The rapid build-up of variable renewable power sources such as wind and solar are leading to increased instability in the electrical grid. Many methods of controlling this instability have been proposed but existing hydroelectric power plants in many cases already been enlisted to fill the regulating power needs of industry and the population. Filling this regulating power role necessitates that a machine changes load state more often and experiences an increase in starts and stops. Likewise, the push for a less environmentally intrusive society has raised the importance of utilizing equipment with reduced impact.This situation has created a host of opportunities to improve existing power plants and upgrade designs of new power plants to allow for reduced impact, better reliability, and increased efficiency. As one of the most critical and failure prone components of the power plant, the bearings hold great potential for improvements that together can reduce impact while increasing efficiency and reliability.To accomplish these opportunities, this work investigates the potential of new, environmentally adapted, lubricants to improve power plant efficiency. It then continues by developing guidelines for power plant operators when considering changing lubricants. Finally, the potential of polymer faced bearings to improve plant reliability at start up is investigated.A journal bearing test machine was constructed to investigate a number of new synthetic lubricants and polymer bearing materials. These tests found that a significant reduction in power loss could be accomplished without significantly affecting the bearing's minimum film thickness by changing from a traditional mineral oil based lubricant to a high viscosity index lubricant of much lower base viscosity grade.Further experimental work led to the development of practical guidance for power plant operators contemplating a lubricant change. This technique focuses on the importance of maintaining equivalent viscosity in the minimum film thickness region after a lubricant change. Efficiency improvements can then be calculated by comparing the viscosity in the bulk of the bearing to that with the original lubricant.Experimental work with polymer bearing facing materials demonstrated the dramatic reductions in break away friction that these materials can provide. A number of polymer composite materials were investigated for their friction characteristics at the moment of the start of sliding, finding that PTFE based materials were far superior to traditional Babbitt metal. The break away friction of PTFE materials was much lower than that of Babbitt and furthermore, the PTFE materials provided a much more stable friction than Babbitt through large variations in both pressure and oil bath temperature.Finally, experimental work with a full scale polymer faced bearing provided insight into the function of polymer faced bearings as well as valuable lessons in the further development of these bearings and their monitoring systems.

Engine bearings are amongst the most critical components of an internal combustion engine that support and allow smooth rotation of the crankshaft. They are designed to operate under hydrodynamic lubrication condition where the bearing and shaft surface are separated by a thick lubricant film. However, they also occasionally operate under mixed and boundary lubrication conditions particularly during starting, stopping and load, speed and temperature variations. Under these conditions, tribological performance of bearing materials is crucial for the satisfactory performance of theengine. Traditionally, the most extensively used engine bearing materials have been Copper-Lead (Cu-Pb) based linings and Pb based overlays because of the friction reducing properties of Pb. However, due to the adverse health and environmental impact of Pb, there is growing emphasis on restricting the usage of Pb in engine bearings. Owing to this, new Pb-free bearing materials that provide at least comparable or superior tribological performance to that of Pb containing materials are being developed. Someof these materials have already been introduced in engine bearing applications. There are, however, only few research results in the open literature as to how these new engine bearing materials would perform in mixed and boundary lubricated conditions. The objective of this work is to evaluate and understand the tribological performance of selected Pb-free engine bearing materials and compare their performance with that of the traditional Pb-containing material. To understand the damage mechanisms in thetraditional Pb-containing bearings, a full set of main and connecting rod bearings from field test run in Euro V truck engines in long haulage application with European diesel fuel and with slightly longer oil drainage interval were investigated. Furthermore, laboratory tests on Pb-free engine bearings with different compositions of lining and overlay materials were carried out with a block-on-ring test set up in order to evaluate their tribological performance. For this study, aluminum-tin (Al-Sn) based lining withno overlay; Cu-based linings with overlay of polyamide-imide (PAI) containing MoS, 2 and graphite, Al-Sn based overlay and Sn based overlay were studied. Cu-Pb lining with Pb-based overlay was also studied as a reference.Investigations on a full set of main bearings and connecting rod bearings from field test revealed that the major damage mechanisms were 3-body abrasive wear leading to exposure of lining material, flaking of overlay material due to surface fatigue, formation of compound layer composed of Sn, Cu and Ni and cavitation damage.Laboratory tests on Pb-free bearing materials have shown that Al-Sn based lining with no overlay shows higher friction than the other materials at lower rotational speed. For Al-Sn based lining and Pb-based overlay materials, the decrease in friction is relatively sharp as rotational speed increases compared with the PAI based overlay. Test samples with overlay of PAI containing graphite and MoS 2 exhibited better friction and wear properties than Al-Sn based and Pb-based materials. Under steady-state conditions,Pb-containing bearing material shows higher wear and Al-Sn based material hasshown higher friction. In addition, Sn-based and Pb-based overlays have shownsimilar friction behaviour when rotational speed is varied. For relatively longer test durations, samples with Sn overlay exhibited comparable friction and wear with that of Pb-based overlay material.

