Antiwear Films Research Articles

Tribological and physicochemical characterization of strontium colloidal additives in mild wear regime

The morphology, the structure and chemical nature of colloidal strontium dimethyl-3,5-hexanoate (SDMH) reverse micelles are investigated by atomic force microscopy (AFM) and analytical transmission electron microscopy (ATEM). Their anti-wear properties first measured through a classical tribology experiment are then investigated via the characterization of the structural and physicochemical properties of the surface film generated during friction tests between the two ferrous surfaces in the presence of the lubricant containing SDMH as additive. AFM studies demonstrate that the film is constituted of platelets of 40 nm thickness with a surface roughness smaller than 1 nm. The ATEM investigations on anti-wear film fragments show that the film is mainly composed of strontianite crystallites formed from the amorphous mineral cores of the micelles. These crystallites, which sizes are significantly larger than the mineral cores of the micelles, are linked to each other by an amorphous intergranular phase. Electron energy loss spectroscopy shows that during friction the colloidal particles loss their organic shell in the contact and that carbon under the carbonate form only remains in the anti-wear film. The good anti-wear properties of this additive are explained by the microgranular structure and the adherence of the anti-wear film on the surfaces.

A Quantum Chemical Study of the Unimolecular Decomposition Mechanisms of Zinc Dialkyldithiophosphate Antiwear Additives

Density functional theory calculations were applied to study a set of isomerization and decomposition reactions that zinc dialkyldithiophosphate (ZDDP) engine oil antiwear additives may take part in. The products of these reactions comprise a set of chemical species that can lead to the formation of poly(thio)phosphate films that are thought to be responsible for the antiwear protection offered by ZDDPs. The specific reactions examined involved either intramolecular alkyl group transfer within ZDDP or the intramolecular elimination of olefins from this molecule. A series of substituents were employed to examine how the nature of the substituent on the ZDDP molecule affects the thermodynamic and mechanistic details of these reactions, which in turns may have ramifications regarding various aspects of antiwear film formation. It was found that qualitative and quantitative aspects of these reactions were markedly different when hydrogen atoms were used as substituents instead of alkyl groups. The details of ...

The formation of zinc dithiophosphate antiwear films

In situ spacer layer interferometry has been used to monitor and compare the formation of both tribo- and thermal zinc dialkyldithiophosphate (ZDDP) films on steel surfaces. For the ZDDP solution studied, negligible thermal film formation takes place below a temperature of 110°C, but a ZDDP tribofilm is generated on rubbed surfaces even at room temperature. The ZDDP tribofilm formation is critically dependent on the extent of direct solid solid contact during rubbing and does not take place if the elastohydrodynamic film thickness is significantly greater than the surface roughness. Tests at different slide-roll ratios indicate that the rate of ZDDP tribofilm formation is proportional to the product of sliding distance and rubbing time. Based on the results, it is not possible to explain ZDDP tribofilm formation in terms of a response to contact pressure or flash temperature. It is likely that the tribofilm reaction is either strongly catalysed by species released during rubbing, such as soluble FeII or FeIII, or is triggered by triboelectronic processes such as exoelectron emission.

Molecular basis of lubrication

Lubrication is an art that has been practiced for thousands of years from the early days of our human civilization. The study of lubrication as a science began in the 17th century with the development of bearings and axles. In the early 21st century, the advent of automobiles and steam engines spurred the development of modern complex lubricants consisting of base oils and chemical additives. The development, however, has been mostly empirical in nature. The detailed mechanisms of the chemistry and why they worked were not understood. Rapid advancements in analytical instrumentations and techniques in the last several decades offer an unprecedented opportunity to analyze the complex chemistry and probe the surfaces for chemical evidence. Recent developments in nanotechnology provide further ability to examine phenomena and mechanisms at the nanometer level. As a result of these advances, our understanding of the complex lubrication system has improved significantly. This paper will attempt to provide a molecular basis of how lubricant and additives function in lubrication. Monomolecular thin films have been developed to investigate the fundamental mechanism of boundary lubricating films. Results provide additional insights of how antiwear films work in the lubrication system. Prospect for applying this know-how may result in a revolutionary change in our current lubricating technology.

Investigating binary oil additive systems containing P and S using X-ray absorption near-edge structure spectroscopy

