REACTIVE FORMATION OF COATINGS AT BORON-CARBIDE INTERFACE WITH TI AND CR POWDERS

Boron carbide, B4C, is an attractive candidate mater for reinforcement in metal matrix composites, whose application is severely hamper ed by its reactions with most engineering alloys at the high processing or service temperatures. The reactivity of B4C 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 B4C with Ti and Cr powders at 1000-1200 degrees C was investigated employing X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy. Coatings obtained by treating B4C in Ti powder were found to contain Ti carbide, TiC1-X, and Ti borides (TiB2 and TiB). A relatively thin inner layer of the coating was carbide-free ee and contained only borides, while the major part of the coating was a mixture of TiC1-X and TiB. In contrast to this, coatings formed by reaction of B4C with Cr powder contained no carbides, and were shown to consist of Cr borides (CrB2, CrB, Cr5B3 and Cr2B) 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 B4C/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.