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 study of triazine derivatives as additives in rapeseed oil

Three novel S‐P‐N type triazine derivatives were synthesised, and their structures were analysed through elemental analysis, and their tribological behaviours as additives in rapeseed oil were evaluated using a four‐ball tester. The extreme pressure and antiwear (AW) mechanism was investigated by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results indicate that the additives possess excellent load‐carrying capacity and AW ability but have some friction‐reducing property only under the low load condition. Based on the results of XPS and SEM analysis, it can be seen that tribochemical reaction occurred between the synthesised compounds and metal surfaces during the sliding process, to form a complex film which has excellent load‐carrying capacity and AW ability. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.     

含氮硼酸酯添加剂性能研究

合成了四种含氮硼酸酯,对其摩擦学性能和抗腐蚀性能进行了研究,结果表明,B-N添加剂具有优良的抗磨减摩性能和良好的极压性能,同时具有优异的抗腐蚀性能。XPS表面分析表明,B-N添加剂在磨痕表面形成了一层含B2O3、BN、O-Fe-B,氧化铁和有机化合物的复杂保护膜。     

Tribochemical Behavior of Ashless Dialkyl Dithiophosphate Ester as an EP/AW Additive in Mineral Oil for Steel–Aluminum Contact

Dialkyl dithiophosphate ester (DDPE) used as an extreme pressure/antiwear (EP/AW) additive in mineral base oil (BO) was introduced to a steel–aluminum contact in this study. The tribological performance of DDPE was explored by means of a universal tribotester under different loads and durations. The worn aluminum surface topographies were observed and photographed via laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). Tribochemical interactions between the additive and aluminum surface were investigated using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The distinction of chemical structure between lubricant untapped and the counterpart retrieved after a 3-h sliding process was detected by Fourier transform infrared spectroscopy (FTIR). The friction coefficient of a BO + DDPE-lubricated friction pair under 300 N shows the lowest value. LSCM and SEM images show that the aluminum surface lubricated with BO + DDPE was well protected under a high loading condition of 300 N, and the 3-h sliding process deteriorated the surface topography. However, DDPE was not able to offer an effective lubricating film under a mild condition of 50 N. EDS results of S and P elements on the worn surface indicate that a tribochemical film was generated under 300 N in the sliding process. XPS results further show that the chemical compounds in the tribochemical film included Al₂S₃, Al₂(SO₄)₃, AlPO₄, and Al₂O₃. The P-containing compound in the tribofilm acted as a sacrificial layer, whereas the S-containing compounds were more durable. FTIR analyses demonstrate that the phosphorus–sulfur double bond was broken up due to the tribochemical interactions.     

A method for testing lubricants under conditions of scuffing. Part II. The anti-seizure action of lubricating oils

This paper describes an investigation of lubricating oils under extreme-pressure (EP) conditions in a specially modified four-ball tester. A new test method developed at the Tribology Department of ITeE described in Part I of this paper was used. In this, during a test run, the applied load is increased continuously and the friction torque is measured. A sudden increase in the friction torque indicates the collapse of the lubricating film — where scuffing is initiated. The load at this moment is called the scuffing load. If the load is increased further, it is possible to observe scuffing propagation until seizure occurs, i.e., a defined, maximum friction torque is reached. Thus, scuffing is considered as a process leading to seizure. Using the method, tribological experiments were performed employing various lubricating oils consisting of viscosity-index improvers and antiwear (AW) and extreme-pressure (EP) additives added to a base oil. Mineral and synthetic base oils of different kinematic viscosities were used. The aim was to investigate the influence of such lubricants on scuffing initiation and propagation with the present methodology. In Part I it was shown that scuffing initiation depends strongly on the kinematic viscosity of the lubricant; the higher the viscosity, the greater the scuffing load. The presence of AW and EP additives in the lubricant increases the scuffing load significantly. It was also shown that the kinematic viscosity of the lubricant oils has no effect on scuffing propagation. However, scuffing propagation is significantly mitigated by AW and, to a greater extent, by EP additives. The results of surface analyses show the decisive nature of the chemical reactions of AW and EP additives with the steel ball surface under scuffing conditions, as well as the possible diffusion of sulphur and phosphorus. Chemical reactions and diffusion lead to the creation of an inorganic surface layer (probably iron sulphide), the good anti-seizure properties of which limit scuffing propagation.     

