Some mechanisms of tribofilm formation in metal/metal and ceramic/metal sliding interactions

Tribological interaction often generates new structures and materials which form the interface between the sliding pair. The new material designated tribofilm here may be protective or tribologically deleterious. The tribofilm plays a major role in determining the friction and wear of the interaction. Here, we give three examples: mechanically mixed, chemically generated and thermally activated, of tribofilms formed in three different tribological systems and speculate on the mechanism of their formation.

Transition in wear performance for ionic liquid lubricants under increasing load

The effect of ionic liquid (IL) lubrication for aluminium/steel systems is highly dependant on the applied load and the IL structure. This study illustrates that a change in anion of an IL lubricant results in different physicochemical properties that will alter its performance at a given load. As the load is increased there is a shift in lubricant performance and mechanism of the IL. Up to a load of 30 N the lowest wear coefficient was achieved by a phosphonium diphenylphosphate IL, whilst above 30 N a phosphonium bis(trifluoromethanesulfonyl)amide IL was able to form a more tenacious tribolayer that resulted in the lowest wear.

A comparison of phosphorus and fluorine containing IL lubricants for steel on aluminium

Ionic liquids have been shown to be highly effective lubricants for a steel on aluminium system. This work shows that the chemistry of the anion and cation are critical in achieving maximum wear protection. The performance of the ILs containing a diphenylphosphate (DPP) anion all showed low wear, as did some of the tris(pentafluoroethyl)trifluorophosphate (FAP) and bis(trifluoromethanesulfonyl)amide (NTf2) anion containing ILs. However, in the case of the FAP and NTf2 based systems, a cation dependence was observed, with relatively poor wear resistance obtained in the case of an imidazolium FAP and two pyrrolidinium NTf2 salts, probably due to tribocorrosion caused by the fluorine reaction with the aluminium substrate. The systems exhibiting poor performance generally had a lower viscosity, which also impacts on their tribological properties. Those ILs that exhibited low wear were shown to have formed protective tribofilms on the aluminium alloy surface.