Tribological Properties of AW/EP Additives Under Different Thermal Conditions

The authors have investigated the action of compositions containing AW/EP additives under scuffing conditions and the influence of temperature on the wear of friction elements lubricated with selected compositions. It has been stated that postponing of scuffing initiation is not always sufficient to obtain very good strength of the surface layer and decreased wear. Furthermore, the results show the influence of temperature and load on the additive action. Extreme friction conditions result in greater wear intensity and change in the lubricating action of additives.     

The temperatures at scuffing and seizure in a four‐ball contact

The temperature of surface asperities affects lubricant‐surface tribochemical interactions. It is important to know the nature of this to identify ways of preventing scuffing and seizure under extreme‐pressure (EP) conditions. A new model for the determination of the temperature of contacting asperities is presented in this paper. It assumes the superposition of thermal processes occurring on the macroscale and thermal phenomena in the contact of asperity tips (microscale). Numerical results have been obtained for conditions of four‐ball testing of various lubricating oils — a mineral base oil with and without antiwear and EP additives. To calculate the scuffing and seizure temperatures, knowledge of the mechanical and physical properties of the test ball material (bearing steel) and lubricants, as well as the parameters describing the surface topography of the balls, was necessary. Friction coefficient curves were also needed; they were determined during four‐ball tests with a continuously increasing load. For the base oil with lubricating additives, the temperature of contacting surface asperities at the moment of scuffing initiation was calculated to be about 230°C and increased to over 1000°C at the highest loading of the four‐ball tribosystem. This suggests the possibility of tribochemical reactions of the lubricating additives with the steel surface, and diffusion of some elements, a modified surface layer having good antiseizure properties being produced. Such a layer prevents seizure of the tribosystem. For the base oil without lubricating additives, scuffing initiated at about 150°C and the temperature exceeded 1200°C at seizure. The temperature values obtained agree with results in the literature.     

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.     

The lubricant-coating interaction in rolling and sliding contacts

The research presented in this paper was aimed at elaboration of a new technology for heavy-loaded machine elements, lubricated with ecological oils.The tribological experiments were performed using four-ball tester (scuffing resistance), cone-three balls pitting tester (fatigue life), as well as gear test rig (resistance of lubricated gears to scuffing). The tribosystems were lubricated with various base oil and vegetable-based eco-oil.The tested components were coated with standard single coatings (TiN, CrN) and low-friction coatings (a-C:H:W, MoS₂/Ti). The results obtained confirm that low-friction a-C:H:W coating has a great potential for application in heavy-loaded machine components. Under extreme-pressure conditions this coating can take over the functions of anti-wear/extreme-pressure (AW/EP) additives and through this it is possible to minimise the application of toxic lubricating additives and achieve “ecological lubrication”.     

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.     

Effect of the type and concentration of lubricating additives on the antiwear and extreme pressure properties and rolling fatigue life of a four‐ball tribosystem

Tests were performed on two different four‐ball testers. The first was used to determine antiwear (AW) and extreme pressure (EP) properties at sliding friction. The second was used to assess the surface fatigue (pitting) life at rolling movement. Lubricating oils of various chemical compositions were tested. A base mineral oil was blended with two different commercial packages of lubricating additives of AW and EP types. The AW additives contained ZDDP and were blended with the base oil at 0.2 and 3wt %. The EP additives were organic compounds of sulphur and phosphorus, blended with the base oil at 1 and 10wt %. It is shown that AW additives not only improve AW and EP properties but also — at 0.2% — are beneficial for the fatigue life. An increase in the concentration of AW additives leads to an improvement of AW and EP properties but — for one of the packages — reduces the fatigue life. EP additives — at 1% concentration — significantly improve EP properties, and to a lesser extent AW properties. Such a concentration of EP additives has no influence on the fatigue life. An increase in the concentration of EP additives leads to a further improvement of EP and AW properties. However, this is accompanied by a considerable decrease in the fatigue life. By using a scanning electron microscope and energy dispersive spectrometer for analysis of the worn surface, mechanisms of action of various lubricating additives under different friction conditions were identified. Copyright © 2006 John Wiley & Sons, Ltd.     

有机硼酸酯添加剂对摩擦副抗咬死性能影响研究

通过对稀土金属硼系列有机硼酸酯（Ｃ９Ｈ２１ＢＯ３）添加剂的润滑性能抗咬死性能试验研究,发现有机硼酸酯添加剂能显著改善滑动摩擦副的抗咬死性能及润滑油的润滑性能,通过分析磨斑表面Ｘ射线衍射图谱（ＸＰＳ）,发现该种添加剂作用下,摩擦副表面形成了聚合物膜,使得摩擦力减小了,随着载荷的增大,添加剂在摩擦热产生的高温作用下,部分原子如Ｃ、Ｂ同表面金属发生化学反应,生成了ＦｅＣ,Ｂ２Ｏ３,Ｈ３ＢＯ３等物质。结合     

Tribological behaviour of antiwear additives used in hydraulic applications: Synergistic or antagonistic with other surface-active additives?

