Boron carbide formation from BCl3-CH4-H-2, mixtures on carbon substrates and in a carbon-fibre reinforced Al composite

A coating of boron carbide on carbon fibres was found to be a suitable method for preventing the chemical reactions between the fibre carbon and an aluminium matrix during the fabrication of a composite material by a squeeze-casting process. Such a coating is also interesting since it has an excellent oxidation resistance. Unfortunately, the mechanical properties of composites are not always reproducible; they can be related rather to the microtexture and the physical properties of the coating than to a possible reaction between the two components of the composite. The interaction between boron trichloride and graphite or carbon fibres in the presence of hydrogen and/without methane have been studied in order to obtain a continuous layer of ByC without defects. A thermodynamic approach from the BCl3-CH4-H-2-C system is presented and solid phase diagrams calculated. The coatings were obtained on various substrates such as bulky graphite and some PAN-based and pitch-based carbon fibres. They were observed by scanning electron microscopy, transmission electronic microscopy, X-ray diffraction and electron probe analysis. Whatever the CH4 content of the input gas phase, the coatings were polycrystalline and the grains contained numerous twins. For application as mechanical reinforcement components, strength measurements were performed on the coated filaments. A study by TEM showed the absence of reaction products between the carbide and an aluminium matrix and the presence of micropores at B4C-Al interface in a composite, which suggests a poor wettability of the boron carbide by the liquid metal. Copyright (C) 1996 Elsevier Science Ltd