Effects of organic friction modifiers on properties of ZDDP tribo-film and friction behavior under boundary lubrication

Lubricants are composed of base oils and variety of additives. In particular, additives such as friction modifiers (FMs) and anti-wear additives (AWs) are important components to control friction and wear of sliding surfaces. Owing to a considerable number of previous studies, the lubrication mechanisms of individual additives has been gradually revealed. However, the chemical interactions between additives in the formulation oils should be investigated since their performances strongly depend on the synergistic or competitive effects between each additive. Zinc dialkyldithiophosphate (ZDDP), which is multifunctional additive to provide anti-oxidancy, seizure resistance and anti-wear, has been widely used for many years as additives in engine oils. It is well-known that the ZDDP controls friction and wear by the characteristic tribo-films on sliding surfaces. There are several reports that the tribo-film characteristic and the lubricity of ZDDP are affected by the combination with other additives. However, the competitive mechanism has not been revealed completely. In this study, the interaction mechanisms between ZDDP and organic friction modifiers were investigated. Friction tests were carried out by using a cylinder-on-disk tribometer. ZDDP, glycerolmono-oleate (GMO) and tallow diethanolamine (TDEA) were investigated as lubricant additives for the base oil of PAO. Surface analyses were conducted on worn surfaces using X-ray photoelectron spectroscopy (XPS). Based on the experimental results, the relationship between the lubricity and the tribo-film are discussed with the interaction mechanism of each additive.