High temperature wear behaviour of selective laser melted (SLM) 316L stainless steel (SS) was studied to elucidate the influence of characteristic microstructure of SLM 316L SS on the wear properties. The wear tests were conducted from room temperature (RT) to 600 °C using ball-on-disc setup with alumina counter ball. The effect of temperature on the wear rate and the underlying mechanisms were evaluated and compared with conventional 316 SS. The RT coefficient of friction (COF) and wear rate of SLM 316L SS and conventional 316 SS were 0.5 and 4.6 ± 0.4 x 10−4 mm3/Nm and 0.7 and 4.5 ± 0.1 x 10−4 mm3/Nm, respectively. The wear rate of conventional 316 SS slightly decreased with increasing temperature from 4.5 ± 0.1 x 10−4 mm3/Nm at RT to 3.2 ± 0.1 x 10−4 mm3/Nm at 300 °C, followed by increasing to 4.9 ± 0.4 x 10−4 mm3/Nm at 400 °C, while the wear rate of SLM 316L SS was twofold lower with 2.3 ± 0.6 x 10−4 mm3/Nm at 300 °C and 2.7 ± 0.3 x 10−4 mm3/Nm at 400 °C. The wear rate at 600 °C was found to be comparable between SLM 316L SS and conventional 316 SS with a wear rate of 6.4 ± 0.7 x 10−4 mm3/Nm and 6.6 ± 0.6 x 10−4 mm3/Nm, respectively. The lower wear rate in SLM 316L SS at higher temperatures of 300 °C and 400 °C was due to its stable hierarchical microstructure, cellular subgrains, formation of stable oxide glaze and higher hardness. Moreover, the cross-sectional microscopy of wear track after 600 °C wear tests showed that the deformation zone below the wear track in SLM 316L SS was 10–15 μm compared to 30–40 μm for conventional 316 SS. The two folds low wear rate of the SLM 316L SS at 300 °C and 400 °C compared to conventional 316 SS could potentially render it for usage in applications where high temperature wear resistant SS are needed.

A wear model including a deterministic FFT-accelerated contact mechanical tool to calculate pressure and elastic-plastic deformation, is employed to simulate the time dependent wear in a sphere on flat contact. The results of the wear simulations compared to experimental results from a reciprocating test in a ball on disk tribometer. The conditions of the simulations and the experiments are independently adjusted to match up. Similarities and differences shows upon the usefulness and limitation of wearmodelling of this type.

This paper deals with field validation of the force response from a defective wheel after normal service. The defective wheel represents a typical defect arising from normal winter operation in a cold climate. The current detector is a typical wheel load impact detector that has been in use for about fifteen years; this type of detector is widely used in the infrastructure. The wheel has defects of up to1.8 mm in depth and an un-roundness of 0.2 mm. The results from this investigation, pertaining to the particular vehicle and wheel defect, show a linear correlation of speed and force response. Each change in speed of 1 km/h changes the force response by about 0.9 kN.

The abrasive wear of carbide-free bainitic steel under dry rolling/sliding conditions has been studied. It is demonstrated that this nanostructure, generated by isothermal transformation at 200 °C, has a resistance to wear that supersedes that of other carbide-free bainitic steels transformed at higher temperatures. The experimental results, in combination with a theoretical analysis of rolling/sliding indicates that under the conditions studied, the role of sliding is minimal, so that the maximum shear stresses during contact are generated below the contact surface. Thus, the hardness following testing is found to reach a maximum below the contact surface. The fine scale and associated strength of the structure combats wear during the running-in period, but the volume fraction, stability and morphology of retained austenite plays a significant role during wear, by work-hardening the surface through phase transformation into very hard martensite

Computational movies were used to analyse the formation and collapse of vapour cavitation bubbles in a submerged journal bearing. The effect of vibration amplitude on vapour cavitation was studied for a journal undergoing circular whirl. The boundary conditions were implemented using Elrod's algorithm which conserves mass flow through the cavitation bubble as well as the oil film region of the bearing. In the calculations, 0.1 ε εmax, where ε is the instantaneous eccentricity and 0.4 εmax 0.9 for the different cases studied. For the case 0.1 ε 0.4, no vapour cavitation occurred. For the case in which 0.1 ε < 0.9, vapour cavitation was present for 76% of the total time

This work focussed on assessing the contact conditions driving the adhesion tendency of PVD TiAlN coated cemented carbide during reciprocating sliding against a normalized AISI 4137 carbon steel. A special emphasis is given to the surface topography of the coating. Results are analysed in terms of friction and material transfer over a large range of temperatures (up to 800 °C) and contact pressures. The post-test surface analysis of the specimens is conducted in order to understand the tribological behaviour and elucidate the formation mechanisms of transfer layers. A numerical model is developed to assess the amount of heat effectively transmitted into the first bodies and the temperature of the surfaces in contact.Whereas temperatures close to 400 °C ensure the formation of a stable tribofilm reducing friction, the highest temperatures lead to unstable frictional behaviour. Coating surface topography has been seen to be a major parameter driving material transfer during the first stages of the contact and the formation of a transfer layer. Thin and homogeneous layers are almost instantaneously formed with a polished surface whereas some time is required with a rougher one to form a film. A large amount of the frictional power is dissipated into these layers and high temperatures can be reached at the surface due to frictional heating. Contact pressure is found to be a parameter promoting transfer and oxidation