X-ray absorption near-edge structure (XANES) spectroscopy to monitor the chemical properties of tribochemical films generated from oil solutions comprised of two additives containing phosphorus and sulfur. Oil solutions containing diphenyl phosphate (DPP) or triphenyl phosphate (TPP), were blended with sulfurized ester (SE) and dithiocarbamate (DTC) extreme-pressure (EP) agents to generate tribochemical films under both antiwear (AW) and EP conditions. This study found that the chemistry of all tribofilms, was clearly dominated by the phosphate ester additives. XANES spectroscopy of the AW films revealed that in the presence of any S additive, phosphate esters were able to generate polyphosphate films, mainly comprised of an iron polyphosphate. S K-edge XANES spectra (probing the bulk) revealed that in the presence of either phosphate ester, sulfur was oxidized to sulfate. Noisy S L-edge XANES spectra (probing the surface) indicated that sulfur was mainly present in the bulk of the film. Thickness measurements of the AW films obtained directly from both P and S K-edge fluorescence yield (FY) spectra showed that the proportion of S was significantly reduced when compared to AW films generated by the S additives in the absence of either phosphate ester. In addition to the reduction of S present, a decrease in the amount of phosphorus was also apparent, indicative of competitive interactions at the surface between the additives. The reduction in phosphate in the tribofilms can be attributed to the reaction of either SE or DTC with steel during the heating of the oil solution prior to rubbing. Wear scar width (WSW) measurements revealed that wear protection is determined completely by the action of phosphorus and sulfur has little role, if any, in further protecting the substrate during AW conditions. XANES spectroscopy of the EP films containing DPP showed that DPP was able to generate a relatively thick phosphate EP film, in the presence of sulfur, leading to a decrease in the amount of sulfur present. Conversely, when TPP was used, only a thin phosphate film could form leading to an increased amount of sulfur present. Sulfur present, was in the form of FeS. The ability of DPP to react with the substrate at lower temperatures than TPP, thereby reducing the number of adsorption sites at the metallic surface, was responsible for the differences in the amount of S present.

The Compatibility of Crankcase Lubricant–Material Combinations in Internal Combustion Engines

Recent advances in engine technology have seen the introduction of new materials and surface coatings leading to an overall reduction in weight and improved durability. However, the lubrication requirements of these materials are not fully understood. In this paper the lubrication of traditional Cr-bearing steel and Al–Si alloy-based components is investigated using two fully formulated lubricants. These two lubricants have the same viscosity grade and phosphorous levels but differing base stock and detergent systems. A pin-on reciprocating plate configuration was used for the tribological experiments and analysis was performed using X-ray Photoelectron Spectroscopy (XPS), Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray analysis (EDX). It has been found that in a completely ferrous-based system, fully formulated lubricants are effective in reducing wear and friction and a relatively thick anti-wear film is formed. In partially aluminium systems the fully formulated lubricant is less effective and a thin (and therefore unstable) anti-wear film is formed when a cast iron pin is rubbed on the Al–Si alloy plate. In completely non-ferrous systems the wear film is absent. In this paper the fundamental aspects of the film formation as well as the practical aspects of the results will be discussed.

Tribological performance of an ionic liquid as a lubricant for steel/aluminium contacts

AbstractThe wear and friction behaviour of an ionic liquid 1‐ethyl‐3‐hexylimidazolium tetrafluoroborate (L206) was investigated as a lubricant for steel/aluminium contacts using an Optimol SRV® oscillating friction and wear tester. The elemental composition and chemical nature of the antiwear films generated on the aluminium surface were analysed using a scanning electron microscope with a Kevex energy dispersive X‐ray analyser attachment (SEM/EDS) and X‐ray photoelectron spectroscopy (XPS). A low friction coefficient (˜0.05) was recorded when lubricating with L206; a small amount of water (5 wt. %) in L206 effectively reduced the wear volume and greatly increased the microhardness of the aluminium alloy, but had little effect on the friction coefficient. The SEM/EDS results showed that severe corrosive wear occurred on the aluminium alloy when lubricating with neat L206, which could be avoided by the addition of water in L206. The XPS results indicated that the species AlF3, Al2O3, AlO(OH), and Al(OH)3 formed during friction; there was no indication of boron on the worn surfaces.

Tribological performance and action mechanism of S-[2-(acetamido) thiazol-1-yl] dialkyl dithiocarbamate as additive in rapeseed oil

There has been a growing concern for the use of environmentally friendly lubricants because of the worldwide interest in environmental issues. This has promoted the development of new environmentally friendly lubricants and additives. In this paper, three potential ashless additives, S-[2-(acetamido) thiazol-1-yl] dialkyl dithiocarbamate, were prepared. The tribological behavior of the synthesized compounds as additives in rapeseed oil (RO) were evaluated with a four-ball wear tester. The elemental composition and chemical nature of the antiwear film generated on steel counterfaces were investigated with electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). It was found that the synthesized compounds as additives in rapeseed oil had shown improved extreme pressure and antiwear properties and friction-reducing behavior as compared with the rapeseed oil alone. The protective film containing a reaction layer and adsorption layer was found to be formed on the wear surface. The elemental sulfur contained in the dialkyl dithiocarbamate group and thiazole group may react with counterface metal, and result in the formation of reaction films containing sulfate, organosulfur compounds and FeS 2. The nitrogen element was merely adsorbed on the lubricating surface and did not react with the metal. In addition, the polymerized ester produced by rapeseed oil also played an important role in the adsorption film.

