Oxidation behavior of tungsten and germanium alloyed molybdenum disilicide coatings.

摘要

A two-step coating process was used to produce a (Mo,W)(Si,Ge)(sub 2) coating on niobium. After exposure to high temperatures, a lower silicide layer forms underneath and is effective in arresting cracks. The oxidation weight-gain kinetics are parabolic following an initial transient period. Test coupons coated with (Mo,W)(Si,Ge)(sub 2) passed 200 one-hour cycles at 1370(degree)C and 60 one-hour cycles at 1540(degree)C. These results, along with evidence of a thick protective glass layer, suggest that the germanium additions help cyclic oxidation resistance. The beneficial effects of the tungsten include the formation of microvoids, which provides a lower effective elastic modulus, and mechanical strengthening. No accelerated low temperature or ''pest'' oxidation was observed in the temperature range between 500--700(degree)C. Thus, a (Mo,W)(Si,Ge) multicomponent silicide coating offers significant promise for the protection of Nb-base alloys exposed to cyclic oxidizing environments over a broad range of temperatures.