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Additives are important components to reduce friction and wear of tribological systems. Zinc dithiophosphate (ZDDP), the most commonly used anti-wear additive prevents the tribological system from the friction and wear loss without any commercially available antagonist additive. However, its environmentally harmful content enforces to lubricating oil manufacturers to replace it with an alternative one. Therefore, in this study, the H3BO3 + MoS2 combination was tested in the boundary lubrication regime in a tribometer with a pin on flat reciprocating test module to be a possible alternative anti-wear additive against ZDDP. The friction and wear behavior of Boric acid (H3BO3) + Molybdenum Disulfide (MoS2), ZDDP, MoS2, and H3BO3 evaluated in terms of friction coefficient (COF), wear rate, surface energy and tribo-chemical analysis of tribofilms. SEM/EDX and XPS were employed to explore tribofilm formations on the wear scars. According to the results, the addition of MoS2 into H3BO3 presented a similar tribological performance to ZDDP by forming a strength tribofilm on the surface containing B2O3, MoO3, and metal sulfate.

Zinc dithiophosphate is the most commonly used antiwear additive in lubricating oil. However, zinc dithiophosphate has a poisoning effect on engine catalysis via phosphorus and zinc content that reduces the efficiency causing hazardous emission increase, therefore, it needs to be replaced with an alternative additive. In this study, the antiwear performance of molybdenum disulfide (MoS2) is enhanced by zinc sulfate (ZnSO4) addition and subjected to tribometer tests at different contact pressures to explore the MoS2 + ZnSO4friction and antiwear performance against MoS2and zinc dithiophosphate. Wear rates and surface morphology changes were carried out by an optical microscope, optical profilometer, and atomic force microscope analysis. Furthermore, tribochemical and surface energies of tribofilms were evaluated via scanning electron microscope/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscope adhesion force mapping analysis. Results showed that ZnSO4addition to MoS2 + base oil improves the antiwear performance of the lubricating oil significantly and it presents similar friction characteristics to zinc dithiophosphate.

The friction characteristics of a shock absorber are very complex because the reciprocating motion is not always identical. In this study a device was developed and used to analyze the dynamic friction characteristics under various reciprocating sliding conditions to determine the sliding materials and hydraulic oils that improve the shock absorber performance. This study describes the influence of hydraulic oil additive on the fine reciprocating friction characteristics of steel and copper alloy. Hydraulic oils were prepared by blending a paraffinic mineral oil with zinc dithiophosphate (ZnDTP) and polyhydric alcohol ester as additives. The results show that the dynamic frictional characteristics vary mainly depending on the additive concentration. A specific additive formulation induces a unique amplitude-dependent friction behavior. In addition, the influence of different additives on the lubrication mechanism is investigated based on the instrumental analysis of the friction surface.

Experimental studies reveal that the simultaneous addition of zinc dialkyl dithiophosphates (ZDDPs) and multi-wall carbon nanotubes (MWCNTs) to a poly-alpha-olefin base oil strongly reduces wear. In this paper, it is shown that MWCNTs promote the formation of an anti-wear (AW) layer on the metal surface that is much thicker than what ZDDPs can create as a sole additive. More importantly, the nanotubes&rsquo, action is indirect, i.e., MWCNTs neither mechanically nor structurally strengthen the AW film. A new mechanism for this effect is also proposed, which is supported by detailed tribometer results, friction track 3D-topography measurements, electron diffraction spectroscopy (EDS), and Raman spectroscopy. In this mechanism, MWCNTs mediate the transfer of both thermal and electric energy released on the metal surface in the friction process. As a result, this energy penetrates more deeply into the oil volume, thus extending the spatial range of tribochemical reactions involving ZDDPs.

The effect of the alloying of diamond-like orienting carbon coatings by carbide-forming elements (tungsten, molybdenum, and silicon) exerted on the antifriction properties of lubricant PAO-4 with a surfactant additive such as oleic acid and two chemically active additives (zinc dithiophosphate and molybdenum dithiocarbamate) is studied. The antifriction properties of the compared environments containing sulfur abruptly increase upon the friction of steel against coatings alloyed with tungsten or molybdenum (the friction coefficients decrease to 0.02–0.05), whereas in the case of environments containing no sulfur, the friction coefficients are about 0.15. This phenomenon can be explained by the formation of molybdenum and tungsten dichalcogenides as a result of the tribochemical interaction of these metals with the sulfur contained in the lubricating lubricant.

