The high-temperature oxidation resistance of molybdenum can be significantly improved by coating it with MoSi_2. But, at the high temperature of operation, silicon from MoSi_2 diffuses into the molybdenum substrate and the oxidation resistance of the system deteriorates. Further, because of the CTE mismatch between Mo and MoSi_2, the composite breaks down and spalls on thermal cycling. To alleviate the problem of the CTE mismatch, the CTE of MoSi_2 is matched with that of Mo by the addition of 50wt% SiC. Silicon and carbon still diffuse into the molybdenum substrate thereby changing the chemistry of the overlayer and deteriorating the oxidation resistance of the composite. Incorporating a diffusion barrier layer between the Mo Substrate and the MoSi_2+SiC composite layer on the top solved this problem. The newly developed amorphous diffusion barrier layer prevents diffusion of both carbon and silicon into the substrate. Finite element modeling studies were used to evaluate the effect of the diffusion barrier on thermal stress in the coating. finite element models have calculated the thermal stresses produced in the coating upon cycling from 25 °C to 1600 °C. For reference, the stresses generated by a sharp interface between MoSi_2 and Mo were also modeled. Applying a diffusion barrier whose CTE falls in between that of Mo and MoSi_2 reduced the stress level present in the MoSi_2 coating. Changing the coating from MoSi_2 to Mosi_2+50wt% SiC composite effectively eliminated the stress in the coating. The stress in the MoSi_2+50wt% SiC composite coating turned compressive when the diffusion barrier had a CTE lower than Mo. When the diffusion barrier thickness increased, so did the stress in the coating. The present calculations have shown that a CTE of less than 6.4*10~(-6)/°C and a thickness of 50 nm for the diffusion barrier layer produce compressive stresses in the MoSi_2+50wt% SiC composite coating.
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