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Composite materials

In a ceramic matrix composite (CMC) reinforced with SiC fibers, what is the main purpose of applying a thin coating of boron nitride (BN) or pyrolytic carbon at the fiber–matrix interface?

  1. ATo bond the interface as strongly as possible, maximizing load transfer from the matrix to the fibers so that the fibers stop cracks
  2. BTo act as a sintering aid that lowers the melting point of the ceramic matrix so it can be sintered at a lower temperature
  3. CTo make the interface suitably weak so that cracks are deflected and energy is absorbed by fiber debonding and pull-out
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Answer: C. To make the interface suitably weak so that cracks are deflected and energy is absorbed by fiber debonding and pull-out

The ceramic matrix is brittle, so under load cracks first form in the matrix. If the interface is too strong, these cracks run straight through the fibers and the material fractures all at once, just like a monolithic ceramic. A layered, easily sliding coating such as BN or pyrolytic carbon makes the interface suitably weak, so cracks are deflected along the interface, and the fibers bridge the crack while absorbing considerable energy through debonding and pull-out. The result is a fracture mode that does not fail suddenly, with a large increase in apparent toughness. Bonding the interface as strongly as possible is the approach for improving load transfer in resin matrices, and it is counterproductive in CMCs. The coating is not meant to lower the sintering temperature.

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  1. Which correctly describes the main role of the matrix in a fiber-reinforced composite?
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