Tribological characteristics of ashless dithiocarbamate derivatives and their combinations with ZDDP as additives in mineral oil

Two ashless dithiocarbamate derivatives, octyl 2-(dibutylcarmothioylthio) acetate (DDCO) and S-dodecyl 2-(dibutylcarbamothioylthio) ethanthioate (DDCS), were prepared. Thermal stabilities tests were conducted with a thermo-gravimetric analyzer (TGA). The tribological properties of each compound and their combinations with ZDDP in a mineral oil (HVI WH150) were evaluated using a four-ball tester. X-ray absorption near-edge structure (XANES) spectroscopy was used to characterize the chemical properties of tribofilms generated from DDCO, DDCS and their combinations with ZDDP. According to the TGA results, the synthesized compounds possess good thermal stability (initial decomposition temperatures are above 270 °C). It can be found that all the prepared compounds have better friction-reducing capacity than ZDDP, with anti-wear performance and extreme pressure property worse than ZDDP. However, their combinations with ZDDP perform better than ZDDP in tribological properties. The results of the XANES analyses indicate that the composition of the tribofilms from DDCO or DDCS is organic sulphide on the outer surface and pyrite with a little sulphite in the inner layer, which also suggests the -SC(=S)-N- part in additives structure plays key role in tribol-chemical behaviour. The XANES spectra of the combinations exhibit interestingly that the addition of DDCO or DDCS can increase the length of polyphosphate chain in the tribofilms.     

苯并噻唑衍生物的抗磨性能及作用机制

利用四球试验机考察了所合成的苯并噻唑衍生物添加剂在基础油中的摩擦学性能,并用X射线光电子能谱仪和扫描电子微探针观察分析了磨斑表面的元素化学状态。研究结果表明:苯并噻唑衍生物可以显著改善基础油的抗磨性能和承载能力;添加剂在摩擦过程中发生了摩擦化学反应,生成了含有机硫化物、硫酸亚铁等的边界润滑膜,从而改善了基础油的抗磨性能。     

Investigation of decomposition of hydrocarbon oil on the nascent surface of steel

Hydrocarbon oil with low vapor pressure has been used as a lubricant in high-vacuum conditions. Decomposition of the oil under boundary lubrication conditions was studied by investigating desorption of hydrogen and hydrocarbons, generated by tribochemical reaction occurred on nascent surface of 52100 steel during the sliding process in a ball-on-disk type sliding tester. Gaseous products by tribochemical decomposition were monitored by a quadrupole mass spectrometer (Q-MS), which would reveal the decomposition mechanism of hydrocarbon oil. It is found that tribochemical reactions of hydrocarbon oil occurred on active sites on steel generated by sliding, the desorption amount of hydrogen and gaseous hydrocarbons increased linearly with sliding velocity, and parabolically with load. A critical load for the activation of decomposition of the hydrocarbon oil on bearing steel was found to be about 1.1 N. In this paper, the decomposition mechanism of hydrocarbon oil was also explored.     

The tribological properties and action mechanism of non-active organic molybdate ester and its combination with ZDDP

A non-active molybdate ester (ME) was synthesized in a batch process. Its tribological performance and its synergistic effect with ZDDP in 5CST were evaluated using a four-ball machine, and the chemistry of tribofilms was analyzed with XANES. The results indicate that ME possesses excellent anti-wear and friction-reducing properties, not load-carrying capacity. Both ME and ZDDP show excellent synergistic tribological behavior in 5CST. According to the XANES results, the tribochemical films generated from ME alone are mainly composed of MoO₃, and the tribochemical films generated from the oil blends containing ME and ZDDP consist mainly of MoS₂, sulphate and polyphosphate.     

