The Correlation of Microchemical Properties to Antiwear (AW) Performance in Ashless Thiophosphate Oil Additives

X-ray absorption near-edge structure (XANES) spectroscopy at macro-scale (mm2) and X-ray photoelectron emissions microscopy (X-PEEM) at micro-scale (m2) have been used to investigate the chemistry and spatial distributions of chemical species in tribochemical films generated from ashless thiophosphate oil additives on steel. Two different ashless thiophosphate additives were used: a triaryl monothiophosphate (MTP) and a dialkyldithiophosphate (DTP). Atomic force microscopy (AFM) and secondary electron microscopy (SEM) were also used to investigate the thickness and the topography of the tribofilms. Macro-scale XANES analysis showed that both ashless thiophosphates reacted with the steel surface to produce short to medium chain polyphosphates as the main constituent and sulfur species as minor component. From the PEEM experiment, it was found that the DTP tribofilm was microchemically heterogeneous, with areas of varying degrees of polyphosphate chain length. Conversely, MTP formed a tribofilm microchemically homogeneous, with areas comprised of only short chain polyphosphates. From, the different areas of polyphosphate chain length within the DTP tribofilm, colour-coded polyphosphate distribution map was generated. AFM, X-PEEM and SEM revealed that the DTP film was thicker and was composed of AW pads that were wider in area than MTP. This resulted in a smaller wear scar width (WSW) value for DTP. This is the first time that all these analytical techniques have been combined to better understand the nature of the tribofilms from ashless additives. We have concluded that an ideal AW film is comprised of a thick film with pad-like structures that are wider in area and microchemically heterogeneous, with areas of varying polyphosphate chain length.     

Study of the Chemistry of Films Generated from Phosphate Ester Additives on 52100 Steel Using X-ray Absorption Spectroscopy

X-ray absorption near-edge structure spectroscopy (XANES) has been used to investigate the chemistry and thickness of thermal and antiwear (AW) films generated on steel from oil solutions containing phosphate ester additives. DPP, a diaryl phosphate, reacted with steel to form a thermal phosphate film at lower temperatures than TPP, a triaryl phosphate and Irgalube 349, an amine phosphate. This phosphate film formation at lower temperatures resulted in better wear protection to the metal in tribochemical experiments, as indicated by a smaller wear-scar measurement for oil solutions containing the DPP additive. For TPP, a brief period of wear to the metal was necessary to initiate the tribochemical reaction between the additive and substrate. Once the tribochemical reaction begins, TPP is able to generate a tribochemical film of relatively the same thickness and chemistry as DPP. Irgalube 349 generated the thickest thermal films at temperatures greater than 150 °C, significantly thicker than any of the films generated from DPP and TPP. The substantial difference in thickness is believed to be due to the availability of alkyl/ammonium cations which enables continued growth of the phosphate film.     

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.     

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     

Spectromicroscopy of tribological films from engine oil additives. Part I. Films from ZDDP's

Antiwear films formed from pure neutral alkyl‐ and aryl‐ZDDP's, and a commercial ZDDP, have been studied with high resolution synchrotron‐based photoemission spectromicroscopy with a new instrument, MEPHISTO. Good P L‐edge XANES spectra have been taken on areas between 12 and 400 μm², and good images of phosphates and ZDDP have been obtained at ∼1 μm resolution on both smooth and rough steel. These spectra, and corresponding images, show immediately that both the chemistry and the morphology of the alkyl and aryl films are very different. The alkyl film contains a range of smaller and larger protective polyphosphate pads from a few to ∼25 μm² in area. We have shown that the chemistry of small and large pads are different. The large pads contain very long chain polyphosphate; while the smaller pads contain short chain polyphosphate. The aryl films contain ortho‐ or pyro‐phosphates, are much thinner and more uniform, with obviously more streaking from initial wear, and no obvious protective pad formation. Antiwear films generated from the commercial ZDDP, rubbed in base oil, show that the long chain polyphosphate is converted to ortho‐ or pyro‐phosphate, but the amount and distribution of phosphate does not change noticeably. The antiwear films are remarkably stable physically. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Morphology and Nanomechanical Properties of ZDDP Antiwear Films as a Function of Tribological Contact Time

We have employed scanning force microscopy (SFM) and nanoindentation analysis to track the evolution of tribologically generated antiwear films derived from zinc dialkyldithiophosphate (ZDDP) as a function of rubbing time. The SFM images reveal that film morphology evolves with time through a growth mechanism consisting of three stages. In the first stage nucleation on active sites at the steel surface leads to the growth of distinct segregated islands. In the second stage the islands coalesce causing the film to spread over a larger fraction of the surface. In the final stage continuous rubbing induces the large islands to divide into smaller, densely packed structures. In contrast to the observed morphological changes, rubbing time does not strongly influence the nanomechanical properties of the films. This highlights the importance of film morphology in determining the effectiveness of ZDDP antiwear films. We also observe large variation in both the morphology and nanomechanical properties that are likely due to the heterogeneity in contact pressure at the pin-sample interface of the wear rig.     

