WCA March 2015

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 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 nano- crystalline 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. For nanocrystalline diamond, the diamond band at 1,332cm -1 is significantly broadened, and Raman scattering intensity near the 1,560cm -1 region is pronounced. Broadening of the diamond band is a result of decreasing the grain size to the nanometer scale, and the presence of scattering intensity at 1,560cm -1 is due to increasing graphite-like or amorphous carbon sp 2 -bonded components at the grain boundaries in films. Raman scattering is 50~60 times higher for sp 2 -bonded carbon compared to sp 3 -bonded carbon, hence the diamond component dominates in films. The Raman spectrum of the nanocrystalline diamond film in Figure 3 can be regarded as the representation of the surface of the composite diamond film because the Raman spectrum reveals the structure of the top layer of the film. The surface profiles of conventional multi-crystalline coating and nanocrystalline diamond composite coating were tested with the same thickness at five sampling sites with surface profilometer, and the results of the surface roughness (Ra) were 309.64nm and 104.71nm, respectively. By comparison, Ra of the nanocrystalline diamond composite coating could reach 30nm or even lower after mechanical polishing. According to the characterisation and analysis, the nanocrystalline diamond coating has a smooth surface with a grain size of about 50nm, which is much smaller than conventional multi-crystalline diamond coatings. This is advantageous to the surface polishing of diamond coatings. Therefore, nanocrystalline diamond composite coatings dies (nano-dies) can easily be fabricated,

see Figure 4 (with die casing), to meet the requirements of both excellent wear resistance, very high finish and low friction with aluminium. 4 Application tests of nano-dies for aluminium wire drawing 4.1 Conventional application tests of nano-dies for aluminium wire drawing We prepared nano-dies with various specifications (bore diameter smaller than 4mm) for aluminium wire drawing, using the above nanocrystalline diamond composite coatings technology and subsequent polishing technology. According to practical aluminium enamelled wire and aluminium wire drawing tests performed on the production lines of customers in China, nanocrystalline diamond composite coatings show very good adhesion performance, wear resistance and much lower surface friction. The nano-dies showed a die life improvement by a factor of about 10 to 25, when drawing speed was about 15m/second, so it can not only save cost of drawing die itself, but also largely reduce change-over time (only 4% to 10% of previous process) and dramatically increase productivity. Because the friction coefficient between diamond coating of nano-dies and aluminium conductor is so small (only about 0.09), use of nano-dies can obviously improve the surface finish of drawn products and avoid aluminium galling on drawing dies. The super wear resistance of the diamond coating of nano-dies yields ❍ ❍ Figure 5 : The water lubricating application in Al-plastics compound pipe drawing process with nano-dies and appearance of the aluminium product

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Wire & Cable ASIA – March/April 2015