K.H. Tan

Application of soft X-ray absorption spectroscopy in chemical characterization of antiwear films generated by ZDDP. Part I : the effects of physical parameters

X-ray absorption near edge structure (XANES) spectroscopy has been used to study the chemical nature of the antiwear films generated on steel surfaces using zinc dialkyldithiophosphates (ZDDPs). The spectra were recorded using total electron yield (TEY) and fluorescence yield (FY) to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. In the first part of this study, the effects of physical parameters on the composition and mechanism of antiwear film formation is discussed. It has been found that lower concentration of ZDDP, higher temperature and higher load all increase the rate of ZDDP decomposition; and longer rubbing time, higher concentration of ZDDP, moderate temperature, higher load and smooth surfaces help to form long chain polyphosphates. The sulphur in the film in most of the cases is in the reduced form. The presence of sulphate in very short rubbing times or high temperatures has also been detected. When the spectra from the TEY mode and FY mode were compared, a layered structure was found in most of the films. In these films, there is a longer chain polyphosphate on the topmost surface and a shorter chain polyphosphate in the bulk. At short rubbing times and low temperature, unchanged ZDDP is also present in the film. Depth profiling using X-ray photoelectron spectroscopy showed that the antiwear film formed in 30 min is thinner compared with a 12 hour film. Based on the above information, a new mechanism for antiwear film formation is proposed.

Sampling depth of total electron and fluorescence measurements in Si L- and K-edge absorption spectroscopy

High resolution Si L-edge and K-edge X-ray absorption near edge structure (XANES) spectra for SiO2 on Si substrates have been recorded using total electron yield (TEY) and fluorescence yield (FY) techniques. The sampling depths of TEY and FY for Si L-edge and Si K-edge, respectively, have been investigated in the energy range 95–120 eV and 1830–1900 eV. The maximum sampling depth for TEY is found to be ∼ 5 nm for the Si L-edge and ∼ 70 nm for the K-edge regions. The FY sampling depth at the L-edge is ∼ 70 nm whereas for the K-edge, the sampling depth is several hundred nm. Based on these data, and using a theoretical model, electron escape depths for the TEY measurements in both energy ranges have been deduced.

Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using X-ray absorption spectroscopy

X-ray absorption near edge structure (XANES) spectroscopy at the phosphorus L-edge and sulphur L-edge has been used to characterize the chemical nature of tribochemical and thermally generated films from several ZDDP antiwear agents in the neutral and basic forms. Using the P and S L-edge XANES spectra of model compounds with known structure as fingerprints, the chemical structures of P and S species in the films have been identified. P appears in all the films as polyphosphates in different proportions of short and long chain polyphosphates. In some films, polyphosphates are accompanied by unchanged ZDDP. Generally films generated from neutral and basic ZDDPs show similar P and S chemistry (polyphosphates and sulphides) but contain different proportions of unchanged ZDDP. However, the aryl ZDDP films have different polyphosphate structure compared to the alkyl ZDDP films. The sulphur proportion in the tribochemical films is decreased a great deal, but remains in the reduced form. However, S in the thermo-oxidatively generated films, appears both in the reduced and oxidized form, depending on the ZDDP and the temperature.

Solution decomposition of zinc dialkyl dithiophosphate and its effect on antiwear and thermal film formation studied by X-ray absorption spectroscopy

A detailed study was undertaken to investigate the effect of ZDDP oil solution chemistry changes due to thermal decomposition, on antiwear and thermal film chemistries, film thickness and wear. P and S K- and L-edge X-ray absorption near edge structure (XANES) spectroscopies were used to characterize film chemistry, and 31-P NMR spectroscopy was used to monitor the ZDDP oil solution chemistry. P L-edge XANES results of antiwear films prepared from ZDDP oil solutions preheated at 150°C for various lengths of time, showed a decrease in polyphosphate chain length as ZDDP thermal solution decomposition progressed. Film thickness and wear increased with increasing ZDDP oil solution preheating time (decomposition). Antiwear films formed from ZDDP oil solutions preheated at a higher temperature (200°C) for 1 and 3 h, yielded thinner films and showed catastrophic wear. 31-P NMR spectra showed that no oil soluble P containing species were left in solution after heating at 200°C for 1 h and yet the 200°C, 6 h antiwear film was found to be as thick as that generated from previously unheated solution. Wear was comparable to that obtained by using base oil alone. These films were found to be of short chain polyphosphate structure. ZDDP oil solution chemistry was also shown to have an effect on the chemistry of thermally generated films. Film chemistry changed with ZDDP oil solution heating time. A linkage isomer of ZDDP is proposed as an important precursor for film formation after analysis and comparison of an oil insoluble ZDDP decomposition product with the thermal and antiwear film chemistries. As with the related antiwear films, thermal film thickness was also shown to increase dramatically when ZDDP decomposition in solution increased. An overall mechanism for film formation, taking into account the ZDDP linkage isomer and the deposition of colloidal polyphosphate material, is proposed.

