Study of zinc dialkyldithiophosphate in di-ethylhexyl sebacate using electrochemical techniques

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.     

Films formed by antiwear additives and their incidence in wear and scuffing

Lead naphthenate, zinc dithiocarbamate, antimony dithiocarbamate and zinc dithiophosphate have been tested as antiwear additives in the four-ball machine.The chemical nature, thickness and distribution of the films formed on the wear track have been determined by X-ray energy dispersive spectroscopy using a method for analysing films.It has been found that the film thickness arises from a balance between the rate of growth, depending on the type of additive, its concentration in the base oil, the contact temperature and the rate of removal determined by wear. Under mild conditions the first process dominates and thicker films are formed which reduce wear. With increasing severity the films are strongly thinned until scuffing occurs when the films have been worn off.     

Friction Reduction and Antiwear Capacity of Engine Oil Blends Containing Zinc Dialkyl Dithiophosphate and Molybdenum-Complex Additives

The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of boundary-lubricated steel surfaces was investigated experimentally. The performance of oil blends containing three different types of Mo-complex additives of varying Mo and S contents with or without primary/secondary ZnDTP additions were investigated at 100°C. The formation of antiwear tribofilms was detected in situ by observing the friction force and contact voltage responses. Wear volume and surface topography measurements obtained from surface profilometry and scanning electron microscopy studies were used to quantify the antiwear capacity of the formed tribofilms. The tribological properties are interpreted in terms of the tribofilm chemical composition studied by X-ray photoelectron spectroscopy. The results demonstrate that blending the base oil only with the Mo-compound additives did not improve the friction characteristics. However, an optimum mixture of Mo complexes and ZnDTP additive provided sufficient amounts of S and Mo for the formation of antiwear tribofilms containing low-shear strength MoS ₂ that reduces sliding friction. In addition, the formation of a glassy phosphate phase due to the synergistic effect of the ZnDTP additive enhances the wear resistance of the tribofilm. This study shows that ZnDTP- and Mo-containing additives incorporated in oil blends at optimum proportions improve significantly the tribological properties of boundary-lubricated steel surfaces sliding at elevated temperatures.     

Friction and Wear Phenomena of Vegetable Oil–Based Lubricants with Additives at Severe Sliding Wear Conditions

The tribological responses of palm oil and soybean oil, combined with two commercial antiwear additives (zinc dialkyl dithiophosphate and boron compound), were investigated at a lubricant temperature of 100°C and under severe contact conditions in a reciprocating sliding contact. The friction coefficient of palm oil with zinc dialkyl dithiophosphate was closest to the commercial mineral engine oil, with a 2% difference. The soybean oil with zinc dialkyl dithiophosphate produced a 57% improvement in wear resistance compared to its pure oil state. The existence of boron nitride in vegetable oils was only responsive in reduction of wear rather than friction. The response of commercial antiwear additives with vegetable oils showed a potential for the future improvement in the performance of vegetable oils.     

Copolymeric succinamic acids as antiwear additives: Synergistic and adverse effects

Copolymeric succinamic acid (COSMA) additives have been synthesised in the laboratory and evaluated for their antiwear performance, both alone and in combination with zinc dialkyl‐dithiophosphate (ZDDP) in HVI light neutral oil. COSMA additives show antiwear behaviour and, in combination with ZDDP, exhibit a good synergistic effect, reducing the wear‐scar diameter by 60% and increasing the initial seizure load from 50 kg to 85–95 kg.     

The effects of engine oil additives on valve train wear

Preventing valve train wear in automotive engines is one of the most important properties of an engine oil. However, the influence of engine oil additives on valve train wear has not been sufficiently clarified. In this investigation, the antiwear performance of additives (such as ashless dispersants, metallic detergents and zinc dithiophosphate - ZDTP) and the influence of the interaction of the additives were evaluated. Secondly, metallic detergents were considered. An overbased calcium sulphonate and an overbased phenate were found to have good anti-scuffing performance when evaluated in engine tests and in Falex wear tests. However, from the results of four-ball tests, these additives did not appear to have many extreme pressure properties. From surface analyses, it was determined that a calcium carbonate film was formed on the sliding surface of the Falex test piece, and this film provided good protection against wear. Finally, the interaction of ZDTP, succinimides and calcium detergents and their influence on valve train wear were studied. The decomosition temperature of ZDTP increased with certain additives. including the succinimide. As a result, scuffing was more prone, at temperatures below those increased temperatures. Also, changes in additive concentration on the sliding surface, due to competitive adsorption, altered the antiwear performance of the oil.     

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.     