The tribological behaviour of some triazine–dithiocarbamate derivatives as additives in vegetable oil

Three novel ashless triazine–dithiocarbamate derivatives, which are potential environmentally friendly oil additives, were synthesized. Their tribological properties as additives in rapeseed oil (RSO) were evaluated using a four-ball tester. Their anticorrosive properties and thermal stabilities were also investigated. The results indicate that the three novel triazine–dithiocarbamate compounds possess excellent load-carrying capacity, good corrosion inhibiting performance and high thermal stability. Although these additives show friction-reducing property at high additive concentration (5 wt.%), they have friction increase and wear promote natures in most cases. The elemental composition and chemical state of the antiwear films generated on steel counterface were investigated with scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). It was found that the synthetic additives reacted with the counterface metal and generated a surface protective film consisting of sulfate, FeS and absorbed compounds containing organic-N.

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. Presented at the 55th Annual Meeting in Nashville, Tennessee May 7–11, 2000

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.

Friction-Enhancing Properties of ZDDP Antiwear Additive: Part I—Friction and Morphology of ZDDP Reaction Films

Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiphosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films. In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness (13). Presented at the 57th Annual Meeting in Houston, Texas May 20–24, 2002

Friction property study of the surface of ZDDP and MoDTC antiwear additive films using AFM/LFM and force curve methods

AbstractThe friction properties and material differences of the surface of ZDDP and MoDTC antiwear additive films, which give clear evidence of different friction coefficients in a pin‐on‐disc test, have been studied using atomic force microscopy (AFM)/lateral force microscopy (LFM) and force curve methods. The AFM/LFM observations show that the friction force on the surface of MoDTC additive films over the sliding area of a steel disc is lower and the friction force of ZDDP additive films is higher than that of afilmless area. Lateral force scope‐trace evaluations reveal that the ratio of the friction forces on the surface of the ZDDP film, the filmless area, and the MoDTC film under the same normal force is approximately 1.5:1.0:0.7. Force curve measurements indicate that the surface materials of the ZDDP film, thefilmless area, and the MoDTC film differ according to their attractive forces, that is 29 nN for the ZDDP film, 22 nN for the filmless area, and 12 nN for the MoDTC film. These results correspond to the friction behaviour in the pin‐on‐disc test and also agree with the idea of the formation of solid MoS2 lubricant from MoDTC additives on the surface of the antiwear film.

Tribological behavior of sialon ceramics sliding against steel lubricated by fluorine-containing oils

The friction and wear behavior of sialon ceramics sliding against steel and lubricated by perfluoropolyethers (PFPE), tetrakis (3-trifluoromethylphenoxy)-bis(4-fluoro-phenoxy)-cyclotriphosphazene (X-1P) and ionic liquid (1-methyl-3-octylimidazolium tetrafluoroborate, coded as L108) were investigated. It was found that the three fluorine-containing lubricants reduced friction coefficient and wear volume effectively. The effectiveness of the three lubricants in reducing wear volume could be ranked as L108>X-1P>PFPE. The antiwear films mainly consisting of organic oxyfluoride or carbonfluoride species and silicon fluoride are all observed for the three lubricants, while the degradation of PFPE during friction might account for the higher wear volume therewith. The lowest friction coefficient 0.065 was recorded for L108 under load of 0.5–400 N. This is dependent on the physically adsorbed ionic liquid on the rubbing surface and the formation of BN under the harsh conditions.

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.

Morphology and Nanoindentation Profiles of Automotive Engine Components

Scanning probe microscopy has been used for the analysis of a wear surface from a component removed from a heavy duty diesel engine. This component is analysed and compared to a wear surface that has been generated in a rig that simulates engine conditions. It was found that the wear surface from the engine component is highly pitted in the regions where contact pressures are exceedingly high, whereas regions further away from the high pressure points are covered with an anti-wear film that resembles the films formed in the wear rig. These results are confirmed by both scanning force microscopy images and nanoindentation results.

Study of the tribological behavior of S-(carboxylpropyl)- N-dialkyl dithiocarbamic acid as additives in water-based fluid

There has been a growing concern for the use of water-based fluids because of the worldwide interest in environmental issues. This has promoted the research and use of water-soluble additives as environmental friendly lubricants. A kind of potential water-soluble additive, S-(carboxylpropyl)- N-dialkyl dithiocarbamic acid was prepared in this work. The friction and wear behaviors of the synthesized compounds as an additive in water-based liquid were evaluated with a four-ball tester and a ring-on-block rig. The wear scar morphology of the ball and the chemical nature of the antiwear films generated on the steel counter face were investigated with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It was found that the synthesized additives had excellent antiwear, load-carrying and extreme pressure performance. The additive reacted with the counter-face metal and generated a surface protective film consisting of FeS, FeS 2, FeSO 4 and an absorbed compound containing N and acid.

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.

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.

2423 高分子上に形成した DLC 膜の摺動特性

Our study is to search for tribological properties of diamond-like carbon (DLC) films as known as anti-wear hard thin film on various polymers. This report deals with the deposition of DLC films on various polymer substrates in vacuum by magnetron radio frequency (RF) sputtering method with using argon plasma and graphite, titanium target. The properties of friction and wear are measured using a ball-on-disk wear-testing machine. The properties of friction and wear have been remarkably improved by DLC coating. Moreover the composition of DLC films has been analyzed by using auger electron spectroscopy(AES). The wear rate of titanium-containing DLC film is lower than that of no-metal-containing DLC film.