Purpose This paper aims at understanding tribological properties of lubricating oils doped with zinc dithiophosphate(ZDDP) with and without electromagnetic field impact. Design/methodology/approach The friction and wear properties of the oils formulated with zinc butyloctyl dithiophosphate (T202) or zinc dioctyl dithiophosphate (T203) under electromagnetic field or nonelectromagnetic field were evaluated on a modified four-ball tribotester. The characteristics of the worn surfaces obtained from electromagnetic or nonelectromagnetic field conditions were analyzed by scanning electronic microscopy, energy dispersive spectrometer and X-ray photoelectron spectroscopy. This paper focuses on understanding influence of electromagnetic field on lubrication effect of the ZDDP-formulated oils. Findings The electromagnetic field could effectively facilitate anti-wear and friction-reducing properties of the oils doped with T202 or T203 as compared to those without electromagnetism affection, and the T203-doped oils were more susceptible to the electromagnetic field. The improvement of anti-wear and friction-reducing abilities of the tested oils were mainly attributed to the promoted tribochemical reactions and the modification of the worn surfaces (forming Zn-Fe solid solution) induced by the electromagnetic field. Originality/value This paper has revealed that tribological performances of ZDDP-doped oils could be improved by the electromagnetic field and discussed its lubrication mechanisms. Investigating tribological properties of additives from the viewpoint of electromagnetics is a new attempt, which has significance not only for the choose and designing of additives in electromagnetic condition but also for development of tribological theories and practices.

International audience; Amide molybdate has been recently introduced as a friction modifier for tribological applications. Combined with zinc dithiophosphate (ZDDP) and fatty amines, it provides an ultralow friction coefficient. The ultimate product of Mo compound transformations in tribological contact, due to frictional heating and shearing, as well as chemical interactions with oil additives, is molybdenum sulfide (MoS2). Understanding the decomposition of amide molybdate leading to MoS2 is of primary importance to the optimization of the design of lubricant formulations. This study focuses on the investigation by Raman spectroscopy of amide molybdate decomposition intermediates. Raman spectra of tribofilms, obtained after friction tests under different temperatures and pressures, revealed the formation of an amorphous MoS3 intermediate coexisting with MoS2. However, under severe conditions, the tribofilms are mostly composed of MoS2.

Although diamondlike carbon coatings like WC/a-C:H have a proven history of tribological performance in many mechanical applications, studies have reported that these coatings can sometimes experience high wear rates when operating in lubricants with large concentrations of phosphorous- and sulfur-based additives. In this study, WC/a-C:H and TiC/a-C:H coatings have been tribologically tested against AISI 52100 steel and Si3N4 counter faces under boundary lubrication in ISO 320 mineral oil, mineral oil with 1% aryl zinc dithiophosphate and 1% molybdenum dithiocarbamate, and a fully formulated wind turbine gear box oil. Results revealed that the wear coefficients of TiC/a-C:H were much smaller than those of WC/a-C:H in testing performed in lubricants with additives. A thionization wear mechanism was proposed to explain these results.

Tribofilm formation by several zinc dialkyl- and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet.

This article is part of a series of articles on the mechanism of tribocatalytic processes. The aim of this study was to analyze the behavior of lubricants containing zinc dialkyldithiophosphate (ZDDP) and ordered molecular structures such as carbon nanotubes (CNTs), fullerenes (C60) and antistatic additive (ASA). The mechanism of lubricant additives has been studied for many years and has been described in numerous publications, including [11,12,30,31]. Two types of tribological tests were used: the high frequency reciprocating rig (HFRR) test and the triboelectric (TET) test, with the latter developed at the Air Force Institute of Technology. It was found that the effectiveness of the lubricating additive (ZDDP) depended on the presence of ordered molecular structures, which were able to conduct energy from the surface of the specimen to the ZDDP reaction zones inside the boundary lubricating film. CNTs and ASA were reported to conduct energy effectively. Fullerenes (C60), on the other hand, were not suitable as energy conductors in the lubrication process.

