Oxidation of BN/Nicalon fiber interfaces in ceramic-matrix composites and its effect on fiber strength

Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after t hermal exposure at different temperatures (in the range 800 degrees-1400 de grees C) and in different environments (air and argon), The outer SiC coati ng acted as a barrier to oxygen, which penetrated into the composite via pi peline diffusion along the BN/fiber interfaces. Oxygen penetration led to t he formation of an SiO2 layer by oxidation of the fiber surfaces. The in si ts fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO 2 layer severely degraded the fiber strength. Oxidation was highly dependen t on the nature of the BN/fiber interface. The presence of a thin carbon-ri ch interlayer, which burned out rapidly at high temperature, favored the en try of oxygen and accelerated oxidation of the fibers.