Friction reduction mechanisms in cast iron sliding against DLC: Effect of biofuel (E85) diluted engine oil

Parasitic friction losses during piston ring-cylinder liner interactions in an internal combustion (IC) engine consume large portions of the available fuel energy. This work investigates whether the tribological performance of grey cast iron (CI) cylinder liner would be improved using a piston ring coated with non-hydrogenated diamond-like carbon (NH-DLC) in comparison to uncoated CI and steel rings. Coefficient of friction (COF) values and volumetric wear losses of CI were determined using ball-on-disk type tests as a function of sliding distance. CI tested against itself during boundary lubricated sliding with synthetic engine oil showed a COF of 0.14 and a volumetric wear of 11.0×10−4 mm3 after 6×105 cycles, whereas the use of NH-DLC coated counterface resulted in a COF of 0.11 and lesser wear of 0.5×10−4 mm3. Dilution of the engine oil by ethanol containing E85 biofuel was beneficial as COF was further reduced to 0.08 for CI tested against NH-DLC while maintaining low wear of 0.2×10−4 mm3. According to TEM and XPS analyses, an oil residue layer (ORL) formed on the CI contact surfaces as a result of sliding-induced degradation of zinc-dialkyldithiophospahte (ZDDP) additive in the oil. The ORL, which consisted of nano-crystalline particles of sulphides and phosphates of zinc (anti-wear components) embedded in an amorphous carbon matrix, was responsible for maintaining low wear. Ethanol dilution of the synthetic oil facilitated the formation of ORL. TEM/EELS studies of the NH-DLC counterface provided evidence for OH adsorption and passivation of carbon bonds at the surface that reduced the friction. It is anticipated that piston rings coated with NH-DLC run against CI liners would show low friction and wear during boundary lubricated sliding, an effect that could be enhanced when ethanol diluted oil is used.