A model of friction for a pin-on-disc configuration with imposed pin rotation

A friction model is developed by considering the Coulomb friction model, a probabilistic approach of wear prediction, the kinematics of the pin-on-disc configuration and the elastic theory of bending. The model estimates the magnitude and direction of the frictional force, the pin torque, the probability of asperity contact and the real area of contact distinguishing between the part due to elastic and plastic asperity contacts respectively. Therefore, the proposed model is suitable for the prediction of adhesive wear. It can be applied to metal contacts for conductance characterisation through the plastically deformed asperities which is of great interest for electrical contact resistance studies.     

Tribological behaviour analysis of hydrogenated and nonhydrogenated DLC lubricated by oils with and without additives

Diamond‐like carbon (DLC) is an essential element of friction reduction made major by the desire to reduce energy consumption for environmental and economic reasons. The contribution of the DLC depends on its tribological behaviour. The analysis of the wear of DLC in DLC/steel contact is important because of the high exposure to the wear of coated parts and the low degree of mastery of behaviour of DLC in the lubricants studied and optimised for steel. In this study, we have analysed the tribological behaviour of hydrogenated DLC (a‐C:H) and nonhydrogenated DLC (ta‐C) under various lubrication conditions. Oils with and without additives were used. The results show that the wear of a‐C:H is considerably lower than that of ta‐C in base oils, and in the investigated oil with additives, the wear of ta‐C is substantially reduced. Copyright © 2012 John Wiley & Sons, Ltd.     

Boundary lubrication of low‐sulphur diesel fuel in the presence of fatty acids

This paper presents an overview of the effect of aliphatic acids on the tribological properties of selected hydrocarbons and petroleum fractions. The major experimental part of the work focuses on improvements to the lubricity of low‐sulphur diesel fuel. Experiments were carried out using a pin‐on‐disc friction machine and HFRR test rig. The wear results obtained clearly show a specific effect of the test acids dissolved in hydrocarbons in the concentration range 0.005–0.1% (50–1000 ppm). Although the overall picture presented by these results is very complex, it can be concluded that a very small amount of the selected acids dramatically improves low‐sulphur diesel fuel lubricity.     

The effect of surface chemistry in lubrication on the tribological behaviour of steel rollers under rolling-sliding contacts

Experiments were carried out on a wear test machine utilising a gearcam adapter to simulate line-contact lubrication. Due to the formation of a surface adsorption layer on the steel substrate, the tribological performance was detected by measuring the voltage for a lubricant with various additive concentrations. The roller wear rate was found to be strongly dependent upon both the rising rate of voltage in the wear process and the time period needed to create a positive voltage. The antiwear effect of differing additive concentrations was evaluated using the measurements of voltage for various operating conditions. An increase in rotational speed, with a lubricating oil with a low additive concentration, decreases the wear rate. A lubricating oil with a high additive concentration does little to decrease the wear rate at low rotational speeds.     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

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.     

Tribological behaviour of stainless steel with respect to that of DLC in terms of energetic aspects

Abstract

The tribological behaviour of stainless steel (SUS 440C) relative to that of diamond-like carbon (DLC) was investigated in terms of tribometer input energy. The DLC was prepared on tungsten carbide (WC) substrates using radio frequency plasma chemical vapour deposition with benzene (C₆H₆) as a gas source. The stainless steel ball, as the counterpart, was tribotested. The input energy was calculated using the applied load, friction coefficient and sliding distance obtained from each tribotest. The wear loss of the ball increased as the sliding distance increased, whereas wear loss of the DLC was not directly observed. During evaluation of the input energy, the wear rate of the stainless steel ball decreased as the input energy increased. We propose a method for evaluating tribological properties using the input energy and discuss the wear behaviour of the stainless steel based on the input energy.     

The synergistic effects of EP and AW additives with a sulphurised steel surface under boundary lubrication

In this paper the synergistic effect on tribological behaviour of two additives, sulphurised olefin and tricresyl phosphate, in paraffin oil, and 1045 steel, treated by sulphurisation, was studied on a ball-on-disc test machine. An excellent load-bearing capacity was obtained by the combination of sulphurisation and the lubricant paraffin+sulphurised olefin. The mechanism of this synergistic effect is discussed, based on micro-analysis of the surface film by AES and XPS.     

Measurement and simulation of fuel injection pipe pressure and study of its effect on the heat release in a direct injection diesel engine

Fuel injection pipe pressures are measured and simulated to study the effect of fuel injection system characteristies on the heat release in a direct injection diesel engine. The fuel injection simulation is based on a linear model. The governing equations are solved by the finite difference method. The measured fuel pipe pressures and the simulated fuel pipe pressures matched well to each other except for the interval when the nozzle is closing. The effects of the fuel pipe length and the nozzle opening pressure are tested. The longer fuel pipe length causes proportional retardation of the fuel injection time. The higher nozzle opening pressure results in increase of the maximum fuel pipe pressure and shorter combustion duration.     

Development of a detailed model for piston-ring lubrication in IC engines with circular and non-circular cylinder Bores

Over recent years a detailed model for piston-ring lubrication has been developed by the authors. The computer model incorporates a finite difference solution of the two-dimensional Reynolds equation with the squeeze effect for fully flooded lubrication as well as a flow-continuity algorithm for ‘starved' lubrication. It can be used to evaluate the tribological performance of piston rings operating in both circular and distorted bores. The influences of many factors, such as level of bore distortion, ring conformability, axial motion of the ring, and circumferential variation of the ring face profile are taken into account. An improved method for determining oil availability in a ring pack was also developed by considering the effect of relative locations of rings on the piston and oil accumulation in front of the ring. This paper summarises the development of the model and presents some selected results. It should be noted that much of the material presented has been drawn from other publications by the authors for the purpose of review and summary.