Interfaces in BN coated fiber reinforced glass-ceramic matrix composites

Abstract

It has been found in studies at UTRC and elsewhere, that polymer derived SiC type fibers such as Nicalon that contain excess carbon and oxygen over stoichiometric SiC, form a thin (20--50nm) carbon rich fiber/matrix interfacial layer when incorporated into glass-ceramic matrices at elevated temperatures. The formation of this weak interfacial carbon layer is responsible for the high toughness and strength observed in these composites, but is also responsible for composite embrittlement and concurrent strength and toughness degradation when either stressed at elevated temperatures in an oxidizing environment or thermally aged in an unstressed condition in oxidizing environments. This embrittlement and strength degradation is a result of oxidation of the carbon layer and its replacement by a glassy oxide layer that is bonded strongly to both the fiber and matrix, thus inhibiting matrix crack deflection at the fiber/matrix interface. In the case of nitride or oxide based fibers that do not form an in situ carbon interface during CMC composite processing, the fiber/matrix bonding is usually strong, with resultant composite behavior that is weak and brittle. It is thus imperative that the fiber/matrix interface in all of these fiber reinforced ceramic matrix composites be controlled, or engineered,'' so that relativelymore » weak interfacial bonding exists for matrix crack deflection while maintaining oxidative stability. An approach to accomplish this is to utilize coatings on the fiber surfaces that are applied before composite processing. Not only must these interfacial coatings be weak and oxidatively stable, they must also be resistant to matrix and/or fiber interdiffusion so that interfacial reactions do not occur. The utilization of CVD BN based fiber coatings to accomplish this is the subject of this paper.« less