This paper examines the friction and antiwear (AW) properties using SRV (Schwing–Reib–Verschleiss) tribometer and film-forming properties using atomic force microscope (AFM) of one simple model formulation containing solely AW additive and seven oils containing mixture of additives including three zinc-based packages (ZP), ZP with additional AW additives, ZP with extreme pressure (EP) additives, ZP with viscosity index improvers (VII) and one zinc-free ashless package in steel/steel contacts. VII-containing oil show lower boundary and mixed friction coefficients than the other oils. Although all AW additive-containing oils formed tribofilms, AW properties of ZPs appear to be affected antagonistically by EP additives while synergistically by VII. Zn-free additives investigated in this study show higher wear than ZPs.     

纳米硼酸镧添加剂的摩擦学性能研究

通过对含纳米硼酸镧粒子添加剂润滑油的摩擦学性能试验研究 ,发现纳米硼酸镧添加剂能改善滑动摩擦副的摩擦学性能、抗胶合能力及润滑油的润滑性能 ,分析滑动摩擦副胶合失效后的表面形貌 ,结合失效后滑动摩擦副表面 XPS图谱 ,发现这是由于这种添加剂能在摩擦副表面形成吸附膜及聚合物膜 ,且摩擦副表面有渗硼层出现所致     

Understanding the tribological chemistry of chlorine-, sulfur- and phosphorus-containing additives

It is demonstrated that the surface chemistry of simple-model extreme-pressure lubricant additives, measured in ultrahigh vacuum, corresponds to that measured at higher pressures, where film growth rates are monitored using a microbalance. This chemistry and reaction kinetics are used to explain the extreme-pressure lubrication behavior by successfully modeling the measured seizure load vs. additive concentration curves. It is also demonstrated, by growing ferrous chloride films on iron substrates in ultrahigh vacuum, that these have the same friction coefficients as those found for model extreme-pressure lubricants. It is found that a monolayer of a solid boundary lubricant film is sufficient to lower the interfacial friction coefficient to its minimum value. These results demonstrate that the chemistry measured under conditions of thermodynamic equilibrium at some temperature can be successfully applied to the formation of a boundary film, in the extreme-pressure regime.     

The effect of the addition of phosphate esters to paraffinic base oils on their lubricating performance under sliding conditions

Using a modified Timken machine, the effect of the addition of different phosphate esters to paraffinic base oils on their lubricating performance was investigated. The aryl phosphate ester tricresyl phosphate (TCP) has a greater resistance to scuffing than the alkyl phosphate ester trioctyl phosphate (TOP) when used as a lubricant. In contrast, base oils containing TOP are superior to those containing TCP, although phosphate esters are effective only in base oils of higher viscosity than the esters.The lubricating performance of high viscosity base oils containing TOP is improved by coupling the ability of TOP to form a beneficial surface film with the ability of the base oils to build up thick oil films. The addition of TCP, which has a higher adsorption ability but a lower reactivity than TOP, seems, however, to prevent the formation of the oxide film which is formed with base oils alone owing to the marked ability of TCP to adhere to a metal surface. In addition, because of the low reactivity of TCP, iron phosphate is hardly formed. Consequently, the addition of TCP increases the scuffing load of the base oils but has the disadvantage of increasing the coefficient of friction.     

Enhancing AW/EP property of lubricant oil by adding nano Al/Sn particles

This paper presents results of experiments to enhance antiwear/extreme pressure (AW/EP) properties of a lubricant oil by adding metal nano particles. In this experiment, Al, Sn and Al + Sn nano-particles were selected as trial additives. The AW and EP properties were evaluated on Four-Ball test machine, while the feature and composition of the wear scar surface were investigated by scan electron microscope (SEM) and energy dispersion spectrum (EDS). The test results show that the AW and EP performance can be improved within a wide load range by adding Al + Sn nanoparticles. Analysis of the enhancement mechanism has also been conducted in this experiment and presented in this paper. It is found that nano-Sn particles can be deposited on the friction surface when the pressure was moderate and act as AW additive. It is also found that the nano-Al particles can be deposited under the condition of high load pressure and act as EP additive. Thus, the AW and EP properties of tested lubricant oil have been improved at the same time due to adding both Al and Sn.     

