The effect of ZDDP and MoDTC additives in engine oil on the friction properties of DLC‐coated and steel cam followers

Friction tests simulating cam follower sliding conditions were conducted using a pin‐on‐disc test rig. In the case of SAE 5W‐30 class engine oil, the friction coefficient of the combination of steel pins sliding on a steel disc increased from 0.11 to 0.12, while that of steel pins sliding on a diamond‐like carbon (DLC)‐coated disc decreased from 0.12 to 0.10. For 5W‐20 oil containing the friction modifier MoDTC (molybdenum dithio‐carbamates), the friction coefficient of steel pins sliding on a steel disc decreased markedly from 0.12 to 0.04. In contrast, that of steel pins sliding on a DLC‐coated disc decreased more moderately, from 0.11 to 0.08. In both cases, Zn, P, S, and Mo elements derived from ZDDP (zinc dialkyldithiophosphate) and MoDTC additives were not detected on the DLC‐coated disc, while these elements were detected on the steel disc and pins using electron probe microanalysis and X‐ray photo‐electron spectroscopy surface analysis. It appears that a tribochemical reaction film did not form on the DLC material due to its inactive chemical properties. Therefore, an increase in friction due to the formation of the film derived from ZDDP and a decrease in friction due to the formation of the film derived from MoDTC were clearly suppressed in the case of the steel pins sliding on the DLC‐coated disc. It is thought that the tribo‐chemical reaction film was instrumental in reducing friction substantially. The lateral force of the film formed on the steel disc was then measured using an atomic force microscopy lateral force microscopy test. The lateral force of the film resulting from the 5W‐30 oil was much higher than that of the film formed from the 5W‐20 oil with MoDTC. This result coincided well with the results of the friction tests. Judging from these results, it is thought that the high friction coefficient observed for the steel pins on the steel disc for the 5W‐30 oil was caused by the higher shear strength of the film formed from ZDDP. On the other hand, the very low friction coefficient observed for the steel pin‐steel disc combination for the 5W‐20 oil was presumably caused by the formation of a solid MoS₂ lubricant from the MoDTC additive.     

Applicability of ring‐on‐disc and pin‐on‐plate test methods for Cu–steel and Al–steel systems for large area conformal contacts

This study compares two methods for model scale testing of ferrous/non‐ferrous tribocouples under large area conformal contact condition, the kind existing in engine components such as journal bearings. Results show that the ring‐on‐disc method is better suited to visualise the performance of such tribosystems compared with the pin‐on‐plate method. The former offers greater sensitivity to minor changes in coefficient of friction and contact potential and is able to determine the thermal stability of the tribosystem under given conditions. Post‐test surface characterisation revealed protective phosphorus‐rich tribofilms on the surface of the steel counterparts from both test methods. Copyright © 2012 John Wiley & Sons, Ltd.     

Vacuum tribological behaviour of self‐lubricating quasicrystalline composite coatings

High‐temperature‐resistant self‐lubricating coatings are needed in space vehicles for components that operate at high temperatures and/or under vacuum. Thick composite lubricant coatings containing quasicrystalline alloys as the hard phase for wear resistance can be deposited by a thermal spray technique. The coatings also contain lubricating materials (silver and BaF₂ CaF2 eutectic) and NiCr as the tough component. This paper describes the vacuum tribological properties of TH103, a coating of this type, with a very good microstructural quality. The coating was deposited by high‐velocity oxygen fuel spraying and tested under vacuum using a pin‐on‐disc tribometer. Different loads, linear speeds, and pin materials were studied. The pin scars and disc wear tracks were characterised using a combination of scanning electron microscopy and energy dispersive spectrometry. A minimum mean steady friction coefficient of 0.32 was obtained when employing an X750 Ni superalloy pin in vacuum conditions under 10 N load and 15 cm/s linear speed, showing moderate wear of the disc and low wear of the pin.     

Cooling by sub-zero cold air jet in the grinding of a cylindrical component

This paper investigates the cooling effect when using an air jet at sub-zero temperature of ?15 °C in the plunge grinding of a cylindrical component made of high strength steel EN 26. A three-dimensional finite element heat transfer model with a moving heat source was developed to reveal the complexity of the heat transfer mechanism involved. It was found that the use of cold air does not significantly reduce the temperature rise in grinding and that the cooling effectiveness is mainly limited by the following facts: (a) the air jet is difficult to penetrate into the grinding zone, (b) the heat transfer coefficient provided by an air jet is small and (c) cooling is limited by the time which the rotating workpiece surface can be exposed to the jet impingement. The study also showed that the present modelling method can be used as a first tool to assess the feasibility of a new cooling medium for grinding operations.     

