ULTRASONIC CHARACTERIZATION OF OXIDATION MECHANISMS IN NICALON/C/SIC COMPOSITES

The nonbrittle fracture of composites consisting of ex poly-carbosilane SiC (Nicalon) fibers in a SiC matrix prepared by chemical vapor infiltration is strongly dependent on the presence of a pyrocarbon layer at the fiber/matrix interface (Nicalon/C/SiC composites). The mechanical properties of such materials are known to be influenced by oxidation reactions. Elastic modulus measurements, using ultrasonic wave propagation in the ''long bar'' mode, have been used to show the influence of the environmental parameters temperature, atmosphere, and pressure on the mechanical behavior of bidirectional Nicalon/C/SiC. In situ measurements of elastic modulus performed in parallel with thermogravimetric analysis allow examination of oxidation mechanisms which affect interfacial properties. Results showed the modulus to be affected by two interfacial oxidation mechanisms: (1) oxidation of pyrocarbon coating and (2) closure of the resulting interphase gap by silica formation.