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.     

Combination of ashless antiwear additives with metallic detergents: interactions with neutral and overbased calcium sulfonates

The interactions of neutral and overbased calcium sulfonate detergents with ashless thiophosphate oil additives under boundary lubrication were studied. The ashless additives used were neutral and acidic dialkyldithiophosphate (DTPs) and neutral triaryl monothiophosphate (MTP). This study uses three surface analytical tools to provide elemental and chemical information at the surface and in the bulk of the derived tribochemical films. The elemental composition of the tribofilms was studied using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDX) (combined with scanning electron microscopy (SEM)). Corresponding P and S X-ray absorption near edge structure (XANES) spectroscopy was also used to provide further insight into the chemical composition of the films. SEM images of the derived tribofilms revealed that each film had distinct topographical features associated with it. XPS and EDX revealed that when oil is blended with calcium sulfonate, considerable amount of calcium is incorporated into all the tribochemical films. The phosphorus content of the tribofilm was reduced substantially when the overbased detergent was combined with MTP additive. XANES spectroscopy of the P L-edge provided direct evidence for the formation of calcium phosphate in tribofilms from the ashless additives in combination with the detergent.S K-edge revealed that sulfate is the main sulfur species formed in the bulk for all three tribofilms in the presence of the neutral detergent while a mixture of sulfite and sulfide species are formed when the overbased was used. S L-edge XANES showed that calcium sulfonate has undergone some oxidation at the surface. A more antagonistic effect was observed for MTP, with the formation of a very thin phosphate film.Tribological performance was also evaluated. Surprisingly, combination of the neutral detergent with any of the AW additives did not result in any significant change in wear to the substrate. For MTP plus neutral detergent, the thinner phosphate film produced, combined with very little change in wear protection confirms that, not only is the tribochemistry dominated by calcium sulfonate, but also confirms the anti-scuffing and AW properties that are associated with it as well. Even more surprising, was the significant decrease in wear when the overbased detergent is used. This illustrates not only that the tribochemistry was dominated by the detergent, but also the exceptional AW properties of calcium carbonate.     

摩圣修复剂减摩机理的研究

摩圣技术是一种新型的摩擦表面再生技术，其实质是通过在润滑油中添加一种“摩圣修复剂”以达到增强摩擦件减摩抗磨能力、对摩擦表面进行原位免拆卸修复的目的。以透射电镜、X射线光电子能谱仪、俄歇电子能谱仪和拉曼光谱仪分析了在添加摩圣修复剂的润滑油中运行过的齿轮表层，对摩圣修复剂的减摩机理进行了分析。结果表明：摩擦表面的修复再生层为类金刚石膜和硅氧玻璃膜。     

一种硫磷氮衍生物在菜籽油中的摩擦学研究

通过Mann ich合成了2种有机环胺的硫磷酸酯衍生物,在四球摩擦磨损试验机上研究了它们作为菜籽油添加剂的摩擦学性能。实验结果表明,该类化合物具有良好的极压抗磨性能,能提高菜籽油的极压抗磨性能。通过用X-射线光电子能谱(XPS)分析了钢球磨损表面典型元素的化学状态,显示在摩擦过程中,钢球表面形成了一层含硫、磷无机膜和含氮的有机膜。     