The data from the authors&rsquo, earlier investigations show that molecules of zinc dithiophosphate (ZDDP) added to a lubricant can absorb energy emitted by a solid surface, which is where triboreactions occur. If the lubricant contains structures able to conduct energy, the ZDDP reactions can occur even at a relatively large distance from the solid surface, which should increase the effectiveness of ZDDP as an antiwear additive. The purpose of this paper was to verify the thesis that the tribocatalytic effect depends on the ability of the solid surface to emit electrons/energy and the ability of ordered molecular structures, such as carbon nanotubes (CNTs), to conduct energy and, most likely, to enhance the energy transfer. The tribological tests were performed using a TRB3 tribotester for 100Cr6 steel balls and uncoated or a-C:H coated HS6-5-2C steel discs. Polyalphaolefin 8 (PAO8) and PAO8 mixed with ZDDP and CNTs were used as lubricants. The results of the tribological tests suggested that: (a) the effect of the interactions between ZDDP and CNTs was clearly visible, (b) the structure and properties of the solid surface layer had a significant influence on the antiwear action of the ZDDP additive.

The anti-wear and anti-seizure performance and action mechanisms of zinc dithiophosphate (ZDDP) have been investigated under reciprocating pure sliding conditions to simulate piston ring and cylinder liner assembly, using new techniques. The Mini Traction Machine–Space Layer Imaging is a useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions and ZDDP film formation in reciprocating pure sliding conditions is rarely investigated. In this paper, the authors describe an investigation of ZDDP film formation in stationary ball on reciprocating disc pure sliding conditions and compare the results to those obtained in unidirectional pure sliding conditions. In unidirectional pure sliding conditions, the worn area on the ball expands with test time. By contrast, in reciprocating pure sliding conditions, tribofilm forms on the stationary ball and no significant damage occurs. In the initial tribofilm formation under reciprocating pure sliding conditions, solid particulate tribofilm with a high concentration of S forms initially in the contact area and subsequently breaks up. During further rubbing, a Zn- and P-rich tribofilm forms on the comminuted sulphur-rich tribofilm and also the area where the initial tribofilm was removed.

The anti-wear performance and action mecha- nisms of zinc dithiophosphate (ZDDP) have been investi- gated under various test conditions. The Mini Traction Machine–Space Layer Imaging (MTM–SLIM) is a widely used and useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions, typically between 50 % SRR (slide–roll ratio) and 100 % SRR. In this paper, the authors describe an investigation of ZDDP film formation at SRRs much higher than 100 % SRR, including pure sliding conditions using a novel MTM–SLIM technique. At high SRRs, ZDDP tribofilms form without damaging the ball surfaces so long as both surfaces move above a threshold speed with respect to the contact, regardless of whether the two surfaces move in the same or opposing directions. In pure sliding conditions, although the worn area expands with time under pure sliding conditions showing that wear takes place, tribofilms are still built up throughout a test and the ZDDP has a beneficial effect on wear rate. The very early stages of film formation are studied to show that a tribofilm with a high concentration of S is formed initially and then replaced with a film having a high concentration of Zn and P.

The development of improved anti-wear additives would enable the use of lower viscosity oils that would lead to improved efficiency. Ionic liquids have the potential to be this type of new anti-wear additive. However, currently the best performing ionic liquids that are miscible in non-polar base oils, the phosphonium phosphates, contain phosphorus on both the cation and anion. Manufacturers are seeking to reduce the presence of phosphorus in oils. Here, as a first step towards phosphorus-free anti-wear additives, we have investigated ionic liquids similar to the phosphonium phosphates but having either a phosphorus-free cation or anion. Two quaternary ammonium phosphates (N6,6,6,14)(BEHP) and (N8,8,8,8)(BEHP) and a phosphonium silyl-sulfonate (P6,6,6,14)(SSi) were compared to a phosphonium phosphate (P6,6,6,14)(BEHP) and a traditional zinc dithiophosphate (ZDDP) as anti-wear additives in mineral oil. The change from a phosphonium to a quaternary ammonium cation drastically reduced the miscibility of the Ionic liquid (IL) in the oil, while the change to a smaller silicon containing anion also resulted in limited miscibility. For the pin-on-disk wear test conditions used here none of the ionic liquids outperformed the ZDDP except the (P6,6,6,14)(BEHP) at a relatively high loading of 0.10 mol·kg−1 (approximately 8 wt%). At a more moderate loading of 0.025 mol·kg−1 the (P6,6,6,14)(SSi) was the best performing ionic liquid by a significant amount, reducing the wear to 44% of the neat mineral oil, while the ZDDP reduced the wear to 25% of the mineral oil value. Electron microscopy and energy dispersive X-ray spectroscopy showed that the presence of a silicon containing tribofilm was responsible for this protective behaviour, suggesting that silicon containing ionic liquids should be further investigated as anti-wear additives for oils.

