Reaction mechanism and microstructure development of strain tolerant in situ SiC-BN composites

The reaction mechanism and microstructure development of strain tolerant in situ SIC-BN composites fabricated from the in situ reaction of Si3N4, B4C and C were investigated. This exothermic reaction took place at about 1400 degreesC in an argon atmosphere according to the results of X-ray diffraction analysis and differential thermal analysis. The reaction finished after hot pressing at 1700 degreesC for 60 min, and densification occurred mainly in the temperature range of 1700 degreesC to 2000 degreesC. In spite of poor sinterability of BN, composites with rather high density were obtained. Chemical composition analysis of the composites obtained showed that there was no obvious change in the composition after hot pressing. The in situ formed SiC was of the beta -type with a quasi-spherical shape, whereas the in situ BN was graphitic hexagonal with a Bake shape, and was located at the grain boundaries of SiC. The composite obtained showed a very fine and homogeneous microstructure. The bending strength of the composite was high, while the elastic modulus decreased substantially. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.