Effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates

Abstract

The effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition (CVD) of Si on Mo substrates at 1100 degreesC was investigated. The microstructure of monolithic MoSi2 coating formed without a prior nitridation of Mo exhibited a typical columnar structure perpendicular to the Mo substrate. On the other hand, a two-step deposition process (ammonia nitridation followed by CVD of Si) produced a MoSi2/alpha-Si3N4 composite coating, which consisted of equiaxed MoSi2 grains with an average size of 0.5 mum, where the alpha-Si3N4 particles with an average size of 120 nm were mostly located at the grain boundaries of MoSi2. The shape of alpha-Si3N4 particles was oblate-spheroidal type and its volume percentage ranged from 12.9 to 17.7%. The density and average width of cracks in MoSi2 layer formed by the mismatch of thermal expansion coefficients between the MoSi2 coating and the Mo substrate was smaller for the two-step deposition process than that for CVD process only. The columnar MoSi2 coating grew by the diffusion of Si and conversion of Mo5Si3 phase to MoSi2 phase at the interface of MoSi2/Mo5Si3, while the MoSi2/alpha-Si3N4 composite coating grew by successive displacement reaction of Mo2N phase and Si. The growth of MoSi2 grains was inhibited by the nanosize alpha-Si3N4 particles. (C) 2002 Elsevier Science B.V. All rights reserved.