The CVD of pyrolytic carbon and titanium diboride coatings on carbon fibre yarns for use in aluminium-based composites

In order to optimize the interfaces in carbon fibre (high-resistance Toray T800) reinforced aluminium matrix composites, a duplex coating consisting of a pyrolytic carbon (C-pyr,,,) layer and a titanium diboride external layer has been developed by low-pressure chemical vapor deposition (LPCVD). Pyrolytic carbon, which has a weak shear strength, is introduced to promote adequate debonding at the fibre-matrix interface and the external TiB2, film is used to protect carbon fibres from Al4C3, formation and to promote fibre wettability during composite fabrication. LPCVD studies were conducted on planar carbon substrates with a parametric investigation of the effects of temperature, pressure, reactant flow rates on the growth rate and morphology of the selected coatings. For TiB2,, a change from a mechanism controlled by surface reaction 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 from thermal decomposition of ethylene, a very high activation energy (310 kJ mol(-1)) and a decrease in coating growth rate with the reactant residence time indicate that the rate-limiting step occurs in the gaseous phase. In both cases, optimum deposition parameters have been defined to ensure a satisfactory infiltration of 6 k filaments yarns aiming at a uniform thickness. (C) 1997 Elsevier Science S.A.