Chemistry of Antiwear Films from Ashless Thiophosphate Oil Additives

X-ray absorption near-edge structure (XANES) spectroscopy has been combined with atomic force microscopy (AFM) to investigate the interaction of ashless thiophosphate oil additives on steel. Both mono- and dithiophosphates were studied and compared with one another in terms of chemistry and tribological performance. XANES revealed that, thermally, all three thiophosphate additives behaved similarly with steel to form a thermal film at temperatures of 150 °C. The thermal films all consisted of a layered structure comprised of Fe(II) polyphosphate and FeSO₄ in the bulk and iron polyphosphate of various chain length towards the surface. Tribochemical films generated at 5min, 1 h, and 6 h of wear testing revealed that for all three additives, the phosphorus chemistry of an antiwear (AW) film remained chemically consistent throughout all rubbing times. This suggests that the phosphorus chemistry of the AW film is determined in the initial stages of tribofilm formation. The iron polyphosphate chain length remained uniform throughout the AW film with short chain iron polyphosphates found both at the surface and in the bulk of the films. Mild AW conditions produced several different forms of sulfur at the various stages during wear testing. S K-edge XANES spectra for the 5-min tribofilms (both total electron yield and fluorescence yield) showed oxidized and reduced forms of sulfur throughout the films for all three additives. Over extended periods of rubbing (6 h), the more thermodynamically stable product, FeSO₄, was produced and became the major constituent of the tribofilms formed. Iron sulfate was present throughout the films with only traces of reduced sulfur present.AFM imaging of the AW films revealed that the morphology of the films varied from additive to additive and changed over the duration of wear testing. Generally, the AW films were composed of elongated pads orientated in the sliding direction. As rubbing continued, the pads of each AW film became more homogeneous. The larger pads of AW film appeared to have supported most of the load throughout the course of wear testing, resulting in better AW protection to the metal over increased periods of rubbing     

镍、硼、碳纳米管复合镀层的性能研究

用化学复合镀的方法,把碳纳米管作为第二相加入到化学镀镍、硼镀液中得到复合镀层;改变还原剂二甲胺硼烷在镀液中的相对浓度,得到各种镀层表面成分;利用摩擦磨损实验机、电化学综合测试仪、X射线光电子能谱（XPS）和扫描电子显微镜等分析复合镀层的摩擦磨损性能、耐腐蚀性能、表面形貌特性。结果表明：通过化学复合镀的方法得到镍、硼和碳纳米管复合镀层,其平均厚度为7~8μm;随着二甲胺硼烷在镀液中的相对浓度增加,镀层的摩擦因数减小,磨损量和腐蚀电流也减小,说明随着二甲胺硼烷在镀液中浓度的增加,其耐磨性和抗腐蚀性逐渐提高;复合镀层中主要含有N i,C,O 3种元素,其中N i以单质的形态出现,B以N i2B化合物的形态出现,元素C以碳纳米管形式沉积在镍硼基体中。     

二聚酸盐的合成及在水性基础液中的摩擦学及防腐防锈性能

采用二聚脂肪酸和二乙醇胺合成一种二聚酸二乙醇胺盐,在四球摩擦磨损试验机上考察其在水性基础液中的摩擦学性能,用 X 射线光电子能谱仪（XPS）分析试验后钢球磨斑表面典型元素的化学状态。结果表明：该添加剂在水性基础液中具有优良的承载能力和抗磨减摩性能以及良好的液相防锈和抑制铜片腐蚀的性能;在摩擦过程中,磨斑表面形成了含氮的吸附膜和含 Fe2 O3的化学反应膜,二者协同作用使添加剂在摩擦过程中具有良好的抗磨减摩性能。     

Low-Temperature Friction in the XPS Analytical Ultrahigh Vacuum Tribotester

In this work, we present the capability of ultrahigh vacuum analytical tribometry for studying mechanisms of friction at low temperatures. We investigated the low-temperature frictional behavior of two different materials: ice and polyethylene (PE). We successfully formed a thin layer of ice on a steel surface, at temperature as low as 123 K in an ultrahigh vacuum. The surface characterization technique used for this study was X-ray photoelectron spectroscopy (XPS). We investigated the frictional behavior of such a thin ice layer. The changing friction as a function of temperature indicates that the ice might undergo pre-melting even at temperatures below 123 K. A polyethylene (PE) film previously deposited on a metal surface also showed changing friction as a function of temperature in the range 123 to 400 K. As there is no change in the nature of the surface chemistry of the polymer, as indicated by XPS, the results are therefore interpreted in terms of change in ductile-to-brittle transition of the polymer film over the temperature range. This work enables the fundamental investigation of friction at low temperatures with the help of surface analysis.     

