Tribochemical Reactions on Surfaces

To understand the wear mechanism in valve train wear tests, samples of the worn tappet surface were analyzed for surface elements by electron probe microanalysis (EPMA) and X-ray photo electron spectroscopy (XPS). Results of EPMA analysis of the worn surface in terms of concentration of phosphorus and sulfur atoms for oil with primary ZnDDP without MoDTC, showed an increase of zinc and sulfur intensity after 100 hrs of test time, in spite of decreasing phosphorus intensity. Examination of the worn surface by XPS with primary and secondary ZDDP with addition of MoDTC showed the presence of MoS2 in the tribofilm. Using mixtures of ZDDP and MoDTC, the friction coefficient is reduced, and wear is comparable to that of using ZDDP alone (Kasrai et ah, 1997). 

In the presence of friction reducing additives in engine oil, a friction coefficient of 0.04 was observed which increased up to 0.12 after the additives had been depleted (Johnson et al., 1997a). 

X-ray Photoelectron Spectroscopy (XPS) tests were conducted on surfaces lubricated with a sulfur-containing extreme pressure additive, dibenzyl sulfide (Baldwin, 1976 Bird and Galvin, 1976). The films can arise from the use of additives that contain sulfur, phosphorus, chlorine, bromine, or boron and the differences in reactivity are affected by the formation of protective layers. Triboinduced electrons are said to activate the formation of iron halides, iron phosphates and iron sulfides (Dorison and Ludema, 1985 Grunberg, 1966 Kajdas, 2001 McFadden et al., 1998 ). When a chemical reaction takes place, e.g., oxygen interacts with aluminum to form aluminum oxide, a large oxygen peak is seen at approximately 500 eV in the Auger electron spectra (Benndorf et al., 1977 Nakayama et al., 1995). 

Phosphorus containing additives include esters of phosphoric acids, derivatives of thiophosphoric acids, phosphites (Davey, 1950 Sakuri and Sato, 1970), and the metal salts of dithiophosphoric acid diesters, the best known of these compounds being the ZDDPs. The structure of ZDDP films, the composition of the film formed, and the mechanism of action on the molecular level are summarized in Chapter 4. 

Lubricants containing chlorinated hydrocarbons are typically used as antiseizure additives in the metalworking industry. Some chlorine-containing hydrocarbons function by producing iron chloride on the metal surface (Kotvis et al., 1991). The correlation between chemical reactivity and load carrying capacity of oil containing extreme pressure additives can be assumed to be as follows  

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