Hot stamping is characterised by severe contact conditions, especially when forming aluminium components. In order to improve the tool lifetime, process economy, and component quality, understanding the initiation mechanisms behind aluminium transfer onto the tool surface at high temperatures is critical. To date, the tribological interaction between tools and aluminium sheets at high temperature has received limited attention. Lubricants, combined with surface engineering techniques (e.g. coatings, nitriding and surface topography control), show great potential for reducing the severity of material transfer at high temperatures. However, there is still, limited knowledge about their interaction and performance in this tribological context. In this study, high temperature tribological tests were carried out to characterise the synergetic effects of surface coatings/treatments with and without lubrication on friction and wear. A commercially available lubricant was evaluated when used in combination with uncoated, nitrided and CrWN- or DLC ta-C-based PVD coated tool steel. The tests were carried out on a hot strip drawing tribometer, employing an open contact configuration representative of the hot stamping contact conditions at two different temperatures. The counter-material was a 6082 aluminium alloy, heated up following a thermal cycle relevant for the hot stamping process. The results showed that the tribological response was highly dependent on the retention of the lubricant in the contact and the type of surface modification technique. The results show that bonding of the lubricant to the tool surface is critical. In the case of lubricant failure, severe adhesive wear and aluminium transfer onto the tool surface occurred, correlated with an increase in friction. The use of different surface engineering methods led to different results: lower friction levels could be reached when combining use of lubricant and PVD coatings compared to using uncoated or plasma nitrided tool steel. In this study, the best combination to minimise aluminium transfer and friction is the association of the lubricant with CrWN PVD coating in this study.

Increased awareness of environmental problems has stressed the importance of switching from traditional lubricants to more environmentally friendly alternatives. Different investigations with standard test methods indicate that such a switch is possible without loss of lubricating power. The question arises if the statement above is true for industrial lubricating conditions, without laboratory cleanliness. This paper presents a study of friction and wear in a two grooved journal bearing at different shaft speeds, oil temperatures and contamination levels. A number of tests have been conducted combined with a theoretical analysis of film thickness and lubricating regime. The aim was to investigate whether an environmentally adapted rape seed-synthetic ester oil could replace a traditional mineral oil in a full scale application. The results show that the rape seed-synthetic ester oil gives significantly lower values of wear regardless of the operating conditions and there is also a tendency of lower values of frictional torque compared with the mineral oil. Mainly operating in the mixed lubrication regime, no signs of impending bearing failure have been registered, even though a large amount of silica particles added to the oil gave higher wear values than with uncontaminated oil.

A pin-on-disc machine has been used to make in situ observations of the dynamics of the contact between brake pad materials and a glass disc, while simultaneously monitoring the friction force. The machine, that is normally used to study elastohydrodynamical film formation, has been slightly modified and fitted with a brake pad material sample instead of a rolling ball. The glass disc facilitates direct microscopy and video recording of the dynamic contact situation using a microscope equipped with a CCD-camera. The most obvious features of the topography of a brake pad are the contact plateaus, small flat islands rising typically a few microns above the rest of the surface. These plateaus are based on the more wear resistant constituents of the pad such as fibres and abrasive particles, but also include softer ingredients. The study shows that the softer and more porous areas surrounding the contact plateaus are worn mainly through three-body abrasion. When the disc is sliding against the plateaus, a large number of small particles (wear debris) are transported through the narrow labyrinth between the surfaces, milling down the weaker constituents. The wear debris can also form very small particles or a continous friction film that may become compacted in front of the initial contact plateaus based on, e.g. a metal fibre. In this way larger but softer plateaus form. The investigation elucidates the rapid changes of the contact situation on a microscale.

Detailed understanding of wear processes is required to improve the wear resistance and lifetime of machine components. Atomic force microscopy (AFM) is used to measure surface height profiles with high precision, before and after a wear experiment. The distribution and depth of wear on steel surfaces is then calculated using a relocation method. A numerical investigation of wear based on Archard's equation is conducted on the same measured surfaces. A good correlation was found between the model and experiment for wear larger than a hundred nm. The wear mechanisms considered in the numerical simulation was thus found to be the cause of the majority of the wear. On the scale of tens of nm the correlation was limited, but the measured wear was still analysed in detail.

Traditional bearing materials contain different amounts of lead (Pb) because of its friction reducing properties. However, in view of the negative health and environmental impact of Pb, there is growing emphasis on restricting the usage of Pb in engine bearings. Owing to this, new bearing materials that provide at least comparable tribological performance to that of Pb containing alloys are being developed and some new Pb-free materials are being already used in engine bearing applications. It is, however, still unclear how these new engine bearing materials would perform in mixed and boundary lubricated conditions. In this study, a block-on-ring test setup was employed to investigate the tribological performance of several bimetal and multi-layer Pb-free bearing materials with different compositions of bearing lining and overlay plating. Pb-containing bearing material was also studied as a reference material. Friction and wear properties of these bearing materials were investigated and their wear mechanisms under lubricated conditions have been analysed. Bearing material with Polyamide-Imide based overlay containing graphite and MoS2 exhibited better friction and wear properties than Pb-based and Al-Sn based materials. Pb-containing bearing material shows higher wear of material and Al-Sn based material has shown higher friction compared with the other test materials.

The present study aims at investigating the tribological behaviour of polyphenylene sulfide (PPS) composites in water lubricated sliding contact against a metallic (Inconel) counterpart. Factorial combinations of three commercially available carbon based fillers namely short carbon fibers (SCF), multi-walled carbon nanotubes (MWNT) and graphite (Gr) were utilized as single, dual- and multi-modal reinforcement.Reduction in wear of more than three orders of magnitude and a reduction in friction of more than 60% were achieved for the composite containing short carbon fibers as compared to neat PPS. However, incorporation of MWNTs and/or Gr marginally influenced the friction and wear behaviour of the PPS composites. X-ray photoelectron spectroscopy (XPS) of the worn counter surfaces confirmed the formation of a reaction layer on the Inconel counterpart. Further investigations utilizing Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) revealed the formation of nano-tribo-layers on load bearing plateaus of the Inconel counter surface.It can be concluded that the sliding wear resistance of the short carbon fibers, the enhanced polymer transfer, and the formation of a nano-tribo-layer on the counter surface are mainly responsible for the outstanding tribological behaviour of short carbon fiber reinforced PPS composites in water lubricated sliding conditions.

The effect of ZDDP on fretting wear was investigated in a ball on flat machine. The results confirm previous work that anti-wear agents may reduce friction and wear in fretting contacts. It was further found that temperature, adsorption time, base oil polarity as well as the presence of other surface active additives in the oil were all important parameters affecting the ability of ZDDP to protect the surfaces against fretting wear.

