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The influence of temperature, stress, and environment on the oxidation and life of C/SiC composites

Experimental tests were conducted to determine the effect of temperature, sustained tensile load, and environment on the oxidation of continuous carbon fibers within a C/SiC composite. Stressed oxidation (stress rupture) tests were conducted in air and oxygen environments at temperatures of 800°C and 1200°C at sustained loads corresponding to initial stresses of 35, 69, and 138 MPa. The influence of these variables on carbon consumption and composite lives will be discussed. The microstructural analysis of polished cross-sections showing oxidation trends will be presented. It was found that stress and temperature influenced crack openings and thus the oxidation of carbon fibers in the pre-cracked composites. Temperature determined the kinetic regime in which carbon-oxygen reactions occurred as demonstrated by oxidation trends. The oxygen environment contributed to faster rates of carbon consumption compared to the rate in an air environment and therefore time to failure was dependent on the oxidizing environment as well as temperature. Coupon failure was dependent on the volume of carbon fibers consumed and the applied load.