Stability and heat resistance of silicide coatings on refractory metals - II. Stability and heat resistance of silicide coatings on tungsten and molybdenum at 1500-2000 degrees C

The kinetics of diffusion redistribution of phases within the system WSi2-W on heating tungsten silicide in air in the temperature range 1500-2000degreesC is studied. The stability and heat resistance of silicide coatings on tungsten is mainly governed by the diffusion of silicon towards the interphase boundaries W - W5Si3, W5Si3 - WSi2, and WSi2 - SiO2, formation at them of diffusion barriers of lower silicide W5Si3, and also a protective SiO2 film at the outer boundary of the silicide coating. It is established that the transition ratefor the higher to the lower tungsten silicide WSi2 --> W5Si3 is on average four times slower than the transition ratefor MoSi2 --> Mo5Si3. It is shown that an increase in silicon concentration in the WSi2 surface layer stimulates formation of diffusion barrier compounds at interphase boundaries. This leads to an increase in the stability of the phase composition and heat resistance of a silicide coating on metals. In particular at 1700degreesC the transition rate for molybdenum silicide on tungsten MoSi2 --> (Mo, W)(5)Si-3 is about twenty times slower than the transition ratefor MoSi2 --> Mo5Si3, and less by a factor of about eleven than the transition rate for WSi2 --> W5Si3. Here there is also an increase in the heat resistance of silicide coatings on tungsten and molybdenum. It is shown that the SiO2 film on tungsten silicide does not lose its protective properties lip to 2000degreesC.