Friction and wear characteristics of organic thiophosphoro derivatives — a critical appraisal

The friction- and wear-reducing characteristics of a number of oil soluble sulphurised and phosphosulphurised derivatives for automotive and industrial applications have been reviewed. A critical appraisal of the suggested causes and mechanism of friction reduction and antiwear characteristics has been made. A review of the investigations carried out so far indicates that the creation of in situ films of simple inorganic salts, such as molybdenum disulphide, iron sulphide or phosphates of low shear strength and lamellar structures, does not appear to be the primary cause of low friction and wear. The authors have, therefore, undertaken a programme to synthesise and study the chemistry and tribochemical reactions in relation to friction and wear characteristics of hydrocarbon-soluble thiophosphoro derivatives of alkyl phenol, alcohols and fatty esters, in order to establish relationships between the reactivity, chemical nature and crystal structure of films formed on rubbing surfaces, and antifriction and antiwear characteristics. Molybdenum salts of various phosphorothio derivatives of pentadecylphenol and lauryl oleate of specific structures have been synthesised, and the friction and wear properties of their blends in mineral oil base stock have been studied. It can be clearly inferred from the results that type of bonding between sulphur, phosphorus and molybdenum, and reactivity of these derivatives with rubbing surfaces, determine their friction-reducing and antiwear characteristics. Their reactivity with iron and the nature of films formed are under investigation.     

Antiwear, extreme pressure and antifriction action of friction polymer forming additives

A new type of additive for oils and fuels has been developed. These additives, ashless organic compounds forming friction polymers, provide simultaneous antiwear, extreme pressure and antifriction action. The results of four-ball testing of antiwear, extreme pressure and antifriction action of the additives are presented. Their ability to form friction polymers on rubbing metal surfaces was confirmed by radiotracer investigations.     

Lubricity characteristics of ethyl hexyl esters of mono‐, Di‐, and triethenoic acids

Ethyl hexyl esters of oleic, linoleic, and linolenic acids were synthesised and thermally polymerised to obtain products with viscosities in the range of hydrocarbon lubricants at 100°C (11–22 cSt). Molecular weight, elemental analysis, IR, ¹³C NMR, and intrinsic viscosity data showed that most of these derivatives are mixtures of monomers, dimers, and trimers and have linear and cyclic products with predominantly trans characteristics. The lubricity characteristics were determined on a friction and wear tester under conditions of thick‐film lubrication. A comparison was made with hydrocracked hydrocarbon lubricants of comparable viscosities at 100°C and comparable viscosity indices. It is inferred that all the esters maintained relatively thicker surface films and much lower friction coefficients than the hydrocarbon oils. The wear‐scar data show that the antiwear characteristics of polymerised ethyl hexyl oleate and linoleate are only slightly inferior to those of the hydrocarbon oils, but at higher temperatures their antiwear characteristics rapidly deteriorate while the friction coefficients markedly increase and become comparable to those of hydrocarbon oils. These studies are being undertaken with a view to selecting and modifying vegetable oils containing mixtures of fatty acids for obtaining esters of outstanding friction, wear, and film‐forming characteristics.     

Synthesis and Tribology of Micro-Carbon Sphere Additives for Enhanced Lubrication

Poor or inefficient lubrication often gives rise to high friction and wear losses in machine components, which adversely affect their performance, efficiency, and durability. Many approaches are being explored to enhance the antifriction and antiwear properties of sliding machine components. In this study, the antifriction and antiwear properties of carbon spheres, synthesized from plastic waste by an autogenic process, were investigated as an additive to a poly-alpha-olefin (PAO-4 grade) oil. When dispersed at 1 wt% concentration, the carbon spheres reduced both friction and wear under boundary-lubricated sliding conditions. In particular, the reduction in wear was quite dramatic and appeared to be enabled by the formation of a fairly thick (≈200 nm) carbon-rich boundary film, the formation of which is attributed to tribochemical interactions between the carbon particles and sliding contact surfaces.     

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.     

