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 wel ding contact tips is reported. Both studies and tests were made in pra ctical conditions, i.e. in industrial manufacturing. The first case st udy was on protection of the surface of a precision mould with a compl icated shape and used for artificial tooth production. The substrate w as 4 mm thick Ni, electrodeposited and electropolished. Such a sensiti ve surface is used together with polymethylmethacrylate paste in a too th production pressure process at 150 degrees C. We found that in this case, the only feasible deposition technique was DC reactive sputteri ng of Cr and CrN at 140 degrees C in the SPUTRON plasma beam sputterin g equipment of Balzers. A 3 mu m thick CrN coating with high microhard ness (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 prod uction cycles, even after its use for 18 months. The second case was i mprovement of copper welding contact tips used in the MIG welding tech nique. TIN and CrN coatings were used both as single and double coatin gs, directly deposited on the copper substrate and also using 5 and 16 mu m thick Ni galvanic coatings as an intermediate layer. Good adhesi on (L-c AE=15-22 N), low scratching coefficient, high microhardness (1 400-2500 HV, 0.10 N) and temperature resistance up to 600 degrees C (T IN) or 800 degrees C (CrN and TIN + CrN) provided an improvement in we lding contact tip life from the standard value of 10-16 h of interrupt ed work to 85-210 h. The best results were obtained with the ''hard'' tip substrate made of CuZnZr alloy. For all substrate-coating combinat ions - various substrate quality and applied coatings - the abrasion w ear of the electrode wire was predominant, while any welding material stuck onto the coated contact tip was easy to clean off. (C) 1998 Else vier Science S.A.