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 °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 °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 °C. The oxidation product of WSi₂ coating was composed of the WO₃ particles embedded in the amorphous SiO₂ matrix at below 1200 °C but consisted of only SiO₂ phase at 1300 °C. The fast oxidation behavior of WSi₂ coating at below 1200 °C was attributed to the formation of many cracks and pores, i.e. short-circuit 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 °C was due to the exclusive formation of a slow-growing continuous SiO₂ scale by the rapid evaporation of WO₃ phase.