EuroWire January 2015

Technical article

nanocrystalline 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 consistency of wire diameter, saving raw metal material up to 1~2%. Additionally, nano-dies are welcomed in high-speed wire drawing and they are highly applicable to environ- mentally friendly ‘non-pickling’ wire drawing processes. 4.2 Water lubricating application diamond There is another dominant advantage of nano-dies studied in this paper. That is the change of lubricating method with a most attractive green element in addition to direct cost savings. Most production lines with TC dies use oil-based lubricants to cool the dies and decrease the friction in the reduction and bearing zones of the die. Another consideration is the finish of aluminium products. TC dies need an oil-based lubricant in order to achieve a satisfactory surface finish. However, oil-based lubricating systems also bring lots of concerns. For example, the cost of lubricants is high, expensive accessory equipment is required, and there is consumption of oil resources (in oil-based lubricant manufacturing processes). Additionally, oil-based lubricants carry pollutants into the workshop (smog, chemical particles and microbes) and there are great difficulties in recycling aluminium scraps. For drawing customers, the latter two factors are big problems, especially aluminium scrap recycling. A kind of water-based lubricating system can be successfully introduced to replace the conventional oil-based lubricating system when using nano-dies for producing coaxial cable and Al-plastics compound pipe. This is becoming very popular in Chinese production in recent years [5][6] . perspective in Al wire and coaxial cable/Al-plastics compound pipe drawing process with nano-dies

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 nano- crystalline 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 nanocrystal- line 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 com- posite 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 aluminiumwire drawing 4.1 Conventional application tests of nano-dies for aluminiumwire 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. aluminium enamelled wire and aluminium wire drawing tests performed on the production lines of customers in China, According to practical

▲ ▲ Figure 5 : The water lubricating application in Al-plastics compound pipe drawing process with nano-dies and appearance of the aluminium product t

Success is the direct result of the unique structure of sp 2 -bonded carbon in the layer of nano-crystalline diamond coating of nano-dies. It brings not only very low friction in the interface with aluminium (and with good self-lubricating, similar to some graphite-like structures), but also better wetting with lubricating film in water-based lubricants (vs conventional multi-crystalline coating). (to form abrasive dust etc) when using water-based systems. By contrast, the process is completely worry-free when using nano-dies because of the high degree of chemical inertness and very low wear rate of diamond coatings. Figure 5 shows the water lubricating application in the Al-plastics compound pipe drawing process with nano-dies at a customer’s plant in Zhejiang Province of China and the final appearance of the aluminium product. Used in this manner, the customer saves a large amount of cost of lubricating oil, accessory thermostat cleaning equipment and other processes, and significantly reduces environmental pollution. The Al-plastics compound pipes they produce are more consistent, have a more accurate diameter and a better finish than anything previously achieved using TC dies and an oil-based lubricating system. The same story has been repeated in multiple aluminium wire drawing applications in China. Conventional TC dies will also meet difficulties of strong oxide etching and adhesive wear

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January 2015