Antiwear,Antifriction, and Extreme Pressure Properties of Motor Bike Engine Oil Dispersed with Molybdenum Disulfide Nanoparticles

This work presents studies on the antiwear, antifriction, and extreme pressure properties of motor oil dispersed with MoS₂ nanoparticles. Commercial oil (SAE 20W-40 grade) is dispersed with stabilized MoS₂ nanoparticles in 0.25, 0.5, 0.75, and 1 wt%. The test oils are tested for antiwear, antifriction, and extreme pressure properties on a four-ball wear tester. The wear and friction offered by nanolubricants has decreased remarkably compared to the commercial base oil. The weld load and load wear index of oils dispersed with nanoparticles were improved substantially compared to the commercial base oil. Metallographic studies conducted on the wear balls from the extreme pressure test show that nanoparticles are deposited on the worn area along with additives in the oil, thereby preventing welding of the surfaces. An optimum weight fraction is arrived at for best performance. A synergy between the additives in the oil and dispersed nanoparticles has been observed, resulting in less dispersion for best results. It is found that beyond an optimum weight percentage of nanoparticles, the trends tend to reverse, resulting in greater wear and friction.     

Lubricating properties of thixotropic systems based on overbased calcium sulphonate

The lubricating properties of model systems based on overbased calcium sulphonate have been studied by simulating various kinematic, temperature, and load‐speed conditions of friction machines as well as by investigating the physical chemistry of friction surfaces. It is found that overbased sulphonate complex grease shows good antifriction, antiwear, and extreme‐pressure properties. A synergistic action of calcium tetraborate with overbased sulphonate has been shown. It is found that calcium 12‐hydroxystearate, however, has little effect on lubricating properties, but it improves the thixotropic characteristics of a system.     

Origins of the friction and wear properties of antiwear additives

Experimental techniques have been developed to measure the friction, antiwear film‐forming and wear properties of lubricants in rolling–sliding contact. Friction measurements show that zinc dialkyldithiophosphates (ZDDPs) and also some other phosphorus‐based additives increase friction in mixed lubrication. Film thickness measurements show that this increase in friction correlates with the thickness of antiwear film. They also reveal some of the drivers of antiwear film formation and removal. A novel wear tester is described which enables the mild wear resulting from ZDDP‐containing oils to be monitored. Copyright © 2006 John Wiley & Sons, Ltd.     

S—P—Mo减摩剂的摩擦磨损性能的研究

本文用四球摩擦磨损试验机、Ｍ－２００型磨机试验机考察了Ｓ－Ｐ－Ｍｏ减摩剂在６８＾＃汽轮机油（简称６８＾＃油）中的减摩抗磨性能；用ＣＬ－１００型齿轮试验机考察了该减摩剂在６８＾＃油中的齿轮承载能力；并用光学显微镜观察了它们的磨斑表面形貌。试验表明，Ｓ－Ｐ－Ｍｏ减摩剂具有良好的减摩抗磨性能，可以大幅度提高６８＾＃油的齿轮承载能力。     

Effect of structure and composition of solid carbon coatings of steel parts on the lubricating properties of synthetic oil

The authors studied the friction and wear properties of PAOM-4 synthetic oil under the friction of steel samples, the working surfaces of which are coated by a thin layer of hard carbon coatings of different composition and structure. It was found that some of them result in improved antiwear and antifriction behavior of the test oil. The polyalphaolefin oil shows the highest lubricity when steel rubs against carbon diamond-like coatings, namely, monocrystalline tungsten-alloyed coatings and amorphous coatings with increased sp ³ phase.     

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.     

