The two-layer structure of Zndtp tribofilms: Part I: AES, XPS and XANES analyses

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.     

Limitations on use of ZDDP as an antiwear additive in boundary lubrication

Wear tests on rubbing elements under various contact pressures, oil temperatures and surface roughness were performed to elucidate the limitation conditions on antiwear performance of a typical zinc dialkyl dithiophosphate (ZDDP) added to a paraffin base oil. The analysis results indicate that the antiwear performance of ZDDP depends on the rates of recovery and growth of effective surface films. If these rates exceed that of scraping the films off the rubbing surfaces, antiwear performance is evident. However, the rates of recovery and growth of surface films are influenced by some essential factors, including the oil temperature, contact pressure and the surface roughness and hardness of rubbing elements. Under some suitable conditions, ZDDPs show favorable antiwear performance, but under some severe conditions, such as high pressure (over 90 MPa), high temperature (over 200 °C) with an insufficient concentration of ZDDP and the rubbing of a rough and harder surface on a softer surface, the antiwear performance of ZDDPs is negligible.     

The influence of tribochemical reactions of antiwear additives on heterogeneous surface layers in boundary lubrication

The article discusses methods for the deposition of antiwear, composite coatings with a metallic matrix. The Ni–P–Al₂O₃ composite layers were superimposed on carbon steel elements using the electroless plating technique. A comparative analysis of adsorptive interactions was carried out for model systems with antiwear characteristics, i.e., for the Ni–P matrix, and the composite material containing micro-dispersive corundum, i.e., Ni–P–Al₂O₃. The paper presents the results of tribological tests and analyses the effects of tribochemical interactions in the macroscale (using in a pin-on-disc system device) and in the micro/nanoscale—performed with the use of Atomic Force Microscopy (AFM).     

First principles study of organophosphorus additives in boundary lubrication conditions: Effects of hydrocarbon chain length

We apply first principle calculations to investigate the effects of the hydrocarbon chain length in additive molecules under boundary lubrication conditions. In these conditions, occurring in high‐pressure applications, the thickness of the oil film becomes extremely thin, and the additive molecules remain confined between the two solid surfaces in contact. We consider two types of organophosphorous additives having the same phosphite group but hydrocarbon chains of different lengths. By comparing the molecular behavior under uniaxial stress applied, we elucidate the atomistic mechanisms that control the molecular capacity of maintaining an interfacial spacing under compression and the load‐induced molecular dissociation. This insight is relevant not only for a rational design of lubricant additives but also to provide understanding on the activation mechanisms of tribochemical reactions.     

Friction reduction by metal sulfides in boundary lubrication studied by XPS and XANES analyses

In this work, the antiwear action of zinc dithiophosphate (ZnDTP) in combination with friction modifier agent molybdenum dithiocarbamate (MoDTC) has been investigated. A Cameron-Plint friction machine was used to generate large tribofilm areas in mild tribological conditions. Two analytical techniques, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy, have been employed to characterize the chemical species in tribofilms. XANES spectroscopy at the P K and L-edge, Mo L and M-edge and S K-edge was carried out in order to investigate phosphates and sulfides species in the tribofilms This was followed by XPS analysis in the same location in the tribofilms. Special attention has been paid to the peak fitting of Fe 2p, Zn 2p, S 2p, Mo 3d and O 1s photopeaks and Auger ZnLMM lines. The combined analyses have shown that the MoDTC+ZnDTP containing oil produces mainly shorter chain metal polyphosphate material in addition to ZnS and MoS₂. No zinc sulfate has been detected. XPS confirms that the formation of MoS₂ is enhanced by the presence of ZnDTP. For all additives combinations; no iron has been detected in the tribofilms. Analytical results are discussed and compared with theoretical predictions from the Chemical Hardness model for ZnDTP, MoDTC and ZnDTP+MoDTC tribofilms formation.     

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms.

The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and the formation of phosphate in tribofilms and thermal films. B K-edge XANES shows that boron has a trigonal coordination in the untreated additive, but the coordination changes partially to a tetrahedral coordination in the tribofilm upon rubbing. No BN was detected in the film analyzed by B K-edge or N K-edge. Boron 1s XPS also did not show the presence of BN in the film.     

The tribological behaviour and tribochemical mechanism of action of oxovanadium (IV)-dithiophosphate

Vanadium (IV)-oxo-dicyclodithiophosphate (VDTP) was prepared and its additive properties evaluated using a four-ball machine. The results show that VDTP exhibits better extremepressure performance, but lower antiwear activity, when compared with ZDTP. The reaction film composition, characterised by scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS) and diffractometer surface techniques, reveals that the antiwear and friction layer containing vanadium oxide, phosphate/phosphide, and sulphide, prevent the rubbing pairs from contacting directly, resulting in better extreme-pressure and antiwear behaviour.     

