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.

The growth and morphology of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and an ashless dialkyldithiophosphate (DDP) over a wide range of rubbing times (10 s to 10 h) and concentrations (0.1-5 wt% ZDDP), have been examined using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy at the O, P and S K-edges and the P, S, and Fe L-edges. The physical aspects of the growth and morphology of the tribofilms will be presented in Part I and the chemistry of the films will be discussed in Part II. The major components of all films on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 Å thick) forms after 10 s, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600-800 Å after 30 min of rubbing, before leveling off or decreasing. The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations have also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed. The AFM results show that ZDDP forms larger and better developed ''pads'' than DDP at short rubbing times. At longer rubbing times, both films become more uniform. For the 1 h ZDDP films, the film thickness is surprisingly independent of the ZDDP concentration from 0.1 to 5 wt% ZDDP. The film thickness is also independent of the ratio of ZDDP/DDP concentrations.

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The chemical interaction of two kinds of dispersants (bis-succinimide dispersant and borated bis-succinimide dispersant) with four kinds of antiwear (AW) and extreme pressure (EP) additives (zinc dialkyldithiophosphate, dialkyldithiophosphate ester, diphenylphosphate ester and dialkyldithiocarbamate) has been investigated under different contact pressures. The chemical compositions of the tribofilms have been studied by B, N, P and S X-ray absorption near edge structure (XANES) spectroscopy. The N K-edge XANES analysis has been used to follow the reaction pathway of amine and imide functional groups in the dispersants and their interactions with EP and AW additives. It has been found that AW additives react with amine to form amine phosphate at low load. However, at high load, there is a good evidence for the formation of a nitrate phase in the tribofilms, the first direct observation of oxidative dispersant loss in the rubbing contact. On the other hand, EP additives behave differently and in general are less reactive. The B K-edge XANES has been employed to follow the interaction of borated dispersant with the EP and AW additive. In general, boron originally in the trigonal coordination, is converted to a tetrahedral coordination form in the process of tribofilm formation.

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