EFFECTS OF CARBON ADDITIONS ON THE HIGH-TEMPERATURE MECHANICAL-PROPERTIES OF MOLYBDENUM DISILICIDE

Additions of up to 4 wt.% C to MoSi2 caused elimination of the siliceous grain boundary phase in hot pressed samples as well as the formation of small quantities of beta-SiC and Mo5Si3C. Both the hardness and fracture toughness of the carbon-containing alloys exceeded those of the carbon-free (and oxygen-rich) materials. The toughness for all carbon-containing alloys at 800-1400-degrees-C was independent of the level of carbon addition, although the hardness at all temperatures up to 1000-degrees-C increased with the level of carbon addition. Unstable fracture was exhibited by all alloys tested below 1200-degrees-C, while stable cracking and high toughness (greater than or equal to 11.5 MPa m1/2) was obtained in the 2 wt.% C alloy at 1400-degrees-C.