The mechanism of synergism between ZDDP and CeF3 additives

The interaction of zinc dialkyldithiophosphate (ZDDP) with cerium fluoride and cerium dioxide in lithium grease has been studied for friction, wear, and EP characteristics on a four-ball and SRV tester. The combination of ZDDP and cerium trifluo-ride has been shown to be beneficial in reducing wear, especially over a long period of friction and with increasing EP load. The test results show that adding cerium dioxide to lithium grease does not improve the antiwear and friction performance of the paste. The analytical results of X-ray photoelectron spectrum (ESCA, XPS) reveal that ZDDP inhibits the decomposition of cerium fluoride and improves its film-forming property. The wear scar reaches a minimum at an atomic concentration ratio of 3P:2Zn:1S:25F, and an atomic concentration ratio of 2S:2P:1Zn:4F gives the highest EP load. The scratch test results show that combining ZDDP with cerium trifluoride improves the tenacity of the surface film. The stabilisation of cerium trifluoride by ZDDP is proposed.     

Deactivation Effect of Tricresyl Phosphate (TCP) on Tribochemical Decomposition of Hydrocarbon Oil on a Nascent Steel Surface

A nascent surface has high activity to catalyze the decomposition of a lubricant under boundary lubrication conditions. To reduce the decomposition of a lubricant (multialkylated cyclopentane, MAC), tricresyl phosphate (TCP) was introduced as an additive. The tribological properties and decomposition process of lubricants on the nascent surface of bearing steel 52100 were investigated by a ball-on-disk friction tester in a vacuum chamber with a quadrupole mass spectrometer (Q-MS). The addition of TCP prolonged the induction period for decomposition of the lubricant. During the friction processes, hydrogen and gaseous hydrocarbons desorbed as tribochemical reaction products. XPS analysis revealed that the tribofilm from the additive was mainly composed of iron phosphate, which decreased the probability of generating a nascent surface, resulting in the reduction of desorption rate of gaseous products. The critical load for the mechanical activation of the decomposition correspondingly doubled.     

Additive–additive interaction: an XPS study of the effect of ZDDP on the AW/EP characteristics of molybdenum based additives

The effect of zinc dialkyldithiophosphate (ZDDP) addition on the antiwear (AW) and extreme pressure (EP) properties of molybdenum dialkyldithiocarbamate (MoDTC) and molybdenum dialkyl dithiophosphate (MoDTP) are evaluated by standard Four-Ball friction test and also by the determination of coefficient of friction using an oscillating SRV apparatus. The boundary lubrication film formed on the worn surface using the two molybdenum additives and their combination with ZDDP is investigated by depth profile X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to find out the chemistry of tribochemical reaction occurring at the surface during friction. It is seen that MoDTP possesses better AW properties than MoDTC due to its high reactivity with the metal surface. Nevertheless, the AW characteristics of MoDTC could be improved by the addition of ZDDP. The frictional properties of MoDTP, however, do not change by the addition of ZDDP. The synergistic action of ZDDP on MoDTC is attributed to the enhanced decomposition of MoDTC in presence of ZDDP. This is tentatively explained in terms of some interaction of zinc with the electron donating nitrogen present in MoDTC, which would have helped to increase its tribo-reactivity. XPS studies revealed that in the presence of ZDDP, MoDTC form mainly metal sulphides like MoS₂ and FeS under friction. The MoDTP+ZDDP derived surface, on the other hand, produced mainly metal phosphate along with molybdenum oxysulphides and small amount of MoS₂ and FeS. The mechanism of action of additives is explained.     

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.     

Tribochemical Effects on Tribological Properties of Self-Mated Zirconia Ceramic in HFC-134a Gas

Tribochemical effects on the tribological properties of self-mated zirconia ceramic in CF₃CH₂F (HFC-134a) were investigated using a ball-on-disk type environmental tribometer. The friction chamber of the tribometer was attached to a micro-spot X-ray Photoelectron Spectrometer (XPS) for ensuring that surface analysis be conducted without exposuring the frictional surfaces to air. It was found that HFC-134a gas was an effective lubricant for zirconia ceramic, especially at a pressure higher than 10³ Pa. The products of tribochemical reactions between zirconia and HFC-134a molecules were detected. The amount and chemical state of the tribochemical products seemed to control the tribological behaviors. Thus, the role of tribochemical products on the tribological properties of zirconia in HFC-134a gas at 10⁴ Pa was studied in detail under applied loads of 0.6–5.0 N and sliding speed of 0.04–0.35 m/s. It was found that severe tribochemical reactions occurred at low speeds and high loads. The formation of ZrF₄ accelerated the chemical wear of zirconia, and raised the friction. Zirconia ceramic is suitable for use at moderate load and sliding speed under a reactive environment.     

