Insights into the chemical behavior of zinc dialkyldithiophosphate anti-wear additives in their isomeric and decomposed forms through molecular simulation

Ab initio molecular dynamics simulations and static quantum chemical calculations at the density functional level of theory have been applied to study zinc dialkyldithiophosphate (ZDDP) engine oil anti-wear additives in their parent, isomeric and decomposed forms to identify differences in the basic finite temperature chemical behavior of these species that may be related to the formation of ZDDP anti-wear films. It was found that the isomers, which contain Zn–O bonds, exhibited finite temperature behavior leading to the dissociation of these bonds, while the parent ZDDP molecule, which contains only Zn–S bonds, did not. The dissociation of the Zn–O bonds improves the abilities of these molecules to participate in film formation reactions as electron donors and hence the transformation of the ZDDP molecule into species with Zn–O bonds, such as the isomers and decomposed species, may aid in anti-wear film formation. It was observed that the dissociation of the Zn–O bonds allowed the molecules to attain rearranged structures where the coordination at the zinc atom had significantly changed and it was found that these rearranged structures could decompose through the elimination of metathiophosphates (MTPs). The results of static density functional theory calculations on all species indicated that the elimination of MTPs was energetically favorable at high temperatures, however, the ability of the parent molecule to participate in rearrangement determines whether decomposition will occur. The fact that these species readily participate in reactions that lead to MTP formation is relevant within the context of ZDDP anti-wear film formation since the MTP molecules possess properties that make them ideal precursors to the thiophosphate networks that are thought to comprise these films. Overall, the results of this study indicate that differences between the ZDDP molecule, its isomers and decomposition products exist that may affect the abilities of these species to participate in the formation of anti-wear films. This information will be useful within the development of an overall mechanism for the formation of ZDDP anti-wear films.     

Development of novel lubricity additives: hydroxyalkyl ester of ortho-phenylene phosphate

Hydroxyalkyl esters of ortho-phenylene phosphate have been designed as novel lubricity additives for carboxylate-type synthetic oils. Excellent wear prevention and load carrying properties have been observed with hydroxyalkenyl esters. It is suggested that the presence of carbon-carbon double bonds is a key functional group to improve the lubricity of phosphate additives.     

The binding of antiwear additives to iron surfaces: quantum chemical calculations and tribological tests

The molecular orbital parameters of lubrication additives and iron atom cluster were calculated by use of a quantum chemical method. The interaction pattern between the additive and metal atom cluster was discussed base on the approximate rule of orbital energy. By comparing some parameters that characterize the action strength between additive and metal, the correlations between the additive molecular structures and additive action abilities with metal are elementally obtained, which may be used as the basis for forecasting the tribological performances of additives. The prediction results obtained by quantum chemical calculations are in good accordance with the results of tribological tests. The molecular orbital parameters exhibit advantages for studying the quantitative structure–activity and structure–property relationships of lubrication additives.     

Effects of anti-wear additives on friction and wear properties of Cr2O3 coating

The effects of some anti-wear additives on the friction and wear behaviour of plasma-sprayed Cr₂O₃ coating were investigated using a block-on-ring tester at ambient conditions. The results show that zinc dialkyldithiophosphate (ZDDP), tricesyl phosphate (TCP) and tributyl phosphate (TBP) significantly reduce the wear of Cr₂O₃ coating lubricated by paraffin oil. Additive concentrations as well as sliding time have great influence on the wear. The friction coefficient varies slightly with test conditions. The analysis by XPS of worn surfaces indicates that the wear resistance of these additives is due to the formation of tribochemical reaction films by reacting with Cr₂O₃ coatings.     

