Roles of mirror-like surface finish and DLC coated piston rings on increasing scuffing resistance of cast iron cylinder liners

This study considers two approaches towards increasing the dry scuffing resistance of grey cast iron (CI) cylinder bore (or liner) surfaces, namely by i) decreasing the initial roughness of the CI liner surfaces and ii) by applying a diamond-like carbon (DLC) coating to the top compression piston rings. CI liners were polished to different surface roughness values ranging between 0.08 µm and 6.83 µm and were subjected to unlubricated reciprocating sliding tests with progressively increasing loads (50-600 N) at a constant frequency of 5.0 Hz (0.15 m/s) using a Cameron-Plint TE77 tribometer. The counterface materials used were CrN coated steel as well as a hydrogenated DLC (H-DLC) coated steel. Microscopic and spectroscopic observations suggested that at low loads a mixed tribolayer comprising of iron oxide (Fe2O3) and graphitic carbon that was formed on wear tracks maintained low wear rates. At higher loads this tribolayer was removed resulting in metallic contact and leading to scuffing. Mirror-polished CI liner contact surfaces (with an average roughness of 0.08 µm) provided higher scuffing resistance; the critical scuffing load was 300 N compared to 200 N for liners with >0.25 µm average roughness. The improved scuffing resistance could be attributed to the lower contact stress experienced by the mirror-polished liners with a bearing ratio of 83% compared to medium-finished liners with a bearing ratio of 69%. CrN coated counterface contributed to scuffing of the CI liners at above 300 N load accompanied by a sharp increase in COF (>0.4). The H-DLC coated counterfaces prevented scuffing throughout and maintained a low steady state coefficient of friction (COF) of 0.13 up to 600 N load. The low wear of the CI liners run against H-DLC counterfaces was attributed to the formation of the mixed tribolayer on the CI surfaces that remained intact at all tested loads. The results suggest that the use of DLC coated piston rings and a mirror-polish finish of cylinder liners would avert scuffing and provide low COF values while maintaining low wear rates.