Optimisation of Industrial Tribological Systems with the Pin‐on‐Disc Tribometer Method

The purpose of tribology is to improve the service life and efficiency of machines and their elements. This paper describes certain tests that can be done on a modified pin‐on‐disc apparatus to match in‐service conditions more closely. The three examples considered incorporate a liquid heating function, a reciprocating modification to allow for linear sliding rather than circular sliding, and a modification to allow the simulation of different lubrication regimes.     

On the feasibility of using cryo‐treatment as a tool to enhance the abrasive wear behaviour of solid‐lubricated polymeric composites

Cryo‐treatment, a bulk modification technique, is fast emerging as a way with which to improve the wear resistance of metals. This technique has also shown the ability to enhance significantly the abrasive wear performance of some polymers and their short glass‐fibre reinforced composites. In this work, short carbon‐fibre reinforced composites of some heat resistant polymers, such as polyetherimide, polyethersulphone, polyamide 6,6, polyetheretherketone, and polytetrafluoroethylene, were selected to explore the potential of cryo‐treatment. The selected materials were cryogenically treated by cooling them to the temperature of liquid nitrogen. The abrasive wear tests were carried out at ambient temperature in single pass conditions at various loads, on a pin‐on‐disc machine, using silicon carbide paper as a counterface. The investigations revealed that this technique has definite potential to increase the wear performance of carbon‐fibre reinforced composites. An increase in hardness due to cryo‐treatment was thought to be responsible for an observed improvement in wear performance. However, the extent of improvement in the wear performance was not matched by an increase in the hardness value. Scanning electron microscopy proved useful in examining the morphological changes in the composites due to cryo‐treatment.     

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.     

An experimentally based thermo-kinetic phase transformation model for multi-pass laser heat treatment by using high power direct diode laser

Laser-based phase transformation hardening (LPTH), based on rapid heating and cooling cycles produces hard and wear-resistant layers only at the selective region of the components. However, the bulk mass of the material’s core property is retained. The advantages of high power direct diode laser in comparison with other high power lasers (CO₂ and Nd:YAG) have put this type of laser as a main heat source for localized heat treatment. However, a tempered zone is formed in overlapping regions of a large heat-treated area during multi-pass laser heat treatment (MPLHT) that affects the uniformity of heat-treated depth of material. This study is focused on the development of a uniform hardness distribution model to minimize the tempering effect during the MPLHT process. A tool steel AISI S7 is heat treated by using different levels of laser power (1,400–1,800 W) and scanning speeds (15–25 mm/s). An experimentally based finite element (FE) thermal model is developed to predict the cross-sectional as well as surface temperature history of the MPLHT process. The temperature-dependent material properties and phase change kinetics are taken into account in the model. The laser beam is considered as a moving rectangular-shaped heat source (12 mm?×?1 mm) with a uniform distribution (top-hat) of laser power. The temperature history acquired from the FE thermal model is coupled with thermo-kinetic (TK) equations to determine the corresponding phase transformations and hardness. The tempering effect of MPLHT is studied for different sizes of overlap (1 mm–3 mm) and lengths of scan (10 mm–35 mm). The TK model results are verified with experimental ones to optimize the processing parameters. The optimized processing parameters, including laser power, scanning speed, size of overlap, and the length of scan are used to achieve a uniform hardness distribution and an even depth of heat treatment in the MPLHT area.     

A study of the role of solid lubricant and fibrous reinforcement in modifying the wear performance of polyethersulphone

High‐temperature polymers are generally preferred for those tribology applications where cost is secondary and performance is the primary consideration. Since frictional heat dissipation limits the usefulness of polymers because of their poor thermal conductivity, high‐temperature polymers are preferred in applications which have harsh operating conditions. In this paper, a high‐temperature polymer, polyethersulphone (PES), was selected for some adhesive wear studies, along with two PES composites containing 18% glass‐fibre (GF) reinforcement and two solid lubricants, i.e., PTFE and MoS₂ (2% each). Adhesive wear studies of these materials on two pin‐on‐disc machines indicated that neat PES was not a good tribo‐material. However, incorporation of GF and solid lubricants enhanced the wear performance by an order of two. PTFE was found to behave better than MoS₂. However, after long sliding duration both the lubricants performed almost equally well. The topography of the surface of the pins and the disc was studied using SEM to investigate the wear mechanisms.     

