The tribological performance of plain and fully formulated commercial engine oil under 2 different rotational speeds and extreme pressure contact using design of experiment

Abstract

The friction coefficient under boundary lubrication is affected by many factors. In order to study the friction characteristic of the optimized tests tribofilms, a design of experiment model that includes the contact loads (297 and 405 N or 2.5 and 2.77 GPa Hertzian contact pressure) and two different rotational speeds (a lower speed of 100 rpm was used for the first 5,000 revolutions and a higher speed of 700 rpm was used until failure or 100,000 revolutions, whichever came first) instead of the single rotational speed of 700 rpm, the temperature treatment of test cylinders at 100°C vs. nontreatment was used as part of a design of experiment (DOE) software to simplify experiments and to investigate the failure and wear responses in plain zinc dialkyl dithiophosphate (ZDDP) oil (0.1 P% (phosphorus percentage)) and commercial fully formulated oil (0.1 P%). Findings indicate that the factors affecting friction and wear are extreme load of 405 N under the rotational speed of 700 rpm. Properties were evaluated using scanning electron microscopy (SEM) to analyze the wear track and transmission electron microscopy (TEM) to characterize the wear debris. It was found that the optimized loading sample for the two different rotational speeds in plain ZDDP oil had better friction and wear properties than the commercial fully formulated oil sample. The overall results indicate that a sample tested at two different rotational speeds under extreme loading (2.77 GPa Hertzian contact pressure) has better wear protection than one tested at a steady 700 rpm rotational speed. Chemistry analysis shows more stable oxide nanoparticles of Fe3O4 in the tribofilm of the plain ZDDP oil (0.1 P%) sample tested under an optimized load of 307 N (2.55 Hertzian contact load) and two different rotational speeds.