The wear volume and wear distribution of chromium steel cam rollers in a high-torque hydraulic motor has been investigated. The cam roller is a part of a novel silicon nitride/chromium steel journal bearing system. An atomic force microscope (AFM) was used to obtain topographical images of the cam roller surfaces before and after use in a full-scale test of the hydraulic motor. The surfaces were marked with small Vickers indentations to make it possible to reposition the AFM to the same locations. To measure the microscopical wear and produce high resolution maps of the local distribution of wear, a recently developed method was utilised. The method is based on two techniques to treat digital topographical images. To map the distribution of wear, the image of the unworn surface is substracted by the image of the worn surface. To measure the wear volume, the bearing histogram is used to calculate a volume relative to a fixed depth. The calculated volume of the unworn surface is then subtracted by the volume of the worn surface. The ceramic/metal system displayed an extremely low wear rate. corresponding to a typical total mean wear depth of about 30 nm. The surface topography showed very limited changes with the minute wear mainly localised to the uppermost part of the surface ridges. The adopted method thus allowed a unique high resolution mapping and volumetric measurement of the initial stages of wear (1 mg lost out of 600 g) on a real machine element. This high resolution analysis is promising for improving tribological testing of real machine elements with long expected wear lives, by reducing the need for excessively accelerated tests or extremely long and costly test durations.

Recent years have seen a continuously growing interest in high temperature tribological research. A significant part of this is driven by the need for improved understanding and knowledge pertaining to friction and wear and their control in the context of hot forming of high strength steels. Friction and wear characteristics of a sliding system are highly dependent on the properties of the two interacting surfaces. At high temperatures, the surface and material properties become extremely important since these systems often operate under unlubricated conditions. High temperature tribological processes are highly complex as these involve changes in mechanical properties due to microstructural changes; thermal softening; surface chemical and morphological changes due to oxidation and diffusion; deterioration of the surface and bulk material as a result of adhesive/abrasive wear and thermal fatigue. Many of these changes occur on the surfaces and/or in the near surface region. The formation of surface oxide layers and near surface layers with a highly refined microstructure (nano-structured) has been reported to have a significant influence on the tribological behaviour. An improved understanding of these effects is a prerequisite in an attempt towards controlling friction and wear at high temperatures. The main aim of this work is to investigate the formation of oxide layers and near surface transformed layers during tool steel and boron steel interaction at elevated temperatures and their relation to the friction and wear response. The results from sliding wear tests showed that under favourable conditions of temperature and load, a reduction of wear by three orders of magnitude and reduced friction by 50% was obtained. This was attributed to the formation of a composite layer structure involving a refined workhardened layer and a protective oxide layer on top. In the case of three body abrasive wear of boron steel, a reduction in wear rate when temperature increased (100–200 °C) has also been found. This reduction in three-body wear is due to the formation of a workhardened layer with a mechanically mixed layer of wear debris and fragmented silica particles on top. At higher temperatures (>500 °C), the softer matrix due to recrystallisation and phase transformations was unable to maintain a lower wear rate despite the presence of embedded fragmented silica particles.

There has been a growing usage of high strength steels, particularly in automobile applications mainly as structural parts in view of their light weight and high strength properties. These materials are also being considered for dynamic applications. However, the understanding of their tribological behaviour vis-a-vis their hot forming and also as tribological materials is highly inadequate. The present work thus aims at creating new knowledge about the tribological characteristics of high strength steels and bridging this existing gap. High temperature tribological studies on different tool steels (with and without surface treatment) sliding against high strength boron steel (with and without coating) and studies on self-mated hardened high strength boron steel under dry reciprocating sliding conditions have been conducted. High temperature tribological studies keeping in view the hot metal forming aspects were conducted by using an SRV machine whereas a two-disc machine was employed for investigating their fundamental friction and wear behaviour. The results from the high temperature studies indicate that the friction is dependent on temperature since a reduced friction level was observed with increasing temperature. The wear of the tool steels increased with increasing temperature and nitriding of the tool steels provided better protection against severe wear. The results from the study on self-mated hardened high strength boron steel showed that sliding speed has a marginal effect on friction whereas the effect of contact pressure is more pronounced (decreasing friction with increased contact pressure). The specific wear rate decreased with increased sliding speed.

In many industrial applications the occurrence of abrasive wear results in failure and replacement of components. Examples of these applications are found in mining, mineral handling, agriculture, forestry, process and metalworking industry. Some of these applications also involve operation of relatively moving surfaces at elevated temperatures which increases the severity of wear. A typical example of high temperature wear phenomena is that of tool steels during interaction with boron steel in hot forming. Some studies have been carried out regarding the high temperature tribological behaviour of these materials but results pertaining to their high temperature three body abrasive behaviour have not been published in the open literature. In this work, the high-temperature three body abrasive wear behaviour of boron steel and two different prehardened tool steels (Toolox33 and Toolox44) was investigated using a high temperature continuous abrasion machine (HT-CAT) at different temperatures ranging from 20 °C to 800 °C using a load of 45 N and a sliding speed of 1 ms-1. The wear results were correlated to the hot hardness of the different materials measured by means of a hot hardness tester (HHT) at a load of 10 kgf. Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) techniques were used to characterise the worn surfaces. The hot hardness measurements of the three different materials showed a slight but continuous decrease of hardness from room temperature to 600 °C. At temperatures above 600 °C the hardness showed a sharp decrease. The wear rate of Toolox 44 was constant from 20 °C to 400 °C. On the other hand, Toolox33 and boron steel, showed a reduced wear rate from 20 °C to 400 °C attributed to an increased toughness and the formation of wear-protective tribolayers respectively. At higher temperatures (from 400 °C to 800 °C), the wear rate for these materials increased mainly due to a decrease in hardness and the occurrence of recrystallization processes.

