber, the fiber can bridge the crack and hold the composite together

~ if forces are sufficient for complete crack propagation through the matrix, the fibers may still carry the load

~ after fiber fracture at different locations, energy is still required to pull the fibers out of the matrix

~ result of these mechanisms in non catastrophic metal-like stress strain behavior