Impacts of Friction-Modified Fully Formulated Engine Oils on Tribological Performance of Nitrided Piston Rings Sliding against Cast Iron Cylinder Bores

Abstract

The tribochemical interaction in a nitrided piston ring/cast iron cylinder bore sliding contact with two commercial fully formulated engine oils (ILSAC GF-2 and GF-3 category, referred to as modified GF-2 and modified GF-3, respectively) supplemented with molybdenum dithiocarbamate (MoDTC) friction modifier was investigated in a laboratory bench test. Test results indicated that the modified GF-2 oil could produce a rapid friction reduction while the modified GF-3 oil maintained a stable and lower coefficient of friction (μ = 0.03) for a longer testing time. The modified GF-3 oil also generated a lower wear rate compared with the modified GF-2 oil under the same tribotest conditions. Based on SEM/EDX analyses of wear tracks on the tested tribo-components, it appears that the better formed tribofilm on both the lower and upper specimens by the modified GF-3 oil made a significant contribution to its tribological performance compared with the modified GF-2 oil when sliding under the same test conditions.