The use of X‐ray absorption spectroscopy for monitoring the thickness of antiwear films from ZDDP

X‐ray absorption near edge structure (XANES) spectroscopy at the P K‐edge was used to monitor ZDDP antiwear film thickness with rubbing time. Thermal immersion films of varying thickness were generated from the ZDDP and analysed using XANES spectroscopy and the particle induced X‐ray emission (PIXE) technique. P K‐edge XANES edge jumps and (1s → np) peak heights of the spectra were plotted against PIXE mass thickness values in order to establish calibration curves. Antiwear films were analysed using XANES spectroscopy, and average mass thicknesses were extrapolated from the calibration curves. A set of antiwear films formed in the presence of ZDDP and then further rubbed in base oil (no ZDDP) showed no significant decrease in film thickness. A set of antiwear films rubbed in the presence of ZDDP for various lengths of time showed an increase in film thickness, followed by thinning of the film. The decrease in film thickness is believed to be due to wear caused by the ZDDP solution decomposition products acting as an abrasive in the contact region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum–silicon alloys under cylinder/bore wear conditions

summary  Understanding the lubrication of aluminum–silicon (Al–Si) alloys (>18 Si) under conditions similar to those in the cylinder/bore system is vital to determining their applicability to current engine designs. A novel investigation of the location of zinc-dialkyl-dithiophosphate (ZDDPs) antiwear (AW) film formation on an Al–Si alloy has been performed using X-ray absorption near edge structure (XANES) analysis, X-ray photoelectron emission spectroscopy (X-PEEM), and imaging nanoindentation techniques. A study of the initial stages of wear (10 min) to prolonged rubbing (60 min) was performed. The findings show that the film forms primarily on the raised silicon grains and is consistent with a zinc polyphosphate glass. The film has an elastic modulus of ~70 GPa and a similar elastic response to a ZDDP AW film formed on steel under the same conditions. This provides the first direct observation and characterization of a ZDDP antiwear film on Al–Si alloys using spatially resolved chemical and mechanical techniques at the nanoscale.     

Characterization of Tribofilms Generated from Serpentine and Commercial Oil Using X-ray Absorption Spectroscopy

The chemical and tribological properties of serpentine particles suspended in lubricating oil were investigated using a pin-on-disk high frequency friction machine at 100 °C. The wear scar width of the upper steel pins was measured by an optical microscope. The tribofilm was characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) elemental mapping, and X-ray absorption near-edge structure (XANES) spectroscopy. It was found that the addition of serpentine to commercial engine oil improves its tribological properties. The SEM and EDX elemental mapping shows that a tribofilm formed by the commercial oil with serpentine contains silicon, magnesium, oxygen, phosphorus, sulfur, zinc, calcium, and carbon on the worn surface, which is different from the tribofilm formed by the commercial oil without serpentine. The results of the XANES analysis show that the addition of serpentine to the commercial oil changes the chemical compositions of the tribofilms. This change may account for the better tribological properties of the lubricating oil containing serpentine. The formation mechanism of the tribofilm is discussed.     

Improvement of PEEM images from thick inhomogeneous antiwear films using a thin Pt coating

Antiwear films formed from zinc dialkyl-dithiophosphate, in base oil, are known to create inhomogeneous agglomeration of patches on metallic surfaces up to 400 nm thick. It has been found that these patches (termed antiwear pads) are also non-conducting. These two features create difficulties in analyzing data obtained using X-ray photoelectron emission microscopy (X-PEEM). Topography and near-surface charging dominate images obtained using X-PEEM techniques, which can alter electron trajectories and lower signal-to-noise counts. It has been found that the application of a thin continuous platinum coating provides sufficient neutralization to eliminate the positive charge-buildup and improve signal-to-noise. This improves data analysis even with the thickest pads. Examples of charging alleviation and improved signal-to-noise ratios (obtained in the P L-edge spectroscopy) are shown. Furthermore, data analysis of the spectromicroscopy stacks show improved fitting and better polyphosphate distribution mapping for the films.     

Nanometer Scale Chemomechanical Characterization of Antiwear Films

We report the first nanometer scale chemical and mechanical (chemomechanical) characterization of selected features of a tribologically derived zinc dialkyl-dithiophosphate (ZDDP) antiwear film. AFM permits identification of the features responsible for preventing wear. These features are identified by nearby microscale fiducial marks, and their mechanical properties are determined by imaging nanoindentation. The same features are then studied by X-ray photoelectron emission microscopy (X-PEEM), which provides both elemental and chemical information at 200 nm spatial resolution. The mechanical properties are then determined for the same features, which are formed of a polyphosphate glass. This information provides new insights into the mechanisms by which ZDDP antiwears films are effective at inhibiting asperity contact between two metal surfaces     

X-ray Absorption Spectroscopy and Atomic Force Microscopy of Films Generated from Organosulfur Extreme-Pressure (EP) Oil Additives

