Ed Nicholson

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.

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.

The effect of varying deposition conditions on the Young's modulus of diamond coated wires

Abstract Diamond Coated Tungsten (W) fibres wires have been produced by hot filament chemical vapour deposition (HFCVD), and their Youngs modulus measured using a resonance test. Changing the diamond deposition conditions results in changes in morphology, grainsize, film thickness, and quality, which in turn affects the composite fibre modulus and other mechanical properties (e.g. strength and fracture toughness). The aim of this investigation was to see the effect of varying the methane (CH 4 ) concentration and the deposition time on fibre modulus.

Excimer laser processing of chemical vapour-deposited diamond fibres

Excimer laser radiation (XeCl, λ = 308 nm and ArF, λ = 193 nm) has been used both to cut chemical vapour-deposited diamond-coated tungsten wires (diamond fibres) of diameter ≤200 μm and to smooth and profile the diamond surface. The effects of ablation and localized heating in the focal volume on the diamond and the tungsten core are described, and compared with results obtained for a Nd:YAG laser. The laser fluence and/or intensity, beam homogeneity and angle of incidence were identified as key parameters in ultraviolet laser-induced ablation of diamond-coated wires and fibres. Surface smoothing may lead to increase fibre strength.

Microstructure and interfaces in diamond coated steel wires

Diamond fibres were produced in a hot filament reactor by chemical vapour deposition of a 40 μm thick layer of diamond on to titanium-coated iron and iron–chromium wires. In the 40 h deposition time carbon diffused through the titanium and steel and produced carbide microstructures with hardness values of ∼ 3234 and 5390 N mm-2 for the Fe and Fe–Cr cores, respectively. During cooling thermal stress fracture of the diamond was avoided by stress relief cracks in the disordered graphite layer at the diamond/Ti-rich layer interface.

Nanoscale grinding of ceramics using diamond fibres

Abstract Diamond fibres have been manufactured by CVD on to W wires. The fibers were used to grind optical glass and alumina. The surfaces were studied using Talysurf, SEM, and atomic force microscopy. Nanoscale surface roughness values have been obtained and ductile grinding was produced on glass. The results suggest that the shallow depth of cut produced by the surface diamond facets will lead to reduced surface damage, and the high density of cutting sites will allow high rates of material removal.

Laser ablation of diamond fibres and a diamond fibre metal matrix composite

Continuous chemical vapour-deposited diamond-coated fibres with tungsten wire or SiC fibre cores are attractive for reinforcing metals and ceramics. The fibres have been embedded in Ti-6A1-4V alloy to produce a diamond fibre-reinforced composite. Both the fibres and the composite material are extremely difficult to cut without damage by conventional mechanical methods. The use of a Nd-YAG laser to cut these materials is described.

Microstructure and tensile properties of continuous diamond fibre reinforced titanium alloy

Abstract Preliminary experiments on a diamond fibre reinforced Ti-6Al-4V alloy are reported. The titanium alloy was electron beam evaporated from a molten bath and condensed on to diamond fibres. Titanium alloy coated fibres were consolidated into a composite by hot isostatic pressing. Round bar tensile test pieces were machined from the composite and tensile properties determined. The composite microstructure was characterised using Auger analysis and scanning electron microscopy. Excellent composite modulus values were obtained in the fibre direction, with values increasing from 115 to 222 GPa with fibre volume fraction, whilst corresponding strength and elongation values decreased from 1000 to 626 MPa and 10% to 0%, respectively. The results are compared with data for current titanium alloy MMCs.