Tribochemistry of Boron-Containing Lubricant Additives on Ferrous Surfaces for Improved Internal Combustion Engine Performance

Global concerns for the environmental impact of pollutants from automotive sources require considerable reduction of phosphorus and sulphur-based antiwear additives in lubricating oils. One of such additives used as antiwear/extreme pressure (EP) additives in lubricating oils is Zinc Dialkyl DithioPhosphate (ZDDP). Potential replacements for ZDDP are antiwear/EP boron-based additives. In this study, a comprehensive evaluation of the tribological properties of model oils of different types of borate antiwear additives are considered for comparison to ZDDP on steel surfaces in tribo-contact. In this thesis, tribological experiments in pure sliding under boundary lubrication conditions were performed using pin-on-reciprocating plate test rig with variations of: additive concentrations in the oil, bulk-oil temperature, sliding process, dissolved, and free water contamination tests. The coefficient of friction response and antiwear performance of tribofilms were evaluated. Bulk oil analysis of the model oils were performed to evaluate their response to different thermal and oxidative conditions in comparison to ZDDP. In addition, chemical characterization of key crystalline boron compounds was done. The physical and chemical aspects of tribofilms generated during tribological tests were evaluated using surface analysis techniques such as: Optical white-light interferometry, Atomic Force Microscopy (AFM), Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Nanoindentation. One of the key findings of this study is that tribofilms from hydrolytically unstable borates additives gave poor antiwear performance compared to ZDDP and other synthetic borate additives with better resistance to hydrolysis. This study has revealed that boric acid is not directly responsible for the poor antiwear performance as previously understood. Tribochemistry results by this thesis has shown that high atomic concentration of boron and particles such as; boron nitride and carbides acts as third body abrasives, are responsible for the poor antiwear performance. The established antiwear mechanism of borate tribofilms relies on the digestion of abrasive iron oxides by trigonal structural groups in boron oxide. Results at different test conditions from this research have indicated that boric acid, iron oxyhydroxide, and tetragonal structural units in boron oxide plays a major role in this process. The established friction reducing mechanism of boron-containing tribofilms relies on the weak van der Waal’s of boric acid and passivation of its high energy edge-sites by moisture. Another major finding of this PhD thesis is that decomposition and volatility of boric acid at certain temperatures affects the easy shear of the lamellar. An important finding from this study indicated that the abnormal behaviour of boron oxide effect in borosilicate glass manufacture could also affect the antiwear performance and durability of tribofilms containing nanoparticle alkali borate ester. In addition, the hardness of built-up tribofilms from oils containing hydrated potassium borate was found to be comparable to ZDDP, unlike organic borates which gave significantly higher hardness. Another major finding of this research is that in moisture-rich atmosphere, borate tribofilms formed on ferrous surfaces was more wear-resistant than ZDDP due to their different tribochemistry.