Biobased dry‐film metalworking lubricants

Metalworking lubricants must allow the manufacture of acceptable products at competitive cost without causing harm to operators or the environment. One way of attaining such a goal is through the use of biobased raw materials in lubricant formaulations. Biobased materials are generally non‐toxic, easily biodegradable, and abundantly available from renewable agricultural sources. However, successful application of biobased marterials in lubrication requires a thorough understanding of the tribochemical properties of these agricultural products. Recent studies have shown that biobased lubricants comprising starch and vegetable oils have promising lubrication properties. This paper discusses investigations into the effect of film thickness on the friction properties of dry‐film lubricants formulated from starch‐soybean oil composites.     

Friction properties of chemically modified starch

Starches of varying chemical structures were esterified with anhydrides, formulated into dry film lubricants with the addition of canola oil, and its friction properties investigated. In the absence of canola, chemically modified starch displayed a very high coefficient of friction (COF). Addition of canola resulted in a sharp decrease in COF, which decreased with increasing [canola] until it attained a minimum and constant value that was independent of the further increase in [canola]. This observation was attributed to the adsorption of the canola oil onto starch due to H‐bonding between the ester groups of the canola triglycerides and the free hydroxyl groups of starch. Analysis of the [canola] vs COF data using the Langmuir adsorption model showed stronger adsorption of canola to modified starch with a lower degree of substitution (DS). This was attributed to the availability of more free hydroxyl group adsorption sites on starches with lower DS than those with higher DS. Copyright © 2008 John Wiley & Sons, Ltd.     

Tribological properties and lubrication mechanisms of a Ag–Mo composite

A composite material containing silver and molybdenum metals was fabricated by powder metallurgy method with a Ag/Mo molar ratio of 2 : 1 and the sintering temperature is 700°C. Tribological properties, especially the solid lubrication behaviours during oxidation of the composite in air, were considered from room temperature to 600°C. Phase composition, microstructure and thermal behaviour of the composite were analysed before and after tests to investigate the lubrication mechanisms. The friction coefficients of the composite are ~0.7 below 400°C but decrease sharply to ~0.18 above 500°C. Characterisations of this composite indicate that several silver molybdates (Ag₂MoO₄, Ag₂Mo₂O₇ and Ag₂Mo₄O₁₃) formed from oxidation of Ag–Mo composite at high temperatures benefit lubrication effects and lead to the decrease of friction coefficients. Formation mechanism of these silver molybdates during oxidation and wear was therefore studied, and a model based on solid reaction processes in the Ag–Mo–O₂ system was promoted. Copyright © 2015 John Wiley & Sons, Ltd.     

Suspension of molybdenum–sulphur complexes in paraffin oil as extreme pressure lubricants

Molybdenyl aryl thiobiurets were evaluated as extreme pressure lubrication additives in a four ball test using steel balls of 12.7 mm diameter. A comparative account of performance of the tested compounds is presented on the basis of various tribological parameters such as initial seizure load, 2.5 s seizure delay load, weld load, mean Hertz load, flash temperature parameter, pressure wear index, friction coefficient (μ) and wear scar diameter (d), etc. All the tested complexes have been found to possess significant Extreme Pressure (EP) efficiency under the experimental conditions of four ball test. The best performance was shown by p-methoxyphenyl and p-chlorophenyl thiobiurets. The surface topography and tribochemistry of wear scar surface of balls in the presence of p-methoxyphenyl derivative at different loads have been studied by scanning electron microscopy and Auger Electron Spectroscopy (AES). The presence of molybdenum, sulphur, oxygen, nitrogen, etc. on the ball surface, detected by AES, suggests that these compounds act as precursors of tribologically active chemical layers formed under extreme pressure conditions.     

Wear characterization of electrodeposited Ni–WC composite coatings

The present contribution reports the tribological properties of Ni–WC composite coatings, electrodeposited on steel substrate. Commercial WC particles with an average size of 5 μm were codeposited with Ni on a mild steel substrate using a Watts bath at 50°C. The effect of plating variables on deposition behavior was studied. The amount of WC in the deposited layer decreased and plating efficiency increased with an increase in current density from 0.1 to 0.3 A/cm². The tribological properties of the coatings were studied using a small amplitude reciprocating friction wear tester. The addition of WC in Ni increases the microhardness of the electrodeposited coatings. An important result is that the presence of embedded WC particles in the electrodeposited coatings results in a much lower coefficient of friction (COF) of 0.34, when compared with pure Ni (COF 0.62) and mild steel (COF 0.54).     

Influence of fatty acid additives on the tribological performance of sunflower oil

Vegetable oils are potential substitutes for petroleum‐based lubricants because they are environmentally friendly, renewable, less toxic and readily biodegradable. The addition of free fatty acids has been shown to increase the lubrication performance of vegetable oils at elevated temperatures. The purpose of this study was to evaluate the relationship between the length of the carbon chain in the fatty acid and its effectiveness as an additive for a range of elevated temperatures. Stearic, arachidic and behenic fatty acid additives were added to commercial sunflower oil. All fatty acid additives were shown to be effective in lowering the wear rate and coefficient of friction in ball‐on‐disc tribological tests. The overall carbon chain length was not observed to have a consistent influence on the effectiveness of the additive. All additives were less effective at temperatures above 100°C. Copyright © 2010 John Wiley & Sons, Ltd.     

