Microstructure of Nb substrates coated with Mo(Si,Al)2–Al2O3 composite and B-doped Mo5Si3 layers by spark plasma sintering

Niobium substrates were coated with Mo(Si,Al)2–Al2O3, B-doped Mo5Si3–SiO2, Nb3Si5Al2, Cr-doped Nb(Si,Al)2 and NiAl layers by spark plasma sintering (SPS) and low-pressure plasma spraying (LPPS), and microstructure and oxidation behavior of the coated specimens were examined. Only B-doped Mo5Si3 layer bonded to the Nb substrates, when Mo(Si,Al)2, B-doped Mo5Si3, Cr-doped Nb(Si,Al)2 and NiAl powders were directly put on the substrates before the sintering. Nb substrates coated with B-doped Mo5Si3–5 wt% SiO2 interlayer and MoSi2–2 wt% SiO2 outerlayer by SPS showed extremely good oxidation resistance in the temperature range of 1473–1673 K. Al2O3 interlayer, which is expected to work as a diffusion barrier, was formed between Mo(Si,Al)2 layer and Nb substrates, when thin Al foil was inserted between the Mo(Si,Al)2 powder and the substrates before the sintering. In addition, only B-doped Mo5Si3 layer bonded to the Nb substrates, when they were coated with Nb3Si5Al2, B-doped Mo5Si3 and Cr-doped Nb(Si,Al)2 by LPPS.