Tribological performances of heterocyclic‐containing ether and/or thioether as additives in the synthetic diester

The load‐carrying capacity, wear and friction properties of 2‐octoxyl methylthio‐benzothiazole (DEOY) and 2‐dodecylthio methylthio‐benzothiazole (DEMB) added to a synthetic lubricant (diester) were evaluated using a four‐ball test machine. The results indicate that the two compounds added to the diester possess good load‐carrying capacities and excellent anti‐wear and friction reduction properties. The thermal stability of the two compounds under nitrogen atmosphere was investigated by thermogravimetric analysis (TGA). It was found that the compounds DEOY and DEMB possess excellent thermal stability. The surface topography of the rubbed surface was investigated with scanning electron microscopy, the elemental chemical nature of the anti‐wear films generated on steel counterface were investigated with X‐ray photoelectron spectroscopy. Copyright © 2009 John Wiley & Sons, Ltd.     

Load-carrying synergism of binary additive systems: dibenzyl disulphide and halogenated hydrocarbons

In this study, the tribological performance of binary additive blends with dibenzyl disulphide and some chlorinated and brominated compounds was investigated. The tribological tests were done on a four-ball and ball-on-disc machines. For the test conditions explored, chlorinated paraffin (40% Cl), benzyl chloride, hexachloroethane, benzyl bromide and two brominated paraffins which contained 29 and 49 mass% of bromine in the presence of dibenzyl disulphide produced strong synergism with regard to their load-carrying capacities. Load-carring synergism increased with the increase of both additive concentrations. Some chlorinated and brominated hydrocarbons produced almost no synergism. Chlorinated hydrocarbons with chlorine atoms on adjacent carbon atoms and when between carbon atoms bonded with chlorine there are one or two no chlorinated carbon atoms did not exhibit antiwear and extreme pressure synergism. Increased chlorine substitution in ethane increased synergism. Amounts of iron sulphide, benzyl thiol and elemental sulphur formed from dibenzyl disulphide as well as sulphur surface concentrations for brominated compounds were strongly reduced in the case of binary additive systems' strongly improved load-carrying properties. The bromine surface concentration after wear test is very low, and independent of additive concentration. A possible explanation for the load-carrying synergism of DBDS and halogenated hydrocarbons binary additive systems is proposed.     

Mechanisms of oiliness additives

Ultrathin film interferometry has been used to measure the boundary film-forming behaviour of long chain, carboxylic acid oiliness additives. It has been shown that in dry conditions, these acids form very thin films of around 2–3 nm thickness. However when water is present, some acids form significantly thicker films, around 10 nm in thickness. The behaviour of these films is very similar to that previously seen with metal carboxylate additive films, including thick film collapse at high rolling speeds followed by film reformation at slow speeds. It is suggested thick films formed by long chain carboxylic acid additives result from reaction of the acids at the rolling solid surfaces in the presence of water to form deposits of insoluble iron carboxylate.     

Effectiveness of lubricant additives for copper‐alloy‐steel sliding contacts

A selection of additives and their performance and compatibility with a variety of copper alloys have been evaluated in an SRV test set‐up. The tests show a remarkable variation of tribological behaviour with a clear relation to both the type of lubricant / additive and the type of alloy. One ester‐based additive showed outstanding friction and wear reduction for some groups of copper alloys. In order to better understand the fundamental mechanisms, we applied a variety of surface analyses, such as 3D confocal white light microscopy, scanning electron microscopy and X‐ray photoelectron microscopy. Copyright © 2010 John Wiley & Sons, Ltd.     

The tribological study of S-(1H-Benzotriazol-1-yl) methyl N,N-dialkyldithiocarbamates as additives in synthetic lubricants

The load-carrying capacity, wear, and friction properties of four S-(1 H-Benzotriazol-1-yl)methyl N,N-dialkyldithiocarbamates added to a synthetic lubricant (diester) were evaluated using a four-ball test machine and an Optimal SRV tester. The results indicate that these compounds added to the diester possess good load-carrying capacities and antiwear properties. The rubbed surfaces were investigated using X-ray photoelectron spectroscopy (XPS) analysis and the antiwear mechanism is discussed.     

