MORPHOLOGICAL DESCRIPTION OF COATING CRACKS IN SIC COATED CARBON-CARBON COMPOSITES

Thermal expansion mismatch between silicon carbide coatings on carbon- carbon composites creates a complex pattern of microcracks in the coat ings. A better understanding of the mechanics and environmental perfor mance of these coatings requires an improved ability to quantify the c oating crack morphology. The work described here proposes and measures a variety of coating crack descriptions, such as crack type, spacing, width, cell area, and cell feret orientation. Characterization has be en done from a section view and a plan view. Specialized specimen prep aration techniques and computer-aided image analysis approaches were d eveloped to make the measurements feasible. Data from a multilayer che mical vapor deposited SiC coated carbon-carbon composite illustrate th e techniques. From the section views it is found that about 67% of the cracks do not penetrate to the surface of the specimen. From the plan view it is found that the crack pattern is systematically influenced by the underlying weave geometry of the cloth reinforcement. This resu lt, along with the measured crack area/surface area, can be explained with simple thermal expansion mismatch arguments. The data is used to build two tractable but realistic models of coating crack morphology t hat would be useful in oxidation rate predictions. The observations al so suggest several strategies for tailoring the coating crack morpholo gy.