Elucidation of the Action of Functional Groups in the Coexisting Ashless Compounds on the Tribofilm Formation and Friction Characteristic of Zinc Dialkyldithiophosphate-Formulated Lubricating Oils

Zinc dialkyldithiophosphate (ZnDTP) is well known to show supreme antiwear performance by forming a thick polyphosphate-based tribofilm on the friction surface. However, it has also been demonstrated that a coexisting dispersant can interfere with the performance of ZnDTP. Because many lubricating oils contain a variety of additives in addition to ZnDTP, the effect of functional groups in the coexisting polar compounds on the tribofilm formation of ZnDTP should be clarified. In this study, several kinds of functionalized polyalkylmethacrylates (PMAs), a dispersant, and friction modifiers were used with ZnDTP, and their effects on the tribofilm formation and the friction characteristic of ZnDTP were discussed. The formulation of polar compounds with different types of functional groups significantly changed the state of ZnDTP compounds in the bulk solutions. Amino groups tended to generate coordination complex compounds and reduced the neutral ZnDTP content in the sample solutions. PMA-OH and glycerol monooleate (GMO) both had hydroxyl groups, and nonfunctionalized PMA 0 did not significantly influence the neutral ZnDTP content. PMA-COOH and stearic acid (StA) containing carboxyl group(s) significantly increased the neutral ZnDTP content. The composition of neutral ZnDTP and basic ZnDTP in the lubricating oil is a key to understanding the influence of coexisting ashless polar compounds on the formation of the ZnDTP-based tribofilms and the friction characteristics of ZnDTP.     

Friction Reduction and Antiwear Capacity of Engine Oil Blends Containing Zinc Dialkyl Dithiophosphate and Molybdenum-Complex Additives

The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of boundary-lubricated steel surfaces was investigated experimentally. The performance of oil blends containing three different types of Mo-complex additives of varying Mo and S contents with or without primary/secondary ZnDTP additions were investigated at 100°C. The formation of antiwear tribofilms was detected in situ by observing the friction force and contact voltage responses. Wear volume and surface topography measurements obtained from surface profilometry and scanning electron microscopy studies were used to quantify the antiwear capacity of the formed tribofilms. The tribological properties are interpreted in terms of the tribofilm chemical composition studied by X-ray photoelectron spectroscopy. The results demonstrate that blending the base oil only with the Mo-compound additives did not improve the friction characteristics. However, an optimum mixture of Mo complexes and ZnDTP additive provided sufficient amounts of S and Mo for the formation of antiwear tribofilms containing low-shear strength MoS ₂ that reduces sliding friction. In addition, the formation of a glassy phosphate phase due to the synergistic effect of the ZnDTP additive enhances the wear resistance of the tribofilm. This study shows that ZnDTP- and Mo-containing additives incorporated in oil blends at optimum proportions improve significantly the tribological properties of boundary-lubricated steel surfaces sliding at elevated temperatures.     

Film Formation of Non-Phosphorus Wear Inhibitors

The time-dependent film formation of two potential non-phosphorus supplemental wear inhibitors in the presence of secondary zinc dialkyldithiophosphate (ZnDTP) was studied by electrical contact resistance (ECR), auger, and X-ray photoelectron (XPS) spectroscopy. One weight percent of a molybdenum dithiocarbamate or an ashless dithiocarbamate was blended with a secondary ZnDTP, sufficient in quantity to yield 0.05 wt% phosphorus at blend level. A thorough surface examination by auger and XPS, coupled with the ECR results, detailed the deleterious effects that these supplemental antiwear additives had on ZnDTP antiwear film formation. Both carbamates interfered with antiwear film formation by secondary ZnDTP. It is speculated that MoDTC generated a competing molybdenum sulfide film that oxidized over time to form MoO ₃ , which promotes wear in the ECR bench test based on literature insight. Ashless DTC also formed a competing antiwear film but not as good a film as from ZnDTP alone.     

