Growth kinetics and oxidation behavior of WSi2 coating formed by chemical vapor deposition of Si on W substrate

The growth kinetics of WSi2 coating formed by chemical vapor deposition (CVD) of Si on a W substrate at temperatures between 1000 and 1200 deg C using SiCl4-H2 gas mixtures was investigated and its isothermal oxidation resistance in 80% Ar-20% O2 atmosphere was evaluated at temperatures between 800 and 1300 deg C. WSi2 coating grew with a parabolic rate law after an initial incubation period, indicating the diffusion controlled growth. The activation energy for growth of WSi2 coating was about 42.5 kcal/mol. The isothermal oxidation rate of WSi2 coating increased with increasing oxidation temperature but rapidly decreased at 1300 deg C. The oxidation product of WSi2 coating was composed of the WO3 particles embedded in the amorphous SiO2 matrix at below 1200 deg C but consisted of only SiO2 phase at 1300 deg C The fast oxidation behavior of WSi2 coating at below 1200 deg C was attributed to the formation of many cracks and pores, i.e. shortcircuit diffusion path of oxygen, within the oxide scale, which resulted from the internal stress generated both by the large volume expansion caused by the oxidation reactions of WSi2 and by the evaporation of WO3 phase. The slow oxidation behavior of WSi2 coating at 1300 deg C was due to the exclusive formation of a slow-growing continuous SiO2 scale by the rapid evaporation of WO3 phase.