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.     

Tribological properties of phosphate esters as additives in rape seed oil

With the growing concern about environmental pollution, vegetable oils have begun to be applied as base oils for environmentally friendly lubricants. Additives containing phosphorus can be used for their excellent antiwear properties and low toxicity. In view of this, a study of a series of phosphate esters as additives in rape seed oil (RSO) was carried out using a four‐ball tester. The results indicate that phosphate esters possess very good load‐carrying capacity and good antiwear and friction‐reducing properties compared with RSO by itself. Surface analysis of the worn balls was carried out using X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The tribological mechanism of the additives is discussed on the basis of the experimental results.     

Antiwear Action of Mineral Lubricants Modified by Conventional and Uncoventional Additives

The authors of this paper investigated the influence of esters of rapeseed oil fatty acids on the lubricating properties of mineral lubricants containing chosen AW/EP additives. Methyl esters, ethylene glycol esters, and glycerol esters as well as some commercial AW/EP packages based on zinc dialkyldithiophosphate, S–P organic compounds, and sulphurized esters of fatty acids were tested. The tribological tests were carried out with the use of a four-ball machine. Antiwear (AW) properties of tested compositions were determined using their limiting load of wear (G_oz(40)). It appears that the AW action of esters of rapeseed oil fatty acids depends on their structure. The best AW action is shown by compositions of mineral oil lubricants containing AW/EP additives and methyl esters of rapeseed oil fatty acids. The SEM/EDS analysis of the scar surface layer indicated that the presence of these esters in lubricants causes a change in the interaction between AW/EP additives and the metal surface. These observations were confirmed by the XPS surface analysis.     

The tribological behavior of diester-containing polysulfides as additives in mineral oil

The diester-containing polysulfides were synthesized and their anti-wear (AW) and friction-reducing behaviors as additives in HVIW H150 base oil were evaluated using a four-ball tester. The copper corrosion test was conducted to identify their anti-corrosion property. The nature of the film formed in the rubbed surface was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on the experimental results, diester-containing polysulfides show good anti-corrosion properties and excellent AW abilities similar to that of ZDDP. The results of XPS analyses illustrate that an S-containing inorganic layer, generated by the reaction of diester-containing polysulfides with the metallic surfaces, mainly contains sulfate, sulfite and little sulfide, and there exists a polymer film protective layer over the inorganic layer surface.     

A study of the tribological behaviour of dialkyldithiophosphate esters as additives in rape seed oil

Zinc dialkyldithiophosphates (ZDDP) cannot be used as additives in biodegradable lubricants because of their zinc content. In investigating substitutes for ZDDP, dialkyldithiophosphate esters have been synthesised, and their tribological behaviour as additives in rape seed oil has been evaluated using a four‐ball friction and wear tester and compared with that of ZDDP. The results show that these additives have better antiwear properties and load‐carrying capacity than rape seed oil alone. The morphologies and the elemental chemical states on the worn surfaces of the lubricated steel balls of the tester were examined using X‐ray photoelectron spectroscopy and scanning electron microscopy. The tribological mechanism is discussed on the basis of the experimental results.     

A study of P–N compound as multifunctional lubricant additive

A novel P–N compound was synthesised, and its tribological and anti‐oxidation behaviours used as lubricant additive for polyalphaolefins (PAO4) were evaluated. The results demonstrate that adding only low amount (<2 wt.%) of P–N compound into PAO4 could improve its anti‐wear performance by 10 times, enhance its load‐carrying capacity and oxidative stability, verifying the P–N compound is a kind of multifunctional and high‐performance additive. The morphology and the chemical composition of the worn surfaces were characterised by scanning electron microscopy and X‐ray photoelectron spectroscopy, indicating that the excellent anti‐wear and load‐carrying performance could be attributed to the forming of boundary lubrication film composed of iron oxide, iron phosphate, organic phosphine and organic amine, and so on. The work reported here is helpful to pave the practical applications and to understand the action mechanism of the multifunctional lubricant additives. Copyright © 2011 John Wiley & Sons, Ltd.     

