P. G. Partridge

CVD diamond coated fibres

Abstract Diamond-coated fibres have been fabricated using hot filament chemical vapour deposition (CVD) and tested for mechanical stiffness. The fibres coated include small (

Auger electron spectroscopic analysis of chemical vapour deposited diamond/substrate interfaces

Auger electron spectroscopy has been used to identify the allotropes of carbon in chemical vapour deposited diamond films deposited on copper and tungsten wires and on SiC and silica fibres and to measure the thickness and composition of the diamond/substrate reaction layers. The significance of these results for the manufacture of diamond fibres is discussed.

Young's modulus of diamond-coated fibres and wires

Abstract Diamond-coated fibres and wires were produced by hot filament chemical vapour deposition (HFCVD) of diamond on a variety of core materials including tungsten(W) and silicon carbide (SiC). Fibres with a diamond volume fraction exceeding 95% have been produced. Three different methods of measuring the fibre Youngs modulus(a resonance method, a bend test and a tensile test) are presented, together with recent results. Possible applications for such fibres include reinforcements in metal matrix composites (MMCs).

Preparation of solid and hollow diamond fibres and the potential for diamond fibre metal matrix composites

The development of techniques to grow diamond thin films using chemical vapour deposition (CVD) is now an area of active world-wide research [1, 2] and the unique physical and chemical properties of diamond promise many potential applications in optical components, semiconducting devices and hard wear-resistant coatings [3, 4]. Previous work has focused primarily on planar silicon on molybdenum substrates, but some preliminary results have recently been reported for coatings on wires [5, 6]. This paper describes a technique for producing uniform diamond coatings on the surface of metallic wires or ceramic fibres and the production of free-standing diamond tubes. The factors that affect the quality of the diamond fibres are discussed and their potential use in reinforced composites is considered. In the present experiments, diamond-coating was carried out in a standard hot filament CVD reactor [1, 2], in which CH 4 and H2 in a ratio of 1:100 were passed into a vacuum chamber at a total flow rate of 200 standard cm 3 min -I and a pressure of about 4000 Pa. A Ta filament held at 2000 °C dissociated the gases allowing carbon to deposit on to the surface of the wires and fibres in the form of a polycrystalline diamond film at a rate of about 0.5/xmh -1. If the wire was placed parallel to, and a few millimetres from, the filament (as for planar substrates) the uniformity of the diamond-coating was limited by the thickness of the wire, since diamond grew fastest on the side of the wire facing the filament. This effect became noticeable for wires and fibres with diameter > 250/zm, placing an upper limit upon the thickness of wires or fibres that can be uniformly coated by this method of around 300/zm. Alternatively, if the wire was positioned centrally and coaxially within the coils of the filament, uniform coatings on wires and fibres with a wide range of diameters were achieved. In this case, for thicker wires or fibres (even up to a few millimetres diameter), the diameter of the filament coils was simply increased to maintain an optimum distance of about 4-5 mm between the surface of the wire and the filament. This ensured that the wire was heated to a sufficient temperature to favour diamond deposition (typically about 900 °C), and also that the

Potential high-strength high thermal conductivity metal-matrix composites based on diamond fibres

Abstract Thermal conductivity data reported for chemical vapour deposited (CVD) diamond films have been used to calculate the effective thermal conductivity of continuous diamond-fibre-reinforced metal-matrix composites. It is concluded that the very high thermal conductivity values (about 2000 W m −1 K −1 ) reported for CVD diamond may be difficult to achieve in practical composites. However, the combination of properties such as thermal conductivity, elastic modulus and density predicted for diamond-fibre composites are far superior to those offered by alternative materials. With hollow fibres the ability to combine conductive and convective cooling increases the design options in thermal management systems.

Laser cutting of diamond fibres and diamond fibre/titanium metal matrix composites

Abstract Continuous diamond fibres were produced by chemical vapour deposition of diamond onto tungsten wire. The fibres were embedded in Ti-6A1-4V alloy to produce a diamond fibre reinforced composite. The diamond fibre reinforced titanium alloy composite contained a high volume fraction of fibres uniformly spaced at a distance of about 50–100 μm. Both the fibres and the composite material were extremely difficult to cut without damage by conventional mechanical methods. The use of an Nd-YAG laser to cut these materials is described.

The effective chemical vapour deposition rate of diamond

The effective chemical vapour deposition (CVD) rate of diamond, defined as the total thickness of diamond or as the mass of diamond deposited per unit time, may be increased by orders of magnitude by increasing the substrate area per unit volume. To obtain these high deposition rates, novel substrate designs are proposed that exploit three-dimensional arrays of small diameter wires or fibres. The analysis suggests that the increased diamond output should be achieved with no increase in the net gas flow or power consumption, which could lead to the more economic production of solid diamond shapes and of composites containing continuous or short diamond fibres, or particulate diamond. Estimates for the cost of CVD diamond made by the fibre array technique are compared with reported current and predicted costs for CVD diamond and estimates for the cost of CVD SiC.

Nanoscale ductile grinding of glass by diamond fibres

Chemical vapour-deposited diamond fibres have been used to grind soda glass. The surface topography was studied using scanning electron microscopy and atomic force microscopy techniques. The diamond surface facets and edges led to grinding without surface cracking and to surface roughness, Ra, values in the range 3–50 nm. The grinding mechanism involved the formation by ductile flow of glass ribbons adjacent to the grinding grooves. This grinding mechanism was similar to that reported for single-point diamond machining. The potential for ductile grinding with diamond fibres is discussed.

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

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

Diamond fibre metal matrix composites

Abstract Solid and hollow diamond fibres have been manufactured by chemical vapour deposition (CVD) on to tungsten wires and coils. The fibres were coated with metal matrices by sputter deposition. The matrices were Cu, Al, and Ti-alloy. The coated fibres were then consolidated by hot pressing to metal-matrix composites. The properties of the composites are compared with current composites. Copper and aluminium composites have potential for high stiffness thermal conductors, and Al and Ti-alloy are attractive light-weight high stiffness materials for aerospace applications. In combination with hollow fibres, lower density and higher compressive stiffness are expected.