A comparative study of rhenium coatings prepared on graphite wafers by chemical vapor deposition and electrodeposition in molten salts

The purity,preferred orientation,microstructure,microhardness,bonding strength,thickness uniformity and thermal stability of rhenium(Re)coatings prepared on graphite wafers by chemical vapor deposition(CVD)and electrodeposition(ED)in molten salts were comparatively studied in this paper.It was found that carbon(0.0140 wt%)and oxygen(0.0067 wt%)were the primary impurities for CVD and ED Re coatings,respectively.The diffusion of carbon into CVD Re coating caused higher microhardness near the substrate and helped to improve the bonding strength at the same time.The preferred orientation,microstructure and microhardness of ED Re coating were all susceptible to oxygen.The coating deposition uniformity of ED Re is obviously better than that of CVD Re coating,due to its intrinsic characteristics.The(002)-oriented,coarse columnar CVD Re coating exhibited better thermal stability compared with that of the<110>-oriented,fiber-like columnar ED Re coating,while the ED Re grains grew remarkably and the microstructure evolved toward the similar structure of CVD Re after annealing treatment.The diversity of Re coatings in microstructure could be attributed to the mobility of grain boundaries(affected by temperature and impurity)during deposition processes.