Tribological performance of sulfonamide derivatives as lubricating oil additives in the diester

The load‐carrying capacity, anti‐wear and friction reduction properties of two sulfonamide derivatives added to a synthetic lubricant (diester) were evaluated using a four‐ball test machine. The results indicate that both additives possess good load‐carrying capacities and excellent anti‐wear and friction reduction properties. The surface topography of the rubbed surface was investigated by scanning electron microscopy, and the chemical nature of the anti‐wear films generated on the steel counter‐face was investigated by X‐ray photoelectron spectroscopy. Surface analysis results show that a stable lubricating film consisting of the reaction layer and adsorption layer was formed on the worn surface. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of molybdenum polyisobutenyl succinate (MoPIBS) and a study of its tribological characteristics

In view of the increasing awareness of the need for environmental protection, it is considered important to have more low‐ or non‐phosphorus additives in engine oils. To this end, molybdenum polyisobutenyl succinate (MoPIBS) was synthesised and its structure investigated using spectroscopy. The antiwear and friction‐reducing behaviour of MoPIBS as an oil additive was investigated and compared to that of zinc dialkyldithio‐phosphate (ZnDDP) using a four‐ball tester. The results indicated that MoPIBS in white oil exhibits very good antiwear and friction‐reducing properties, better than those of ZnDDP, but that the load‐carrying capacity is poorer than that of ZnDDP. The worn surfaces were investigated using Auger electron spectroscopy (AES) and X‐ray photoelectron spectroscopy (XPS). It was found that the boundary lubrication film on the worn surfaces was mainly composed of MoO₃ and Fe₂O₃, which contributed to improving the tribological properties of MoPIBS as an additive in white oil.     

2-烷基硫甲基噻吩系列添加剂对菜籽油磨擦学性能的影响

利用四球摩擦磨损试验机考察了噻吩及实验室合成的2－烷基硫甲基噻吩对菜籽油摩擦学性能的影响，用X射线光电子能谱仪（XPS）和扫描电子显微镜（SEM）观察分析了磨损表面的形貌和元素存在状态。用差示扫描量热法（DSC）评价了合成添加剂对菜籽油氧化稳定性的影响，结果表明：2－烷基硫甲基噻吩系列化合物大大提高了菜籽油的承载能力，但加剧了钢－钢摩擦副的磨损。含上述添加剂的菜籽油在摩擦副表面发生摩擦化学反应，生成菜籽油和添加剂共同作用所产生的边界润滑膜，从而改变了菜籽油的润滑性能，2－烷基硫甲基噻吩可明显改善菜籽油的抗氧化性能。     

Tribological Performance and Wear Mechanism of Compound Containing S,P, and B as EP/AW Additives in Copper Foil Oil

Research and development on the high biodegradability of additives is indispensable for environmentally friendly lubricants, which is one of the key factors to advance lubricant technology toward “greener” chemistry. The tribological performance of fatty alcohol polyoxyethylene phosphate acid ester (EK), boron-containing amide (BT), dialkyl dithiophosphate ester (DDE), and a mixture of these (compound) as extreme pressure (EP)/antiwear (AW) additives in hydrogenated base oil (GH) were investigated using a four-ball testing machine. The elemental composition and chemical characteristics of the AW films generated on the surfaces of the steel balls were studied using X-ray photoelectron spectroscopy (XPS), and their AW mechanisms are hereby proposed. Thermal degradation tests were conducted to identify their thermal stabilities using thermogravimetry and differential scanning calorimetry. The results show that these additives can greatly improve the EP/AW properties of GH. XPS analyses of the worn surfaces indicate that decomposed borate esters and organic sulfide or nitrides were adsorbed on the worn surface, and the P and S elements of the compound reacted with the metal and existed in the form of phosphates and sulfates, both of which contributed to the formation of a boundary lubricating film. Moreover, these additives provide the lubricants with excellent oxidation resistance and thermal stability.     