Friction and wear behaviour of liquid paraffin containing an S-P type additive and amine

Sulphurised olefin (SO) and dibutyl phosphite (DBP) are two popular antiwear and extreme-pressure additives for gear oils. Investigation of the effects of the weight ratio of SO to DBP in liquid paraffin, and the effects of dodecyl amine, on the friction and antiwear properties of oil, were conducted with a four-ball wear tester. The chemical composition of the film, formed was examined using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflecting infrared spectro-scopic (IR) microscopy. The four-ball test results indicate that the weight ratio of SO to DBP affects the friction and wear behaviour of the oil and, with a weight ratio of SO to DBP of 20:1, a stable friction coefficient, and the smallest wear scar, were observed. Wear tests also showed that dodecyl amine improves or reduces the antiwear ability of the oil at certain concentrations. AES profile analyses demonstrate a thick boundary film formed on the rubbed surface with the weight ratio of SO to DBP at 20:1. Both AES and IR results indicate that the addition of dodecyl amine to an S-P type oil results in competitive adsorption, and hinders the reaction of SO and DBP with the steel surfaces.     

Antiwear Behavior of ZDDP and Fluorinated ZDDP in the Presence of Alkylated Diphenyl Amine Antioxidants

Environmental regulations have called for a reduction of phosphorus content in engine oils in recent years. The anti-wear additive zinc dialkyl dithiophosphate (ZDDP), which is also an antioxidant, is one of the most important components of engine oil additives. ZDDP is a major source of phosphorus. One way to reduce phosphorus levels is to replace ZDDP with new environmentally friendly antiwear agents that have similar or superior wear performance compared to ZDDP. Another way to address the environmental issue is to reduce the amount of ZDDP in engine oils. At the same time, it is necessary to increase the efficiency of ZDDP by finding optimum conditions that would result in improved antiwear performance. The antiwear mechanism of ZDDP involves its degradation thermally and tribologically, leading to the formation of an antiwear film that consists of polyphosphates and sulphides. The structure of the antiwear film is almost similar in both types of degradation. But the breakdown efficiency of ZDDP is diminished by the parallel reaction of ZDDP with other additives, as well as the antagonistic effects of these additives. The new fluorinated ZDDP complex developed has shown better wear performance compared to ZDDP. This would allow the possibility of further reduction of phosphorus in engine oils compared to current levels. In this paper we study the interaction of ZDDP and fluorinated ZDDP with alkylated diphenylamine. The impact of antioxidant on wear performance was examined using a ball-on-cylinder tribometer. The interactions between ZDDP and the fluorinated ZDDP with the antioxidant were studied using NMR and the surface of the tribofilm was examined using SEM, TEM, and Auger spectroscopy.     

Oil-Soluble Fluorinated Compounds as Antiwear and Antifriction Additives

Many studies have been published on the use of solid fluorinated compounds as lubricants and lubricant additives, but much less has been done with oil-soluble fluorinated additives.

This paper describes a study of fluorinated telomers, especially fatty acids and their amine salts, for boundary lubrication (antiwear conditions). The antiwear effectiveness of these fluorinated compounds is compared with a commercial additive, zinc di-n-butyl dithiophosphate (ZDTP).

Modern analytical tools are used to study surface layers (XPS, automatic wetting balance), and wear particles (IR) to determine their antiwear mechanism. Special attention is given to fluorinated reaction film formation.     

Relationship between Surface Composition and Wear: an X-ray Photoelectron Spectroscopic Study of Surfaces Tested with Organosulfur Compounds

An examination of steel surfaces, after wear-testing in oils containing organosulfur compounds, was conducted to investigate possible reasons for the differences in the efficiency of organosulfur compounds as antiwear additives. The tested surfaces were characterized both qualitatively and quantitatively by x-ray photoelectron spectroscopy. Qualitative analysis showed that wear-testing of several different organosulfur compounds produced metallic sulfide on the external surface. This observation indicates that the differences in efficiency cannot be ascribed to chemically different surface species. From quantitative analysis, a correlation was determined between the relative surface concentration of sulfide and wear; the higher the sulfide concentration at the end of the wear-test, the lower the wear during that test. The sulfide concentration was constant throughout the three-hour test of a given compound, but differed from one compound to another. The addition of amines to the oil formulation decreased the sulfide concentration and increased the wear.

These results indicate that the surface sulfide, or its direct precursor, is the species which provides antiwear protection of steel surfaces, under the present lest conditions, when organosulfur compounds are used as anti-wear additives.     

Thickness and chemical composition of films formed by antimony dithiocarbamate and zinc dithiophosphate

Commercial dithiocarbamate and dithiophosphate were used as antiwear additives in mineral oil in four-ball tests. The surface films formed were analysed by energy dispersive spectroscopy in a scanning electron microscope, using a convenient method for analysing films on substrates. The film thickness and chemical composition determined show the thermal conditions existing at the surface. Seizure and scuffing are extreme manifestations of severe wear occurring respectively at low and high temperatures.     

