CHEMICAL-REACTIONS IN THE PROCESSING OF MOSI2+ CARBON COMPACTS

Hot-pressing of MoSi2 powders with carbon at high temperatures reduces the siliceous grain boundary phase in the resultant compact. The chem ical reactions in this process were examined using the Knudsen cell te chnique. A 2.3 wt% oxygen MoSi, powder and a 0.59 wt% oxygen MoSi2 pow der, both with additions of 2 wt% carbon, were examined. The reduction of the siliceous grain boundary phase was examined at 1350 K and the resultant P(SiO)/P(CO) ratios interpreted in terms of the SiO(g) and C O(g) isobars on the Si-C-O predominance diagram. The MoSi2 + carbon mi xtures were then heated at the hot-pressing temperature of 2100 K. Lar ge weight losses were observed and could be correlated with the format ion of a low-melting eutectic and the formation and vaporization of Si C.