Microstructure characterization of Mo-Si-C-N anti-oxidation coatings on C/C composites

The microstructures of a Mo-Si-C-N coating, synthesized in a flowing nitrogen atmosphere by fused slurry, have been characterized in detail by means of X-ray diffractometry, scanning electron microscopy (SEM) and transmission electron microscop y (TEM). The results indicate that the Mo-Si-C-N coating has a four layer structure consisting of a SiC bonding layer, MoSi ₂/Si main layer, sialon-ceramic subsurface layer and SiC nanowire surface layer. The SiC bonding layer formed by the diffusion of Si and C atoms is very dense and consists of β-SiC nano-grains. No crack or defect is found on the two-sided interface of the SiC bonding layer, indicating that the SiC bonding layer has strong bonding with the C/C composite substrate and MoSi ₂/Si main layer. The MoSi ₂/Si main layer is covered with a sialon-ceramic subsurface layer of about 5 μm in thickness. The sialonceramic subsurface layer plays an important role in further improving the oxidation resistance of the Mo-Si-C-N coating due to its excellent anti-oxidation properties. β-SiC nanowires with a substructure of twin and stacking faults growing alongthe 〈111〉 direction constitute the surface layer of the Mo-Si-C-N coating with a thickness of about 10 μm. To minimize the surface energy as well as total free energy, the SiC nanowires have surfaces with a fixed faceted angle, which accords with the intersection angle of the adjacent (111) close-packed plane in the faced-centred cubic structure.