Tribological Performance and Wear Mechanism of Compound Containing S,P, and B as EP/AW Additives in Copper Foil Oil

Research and development on the high biodegradability of additives is indispensable for environmentally friendly lubricants, which is one of the key factors to advance lubricant technology toward “greener” chemistry. The tribological performance of fatty alcohol polyoxyethylene phosphate acid ester (EK), boron-containing amide (BT), dialkyl dithiophosphate ester (DDE), and a mixture of these (compound) as extreme pressure (EP)/antiwear (AW) additives in hydrogenated base oil (GH) were investigated using a four-ball testing machine. The elemental composition and chemical characteristics of the AW films generated on the surfaces of the steel balls were studied using X-ray photoelectron spectroscopy (XPS), and their AW mechanisms are hereby proposed. Thermal degradation tests were conducted to identify their thermal stabilities using thermogravimetry and differential scanning calorimetry. The results show that these additives can greatly improve the EP/AW properties of GH. XPS analyses of the worn surfaces indicate that decomposed borate esters and organic sulfide or nitrides were adsorbed on the worn surface, and the P and S elements of the compound reacted with the metal and existed in the form of phosphates and sulfates, both of which contributed to the formation of a boundary lubricating film. Moreover, these additives provide the lubricants with excellent oxidation resistance and thermal stability.     

The nature and origin of wear particles from boundary lubrication with a zinc dialkyl dithiophosphate

It is well known that in many tribological situations antiwear additives act by reaction film formation on contacting surfaces. In the present paper we describe our observations on the reaction film formed with a zinc dialkyl dithiophosphate and the wear particles. Detailed structural and chemical information has been obtained by using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray energy spectrometry. The results show a clear connection between the reaction film material and the nature of the wear particles present in the lubricant; both a highly dispersed phase system and a layered structure are present. The origin of the wear particles is discussed in terms of crack initiation mechanisms.     

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

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. Formation of MoS ₂ , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film.

     

A study of the tribological behaviour of dialkyldithiophosphate esters as additives in rape seed oil

Zinc dialkyldithiophosphates (ZDDP) cannot be used as additives in biodegradable lubricants because of their zinc content. In investigating substitutes for ZDDP, dialkyldithiophosphate esters have been synthesised, and their tribological behaviour as additives in rape seed oil has been evaluated using a four‐ball friction and wear tester and compared with that of ZDDP. The results show that these additives have better antiwear properties and load‐carrying capacity than rape seed oil alone. The morphologies and the elemental chemical states on the worn surfaces of the lubricated steel balls of the tester were examined using X‐ray photoelectron spectroscopy and scanning electron microscopy. The tribological mechanism is discussed on the basis of the experimental results.     

二烷基二硫代氨基甲酸锑在聚α-烯烃中的摩擦性能及其摩擦化学作用机制的研究

合成了2种二烷基二硫代氨基甲酸锑,并经红外、核磁对其分子结构予以确认。研究了它们的热稳定性,系统地考察了其在聚α-烯烃（PAO）中不同含量、不同载荷下的摩擦学性能。探讨了二烷基二硫代氨基甲酸锑的抗磨作用机制。结果表明,二烷基二硫代氨基甲酸锑盐的热分解温度较高,热稳定较好,具有良好的极压抗磨性能。EDS分析表明,2种添加剂存在的润滑条件下,钢球磨斑表面上存在着S、Fe、Sb等元素。XPS分析表明,S元素以硫酸亚铁及少量的FeS形式存在,Sb元素以Sb2O3形式存在,而N元素则以复杂的吸附膜形式存在,这些因素一同起极压抗磨作用。     

Friction‐induced two‐dimensional solid films from lubricant additives

Tribochemical interactions between some lubricant additives, namely zinc dithiophosphate (ZnDTP), an antiwear additive, molybdenum dithiocarbamate (MoDTC), a friction modifier, and overbased calcium borate (OCaB) detergent, have been investigated. The nature of the tribofilms formed was studied by combining high‐resolution transmission electron microscopy of wear fragments and inside wear scars with micro‐spot X‐ray photoelectron spectroscopy at the same location of the wear track (‘dual’ analysis). The OCaB/ZnDTP/MoDTC ternary system provides both a low wear rate and ultralow friction, while adding detergent and anti‐corrosion properties to the formulation. The analytical data indicate that such a synergistic effect can be attributed to an outstanding feature of the tribofilm nanostructure: it is composed of perfectly oriented MoS₂ single sheets embedded in a single‐phase calcium and zinc borophosphate glass. Compared to phosphate alone, MoS₂ sheets are oriented by the borophosphate phase. This could be related to a template effect of friction‐aligned planar molecules of the glassy borate polymer.     

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.     

Antiwear Additives in Neopentyl Ester Oils

A variety of organophosphorus and other selected compounds were studied as antiwear additives for neopentyl polyol ester lubricants on 440C stainless steel rubbing surfaces using a four-ball wear test machine. Their effectiveness as additives was compared with that of tricresyl phosphate. Although 440C steel is relatively unreactive, it was possible to improve strikingly its wear and friction characteristics with organophosphorus additives. Some of the best results were obtained with acid phosphates. Alkyl chain length also appeared to be a significant factor. It was demonstrated that small concentrations of extraneous acidic substances are responsible for the antiwear action of phosphites at high load and that larger amounts of such acidic materials are needed as the load increases. Experiments involving addition of amines to neutralize formulations containing an acid phosphate confirmed that free acids promote antiwear action at the higher loads. Vinyl stearate and selected compounds containing chlorine or sulfur proved ineffective as additives for 440C steel in the antiwear region.