The two-layer structure of Zndtp tribofilms: Part I: AES, XPS and XANES analyses

The nature and properties of polyphosphate glasses formed in the antiwear action of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on the same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The data show the two-layer structure of the Zndtp film and permit a clear identification of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phosphate. A Chemical Hardness model is found to predict the formation of such a layered tribofilm. Moreover, a tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the Hard and Soft Acids and Bases (HSAB) principle. This reaction explains the anti-abrasive mechanism of Zndtp and also predicts a depolymerisation of the long chain zinc polyphosphate glass, in very good agreement with AES/XPS/XANES analytical data. The role of residual sulphur in the lubricant is also explained and the model is in agreement with the formation of metal sulphides embedded in the short chain phosphate matrix. This first paper serves as a basis for a detailed study of the mechanical properties of each film.     

EP/AW performance evaluation of some zinc alkyl/dialkyl/alkylaryl‐dithiocarbamates in four‐ball tests

Tribochemistry, the chemistry of interacting surfaces under the influence of a lubricant, helps in the appropriate selection of suitable lubricant additives for specific uses. Modern lubricants are usually formulated from a range of petroleum base oils or synthetic fluids incorporating a variety of chemical additives for performance enhancement. Extreme‐pressure (EP) and anti‐wear (AW) additives are used extensively in lubricants for hypoid gears and metal cutting and forming operations to reduce wear, modify friction, and prevent scuffing of moving metallic parts. The present paper includes the synthesis and the evaluation of the tribological properties of 0.5% (w/v) solutions of some zinc bis‐(alkyl/dialkyl/alkylaryldithiocarbamates) in paraffin oil using 12.7 mm diameter steel bearing ball specimens in four‐ball tests. All the synthesised zinc dithiocarbamate additives in general, and zinc bis‐(morpholinodithio‐carbamate) (A₄) in particular, exhibited good AW, EP, and friction‐reducing properties. Additive A₄ especially gave low values of wear‐scar diameter and coefficient of friction at higher loads and higher values of load wear index and flash temperature parameter during EP tests (ASTM D 2783) and afforded lower values of wear‐scar diameter in a one‐hour wear test (ASTM D 2266–67). The surface topography of the wear‐scar matrix of the used ball specimens was investigated by scanning electron microscopy.     

Synergistic effects in binary systems of lubricant additives: a chemical hardness approach

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and detergent overbased calcium borate (CB) lubricant additives have been investigated by coupling analytical TEM and micro‐spot XPS in the tribotester Optimol of SRV GmbH (mild wear conditions in boundary lubrication). Synergistic effects have been observed on both friction and wear data, especially in the Modtc/Zndtp combination. Results have been interpreted on the basis of a chemical hardness concept: the hard and soft acids and bases (HSAB) principle, stabilisation of hard–hard pairs and the maximum hardness principle. The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS₂ single sheets and the digestion of MoO₃, which is also formed, by the zinc polyphosphate glass. The final result of the tribochemical reaction is a tribofilm composed of MoS₂ sheets embedded in a mixed Mo/Zn polyphosphate glass. The CB/Modtc mixture has a similar mechanism except that the oxide is not completely eliminated, due to the softer borate anion compared with the phosphate one. This revised version was published online in July 2006 with corrections to the Cover Date.     

二烷基二硫代氨基甲酸锑在聚α-烯烃中的摩擦性能及其摩擦化学作用机制的研究

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

Friction and wear of tribofilms formed by zinc dialkyl dithiophosphate antiwear additive in low viscosity engine oils

Friction and wear characteristics of low viscosity SAE 5W-20 engine oils containing different amounts of phosphorus were studied using two different test devices. One was a laboratory high frequency reciprocating rig (HFRR) testing new and used oils at low and elevated temperatures. A direct acting mechanical bucket (DAMB) sliding valvetrain bench test apparatus was used to measure the friction and wear performances of fresh engine oils containing 0, 0.05 and 0.1 wt% phosphorus for a cam lobe rubbing against a tappet insert. The tester was coupled with a radioactive tracer machine (RTM). The results show that in the region of low phosphorus concentration, friction is inversely correlated to temperature. The friction coefficient slightly drops with increasing temperature and increases with increased phosphorus concentration at elevated temperatures. Significant wear is produced at phosphorus concentrations lower than 0.02 wt% at most temperatures. Friction and wear are reduced with the addition of supplemental antiwear additives. MoDTC reduces wear more effectively than ZnDTC in the presence of ZDDP.     

An approach to improve the load-carrying capacity of zinc dithiophosphate

By mixing 0.5wt.% silver complex with 2-ethylhexyl phosphonic acid mono 2-ethylhexylester (AgL·HL) into a conventional zinc dithiophosphate (ZDTP), a new compounded extreme-pressure and antiwear as well as antiseizure additive was obtained. The antiseizure and load-carrying performance of the mixture (Zn-AgDTP) was significantly improved in comparison with ZDTP and AgL·HL. The synergistic action and tribochemical mechanism is proposed on the basis of the worn surface analysis by scanning Auger microprobe (SAM), X-ray photoelectron spectroscopy (XPS) and X-ray micro-diffraction (XRD).     

Tribological evaluation and antioxidant capabilities as additives of some organic compounds of zinc

Several organic compounds of zinc synthesized, blended in oil, and evaluated in laboratory bench test for antiwear/extreme pressure friction are presented. The influence of the nature of the structure on the antiwear/extreme pressure properties at wt. 1.5% concentration in a paraffinic oil is studied. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance evaluation of some organic compounds of zinc as additives for lubricating oils

Several organic compounds of zinc were synthesised, blended in oil, and evaluated in laboratory bench tests for AW/EP, friction, and deposition properties. Two of these, a zinc dithiophosphocarbamate and a package based on zinc dialkyldithiophosphate plus a sulphurised compound, appear to give the best overall performance.     