With the increasing requirements of environmental protection, using lead, hexavalent chromium and other heavy metals will be prohibited in different surface coatings. In this paper, the non-ferrous CrN coatings and the rapeseed oil were chosen as friction pair and biodegradable lubricant. The tribology properties were investigated using SRV-IV reciprocating friction wear test. The worn surface of CrN coatings was observed and analysed using scanning electron microscopy and X-ray photoelectron spectroscopy. The results indicate that the rapeseed oil containing 1 wt.% MoZn (MoDTC (molybdenum dithiocarbamate) + ZnDTP (zinc dithiophosphate) (1 : 3)) exhibits better friction-reducing properties than the rapeseed oil containing 1 wt.% PN (an amine salt of an alkoxylphosphate) additives, whereas the rapeseed oil containing 1 wt.% PN additive exhibits better wear resistance properties than the rapeseed oil containing 1 wt.% MoZn additives. This is partly attributed to the tribophysical and tribochemical reactions between the lubricant and the CrN coating sliding surfaces. Copyright © 2012 John Wiley & Sons, Ltd.

Boron-based lubricant additives have recently received significant attention, because of their wear-reducing and frictional properties and low pollution. At the same time, dithiocarbamate complexes with different metals have a long history of being used as multifunctional additives to lubricants. In this study, novel, environmentally friendly additives containing alkylborate and dithiocarbamate groups with alkyl or methylbenzyl substitutes in one molecule were studied. Tribological tests were performed with the additives admixed in a mineral oil using steel-on-steel contacts in a four-ball tribometer. Borate derivatives of different dithiocarbamate ligands were synthesized by several step reactions to investigate tribochemical properties of boron, sulfur, and nitrogen combined in one selected compound. Viscous liquid products were characterized by multinuclear 1H, 13C, and 11B NMR spectroscopy. The surface morphology and the elemental composition of the tribofilms were investigated using an optical profiler and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). It was found that some of these novel compounds provide better antiwear performance and similar frictional properties compared with a commercially available ZnDTP package. Traces of sulfur in the tribofilms formed with both 0.2 and 1.0 wt% of alkylborate–dithiocarbamate compounds in a mineral oil were detected with EDS.

The influence of metallic and oxidized iron/steel on the reactivity of triphenyl phosphorothionate (TPPT) in oil solution at 473 K was investigated by means of FT-IR, NMR and XPS. The FT-IR and NMR results show that the degradation of TPPT was catalyzed by metallic and oxidized iron powder and started with the breakage of the PS bond to form triphenyl phosphate (TPP). The results of the XPS sputter depth profile show the multilayer structure of the thermal film formed on 100Cr6 steel filings immersed for 72 h in TPPT solution heated at 473 K.

Traditionally, wear protection and friction modification by engine oil is provided by zinc dithiophosphate (ZDDP) or other phosphorus compounds. These additives provide effective wear protection and friction control on engine parts through formation of a glassy polyphosphate antiwear film. However, the deposition of phosphorus species on automotive catalytic converters from lubricants has been known for some time to have a detrimental effect of poisoning the catalysts. To mitigate the situation, the industry has been making every effort to find ZDDP-replacement additives that are friendly to catalysts. Toward this goal we have investigated a titanium additive chemistry as a ZDDP replacement. Fully formulated engine oils incorporating this additive component have been found to be effective in reducing wear and controlling friction in a high-frequency reciprocating rig (HFRR), 4-ball bench wear, Sequence IIIG, and Sequence IVA engine tests. Surface analysis of the tested parts by Auger electron spectroscopy...

