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.     

A variable temperature mechanical analysis of ZDDP-derived antiwear films formed on 52100 steel

The nanomechanical properties of antiwear films formed from zinc dialkyl-dithiophosphates (ZDDPs) on steel have been studied by nanoindentation techniques as a function of temperature. X-ray absorption P K- and L- near edge structure (XANES) spectroscopy has shown that films prepared from oils containing ZDDPs on 52100 steel (pin on flat coupons) consist primarily of medium chain polyphosphates with sulphur (S K-edge) predominantly present as sulphide.Using various scanning probe techniques, high-resolution topographic images and mechanical properties can be extracted at the same length scale. Using focused ion beam (FIB) milling we have compared real cross-sectional film thickness with a value estimated from the P K-edge XANES. We report the first measurements of the elastic modulus of the antiwear films at elevated temperatures relevant to the automobile operating conditions (T ≤ 200 °C). The antiwear films demonstrated a relatively constant indentation modulus over a wide range of temperatures consistent with their efficacy in reducing wear by preventing asperity contact.     

Studies on ZDDP Thermal Film Formation by XANES Spectroscopy, Atomic Force Microscopy, FIB/SEM and 31P NMR

X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of thermal films on steel samples, which were generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive. These films were formed at 150 °C by immersing steel coupons in ZDDP oil solutions. The phosphorus L-edge XANES spectra show that these films are composed of polyphosphates, unreacted ZDDP and other thiophosphate intermediates. Phosphorus K-edge FY XANES was used to monitor the thickness of these films, and the data are consistent with thickness derived by focussed ion beam (FIB) milling and SEM imaging. The sulphur K-edge TEY and FY XANES spectra show that these films are composed of different sulphur components, which depend upon the formation times. The surface morphology of these films was investigated using atomic force microscopy (AFM). These images show that the surface morphology of the thermal films changes with the formation time. 31P NMR spectra show that both primary and secondary ZDDP decomposes gradually at 150 °C.     

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.     

A multi-technique characterization of ZDDP antiwear films formed on Al (Si) alloy (A383) under various conditions

The simulation of the lubrication of aluminum–silicon (Al–Si) alloy cylinder-bore conditions is an important goal in automotive tribology. This study describes the use of X-ray absorption near edge structure (XANES) to determine the macro-chemistry of zinc-dialkyl-dithiophosphates (ZDDPs) antiwear (AW) films formed on A383, an Al-Si alloy. The temperature dependence of the chemistry and mechanical properties were examined using X-ray photoelectron emission microscopy (X-PEEM), and imaging indentation techniques. Our findings suggest that ZDDPs break down to form polyphosphate glasses, which have different chemical natures and depend on the underlying substrate. Furthermore, the chemical nature of the films appears temperature dependent on both the macro- and micro-scale. Not only are the chemical species different, but the mechanical properties also differ, depending on the region upon which an AW pad is formed. Through the use of focused ion beam (FIB) milling, we can determine the film thickness, which was previously estimated from the P K-edge XANES areal density of samples with known thicknesses.     

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.     

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.     

Studies on ZDDP Anti-Wear Films Formed Under Different Conditions by XANES Spectroscopy,Atomic Force Microscopy and 31P NMR

Antiwear (AW) films, generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive, were studied as a function of formation temperature, load and rubbing time. The surface morphology of these films was investigated using atomic force microscopy (AFM), and surface roughness calculated for the observed differing surface morphologies. The morphology of the films is heterogeneous for all the tested conditions, but the surface roughness is dependent on the rubbing condition. X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of these films, and the intensity of the phosphorus K-edge was also used to monitor their thickness. The thickness of these films is in the range of 10–90 nm depending on the running conditions. Phosphorus L-edge spectra show that these films have a similar chemical nature with variable polyphosphate chain-lengths. 31P NMR was used to study the decomposition of ZDDP in the residual oils. The spectra show that the primary and secondary ZDDP react differently under the various conditions. The tribological characteristics of these AW films were probed by measuring the coefficients of friction (μ) and the wear scar width (WSW) of the counter faces. μ is highly related to the applied load and the results of WSW measurements show that the wear performance is related to all the tested parameters, temperature, load and rubbing time.     

