Dry sliding behaviour of hydrogenated amorphous carbon (a-C:H) coatings on Ti-6Al-4V

The present study focuses on the comparison of the tribological performance of three hydrogenated amorphous carbon (a-C:H) coatings, PA-CVD deposited on the Ti–6Al–4V alloy: (i) in standard deposition conditions, (ii) in conditions allowing a higher penetration in holes and cavities, and (iii) in standard deposition conditions, with a PVD-CrN interlayer. The topography and microstructure of the coatings were characterized and the intrinsic hardness, reduced modulus and practical adhesion were determined. The dry sliding friction and wear behaviour of the coated alloy was studied by a slider-on-cylinder tribometer, at room temperature and in laboratory air, under the same sliding speed (0.3 m/s) and distance (1000 m), in the load range 5–90 N. The counterfacing material was the SAE52100-EN100Cr6 bearing steel. A critical load, which corresponds to the end of coating life, was identified for each a-C:H coated system. Below the critical load, all the coatings were effective in significantly reducing wear by comparison to the uncoated alloy. However, the highest critical load (both in dry sliding and in scratch tests) was measured for the coating with the CrN interlayer, thus confirming the key role of interlayers in improving load support by the substrate. The modification of deposition conditions for achieving higher penetration of the a-C:H coating, allows to obtain a very hard and stiff film with relatively low adhesion, which shows low friction and wear values under low applied loads but fails prematurely at high applied load.