Friction and wear behavior of diamond-like carbon coating on plasma nitrided mild steel under boundary lubrication

The friction and wear properties of synthetic ionic liquid functionalized borate esters as additives in poly-alpha-olefin (PAO) were measured for diamond-like carbon (DLC) coating on plasma nitrided AISI 1045 steel. Results show that the borate esters gave much better friction–reduction and antiwear properties for DLC coating/steel and DLC coating/DLC coating sliding pairs than zinc dialkyldithiophosphate (ZDDP). In addition the DLC coating had much better wear resistance than the nitrided mild steel substrate, indicating that duplicate surface modification was more effective in significantly increasing the wear resistance of mild steel.     

Role of complexation in the interaction between antiwear and dispersant additives in lubricants

Chemical interactions between antiwear and dispersant additives have been investigated in the case of mixtures of molybdenum dithiophosphates and nitrogenous compounds. Different types of behaviour have been observed in the friction measurements: in some cases, the additives led to antagonistic effects; in other cases, no (or weak) interactions occurred. Complementary analyses by infrared spectroscopy, transmission electron microscopy, and surface analysis were performed in order to understand the mechanisms of these interactions and to provide new insights that cannot be obtained otherwise. The explanations for these different types of behaviour include: the formation of molybdenum complexes in the bulk oil phase; the stability of those complexes; and the significance of steric effects. The goal of this work is to discover the means to optimise the structure and the nature of oil additives in order to limit the antagonistic effects between these additives.     

Surface topography and tribology of cast iron in boundary lubrication

This work aims to study and understand the influence of the surface topography on wear of grey cast iron used for heavy duty diesel engine cylinder liners. A micro-alloyed grey cast iron was tested with different surface topographies. These were polished surfaces, honed surfaces (with two different honing parameters) and three model surfaces with well defined grooves on a polished specimen.Reciprocating friction tests using a steel ball rubbing against a flat or a cylindrical sample (extracted from a cylinder liner) were carried out on a Cameron Plint test rig. A commercial synthetic oil for diesel engine was used as the lubricant. The friction coefficient and the electrical contact resistance were measured during the tests. The wear volume of the cylinder liner part was also measured at the end of the test.The influence of the surface topography on the tribochemical film formation and on the wear behaviour of cast iron was established. Surfaces exhibiting lots of surface asperities had the highest wear, mainly due to delayed formation of protective tribochemical film. In our test conditions, the spacing between the grooves on model specimens had no influence on the wear behaviour of the cast iron specimens.     

Surface generation and boundary lubrication in bulk forming of aluminium alloy

The electrical contact resistance is measured between the tool and workpiece during plane strain compression of aluminium strip coated with a non-conductive oxide film produced by anodising. Results are correlated with the observed oxide topography after the test. The purpose is to investigate the mechanism of the development of close metal-to-metal contact, the associated material transfer and their effects on the friction coefficient under boundary lubrication conditions. Initially the anodised layer provides electrical insulation between the tool and the strip but, as deformation proceeds, this layer breaks up and fresh metal is extruded through the cracks formed, causing a sharp fall in electrical resistance. Details of this behaviour are explored, showing a dependence not only on strip reduction, but also on the base oil used and the presence of boundary additives. The change in the behaviour is tracked as a transfer layer builds up on the tool.     

A review of zinc dialkyldithiophosphates (ZDDPS): characterisation and role in the lubricating oil

This is a review of the additive, zinc dialkyldithiophosphate (ZDDP), which is found commonly in lubricating oil where it plays a role as both an antioxidant and an antiwear additive. This zinc complex is highly effective but its mechanisms of action have not been definitively reported. This review covers work pertaining to the characterisation and mechanisms of action of ZDDP and includes studies carried out by sophisticated instrumentation as well as laboratory studies. There are some references to the nature of the antiwear films generated by ZDDP and the usefulness of its derivatives.     

