Microstructure of a plasma-sprayed Mo-Si-B alloy

Powders of Mo₅₂Si₃₈B₁₀ were plasma sprayed under inert conditions onto stainless steel substrates to determine if high density free standing forms could be synthesized by this process. Thermal spray conditions were varied to minimize porosity and oxygen impurities while minimizing evaporative metal losses. The assprayed and sintered microstructures were characterized using scanning and transmission electron microscopy and quantitative x-ray diffraction (XRD). The as-sprayed microstructure consisted of elongated splats tens of microns in length and only one to three microns in thickness. The splats contained submicrometer grains of primarily MoB and Mo₅Si₃B _x (T1) and minor amounts of MoSi₂ and a glassy grain boundary phase. The interior of the splats typically consisted of a fine eutectic of MoB and T1. Small pieces were cut out of the cross section of the sample and pressureless sintered for 2, 6, and 10 h at 1800 °C in flowing Ar. After sintering for 2 h at 1800 °C, the samples exhibited a coarser but equiaxed microstructure (1 to 5 μm grain size) containing 78 vol.% T1, 16 vol.% MoB, and 6 vol.% MoSi₂ as determined by XRD. Approximately 8 at.% of the Si formed silica. The high-temperature anneal removed all vestiges of the layered structure observed in the as-sprayed samples.