The antiwear property of zinc dialkyldithiophosphates in used engine oils

The change in the lubrication properties of vehicle engine oils with driving distance was studied by using a cross-pin type of lubricant tester and a Fourier transform IR spectrometer. The effect of the thermal decomposition of zinc dialkyldithiophosphates (ZnDTPs) on their antiwear property was also examined. The amount of the ZnDTP in engine oil decreased and the amounts of the decomposition products increased with increasing running distance but a good antiwear property was still observed. After 60 h of heating at 135°C, the ZnDTP in sample oils had decomposed into a precipitate and the antiwear property of the oil had deteriorated. With sample oils containing ZnDTP and a detergent or a dispersant, the decomposition products dissolved in the oil and the antiwear property was maintained even after 80 h of heating. The results suggest that the decomposition products of ZnDTPs have a good antiwear property and that the good antiwear property of the used engine oils was retained because of the solubilization of decomposition products into the oil with the aid of a dispersant or a detergent.     

Antiwear behaviour of cyclic phosphorus compounds: Comparison with zinc O,O-dialkylphosphorodithioate

The purpose of the present study is to examine the antiwear properties of some cyclic phosphates and phosphites and to compare them with a noncyclic phosphite and a commonly used oil additive zinc O,O-diisooctylphosphorodithioate (ZnDTP).A number of cyclic phosphates and phosphites were tested on a four-ball machine using various temperatures, loads, speeds and base oils. It was observed that cyclic phosphites possess better antiwear properties than cyclic phosphates. Of the phosphites, the compound containing sulphur was more effective than the corresponding molecule with oxygen in place of sulphur and it gave better results. This superior performance was observed in base oils A, B and C. In comparison with ZnDTP, this compound possesses significantly superior antiwear properties at high temperatures and loads.     

Molecular Design of Environmentally Adapted Lubricants: Antiwear Additives Derived from Natural Amino Acids

Novel environmentally adapted lubricant additives were synthesized from cystine (Cys ₂ ), an essential amino acid obtained from natural sources. The structural feature of cystine is a dimeric amino acid with a central disulfide bond. The carboxyl groups in Cys ₂ were converted to corresponding esters by reaction with long-chain alcohols. The resultant diesters were soluble in poly-alpha-olefin (PAO) and ester-type synthetic oils. The structural features of the new additives include multifunctional groups on the same molecule, such as disulfide as a tribologically active moiety and polar functional groups as anchors to friction surfaces. The additives consist of hydrogen, carbon, nitrogen, oxygen, and sulfur; they are free of phosphorus, chlorine, and metals. The tribological properties of the additives in a solution of synthetic oil were evaluated by performing laboratory tribotests under boundary conditions. The Cys ₂ -derived additives exhibited comparable antiwear properties to the conventional additive zinc dialkyldithiophosphate (ZnDTP). The additives showed good antiwear properties even at low concentrations of sulfur (160 ppm) in synthetic hydrocarbons such as PAOs. A high concentration of sulfur (640 ppm) was required to obtain an optimized antiwear performance of the synthetic esters as a base oil. The new additives reduced the friction coefficient of PAOs and synthetic esters. A saturated ester of Cys ₂ reduced the friction of PAOs and synthetic esters up to oil temperatures of 150°C. The lubrication mechanism was discussed with respect to the role of functional groups in the additive molecule. The use of amino acids as versatile building blocks for the synthesis of environmentally adapted additives was also pointed out.     

Structures of adsorbed zinc dithiophosphates and their relationship to engine wear

Zinc dithiophosphates (ZnDTPs) have been used extensively as anti-wear agents for over 30 years. The mechanism of action of these remarkable materials is not fully understood. In particular, the molecular composition of the antiwear film produced by the ZnDTPs remains an area requiring investigation. In this paper, the molecular composition of adsorbed ZnDTPs on aluminium oxide (Al₂O₃) surfaces has been studied using Inelastic Electron Tunnelling Spectroscopy (IETS), a highly sensitive molecular spectroscopy specifically applicable to surface species at low coverages. As such, it complements other surface science techniques for determining surface compositions. A vibrational spectroscopy, IETS utilises the literature on group characteristic frequencies available from the very extensive correlations developed for infrared and Raman spectra. Thus, functional groups may be identified that are formed by the reaction of a lubricating oil additive with a metal surface covered by a native oxide. Using IETS, we show that ZnDTP aryl esters are dissociatively chemisorbed, and do not appear to form P=S groups bonded to the surface. By contrast, primary and secondary ZnDTPs retain the phosphate ester groups and bond to the surface by the P=S bond to different degrees. These spectral differences are qualitatively correlated with the observed antiwear performance of these materials.     

