Synthesis of TiC from porous carbon coating on Si–C–O (Nicalon) fibres by reactive chemical vapour deposition in pressure-pulsed mode or at atmospheric pressure

Abstract

TiC coatings were synthesised on NL202 Nicalon fibres through the treatment of a porous carbon coating at the surface of the fibres by thermal reactive chemical vapour deposition (RCVD) either in pressure-pulsed (P-RCVD) mode at low pressure or at atmospheric pressure (AP-RCVD) with a H2/TiCl4 gaseous mixture. The conversion rate of C into TiC was studied as a function of the H2/TiCl4 pulse number for the P-RCVD method and as a function of the treatment time for the AP-RCVD method. Chemical and micro-structural characterisations of the coatings were carried out by X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning and transmission electron microscopies. Mechanical assessment was achieved by tensile tests at room temperature. For the P-RCVD method, the conversion rate increased when the number of H2/TiCl4 pulses increased from 20 to 100. Once 20 H2/TiCl4 pulses were performed, an interlayer made of TiC was observed at the carbon coating/fibre interface indicating that the H2/TiCl4 gas has diffused inside the porous carbon and has reacted in depth and more particularly at the coating/fibre interface. With 100 pulses, the carbon coating was totally converted. For the AP-RCVD method, the conversion rate increased when the treatment time increased from 30 to 60 min. The reaction of conversion began preferentially at the outermost surface of the carbon coating but several isolated TiC grains were also observed at the coating/fibre interface. At atmospheric pressure, the gaseous mixture was more reactive than at low pressure whereas the diffusion of the gaseous species was limited inside the carbon coating. The mechanical properties of the fibre decreased when the conversion rate of the carbon into TiC increased both in P-RCVD and AP-RCVD.