Formation of crack-free MoSi2/alpha-Si3N4 composite coating on Mo substrate by ammonia nitridation of Mo5Si3 layer followed by chemical vapor deposition of Si

Abstract

The formation of crack-free MOSi2/alpha-Si3N4 composite coating on a Mo substrate by ammonia nitridation followed by chemical vapor deposition (CVD) of Si on the Mo5Si3 layer at 1100 degreesC which was already formed on a Mo substrate was investigated using optical microscopy, field-emission scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM) and X-ray diffraction (XRD). This study demonstrated that the crack-free MoSi2/alpha-Si3N4 composite coating with some pores could be produced on a Mo substrate. Two composite layers (gamma-Mo2N/alpha-Si3N4 and Mo3Si/alpha-Si3N4) with different microstructure were formed by ammonia nitridation of Mo5Si3 layer, and the subsequent CVD of Si resulted in the formation of three composite layers (MoSi2/oblate-spheroidal type alpha-Si3N4, gamma-Mo2N/rod type alpha-Si3N4 and MO3Si/alpha-Si3N4). The volume percentage of alpha-Si3N4 phase in the MoSi2/alpha-Si3N4 composite coating ranged from 30 to 33%, suggesting that the thermal expansion coefficient of composite coating was close to that of Mo substrate. (C) 2002 Elsevier Science B.V. All rights reserved.