In the present work, the wear behaviour of different steels has been investigated under a three body abrasive environment at room and elevated temperatures. High-silicon steel (0.25C-1.42Si) was austempered at 300 and 320 ˚C in order to obtain two carbide-free bainitic steels with different mechanical properties. The same steel subjected to two different quench and temper heat treatments was used as a reference material for mechanical and wear testing. The steels were subjected to three-body abrasive wear by means of a high temperature continuous abrasion tester (HT-CAT). The tests were done at 25, 300 and 500 °C respectively. All samples showed similar wear rates at room temperature. At 500 °C, the material austempered at 320 ˚C showed the highest toughness and the lowest wear rate. High temperature hardness and impact toughness tests showed that abrasive wear is not only influenced by hardness but also by the toughness of the material. Owing to their good strength/toughness combination CFB steels could prove to be an important material for abrasive wear applications

Results from the European 5th Framework project “InfraStar” are presented. The InfraStar project is developing a two-material rail to increase the productivity and reliability of rail infrastructure. The two-material rail aims to be rolling contact fatigue (RCF) resistant and to reduce squeal noise.The InfraStar consortium has built a high level of wheel–rail expertise and understanding regarding RCF initiation and development, level and position of wheel–rail contact forces, and new interface materials and has gained experience in both laboratory and field test operating conditions. Within the project a validated theoretical train–track interaction model and a fatigue design model for two-material rails have been developed.This paper focuses on the field test results of the two-material rail, and how the predictions and expectations from the selection process match with the results observed in track. Results are very promising. The two-material rail prototypes, now over 1 year in track, show no RCF damage where the non-treated rail shows clear RCF damage. The development of the application process and other objectives of the InfraStar project are discussed.

The problem of obtaining smooth gear changing whilst maintaining long life of wet clutches in automatic transmissions demands more knowledge about their behaviour. Experimental and theoretical studies on the engagement of a wet clutch have been carried out. A wet clutch test rig which can apply a drive torque during engagement was developed. The apparatus could also vary the sliding velocity, inertia, force rate, and lubrication. Measured output data included normal force, brake torque, sliding velocity and temperature over time. The input parameters and output characteristics obtained were similar to those in automatic transmissions used in cars. The friction characteristics as well as power and temperature were investigated. A simple model was developed to estimate the engagement performance which gave a good approximation of the performance measured in the tests. The friction is high in the beginning and end of the engagement cycle and lower inbetween. There is a torque peak just before the clutch stops owing to friction characteristics. The maximum developed power occurs at about half of the engagement time, while the maximum temperature is just before clutch stop

This paper describes three cases where image analysis has been used to evaluate transient phenomena in elastohydrodynamic lubrication (EHL). In the first case, a CCD camera and image analysis are used to evaluate the ball trajectory after impact in the "jumping ball apparatus" for subsequent calculation of the limiting shear stress-pressure coefficient. The error of the method is < 1%. In the second case, the motion of the rollers in a roller bearing is traced with a video camera and image analysis. The rolling and sliding motion in the loaded and unloaded parts of the bearing was clearly visible. Although the shaft speed was only 23.6 rpm, the method can easily be used up to 1500 rpm using a conventional high speed video and up to 27 000 rpm with a 6000 pictures per second video camera. In the third case, the method was applied to interferograms of an EHL point contact to investigate lubricant film thickness. The method uses hue, saturation and intensity values from digitized colour interferometric images together with calibration vales. It can be used to evaluate transient as well as quasistatic phenomena in the contact. The method makes it possible to determine absolute values of film thickness without prior knowledge about the fringe order in the interferogram. The method has been shown to work well in the range 140-700 nm with white light but preliminary tests for considerably thicker films are very promising

In hard EHL, encountered in, e.g., gears, rolling element bearings etc. the properties of the lubricant play a significant role in the forming of a lubricating film and reducing friction between the contacting surfaces. The influence of pressure and temperature on viscosity, limiting shear stress and density, has to be taken into account when creating lubricant models to be used in numerical calculations of film thickness and friction. This paper describes some experimental methods to determine these properties of a number of different lubricating oils, both from mineral, vegetable and synthetic origin. The results show that, apart from 5P4E, naphthenic mineral oil has the highest values of pressure-viscosity as well as limiting shear stress-pressure coefficients. It also exhibits the highest density increase with pressure. Rapeseed oil has a rather high pressure-viscosity coefficient but a low limiting shear stress-pressure coefficient as well as a low density increase with pressure. Thus, rapeseed oil is a strong alternative to naphthenic (and also paraffinic) oil since it has the ability to form a relatively thick oil film and at the same time give a low coefficient of friction. Ester, polyglycol and polyalphaolefin oils also exhibit low values of friction but are not equally good as rapeseed oil to form a lubricant film as a result of their lower pressure-viscosity coefficients

The aim of this investigation is to determine how additives in a base oil affect the central oil film thickness in an elastohydrodynamic rolling point contact. The experiments have been carried out using a sapphire disc and a steel ball and the film thickness has been measured by means of optical interferometry. A detailed description of the apparatus is given.Two different additives have been used, polyalkylmethacrylate (PMA) and sulphurized ester (SE). Each of them have been mixed in a superrefined naphthenic base oil at five different concentrations.The results show that the central oil film thickness increases with increasing concentration of additive. This is because the additives increase the oil viscosity. If this effect is compensated for, 0.1 wt.% PMA or 0.63-2.5 wt.% SE give the best relative oil film build-up. There is consequently no reason to use more additive in the base oil unless one wants to have a thicker oil film because of the viscosity-increasing effect

A new method for the experimental evaluation of the shear strength of lubricants at high pressures and temperatures is presented. The main parts of the experimental apparatus are a lubricated sintered-carbide surface and an impacting steel ball. A picture-processing system is used to examine the ball trajectory after impact and to calculate the limiting shear strengthpressure coefficient of the lubricant. Using this apparatus the influence of the chemical composition of the base oil on the limiting shear strengthpressure coefficient has been investigated. It was found that the chemical structure of the oil is of major importance in determining the shear strength. Additives have no significant effect on the shear strength.