In industry today there are increasing demands not only on product performance, but also on environmental performance. In striving to develop high performance environmentally adapted products, optimization of product life is a central issue. The success of design optimization relies on an understanding of the degradation process and of the associated degradation mechanisms. A high level of costs is often associated with downtime of machinery caused by service and by replacement of machine components. Knowledge of ageing mechanisms facilitates evaluation of the remaining useful life, thus optimizing performance during the entire service life of components. In this study the ageing process of wet clutches has been investigated. Wet clutches are used in automatic transmissions and limited slip differentials in private vehicles and are designed to transfer torque. A wet clutch consists of a clutch pack submerged in lubricant. Separator and friction discs are alternately positioned in the clutch pack. The separator discs are connected to the input shaft and the friction discs are connected to the output shaft. When the clutch pack is pushed together friction is generated between the friction and separator discs. Torque transfer is thus created in the interface between contacting surfaces and torque transfer characteristics are determined by the interaction between lubricant and contacting surfaces. The investigations performed in this thesis have been designed to increase the understanding of wet clutch ageing and failure. Tests have been performed from full scale to model tests. Full scale testing describes the actual system which means that all significant degradation mechanisms are present. The disadvantage with full scale testing is that it is difficult to separate and isolate degradation mechanisms. In small scale model tests it is possible to isolate the degradation mechanisms and hence also what effects they have. Correlation of results from full scale to small scale can in turn increase the understanding of which degradation mechanisms are important for the system and how they influence the wet clutch system. The wet clutch lubricant and the contacting surfaces provide the friction characteristics of the clutch. Therefore, this work has focused on how lubricant degradation affects friction characteristics and hence wet clutch performance. Results in this study show that friction levels increase as lubricant degradation proceeds. In accelerated wet clutch test rig ageing, results in this study indicate that high temperatures in the interface between contacting surfaces greatly influence wet clutch degradation.

Wear is a consequence of nature which becomes costly if uncontrolled. Basic wear protection is provided by lubrication which will decrease the severity of the contact between asperities. If the conditions of a contact are such that there can be no hydrodynamic lift off by the oil and most of the contact occurs in between such asperities, the protection is provided by chemically reacted layers, sometimes as thin as just a few nanometers.In such cases where wear is governed by the most basic wear mechanisms, analytical models and numerical simulation tools have been developed and used to predict the extent of wear. Few of these models concider the interplay between contact mechanics and wear mechanisms. Wear modelling must keep improving.The goal for this work is to examine the predictive efficiency of current models and initiate construction of reliable models for the chemical growth of wear reducing layers. To achieve this, numerical simulations of contact mechanics are used in Paper A to calculate the wear of contact surfaces and in Paper B as a basis for conditions of chemical growth.The contact mechanics model is based on a solution to Boussinesq’s problem applied to equations for the potential energy by Kalker. The method takes the contact’s surface topographies and substrate material properties as input and outputs elastic and plastic deformation, contact pressure and contact area. The numerical implementation is efficiently evaluated by means of FFT-accelerated techinques. The wear is usually treated as a linear function of contact pressure and in this case the Archard wear equation constitute a feasible approximation. This equation is implemented in the present contact mechanics model to approximately predict the extent of wear, in boundary lubricated contacts, by means of numerical simulations.The chemistry of lubricant additives is discussed. Using chemical theory for adsorption as by Arrhenius, the molecular perspective of antiwear additives is explored. Mechanical properties of tribochemical antiwear layers are taken into account in the developed method. The results in Paper A from wear simulations and comparison with an experiment shows the usefulness of wear equations of geometrical contact mechanics. The chemical model in Paper B for tribofilm growth is applied to rough surfaces allowing comparison of the synergy between contact mechanics and chemistry fordifferent surface contacts. The results show how tribofilms grow on rough or smooth surfaces. The model can be used to compare chemical acitivity for different surface designs.

Wet clutches are machine components using friction to transfer torque and providing interruptible connection between rotating shafts in different automobile applications including automatic transmissions. Like any friction generating machine components, wet clutches are susceptible to continuous wear and degradation during sliding. This regular deterioration process as well as the choice of operating conditions, ultimately change the overall system performance during operation due to resultant change in the system parameters. The first part of this thesis summarizes some of the notable studies on the wet clutch tribological performance and clarifies goals of the investigation.Previously, plenty of experimental studies on wet clutches have been reported but still some effects regarding the water contamination problem and the influence of mechanical design factors are not covered thoroughly. The thesis aims to experimentally analyse these two different aspects for improving wet clutch performance regarding frictional characteristics and reliability. These two investigations are focused on wet clutches in automatic transmission applications. For evaluating friction behaviour during a long clutch engagement period, suitable test equipment is designed where standard paper based friction plates and steel separator plates are tested with commercially available ATF. To investigate a clutch operated in a controlled environment is one of main the research objectives. The vital concern while designing the test rig is to monitor the clutch parameters for achieving the desired operating conditions for individual tests. Instead of using a multiple clutch plate configuration, as in real applications, a single friction and reaction plate arrangement is considered to simplify the analyses.An experimental study on wet clutch frictional behaviour under water contaminated lubrication condition reveals the change in the friction level for a water contaminated lubricant. It was shown that the friction level increased for the addition of water in the system. This is not a desirable clutch frictional behaviour for maintaining frictional stability. The increase of friction for added water was influenced by the water amount but not by the water exposure time. The test results also showed a higher change in the separator plate's roughness parameter (Ra) for water contaminated systems compared to an uncontaminated wet clutch. In the second part of this thesis, the influence of the clutch’s output shaft’s stiffness and inertia on the clutch system‘s friction reliability is experimentally evaluated. Test results show that the choice of these design factors can provide different outcome concerning clutch frictional performance and shudder sensitivity. Shudder tendency is seen to be increased for decreased torsion shaft stiffness. High frictional losses and clutch degradation are observed for systems with less inertia.