含硫极压抗磨添加剂在菜籽油中的摩擦学性能研究

利用四球磨损试验机考察了以硫化异丁烯和硫化棉籽油作为菜籽油极压抗磨添加剂时的摩擦学性能,通过测定2种添加剂在不同含量下的最大无卡咬负荷(pB)和不同条件下的磨斑直径(WSD),分析和研究了载荷、摩擦时间、添加剂含量对菜籽油摩擦学性能的影响。试验结果表明:硫化异丁烯可以明显提高菜籽油的承载能力和抗磨性能,硫化棉籽油对提高菜籽油的承载能力和抗磨性能效果不明显,硫化异丁烯在菜籽油中的承载能力和抗磨性能明显优于硫化棉籽油。试验还表明添加剂的含量并非越高越好,否则WSD值将增大。     

硫、磷系添加剂复合使用在菜籽油中的抗磨性能研究

以菜籽油为基础油,在四球摩擦磨损实验机上分别考察丁两组硫系和磷系添加剂T321与T304、T321与T307复合使用对菜籽油的抗磨性和极压性的影响。结果表明,在菜籽油叶：加入添加剂T321与T304能更好地提高菜籽油的抗磨性能和极压性能。     

Chemical and Mechanical Properties of ZDDP Antiwear Films on Steel and Thermal Spray Coatings Studied by XANES Spectroscopy and Nanoindentation Techniques

X-ray Absorption Near Edge Structure (XANES) has been used to characterize the chemistry of tribochemical wear pads generated from a paraffinic base oil with a zinc-dialkyl-dithiophosphate additive on steel surfaces and high-velocity oxygen fuel (HVOF) thermal spray coatings. The phosphorus K- and L- edge XANES spectra show that the tribofilms formed on steel and the HVOF coatings have the same chemical nature. Also, mechanical properties of these tribofilms were examined by nanoindentation techniques using both Hysitron and Interfacial Force Microscopy (IFM) instruments. The elastic moduli extracted from indentation force-displacement (f-d) curves have demonstrated that tribofilms on steel and HVOF coatings have similar surface topography and mechanical properties.     

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.     

Chemomechanical Properties of Antiwear Films Using X-ray Absorption Microscopy and Nanoindentation Techniques

The first chemomechanical comparison between an antiwear film formed from a solution containing zinc dialkyl-dithiophophates (ZDDPs) to a solution containing ZDDP plus a detergent (ZDDPdet) has been performed. X-ray absorption near-edge structure (XANES) analysis has shown a difference in the type of polyphosphate between each film. The ZDDPdet film has been found to contain short-chain polyphosphates throughout. X-ray photoelectron emission microscopy (X-PEEM) has provided detailed spatially resolved microchemistry of the films. The large pads in the ZDDP antiwear film have long-chain polyphosphates at the surface and shorter-chain polyphosphates are found in the lower lying regions. The spatially resolved chemistry of the ZDDPdet film was found to be short-chain calcium phosphate throughout. Fiducial marks allowed for the re-location of the same areas with an imaging nanoindenter. This allowed the nanoscale mechanical properties, of selected antiwear pads, to be measured on the same length scale. The indentation modulus of the ZDDP antiwear pads were found to be heterogeneous, ~120 GPa at the center and ~90 GPa at the edges. The ZDDPdet antiwear pads were found to be more uniform and have a similar indentation modulus of ~90 GPa. A theory explaining this measured difference, which is based on the probing depths of all techniques used, sheds new insight into the structure and mechanical response of ZDDP antiwear films.     

二烷基二硫代氨基甲酸锑在聚α-烯烃中的摩擦性能及其摩擦化学作用机制的研究

合成了2种二烷基二硫代氨基甲酸锑,并经红外、核磁对其分子结构予以确认。研究了它们的热稳定性,系统地考察了其在聚α-烯烃（PAO）中不同含量、不同载荷下的摩擦学性能。探讨了二烷基二硫代氨基甲酸锑的抗磨作用机制。结果表明,二烷基二硫代氨基甲酸锑盐的热分解温度较高,热稳定较好,具有良好的极压抗磨性能。EDS分析表明,2种添加剂存在的润滑条件下,钢球磨斑表面上存在着S、Fe、Sb等元素。XPS分析表明,S元素以硫酸亚铁及少量的FeS形式存在,Sb元素以Sb2O3形式存在,而N元素则以复杂的吸附膜形式存在,这些因素一同起极压抗磨作用。     

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.     