Tribophyiscal scaling effects in friction and wear of polymers on small‐ and large‐scale tests

Coefficients of friction and wear rates for polyacetal and polytetrafluoroethylene are measured on small‐scale cylinder‐on‐plate tests and large‐scale flat‐on‐flat tests. Three models for extrapolation between small‐ and large‐scale test results, which are based on experimental parameters, are presented: (i) one single mechanical parameter (normal load), (ii) the contact‐pressure–sliding‐velocity model (temperature limit), and (iii) a macroscopic geometry model. The last model is most extensively evaluated and considers thermal effects, sample geometry and contact conditions. After correction for the thermal sliding regime and viscoelastic deformation, the coefficients of friction can be extrapolated while the wear rates are more sensitive to the contact situation. High‐temperature tests on small scale are not representative for high‐load tests on large scale. Besides mechanical effects such as stress concentrations and debris mobility, variations in polymer structure under sliding (formation of rigid amorphous phase) and chemical interactions (wear debris polymerisation) also change with testing scale. The latter effects are explicitly illustrated from spectroscopic measurements on the worn surfaces. Copyright © 2008 John Wiley & Sons, Ltd.     

Galling resistance evaluation of tool steels by two different laboratory test methods for sheet metal forming

Adhesive accumulation of work material on the tool surface is today a major problem in many sheet metal‐forming applications. Different laboratory test methods are used to investigate galling with respect to different tool materials, lubricants and process conditions. In the present study, the galling resistance of a modern nitrogen‐alloyed powder metallurgy tool steel and an conventional ingot cast D2 type tool steel was evaluated under lubricated sliding against ferritic stainless steel sheets using a commercial pin‐on‐disc (POD) and an in‐house made slider‐on‐flat‐surface (SOFS) tribotester. The investigated tool steels ranked similarly in terms of galling resistanc in both test methods. However, sliding distances to galling were longer for the SOFS equipment due to continuous sliding on new lubricated sheet surface. Best performance was demonstrated by the powder metallurgy tool steel treated to 65 HRC. Differences in friction behaviour and galling initiation were analysed on the basis of the two different working conditions, i.e. open (SOFS) and closed (POD) tribosystems. Copyright © 2012 John Wiley & Sons, Ltd.     

Fe thermal analysis of a ceramic clutch

This study involves the finite element (FE) thermal modeling of a dry running ceramic clutch disk to be applied in passenger cars and a comparison with experimental measurement results. The problem can be handled by two independent FE models linked by heat partition changing in time and space. A distributed heat source was applied for modeling heat generation; furthermore, changes in time and space of the heat convection coefficient were also taken into consideration.     

盘式制动器热-结构耦合的数值建模与分析

在充分考虑移动热源且速度可变效应影响、盘与片摩擦界面间热流耦合的基础上,根据制动盘与摩擦片的实际几何尺哌寸,建立一个紧急制动工况下三维瞬态热-结构耦合的计算模型,运用大型有限元软件ANSYS中的非线性有限元多物理场方法,数值模拟盘式制动器的制动过程.揭示制动过程中制动盘瞬态温度场/应力场的分布规律,发现二者之间存在着耦合关系,二者随制动时间明显地呈现周期性变化,这些周期波动是由移动热源产生的热流冲击和对流换热影响的交替作用所引起的,且其变化周期随制动时间的延长而增大.并初步探讨制动盘产生径向裂纹的原因.     

基于长大坡道持续制动的新型制动盘研究

随着我国中西部地区高速铁路的建设,由于长大坡道线路带来的问题日益凸显。在长大坡道行驶时如果电制动出现故障,将采用纯空气制动,长大坡道造成的持续制动会导致制动盘的热负荷急剧上升。由于散热的时效较慢从而导致制动盘温度过高,同时产生较大的温度梯度,从而导致制动盘热疲劳裂纹产生。为了解决长大坡道工况下制动盘的换热效率提升和降温问题,基于兰新线的长大坡道工况,通过设计新型的铝嵌钢结构制动盘,采用有限元分析软件对铝嵌钢结构制动盘和全钢制动盘进行仿真计算,得到温度场和热应力分布。结果表明,在长大坡道采用纯空气制动时,铝嵌钢制动盘可以在实现轻量化的同时明显降低制动盘面的温度和温度梯度,缓解长大坡道制动带来的制动盘热疲劳问题。     

Three-Dimensional Thermohydrodynamic Analyses of Rayleigh Step Air Foil Thrust Bearing with Radially Arranged Bump Foils

