The stability and heat resistant of silicides coatings on refractory metals - II: stability and heat resistance of silicides coatings on tungsten and molybdenum at the temperature of 1500-2000℃

摘要

The kinetics of diffusion redistribution of phases in the WSi{sub}2-W system under heating in air has been investigated in the wide temperature range 1500-2000 ℃. The stability and heat resistance of silicide coatings on tungsten is mainly determined by silicon diffusion towards WSi{sub}2 interfaces and by formation of the lowest-siltcide W{sub}5Si{sub}3 diffusion barriers there, as well as by production of a protective SiO{sub}2 oxide film on the outer side of the silicide coating. It is to und that the rate of higher-to-lower tungsten silicide transition (WSi{sub}2 → W{sub}5Si{sub}3) is, on the average, four times lower than that for the MoSi{sub}2)→Mo{sub}5Si{sub}3 transition. It is shown that an increase in the content of silicon in the surfaceside WSi{sub}2 layer stimulates the process of diffusion barrier compound production at interfaces, and this results in the increase in stability and heat resistance of silicide coatings on metals. In particular, the decay rate of MoSi{sub}2 → (Mo, W){sub}5Si{sub}3 in the MeSi{sub}2-W system at 1700℃ is lower than the corresponding rates for the MoSi{sub}2 → W{sub}5Si{sub}3 systems by factors of about 20 and 11, respectively. In this case, the silicide coatings on tungsten and molybdenum also exhibit their enhanced heat resistance. It is demonstrated that the SiO{sub}2 oxide film on tungsten silicide does not lose its protective properties at temperatures up to 2000℃.