FABRICATION AND EVALUATION OF SiC/C FUNCTIONALLY GRADIENT MATERIAL

Abstract

A plate-like deposit having a compositional gradient from carbon (C) to silicon carbide (Sic) (SiCJC FGM) was obtained on a graphite substrate with chemical vapour deposition (CVD) by changing the SiJC molar ratio in input gas during deposition. Cracking due to heat flux was investigated using a C02 laser setup. Heat flux which resulted in cracking in Sic-coated graphite (Sic NFGM) was 5.8 M W ~ I ~ at a surface temperature (Ts) of 1300 K and a reverse surface temperature (Tb) of 1220 K. In the case of SiCJC FGM, heat flux occurred cracking was 7.4 ~ r n ~ at a Ts of 1650 K and a Tb of 1080 K. The temperature difference between Ts and Tb was 570 K in FGM and 80 K in NFGM. Thermal stress calculation showed that in cracking caused by hoop stress, the stress was close to the fracture strength of Sic. It was clear that SiCIC FGM has effective thermal shock resistance and thermal barrier characteristics. In recent years, much attention has been focused on the development of high-temperature functional materials, especially in the field of aerospace applications 111. Functionally gradient materials (FGMs) have been developed which have a compositional or structural distribution from one side to the other side. It is necessary to have both high oxidation resistance and high fracture strength at high temperatures. We have attempted to fabricate an FGM consisting of S ic and C, taking into consideration the high oxidation resistance and high fracture strength of Sic, as well as the low Young's modulus and easy-processing of C /2,3/. An SiCJC FGM which has a compositional gradient from C to S ic has been fabricated with the CVD method by changing the SiJC molar ratio in input gas. The fabrication and thermal properties of Sic-C composites were also investigated. The authors previously clarified the effect of the addition of C on the lower thermal conductivity of Sic-C composites 141 and on the superior thermal barrier characteristics of Sic-C composites. Furthermore, they clarified the thermal barrier characteristics and thermal shock resistance of SiCJC FGM using a Xenon arc lamp heating method. As a result, it was shown experimentally and by calculation of thermal stress that SiCIC FGM has thermal barrier characteristics because of lower thermal conductivity, as well as thermal stress relaxation due to a lower Young's modulus 15-7J. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991278 C2-650 JOURNAL DE PHYSIQUE IV The authors have already investigated the thermal shock resistance of SiC/C FGM using the C02 laser (50 W) method /8,9/. However, the difference between Sic-single-layer-coated graphite (Sic NFGM) and SiC/C FGM was not clarified of the lower laser power (50 W) of this method. In the present study, we report the thermal shock resistance of SiC/C FGM having a graphite substrate (30 mm in diameter, 5 mm in thickness) using the 5 kW-C02-laser-heating method, and the effect of laser spot diameter.