Antiwear Film Formation by ZnDTP,Detergent, and Dispersant Components of Passenger Car Motor Oils—Part II: Effects of Low-Molecular-Weight Dispersant on Film Formation

Passenger car motor oils (PCMOs) are undergoing a rapid evolution. Studies have found that some exhaust emission catalysts may be deactivated by phosphorus, largely derived from zinc dialkyldithiophosphate (ZnDTP), the mainstay antiwear and antioxidant agent in PCMO formulations for the past 50 years. Consequently, future engine oils will contain significantly reduced phosphorus levels. Since ZnDTP is the dominant antiwear and antioxidant in current PCMOs, lower phosphorus content will impact engine oil formulation strategies.

To better understand the effects of ZnDTP reduction on wear control, electrical contact resistance (ECR) studies have been carried out on blends containing ZnDTP, detergent, and low-molecular-weight (LMW) succinimide dispersant. In contrast to previous results obtained with high-molecular-weight (HMW) dispersant, the combination of ZnDTP and LMW dispersant gave an ECR trace closely resembling that of ZnDTP alone. Thus, the chemical structure of the succinimide dispersant can have a profound effect on ZnDTP antiwear film formation. ECR experiments on three-way combinations of ZnDTP + LMW succinimide dispersant + overbased phenate detergent provided a much better film than that from a similar formulation using an HMW succinimide dispersant. This study demonstrates that the ECR experiment is sensitive to the chemical structures of components controlling the function of modern PCMOs, making ECR a convenient tool to optimize the performance of the remaining ZnDTP in lower phosphorus PCMO formulations.     

Bulk Modulus of Lubricating Oils as Predominant Factor Affecting Tractional Behavior in High-Pressure Elastohydrodynamic Contacts

The phase diagrams corresponding to transition from liquid to viscoelastic solid and that from viscoelastic solid to elastic-plastic solid of Santotrac100 (SN100), mineral oil, synthetic naphthenic oil, polybutene, and tetradecane were first made up by high-pressure density measurements and others. The bulk modulus of lubricating oils under a quasi-static condition was evaluated using a phase diagram. The results indicated that the bulk modulus of lubricating oils is closely related to the oil molecular packing parameter T _VE ?T (where T _VE is the viscoelastic solid transition temperature at pressure p, and T is the oil temperature). The constant values of the bulk modulus in the elastic-plastic range are different depending on the molecular structures of the oils. It has also been shown that SN100, mineral oil, synthetic naphthenic oil, and polybutene converted to amorphous solids at high pressures and tetradecane converted to molecular crystal. Next, the elastohydrodynamic lubrication tractions were measured by a ball-on-disk machine. The results indicated that the maximum traction coefficient is closely related to T _VE ?T. As a result, the importance of the bulk modulus as a predominant factor for traction characteristics of lubricating oil was pointed out.     

Friction reduction by metal sulfides in boundary lubrication studied by XPS and XANES analyses

In this work, the antiwear action of zinc dithiophosphate (ZnDTP) in combination with friction modifier agent molybdenum dithiocarbamate (MoDTC) has been investigated. A Cameron-Plint friction machine was used to generate large tribofilm areas in mild tribological conditions. Two analytical techniques, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy, have been employed to characterize the chemical species in tribofilms. XANES spectroscopy at the P K and L-edge, Mo L and M-edge and S K-edge was carried out in order to investigate phosphates and sulfides species in the tribofilms This was followed by XPS analysis in the same location in the tribofilms. Special attention has been paid to the peak fitting of Fe 2p, Zn 2p, S 2p, Mo 3d and O 1s photopeaks and Auger ZnLMM lines. The combined analyses have shown that the MoDTC+ZnDTP containing oil produces mainly shorter chain metal polyphosphate material in addition to ZnS and MoS₂. No zinc sulfate has been detected. XPS confirms that the formation of MoS₂ is enhanced by the presence of ZnDTP. For all additives combinations; no iron has been detected in the tribofilms. Analytical results are discussed and compared with theoretical predictions from the Chemical Hardness model for ZnDTP, MoDTC and ZnDTP+MoDTC tribofilms formation.     

