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Diamond has been uniformly deposited onto the surface of thin metal wires using hot filament CVD. The diamond-coated wires are stronger and stiffer than the uncoated wires. Subsequent etching of the metal core in a suitable chemical reagent allows free-standing diamond tubes to be made, the typical dimensions being 1 cm long with an internal diameter of 10-150 ~tm. The formation of a thick, chemical-resistant carbide layer at the metal-diamond interface when using Ti and W wires is investigated.

Ultrasound is commonly used to detect and size cracks in a range of engineering components. Modeling techniques are well established for smooth and open cracks. However, real cracks are often rough (relative to the ultrasonic wavelength) and closed due to compressive stress. This paper describes an investigation into the combined effects of crack face roughness and closure on ultrasonic detectability. A contact model has been used to estimate the size and shape of scatterers (voids) at the interface of these rough surfaces when loaded. The response of such interfaces to excitation with a longitudinal ultrasonic pulse over a wide range of frequencies has been investigated. The interaction of ultrasound with this scattering interface is predicted using a finite-element model and good agreement with experiments on rough surfaces is shown. Results are shown for arrays of equi-sized scatterers and a distribution of scatterer sizes. It is shown that the response at high frequencies is dependent on the size, shape, and distribution of the scatterers. It is also shown that the finite-element results depart from the mass-spring model predictions when the product of wave number and scatterer half-width is greater than 0.4.

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 © 1994 British Crown Copyright flux of H atoms at the growing diamond surface was adequate to prevent graphite formation [7].As for planar substrates, diamond grew best upon wires made from materials with relatively low thermal expansion coefficients and preferably having carbide-forming properties. Wires of tungsten, titanium, tantalum and copper, and fibres of silica and silicon carbide were manually abraded with 1-3/zm diamond grit prior to diamond deposition in order to promote nucleation sites for the diamond growth [1]. Fig. 1 shows a section through a 200/zm diameter tungsten wire coated with a 6.5/zm thick layer of diamond. The quality of the diamond was ascertained by laser Raman spectroscopy which showed the strong 1332 cm -1 peak characteristic of diamond, with no corr...

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