Experiments have been conducted into the effect of pressurised hydrocarbons on the viscosity of a polyalkylene glycol. This lubricant is intended to be used in a sub-sea compressor/pump unit and the risk of diluting the lubricant by the surrounding natural gas, thus reducing the lubricant's viscosity, was to be investigated. Dilution can cause insufficient lubrication of the compressor's moving parts. A pressurised Höppler type viscometer was used for the tests which were conducted at 50 °C with a methane based hydrocarbon mixture and with pure nitrogen. The viscosity of the lubricant was reduced to 24% after 24 h exposure at 100 bar with the hydrocarbon mixture. A gas analysis was performed which indicates that the heavier hydrocarbons used are relatively soluble in the lubricant even at relatively low pressures. Nitrogen was found to have a very limited effect on the viscosity even if some was dissolved in the oil. A long duration test of 11 days was also made with nitrogen. This test did not show any significant reduction of the viscosity.

The tribological research pertaining to homogenizers in processing liquid food has received much less attention compared to its potential in enhancing the efficiency and durability of the homogenization process equipment. Homogenization is a process used to disrupt fat globules in dairy products to reduce the formation of creamy layer (separation) and also to enhance the viscosity of certain products. This process takes place in a narrow gap in the homogenizer machine and this region is highly prone to wear. The occurrence of wear during processing not only impairs the homogenization effect but also leads to increased downtime of the machine. The aim of this work is to understand the occurrence of wear and wear mechanisms in the homogenization gap using both experimental and analytical approaches. Two experimental test rigs were used to study the differences in wear during operation, i.e. with and without particles. The trajectories of the particles have been simulated without the influence of cavitation using a CFD-code to investigate whether the particles are the cause of wear. The homogenizer gap has been simulated for a worn geometry to see how the occurrence of wear changes the particles trajectories. The results have shown that the presence of particles accelerates wear and most important parameters are the particle hardness and mass. When cavitation and particles are combined they create a synergistic effect on wear. This can be explained by the fact that cavitation can accelerate particles in random directions through the imploding action of cavities. A change in impact angle and increased velocity increases the amount of wear significantly. CFD-simulations and calculations show that the particles do not fully follow the streamlines and therefore create wear on the gap surfaces.

High water-based hydraulic fluids with slightly higher viscosities than water, for instance 95-5 emulsions and micro-emulsions, do not show any significant deviation from a Newtonian fluid. Adding polymeric viscosity improvers for the purpose of increasing the viscosity will be successful as long as the shear rate is low. However, as the shear rate is increased, higher than about 10**3 s** minus **1, the viscosity will decrease and the advantage of the improver will vanish. The shear rate available was too low to break down the viscosity improvers. This is shown by the fact that the shear stress curve is reversible. Non-Newtonian fluids approach a more Newtonian behavior when the temperature is increased. The viscosity increase with pressure is much lower for water-based fluids than for a mineral oil. The pressure coefficient is about 10 times higher for mineral oils. The pressure coefficient increases if the water content of a water-based fluid is reduced.

When using the inverse hydrodynamic theory to calculate leakage flow and frictional force in an elastomeric seal contact, the pressure distribution is a necessary input. Up till now, assumed or measured pressure distributions usually have been used. One of the authors has earlier presented a semiempirical method for calculating the pressure distribution in an O-ring seal contact. The new calculation method, presented in this paper, is a generalization and an improvement of this earlier work. The method presented is mainly analytical and is based on calculations of the boundary strains in the contact zone and a model of the material behavior, which is based on the pressure dependence of the material compressibility. A computer program has been developed and two test examples are treated. A comparison with experiments is made.

Counterface roughness is known to affect the tribological behavior of carbon fiber reinforced PTFE. However, the effect of roughness in trace moisture environments has not yet been extensively investigated. In this study, the tribological behavior and tribofilm formation were evaluated for a carbon fiber reinforced PTFE composite sliding in a trace moisture environment against 34CrNiMo6 steel counterfaces with different roughness. Tribotests were conducted with a three-pin-on-disc tribometer at a sliding velocity of 2.2 m/s and in a nitrogen environment with moisture content controlled to 11 ppm. Generally, smoother counterfaces gave lower wear, both during running-in and steady-state. Contrarily, the coefficient of friction was only affected by roughness during running-in. Surface analysis from different stages of running-in were done to elucidate the formation of tribofilms and their different characteristics. For the rough countersurface, a loosely adhered transfer film is transitionally formed at the beginning of sliding to enable the formation of a persistent transfer film. Contrarily, for the case of a smooth countersurface, the formation of a persistent transfer film is initiated from the start. Similarly for the rough and smooth countersurface, a micrometer thick tribofilm with excellent low friction properties is observed on the PTFE composite after running-in.

Manufacturers of refrigeration and air-conditioning compressors have had to re-evaluate their knowledge of compressor bearing lubrication following the introduction of hydrofluorocarbon (HFC) refrigerants and polyolester lubricants. The lack of anti wear protection in comparison to traditionally used refrigerants/lubricants makes the lubrication of bearings using these modern systems a much more difficult task than before. This paper presents results from ongoing research activities to develop methods and generate data that can be used to support engineers and chemists designing compressors and lubricants. Viscosity and pressure-viscosity data for three ISO VG 32 polyolester lubricants and also a film forming comparison between two ISO VG 68 polyolester lubricants are presented. This data shows that a high degree of branching has a negative effect on the performance of the lubricant in certain applications. Whilst a branched lubricant offers the benefit of a higher-pressure-viscosity coefficient under normal conditions, the reduction in viscosity and pressure-viscosity coefficient when diluted by refrigerant is far greater than for normal/linear lubricants. This characteristic results in a poorer film forming ability in applications where the lubricant is subjected to refrigerant gas with a low superheat