In order to improve environmental protection, there have been many recent advancements in the purification of exhaust gas from automobiles. In order to prevent poisoning of the catalyst used in after-treatment devices for exhaust gas, it is essential to reduce phosphorus in automobile engine oils. However, this phosphorus comes from the zinc dithiophosphate (ZnDTP) that is included in the oils in order to achieve anti-wear and anti-oxidation properties. In the present study, we examined anti-wear additives that do not include phosphorus as substitutes for ZnDTP. The effects of the number of sulfur atoms in the dimer of mercaptocarboxylate derivatives, the length of the alkyl group and the structure of the end alkyl group were investigated. We found that the disulfide bonding of the short intramolecular alkyl chain and the large end alkyl group not only reduced friction, but also showed the same extreme pressure and anti-wear properties as ZnDTP.

In a previous study, an iron oxide layer (mixture of Fe 3 O 4 and FeO) formed by water-vapor treatment on a steel plate resulted in an anomalous low friction (μ < 0.06) when slid against a steel cylinder in a lubricant containing ZDDP. This value is about a half of a steel/steel friction contact under the same condition. The formation of a tribofilm containing zinc and sulfur has been identified on the iron oxide. In this study, the formation mechanism of such tribofilm is discussed. The adsorption characteristics of ZDDP on the iron oxide and steel are investigated experimentally. The adsorption layer on the iron oxide analyzed by XPS is rich in zinc but contains almost no sulfur, while the one on steel includes phosphorous and oxygen, and probably phosphates. The adsorption plays a significant role in increasing the zinc content in the tribofilm on the iron oxide, while other mechanisms such as mechanical mixing are necessary to incorporate sulfur in the tribofilm.

The increasing dependence on more robust additive chemistry to improve gear pitting resistance requires the additive technology development to rely less on a trial-and-error approach and more on a better basic understanding of the influence of additive chemistry on tribological contact layers' physical and chemical changes. The use of secondary neutral mass spectrometry (SNMS) and nanoindenter to analyse tribological contact layers had been carried out by Inacker and co-workers at NMI. They found that the alkyl structure of zinc dithiophosphate (ZDTP) and the type of cation have a profound effect on the thickness and nanohardness of the tribological layer. An extension to that study has been carried out in this investigation, which involves a design experiment of two variables (oil viscosity and surface roughness) while keeping the additive chemistry constant to determine their impact on the tribological layer. The methods used to analyse the tribological layers include SNMS, nanoindenter and SEM coupled with focused ion beam imaging of the rectangular well-shaped cross section. The results in general are in agreement with the findings of Inacker and his co-workers, namely greater micropitting reduces the thickness of the tribological layer and brings closer the depth of nanohardness maximum to the surface. Copyright © 2006 John Wiley & Sons, Ltd.

Electrochemical techniques have been applied to detect the redox behaviour of the antiwear additive zinc dithiophosphate (ZDDP) in di-ethylhexyl sebacate (DEHS) base fluid solution and to study the interaction of ZDDP with ferrous surfaces. Using cyclic voltammetry it has been shown that the ZDDP is oxidised in a chemically-irreversible process at applied potentials above 1.2 V vs. Pt with an iron electrode. The influence of applied electrode potential on friction and wear characteristics of ZDDP in DEHS has also been investigated. Reciprocating rubbing tests have shown that ZDDP is effective in reducing wear only under oxidising conditions. This electrochemical approach can be used to explore the operating conditions and mechanism of action of antiwear additives.

The crystal structure determination of polymeric [Cd(S2P(OCy)2)2(3-NC5H4C(H) NN C(H)C5H4N-3)·(CHCl3)2]∞, where Cy is cyclohexyl and the bridging ligand is 3-pyridinealdazine, shows that the cadmium atom exists within a trans-N2S4 donor set and that the resultant polymer topology is of a step-ladder. It is argued that this is the expected structure for the compound as it concurs with the structure of the related iso-propyl derivative. Indeed, in most cases polymer topology in related cadmium dithiophosphate systems is controlled by the nature of the bridging diimine-type ligand. By contrast, in the analogous zinc dithiophosphate adducts, controlling polymer topology is possible by systematically varying the steric profile of the dithiophosphate-bound R groups, a phenomenon ascribed to the fact that in the zinc system, the zinc–ligand distances being shorter, bring all the components of the structure in close proximity and thereby exacerbates steric effects. An overview of our accomplishments in this area ‘crystal engineering of coordination polymers’ is presented herein.

