Microstructure and thermal stability of coatings obtained by interaction of SiC and B 4C with Cr and Ti powders

Abstract

SiC and B 4C are known to react with metals at high processing or service temperatures. This interaction affects the properties of metal matrix composite materials reinforced by SiC and B 4C fibres or whiskers. Controlled reactions, especially those accomplished before the final processing, may be helpful in protecting the ceramic-metal interface from further degradation. In the present work the interaction of SiC and B 4C ceramics with Cr and Ti powders was investigated. Experiments were performed on SiC or B 4C plates immersed in the metal powders. During vacuum heat treatments at 1173–1473 K, reactive diffusion of metal atoms along the ceramics resulted in the formation of coatings on the ceramic surface. The microstructure and phase composition of the coatings obtained were studied using X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and Auger spectroscopy. The coatings formed on the SiC surface were found to contain carbides and silicides of the corresponding metal, the outer layer being a carbide or a mixture of carbides. The coating obtained by treating B 4C in Ti powder consisted of Ti borides (the inner part) and Ti carbide mixed with Ti boride (the outer part). Treating B 4C in Cr powder yielded a coating which consisted of CrB and amorphous carbon (the inner part of the coating) and chromium borides CrB, Cr 5B 3 and Cr 2B (the outer part of the coating.) The coatings growth was shown to be controlled by diffusion. The thermal stability of the coated and uncoated ceramic-metal interfaces was estimated by placing SiC and B 4C plates in Fe- and Ni-based matrices and annealing the model composites obtained at 1273–1473 for 1–10 h. The coatings were found to improve the stability of SiC and B 4C in metals.