Plasticizing and wettability-enhancing coatings on carbon, silicon carbide and boron fibers

Abstract

This paper is concerned with two groups of coatings on high modulus and high strength fibers for reinforcing composite materials with metal and polymer matrices. The first group includes coatings for improving and stabilizing the strength of fibers through a plasticizing effect. For carbon fibers the best results were obtained by chemically applying a nickel coating 1 μm thick with subsequent annealing at 1000°C to form a solid solution of carbon in nickel. In the case of boron and silicon carbide fibers, strength stabilization is achieved by applying coatings, 0.5–2 μm thick, of aluminum and some aluminum alloys by pulling the fibers through a melt. Analysis of the physicochemical interaction of the fibers with the coatings indicates a selective dissolution of the atoms of the fiber material in the coating at stress concentrators, with the result that the stress concentrators are smoothed out. In addition, the plasticizing effect is promoted by the relaxation of stresses in the coating at the stage of microplastic strain of the fibers. The second group includes coatings that enhance the wettability of the fibers by metal melts. An essential role in the improvement of the impregmation of carbon fiber strands and fabrics is played by the highly dispersive surface structure of the coatings, which exhibit a capillary effect as the melt spreads. The best results are obtained in double-layer coatings in which the first layer, silicon carbide, is protective and the second, molybdenum, enhances wettability.