FRACTURE-BEHAVIOR OF CHEMICALLY-VAPOR-DEPOSITED SIC-COATED GRAPHITE .1. EXPERIMENTAL RESULTS

CVD SiC coatings on polycrystalline graphite substrates exhibited a cr ack arrest/multiple cracking phenomenon when fractured in four-point f lexure. This fracture behavior appeared to be controlled by the magnit ude of the coating thickness. When the coating thickness exceeded appr oximately 50 mum, catastrophic failure of the sample was observed. How ever, when the coating thickness was less than approximately 50 mum, c rack arrest and multiple cracking were observed in the tensile coating surface before catastrophic failure of the composite. In order to und erstand the fracture behavior of the layered composites, it was necess ary to characterize coating thickness and strength as well as the frac ture toughness of the substrate and the state of residual stress in th e system.