Boron carbide formation from BCl 3CH 4H 2 mixtures on carbon substrates and in a carbon-fibre reinforced Al composite

Abstract

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 B y C without defects. A thermodynamic approach from the BCl 3CH 4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 CH 4 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 B 4CAl interface in a composite, which suggests a poor wettability of the boron carbide by the liquid metal.