An ESCA study of the effectiveness of antiwear and extreme‐pressure additives based on substituted phosphorodithioate derivatives,and a comparison with ZDDP

Ashless substituted dithiophosphoric acid derivatives (ADPs) are a new generation of multifunctional additives with promising antiwear (AW) and extreme‐pressure (EP) characteristics. Three such additives synthesised in the authors' laboratory have been evaluated for their AW and EP properties by standard four‐ball friction and wear tests. The friction‐reducing properties of these additives were compared with those of a commercial zinc dialkyldithiophosphate (ZDDP). It was found that the phosphorodithioate compounds studied here possessed excellent AW/EP properties. Their AW characteristics were found to be comparable to those of ZDDP at low loads. However, at higher loads they show inferior AW characteristics in comparison to ZDDP. Nevertheless, ADP derived from cashew nut shell oil had a higher load‐carrying capacity than ZDDP. The mechanism of the AW and EP behaviour exhibited by the different additives was investigated using X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) of the worn surfaces formed during friction. XPS and AES analyses of the worn surfaces reveal that the tribochemical film formed on the ADP‐tested surfaces consisted mainly of metal phosphates and only a small amount of metal sulphides, even though the ADPs contained twice the number of sulphur atoms than phosphorus atoms. The ZDDP‐tested surface showed a mixture of metal sulphides and metal phosphates. Alkylamino substitution appeared to have no significant effect on the AW/EP properties of the additive. XPS and AES analyses also revealed that the tribochemical film formed on an ADP‐tested surface was thicker than that present on the ZDDP‐tested surface at low loads, whereas at higher loads the reverse was true. The higher weld load obtained for the blend containing cashew nut shell oil‐derived ADP is attributed to the thicker adsorbed reaction film formed on the surface due to the long alkyl groups present in the original additive structure. Short‐chain alkyl groups, however, form only a thin adsorbed layer, which may get rubbed off during the friction at high load. The low sulphide formation on ADP‐tested surfaces was attributed to the absence of any metal atom in the additive, which would help in the formation of metal sulphides during tribofragmentation and further tribochemical reactions.     

水溶性含氮硼酸酯摩擦学性能研究

利用四球机和环块试验机考察了两种水溶性含氮硼酸酯的摩擦学性能,并用X－射线光电子能谱仪（XPS）对摩擦表面进行了分析,摩擦学试验表明：含氮硼酸酯在水中具有良好的减摩抗磨性能和承载能力,表面分析证实含氮硼酸酯在摩擦过程中发生了摩擦化学反应,生成了含硼酸,三氧化二硼,有机氮等的复合膜,有效地提高了水基液的抗磨减摩性能和承载能力。     

Friction and wear behavior of thioether hydroxy vegetable oil

This work describes the tribochemical evaluation of vegetable oil based antiwear additive obtained through chemical modification. The Sulfur was incorporated using a chemical reaction of epoxidized vegetable oil and common thiols, resulting in formation of a hydroxy thioether derivative of vegetable oils. The synthesis retains the vegetable oil structure, eliminates poly-unsaturation in the molecule, and adds polar functional groups that significantly improve adsorption on metal surfaces. These additives are obtained by chemical modification of oils originated from natural resources. The tribochemical behavior of sulfur incorporated vegetable oil was studied by measuring friction coefficient using ball-on-disk configuration and wear scar diameter using four-ball configuration. Comparative tests with commercial antiwear additives demonstrate the effectiveness of these derivatives. The derivatives were found useful as agriculturally based antiwear additives for lubricant applications.     