Combination of DLC coatings and EP additives for improved tribological behaviour of boundary lubricated surfaces

The aim of the present investigation was to obtain some further understanding of the mechanism responsible for low-friction behaviour of W-containing DLC coatings (W-DLC) when lubricated with EP additivated oil. Boundary lubricated wear and friction tests were performed under reciprocating sliding motion using a high frequency test rig and a contact pressure of 1.5 GPa. Additionally, some of the tests were performed in a load-scanning reciprocating test rig, with the contact pressure being in the range from 2.4 to 5.6 GPa. The influence of concentration of a sulphur-based EP additive on the friction behaviour was investigated.This investigation showed that W-DLC coatings greatly improve the tribological properties of boundary-lubricated surfaces, especially when pairing coated and uncoated steel surfaces. The improved tribological behaviour was found to be governed by the gradual formation of a WS₂ type tribofilm on the steel counter-face or on revealed steel substrate. The friction level depends on the additive concentration.     

Anti-wear and friction reducing additives composed of ortho-phenylene phosphate-amine salts for polyether type base stocks

Lubrication properties of amine salts of diaryl phosphates in polyether type base oils were evaluated by means of the four-ball wear test. The unique cyclic phosphate, ortho-phenylene phosphate, prevented wear and reduced friction when combined with tertiary amines. Adsorptivity, solubility, and corrosivity of the additives were taken into account.     

Effect and mechanism of additives for ionic liquids as new lubricants

Ionic liquids are unique compounds, which exhibit low viscosity, non-flammability, low vapor pressure, and extremely high thermal stability. Therefore, they are expected as candidates for advanced lubricants. Several ionic liquids, derived from cations such as imidazolium, pridinium, ammonium, and anions such as , bis(trifluoromethylsulfonyl)imide (TFSI^-) were examined under boundary conditions. It was found that tribological properties of ionic liquids are better than those of conventional lubricants such as synthetic hydrocarbons, synthetic esters, and fluorinated ethers. Careful analysis of worn surface revealed that tribochemical reactions of ionic liquids take place under these conditions. For example, organic fluoride, iron fluoride, iron sulfate derived from anionic moiety of the ionic liquids were detected by the instrumental surface-analysis with TOF-SIMS and XPS. Tricresylphosphate (TCP) and dibenzyldisulfide (DBDS) were found to improve anti-wear properties of ionic liquids to some extent. Interestingly, both additive-derived compounds and anionic moiety derived ones were detected on the worn surface. Moreover, additive response of ionic liquids was found to be superior to those for conventional lubricants. It was speculated that the unique tribochemical reactions will lead us to design tailor-made lubricants. Lubrication mechanism of ionic liquids is discussed from the viewpoint of tribochemistry.     

Textured surfaces for improved lubrication at high pressure and low sliding speed of roller/piston in hydraulic motors

The present study investigates the potential of surface textures to improve the friction situation of the piston/roller contact in hydraulic motors at low speed and high pressure. A self-adjusting flat on flat rig, with conditions representative of an actual hydraulic motor, is used in a reciprocating sliding motion. The samples are made out of the same materials and the surface finishing operations are the same as used in the motor.The piston surfaces are textured using newly developed embossing tools comprising micromechanically formed all-diamond surfaces. The textured surfaces involve parallel grooves and crossed grooves, each of four different spacings. Also polished surfaces were tested. Un-textured—as tumbled—surfaces are used as reference.Generally, it was found that the friction level was only marginally influenced by the textures. However, the friction fluctuations were reduced. SEM investigations disclosed rather severe modifications of the surfaces during use, primarily caused by plastic deformation of the surface layer. The results and their practical implications are discussed.     

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.     

The origins of synthetic lubricants: The work of Hermann Zorn in Germany part 2 esters and additives for synthetic lubricants

The second part of this survey of the work of Hermann Zorn¹ covers his research work on esters, and their introduction as lubricants into aviation and other applications. The additives developed for these applications are also described.     

Metallic sliding friction under boundary lubricated conditions: Investigation of the influence of lubricant at the start of sliding

Experiments are described in which a hard half-wedge, representing a scaled up model asperity, was indented into the horizontal surface of a relatively soft specimen with the specimen then moved in a direction parallel to its surface and normal to the wedge edge. Forces measured at the wedge-specimen interface are used to investigate the state of lubrication at the interface at all stages of the experiment, i.e. from initial indentation to final steady-state wave formation. The results are used to offer possible explanations for the nature of boundary lubrication and of stick-slip motion.     