A three-dimensional (3D) thermohydrodynamic (THD) model for air foil thrust bearings (AFTBs) is presented. The nonisothermal Reynolds equation is solved using pressure boundary conditions at the cooling air plenum considering local temperature-dependent viscosity and density. Air film temperature is calculated using the 3D energy equation with thermal boundary conditions at the top foil, thrust runner, and top foil’s leading edge. The cooling air plenum distributes the cooling air to multiple radially arranged cooling channels. The plenum temperature and pressure are found from mass and energy balance equations applied to the plenum. Temperature fields of the top foil, bump foils, thrust disc runner, bearing plate, and cooling air channels are also solved through appropriate energy balance equations with their surroundings. A robust computational algorithm with multiple iteration loops was developed to find all the temperature fields. THD analyses were performed for AFTB with outer radius of 50 mm up to 100,000 rpm. As the cooling air source pressure is increased, the plenum pressure also increases and its temperature decreases due to more cooling capacity. However, cooling effectiveness is not necessarily proportional to the pressure because the flow residence time inside the cooling channels is inversely proportional to the pressure. The analyses show that the thrust disc temperature is a parabolic function with speed, and thermal expansions of the thrust disc and thrust plates contribute to the most significant driving force of thermal instability. Optimum cooling air pressure was found around 12,500 Pa for the proposed AFTB design at the reference simulation condition.     

Mechanism of boundary film formation from n‐hexadecane

It is widely acknowledged that the process of surface film formation by tribological additives is very complex, because even saturated hydrocarbons — as major components of lubricating oils — undergo chemical changes under boundary lubrication conditions. To gain a better understanding of the tribochemical changes of paraffinic hydrocarbons, research was carried out using n‐hexadecane, which is widely used as a low‐viscosity model base oil. It has been hypothesised that the interactions between the products of n‐hexadecane triboreactions and steel surfaces are mostly initiated by the mechanical action of the system. Wear tests were performed, using a pin‐on‐disc machine. Fourier transform infrared microspectrophotometry and X‐ray photoelectron spectroscopy were used to analyse the wear tracks on the steel discs. Based on the analytical data obtained, a model of boundary film formation from aliphatic hydrocarbons is proposed.     

High‐temperature laser‐scanning confocal microscopy as a tool to study the interface instability during unsteady‐state solidification of low‐carbon steel

Solidification microstructure is a defining link between production techniques and the mechanical properties of metals and in particular steel. Due to the difficulty of conducting solidification studies at high temperature, knowledge of the development of solidification microstructure in steel is scarce. In this study, a laser‐scanning confocal microscopy (LSCM) has been used to observe in situ and in real‐time the planar to cellular to dendritic transition of the progressing solid/liquid interface in low carbon steel. Because the in situ observations in the laser‐scanning confocal microscopy are restricted to the surface, the effect of sample thickness on surface observations was determined. Moreover, the effect of cooling rate and alloy composition on the planar to cellular interface transition was investigated. In the low‐alloyed, low‐carbon steel studied, the cooling rate does not seem to have an effect on the spacing of the cellular microstructure. However, in the presence of copper and manganese, the cell spacing decreased at higher cooling rates. Higher concentrations of copper in steel resulted on an increased cell spacing at the same cooling rates.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

Contact and thermal analysis of an alumina—steel dry sliding friction pair considering the surface roughness

In the present study, finite element transient contact and thermal sliding simulation and temperature measurement of dry sliding friction were performed in order to analyse the real contact area and temperature developed in the contact region. Real 3D surface worn topographies were taken into consideration, at macro and intermediate stages. The calculated real contact area has been changing in time and space in the course of sliding. The sliding components were high purity alumina ceramic palettes and 100Cr6 steel with constant accelerated motion. The calculated temperature results are in good agreement with the temperature data measured. Heat partition was changing in time during sliding. The developed algorithm based on incremental FE technique can characterize real processes.     

Transient thermo‐elastohydrodynamic lubrication of gear teeth

During a gear mesh cycle, load, rolling and sliding velocities, curvature, and temperature change rapidly. In this paper, a transient, thermo‐elastohydrodynamic lubrication model is presented that has been used to study the lubrication parameters at 250 contact points along the path of a contact. The working flanks were assumed to be smooth. The line load was calculated at every contact point, using the finite‐element method. Dynamic loading was not considered. The steady‐state temperature field of the working flanks was used to obtain the surface temperature at the inlet of each contact point, determined by an experimental‐analytical method presented previously. In this model, the lubricant is considered to follow the Ree‐Eyring constitutive equation. The influences of both pressure and temperature on density, thermal conductivity, and specific heat of the lubricant are also taken into account. Results are presented for an FZG type A gear pair lubricated with an FVA No. 4 reference oil; these include film shape, pressure and temperature distributions, as well as friction coefficient.