Microstructure of MoSi2-base nanocomposite coatings formed on Mo substrates by chemical vapor deposition

Abstract

The microstructures and formation processes of MoSi2-Si3N4 or MoSi2-SiC nanocomposite coatings on Mo substrates by ammonia nitridation or carburizing process followed by chemical vapor deposition of Si was investigated using optical microscopy, field-emission scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and X-ray diffraction (XRD). The present study demonstrated that the crack density formed in the MoSi2-base nanocomposite coatings due to mismatch in the coefficient of thermal expansion (CTE) between the nanocomposite coatings and the Mo substrate could be reduced by the control of volume percentage of the Si3N4 or SiC particles with the low CTE values. Especially, no cracks were observed in the MoSi2-(30-33) vol.% Si3N4 nanocomposite coating, indicating that its CTE value was close to that of the Mo substrate. The microstructure of MoSi2-base nanocomposite coatings showed the oblatespheroidal-type Si3N4 or SiC particles dispersed in the equiaxed MoSi-, grains. The average size of equiaxed MoSi2 grains ranged from 300 to 100 nm depending on the processes, and that of Si3N4 or SiC particles ranged from 150 to 90 nm. The growth of MOSi2 grains was inhibited by the nanosize Si3N4 or SiC particles located mostly at the grain boundaries of MoSi2.