No.1008E 2011 Boundary Lubrication from the Viewpoint of Surface Chemistry −Role of nascent surface on tribochemical reaction of lubricant additives−

Author Professor Shigeyuki MORI
Iwate University, Faculty of Engineering, Department of Chemistry and Bioengineering
Abstract Chemical properties of nascent surfaces of tribo-materials were reviewed from the viewpoint of tribochemical activation. In general, material surfaces are covered with metal oxides and organic contaminants, and are chemically stabilized. Once the surface layers are removed mechanically, nascent surfaces are formed at mechanical contact. The chemical nature of the nascent surfaces of metals and ceramics was estimated by a method developed by the author. Chemisorption and surface reactions of model compounds of lubricant components on nascent surfaces were monitored during friction tests in vacuum by a quadrupole mass spectrometer. The chemical activity of nascent steel surfaces can be explained by Pearson's HSAB principle. The chemical nature of nascent aluminum surfaces was found to be different from that of nascent steel surfaces as a transition metal. Nascent ceramic surfaces exhibited such high activity that even hydrocarbons could chemisorb on them. It was found that synthetic-hydrocarbon base oil was decomposed during lubrication tests in a vacuum, and hydrogen and hydrocarbons with low molecular weight were evolved under the effects of the nascent steel surface and temperature rise at the contact. The role of the tribochemical process on boundary lubrication properties of lubricant additives is discussed by the chemisorption activity of nascent surfaces of metals and ceramics based on tribological performance of additives. For example, tribological behavior of extreme pressure additives under severe conditions can be explained by the chemisorption and reaction of EP additives on nascent steel surfaces. In conclusion, the surface chemistry of boundary lubrication is closely dependent on the contact condition.
Keyword boundary lubrication, tribochemistry, nascent surface, adsorption, surface reaction, lubricant additive,
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