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 m oduli of the composite, which are then used to determine the elastic m oduli of the fiber-matrix interphase, Thus the interphasial damage may be estimated quantitatively, As a model system we used, to demonstrat e applicability of the method, a unidirectional SiC-fiber-reinforced r eaction-bonded silicon nitride matrix composite (SiC/RBSN), The compos ite samples mere oxidized in Bowing oxygen for 0.1, 1, 10, and 100 h a t 600 degrees, 900 degrees, 1200 degrees, and 1400 degrees C, The ultr asonic phase velocity in the composite was measured at room temperatur e before and after oxidation; the data were then used to find the comp osite moduli, which quantify the induced damage, Significant changes i n ultrasonic velocities and composite moduli were found as a result of oxidation, Fiber-matrix interphasial moduli were determined by multip hase micromechanical analysis, We found that oxidation of the carbon i nterphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite, The critical exposure time for transition fro m the nondamaged to the damaged state at different oxidation temperatu res has been determined.