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.