A tribological study of a ternary complex of zinc with dioxyethylenated octylphenol phosphate in water

A novel aqueous antiwear additive, a ternary complex of zinc with dioxyethylenated octylphenol phosphate and triethanolamine (TXOZ + TEA), has been synthesised. Its tribological behaviour in water was evaluated using a four‐ball tester. The results indicate that the complex in water exhibits higher load‐carrying capacity than mineral oil plus ZDTP, and excellent antiwear behaviour. The elemental composition of its rubbed surface was investigated with Auger electron spectroscopy (AES), and the results of this analysis are presented in the paper. The action mechanism of the additive is also discussed.     

X-ray absorption near-edge structure analysis of the chemical environment of zinc in the tribological film formed by zinc dialkyl dithiophosphate decomposition on steel

Calculation of X-ray absorption near-edge structure (XANES) has been used to investigate the chemical nature of zinc in thermal films formed on the surfaces of steel coupons by the additive zinc dialkyl dithiophosphate (ZDDP) in oil solution. Carbon K, oxygen K, and zinc L edge XANES spectra were used to characterize the thermal films and a model film formed by addition of cumene hydroperoxide (CHP) to the ZDDP. Thermal films formed by heating at up to 100 °C for 5 h showed little change from unreacted ZDDP. At 160 °C for 5 h, the ZDDP underwent thermal solution decomposition with similarities to the oxidation induced by CHP. The Zn L edge XANES spectra were compared to calculated spectra of the model surface species ZnO and ZnS in the wurtzite structure and ZnS in the zinc blende structure. Agreement between experiment and simulation is found for the 160 °C thermal film with a roughly two monolayer film of ZnO and for oxidation by CHP with a three to four monolayer film of ZnS.     

Tribochemical Transformation of Nano TiO2 to Ilmenite on the Surface of Wearing Steel Parts: Antiwear Action of Nano TiO2 as an Additive in Engine Oil

TiO₂ nanoparticles of average size about 20–30 nm were hydrothermally synthesized from TiCl₄ under mild acidic conditions. The nanoparticles were mixed with dispersant and base oil to give a partially transparent concentrate with 1.5 wt% of Ti content. The concentrate was dispersed in hexane and base oil to characterize, respectively, by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The concentrate was diluted with base oil to a parts per million level of Ti containing dispersion blends that were evaluated for wear and friction control performance. Nano TiO₂ containing fully formulated oil blend showed excellent load-bearing capability in Swingung, Reibung, Verschleiβ (SRV; oscillation, friction, wear) tests. Four-ball test results show that the wear scar diameter was considerably reduced to 0.30 mm for TiO₂-added blend compared to neat base oil (0.60 mm). The performance of TiO₂-added blend was comparable to secondary zinc dialkyl dithiophospate (ZDDP)-added blend under identical condition. Raman spectra of the worn surface on the tested ball revealed the presence of ilmenite (FeTiO₃) and no deposits of pure TiO₂.     

Physical and chemical mechanisms of influence of Cu-containing additives on grease properties

The mechanisms governing the influence of copper additives (powders, oxides, organic and inorganic salts, complex compounds) on tribological, rheological and basic physical properties of greases with various thickeners (lithium, complex lithium, complex aluminium and polyurea) are discussed. The copper plating effect was found to occur only under EP conditions. The physical and chemical patterns of copper-containing grease additives under antiwear and extreme pressure conditions are discussed together with rational application alternatives in friction couples of equipment and machinery.     

Antiwear properties of complex compounds of Sn(II) and Sn(IV) in lubrication of a bronze-steel contact

The wear characteristics of metal-organic chelate additives containing tin bonded to aliphatic dicarboxylic acids were investigated on a reciprocating motion test machine in lubrication of a steel-bronze contact. It was found that both the valence state of the tin and the chain lengths of the dicarboxylic acids affected the wear results. All the compounds reduce wear effectively and, at pressures below 1.76 MPa, they assure zero ZDTP and SnDTP additives in the steel-steel concentrated contact using the standard four-ball machine.     

A tribological study of an S,P-containing benzotriazole derivative as an additive in pentaerythritol ester

The load-carrying capacity, antiwear and friction reduction properties of an S,P-containing benzotriazole derivative (BMDDP) added to a synthetic lubricant (pentaerythritol ester) were evaluated using a four-ball test machine. The results when compared with those of tricresyl phosphate (TCP) and zinc dialkyldithiophosphate (ZDDP) show that, in pentaerythritol ester, the novel compound (BMDDP) is an excellent multifunctional additive, which possesses better load-carrying capacity, antiwear and friction reduction properties than either TCP or ZDDP. The rubbed surfaces were analysed using XPS and EPMA, and the antiwear mechanism is discussed.     

Additives for Rape Seed Oil 1: Preparation and Tribological Behaviour of a Sulphurised Rape Seed Oil Lubricant Additive

A new type of environmentally friendly lubricant additive ‐ a sulphurised rape seed oil additive ‐ was prepared, and the chemical structure characterised using infrared spectroscopy. The tribological properties of the additive in a rape seed base oil were evaluated using a friction tester. The morphologies and tribochemical species of worn surfaces were analysed by means of X‐ray photoelectron spectroscopy. The results show that the additive increased the load‐carrying capacity and improved the antiwear and friction‐reducing properties of the rape seed oil. The inferred lubrication mechanism is that a high‐strength adsorption film and a tribochemical reaction film form on rubbing surfaces due to the carrier effect of the long‐chain rape seed oil molecules, the high reactivity of sulphur, and their synergism.