Electrodepositing a Mo layer and composite siliconized coating on Nb-based alloy for high temperature protection

In order to protect Nb-based alloy from oxidation attack, a Mo layer was first electrodeposited under different current densities and then its siliconized coating was obtained via halide activated pack cementation (HAPC) process. Microstructure, chemical composition, hardness and adhesive strength of the Mo pre-layer on the Nbbased alloy were analyzed and oxidation behavior of the siliconized coating exposed at 1200 degrees C and 1300 degrees C was investigated. The results showed that the electrodeposited Mo layer was composed of amorphous Mo phase and MoO3. The oxygen content in the Mo layer was low as 38.2 at% when the current density during electrodeposition was 400 mA/cm2 with the microhardness as 238 +/- 18 HV. The HAPC siliconized coating exhibited a multilayer structure of MoSi2/(Mo, Nb)5Si3/NbSi2, which showed an increase in the cohesive/adhesive strength compared to the Mo layer. Mainly Nb2O5 and SiO2 were formed on the exposed siliconized coating. An accelerated mass gain and a thicker oxidation layer were found as the exposure temperature increased with a thinning of NbSi2 inner layer. The siliconized coating provided an effective oxidation resistance for the Nb-based alloy and the increased exposure temperature made the protection of the coating deteriorate.