Tribofilms generated from ZDDP and DDP on steel surfaces: Part 2, chemistry

Abstract

The chemical constitution of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and ashless dialkyldithiophosphate (DDP), has been examined by X-ray Absorption Near Edge Structure (XANES) spectroscopy. The identification of spectral features and interpretation of the results for P, O, Fe, and S species are given, allowing an overall mechanism to be deduced. The role of Fe in these films was investigated in some detail using P L-edge, O K-edge and Fe L-edge XANES spectra. From the P L-edge XANES spectra, the DDP films are uniformly very short chain iron polyphosphates. In contrast, the ZDDP films are formed initially as short chain polyphosphates; but after more rubbing, a bilayer phosphate film is formed with long chain Zn polyphosphates on the surface and shorter chain in the bulk of the film. The O K-edge XANES spectra show that there is, as expected, more Fe in the DDP phosphate films than in the ZDDP phosphate films. The S K-edge spectra of ZDDP films show the presence not only of ZnS as previously observed, but also the presence of FeS for the first time in the early stages of film formation. The predominant S species in the DDP films is FeS.

Tribochemical study of micropitting in tribocorrosive lubricated contacts: The influence of water and relative humidity

“…Using XPS is not possible to conclusively distinguish between (thio)phosphate and metal sulphide. as ZnO rather than ZnS in agreement with the XANES study of similar tribofilms [55,56].…”

The Influence of Slide–Roll Ratio on ZDDP Tribofilm Formation

“…Many studies on the mechanisms of ZDDP's anti-wear and anti-seizure performance have been carried out under pure sliding conditions using friction machines such as the Shell four-ball, Falex block-on-ring, Optimol SRV and Cameron Print reciprocating rig. In these researches, after rubbing tests the physical and chemical properties of ZDDP tribofilms have been investigated using surface characterisation methods such as atomic force microscopy (AFM), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX) and X-ray absorption near-edge structure (XANES) [1][2][3][4][5][6][7].…”

The Tribofilm Formation of ZDDP Under Reciprocating Pure Sliding Conditions

“…Since zinc dialkyldithiophosphates (ZDDPs) are widely used in engine oils, primarily as anti-wear additives, many researchers have studied their anti-wear and anti-seizure performance under reciprocating pure sliding conditions which simulate piston ring and cylinder liner assembly. In previous research, friction machines such as the Optimol SRV and Cameron Print reciprocating rig have been employed and tribofilms formed on the specimens after rubbing tests have been investigated [3][4][5][6][7][8][9]. Recently, a new approach to monitor ZDDP film formation process under pure sliding conditions using AFM (atomic force microscopy) was introduced by Gosvami et al [10].…”