Simultaneous coating of Si and B on Nb–Si–B alloys by a halide activated pack cementation method and oxidation behaviors of the alloys with coatings at 1100 °C

Abstract

Phase formations during Si and B co-deposition by a halide activated pack cementation on Nb–Si–B alloys and the oxidation behaviors of the alloys were investigated together with diffusion pathway analysis. Coatings comprising Nb-borides and Nb-silicides were successfully fabricated on the surfaces of α-Nb, T2 single-phase and Nbss+T2 two-phase alloys at 900°C, respectively. For the coating process, powder mixtures with different Si to B ratios of (12.5:12.5) and (18.75:6.25) were used, respectively. Regardless of the Si to B ratio in the powder mixture used, the coating layers were identified as NbB2/NbSi2/Nb5Si3/substrate. The diffusion pathway followed the mass balance and showed one route regardless of the compositions of powder mixtures used in this study. The sequential oxidation product layers of the α-Nb and the T2 single phase alloy were identified as borosilicate/NbO2/NbB2/NbSi2/α-Nb and (borosilicate+3Nb2O5⋅B2O3)/Nb2O5/NbB2/NbSi2/T2, respectively. The diffusion pathway for the oxidation reaction was proposed using a schematic quaternary isothermal diagram.