WCA March 2015

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. Nanocrystalline diamond thin 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. ❍ ❍ Figure 3 : Raman spectra of the multi-crystalline coating (black) and nanocrystalline diamond coating (red)

product quality and a huge amount of wasted material, eg copper and aluminium, which would be unacceptable in mass production. A solution to both of the problems has appeared with the development of the technique of chemical-vapour-depos- ited (CVD) diamond films. Synthesis and characterisation of diamond coatings have gained wide spread research interests [1] and this wear resistant coating can be easily exploited for drawing dies. The hot-filament CVD (HFCVD) diamond coating on the interior hole surfaces of WC-Co drawing dies provides particularly good results, having the same advantages as traditional diamond drawing dies, but with higher performance in key areas. For example, super high hardness (70~100 GPa), very low friction coefficient (~0.1), super high thermo-conductivity (8~20W/cm ⋅ K) and chemical inertness. Significantly, the HFCVD techniques provide tremendous economic advantages at larger bore diameters where traditional diamond dies show relatively weak economic perfor- mance. Specifically, nano-dies enjoy spectacular success replacing TC dies and PCD dies for copper and aluminium power cable compacting applications up to Ø60mm bore diameter [2]. One of the key advantages of HFCVD is production of diamond coatings with low roughness. This has always been a big challenge for conventional multi-crystalline diamond surface films. Because synthetic diamond films deposited by conventional CVD processes are multi-crystalline with a large grain size, 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 diamond composite coating dies (nano-dies) 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 ❍ ❍ Figure 2 : The plane-view SEM images of the multi-crystalline and nanocrystalline diamond coating

surface NCD film

intensity (au)

underlying MCD film

wavenumber (cm -1 )

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

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Wire & Cable ASIA – March/April 2015