Are thin hard coatings (TiN, DLC, diamond) beneficial in tribologically stressed vibrational contacts? Effects of operational parameters and relative humidity

Vibrating contacts often fail by triboinduced damage. In the frame of a BRITE/EURAM project thin hard coatings have been considered as a solution to overcome such contact problems. TiN, Me-C:H (metal-containing DLC) and diamond coatings were investigated under oscillating sliding conditions in a ball-on-disk configuration. The tribological behaviour of flat coated specimens was investigated in comparison to uncoated steel specimens against steel and alumina balls in the gross-slip regime in a broad range of strokes (25 mu m to 10 mm), frequencies (1-20 Hz) and loads (1-20 N). Tests were performed at room temperature in dry air, normal air and moist air, respectively. friction force, total linear wear and electrical contact resistance were measured continuously. In the present study, for TiN, Me-C:H and diamond coatings, friction coefficients of 0.15-0.9, 0.03-0.20 and 0.05-0.8, respectively, were determined and wear coefficients varying by orders of magnitude were found in dependence of test conditions. Sharp friction and wear transitions were observed in dependence of stroke, frequency, load, test duration and relative humidity. Preselection and development of coatings for vibrating contacts, development of testing strategies, standardisation activities, as well as valuation of tribotest results of thin hard coatings have to consider effects of test parameters. For comparison and preselection of coatings under conditions of oscillating sliding a data base was developed. The results reveal that thin hard coatings, especially carbon-based coatings can reduce friction and wear in vibrating contacts for a broad range of test conditions. Investigations on wear scars with different methods of surface analysis indicate that the tribological behaviour is determined in many cases by tribochemical formation of surface layers and debris. (C) 1998 Elsevier Science S.A.