Preparation of Si-rich NbSi2 coating on Nb surface by hot dip silicon-plating technology to improve the high-temperature oxidation resistance of pure Nb substrate

The hot dip silicon-plating (HDS) coating with a smooth surface and high Si content is prepared on Nb substrate. The coating surface is made up of NbSi2 grain phase(11.02 mu m2) and Si-rich phases, and the average surface roughness (Sa) and maximum height (Sz) are only 0.283 and 2.603 mu m. As the increase of exposure time, the Si and NbSi2 phases are gradually oxidized, while the oxide and interface layers become more thicker. Besides, the roughness values and mass gain per unit area (Delta m/S) of the coating are also increased with oxidation time. After exposed at 1200 degrees C for 8 h, the Sa and Sz of the coating increased to 1.605 and 19.932 mu m, respectively, and the Delta m/S to 45.68 mg/cm2. By comparison, after oxidation at 1400 degrees C for 2 h, its Sa and Sz values increased to 2.927 and 29.927 mu m, respectively, while the Delta m/S reached as high as 64.13 mg/cm2. During the oxidation process, the formation and diffusion of needle-like Nb2O5 on the coating surface make it difficult to repair defects in the oxide layer, which greatly deteriorates the protective effect of the oxide film and accelerates the disintegration and failure of the coating.