Marina L. Suominen Fuller

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.

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.

Mechanisms of tribochemical film formation: stabilityof tribo- and thermally-generated ZDDP films

Phosphorus L-edge and sulphur L-edge X-ray absorption near-edgestructure (XANES) spectroscopy has been used to characterize thechemical nature of tribochemical and thermo-oxidativelygenerated films from a sec-ZDDP antiwear agent. The chemicalstability of the films has been investigated by rubbing the filmsin base oil without ZDDP. The P L-edge XANES spectra have shownthat the thermal film and in particular the tribo-films are verystable after rubbing in the base oil for a long period of time.The wear scar measurements indicate that best results are givenif the coupon and pin is coated with a tribo-film and then rubbedin oil containing ZDDP.

X-Ray Absorption Study of the Effect of Calcium Sulfonate on Antiwear Film Formation Generated From Neutral and Basic ZDDPs: Part 1—Phosphorus Species

Zinc dialkyldithiophosphates (ZDDPs) from very effective antiwear films in boundary lubrication applications. In most cases, however, the ZDDPs do not work alone. They are formulated with many other additives to provide the performance required by todays modern oils. X-ray absorption near-edge spectroscopy (XANES) has been used to study the antiwear films formed from the commonly used combination of ZDDP and calcium sulfonate in both neutral and basic forms. The results are presented in two papers: Part 1 for the phosphorus species and Part 2 for the sulfur species. XANES showed conclusively that in the presence of LOB (low overbased) or HOB (high overbased) calcium sulfonate under sliding conditions, ZDDPs do not form long-chain polyphosphates that have been associated with antiwear action. Instead, short-chain polyphosphates calcium phosphate are formed. The relative amounts of calcium phosphate formed depend on the ester group of the ZDDP: aryl > n-alkyl > sec-alkyl. Interestingly, this order of ester groups is inversely related to the antiwear effectiveness of the ZDDPs. Thus, it is probable that the addition of either LOB or HOB calcium sulfonate to ZDDP will result in a decrease in antiwear effectiveness of the additive mixture compared to the ZDDP by itself. Wear data support this conclusion. It is suggested that the elimination of long-chain polyphosphates and the formation of calcium phosphates in the tribofilm leads to this decrease in antiwear effectiveness, the latter by abrasion of the antiwear film. Presented at the 55th Annual Meeting in Nashville, Tennessee May 7–11, 2000

X-ray absorption spectroscopy of antiwear films on aluminum alloys generated from zinc dialkyldithiophosphate

The boundary lubrication of Al alloys 6061 (Al-6% Si) and A-390 (Al-18 wt% Si), by a sec-ZDDP (zinc dialkyldithiophosphate) oil blend was examined at 60 and 100°C. The wear performance in the ZDDP blend was an improvement on base stock. At 100°C the ZDDP prevented scuffing, though no antiwear films could be detected due to the severe wear. At 60°C all samples showed initial scuffing followed by a more controlled wear given by an embedded antiwear film. P L-edge X-ray near-edge structure (XANES) spectroscopy using synchroton radiation was used to characterize the antiwear and thermally generated deposition films on the Al alloys. P L-edge XANES results of the antiwear films at 100°C showed the absence of a polyphosphate film. A well-developed polyphosphate film was indicated at 60°C, very similar to that formed on steel at 100°C. P L-edge XANES results of the thermal deposition films showed the presence of only unchanged sec-ZDDP at 125°C with no polyphosphate present. A polyphosphate film was present at 200°C, similar to the antiwear film for Al alloy at 60°C and steel at 100°C.

X-Ray fluorescence spectroscopy and mapping using excitation from white and broad bandpass synchrotron radiation

The bend magnet VESPERS beamline on the third generation CLS synchrotron has been equipped with capabilities for X-ray fluorescence (SXRF) excitation with micro-focussed white radiation, as well as with broad and narrow bandpass monochromatised radiation. Using the former two conditions, SXRF studies have been conducted on several reference materials and metals with known elemental concentrations. The resultant spectral line shapes have been analysed using newly developed software that facilitates ready identification of K, L and M lines present as well as their subsequent spatial mapping. Using both white and broad bandpass radiation, K lines for elements from magnesium to indium and L lines from hafnium to uranium were measured and sensitivity values (S) determined. White radiation provided much higher sensitivity for most elements, and a narrower range of S values makes it possible to use a single white radiation experiment to determine a wide range of elements in the periodic table. Additionally, the variances in S values for several low Z matrices were relatively low, thus making it possible to estimate elemental concentration ranges using a reference material.

X-ray Micro Laue Diffraction and Neutron Diffraction Analysis of Residual Elastic Strains in a 1% Uniaxial Tensile Tested Nickel Alloy 600 Sample

The magnitude and distribution of elastic strain for a nickel alloy 600 (A600) sample that had been subjected to uniaxial tensile stress were measured by micro Laue diffraction (MLD) and neutron diffraction techniques. For a sample that had been dimensionally strained by 1%, both MLD and neutron diffraction data indicated that the global residual elastic strain was on the order of 10 −4 , however the micro-diffraction data indicated considerable grain-to-grain variability amongst individual components of the residual strain tensor. A more precise comparison was done by finding those grains in the MLD map that had appropriate oriented in the specific directions matching those used in the neutron measurements and the strains were found to agree within the uncertainty. Large variations in strain values across the grains were noted during the MLD measurements which are reflected in the uncertainties. This is a possible explanation for the large uncertainty in the average strains measured from multiple grains during neutron diffraction.

The Study of Stress Corrosion Cracking on Alloy 600 C‐Ring Samples by Polychromatic X‐Ray Microdiffraction

Microscopic strains associated with stress corrosion cracks have been investigated in stressed C-rings of Alloy 600 boiler tubing. Polychromatic X-ray Microdiffraction (PXM) was used to measure deviatoric strain tensors and the distribution of dislocations near cracks that had been propagated in electrochemically-accelerated corrosion tests. Stress corrosion cracking (SCC)-generated intergranular cracks were produced in two Alloy 600 specimens after 6h and 18h tests. The diffraction patterns and resultant strain tensors were mapped around the cracked area to a one micron spatial resolution. The strain tensor transverse to the crack growth direction showed tensile strain at the intergranular region just ahead of the crack tip for both specimens. Both cracks were found to follow grain boundary pathways that had the lowest angle of misorientation. Dislocation distributions within each grain were qualitatively obtained from the shapes of the diffraction spots and the effect of “hard” and “soft” grains on the crack pathway was explored for both 6h and 18h specimens.