Comparative study on the effects of oil viscosity on typical coatings for automotive engine components under simulated lubrication conditions

Current emphasis on automotive engine has evoked the desire to improve tribological performance by both reducing friction and making starting easier in the automotive industry. The present study was conducted to determine the effects of viscosity of fully-formulated oil on tribological properties of the typical coatings for automotive engine components, e.g. ta-C, CrN and Cr-diamond coatings, under simulated lubrication conditions. Results showed that compared to Cr-diamond and CrN coating, the ta-C coating exhibited superior tribological properties with the lowest friction coefficients and wear rates under various testing conditions. Low-viscosity oil 0W-20 could significantly improve the tribological properties of ta-C coatings at different testing temperatures. Furthermore, in cyclic start/stop tests, ta-C coatings have been proven to possess excellent low temperature cycle starting performance when lubricated by low-viscosity oil. A synergistic effect of the low-viscosity oil and a nanoscale adaptive tribofilm was speculated for ta-C coatings to be responsible for achieving lower friction and wear simultaneously. The lubrication adaptability of low-viscosity oil to ta-C coating is technologically important and provides a high potential of industrial application for lubrication and wear prevention under actual conditions.