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 role of the cation in antiwear films formed from ZDDP on 52100 steel

Phosphorus L-edge and oxygen K-edge X-ray PhotoEmission Electron Microscopy (XPEEM) have been used to characterize the chemical nature of the cation present in tribochemical films via comparison with model Fe²⁺ and Zn²⁺ compounds. The results are contrasted to the P L-edge, P K-edge and S K-edge XANES data. The findings suggest that antiwear pads containing long chain zinc polyphosphate glass are formed at the points of asperity contact, and a thin, short chain zinc polyphosphate film is formed where no asperity contact is made. SEM/EDX measurements helped to elucidate the distribution of the elements, and strong spatial correlations were observed between P, O, Zn and S in the pads, indicating that they are composed mostly of zinc polyphosphates, especially near the surface. The zinc polyphosphate antiwear pads are characterized by a much lower modulus than that observed on the thin film regions, the latter being characteristic of the substrate steel.     

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.     

Relationship between molecular structures and tribological properties of phosphazene lubricants

Cyclotriphosphazene lubricants were synthesized and the relationship between their structures and tribological properties was investigated using an optimol SRV oscillating friction and wear tester and one-way reciprocating friction tester. The elemental composition and chemical nature of the antiwear films generated on steel surface were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM–EDS) and X-ray photoelectron spectrometer (XPS). It was found that aryloxyphosphazene with polar substituent as a lubricant of steel–steel and steel–aluminum pair gave low wear, while aryloxyphosphazene with nonpolar group on the phenyl pendant led to high wear. Phosphazene provides poor lubricity for the steel–aluminum system under low load (0.5–3 N). The XPS analytical results of the antiwear films generated on the steel and aluminum surface indicate that phosphazene reacted with steel or aluminum counterface and formed a surface protecting film consisting of fluoride and organic compounds containing O, C, F, N, and P during friction. This contributes to reduce the friction and wear of steel–aluminum system.     

Chemistry of Antiwear Films from Ashless Thiophosphate Oil Additives

X-ray absorption near-edge structure (XANES) spectroscopy has been combined with atomic force microscopy (AFM) to investigate the interaction of ashless thiophosphate oil additives on steel. Both mono- and dithiophosphates were studied and compared with one another in terms of chemistry and tribological performance. XANES revealed that, thermally, all three thiophosphate additives behaved similarly with steel to form a thermal film at temperatures of 150 °C. The thermal films all consisted of a layered structure comprised of Fe(II) polyphosphate and FeSO₄ in the bulk and iron polyphosphate of various chain length towards the surface. Tribochemical films generated at 5min, 1 h, and 6 h of wear testing revealed that for all three additives, the phosphorus chemistry of an antiwear (AW) film remained chemically consistent throughout all rubbing times. This suggests that the phosphorus chemistry of the AW film is determined in the initial stages of tribofilm formation. The iron polyphosphate chain length remained uniform throughout the AW film with short chain iron polyphosphates found both at the surface and in the bulk of the films. Mild AW conditions produced several different forms of sulfur at the various stages during wear testing. S K-edge XANES spectra for the 5-min tribofilms (both total electron yield and fluorescence yield) showed oxidized and reduced forms of sulfur throughout the films for all three additives. Over extended periods of rubbing (6 h), the more thermodynamically stable product, FeSO₄, was produced and became the major constituent of the tribofilms formed. Iron sulfate was present throughout the films with only traces of reduced sulfur present.AFM imaging of the AW films revealed that the morphology of the films varied from additive to additive and changed over the duration of wear testing. Generally, the AW films were composed of elongated pads orientated in the sliding direction. As rubbing continued, the pads of each AW film became more homogeneous. The larger pads of AW film appeared to have supported most of the load throughout the course of wear testing, resulting in better AW protection to the metal over increased periods of rubbing     

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.     

