Pulse electric current sintering of ball-milled molybdenum disilicide

The effect of milling on the densification behavior of MoSi₂ powder during the SPS (spark-plasma sintering) process was investigated. MoSi₂ starting powder with an average size of 10 μm was milled to reduce the size to <1 μm. Sintering was performed in a pulse electric current sintering facility, varying the conditions of sintering temperature (1200-1500°C), holding time (1-15 min.) and heating rate (20-200 K/min.). Changes in relative density and the densification rate, as a function of temperature, were determined. The microstructure of sintered compacts was observed and analyzed by SEM and EPMA. The sintered density of milled powder specimens was lower (94-95% relative density) than that of the unmilled ones (94-98%), even though the densification rate of the former was higher than that of the latter in the early and middle stages of sintering. These contradictory results could be explained by oxygen pick-up during the milling process from 0.3 to 1.8 w/o, which seems to be related with to the formation of silicon oxide in MoSi₂ powder and its decomposition at high temperatures above 1200°C accompanied by formation of gas phase