In situ Si3N4-SiC-BN composites:: preparation, microstructures and properties

Boron nitride composites in Si-B-C-N system have been widely studied and applied in industry in these decades. In the present investigation, in situ reaction process was used to fabricate a new kind of silicon nitride-silicon carbide-boron nitride (Si3N4-SiC-BN) composites according to a previously proposed reaction of Si3N4, B4C, and C with extra addition of Si3N4 in the starting powder mixture. The composites were produced by hot pressing at 1800 degreesC in a nitrogen atmosphere. The obtained composites demonstrated relatively high density and high bending strength over those of conventionally-processed ones, although with increasing BN content the densification behavior became poorer especially for composites with BN contents higher than 25 vol.%. The growth of rodlike beta-Si3N4 grains was suppressed in the composites. Different from the previously reported SiC-BN in situ composites in which the strength almost kept unchanged when the BN content was lower than 35 vol.%, the strength of the present Si3N4-SiC-BN composites decreased monotonously with increase in BN content due to the decreased relative density and limited grain growth of rodlike beta-Si3N4 grains. (C) 2002 Elsevier Science B.V. All rights reserved.