Ceramic matrix composites (CMC's) rely on designed weaknesses in order to provide toughening mechanisms during failure. Specifically, the fiber/matrix interface is well established as the parameter having the most influence on CMC fracture behavior, as first noted by Sambell. With proper design, this interface can allow fracture energy to dissipate by two mechanisms: an increase in crack path tortuosity and frictional energy dissipation through fiber pullout.
One design approach in the area of oxide-based CMC's which has seen considerable success recendy has been in the use of “weakly bonding” oxide coating materials placed between the fiber and the matrix phases. Such CMC systems rely on the formation of a weak interface between the fiber and the coating itself to promote the desired fracture behavior. The most notable example of such a coating material is monazite, LaPO4, as observed by Morgan et al.