Application of soft X-ray absorption spectroscopy in chemical characterization of antiwear films generated by ZDDP Part II: the effect of detergents and dispersants

Abstract X-ray absorption near edge structure (XANES) spectroscopy has been used to study the chemical nature of the antiwear films generated on steel surfaces using zinc dialkyldithiophosphates (ZDDPs), in the presence of detergents and dispersants. The spectra were recorded both in total electron yield (TEY) and fluorescence yield (FY) modes, to investigate the chemical nature of P,S, Ca and O on the surface and in the bulk, respectively. The interaction of a commercial secondary ZDDP with a calcium sulphonate (neutral) detergent, a calcium phenate (basic) detergent and a polyisobutylene succinic anhydride polyamide dispersant has been examined. Using a range of concentrations, it has been found that calcium sulphonate interacts with ZDDP only at high concentration (≥2%), whereas calcium phenate affects the film formation even at low concentrations. The strongest interaction has been found for the polyisobutylene succinic anhydride polyamide dispersant. The polyphosphate films formed in the presence of the these additive have shorter chain length compared with ZDDP alone and contain less unchanged ZDDP. These finding are discussed in the framework of a mechanism for the film formation presented in Part 1 of this study.

The Chemistry of Antiwear Films Generated by the Combination of ZDDP and MoDTC Examined by X-ray Absorption Spectroscopy

In order to obtain a better understanding of the decomposition mechanisms of antiwear and antioxidant engine oil additives such as zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC), tribochemical and thermally deposited films were prepared from oil solutions containing varying concentrations of these two additives. The chemistry of these films was studied by using X-ray absorption (XANES) spectroscopy and X-ray photoelectron (XPS). The antiwear properties were examined on a Plint rubbing machine by measuring the amount of wear as a function of time and the concentrations of the additives. The XANES and XPS spectra indicate that the dominant species from MoDTC in the antiwear films resembles that of a MoS2-like film, and in the thermal films, the dominant species are those of a sulphate and MoO3. The spectra also indicate that the phosphorus from ZDDP forms polyphosphate chains of varying lengths. The combined use of organo-Mo compounds and Zn alkyl dithiophosphates showed a synergi...

Chemical characterization of antiwear films generated on steel by zinc dialkyl dithiophosphate using X-ray absorption spectroscopy

Abstract The addition of antiwear and extreme-pressure agents to lubricants produces protective films on the sliding surfaces. Two of the most common elements in antiwear/extreme-pressure additives are sulphur and phosphorus. These are commonly added in the form of Zinc dialkyl dithiophosphate (ZDDP). P and S L-edge X-ray absorption near-edge (XANES) spectra of the films and several model compounds containing both S and P have been recorded at high resolution, utilizing synchrotron radiation. Using the spectra of the model compounds as fingerprints, it has been possible to speciate the nature of the antiwear/extreme-pressure films. It has been found that the chemical state of phosphorus in the films corresponds closely to that of polyphosphates and depends on the alkyl group used in ZDDP. Sulphur is present mostly in the form of sulphide.

High-resolution Si K- and L2,3-edge XANES of α-quartz and stishovite

Abstract Silicon K- and L2,3-edge XANES of α-quartz and stishovite are reported. The XANES spectra reveal the densities of the unoccupied Si 3s, 3p and 3d states in the band gap and the conduction band, and demonstrate the large difference in the bonding of 4:2 coordinate α-quartz and 6:3 coordinate stishovite. The Si K- and L2,3-edge shifts are also systematically related to the coordination of Si, Si-O bond length, Si-Si distance, Si-O-Si angle, Si-O bond valence and Si NMR chemical shift of α-quartz and stishovite. These XANES spectra may be used as a fingerprint in studying the structure of silicate glasses and melts.

X-ray fluorescence measurements of x-ray absorption near edge structure at the Si, P, and S L edges

High‐resolution (≤0.2 eV) x‐ray absorption near edge structure (XANES) spectra have been recorded at the Si, P, and S 2p edges of several compounds using microchannel plates to detect the ultrasoft x‐ray fluorescence after 2p excitation or ionization. The fluorescence yield (FY) XANES of SiO2, Si, InP, FeS2, and Na2S2O3 are of at least as good quality as the XANES recorded using total electron yield (TEY), despite the extremely small fluorescence yield. The FY spectra obtained show two significant advantages over the TEY spectra. First, the FY XANES is much more bulk sensitive; and second the resolution of the FY spectra is usually considerably better.

Sulphur characterization in coal from X-ray absorption near edge spectroscopy

Abstract High resolution (≤ 0.2 eV) S L-edge X-ray absorption near edge structure (XANES) spectra of several inorganic and organic sulphur model compounds are presented. The spectra were recorded in both total electron yield (TEY) and fluorescence yield (FY) modes. A qualitative interpretation is used to explain the origin of the fine features in the XANES spectra of certain model compounds. The XANES spectra of several coals such as the Rasa coal (Cretaceous, Yugoslavia), P-803 coal (Cretaceous, Canada) and fossilized Latex (Eocene, Germany) have also been recorded. Using the spectra of model compounds as fingerprints, several organic functional groups were identified in untreated coal. These include alkyl and aryl sulphides, alkyl and aryl disulphides, and heterocyclic sulphurs. Sulphonic acid and sulphate were observed for aerially oxidized coal. The S K-edge of several model compounds and coals were also recorded to compare with the L-edge. Advantages and drawbacks of the L-edge and K-edge techniques are described.