The effect of surface texturing in soft elasto-hydrodynamic lubrication

A theoretical model is developed to study the potential use of laser surface texturing (LST) in the form of spherical micro-dimples for soft elasto-hydrodynamic lubrication (SEHL). The model consists of mutual smooth elastomeric and LST rigid surfaces moving relatively to each other in the presence of viscous lubricant. The pressure distribution in the fluid film and the elastic deformations of the elastomer are obtained from a simultaneous solution of the Reynolds equation and the equation of elasticity for the elastomer. An extensive parametric investigation is performed to identify the main important parameters of the problem, which are the aspect ratio and area density of the dimples. The parametric analysis provides optimum parameters of the surface texturing and shows that LST effectively increases load capacity and reduces friction in SEHL.     

ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio

Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 °C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 °C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high- and low-friction species in the tribofilm.     

Surface analytical studies of surface-additive interactions, by means of in situ and combinatorial approaches

The influence of tribological conditions on the surface reactions occurring between zinc dialkyldithiophosphate (ZnDTP) and steel surfaces has been studied by means of a combination of X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), in situ attenuated total reflection (ATR) infrared spectroscopy, and high-throughput combinatorial approaches. Purely thermal treatment at 150 °C appears to lead to the formation of zinc polyphosphates. However, in the presence of tribological stress, simple phosphates appear to dominate, with some indication that higher load conditions lead to an increase in the surface concentration of both phosphate and, at higher temperatures, polyphosphate.     

A computer model of mixed lubrication in point contacts

A numerical model of mixed lubrication is presented in this paper. The idea introduced here is that asperity contact may be viewed as a result of a continuous decrease in film thickness, so that the transition between contact and non-contact is continuous and the same mathematical model should work for both regions. The pressure over the thin films is assumed to obey the Reynolds equation, and the solution of the equation, under the condition of h→0, is expected to be the same as that predicted by the theory of elasticity. To achieve convergent and stable solutions, the left-hand side terms of the Reynolds equation are switched off when the local film thickness approaches zero, leading to a reduced Reynolds equation. Pressure distributions over the entire computation domain are thus obtained through solving a unified equation system without identifying hydrodynamic or asperity contact regions. Computations were conducted for several example cases and results show that convergent solutions are achievable on different types of roughness, over a wide range of λ ratios (0.01 to infinity), and for different slide-to-roll ratios (0.0–2.0).     

Micromechanics and electrical conductivity of point contacts in boundary lubrication

Research methods based on recording parameters of electrical conductivity are considered for future application in nanotribology. Design relationships of conductivity are presented for idealized models of interface under conditions of boundary lubrication; contributions of various conductivity types in the total contact conductivity are estimated. An application methodology is described for the methods to study the properties of boundary lubricating layers. Some of the results obtained from the study of strength and frictional properties of boundary lubricating nanometre layers under normal load and sliding conditions are discussed.     

Tribocatalysis reaction during antiwear synergism between borates and Sn(IV) compounds in boundary lubrication

The combination of an oil-soluble Sn(IV) compound such as dibutyl tin dilaurate and organic borates gave better antiwear protection to an oil than either component separately. The surface examination of the rubbing zone indicated that the atomic concentration of tin produced on the boundary layers by such combinations was greater than those without borates. Based on this, an antiwear synergistic mechanism can be postulated in which borates with electron-deficient p orbits in boron catalyse the triboreduction of Sn(IV) compounds on rubbing surfaces.     

在线紫外/过硫酸盐消解流动分析法测定地表水中总氮

应用FS Ⅳ型流动分析仪,采用在线紫外/过硫酸盐消解测定地表水中总氮。通过试验,确定最佳测定参数,对该方法的灵敏度、准确度和精密度等进行试验。测得的检出限0.019mg/L、相对标准偏差? 5.0%及回收率在95%～100%范围内。     

Behavior of thin viscous boundary films in lubricated contacts between micro-textured surfaces

Thin film colorimetric interferometry was used to map changes in film thickness in the vicinity of micro-dents of various depths produced on rubbing surfaces. It has been shown in recent studies that shallow micro-features within concentrated contact can increase mean film thickness by supplying more lubricant to the contact; however, this beneficial effect can also be accompanied by a local film thickness reduction. Nevertheless, these observations were done with mineral base oils that exhibited no boundary films formation. In this study the behavior of micro-textured surfaces are observed using formulated lubricant containing polyalcylmethacrylate (PAMA), viscosity index improver with boundary film forming properties. Obtained results show that an enlarged film thickness due to the presence of viscous boundary films is formed within the whole contact and these boundary films minimize the local film thickness reduction caused by micro-dents and further increase the efficiency of surface texturing within non-conformal contacts. It can be suggested from the obtained results that joint action of both boundary film formation and surface texturing combines both contributions that can help to increase tribological performances in different stages of machine parts operation by increasing lubrication film thickness.