Abstract
Ionic liquids (ILs) have recently been developed as a novel class of lubricant anti-wear (AW) additives, but the formation mechanism of their wear protective tribofilms is not yet well understood. Unlike the conventional metal-containing AW additives that self-react to grow a tribofilm, the metal-free ILs require a supplier of metal cations in the tribofilm growth. The two apparent sources of metal cations are the contact surface and the wear debris, and the latter contains important ‘historical’ interface information but often is overlooked. We correlated the morphological and compositional characteristics of tribofilms and wear debris from an IL-lubricated steel–steel contact. A complete multi-step formation mechanism is proposed for the tribofilm of metal-free AW additives, including direct tribochemical reactions between the metallic contact surface with oxygen to form an oxide interlayer, wear debris generation and breakdown, tribofilm growth via mechanical deposition, chemical deposition, and oxygen diffusion.
Highlights
ZDDPs are believed to decompose and self-react to deposit a tribofilm primarily composed of zinc and iron phosphates and oxides[16,17,18]
We investigated the tribofilms and wear debris particles generated in a steel–steel sliding contact lubricated by a base oil containing a phosphonium–phosphate Ionic liquids (ILs)
The IL tribofilm formation is proposed as a multi-step process: direct tribochemical reactions between the metallic contact surface with oxygen to form an oxide interlayer, wear debris generation and breakdown, tribofilm growth via mechanical deposition, chemical deposition, and oxygen diffusion
Summary
ZDDPs are believed to decompose and self-react to deposit a tribofilm primarily composed of zinc and iron phosphates and oxides[16,17,18] Such a tribofilm growth model is not directly applicable to ILs, because ILs do not self-supply metal cations. We investigated the tribofilms and wear debris particles generated in a steel–steel sliding contact lubricated by a base oil containing a phosphonium–phosphate IL. This particular IL was selected because of its superior anti-scuffing and wear protection compared with conventional ZDDP or amine-phosphate[8, 11, 19]. Mechanism is expected to shed light on the fundamental understanding of the AW nature of IL and the design of next-generation lubricant additives
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