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Zinc dithiophosphates (ZnDTPs) are ubiquitous lubricating oil additives in today's passenger car motor oils, providing the important functions of wear and oxidation inhibition. However, the molecular-level mechanism by which these materials reduce wear is not understood. As a first step in developing an understanding of this mechanism, we used ab initio quantum chemical methods to examine the structures, vibrations, and energetics of these systems. The results show that the two phosphorus-sulfur bonds of the dithiophosphate of ZnDTPs are equiValent and have character intermediate between single and double bonds. This contrasts with the paradigm of one double bond (PdS) and one single bond (P-S) often used. Vibrational studies of DTP systems lead to a strong IR transition at about 650 cm -1 and a weak transition at about 530 cm -1 . We find modes in good agreement with experiment, where the high-frequency mode is antisymmetric PS stretch (not PdS), while the lower mode is symmetric PS stretch (not P-S). On the basis of the ab initio calculation results, we used the biased Hessian method to develop a vibrationally accurate force field (FF) for ZnDTPs. This FF can be used to examine the binding of DTPs to metal and metal oxide surfaces.

Zinc dithiophosphate (DTP) molecules have long been used as wear inhibitor oil additives for automotive engines. In order to obtain an atomistic understanding of the mechanism by which these molecules inhibit wear, we examined the geometries, energetics, and vibrations of an oxidized iron surface [(001) surface of α-Fe2O3] using the MSX force field (FF) based on ab initio quantum chemistry (QC) calculations. The DTP molecules studied include (RO)2PS2 with R = methyl, isobutyl, isopropyl, and phenyl. The α-Fe2O3 surface is described using the generalized valence bond (GVB) model of bonding. The geometries, binding energies, and vibrational frequencies from ab initio calculations on simple clusters are used with the biased Hessian method to develop the MSX FF suitable for describing the binding of DTP molecules to the surfaces. We find that the cohesive energies for the self-assembled monolayers (SAM) of the DTP molecules on the Fe2O3 surface correlate with the antiwear performance observed in experimental engine tests. This suggests that the search for more effective and environmentally benign wear inhibitors can use the cohesive energies for SAM formation as a criterion in selecting and prioritizing compounds for experimental testing.

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.

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.

No abstract

In previous studies of dithiophosphate [DTP = S2P(OR)2] wear inhibitors bound to an oxidized iron surface, we found that the cohesive energy of the self-assembled monolayers (SAM) for DTP molecules with various organic R groups correlates with the wear inhibition observed in full engine experiments. In this paper we expand these calculations to consider dynamics at 500 K and then use the SAM model to predict new candidates for wear inhibitors. Using molecular dynamics (MD) simulations at 500 K, we show that the SAM has one DTP per two surface Fe sites of iron oxide. At this coverage we find that the cohesive energy of the SAM at 500 K is in the sequence 2-alkyl > 1-alkyl > aryl (e.g., iPr > iBu > Ph) which again correlates with wear inhibitor performance observed in engine tests. We then considered 7 novel DTPs and predict that R = cyclo-hexyl, nPr, and benzyl may perform as well as iPr. We then used the SAM wear inhibitor model to assess the likely performance of 11 novel classes of potential wear inhibitors. On the basis of this model we selected dithiocarbamates (DTC) as the best candidate to supplement DTP. We then considered a number of possible alkyl substitutions for DTC. The SAM model suggests that iC5 and nC3 are the best candidates, followed closely by iC3.