Functional mechanism of formation of friction polymer with dihydroxydocosanoic acid

The results of wear tests with a four-ball machine showed that dihydroxydocosanoic acid has good antiwear properties, comparatively approaching those of zinc dialkyldithiophosphate (ZDDP): the four-ball wear-scar diameters, d 30 min/294 N, with dihydroxydocosanoic acid, ZDDP, and 400 neutral base oil are 0.36, 0.35, and 0.62 mm, respectively. In order of effectiveness the antiwear ability of the C₂₂ acids is: 13,14-dihydroxydo-cosanoic acid (I) 13(14)-mono-hydroxydocosanoic acid (II) docosanoic acid (III), showing that their antiwear properties are remarkably improved after introduction of the hydroxy group into fatty acids. Work on dihydroxy acid as an antiwear agent is not much reported in the literature up to now. In the present work, it was verified with infrared spectroscopy that polyester is formed on the rubbing surfaces with I and II by tribochemical reaction. An oxygen-rich protective film on the rubbing surfaces was identified with Auger Electron Spectroscopy analysis. These results are identical to the high oxygen-containing polyester film shown by IR analysis. It is preliminarily confirmed that the functional antiwear mechanism is, principally, the formation of a polymeric friction film on the rubbing surfaces.     

The mechanism of lubrication by molybdenum disulphide dispersed in oil and the effect of a zinc dialkyldithiophosphate additive

The effectiveness of the lubrication of hard smooth steel surfaces by molybdenum disulphide (MoS₂) dispersed in a mineral oil, both with and without a zinc dialkyldithiophosphate (ZDDP) additive, has been investigated using a four-ball extreme pressure lubricant test machine.Each oil, additive-oil or MoS₂ dispersion, exhibited a wear scar diameter-load curve with an abrupt transition from mild to severe wear at a characteristic load. When ZDDP was present in the oil, MoS₂ was effective at pre-transition but not post-transition loads, whereas in the absence of ZDDP the converse applied, with MoS₂ only beneficial above the transition load.These results are explained in terms of the adherence (or not) of MoS₂ particles to the films physically adsorbed on the steel surfaces at pre-transition loads. At the transition load the surface temperature reaches the desorption (or decomposition) temperature of the particular oil or additive film and MoS₂ then adheres to exposed metal, unless the latter chemically reacts with oil constituents to form a chemisorbed film.     

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

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

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

Tribological Characteristics and Tribochemical Reactions of Various Ceramics Lubricated with HFC-134a Gas

The role of tribochemical products in the friction and wear reduction of ceramics with different fractional ionic character in CF₃CH₂F (HFC-134a) gas was investigated using a ball-on-disk type tribometer. Without exposure to air, the wear tracks on the disks were characterized with the aid of a micro-spot X-ray Photoelectron Spectroscope (XPS) whose analytical chamber was connected to the friction chamber of the tribometer. Further, the adsorption and desorption behaviors of HFC-134a molecules on the nascent surfaces of the ceramics were studied using an adsorption test apparatus in high vacuum. It was found that the lubricating effect of HFC-134a gas was closely related to the fractional ionic or covalent characters of the ceramics. HFC-134a gas was more effective in lubricating ionic ceramics than the covalent ceramics. XPS analysis revealed that metal fluorides were mainly formed on the frictional surfaces of the ionic ceramics, whereas the composition of the tribochemical products on the frictional surfaces of the other ceramics was complicated. The adsorption tests proved that HFC-134a was decomposed to an olefin CF₂=CHF on the nascent surfaces of the ionic ceramic Al₂O₃ and the covalent ceramics. However, the formation of organic fluorine-containing compounds was not detected on the frictional surfaces of the ionic ceramics by XPS. This result implies that the mechanism of tribochemical reactions is strongly dependent on the bond type of ceramics. It is concluded that the low friction and wear of the ionic ceramics in HFC-134a gas result from the metal fluorides formed with high surface concentration on the sliding surfaces.     

Antiwear mechanisms of zinc dithiophosphate: a chemical hardness approach

The role of zinc polyphosphate in the antiwear mechanisms of zinc dithiophosphate (ZDDP) is investigated in the light of recently published analytical data carried out on thermal films and tribochemical films obtained from the additive. Special attention is paid to explaining the mechanisms of the polyphosphate in eliminating abrasive wear due to the presence of transition metal oxide species in boundary lubrication. A set of tribochemical reactions between the polyphosphates and the oxides is proposed on the basis of the hard and soft acids and bases (HSAB) principle. The antiwear action of ZDDP is found to be very adaptive and in severe conditions, the model predicts a layered tribofilm with the presence of polymer-like zinc metaphosphate overlying a mixed-transition-metal phosphate glass, in good agreement with recent analytical data. The role of residual sulphur in the lubricant is also envisaged and the model is in agreement with the formation of metal sulphides embedded in the phosphate matrix. The model, based on the chemical hardness concept, could be used as a basis for the prediction of interactions between ZDDP and other additives in motor oils. This revised version was published online in August 2006 with corrections to the Cover Date.     