Tribochemistry can be defined as the chemical reactions between the surface and the lubricant molecules inside a sliding contact under boundary lubrication conditions. These reactions are important because they control the lubrication processes, and therefore they determine the reliability and durability of moving parts in machineries. In spite of its practical significance, our understanding of the nature of tribochemistry is limited, and to a large extent, empirical in nature, and mainly restricted to the steel?hydrocarbon system. As new materials emerge and are being applied to various applications, users are often hampered by the lack of knowledge of how these new materials can be effectively lubricated via tribochemical reactions, resulting in occasional premature failure and product recall.There are many tribochemical reactions which occur in a sliding system, such as tribocorrosion, electro-chemistry, hydrolysis of ceramics and anti-wear additive film formation. However, the fundamental basis of lubrication in most of the practical systems starts with the reactions between materials surfaces and hydrocarbon base oils. Without this reaction, for example, an anti-wear additive such as zinc dithiophosphate would not be effective. Therefore, in this paper, we will focus on the effect of materials on the basic tribochemical reactions between hydrocarbons and surfaces.This paper integrates existing information in the literature on the lubrication chemistry of various materials and provides a consistent framework in the context of tribochemistry. For discussion purposes, we will examine metals, semiconductors, and insulators. The preponderance of data in the metals?hydrocarbon systems suggests that thermochemistry and organometallic chemistry dominate the interfacial chemical reactions. For semiconductors and insulators, there is evidence that electrostatic charge induced electron emission plays an important role in the tribochemistry of these materials. Various measurement methods used to characterize tribochemical behaviours are introduced and discussed in terms of reactivity analysis and tribochemical mechanisms.

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS 2 on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Form...

Replacing zinc dithiophosphate (ZnDTP) with ashless anti-wear alternatives will lead to a significant reduction of ash and give more flexibility in the use of metallic detergents. The question of the contamination of exhaust gas after treatment devices by volatile phosphorus-containing compounds has recently been raised. We report the results of investigations on the amount of gaseous emissions of phosphorus released from engine oils formulated with a range of phosphorus-containing anti-wear additives. To relate the structure of additives to the observed phosphorus emissions, we have investigated a variety of phosphorus-containing anti-wear agents, metal-containing as well as ashless. We have used established and new laboratory tests in addition to measurements on fired engines to assess the phosphorus emissions. We discuss the anti-wear capability and friction control of these phosphorus-containing additives. We conclude that it is possible to formulate zinc-free, low-ash engine oils which will control wear and corrosion and not release more phosphorus into the exhaust gas after treatment systems than the traditional ZnDTP based oils. Copyright © 2006 John Wiley & Sons, Ltd.

The fuel economy and reduction of harmful elements of lubricants are becoming important issues in the automotive industry. One approach to these requirements is the potential use of low-friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like carbon (DLC) coatings, extensively studied as ultra-low friction films to protect ductile metals surfaces for space applications, are expected to fit the bill. The main purpose of this work is to investigate the friction and wear properties of DLC coatings lubricated with molybdenum dithiocarbamate (MoDTC) and zinc dithiophosphate (ZDDP) under boundary lubrication conditions. The mechanisms by which MoDTC reduces the friction in the centirange were studied using ultra-high vacuum (UHV) analytical tribometer. The UHV friction tests were performed on a tribofilm previously formed on selected DLC material with MoDTC and ZDDP containing oil. Ex-situ characterizations show that the composition of this tribofilm is similar to that of a tribofilm obtained on steel surfaces in the same lubrication conditions with MoS2 single sheets dispersed inside zinc phosphate zones. However, analyses by X-ray photoelectron spectroscopy (XPS) indicate that MoDTC and ZDDP additives seem to be more active on steel surfaces than carbonaceous ones. After UHV friction with the tribofilm formed on selected DLC and steel pin counterpart, the wear scars of both sliding surfaces were characterized by in-situ analytical tools such as Auger electron spectroscopy, scanning Auger microscopy and micro-spot XPS. Low friction is associated with the transfer of a thin MoS2 film to the steel pin counterpart. Copyright © 2006 John Wiley & Sons, Ltd.