Applications of the concept of tribopolymerisation in fuels,lubricants, metalworking,and ‘minimalist’ lubrication

Tribopolymerisation, a novel concept of molecular design developed by Furey and Kajdas, involves the continuous formation of thin polymeric films on rubbing surfaces; the protective films formed are self‐replenishing. The antiwear compounds developed are effective with metals and ceramics and in the liquid and vapour phases. Furthermore, they are ashless and contain no harmful phosphorus or sulphur, and many are biodegradable. The potential applications are diverse and have cost, performance, energy, and environmental advantages. The present paper describes some applications of the concept of tribopolymerisation as a mechanism of boundary lubrication, including recent laboratory research and in‐plant industrial demonstrations. Applications include: (a) ashless antiwear or ‘lubricity’ additives for fuels; (b) ashless lubricants for automotive engines to reduce exhaust catalyst poisoning and environmental emissions; (c) lubrication of ceramic engines or ceramic components; (d) machining and cutting using thin films to reduce friction and ceramic tool wear; (e) vapour‐phase applications in high‐temperature gaseous systems or to counter fuel injector wear problems in natural gas engines; (f) special ‘minimalist’ pre‐treatment compositions for engine assembly and running‐in; and (g) enabling technology in the development of new engines and propulsion systems.     

Investigation of wear mechanism by organic sulphides in vegetable oils

Tribological properties of organic sulphides in esters were examined under boundary conditions. Magnification of wear by sulphides was observed in unsaturated esters. A dumb‐bell‐shaped wear scar, composed of a ‘central area’ and a ‘border area’, was found as an intermediate of wear growth. The ‘border area’ then expanded, resulting in a large wear scar. An induction period was observed before the generation of the ‘border area’. Influences of additive concentration on the induction period were also observed. Surface analyses of worn surfaces revealed that organic oxides, sulphides (unreacted additive) and sulphoxides are present in the boundary film. Influence of auto‐oxidation of the base oil on wear mechanism was proposed. Copyright © 2007 John Wiley & Sons, Ltd.     

Elastohydrodynamic study of vegetable oil–polyalphaolefin blends

Two polyalphaolefins, of higher and lower viscosity than vegetable oils, were used to make binary blends of varying compositions with soy bean and canola oils. The pure oils and the blends were used in viscosity and film thickness investigations. The effects of composition and temperature on viscosity were found to agree well with the theoretical predictions of a simple mixing law. The film thicknesses of the various blends under elastohydrodynamic conditions were measured at 20 N load, and varying entrainment speeds and temperatures. From the data, pressure–viscosity coefficients, α, as a function composition and temperature were obtained. The resulting α values were compared with theoretical predictions. Experimental values of α as a function of composition showed a slight negative or no deviation from the values predicted by an ideal mixing model. On the other hand, experimental values of α displayed a mild decrease with increasing temperature, while the model predicted a sharp decrease with increasing temperature. Published in 2008 by John Wiley & Sons, Ltd.     

Influence of temperature on the friction performance of gear oils in rolling–sliding and pure sliding contacts

The friction behaviour of five different gear oils in rolling–sliding and pure sliding contacts and how temperature influences their friction properties were investigated. It is found that increasing temperature decreases boundary friction with gear oils that contain friction modifiers while not for other gear oils, at all contact pressures investigated. In mixed lubrication region, temperature decreases friction at low contact pressures while increases friction at high contact pressures. The effect of slide–roll ratio on friction is significant in boundary lubrication region especially at higher temperature while less significant in mixed lubrication region at both low and high temperatures. The ranking of gear oils for friction in boundary and mixed lubrication regimes is similar both in rolling–sliding and pure sliding contacts, regardless of temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

球面配流副油膜的几何成型计算

以球面配流副为研究对象,通过引入欧拉角来表征缸体位姿,建立起球面配流副油膜分布的数学模型,从而获得油膜二维厚度值分布图以及三维球面分布图;进一步,在考虑实际配流副几何空间限制的约束下,建立基于球面油膜厚度空间的优化模型,通过GAS算法获得了油膜空间最小和最大油膜厚度以及所在位置.研究表明,缸体姿态对油膜几何场分布影响显著,为进一步研究油膜流场分布奠定了基础.     

Effects of supply of fine oxide particles onto rubbing steel surfaces on severe–mild wear transition and oxide film formation

This study is the first to show a quantitative condition required for the establishment of severe–mild wear transition with sliding distance, by studying the effects of supply of Fe₂O₃ particles onto rubbing steel surfaces on the transition and oxide film formation process. The supply of fine Fe₂O₃ particles was found to accelerate the wear transition, and the sliding distance at which the transition occurs was found to increase with particle diameter and applied load. Oxide films are produced on the rubbing surfaces by sintering of the supplied Fe₂O₃ oxide particles. At the severe–mild wear transition, the relative area of oxide films is the same for all diameters of supplied Fe₂O₃ particles. This finding suggests that the transition occurs when the relative area of oxide films reaches a specific value, which is proportional to the area of real contact.     

Impact of Cr3C2/VC addition on the dry sliding friction and wear response of WC–Co cemented carbides

Two grades of WC–10 wt.%Co cemented carbide with or without addition of Cr₃C₂/VC grain growth inhibitor during liquid phase sintering were produced with the goal to investigate their reciprocating sliding friction and wear behaviour against WC–6 wt.%Co cemented carbide under unlubricated conditions. The tribological characteristics were obtained on a Plint TE77 tribometer using distinctive normal contact loads. The generated wear tracks were analyzed by scanning electron microscopy and quantified topographically using surface scanning equipment. The post-mortem obtained wear volumes were compared to the online assessed wear. Correlations between wear volume, wear rate and coefficient of friction on the one hand and sliding distance and microstructural properties on the other hand were determined, revealing a significant influence of Cr₃C₂/VC on the friction characteristics and wear performance.