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

The nonbrittle fracture of composites consisting of ex poly-carbosilan e SiC (Nicalon) fibers in a SiC matrix prepared by chemical vapor infi ltration is strongly dependent on the presence of a pyrocarbon layer a t the fiber/matrix interface (Nicalon/C/SiC composites). The mechanica l properties of such materials are known to be influenced by oxidation reactions. Elastic modulus measurements, using ultrasonic wave propag ation in the ''long bar'' mode, have been used to show the influence o f the environmental parameters temperature, atmosphere, and pressure o n the mechanical behavior of bidirectional Nicalon/C/SiC. In situ meas urements of elastic modulus performed in parallel with thermogravimetr ic analysis allow examination of oxidation mechanisms which affect int erfacial 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.