A technique for the manufacture of long hollow diamond fibres by chemical vapour deposition

Abstract

Diamond possesses a unique combination of physical, mechanical and chemical properties, and many of these properties can now be obtained in diamond produced by chemical vapour deposition (CVD) [13]. Hitherto, CVD diamond has only been available as thin (micrometre) films on suitable substrates, which has limited CVD diamond applications to optical or wear resistant coatings and cutting tools [4]. Recently, continuous CVD diamond fibres have been produced by coating CVD diamond onto wires or ceramic fibre cores [5-7]. The diameter of the fibre cores can vary from < 50 gm, typical of the commercial fibres present in flexible multifilament tows such as HI-nicalon, Altex and Nextel 480, t o about 100-150gm, typical of commercial fibre monofilaments such as SiC and A1203 (Saphikon) [5, 8]. CVD diamond fibres can have tensile modulus values of 1 0 0 0 GPa [8,9], over twice that of monofilament SiC fibres [5], and are suitable for reinforcing polymer, metal [6, 10] or ceramic matrices. Thus, diamond fibres offer the engineer, for the first time, the possibility of exploiting the properties of diamond on a large scale in conventional engineering components and structures [5, 10]. In order to increase the compressive stiffness and decrease the density of glass fibre reinforced composites, hollow glass fibres, made by extruding and drawing, have been developed with the same mass per unit length as solid glass fibres [11]. Hollow diamond fibres have also been produced by etching out the diamond fibre core [6]. The possibility of obtaining the properties of diamond in hollow fibres is particularly attractive, but the etching method is unsuitable for very long, small diameter fibres. This letter describes a method for manufacturing long hollow fibres that does not rely on etching. The manufacturing technique involves CVD of diamond onto a helical tungsten wire coil. The diameter of the tungsten wire was in the range d l = 10-20 gm. The coil was made by winding annealed tungsten wire around a stiff wire or ceramic fibre core, as shown schematically in Fig. 1. After winding and relaxing the wire coil, the core was removed to leave a free-standing tungsten coil (Fig. 2a). Diamond deposition was carried out in a specially designed hot filament reactor (designed and built by Thomas Swan, Cambridge, UK) using a 1% CH4/H2 gas mixture and a flow rate of 200 standard