EuroWire January 2015

Technical article

Water lubricating application in wire and coaxial cable drawing process with nano-dies By Zhang Wenhua, Guo Songshou, Zhang Zhiming and Shen Hesheng, Shanghai Jiaoyou Diamond Coating Co Ltd, and Shen Bin, Sun Fanghong andWang Xinchang, Shanghai Jiaotong University, China

Abstract An extremely effective new type of wire drawing, cable compacting and tube drawing die is capable of providing dramatic benefits over PCD and tungsten carbide dies in particular applications. This paper introduces nanocrystalline diamond composite coatings technology applied to dies (nano-dies) and discusses applications in aluminium wire drawing and coaxial cable drawing. The benefits of using nano-dies include significant price/performance improvements, better surface finish, higher productivity and a most attractive green element – water lubricating. 1 Introduction There are many aluminium drawing processes in the electric cable industry and metal wire and tube industries, including aluminium wire, copper clad aluminium wire (CCA), aluminium enamelled wire, aluminium coaxial cable and aluminium-plastics compound pipe. Among these applications, one key factor is preparation of a kind of drawing die suitable for aluminium drawing, with high production efficiency and product quality. Conventional drawing dies for these applications include tungsten carbide (WC-Co) drawing dies (TC dies) and several kinds of diamond drawings dies, such as single-crystal diamond and polycrystalline diamond (PCD) drawing dies. Although diamond drawing dies have the advantages of high hardness and wear resistance, low friction coefficient and good thermal conductivity, it remains impossible to fabricate traditional diamond drawing dies with large bore

1 Filament

power source

2 DC source 3 Electrode holder 4 Reaction gas inlet

5 Drawing die 6 Hot filament 7 Support 8 High temperature spring 9 Gas outlet

▲ ▲ Figure 1 : Illustration showing the appearance of the bias enhancement hot filament CVD fabricating apparatus for nanocrystalline diamond composite coatings drawing dies

diameter at costs that would make them easy or attractive to purchase. Furthermore, these types of dies are easily broken when drawing, especially at high speed and for high strength material. TC drawing dies can be manufactured very economically in a wide range of bore diameters (from 0.4 to 60mm or even larger) and they are used in a wide range of applications (due to their low cost), but they also have serious disadvantages of short working lifetime (easily worn and lose tolerance quickly) and this translates to low 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-deposited (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 advant- ages at larger bore diameters where traditional diamond dies show relatively weak economic performance. 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,

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January 2015