Effect of Boron Content on Microstructure, Mechanical Properties, and Oxidation Resistance of Mo-Si-B Composites

Mo-Si-B composites with boron content from 0 to 8 wt.% were prepared by spark plasma sintering using Mo, Si, and B elemental powders. The effects of boron content on the phase composition, mechanical properties, and high-temperature oxidation resistance of the Mo-Si-B composites were investigated. The results show the evolution of the major phase constitution of the composites with increasing boron content is in the following sequence: (MoSi2, Mo5Si3) → (MoSi2, Mo5Si3, Mo5SiB2) → (MoSi2, MoB) → (MoSi2, MoB, MoB2). As the boron content increases, both the hardness and flexural strength of the composites increases, but the fracture toughness of the composites gradually decreases. The strengthening mechanisms for the composites are second-phase strengthening and grain refinement strengthening. However, the formation of brittle phases, i.e., Mo5SiB2, MoB, and MoB2, reduces the toughness of the composites. In addition, since a dense barrier layer of SiO2-B2O3 formed on the surface of the composites inhibits the inward diffusion of oxygen, B-doped Mo-Si-B exhibits better oxidation resistance compared with non-doped composites.