The tribological behaviors of some rare earth complexes as lubricating additives Part 1. An investigation of the friction-reducing behaviors and an XPS study of the tribochemical characteristics

Several rare earth coordination compounds of 8-hydroxyquinoline and of di-n-hexadecyldithiophosphate were synthesized. The burnished films of these coordination compounds on a GCr15 bearing steel (SAE 52100 steel) disc were prepared. The friction-reducing behaviors of the burnished films were evaluated on a DF-PM friction tester. The friction-reducing behaviors of these complexes as additives in a lithium grease were examined on an SRV fretting wear tester. The binding energies of some typical elements in the complexes before and after friction were determined on an X-ray photoelectron spectrometer. It was found that these coordination compounds could form a quite complete burnished film on a GCr15 bearing steel surface. The lowest friction coefficient was obtained with respect to the burnished film of neodymium di-n-hexadecyldithiophosphate (NdDDP), while a decreased friction coefficient was reached with respect to the burnished film of rare earth 8-hydroxyquinolinate, as compared with the non-burnished friction pairs. Besides, the SRV fretting wear test results revealed that NdDDP in lithium grease exhibited better antiwear and extreme pressure properties than zinc di-n-butyldithiophosphate (ZDDP), while the antiwear and extreme pressure performance of rare earth 8-hydroxyquinolinates is comparable to that of ZDDP. The related results will be published in a following part of this work. The XPS results indicated that tribochemical reactions were involved in the wear process of these coordination compounds as lubricants or as lubricating additives. Here in the first part of this work, the friction-reducing behaviors of the burnished films and of the additives in a lithium grease are dealt with, while the XPS study of the tribochemical characteristics of these complexes is also reported.     

The decomposition products of zinc dialkyldithiophosphate in an engine and their interaction with diesel soot

Japanese valve train wear engine tests were carried out using oils containing a common zinc dialkyldithiophosphate (ZnDTP). The ZnDTP decomposition products formed in the oils during the engine tests were analysed. It was found that the ZnDTP decomposition results in the formation of zinccontaining compounds and phosphorus-containing intermediate products such as tetraalkylthioperoxidiphosphate. During an engine test, the ZnDTP decomposes rather quickly, whereas the phosphorus-containing intermediate products were detected over a long period. The adsorption test results indicate that diesel soot adsorbs the zinc compounds but that the soot adsorbs little of the phosphorus compounds. From the valve train wear test results and four-ball wear test results it was found that the oils tested in the engine keep their antiwear performance after the ZnDTP has decomposed and that this performance is mainly attributable to the phosphorus-containing compounds.     

Tribological behaviors and mechanism of sulfur- and phosphorus-free organic molybdate ester with zinc dialkyldithiophosphate

An oil-soluble sulfur- and phosphorus-free organic molybdate ester (ME) was synthesized. The antiwear and friction-reducing properties of ME with zinc dialkyldithiophosphate (ZDDP) in base oils were evaluated by four-ball tester. The results show that ME addition effectively reduced wear scar diameter (WSD) and friction coefficient (μ) as well as good antiwear synergism with ZDDP. The topography, composition and chemical states of typical elements on the worn scar were analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) and X-ray photoelectron spectrometer (XPS). Smooth and light topography of worn scar further confirms the good antiwear synergism of ME with ZDDP. EDX and XPS analyses indicate that tribo-chemically boundary films formed on the rubbing surface consist of metal oxides, sulfides and phosphates, leading to enhancement of the antiwear and friction-reducing properties of the lubricants, and that the considerable MoS₂ layer especially plays an important role in improving antiwear and friction-reducing properties of oils. The proposed antiwear mechanism involves a synergy between ME and ZDDP.