ULTRASONIC EVALUATION OF INITIATION AND DEVELOPMENT OF OXIDATION DAMAGE IN CERAMIC-MATRIX COMPOSITES

In this paper we report on the development of a method for ultrasonic nondestructive characterization of oxidation damage in ceramic-matrix composites, The method is based on ultrasonic measurement of elastic moduli of the composite, which are then used to determine the elastic moduli of the fiber-matrix interphase, Thus the interphasial damage may be estimated quantitatively, As a model system we used, to demonstrate applicability of the method, a unidirectional SiC-fiber-reinforced reaction-bonded silicon nitride matrix composite (SiC/RBSN), The composite samples mere oxidized in Bowing oxygen for 0.1, 1, 10, and 100 h at 600 degrees, 900 degrees, 1200 degrees, and 1400 degrees C, The ultrasonic phase velocity in the composite was measured at room temperature before and after oxidation; the data were then used to find the composite moduli, which quantify the induced damage, Significant changes in ultrasonic velocities and composite moduli were found as a result of oxidation, Fiber-matrix interphasial moduli were determined by multiphase micromechanical analysis, We found that oxidation of the carbon interphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite, The critical exposure time for transition from the nondamaged to the damaged state at different oxidation temperatures has been determined.