The measurement of water in lubricating oils is important because water accelerates the corrosion of metal parts and bearings in motors. Some of the additives added to lubricating oils to improve their performance react with the Karl Fischer reagent (KFR) causing a positive bias in the water measurement. A new oven evaporation technique for measuring water in oils has been developed that is automated, requires less sample handling, is easily calibrated, and is capable of measuring relatively small mass fractions of water (or =50 mg/kg sample). A series of motor oils was analyzed with the standard KFR, a reagent that detects interfering substances that reduce iodine, and the aldehyde-ketone reagent that does not detect substances that react with methanol and form water. The oil samples were heated to 107 degrees C and then reheated to 160 degrees C. At both temperatures, material was measured by both KFRs, but only zinc dithiophosphate released sulfur compounds that would react with the reagent that detects interfering substances. Mass fractions of between 20 and 70% of the volatile material released at either temperature were measured with the standard KFR but not with the aldehyde-ketone reagent. These results demonstrate that there are a number of sources of positive bias in the measurement of water in motor oils and that the standard KFR cannot be used to measure water in motor oils and motor oil additives. These results also indicate that some of the material reacts with methanol to form water. Finally, these results suggest that some of the material that is volatile at 160 degrees C and not at 107 degrees C may be water that is physically occluded or may be substances that react with diethyleneglycol monomethylether to produce water.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (static and dynamic modes) has been applied to chemical characterization of zinc dithiophosphate additives tribofilm. Main result concerns insights in the spatial distribution of phosphate and sulphide species in the whole tribofilm thickness, at a sub-micrometer scale. The disappearance of oxide layer at the interface between steel and the tribofilm is also noticed.

In situ spacer layer interferometry has been used to monitor and compare the formation of both tribo- and thermal zinc dialkyldithiophosphate (ZDDP) films on steel surfaces. For the ZDDP solution studied, negligible thermal film formation takes place below a temperature of 110°C, but a ZDDP tribofilm is generated on rubbed surfaces even at room temperature. The ZDDP tribofilm formation is critically dependent on the extent of direct solid solid contact during rubbing and does not take place if the elastohydrodynamic film thickness is significantly greater than the surface roughness. Tests at different slide-roll ratios indicate that the rate of ZDDP tribofilm formation is proportional to the product of sliding distance and rubbing time. Based on the results, it is not possible to explain ZDDP tribofilm formation in terms of a response to contact pressure or flash temperature. It is likely that the tribofilm reaction is either strongly catalysed by species released during rubbing, such as soluble FeII or FeIII, or is triggered by triboelectronic processes such as exoelectron emission.

The influence of load on the chemistry of tribofilms formed on a steel surface in solution of pure di-isopropyl zinc dithiophosphate (i-ZnDTP) in n-decane has been investigated by means of a combinatorial tribological experiment involving X-ray photoelectron spectroscopy. The experiment consisted of the preparation of a set of spatially separated areas, produced under various tribological test conditions, and the subsequent spectroscopic probing of the chemical composition of the tribofilm. The experiment was carried out at room temperature under boundary-lubrication conditions and revealed a physically adsorbed layer of the additive in the non-contact area and a thin (ca. 5 nm), inhomogeneous phosphate film covering the tribostressed areas. The total amount of phosphate present in the tribostressed area was found to increase with increasing load. In the contact areas, iron oxides and metal sulfides have also been detected.

Over the years many analytical tools have been developed to measure zinc dithiophosphate depletion in engine oil formulations. Many of these tools are obvious ones, such as infrared spectroscopy and thin layer chromatography; however, in this paper we report on our study using X-Ray Absorption Near-Edge Structure (XANES) Spectroscopy for examining zinc dithiophosphate depletion in oils. Thus, XANES joins the constellation of techniques available to the additive chemist for study of the status of the zinc dithiophosphates in used oils. Comparison of this XANES technique with a more traditional technique, 31P nuclear magnetic resonance (NMR) spectroscopy, showed that XANES spectroscopy gave data consistent with NMR. Since these were diesel engine oils, the authors were also able to examine, by solid-state NMR, the soot from two fully formulated engine oils, one a high wear and one a low wear oil. By this study the authors were able to show that the Rounds' hypothesis for zinc dithiophosphate depletion and w...

