REACTION MECHANISM AND MICROSTRUCTURE DEVELOPMENT OF STRAIN TOLERANT IN SITU SiC--BN COMPOSITES

摘要

The reaction mechanism and microstructure development of strain tolerant in situ SiC-BN com- posites fabricated from the in situ reaction of Si_3N_4B_4C and C were investigated. This exothermic reaction took place at about 1400deg.C in an argon atmosphere according to the results of X-ray diffraction analysis and differential thermal analysis. The reaction finished after hot pressing at l700deg.C for 60 min, and densification occurred mainly in the temperature range of l700deg.C to 2000deg.C. In spite of poor sinterability of BN, com- posites 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 β-type with a quasi-spherical shape, whereas the in situ BN was graphitic hexagonal with a flake 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.