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

Powders of Mo52Si38B10 were plasma sprayed under inert conditions onto stai nless steel substrates to determine if high density free standing forms cou ld be synthesized by this process, Thermal spray conditions were varied to minimize porosity and oxygen impurities while minimizing evaporative metal losses, The as-sprayed and sintered microstructures were characterized usin g scanning and transmission electron microscopy and quantitative x-ray diff raction (XRD). The as-sprayed microstructure consisted of elongated splats tens of microns in length and only one to three microns in thickness. The s plats contained submicrometer grains of primarily MoB and Mo5Si3Bx (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 pressurele ss sintered for 2, 6, and 10 h at 1800 degrees C in flowing Ar. After sinte ring for 2 h at 1800 degrees C, the samples exhibited a coarser but equiaxe d microstructure (1 to 5 mu m 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 lay ered structure observed in the as-sprayed samples.