Christian Stein, Markus Höfer, Daniel Schulze, Sarah Baron, Tino Harig, Volker Sittinger, Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
Wire drawing is a metalworking process for the fabrication of wires with diameters ranging from less than one up to a few tens of millimetres. To enable high production volumes of typically many tons of metal-wire with good surface quality and constant diameter, the employed drawing dies must be highly wear-resistant. Therefore, cemented carbide (mainly WC-Co) is broadly used as a tool material. WC-Co’s industrially relevant range of hardness begins at about 1000 HV and reaches up to 2300 HV, mainly controlled by the WC grainsize and the amount of Co binder. Higher hardness and wear resistance can be reached with sintered polycrystalline diamond (PCD, hardness >5000 HV depending on the diamond fraction), resulting in significantly higher tool costs, especially for large diameters. CVD-Diamond coatings with a typical thickness of about 10 µm have excellent material properties comparable to natural diamond with a possible hardness of up to 10000 HV and can be deposited by hot-filament CVD (HFCVD) on qualified carbide grades. On flat samples, different carbide grades with binder contents of 3, 6 and 10 % Co were pre-treated by chemical etching to remove the binder in the surface near region, and subsequently a diamond coating was deposited with a standard HFCVD setup. Standard and commercially available diamond deposition equipment is capable to coat a variety of complex tool shapes, such as drills and end-mills, but is very limited for the coating of geometries with high aspect ratios and is hence unsuitable for drawing dies. In this work a modified HFCVD system optimized for the deposition of diamond into hole-like geometries was employed. The diamond thickness distributions inside of cylindric test samples were studied in detail to identify suitable deposition parameters for different tool geometries. Cemented carbide drawing dies with diameters ranging from 1.4 mm up to more than 8 mm were coated successfully. SEM investigations on coated dies show a microcrystalline diamond growth right down to the smallest section of the tools for all tested diameters. The CVD-diamond coated dies were tested in an industrial production process already showing four times longer service life in steel wire drawing.