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.     

Room temperature ionic liquid 1-ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide as lubricant for steel–steel contact

The ionic liquid 1-ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide was synthesized and evaluated as lubricant for the contact of steel/steel. The tribological properties of the ionic liquid as lubricant were investigated on an Optimol SRV oscillating friction and wear tester in ambient condition. The synthetic ionic liquid shows excellent tribological performance and is superior to the ionic liquid of alkylimidazolium tetrafluoroborate and the conventional high temperature lubricants X-1P and PFPE in terms of antiwear performance and load-carrying capacity. The elemental composition and chemical nature of the boundary film generated on the 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), and the XPS results indicate the occurrence of a complicated tribochemical reaction of ionic liquid with iron on the rubbing surface during the sliding process, with the formation of FeS, organic fluoride, inorganic fluoride, etc.     

磨损条件对轴承钢低温离子渗硫层摩擦学行为的影响

采用低温离子渗硫技术在GCr15钢的表面制备了FeS固体润滑薄膜。在球-盘摩擦磨损试验机上对比研究了GCr15钢渗硫在不同磨损条件(工况和润滑条件)下的摩擦学性能。利用SEM观察分析了磨损表面形貌及成分,利用XPS分析了磨损表面边界润滑膜化合物的价态。结果表明:在改变工况的情况下,转速的增加会使FeS层作用下降,载荷的增加也会降低FeS层抗磨性能,但在一定转速下都存在一个可充分发挥FeS层减摩特性的载荷。使用极压抗磨添加剂不能改善渗硫层的减摩抗磨性能,但选用合适的添加剂可增强渗硫处理件的承载能力,提高其在恶劣工况下的磨损寿命。     

纳米Al/Sn金属颗粒对润滑油抗磨极压性能的影响

利用四球试验机分别对添加有纳米铅粉、锡粉以及Al Sn金属粉的润滑油进行极压和抗磨性能实验。采用SEM(扫描电子显微镜)对摩擦表面进行观察，采用EDS(能量色散谱仪)对表面进行元素测定。测试结果表明．纳米Al Sn金属粉可在较宽的载荷范围内明显改善润滑油的极压抗磨性能。其作用机理是锡粉在低载荷阶段沉积到摩擦表面起到抗磨剂作用，铝粉在高载荷阶段沉积到摩擦表面起到极压剂作用．从而实现了在低载荷到高载荷范围内对润滑油抗磨极压性能的提高．     

The tribological behaviors of hydroxyl-containing dithiocarbamate-triazine derivatives as additives in rapeseed oil

Two ashless and non-phosphorus hydroxyl-containing dithiocarbamate-triazine compounds, 2,4,6-tri[N,N-di-n-butyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDBA) and 2,4,6-tri[N,N-di-i-octyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDIOA), were prepared and their tribological behaviors as additives in rapeseed oil (RSO) were evaluated using a four-ball tester. Thermal degradation tests were conducted to identify their thermal stabilities using a thermo-gravimetric analyzer. The worn surfaces of the steel balls were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The results indicate that the additives possess high thermal stabilities and good load-carrying capacities. Moreover, they both have good anti-wear and friction-reducing property at relatively high concentration (>1.5 wt%) and under all test loads. The results of XPS analyses illustrate that the prepared compounds as additives in RSO forms a protective film containing inorganic sulfide, sulfate, oxidized compounds and organic nitrogen-containing compounds on the metal surface during the sliding process.