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．     

Additive influence on wear and friction performance of environmentally adapted lubricants

In this paper, the influence of concentration level and chemical composition of three different additive types on friction and wear coefficient are presented for a synthetic ester base fluid and a mineral base oil. One extreme-pressure (EP), two antiwear (AW) and two yellow metal passivator (Cu-passivators) additives were used. Factorial experimental design was used as the basis for a systematic evaluation of wear rates under mixed and boundary lubrication conditions. A total of 33 different lubricant blends were tested in a Plint and Partner High Frequency Friction Machine. For the synthetic ester, the extreme-pressure (EP) additive, containing phosphorus and nitrogen, was found to be much more effective in reducing wear than either of the two antiwear (AW) additives tested. In fact, the AW and Cu-passivator additives made little or no contribution to the wear protection in most of the cases studied. A synergy effect between the three additive combinations was observed only for the reference mineral oil blend. A significant difference between the antiwear performance of the test lubricants was found. This study suggests that the traditional “AW” and “EP” labels associated with commonly used additives are poor aids when designing of ester based lubricants.     

A method for testing lubricants under conditions of scuffing. Part I. Presentation of the method

A method for the tribological assessment of lubricants under conditions of scuffing is presented. The method uses a four‐ball tester, and allows one to assess the effect of lubricant on scuffing intensity through an analysis of changes in the friction torque and wear of the stationary balls, at continuously increasing load. The behaviour of a lubricant under scuffing conditions can be characterised using the so‐called limiting pressure of seizure p_oz, which depends on the load at which the balls seize and the average value of the wear area calculated from the wear‐scar diameters measured on the stationary balls. A comparison is made ‐ from the point of view of the resolution, time consumption, and cost ‐ of the new method with the existing, standard tests, using a four‐ball tester and a gear test rig (FZG). It is concluded that the proposed method, unlike standard FZG and certain four‐ball tests, enables one to differentiate between gear oils, in agreement with their API GL performance level. The very short run‐time of the new method enables one to perform more tests and obtain a low standard deviation. The new method is much cheaper than the standard four‐ball and FZG methods.     

DLC coating of boundary lubricated components—advantages of coating one of the contact surfaces rather than both or none

The influence of conventional extreme-pressure (EP) and anti-wear (AW) additives on the wear and friction behaviour of DLC coatings has been investigated. Special emphasis was put on exploring if it is most beneficial to coat only one or both the contacting surfaces and on when and how the coatings may improve the friction situation in sliding contact boundary lubricated systems. Tests were performed in a load-scanning test rig, which allows the normal load to gradually increase during the forward stroke and to correspondingly decrease during the reverse stroke. The sliding speed was set to 0.1 m/s, while the normal load was in the range between 140 and 1700 N (2.4–5.6 GPa).This investigation indicates that, under boundary lubrication conditions, addition of commercial AW and EP additives to PAO oil may significantly improve the tribological properties of DLC coatings. Furthermore, the DLC/steel combination was found to give a smoother running-in process and a better tribological performance than the DLC/DLC and steel/steel combination.     

Study of Influence of Lubricating Oil and Tooth Height on Friction Characteristics of Helical Gears

It is of utmost importance to know the friction loss of helical gears because they are produced quite widely used. However, basic research, e. g., load carrying characteristics of helical gears and friction loss, is extremely scarce.

In this paper the authors focused on the influence of the kind of lubricating oil on the friction loss of helical gears. Further they dealt with the influence of height of teeth on friction loss of helical gears. The authors carried out experiments to study the influence of lubricating oil viscosity, additive, and base oil type as well as rotational speed on friction loss of helical gears. Not only mineral oils but also synthetic oils were used as base oils, e. g., paraffin, poly-α-olefin, and polyglycol type oils. As additives the authors used EP additive and ZnDTP. Further they investigated the influence of tooth height on friction loss.

The experimental results gained make it possible to drastically reduce friction loss of helical gears.     

Additive—additive interaction: A study of the ep and AW performance of sulphurised jojoba oil and sulphurised xanthate in the presence of mocontaining friction modifiers

The extreme-pressure (EP) and antiwear (AW) performance of three soluble molybdenum compounds (commercial molybdenum dialkyldithiophosphate, commercial molybdenum dialkyl dithiocarbamate, and synthesised molybdenum dialkyl dithiocarbamate), and two sulphur-containing EP compounds (sulphurised jojoba oil and sulphurised O-stearyl-S-oleyl xanthate) have been studied alone and in combination, using a four-ball tester. The behaviour of the additives in combination has been found to be selective, and dependent on the chemistry and ratio of the additive components. In general, the addition of Mo-type friction modifiers (FM) to sulphurised EP compounds has shown a synergism, of varying order, in their EP and AW properties.     

Effect of temperature on the scuffing load capacity of EP gear lubricants

Scuffing of gears involves the welding together of locally unprotected metal‐to‐metal contacts when critical limits of pressure and temperature are exceeded. Protection can be maintained by a thick lubricant film, by physically adsorbed layers, or by chemical reaction layers. At higher temperatures, viscosity and film thickness decrease but, using an EP gear oil, due to higher chemical reactivity, the scuffing load capacity is not necessarily reduced accordingly. The reaction temperature of the additives is not always reached for large gears. In this paper the factors that influence the scuffing load capacity are investigated, and test possibilities and calculation methods are outlined.     

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.