Anisotropic oxidation and weight loss in PMR-15 composites

Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. The thermo-oxidative behavior of the composite is significantly different from that of the fiber and matrix constituents as the composite microstructure, including the fiber–matrix interphases/interfaces, introduces anisotropy in the diffusion behavior. In this work, unidirectional G30-500/PMR-15 composite specimens were aged at elevated temperatures in air resulting in oxidation propagation parallel and perpendicular to the fibers. Four different specimen geometries were chosen such that different surface area ratios (i.e., ratios of surface area perpendicular to the fibers to surface area parallel to the fibers) were obtained. Weight loss and volumetric changes were monitored as a function of aging time to study the high-temperature anisotropic oxidation process. Optical micrographs were taken on polished internal sections and viewed in the dark-field mode to measure the degree, depth and distribution of thermal oxidation development from surfaces perpendicular and parallel to the fibers. An empirically based weight loss model is investigated and used to predict weight loss in unidirectional and woven composites.