Health diagnosis of functionally graded C/SiC coating on C/C composites

Specimens made of C/C composites with a functionally graded C/SiC coating were fabricated and subjected to the supersonic hot gas flow to evaluate durability. After repeated heating tests, pin-holes were found in a SiC coating layer. Understanding of the state and the process of damage and the health diagnosis of coating layer are very important for the practical application. Authors introduced the observation of thin film made of cross section of a damaged sample in addition to some kinds of diagnostic technique. On the other hand, an impedance spectroscopy technique was introduced to diagnose the health and the residual life of the coating layer. Initial cracks in the SiC coating layer due to the thermal stress have been said to close as the temperature rises. Authors presumed that chipping of the crack edge of SiC allowed the diffusion of oxygen into the C/C and the combustion of C/C followed by the temperature rise and the active oxidation of SiC. Authors made clear that the electric resistance of CVD-SiC was proportional to the SiC thickness. The measurement of SiC resistance became possible by the introduction of the impedance spectroscopy method even if the high electric resistance layer of SiO₂ was formed on the SiC surface.