Tribological Performance of Lubricating Greases Based on Calcium Carbonate Polymorphs Under the Boundary Lubrication Condition

In this article, we synthesized the calcium sulphonate grease (CSG) based on the calcite using the bright stock (150BS) as the base oil. In order to investigate the tribological performance of lubricating grease containing different calcium carbonate polymorphs under boundary lubrication condition, a calcium sulphonate complex grease (CSCG) based on the vaterite was used as a reference. An oscillating reciprocating friction and wear tester set at a series of applied loads and frequencies was adopted to evaluate the tribological performance under boundary condition. Results showed that the lubricating grease that was composed of crystalline calcite as the partial thickener had excellent friction-reducing and antiwear (AW) properties, regardless of the applied loads and frequencies. The vaterite in CSCG easily experienced a polymorph transformation into calcite or aragonite characterized by Raman spectroscopy. This polymorph transformation was attributed to the highly local friction temperature and activated hydrogen from water or acids oxidated in the rubbing process at high load or frequency. The physical polymorph transformation corresponded to the fluctuations of the friction coefficients, then contributed to the severe wear. XPS analysis indicated that two calcium sulphonate lubricating greases occurred a tribochemical reaction and boundary tribofilms consisted of CaCO₃, CaO, iron oxide and FeSO₄ were formed on the rubbing surfaces. The tribofilm formed by the introduction of the CSG that mainly depended upon the thickeners of calcite structure contributed to an excellent AW protection. The possible boundary friction mechanism for greases based on various calcium carbonate polymorphs was also proposed. Effect of calcium carbonate polymorphs on the tribological performance was discussed.     

The effect of LaF3 nanocluster modified with succinimide on the lubricating performance of liquid paraffin for steel-on-steel system

LaF₃ nanocluster modified with succinimide was prepared. The effect of the modified LaF₃ nanocluster on the lubricating performance of liquid paraffin for steel-on-steel sliding system was investigated on a four-ball machine. The morphology of LaF₃ and its lubricating mechanisms as additive in liquid paraffin were studied by means of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was found that LaF₃ nanocluster modified with succinimide as an additive in liquid paraffin increased the load-carrying capacity and had good antiwear and friction-reduction behaviour. The XPS analysis of the worn steel surfaces indicated that a boundary lubricating film consisting of physically adsorbing film (succinimide) and tribochemically reacting film (lanthanum oxide and ferrous fluoride) was generated during the friction process. This contributed to improving the lubricating performance of liquid paraffin.     

Relation between low temperature fluidity and sound velocity of lubricating oil

Low temperature fluidity is important for lubricating oil. Viscoelastic solid transition temperature at atmospheric pressure T_VE0 represents the low temperature fluidity of lubricating oil, which is estimated from the occurrence of photo elastic effect by lowering the temperature using liquid nitrogen. Sound velocity in lubricating oil is measured using Sing around technique. Then the adiabatic Bulk modulus K is calculated from the measured sound velocity. A relation is found between the adiabatic bulk modulus and the viscoelastic solid transition temperature of lubricating oil. The relation depends on the molecular structure of lubricating oil. The oils of a group belong to almost same molecular structure, follows the same relation. For same molecular structure T_VE0 decreases as decreasing the adiabatic bulk modulus of lubricating oil. It is also found that, the viscoelastic solid transition temperature of blend oils can be predicted from the base oils’ T_VE0 and K.     

Time-Dependent Film Formation from ZnDTPs and Nonphosphorus Antiwear Agents

Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films formed after only 10 min of the ECR experiment showed that both primary and secondary ZnDTPs form a thin film (～70 Å) very rapidly. Those films are rich in Zn, P, and S. Auger and XANES analyses of the same specimens were not as revealing, most likely due to the small wear scar on the balls and the unfortunately relatively large beam cross section. ECR, XPS, and AES were then performed on oils containing nonphosphorus antiwear agents in American Petroleum Institute Group II base oil. Several nonphosphorus supplemental antiwear inhibitors were evaluated. These experiments showed separation in apparent performance among the various components.     