噻唑衍生物在菜籽油中的摩擦学性能研究

合成了两种噻唑衍生物，采用热重分析对其热稳定性进行了评价；利用四球摩擦磨损试验机考察了其在菜籽油中的摩擦学性能，并用扫描电子显微镜和x射线光电子能谱仪观察分析了磨斑表面的形貌和元素化学状态。结果表明：噻唑氨基甲酸衍生物添加剂可显著改善菜籽油的减摩抗磨性能和承载能力；含上述添加剂的菜籽油在摩擦过程中发生了摩擦化学反应，生成了含菜籽油甘油酯、有机硫化物、硫酸亚铁等组成的边界润滑膜，从而改善了菜籽油的摩擦学性能。     

Initial exploration of tribological performance of novel triazine derivatives in water

Two novel ashless and non‐phosphorus triazine derivatives, 2,4,6‐tri‐[N‐hydroxyethyl‐amino‐methylenesulfanyl]‐s‐triazine and 2,4,6‐tri‐[N,N‐bishydroxyethyl‐amino‐methylenesulfanyl]‐s‐triazine, were synthesised and their structures were analysed through elemental analysis. Their tribological behaviours in water were evaluated with a four‐ball machine, and the results indicate that their wear resistance and load‐carrying properties were excellent. The chemical features of the worn steel surface were observed and examined by means of inverted metallurgical microscopy and energy dispersive X‐ray spectroscopy. The analytical results of typical elements on the worn steel surface demonstrated, during the lubricating process, that the additives reacted with counter‐face metal and generated a boundary lubricating and protecting film containing elements of S, N and others, which contributes to improve the tribological properties of the test fluid. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and tribological behaviour of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives

Two types of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives were synthesised and characterised by infrared spectroscopy. Their tribological properties in rape seed oil and in mineral oil were evaluated in a four‐ball tester. The morphologies of the worn surfaces were analysed by scanning electron microscopy. The results show that the modified rape seed oil additives improve the load‐carrying capacity and the antiwear and friction‐reducing properties of rape seed oil more than they do those of mineral oil. The inferred mechanism of lubrication is that a high‐strength adsorption film and/or tribochemical reaction film forms on the rubbing surfaces, due to the carrier effect of long‐chain rape seed oil molecules and to the high reaction activities of phosphorus and nitrogen and their synergism.     

Comparison of boundary lubrication films formed under four‐ball friction test conditions with different AW/EP Additives‐an X‐ray photoelectron spectroscopy study

The antiwear and extreme‐pressure properties of six different types of additive (molybdenum dialkyldithiophosphate, dibenzyl disulphide, molybdenum dialkyldithiocarbamate, zinc dialkyldithiophosphate, chlorinated paraffin wax, and triaryl phosphate) were evaluated by standard four‐ball friction and wear tests. This was followed by scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and X‐ray photoelectron imaging (XPI) analyses of the worn surfaces to determine the structure of the boundary lubrication film and the mechanism of the tribochemical reaction occurring during the friction process. The presence of the additives in the base oil significantly increased the weld load and drastically reduced the wear‐scar diameter, suggesting antiwear and extreme‐pressure properties of the additives. The enhanced antiwear and loadcarrying capacity of the additive‐containing oils was attributed to the formation of a complex boundary lubrication film formed between the surfaces during the friction process as a result of the tribochemical reaction. The antiwear and extreme‐pressure properties of the additives were explained based on the XPS data. The studies indicated that the lubricating properties of the additives depend on their chemical nature and reactivity with metal surfaces.     

Tribological behavior of three novel triazine derivatives as additives in rapeseed oil

Three novel phosphorus-free triazine derivatives, referred to as ZOO, ZOS and ZDION, respectively, were synthesized. Their thermal stabilities and anticorrosive properties were investigated. Their tribological behaviors as additives in rapeseed oil were evaluated using four-ball friction and wear testers as well. The results suggest that all the synthesized compounds have good thermal stability, corrosion inhibiting ability and excellent tribological behavior. That is, ZOS has the best extreme pressure, antiwear and friction-reducing properties under a wide range of test conditions. And, ZOO generally possesses excellent load-carrying capacity and good antiwear and friction-reducing abilities. However, ZDION shows poor antiwear effectiveness in rapeseed oil, though it is capable of improving the load-carrying capacity and friction-reducing ability of the base stock under high concentration (>1.5 wt.%) and low load (<392 N). The worn surfaces of the steel balls were observed using a scanning electron microscope coupled with an energy dispersive spectrometer (SEM/EDS) and X-ray photoelectron spectroscope (XPS). It is supposed that the synthetic additives adsorb and react with the steel surfaces during the rubbing process and generate a surface protective film composed of sulfate, sulfide, iron oxide and N- and/or O-containing organic compounds, which accounts for the better tribological behaviors of the base stock containing the synthetic additives as compared with the base stock alone.     

