MICROSTRUCTURE AND THERMAL-STABILITY OF COATINGS OBTAINED BY INTERACTION OF SIC AND B4C WITH CR AND TI POWDERS

SiC and B4C are known to react with metals at high processing or servi ce temperatures. This interaction affects the properties of metal matr ix composite materials reinforced by SiC and B4C fibres or whiskers. C ontrolled reactions, especially those accomplished before the final pr ocessing, may be helpful in protecting the ceramic-metal interface fro m further degradation. In the present work the interaction of SiC and B4C ceramics with Cr and Ti powders was investigated. Experiments were performed on SiC or B4C plates immersed in the metal powders. During vacuum heat treatments at 1173-1473 K, reactive diffusion of metal ato ms along the ceramics resulted in the formation of coatings on the cer amic surface. The microstructure and phase composition of the coatings obtained were studied using X-ray diffraction, scanning electron micr oscopy with energy-dispersive spectroscopy, and Auger spectroscopy. Th e coatings formed on the SiC surface were found to contain carbides an d silicides of the corresponding metal, the outer layer being a carbid e or a mixture of carbides. The coating obtained by treating B4C in Ti powder consisted of Ti borides (the inner part) and Ti carbide mixed with Ti boride (the outer part). Treating B4C in Cr powder yielded a c oating which consisted of CrB and amorphous carbon (the inner part of the coating) and chromium borides CrB, Cr5B3 and Cr2B (the outer part of the coating.) The coatings growth was shown to be controlled by dif fusion. The thermal stability of the coated and uncoated ceramic-metal interfaces was estimated by placing SiC and B4C plates in Fe- and Ni- based matrices and annealing the model composites obtained at 1273-147 3 K for 1-10 h. The coatings were found to improve the stability of Si C and B4C in metals.