Date of Award
Doctor of Philosophy
The antiwear and antioxidant additives, zinc dialkyldithiophosphates (ZDDPs), are an essential ingredient in engine oil formulations, and have been in use for many years. These additives function by forming antiwear films on surfaces in sliding contact, and thus reduce friction and wear. Among the surface techniques employed, X-ray absorption spectroscopy (including EXAFS and XANES) has been mainly used because of its high resolution, low detection limit and non-destructive characteristics.;This is the first time that P and S L-edge XANES spectroscopy has been used to analyze antiwear films and thermal films. About two hundred antiwear films and thermally prepared films have been measured by XANES spectroscopy. It has been found that longer rubbing time, higher concentration of ZDDP, moderate temperature and higher load help to form long chain phosphates. Sulphur species in most of the cases is sulphide. The presence of sulphate in very short rubbing time or high temperature films is likely from sulphur in base oil.;The chemical composition of additives also affects their behaviour. Secondary ZDDP tends to form long chain polyphosphate while primary ZDDP and aryl ZDDP, in most of the cases, form short chain polyphosphate. The chemical state of sulphur in the same film resembles ZnS or alkyl sulphide. It seems that Zn acts as an antioxidant for sulphur and also catalyses the polymerization of phosphates. These properties are more significant for secondary ZDDP than for primary ZDDP. The presence of detergents and dispersant reduce the amount of unreacted ZDDP on the rubbing surface. The addition of the detergent and dispersant may change the polymerization of phosphate in the antiwear films. The S signals are much weaker than P signals and sulphur is in the reduced sulphide form.;Depth profiling results from XPS showed that the antiwear film formed in 30 minutes is shorter than that formed in 12 hours. The measurements of wear scar widths are general consistent with the XANES study and the results of other researchers. It seems that after film formation in the first a few minutes, further rubbing doesn't increase wear significantly.
Yin, Zhanfeng, "Chemistry Of Antiwear Films By X-ray Absorption Spectroscopy" (1995). Digitized Theses. 2586.