The nature and properties of polyphosphate glasses formed in the antiwear action of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on the same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The data show the two-layer structure of the Zndtp film and permit a clear identification of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phosphate. A Chemical Hardness model is found to predict the formation of such a layered tribofilm. Moreover, a tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the Hard and Soft Acids and Bases (HSAB) principle. This reaction explains the anti-abrasive mechanism of Zndtp and also predicts a depolymerisation of the long chain zinc polyphosphate glass, in very good agreement with AES/XPS/XANES analytical data. The role of residual sulphur in the lubricant is also explained and the model is in agreement with the formation of metal sulphides embedded in the short chain phosphate matrix. This first paper serves as a basis for a detailed study of the mechanical properties of each film.

Zinc and molybdenum dithiocarbamates are used as effective additives in lubricants of various types. Following earlier reported investigations on their antifriction and antiwear properties, it was found that zinc and molybdenum dithiocarbamates had some antioxidant capabilities, so investigations have been carried out to compare the influence of zinc and molybdenum dithiocarbamates and dithiophosphates on the antioxidant properties of lubricating oils over a wide range of temperatures. The antioxidant activity of a number of synthesised additives was determined by a kinetic method in model conditions; the antioxidant properties of these additives were also compared in more severe conditions. It was found that the effectiveness of metal dithiocarbamates depends on the oxidation test conditions. The influence of zinc and molybdenum dithiocarbamates on the antioxidant properties of lubricating oils increases in the presence of zinc dithiophosphate and calcium phenate.

The role of transfer phenomena in the mechanisms of friction reduction by organic molybdenum compounds is studied with the aid of ultrahigh vacuum (UHV) analytical tribometry. Additives used are zinc dithiophosphate (Zndtp), molybdenum dithiophosphate (Modtp), molybdenum dithiocarbamate (Modtc) and Modtc/Zndtp combinations. Experiments involve UHV friction tests on tribofilms formed previously and in situ surface analyses by Auger electron spectroscopy (AES) and imaging. In the presence of Modtc, friction reduction was found to be associated with the transfer of highly dispersed MoS2 as isolated sheets, from the tribofilm to the slider. In the presence of Modtp (or the mixture Modtc/Zndtp), a two-step tribochemical reaction is generally observed. First, phosphate from the film is transferred to the oxide on the pin and the friction coefficient is about 0.3, after an induction period of a few cycles, pure MoS2 single sheets are transferred to the pin and the phosphate is eliminated as wear debris from the contact zone. Friction is then at its lowest value (0.02). The chemistry of the transfer phenomena is modeled using the hard and soft acid and base (HSAB) principle as described by Pearson. The overall data suggest that friction behavior under boundary lubrication with additives may be directly related to molecular scale transfer mechanisms in general.

Tribochemical interactions between zinc dithiophosphate (Zndtp) and polyisobutene succinimide (PIBSI) were studied in the mild wear regime by a dual analysis approach. Both TEM analyses of wear fragments and inside wear scar XPS on tribofilms were obtained in the same location of the wear track. In presence of the succinimide, there are notable modifications in the composition of the zinc polyphosphate tribofilm which is classically formed in presence of Zndtp alone. Apart from amorphous zinc polyphosphate glass, the film contains also large amounts of oxidised species (oxides, sulphates and nitrates) together with residual succinimide function groups. The atomic ratio N/P is about 0.3. Moreover, compared to Zndtp, less tribofilm is produced due to some competition in adsorption process. The results highlight the role of the tribochemical reaction to explain the antagonism between the two additives. The presence of some iron oxide in the tribofilm material indicates that the anti-abrasive wear mechanism of Zndtp is hindered by the presence of the succinimide. The lower content of the tribofilm in sulphide species also suggests a lower efficiency to control the anti-seizure properties of the mixture of the two additives.