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

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-suicides were successfully fabricated on the surfaces of alpha-Nb, T2 single-phase and Nb-ss + T2 two-phase alloys at 900 degrees 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 alpha-Nb and the T2 single phase alloy were identified as borosilicate/NbO2/NbB2/NbSi2/alpha-Nb and (borosilicate + 3Nb(2)O(5)center dot B2O3)/Nb2O5/NbB2/NbSi2/T-2, respectively. The diffusion pathway for the oxidation reaction was proposed using a schematic quaternary isothermal diagram. (C) 2015 Elsevier B.V. All rights reserved.