Nanotribological Studies of Chromium Thin Films

Systematic nanotribological studies of Cr thin films using nanoscratch and AFM techniques are presented. Constant and ramped loading scratches were made using a Nano Indenter II system at various loads (1mN, 2.5mN and 5mN). Extensive AFM studies of the scratch wear tracks have been performed after scratching. The dependence of the displacement, residual wear depth, percent elastic recovery, and friction coefficient on load in constant load and ramped load tests is compared. Under the same (maximum) load, constant load tests exhibit higher displacements, residual depths and friction coefficients but lower percent elastic recoveries. Detailed AFM observations of the wear tracks indicate that significant differences in lateral deformation accompany the observed displacement differences.     

A Method for the Measurement of Hydrodynamic Oil Films Using Ultrasonic Reflection

The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work, a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed.     

The Tribological Chemistry of the Oil Blends of Zinc Dialkyldithiophosphate with 2-Mercaptobenzothiazole Derivatives

X-ray absorption near-edge spectroscopy at the phosphorous and sulfur edges was used to identify the chemical species in thermal films and antiwear films on the macroscale. For the thermal films, it was found that the introduction of the additive MBTT/MBTA to the base oil, zinc dialkyldithiophosphate (ZDDP), tended to retard the formation of the polyphosphate, and has no effect on the thickness of the films. The topmost surface thermal films may be mainly comprised of some new compounds generated from the interaction of ZDDP with the antiwear additives MBTT/MBTA, along with a small amount of unchanged ZDDP, the sub-surface, and the bulk were mainly comprised of Zn phosphate, along with an amount of ZnS. For the antiwear films, the introduction of the additive MBTT/MBTA to the base oil, ZDDP, reduces the chain length of polyphosphate, which the N-containing decomposed products of 2-mercaptobenzothiazole derivatives are responsible for. It is very clear that MBTT or MBTA has indeed interacted with ZDDP to form new phosphate in the overall antiwear films. The topmost surface antiwear films were mainly comprised of short-chain polyphosphate, the sub-surface and the bulk may be mainly comprised of Zn phosphate and ZnS.     

面接触油膜润滑测量系统

介绍了一种能够实现面接触油膜润滑测量的试验系统,该系统以静止的刚性滑块和旋转的透明玻璃盘组成润滑副。滑块的定位采用柔性并联机构,可进行面接触倾角的微小调节。油膜厚度测量采用多光束干涉技术。照明采用激光外部同轴照明,可有效扩大图像视场和膜厚的测量范围。利用该系统测量了恒倾角面接触薄膜润滑的膜厚特性和摩擦力特性,结果与已有理论有较好的一致性。     

Combination of ashless antiwear additives with metallic detergents: interactions with neutral and overbased calcium sulfonates

The interactions of neutral and overbased calcium sulfonate detergents with ashless thiophosphate oil additives under boundary lubrication were studied. The ashless additives used were neutral and acidic dialkyldithiophosphate (DTPs) and neutral triaryl monothiophosphate (MTP). This study uses three surface analytical tools to provide elemental and chemical information at the surface and in the bulk of the derived tribochemical films. The elemental composition of the tribofilms was studied using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDX) (combined with scanning electron microscopy (SEM)). Corresponding P and S X-ray absorption near edge structure (XANES) spectroscopy was also used to provide further insight into the chemical composition of the films. SEM images of the derived tribofilms revealed that each film had distinct topographical features associated with it. XPS and EDX revealed that when oil is blended with calcium sulfonate, considerable amount of calcium is incorporated into all the tribochemical films. The phosphorus content of the tribofilm was reduced substantially when the overbased detergent was combined with MTP additive. XANES spectroscopy of the P L-edge provided direct evidence for the formation of calcium phosphate in tribofilms from the ashless additives in combination with the detergent.S K-edge revealed that sulfate is the main sulfur species formed in the bulk for all three tribofilms in the presence of the neutral detergent while a mixture of sulfite and sulfide species are formed when the overbased was used. S L-edge XANES showed that calcium sulfonate has undergone some oxidation at the surface. A more antagonistic effect was observed for MTP, with the formation of a very thin phosphate film.Tribological performance was also evaluated. Surprisingly, combination of the neutral detergent with any of the AW additives did not result in any significant change in wear to the substrate. For MTP plus neutral detergent, the thinner phosphate film produced, combined with very little change in wear protection confirms that, not only is the tribochemistry dominated by calcium sulfonate, but also confirms the anti-scuffing and AW properties that are associated with it as well. Even more surprising, was the significant decrease in wear when the overbased detergent is used. This illustrates not only that the tribochemistry was dominated by the detergent, but also the exceptional AW properties of calcium carbonate.     

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.