Hard coatings on soft metallic substrates

The application of PVD CrN coatings onto soft, electropolished nickel moulds and PVD TIN, CrN and double-coating (TiN + CrN) onto copper welding contact tips is reported. Both studies and tests were made in practical conditions, i.e. in industrial manufacturing. The first case study was on protection of the surface of a precision mould with a complicated shape and used for artificial tooth production. The substrate was 4 mm thick Ni, electrodeposited and electropolished. Such a sensitive surface is used together with polymethylmethacrylate paste in a tooth production pressure process at 150 degrees C. We found that in this case, the only feasible deposition technique was DC reactive sputtering of Cr and CrN at 140 degrees C in the SPUTRON plasma beam sputtering equipment of Balzers. A 3 mu m thick CrN coating with high microhardness (1400 HV, 0.25 N on the Ni substrate), good adhesion (L-c AE = 12-16 N) and excellent corrosion and oxidation resistance, successfully protected the nickel mould from mechanical damage occuring during production cycles, even after its use for 18 months. The second case was improvement of copper welding contact tips used in the MIG welding technique. TIN and CrN coatings were used both as single and double coatings, directly deposited on the copper substrate and also using 5 and 16 mu m thick Ni galvanic coatings as an intermediate layer. Good adhesion (L-c AE=15-22 N), low scratching coefficient, high microhardness (1400-2500 HV, 0.10 N) and temperature resistance up to 600 degrees C (TIN) or 800 degrees C (CrN and TIN + CrN) provided an improvement in welding contact tip life from the standard value of 10-16 h of interrupted work to 85-210 h. The best results were obtained with the "hard" tip substrate made of CuZnZr alloy. For all substrate-coating combinations - various substrate quality and applied coatings - the abrasion wear of the electrode wire was predominant, while any welding material stuck onto the coated contact tip was easy to clean off. (C) 1998 Elsevier Science S.A.