MODELING THERMAL STRESSES AND MEASURING THIN FILM CTE IN MoSi_2 AND MoSi_2+SiC COMPOSITE COATINGS ON MOLYBDENUM

摘要

The high-temperature oxidation resistance of molybdenum can be significantly improved by coating it with MoSi_2. However, at the high operating temperatures, silicon from MoSi_2 diffuses into the molybdenum substrate and the oxidation resistance of the system deteriorates. Further, because of the coefficient of thermal expansion (CTE) mismatch between Mo and MoSi_2, the coating spalls upon thermal cycling. According to the linear rule of mixtures, the CTE of the coating matches that of Mo by adding 50wt% SiC to MoSi_2. Silicon and carbon diffusion still changes the chemistry of the overlayer and deteriorates the oxidation resistance of the composite. Incorporating a diffusion barrier layer (DBL) between the Mo substrate and the MoSi_2+50wt% SiC layer solved this problem. Non-linear stress analysis utilizing finite element modeling has been employed to evaluate the thermally induced stresses in the coating system during thermal cycling. A description of the model parameters is given. These models examine the effect of varying the DBL thickness and CTE. The results show that the largest axial thermal stresses are in the DBL. The paper proposes two methods to directly measure the CTE of free standing thin films using dilatometry techniques.