The Tribological Performances of Multilayer Graphite-Like Carbon (GLC) Coatings Sliding Against Polymers for Mechanical Seals in Water Environments

Boron-based lubricant additives have recently received significant attention, because of their wear-reducing and frictional properties and low pollution. At the same time, dithiocarbamate complexes with different metals have a long history of being used as multifunctional additives to lubricants. In this study, novel, environmentally friendly additives containing alkylborate and dithiocarbamate groups with alkyl or methylbenzyl substitutes in one molecule were studied. Tribological tests were performed with the additives admixed in a mineral oil using steel-on-steel contacts in a four-ball tribometer. Borate derivatives of different dithiocarbamate ligands were synthesized by several step reactions to investigate tribochemical properties of boron, sulfur, and nitrogen combined in one selected compound. Viscous liquid products were characterized by multinuclear ¹H, ¹³C, and ¹¹B NMR spectroscopy. The surface morphology and the elemental composition of the tribofilms were investigated using an optical profiler and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). It was found that some of these novel compounds provide better antiwear performance and similar frictional properties compared with a commercially available ZnDTP package. Traces of sulfur in the tribofilms formed with both 0.2 and 1.0 wt% of alkylborate–dithiocarbamate compounds in a mineral oil were detected with EDS.     

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS ₂ on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Formation of MoS ₂ , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film.

     

基础油组成对润滑脂抗磨性能的影响

采用液固吸附色谱、红外光谱和X射线荧光光谱对2种润滑脂基础油进行了分离和分析,用SRV摩擦性能实验考察基础油和润滑脂的抗磨性能。实验结果表明,增加基础油的芳烃和硫化物含量能够提高基础油的抗磨性能,基础油的抗磨性与润滑脂的抗磨性具有一致性,可以通过改变基础油的组成来改善润滑脂的抗磨性能。     

Synergistic effect on frictional characteristics under rolling-sliding conditions due to a combination of molybdenum dialkyldithiocarbamate and zinc dialkyldithiophosphate

The frictional characteristics of molybdenum dialkyldithiocarbamate (MoDTC) and/or zinc dialkyldithiophosphate (ZnDTP) in a paraffinic mineral oil were studied using a two-roller machine, in which the synergistic effect for reducing friction was found for the oil containing ZnDTP together with MoDTC. Variation in chemical composition of the surface film over time indicated preferential formation of the products through decomposition of ZnDTP. The reduction in friction was mainly due to MoS₂ derived from MoDTC, while ZnDTP had a role in enhancing the wear resistance and promoting the formation of MoS₂. More severe conditions such as an increase in sliding speed tended to more significantly decrease friction.     

Silicon Nitride Boundary Lubrication: Effect of Oxygenates

A ball-on-three-flat (BTF) wear tester was used to investigate the boundary lubricating characteristics of oxygenates on a commercial silicon nitride. A wide variety of oxygen-containing compounds were tested neat and/or at 1% by weight in a paraffin oil. Compounds containing hydroxyl functional groups were more effective compared to a base case of neat paraffin oil. Decreases of up to 58% in friction coefficient, and 95% in wear were obtained. In most cases, films were observed in and around the wear scar, suggesting chemical reactions had taken place in the contact.

Additional wear tests, conducted using neat shorter-chain linear primary alcohols, i.e., 6–10 carbons, demonstrated boundary lubrication protection, with longer chain length providing better antiwear performance. A study of several C₈ compounds with specific oxygen-containing functional groups (primary alcohol, secondary alcohols, acid, aldehyde, and ketone) demonstrated that the primary alcohol had the strongest boundary lubricating effect. Varying the amount of water in the alcohols had little effect on friction and wear, suggesting that the boundary lubrication effects observed were not merely due to dissolved water in these fluids, but some characteristic chemical interaction with the hydroxyl functional group of the alcohols and acids.     

Synthesis and tribological properties of ferrous octoxyborate as antiwear and friction-reducing additive of lubricating oil

An oil-soluble compound containing ferrous iron and boron–ferrous octoxyborate was synthesized. The antiwear and friction-reducing properties of the compound as a lubricating oil additive were evaluated using a four-ball and a block-on-ring tribotester. Results indicated that both the wear resistance and the load-carrying capacity of a 500 SN base oil were improved and its friction coefficient was decreased by the additive. The wear scar, after being cleaned using an ultrasonic bath in ligroin and in distilled water, was characterized with SEM and XPS. Diboron trioxide, FeB and Fe₂B were all found to have been deposited on the wear-scar surface. It was this deposition that provided the oil with excellent antiwear and friction-reducing properties. This revised version was published online in July 2006 with corrections to the Cover Date.