Electrochemical formation of MoSi₂ film on Ni-based superalloy

The growth of MoSi₂ films by the electrochemical siliciding of Mo was investigated and MoSi₂ film formation was attempted on a Ni-based superalloy (CMSX-4) via a two-step coating process. An oxidation test of the MoSi₂ film was also performed. A thick MoSi₂ film was obtained at a temperature below 1073 K. At a relatively high current density of 100 mA · cm^-2, the supply of Si to the Mo surface was too fast and pure Si was deposited outward instead of alloying. The growth rate of MoSi₂ at 50 mA · cm^-2 was 57 μm · h^-1, abnormally fast compared to that by normal diffusion. This indicated that Si diffused through the grain boundaries, rather than through grain bodies. A dense and homogeneous MoSi₂ film, approximately 100 μm thick, was successfully formed on CMSX-4. The boundaries in MoSi₂/Mo and Mo/CMSX-4 were coherent. After oxidation testing, a multilayer comprising SiO₂, MoSi₂, Mo₅Si₃, Mo, an intermediate layer, and CMSX-4 was formed. The SiO₂ film was significantly thick at 10-30 μm compared to literature values. To obtain a thin SiO2 film, a MoSi₂ layer with fewer defects might be required.