Effect of Sulfur- and Phosphorus-Containing Additives and Metal Deactivator on the Tribological Properties of Boundary-Lubricated Steel Surfaces

The tribological behaviors of various blends consisting of base oil and additives containing sulfur and phosphorus and a metal deactivator, in conjunction with those of a blend of the same base oil and all the studied additives in the same amounts as in the single blends were investigated at 32°C and 100°C. Balls and flat disks of AISI 52100 steel were tested in baths of the different blends using a ball-on-disk tribometer under a load of 147 N and sliding speed of 0.34 m/s that resulted in sliding in the boundary lubrication regime. Measurements of the coefficient of friction and electrical contact voltage were used to characterize the tribological performance of each blend and to confirm the formation and removal of antiwear tribofilms. Surface profilometry was performed to evaluate the wear resistance of the tribofilms, and scanning electron microscopy to characterize the dominant wear mechanisms. Endurance tests were conducted with the phosphorus-containing blend that presented in-situ evidence of tribofilm formation through the contact voltage response, in order to determine the tribofilm's durability at room temperature in the absence of oil. The results from various tests demonstrate the effects of temperature and additive(s) type on the tribological properties. While the additives reduced the coefficient of friction at 100°C moderately, the effect on the wear resistance at both 32°C and 100°C was more pronounced compared to the base oil. Adhesion, abrasion, surface plastic flow, and microcracking were found to be the prevailing steady-state wear mechanisms, depending on the additive type(s) and oil temperature.