FTIR micro‐reflectance absorption spectroscopic analysis of chemisorbed reaction films for tribological applications

Chemisorbed reaction films (CRFs) were prepared by using iron (Fe) particles (100–200 mess size) and thio (sulfur) derivatives of ethyl octadecenoate and methyl 12‐hydroxy octadecenoate in the light viscosity paraffin liquid medium. The reaction was conducted in a simulated condition of tribo‐chemical situation. CRFs were obtained in solid amorphous phase. The CRFs were examined for elemental composition and layer analysis using C‐H‐N‐O‐S analyzer and Fourier transform infrared spectroscopy micro‐reflectance absorption spectroscopic technique. Further, the CRFs were isolated into organic solvent soluble fractions using polar solvents of increasing polar strength. Their elemental analyses were studied, and chemical constitutions were known. Friction coefficient and wear scar diameter were evaluated by high frequency reciprocating rig, PLINT TE‐77 machine. Thermal stability was studied using thermogravimetric analysis technique in nitrogen environment. These studies inferred that CRFs appeared varying in its composition, luster, phase, chemical structure and thermal stability. Nonetheless, these were also found anomalous in elemental distribution throughout the layer structure of the CRFs. Copyright © 2015 John Wiley & Sons, Ltd.     

Cyclic alkyl disulphides as tribological additives

The use of sulphurised natural oils, fats, hydrocarbons, terpenes and aromatic sulphides, disulphides and poly‐sulphides, as antiwear and extreme‐pressure lubricant additives, has already been established. However, the potential of cyclic disulphides merits attention. The present paper describes the synthesis and evaluation of 0.5% solutions of certain cyclic disulphides as tribological additives, namely 3‐alkylimino‐5‐(N‐methylphenyl)amino‐1,2,4‐dithiazolines, in paraffin oil, on a four‐ball test rig with 12.7 mm diameter steel bearing balls as test specimens. All the additives in general, and 3‐propylimino‐5‐(N‐methylphenyl)amino‐1,2,4‐dithiazoline in particular, exhibited remarkably good antifriction and EP activity, showing a decrease in the coefficient of friction and an appreciable increase in the load‐carrying capacity of the lubricant as indicated by a higher load wear index. The wear surface topography of the used test specimen was studied using scanning electron microscopy.     

Film Formation by Colloidal Overbased Detergents in Lubricated Contacts

It has been reported in the literature that overbased detergents can possess good antifriction and antiwear properties although the origins of these properties are not fully clear. In practice, over-based detergents are colloidal dispersions and this may be important in determining their properties and mechanism of action.

In the current study, the lubricating properties of commercial, overbased magnesium and calcium sulfonates were measured in thin film, lubricated conditions and compared to a neutral sulfonate additive. A range of techniques was employed to evaluate the tribological performance of solutions of these additives. Film thickness measurements were carried out using optical interferometry and in-contact visualization, while friction and wear measurements complemented the study.

It has been found that, when operating in thin film conditions, overbased detergents deposit solid-like boundary films on the rubbing surfaces. These films form in both rolling and mixed rolling/sliding conditions and, unlike many other colloidally-formed boundary films, are able to survive in high speed, thick film conditions. During formation, the film rapidly reaches a thickness corresponding to one colloid particle diameter, between 10 and 20 nm. After prolonged rubbing, however, the film thickness reaches the equivalent of three particle diameters. No such thick boundary films are observed with the neutral sulfonate.

The boundary films formed by overbased detergents produce a significant reduction in wear. However, for the very smooth surfaces used in this study, they also result in an effective roughening of the very smooth surfaces studied. This leads to an increase in friction in the intermediate speed region by promoting solid-solid contact in thin fluid film conditions.     

Tribological behavior of polyoxymethylene in water-lubricated friction against steel

The paper presents the results of testing of block specimens of polyoxymethylene in water-lubricated sliding boundary friction against steel. Under these testing conditions, the polymer shows a combination of good antiwear and antifriction properties. With the addition of 1 wt % of fullerene soot to the polyoxymethylene, its wear rate declines considerably, while its friction coefficient remains quite low; its carrying capacity is high and the wear process is stable. The friction coefficient, wear rate, and carrying capacity of modified pilyoxymethylene are compared to those of carbon plastics under similar testing conditions.     

