Reactive formation of coatings at boron carbide interface with Ti and Cr powders

Abstract

Boron carbide, B 4C, is an attractive candidate material for reinforcement in metal matrix composites, whose application is severely hampered by its reactions with most engineering alloys at the high processing or service temperatures. The reactivity of B 4C with some of the metals, however, may be made use of to create protective coatings on its surface. In the present research, the microstructure of coatings obtained by the interaction of B 4C with Ti and Cr powders at 1000–1200 °C was investigated employing X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy. Coatings obtained by treating B 4C in Ti powder were found to contain Ti carbide, TiC 1− x, and Ti borides (TiB 2 and TiB). A relatively thin inner layer of the coating was carbide-free and contained only borides, while the major part of the coating was a mixture of TiC 1− x and TiB. In contrast to this, coatings formed by reaction of B 4C with Cr powder contained no carbides, and were shown to consist of Cr borides (CrB 2, CrB, Cr 5B 3 and Cr 2B) and amorphous carbon. A thick outer layer of the coating was carbon-free and consisted almost entirely of CrB. In both cases, the growth of the coatings was controlled by diffusion, the activation energy for the growth of B 4C Ti coating being approximately 175 KJ mol . The phase composition, layer sequence and morphology of the coatings obtained were interpreted on the basis of kinetic and thermodynamic data of the ternary systems involved. A good agreement between the experimental results and theoretical predictions was obtained.