A comparison has been carried out between one mineral based hydraulic oil and three environmentally adapted hydraulic oils. Two of these are semi-synthetic oils, i.e. mixtures of vegetable base oil and synthetic esters, and the last is based on synthetic esters only. Technical properties such as viscosity, pour point etc., and chemical properties such as phosphorus and sulphur content etc. were documented using standard test methods. The lubricant capability properties were determined through measurements of the capability of each oil to build a film in an elastohydrodynamic contact. The oil film thickness is important to avoid wear and failure and to guarantee separation of surfaces. The results show that at 40°C the environmentally adapted oils give a thicker film than the mineral oil. At 80°C there is no significant difference between the different types of oil in their capability to build a lubricating film. The maximum shear strength was measured and these results are presented with the constant of proportionality γ, from the theoretical model τ approximately equals γp, which is valid at very high pressures. The measurements were carried out at contact pressures in the range 5-7 GPa. The results show that all environmentally adapted oils give a lower γ value than the mineral oil, which is an advantage since a low shear strength results in lower friction in highly loaded contacts. Measurements of toxicity to aquatic organisms (Daphnia magna) show very large differences between the tested oils, and this implies that a change from a mineral oil to an environmentally adapted oil can give important environmental advantages.

The use of hardened high strength steel is found in applications where high wear resistance is required. The wear properties of high strength Boron steel are well known in applications with abrasive wear from stones, ore and other hard material. A unique concept of wear protection of rails is newly presented, a wear resistant cap made of hardened high strength Boron steel.Reducing the wear of rails and wheels and controlling the frictional behavior in the wheel/rail contact are two key issues for railway owners in order to reduce the increasing costs related to higher axle loads, higher speeds, more frequent traffic, etc. Therefore, the aim of this work has been to investigate and compare the tribological properties of Boron steel and UIC 1100 rail steel in contact with Blue Light wheel steel (AAR Class C (69-JDG-8)) under dry and water lubricated conditions in a two-disc tribometer. Advanced analytical instruments including 3D optical surface profiler, micro-hardness indenter, light microscope and SEM/EDS were used to analyze the results.Results from the experiments show that the friction coefficient in tests with Boron steel is more stable both in dry and water lubricated conditions than tests including UIC 1100 rail steel used in todays application. Surface damages seen from water lubricated tests on UIC 1100 rail steel are not seen on the surface of the Boron steel discs. In all tests, the wear decreased when water was added in the contact and friction was slightly decreased.

The properties of silicon nitride ceramics allow their broad application in extreme tribological conditions. High-temperature sliding contact of Si₃N₄-base materials against metals will be found more often in future applications, in which the ceramic's wear resistance becomes clearly necessary.

In this study, the dry sliding behavior of silicon nitride against Inconel 718 is investigated. Wear experiments were carried out at sliding velocities ranging from 1 to 20 m/s. A finite element wear simulation was constructed by relying on experimentally measured wear rates and COF. The simulations enabled quantifying localized temperature and contact stress fields as a function of geometrical changes due to progressive wear.

The experiments showed a transition in wear mechanisms depending on the sliding velocity and frictional power. Cross-sectional analysis of the ceramic samples provided information on the tribochemical processes and the dominant wear mechanisms. Combining analytical and numerical results enabled proposing a schematic wear model. The agreement of this model with common theories of wear is discussed.

Rolling contact fatigue (RCF) and wear, two major deterioration processes, limit the lifetime of rails. These deterioration processes are even more severe on the curves of tracks used by heavy haul trains. Because wear is a material removing process, it can suppress the formation of RCF (also known as surface initiated cracks). In railways, cracks have a higher risk of instigating a catastrophic failure than wear; hence, it is comparatively better to have wear than to have cracks. By controlling the top-of-rail friction, both of these deteriorating processes can be reduced to enhance the lifetime of rails. In order to achieve these possible advantages, the infrastructure manager of the Swedish railway is planning to implement a top-of-rail friction control technology on the iron ore line in northern Sweden wherein RCF is a major problem on the curves. The present study uses a damage index model in a multi-body simulation software and predicts the probability of RCF formation with suppressing effect of wear for different friction control values. The effect of friction control is simulated on curve radii ranging from 200 to 3,000 m and axle loads ranging from 30 to 40 t at a constant train speed of 60 km/h. Findings show that on a very sharp circular curve, radius < 300 m, RCF can be eliminated without friction control due to the high wear rate. On moderate curves, 300 < radius < 1,000 m, a friction coefficient (µ) of, at most, 0.3 with a Kalker's coefficient of, at most, 30% is required to avoid RCF

In this study, the tribological behaviour of different Diamond-like-Carbon (DLC) coatings sliding against titanium alloy (Ti6Al4V) was analysed in a pin-on-disc tribometer at different applied loads to study effectiveness of tool coatings in titanium alloys machining. Three different DLC coatings were deposited on cemented carbide substrate using HiPIMS (DLC-Ar, DLC-Ne) and arc (DLC-Bn) deposition techniques. A detailed analysis of the wear track and titanium countersurfaces were performed following the tribotest to develop understanding about the wear mechanism and associated variation in the friction response. The results indicated that DLC-Ar presents low friction and reduced wear of coating and respective titanium countersurface at lowest load, seemingly due to its inherent tendency to spontaneously form graphitic transfer-layer at the interface. With an increase in the applied load, the tendency to retain tribofilm decreases as shearing ensue quickly exposing the underneath substrate material. The wear performance of DLC-Ne coatings is better than DLC-Ar under highest load and friction behaviour relatively close to DLC-Ar coatings. In comparison, under increased applied loads, DLC-Bn coatings offered better wear resistance and low friction compared with DLC-Ne and DLC-Ar coatings, which would offer improved performance in machining of titanium alloys.