Biodegradable hydraulic fluids: Rheological behaviour at low temperatures of several oleochemically derived synthetic esters

One of the most important aspects of a hydraulic fluid used in mobile systems (e.g., excavators, wheel loaders) is the viscometric behaviour of the fluid at low temperatures. It has been found that the pour point of the base fluid does not predict this behaviour satisfactorily, and, as a result, the Swedish standard for these products (SS155434) requires that the kinematic viscosity of a hydraulic fluid remains stable at low temperatures (−20 or −30°C) during a period of 72 h. However, in rheological studies under dynamic conditions, it has now been found that certain oleochemically derived esters do not show Newtonian flow behaviour at these low temperatures. The authors have found not only a shear‐dependent behaviour of these esters but also a time‐dependent behaviour, especially after (high) energy input from an external source to simulate pumping of the fluid. Given the non‐Newtonian behaviour of these esters, the kinematic viscosity test method with an Ubbelohde tube was found to be inappropriate for the determination of the low‐temperature viscosity because this method assumes Newtonian behaviour. Dynamic viscosity measurements at higher shear rates (e.g., cold cranking simulator) resulted in much better reproducibility and may offer the best method of assessment.     

Pressure Dependence of ZnDTP Tribochemical Film Formation: A Combinatorial Approach

Combinatorial testing has been performed on zinc dialkyldithiophosphate (ZnDTP)-containing lubricants, to investigate the effects of contact pressure on the formation of tribochemical films. Contact pressures ranging from 25 to 500 MPa were applied in ball-on-disc tribotests with oscillating load. Both the ball and the disc were investigated by means of small-area and imaging X-ray photoelectron spectroscopy (XPS). The thickness and the composition of the reaction layer were estimated from the XPS data. The thickness of the reaction layer in the tribologically stressed areas of the ball and of the disc increased with both temperature and contact pressure. The reaction layer mainly consisted of short-chain poly(thio)phosphates, shorter chains being observed at higher contact pressures. At high pressures, the presence of a thick, high-toughness short-chain poly(thio)phosphate layer can explain the lower friction and dimensional wear coefficients observed. On the ball, similar anti-wear film formation mechanisms were observed as on the disc, zinc sulphide being deposited in the post-contact region.     

Wet clutch transmission fluid for AWD differentials: influence of lubricant additives on friction characteristics

In recent years, several electronically controllable automotive transmission systems using wet clutches as intelligent differentials have emerged on the market. These applications place great demands on the anti‐shudder properties of the transmission fluids used. The aims of this study were (i) to investigate the influence of different additives on the friction characteristics of a transmission fluid for all‐wheel drive systems featuring wet multi‐plate clutch with a sintered brass‐based friction material and, based on this knowledge, (ii) to formulate a new transmission fluid with the desired frictional properties. In addition to excellent anti‐shudder properties, the new fluid was required to lubricate hypoid gears under high load. To meet this requirement, it is necessary to add significant amounts of extreme pressure additives to the base oil, which are known to have an unfavourable influence on anti‐shudder properties, necessitating the adoption of novel additive technologies. The additives studied include anti‐wear additives, friction modifiers, corrosion inhibitors, detergents, antioxidants and extreme pressure additives. This paper shows how different additives affect friction in different ways, and that the interactions between the different additives are important to consider. It was concluded that it is feasible to combine good anti‐shudder properties for wet clutches with good lubrication of hypoid gears. Copyright © 2006 John Wiley and Sons, Ltd.     

Synthetic lubricants for air cooled two cycle engine oils: Effect of esters on lubrication and tribological properties

This paper reports the development of an air-cooled two-stroke engine lubricant using esters as synthetic base stocks, and performance additives. The viscosimetric and terminological properties as well as the performance in two-cycle gasoline engines are described in comparison with standard lubricants. It is shown that the use of suitable esters can make possible the formulation of high performance lubricants for the severity of use.