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.     

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.     

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.     

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.     

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.     

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.     

The Effect of Steel Hardness on the Performance of ZDDP Antiwear Films: A Multi-Technique Approach

X-ray absorption near-edge structure (XANES) analysis has been used to characterize the chemistry of antiwear films formed in a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive. These films were formed by rubbing the AISI 1095 steel samples under a reciprocating boundary contact. The steel samples were tempered to produce different Vickers hardness values. The phosphorus L-edge XANES spectra show that these films differ slightly in their chemical nature, with longer chain polyphosphates being formed on samples with higher hardness value. The surface morphology of the films was investigated using Atomic force microscopy (AFM) and the film thickness was probed by Focussed ion beam and Scanning electron microscopy (FIB/SEM) techniques. Furthermore, the nanomechanical properties of these antiwear films were investigated by nanoindentation methods. Tribological measurements of the coefficient of friction (μ) and wear scar width (WSW) indicate that the poorest antiwear film was formed on the softest substrate, which exhibited the largest WSW and the highest average μ. FIB/SEM images show that the thicknesses of the antiwear pads and the degree of damage on the substrates both change with the hardness value of the samples.     

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 2, chemistry

The chemical constitution of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and ashless dialkyldithiophosphate (DDP), has been examined by X-ray Absorption Near Edge Structure (XANES) spectroscopy. The identification of spectral features and interpretation of the results for P, O, Fe, and S species are given, allowing an overall mechanism to be deduced. The role of Fe in these films was investigated in some detail using P L-edge, O K-edge and Fe L-edge XANES spectra. From the P L-edge XANES spectra, the DDP films are uniformly very short chain iron polyphosphates. In contrast, the ZDDP films are formed initially as short chain polyphosphates; but after more rubbing, a bilayer phosphate film is formed with long chain Zn polyphosphates on the surface and shorter chain in the bulk of the film. The O K-edge XANES spectra show that there is, as expected, more Fe in the DDP phosphate films than in the ZDDP phosphate films. The S K-edge spectra of ZDDP films show the presence not only of ZnS as previously observed, but also the presence of FeS for the first time in the early stages of film formation. The predominant S species in the DDP films is FeS.     

Tribological properties of Mg/Al–CO3 layered double hydroxide as additive in base oil

Abstract

The tribological performance of Mg/Al–CO₃–layered double hydroxide (LDHs) nanoparticles in base oil was studied as a sole additive and in combination with zinc dialkyldithiophosphate (ZDDP) at 100°C. Results show that LDHs could improve antifriction and antiwear properties of base oil. The blend with LDHs alone shows better antifriction properties than that of the mixture of LDHs and ZDDP, but the addition of ZDDP helps to stabilise the friction coefficient. Surface analyses have been performed to study the morphology, nanoparticle distribution and chemical species in the tribofilm. The results show that tribofilms contain Mg, Al, C, O, Zn, P and S, and the structure of Mg/Al–CO₃–LDHs changes under shearing stress and this process can help reduce friction coefficient. Thus, LDHs could be chemically incorporated into tribofilm to reduce the polyphosphate chain length originated from ZDDP decomposition resulting in a medium chain polyphosphate, which provides very good wear protection.     

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.     

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.     

Study of the Interaction of ZDDP and Dispersants Using X-ray Absorption Near Edge Structure Spectroscopy—Part 1: Thermal Chemical Reactions

The interactions of ZDDP and different dispersants have been investigated both in oil solutions and on steel substrates at 150–185°C. X-ray absorption near edge structure (XANES) spectroscopy at P and S L-edge and K-edge has been used to identify the chemical species both in solution and on the surface of the steel. It was found that noticeable ZDDP decomposition in solution starts at 175°C when no dispersant is present. In contrast, thermal oxidative films begin to form on the steel at 150 °C in the same solution. The products of decomposition in solution and in the film are phosphates and sulfides. N K-edge XANES spectroscopy has also been utilized to identify the reaction of the dispersant with the steel surface and ZDDP. The results show that dispersants enhance the decomposition of ZDDP in oil solutions as well on the steel surface. Dispersants, on their own, react/adsorb with the steel surface at 150–175 °C and also interact with ZDDP to form new products. Depending on the composition of the dispersants, the product is different.     

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.     

Study of the Interaction of ZDDP and Dispersants Using X-ray Absorption Near Edge Structure Spectroscopy—Part 2: Tribochemical Reactions

The antiwear properties of zinc dialkyldithiophosphate (ZDDP), dispersants, and mixtures of ZDDP and different dispersants have been evaluated using a pin-on-flat Plint wear machine. Tribochemical interactions between ZDDP and dispersants have been investigated under boundary lubrication conditions by means of X-ray absorption near edge structure (XANES) spectroscopy, probing the phosphorus, sulfur and nitrogen absorption edges. The results show that the dispersants do not give any wear protection by themselves in the base oil. The dispersants also do not affect the antiwear property of ZDDP under the given testing conditions. The N K-edge XANES analysis indicates that dispersants contribute to the chemical composition of the tribofilms and form mixed ammonium/zinc polyphosphates. Phosphorus in the tribofilms is present mainly in the form of medium-chain polyphosphate on the surface and short-chain polyphosphate in the bulk. Sulfur appears in the tribofilms mainly as sulfide S^-II, possibly zinc sulfide. The presence of dispersants in oil blends does not disturb the polyphosphate (and sulfide) formation, but it does decrease the chain length of the polyphosphate in the tribofilms.     

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.     

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     

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.     

Study of silane-based antiwear additives: Wear and chemistry

The chemistry and wear performance of a silane-containing additive in combination with conventional commercial engine oil additives such as zinc dialkyldithiophosphate (ZDDP), calcium-type detergent (Ca detergent), and B- and N-containing dispersant were investigated. The tribological behavior of the low-sulfur base stock 100 N blended with the above additives was investigated using a pin-on-disc Plint friction and wear tester at 100 °C. The wear scar width (WSW) of the upper steel pins was determined using an optical microscope. X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to analyze the chemistry and thickness of the thin tribofilm formed on the disc. The morphologies of the wear scars on the lower steel discs were observed using an atomic force microscope (AFM). It was found when the silane additive is mixed with Ca detergent and B- and N-containing dispersant, the antiwear performance of the blend was greatly improved, while the friction coefficient remained almost unchanged. Indeed, the wear performance was comparable to or better than ZDDP on its own, and much better than a commercial oil blend. The silane additive is converted to hydrous SiO₂ by the water in the oil, and this SiO₂ then interacted chemically with the surface and Ca in the detergent under sliding to form a relatively thick tribofilm containing mainly a Ca silicate species. The incorporation of Si and B had little effect on the tribochemistry of ZDDP in the oil blends. When ZDDP and B- and N-containing dispersants were mixed with the silane additive, polyphosphate-type tribofilms, similar to that of ZDDP alone, were formed. However, addition of ZDDP had adverse effects on the wear performance of the silane-based blend.     

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.