EuroWire January 2015

Technical article

coatings. For conventional multi-crystalline diamond, as shown in Figure 2a , the film displays a well-faceted microcrystalline diamond surface with grain size ranging from 2 to 4μm. The surface is very rough and consists of a combination of {111} and {110} facets. For nanocrystalline diamond, as shown in Figure 2b , the film appears very dense with a fine-grained morphology (grain size about 50nm). The surface morphology is non-faceted and much smoother than multi-crystalline diamond films. Figure 3 shows the Raman spectrum of the composite coatings of multi-crystalline diamond film and nanocrystalline diamond film. For conventional multi-crystalline diamond films, the only sharp characteristic peak for diamond (sp 3 carbon) appears at 1,332cm -1 . For nanocrystalline diamond films, a characteristic peak for diamond (sp 3 carbon) appears near 1,339cm -1 . And a broad peak near 1,580cm -1 corresponds to amorphous carbon or non-diamond carbon (sp 2 carbon). It can be inferred that the conventional multi-crystalline diamond film contains much less non-diamond component. the diamond band at 1,332cm -1 is significantly ▼ ▼ Figure 3 : Raman spectra of the multi-crystalline coating (black) and nanocrystalline diamond coating (red) For nanocrystalline diamond,

(a)

(b)

▲ ▲ Figure 2 : The plane-view SEM images of the multi-crystalline and nanocrystalline diamond coating

Nanocrystalline films were continuously deposited in situ on 10~15μm thickness conventional diamond films by adjusting CVD process parameters (such as gas pressure, hydrocarbon-hydrogen gas mixture ratio and whether or not bias voltage is applied) to significantly enhance the secondary nucleation. Composite diamond films with a smooth surface were achieved by the deposition of alternate rough multi-crystalline diamond and smooth fine-grained nanocrystalline diamond layers [4] . A single straight tantalum filament was arranged to lie on the centre line of the die to be coated (see Figure 1 ). The tantalum wire is held straight by a high temperature spring or a kind of specially designed holder in the case of very small bore diameter dies. A single straight filament located at the axis centre of the die can maintain uniform temperature on the surface of the bore of the drawing die during the deposition process. A DC bias is applied between the filament and the drawing die substrate so as to enhance the diamond nucleation density. 3 Characterisation and discussion of nanocrystalline diamond composite coating dies (nano-dies) Figures 2a and 2b show the surface morphology of conventional multi- crystalline and nanocrystalline diamond diamond thin

this results in a very rough surface, given the very high surface energy of diamond. Since diamond is the hardest material known, the polishing methods are difficult to apply and very time consuming, especially for thin diamond films. The friction coefficient increases as diamond films grow rougher. Such surface roughness is not appropriate for many applications, especially in aluminium conductor drawing applications which benefit greatly from the very low friction of process and very high finish of product, both of which are now provided by HFCVD films.

2 Preparation of nanocrystalline

surface NCD film

diamond composite coating dies (nano-dies)

intensity (au)

The authors of this paper solved the problem of high surface roughness of multi-crystalline diamond coatings by the deposition of nanocrystalline diamond composite coatings. Composite diamond films with a smooth surface were deposited by a two-step chemical vapour deposition procedure, including first the deposition of the conventional rough multi-crystalline diamond and then nanocrystalline diamond layers. Cemented tungsten carbide YG6 (Co 6%) drawing dies were used as the substrates, which were pre-treated by various methods including leaching cobalt by dipping in specific reagents and by scratching the substrate using diamond powders [3] . Conventional and nanocrystalline diamond films were deposited in a bias-enhanced hot filament CVD apparatus.

underlying MCD film

wavenumber (cm -1 )

▼ ▼ Figure 4 : Appearance of nanocrystalline diamond composite coatings die (nano-die)

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January 2015