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 1, growth,wear and morphology

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.     

A reciprocating wear test for evaluating boundary lubrication

A reciprocating wear tester was used to investigated the nature of antiwear boundary lubrication films formed by several ZDDP additives in mineral oil. Under the test conditions examined in this work, antiwear films are relatively thick (approximately 0.1 μm), and so can be readily detected by optical microscopy. Film formation occurs after only a few centimetres of sliding on smooth hard surfaces, whereupon wear essentialy ceases. On rough surfaces, film formation does not take place until the surfaces have run-in, whereupon wear again ceases. Antiwear films did not form on steel pins softer than Rc 25, which wore by an oxidative mechanism. The extent of run-in or rough surfaces before film formation can be used as a measure of the effectiveness of an antiwear additive. Antiwear films are resistant to wear, so once formed they can provide sustained wear protection in base oil. However, antiwear films can be removed by wear in the presence of hydroperoxides, or by running against a new rough countersurface.     

Nanoscale chemistry and mechanical properties of tribofilms on Al–Si alloy (A383): interaction of ZDDP,calcium detergent and molybdenum friction modifier

Abstract

The interaction of a friction modifier and a calcium phenate detergent additive, with zinc dialkyl dithiophosphates (ZDDPs) in the formation of antiwear films on A383, has been studied using synchrotron radiation and nanoindentation techniques. X-ray absorption near edge structure (XANES) spectroscopy has shown that films prepared from oils containing both ZDDP and detergent, and ZDDP and molybdenum dithiocarbamate (MoDTC), are chemically similar to, but thicker than those made from oils containing only ZDDP. In addition, wear was greatly reduced in the presence of the detergent which was correlated with the basicity and the presence of the friction modifier. The phosphorus K and L edge XANES spectra show that the tribofilms are polyphosphate glasses of similar nature to those found on steel, but characterised by a shorter chain length. The sulphur K edge shows a MoS₂ like film and under certain conditions, the presence of a sulphate species is detected. High resolution topographic images and mechanical properties were determined by atomic force microscopy and imaging nanoindentation. The films formed in the presence of the detergent exhibited similar mechanical responses independent of the conditions tested. The indentation modulus of the films on the Al matrix always appear much softer than the films formed on the Si grains whether or not the lubricant contains only ZDDP, or both ZDDP and MoDTC.     

Characterization and mechanical/tribological properties of nano Au-TiO2 composite thin films prepared by a sol-gel process

Nano Au-TiO₂ composite thin films on Si(1 0 0) and glass substrates were successfully prepared with a facile sol-gel process followed by sintering. The morphology and mircostructure of the films were investigated via X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The Au particles, of diameter 14-22 nm depending on the sintering temperatures used, were found to be well dispersed in the TiO₂ matrix, with a small amount of the particles escaped from the film. The surfaces of the films were uniform, compact and crack-free. Hardness and elastic modulus of the films were measured by using the nanoindentation technique. Friction and wear properties were investigated by using a one-way reciprocating tribometer. It was found that the highest hardness and elastic modulus values were obtained for the films prepared with 500 °C sintering temperature. The films displayed superior antiwear and friction reduction performances in sliding against an AISI 52100 steel ball. With 5.0 mol% Au, the friction coefficient was only 0.09-0.10 and the wear life was more than 2000 sliding cycles. The friction coefficient and wear life decreased with increasing sliding speed and load. The failure mechanism of the Au-TiO₂ films was identified to be light scuffing and abrasion. Those films can be potentially applied as ultra-thin lubricating coatings.     

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 today's 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.     

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms.

The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and the formation of phosphate in tribofilms and thermal films. B K-edge XANES shows that boron has a trigonal coordination in the untreated additive, but the coordination changes partially to a tetrahedral coordination in the tribofilm upon rubbing. No BN was detected in the film analyzed by B K-edge or N K-edge. Boron 1s XPS also did not show the presence of BN in the film.