Additive manufacturing of self-lubricating alloys plays a crucial role in the production of complex wear-resistant components and in expanding repair capabilities, especially for intricate wear parts with low tolerances (e.g. with cooling channels). Herein, we report a novel approach providing nickel-based alloy with an excellent tribological performance in dry sliding contacts. Laser-deposited self-lubricating nickel alloys, infused with anti-wear additives of molybdenum disulfide, nickel sulfide, copper sulfide, or bismuth sulfide, were subjected to dry sliding wear tests against an alumina ball counterbody at a temperature range of up to 800 °C. The self-lubricating alloys exhibited a significant decline in friction (43 %) and wear (45 %) at room temperature, 400 °C (friction 40 %, wear rate 55 %), and 600 °C (MoS2-based, friction 58 %, wear rate 75 %). The MoS2-based alloy coating demonstrated excellent performance characteristics up to 800 °C (friction coefficient ⁓0.25, wear rate 11 × 10−6 mm3 N−1 m−1) due to the formation of a ‘glazed’ tribolayer. Wear mapping allowed to identification of a critical condition for self-lubricating alloys where positive transitions in wear mechanisms led to a synergistic lubrication mode involving the formation of tribologically induced new lubricious phases such as silver molybdate or nickel-bismuth intermetallic. This work provides a comparative evaluation of the micromechanisms, surface transitions, and tribochemistry of solid lubricants at a wide temperature range and a variety of applications.

This paper proposes a new procedure to evaluate engine liner wear volume and wear depth particularly for short duration engine tests. The method is based on the fact that honing depth is not removed when small amount of wear occurs. It was considered that 90% point of the bearing area curve is the same before and after wear. Then, the bearing area curve for the used liner was transformed by the depth differences at 90% and the area between the two curves was calculated by numerical procedure. As the diameter of the liner is known, the wear volume was calculated for the various zones selected at different points from TDC and BDC. The methodology involved has been programmed in C++. The wear depth is obtained by the projection on the depth axis of the two bearing area curves, and then finding their difference. This method is considered superior to the usual method of gauging the changes in diameter as such determinations are influenced by the distortions that occur in the liner. The new procedure needs to be applied more extensively to improve confidence in repeatability and reproducibility and to determine the limits of applicability. Controlled laboratory wear tests and further measurements on cylinders from field engines are therefore proposed.

An improved methodology has been developed to characterise running-in and steady-state wear processes. The experimental study was conducted with En 31 steel specimens on reciprocating tester with ball-on-flat geometry under lubricated sliding conditions. The tests were conducted according to a factorial design. The variables selected were the load, temperature and surface roughness. The wear behaviour for a given set of operating conditions has been characterised on the basis of developed methodology. The parametric influence of operating conditions was then analysed on the basis of polynomial relationships. The steady-state wear was found to be significantly influenced by the initial roughness.

Many machine elements are lubricated with oil or grease. The flow of lubricant depends on different parameters such as surface roughness, surface energy, temperature, etc. A known problem with rolling bearings is that lubricants have to be refilled more frequently if a gas stream passes through the bearing. It is therefore important to know the behaviour of the lubricant in order to predict, e.g. bearing fatigue life due to a gas stream. By studying the droplets thrown out into the air after passing an elastohydrodynamic contact, the most important parameters for oil drop formation have been determined. The analysis is based on picture processing and statistical methods. The experiments show that an increase in temperature, viscosity or surface velocity will increase the total drop volume. Decreased contact pressure and pure rolling also increase the total drop volume. The volume flow, passing the Hertzian contact area, varied between 18 and 30% of the air born oil-drop flow and it is reasonable to assume that increased flow of air-borne droplets will decrease the film thickness due to insufficient replenishment.

The meshing of a gear is a typical example where non-stationary conditions exist. Load, velocities and curvature vary along the line of action. In this analysis, pressure and film thickness are computed at several points along the line of action by taking the transient effect into consideration. Isothermal full film lubrication is assumed and the fluid model is a non-Newtonian model which includes the effect of a limiting shear strength.The gear teeth are assumed to be rigid. The load is first assumed to be carried by either two pairs of gear teeth or by only one pair. This transition from two to one pair and vice versa is modelled as a step variation of the load.The results show that the transient effect is most pronounced at the load transitions where the previous pair of teeth goes out of action and where the next pair of teeth comes into action. Due to the squeeze effect, the minimum film thickness increases for a moment when the load is doubled. The opposite happens when the load is halved again, the film thickness decreases for a moment until it stabilises.The friction coefficient is computed at the different contact points and so are also the sub-surface stresses. Results are presented for two different lubricants, a paraffinic mineral oil and an oil of the poly-α-olefin type.

Experimental and theoretical studies of elastohydrodynamically lubricated contacts normally assume static or quasi-static conditions. Non-steady conditions are, however, common. In this paper the case of a ball impacting a flat lubricated surface is investigated theoretically. This case implies transient conditions and the lubricating effect is due to pure squeeze action. Pressure and film thickness distributions are computed during impact and rebound. The results of the analysis show the effects of ball mass, initial impact velocity, lubricant properties and the thickness of the applied lubricant layer on the minimum film thickness. Increased impact velocity increases the minimum value of film thickness achieved during the bounce. The damping capacity of the lubricating film is very high at low impact velocity and small ball mass. In fact, the damping is so high that no rebound occurs if the velocity or the ball mass are smaller than certain critical values. The thickness of the lubricant layer has no influence on the results if it is greater than a certain value. If the pressure-viscosity coefficient is increased, the film becomes thicker

The unfavourable elastohydrodynamic lubrication situation in combined squeeze and sliding motion has been analysed both theoretically and experimentally. In experiments a rotating roller impacted and rebounded on a lubricated surface. It was found that oil film breakdown always occurs at the end of the impact time, when the contact force is low. It has also been found that there exists an upper limit for the sliding velocity. Below this limiting velocity no oil film breakdown occurs. This paper is an initial attempt to explain theoretically why oil film breakdown takes place towards the end of the impact, and why an increasing sliding velocity reduces the capability of the oil film to separate the lubricated surfaces. If the oil film's elastic and damping behaviour are taken into consideration it can be shown that a considerable phase shift between maximum contact force and oil film breakdown will arise. It has been found that the squeeze action dominates the pressure formation in the contact and thus the hydrodynamic effect of sliding motion is moderate. Furthermore, several effects, such as non-Newtonian behaviour, surface roughness, temperature rise, starvation and deformations, which are not included in the theoretical model, may decrease the oil film thickness if the sliding velocity increases

An experimental study of lubricated impacts between a steel ball and a flat glass surface has been performed. The experimental set-up consists of a Ø70 mm ball mounted on a pendulum which impacts onto a lubricated glass disc. The contact region is studied by means of optical interferometry using a monochromatic light source, a microscope and a high-speed video recording equipment. The lubricants are of PAO type and the viscosity ranges from 27 to 2600 mm2 s-1 at the test temperature. The impact velocity is varied between 0.08 and 0.29 m s-1. A dimple occurs at the centre of the contact where the lubricant is trapped. The influence of viscosity and impact velocity on the dimple's depth and diameter is studied.

