THE CVD OF PYROLYTIC CARBON AND TITANIUM DIBORIDE COATINGS ON CARBON-FIBER YARNS FOR USE IN ALUMINUM-BASED COMPOSITES

In order to optimize the interfaces in carbon fibre (high-resistance T oray T800) reinforced aluminium matrix composites, a duplex coating co nsisting of a pyrolytic carbon (C-pyr,,,) layer and a titanium diborid e external layer has been developed by low-pressure chemical vapor dep osition (LPCVD). Pyrolytic carbon, which has a weak shear strength, is introduced to promote adequate debonding at the fibre-matrix interfac e and the external TiB2, film is used to protect carbon fibres from Al 4C3, formation and to promote fibre wettability during composite fabri cation. LPCVD studies were conducted on planar carbon substrates with a parametric investigation of the effects of temperature, pressure, re actant flow rates on the growth rate and morphology of the selected co atings. For TiB2,, a change from a mechanism controlled by surface rea ction to a mechanism controlled by mass transfer occurs as a function of temperature. The surface processes are dependent on the composition and Ti:B ratio in the initial gas mixture. For C-pyr,,,, obtained fro m thermal decomposition of ethylene, a very high activation energy (31 0 kJ mol(-1)) and a decrease in coating growth rate with the reactant residence time indicate that the rate-limiting step occurs in the gase ous phase. In both cases, optimum deposition parameters have been defi ned to ensure a satisfactory infiltration of 6 k filaments yarns aimin g at a uniform thickness. (C) 1997 Elsevier Science S.A.