A study of the synergistic effect of a triazine‐dithiocarbamate derivative with tcp in vegetable oil

A novel, potential ashless S—N‐style additive, 2‐(N,N‐dibutylamino)‐4,6‐bis‐(dibutyldithiocarbamate)‐1,3,5‐triazine (DBTT), was synthesised and its synergistic tribological effect with a traditional additive, tricresyl phosphate (TCP), in rape seed oil was evaluated using a four‐ball tester. The results show that the load‐carrying capacity of the vegetable oil could be improved greatly by the presence of the TCP and/or DBTT additives; at certain mass ratios TCP and DBTT show a good synergistic effect in improving the tribological performance of the base oil. The elemental composition and chemical nature of antiwear films generated at a steel interface were investigated using Kevex energy dispersive X‐ray analysis and X‐ray photoelectron spectroscopy. Scanning electron microscopy was also used to study the rubbed surface morphology. It was found that FeS, organosulphur compound, sulphate, phosphate, and nitrogen‐containing compounds were present in the complex boundary lubrication film.     

Tribological properties of nano‐calcium borate as lithium grease additive

Nano‐calcium borate (NCB) with an average particle size of about 70 nm was synthesised via ethanol supercritical fluid drying technique, and the morphology and microstructures of as‐prepared particles were characterised by means of scanning electron microscope (SEM, JEOL LTD., Tokyo, Japan) and X‐ray powder diffraction. The friction and wear behaviour of the NCB as additive in lithium grease were evaluated with an Optimol‐SRV IV (Optimol Instruments Prüftechnik GmbH, Munich, Germany) oscillating friction and wear tester (SRV tester). The morphology and surface composition of the worn surfaces of lower discs after SRV test were analysed by SEM and X‐ray photoelectron spectroscopy (XPS, Physical Electronics, Inc., USA). The result demonstrated that the anti‐wear and load‐carrying capacities of the lithium grease were significantly improved, and the friction coefficient of the lithium grease decreased with the addition of NCB additive. The analytical results of XPS indicate that the good tribological performance of NCB is attributable to the formation of a boundary lubrication film composed of deposited NCB and the tribochemical reaction products such as B₂O₃, CaO and iron oxides on the rubbing surface. Copyright © 2013 John Wiley & Sons, Ltd.     

Method for scuffing propagation assessment

This paper describes a method for the determination of scuffing propagation using a four‐ball extreme‐pressure tester. The method has been developed at the authors' laboratory and is a completely new approach to the investigation of scuffing phenomena. A series of lubricants was prepared by blending antiwear (AW) and extreme‐pressure (EP) additives with a mineral base oil. Tribological experiments were then performed using the new method. The aim was to investigate the influence of such additives on scuffing propagation. It is shown that there is a significant influence of AW and EP additives on scuffing. Surface analyses (SEM, EDS) show the decisive role of the chemical reactions of AW and EP additives with the steel surface and their creation of a surface layer whose good antiseizure properties mitigate scuffing propagation and reduce wear intensity. It must be emphasised that the authors consider scuffing to be a process leading to the cessation of the relative movement of a tribosystem, known as seizure.     

Tribological properties of P‐ and N‐containing organic compounds as potential extreme‐pressure and antiwear additives

An amine salt of an alkoxylphosphate (a P‐ and N‐containing organic compound, PN) was synthesised. Its extreme‐pressure, antiwear, and friction‐reducing properties were evaluated as an additive in liquid paraffin and a mineral oil. Tests were performed on a four‐ball friction and wear tester with an AISI 52100 steel ball self‐mated pair and the results were compared with those of sulphurised olefin (SO), zinc di‐n‐butyldithio‐phosphate (ZDDP), and dibutylphosphite (DBP). The morphologies of the worn steel surfaces were observed using a scanning electron microscope, while the binding energies of some typical elements on the worn surfaces were determined using X‐ray photoelectron spectroscopy. The additives were found to increase the load‐carrying capacity and to reduce wear and friction coefficient considerably. PN as an additive exhibits better load‐carrying, antiwear, and friction‐reducing properties than SO, ZDDP, and DBP under the same test conditions.     

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.