Formation and performance of niobium silicide diffusion coatings dependence on the halide activators

Nb silicide coatings were processed by HAPC on commercially pure Nb substrates using the activating halides NH4F, NH4Cl, and NaF. Thermodynamic calculations were performed for different activators, together with the analysis of the processed coatings. It is shown that although NH4Cl allows to form more Si halides, coating thickness is strongly impacted by the magnitude of the partial pressures of the halides favoring NH4F to process thicker coatings. The higher stability NaF accounts for the scarce formation of halides and a dual-layer of NbSi2 and Nb5Si3 coatings. Regardless of the activator used, coatings exhibit elongated grains. Processing with NH4F led to a combination of larger oriented grains and smaller grains contrasting with the high hardness coatings processed with NaF, which feature finer grains and equiaxed grains in the intermediate Nb5Si3 layer. Thermogravimetric analysis up to 1350 degrees C revealed a higher mass gain with two growth stages for thick coatings processed with NH4F, a parabolic behavior for the coatings processed with NH4Cl and with NaF with superior performance for the latter. Isothermal oxidation at 1100 degrees C for 2 h showed the formation of Nb2O5 and SiO2 under all processing conditions. Despite the small thickness and the presence of a mixed oxide film coatings processed with NaF are more suitable for equipment operating at high temperatures for extended periods.