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 coatings. A better understanding of the mechanics and environmental performance of these coatings requires an improved ability to quantify the coating 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 been done from a section view and a plan view. Specialized specimen preparation techniques and computer-aided image analysis approaches were developed to make the measurements feasible. Data from a multilayer chemical vapor deposited SiC coated carbon-carbon composite illustrate the 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 result, 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 that would be useful in oxidation rate predictions. The observation also suggest several strategies for tailoring the coating crack morphology.