Phenomenological investigation of cutting oils: Antifriction and antiwear behaviour at low contact temperatures

The basic reason for the use of cutting fluids in machining processes is to increase cutting tool life and to decrease machining costs. An experimental simulation of tribological processes representing rear cutting tool contact was carried out in a ‘block‐on‐disc’ (BOD) tribotester. The coefficient of friction and block wear were measured under boundary (discontinuous fluid film) and elastohydrodynamic (quasi‐continuous fluid film) lubrication conditions for neat and semi‐synthetic cutting oils. Interesting data were obtained concerning a further cutting fluid classification, which is helpful for optimal cutting fluid selection via their antiwear and antifriction characteristics.     

Borate esters used as lubricant additives

Borate esters possess friction‐reducing, antiwear, and anti‐oxidant characteristics when blended in lubricating oils. However, borate esters are susceptible to hydrolysis. The formation of a stable five‐member ring structure in the ester molecules, involving coordination of nitrogen with boron, contributes substantially to the resistance to hydrolysis of borate esters. The susceptibility of borates to hydrolysis can be reduced by introducing N,N‐dialkylaminoethyl groups with alkyl radicals containing more than three carbon atoms. X‐ray photo‐electron spectroscopy and X‐ray diffraction reveal that the borate ester can be adsorbed on the rubbing surface, and some of the adsorbed borate film degrades and forms boron nitride. Four‐ball wear tests indicate that the combination of oil‐soluble copper, tin, and cadmium compounds with organoborates gives better antiwear properties than the components separately. An antiwear synergistic mechanism is postulated in which borates with electron‐deficient boron p orbitals catalyse the tribo‐reduction of the metal compounds on the rubbing surfaces, producing elemental metals.     

矿物质润滑油添加剂对钢/铝锡合金摩擦副摩擦学性能的影响

利用销盘式磨损试验机研究了一种矿物质润滑油添加剂对钢/铝锡合金摩擦副摩擦学性能的影响,并考察了这种添加剂对实际工况下铝锡合金轴瓦的作用效果。采用AFM、SEM/EDS等仪器对摩擦副表面进行了分析。结果表明,矿物质添加剂在低载荷比高载荷条件下的减摩抗磨效果明显;试验时间越长,添加剂的作用越充分,圆盘表面的Al-Sn共晶体的分布越分散,减摩效果越明显;添加剂降低了摩擦表面的粗糙度,显著地提高了摩擦副的减摩抗磨性能,延长了使用寿命;添加剂作用后的摩擦表面发现了少量的Fe元素,并发现个别添加剂粒子和磨损粒子在铝锡合金表面的镶嵌和沉积,添加剂提高了其承载能力。     

Effect of the chemical structure of fluorinated esters on the tribological properties of a steel‐on‐steel system

The effect of the chemical structure of fluorinated esters on the friction and wear behaviour of a steel‐on‐steel system was investigated. The friction and wear testing of a steel disc sliding against a counterpart ball of the same steel was carried out using an Optimal SRV oscillating friction and wear tester. The chemical features of the worn steel surfaces were analysed by means of X‐ray photoelectron spectroscopy, and the morphologies and elemental compositions of the worn steel surfaces observed and determined using scanning electron microscopy. The results indicate that a fluorinated ester with methylene groups that are not substituted by fluorine in the acid structure gives the best friction‐reducing behaviour and a keto‐ester shows the best antiwear properties.     

Friction reduction and antiwear properties of the complex of lanthanum and a phosphate

A complex of lanthanum and di-(2-ethyl hexyl) phosphate (DEHP) was prepared by an extractive method. The friction and wear behaviour of this complex (called LaDEHP) as an oil additive was evaluated using a four-ball wear tester. For comparison, the friction reduction and antiwear properties of DEHP as an oil additive were also investigated. The chemical composition of the boundary film formed after wear tests was examined using X-ray photoelectron spectrometry (XPS). Four-ball tests showed that the prepared LaDEHP was effective in reducing friction and wear as well as in increasing load-carrying capacity. XPS analyses indicate the formation of iron phosphate and lanthanum oxide in the boundary films.