The dry rolling/sliding wear behaviour of Si alloyed carbide free bainitic steel austempered at different temperatures and sliding distances has been evaluated. 60SiCr7 spring steel samples were austempered in a salt bath maintained at 250, 300 and 350 °C respectively for 1 h. Rolling with 5% sliding wear tests were performed using self mated discs for three different test cycles, namely 6000, 18000 and 30000 cycles. The aim was to study the wear performance of the 60SiCr7 steel with a carbide-free microstructure containing different amounts of retained austenite. An in-depth microstructural characterization has been carried out before and after the wear tests in order to link the wear behaviour to the microstructure of each sample. The wear resistance has been expressed by means of the specific wear calculated from the mass loss after the tests. The worn surfaces were analyzed by scanning electron microscopy and X-ray diffraction. Micro-hardness profiles were also obtained in order to analyze strain-hardening effects beneath the contact surfaces. The results indicate that the material with highest hardness—the one austempered at 250 °C—exhibited the lowest wear rate in every case. It was also observed that the hardness increment and thickness of the hardened layer increases with increasing the austempering temperature and number of test cycles. Finally, the results appear to indicate that the initial roughness of the samples has no major effect in the wear rate of the samples above 2500 cycles. The higher wear performance of the sample austempered at 250 °C has been attributed to its superior mechanical properties provided by its finer microstructure. It has been evidenced that all samples undergo the TRIP phenomenon since, after wear; no retained austenite could be detected by XRD.

Specially designed steels with carbon contents from 0.6 to 1.0 wt.% were isothermally transformed at very low temperatures, between 220 and 270 °C, in order to obtain a nano-structured bainitic microstructure. It is shown that the wear resistance in dry rolling-sliding of these nano-structured steels is significantly superior to that ofbainitic steels transformed at higher temperatures with similar hardness values.In addition to the highly refined microstructure, the transformation under strain to martensite (TRIP effect), contributes to the plasticity of the nano-scaled steels, increasing surface hardness during testing, thus reducing the wear rate.

An uncertainty quantification analysis is performed to further investigate the nature of the “two-stage” wear process of the paper-based friction lining in a wet clutch. In this approach, the results of a computerized wear prediction model are examined through sensitivity analysis and a model validation that utilizes the Monte Carlo (MC) method. Extensive computational results that take into account the uncertainty and variability in the input data are presented to gain insight into the evolution of temperature and wear during the engagement process.

Wet clutches are used in many applications today such as automatic transmissions and limited slip differentials in cars as well as in heavy duty equipment such as wheel loaders. The present study is concerned with the wear and engagement behavior of wet clutches in the latter type of application. A test rig is developed in which the wet clutch engagement is monitored during an arbitrary number of test cycles.This rig has many similarities with the SAE #2 test rig in that they are both inertia type test rigs. However, the test rig presented here has several original parts from heavy duty equipment in production incorporated into it. The data collection includes a continuous measurement of the position of the piston used to apply force on the clutch pack in addition to the separator disc temperatures, hydraulic actuating pressure and torque transfer characteristics. The measurements of the piston position can then be related to the clutch wear during a long test series.

An experimental study has been made of the degree of lubrication, which is defined as the number of interacting asperities, when a rotating spherical body approaches a plane during rotation. The normal velocity was varied between 0.1 and 0.5 m s-1 and the sliding velocity between 0 and 9.2 m s-1. The experiments show that the oil viscosity is the most important lubricant parameter. The degree of lubrication is not affected by either the normal velocity, the pressure viscosity coefficient or the shear strength proportionality constant. An increase in the sliding velocity gives a decrease in lubrication of between 25% and 65% depending on the surface roughness and type of lubricant. The surface roughness is also a most important factor impeding good lubrication. To avoid wear one has to increase the viscosity from 8 to 145 mm2 s-1 if the mean surface roughness Ra is increased from 0.01 to 0.14.

The motion when two parts in a machine come into contact can be a normal, sliding or rolling approach, or a combination of the three. The case of combined normal and sliding motion can be very unfavourable from the point of view of lubrication. Nevertheless, this situation does occur, for example in a gear mesh and in heavily loaded rolling-element bearings.The following factors in the case of lubrication of machine elements during combined normal and sliding motion were studied experimentally: oil viscosity, surface texture, shear strength of oil and maximum pressure. The pressure also involves the parameters normal force, Young's modulus, Poisson's ratio and surface curvature.Based on the experimental results, an equation has been deduced which describes how the above-mentioned factors influence the permissible limiting sliding velocity Vsl without oil film breakdown: Vsl=0.127×10-6(v0.1-1.575)(ψ-13.1-1.707)(3840-pmax)This equation agrees well with results from experiments carried out by other authors, and is valid if combined sliding and impact between the machine elements, resulting in a limited contact time, are present.

It has been found experimentally that increased lubricant viscosity will increase the permissible sliding speed without leading to breakdown of the lubricant film while the normal velocity is held at a constant level. The dependence approaches a square root function. It is also apparent that with a squeeze velocity present, increased sliding velocity will decrease the oil-film thickness. An initial attempt at an explanation is given in terms of a combination of Reynolds' equation and further experimental results.