Frictional Behavior and Tribofilm Formation of Organic Friction Modifiers under Severe Reciprocating Conditions

In pursuing environmentally friendly lubrication solutions, it is advantageous to employ organic additives that are free from heavy metals and have low or zero levels of phosphorus and sulfur functionalities. Organic friction modifiers strongly reduce friction and wear when added to engine oils through the formation of an adsorbed boundary film on the contacting surfaces. The mechanism of tribofilm formation and its chemical effects on friction reduction are not entirely understood. In this study, the lubrication mechanism of OFM was investigated with a new approach combining three different acylglycerols with varying ratios. The lubricating performance of mixtures of glycerol monooleate (GMO), glycerol trioleate (triolein), and glycerol dioleate (GDO), as well as individual GMO and triolein in PAO4, was evaluated under the boundary lubrication regime at two temperatures, 60 °C and 100 °C. A synergetic effect on tribological performance has been observed for the mixture formulation. This resulted in lower friction and wear than the single additive in the base oil at both temperatures. The HRTEM analysis indicated that the combination of different acylglycerols provides a thicker tribofilm compared to the single additive. The ToF-SIMS and NEXAFS analyses of the resulting tribofilms showed that at a temperature of 60 °C, the main components of the tribofilm were compounds formed by GMO decomposition and oleate ions, indicating that chemisorption plays a significant role in reducing friction at lower temperatures for the tested OFM additives.