This study examines the interaction of sulfur-based oil additives on steel. Sulfurized isobutylene, dialkyldithiocarbamate and sulfurized esters were the additives investigated in this report. For the first time, X-ray absorption near edge structure (XANES) spectroscopy has provided detailed insight into the chemistry of both the thermo-oxidative and tribochemical films generated from these additives. It was found that the chemical nature of these films was strongly dependent on the operating environment for the additives. The XANES revealed that thermally, all three S additives reacted very similarly with steel to form a film mainly comprised of iron sulfate at temperatures as low as 100 °C. The ample supply of diffused oxygen from the base oil along with oxide naturally present on the substrate allowed for the complete oxidation of the S from the decomposed additive to iron sulfate. Tribochemical films were comprised of different forms of sulfur than observed for the thermo-oxidative films. The moderate AW conditions yielded a mixture of both oxidized and reduced forms of sulfur, with pyrite, FeS₂, being the major constituent. Rubbing between the steel pin and the substrate partially depleted the oxide layer present, allowing the additive to interact intimately with the fresh substrate, yielding FeS₂. Under extreme-pressure conditions, complete removal of the oxide layer occurred with a drastic increase in the interfacial temperatures between the pin and v-block, allowing for the complete thermal decomposition of FeS₂ to FeS to occur. AFM imaging of the AW films revealed the presence of tiny smooth domains randomly oriented, which were completely different from the pad-like structures observed for AW films generated from a typical ashless thiophosphate additive. The inability of the sulfur-based additives to form large pad-like structures, which can ultimately support the load, resulted in poor AW protection to the metal.     

Antiwear film formation of neutral and basic ZDDP: influence of the reaction temperature and of the concentration

Both synchrotron radiation-based techniques (XANES) and transmission electron microscopy (EDX, EELS) are used to draw a comparison of antiwear and thermal films generated from neutral and basic ZDDP salts. Antiwear films were created in a pin-on-flat wear machine and the wear debris was collected. The analysis of the tribofilms did not show any substantial difference between neutral and basic ZDDPs. The wear scar diameter and the P and S chemical environment in the tribofilm were very similar. The chemical analysis of the wear debris revealed differences in the chemical composition. Wear debris from basic ZDDP seems to be mostly composed either of unreacted ZDDP or of a linkage isomer of ZDDP (LI-ZDDP), and zinc polyphosphate; whereas the wear debris as far as neutral ZDDP is concerned seems to be exclusively composed of zinc polyphosphate (and sulphur species). More iron was also detected in the wear debris with basic ZDDP – possibly an indication of the iron content of the tribofilm. Differences in chemical structure could also be detected in the thermal films. While neutral ZDDP reacted with the surface to form polyphosphates at 150°C, the same reaction products were obtained with basic ZDDP at 175°C. The concentration of ZDDP in oil is thought to be the main parameter to explain the differences in the thermal film formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-Ray Absorption Spectroscopy and Morphology Study on Antiwear Films Derived from ZDDP Under Different Sliding Frequencies

X-ray absorption near-edge structure (XANES) spectroscopy has been used to characterize the chemistry of antiwear (AW) films generated from mineral base oil containing a zinc dialkyl dithiophosphate additive. These films were formed on rubbed steel surfaces with a reciprocating boundary contact using different sliding frequencies. The phosphorus L-edge XANES spectra show that these films have slightly different chemical natures. Longer chain polyphosphates were present on the steel surface prepared at the higher sliding frequencies. The surface morphology of these films was investigated using atomic force microscopy. These images show that the surface morphology of the AW films changes with the sliding frequency. Round and bigger antiwear pads were formed at a lower frequency while higher frequencies resulted in thinner films and flattened surfaces. Nanomechanical properties of these antiwear films were investigated by nanoindentation measurement and the elastic moduli extracted from force–displacement (f–d) curves are similar for all antiwear films, 100 ± 10 GPa.     

Tribological behavior and tribofilm composition in lubricated systems containing surface-capped molybdenum sulfide nanoparticles

The tribological behavior of surface-capped MoS₃ nanoparticles (nano-MoS₃) in hydrocarbon oils was studied both alone and in combination with ZDDP. It was found that the nano-MoS₃ additive alone demonstrates pro-wear properties and decreases the friction coefficient only at high temperatures. The combination with ZDDP demonstrates synergism in antiwear and antifriction activity even at low ZDDP content. X-ray Absorption Near Edge Structure (XANES) spectroscopy at the sulfur, molybdenum and phosphorus edges was used to identify the chemical species in tribochemical films. It was established that the nano-MoS₃-formed tribofilms are composed of oxidized sulfur and molybdenum species while tribofilms formed by combination with ZDDP are composed of phosphate layers incorporating MoS₂-type fragments providing friction reduction.     

Interpretation of experiments on ZDDP anti-wear films through pressure-induced cross-linking

We review a recently developed molecular-level theory for the formation and functionality of zinc dialkyldithiophosphate anti-wear films [N. J. Mosey, M. H. Müser and T. K. Woo, Science 307 (2005) 1612]. This theory is based on the idea that pressure-induced cross-linking leads to chemically connected networks. The formation of cross-links modifies the mechanical properties of the films such that wear inhibition may be enhanced. Furthermore, the networks remain intact upon release of the pressure, which resists flow of the film out of the contact area. The ability of the theory to account for a diverse body of experimental data related to anti-wear additives and films is discussed. Routes towards the development of new AW additives are also suggested on the basis of the theory.