The aim of this thesis is to advance the field of solid lubrication science by developing coatings that provide reliable performance in ambient conditions, work on rough surfaces, and are amenable to industrial size and design complexities. Two different coating systems, Ti3C2Tx-MoS2 and Ti3C2Tx-Graphene Oxide blends, were studied in this work. The Ti3C2Tx-MoS2 nanocomposites were spray-coated onto rough 52100-grade steel surfaces, and their tribological performance was evaluated in a ball-on-disk configuration in a unidirectional sliding mode. The test results indicate that Ti3C2Tx-MoS2 coatings achieved superlubricity, which has not been previously reported for either pristine material under macroscale sliding conditions. The observed synergistic mechanism enabled the superlative performance, which was explained by the in-situ formation of a robust tribolayer responsible for sustained lubricity even at high contact pressures (>1.1 GPa) and sliding speeds (0.1 m/s). Processing, structure, and property correlation studies were conducted to understand the underlying phenomena. Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to reveal the formation of the tribolayer. The Ti3C2Tx-Graphene Oxide blends were also spray-coated onto rough-bearing steel surfaces, and their tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental setup. The coatings led to substantial friction reduction compared to uncoated and single-component-coated surfaces, with a friction coefficient as low as 0.065 at 1 GPa contact pressure and 100 mm/s sliding speed, surpassing the state-of-the-art. The coatings also provided excellent protection against wear loss of the substrate and counter-face. The results were explained based on the observations from Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. The in-operando formation of a dense, hard, and stiff tribolayer was observed, which was responsible for the sustained lubricity even at high test loads and sliding speeds. This thesis presents a holistic exploration and correlation of structure-property-processing for the advancement of solid lubrication science. It provides insights into the development of solid lubricant materials and their tribological performance, which can be useful for various industrial applications.

A radial piston hydraulic motor has several components involving sliding or rolling interfaces which may encounter tribological problems. Seizure is one of them and it results in complete stoppage of motion between the relatively moving components and has serious consequences in terms of the operation of a hydraulic motor. Extreme running conditions, like low viscosity and high pressure and speed can cause the rupture of lubricating films and this may lead to the seizure of hydraulic motors. The occurrence of seizure often leads to undesirable damage to the motor components and unscheduled maintenance and can prove to be highly expensive. The work reported in this thesis has focused on investigating the mechanisms of seizure and developing suitable tribo test methods for simulating the sliding interfaces of the hydraulic motor. Extensive motor tests have been performed at low viscosity, high pressure and high speed. The results of this work enabled in describing the seizure progression, namely break down of lubricant film, particle entrapment and thermal expansion. Further, tests have been carried out in a rotary tribometer by using a thrust washer conformal test configuration. In this, the influence of area ratio has been studied and this configuration appears to simulate the piston-cylinder contact effectively during boundary lubricated conditions.

Elasto-hydrodynamic lubricated (EHL) contacts can be found in various machine elements or systems, like rolling element bearings, cam followers or gear transmissions. The service life of these elements and systems are depending to some extent on the performance of EHL contacts. Today most tribological contacts are lubricated with the same type of lubricant throughout the entire service life. However operating conditions can change over the components service life and the contacts will therefore require different lubricant properties. In order to expend the service life of the component, the lubrication of the tribological contacts has to be optimized based on the current operating conditions. A future vision is to develop machine elements which can adopt to the actual operating condition, so called triboactive systems. A first step of necessary research in order to develop such systems is presented in this work.In order to enable operation dependent lubrication the mechanism of monitoring techniques and their interaction with different operating conditions have to be investigated. In this work the effect of surface topography, slide to roll ratio and additives on the running-in and the monitoring by contact impedance were studied. Characteristic dependences between the surface parameter Rq and the contact capacitance and between the surface parameter Rz and the contact resistance were found. Further tests with iron oxide (FE3O4) contaminated bearings, monitored by vibration and acoustic emission were carried out. Premature failure due to iron oxide contamination is the most common problem for rolling element bearings operating in mining environment. Thereby the effect of iron oxide contamination on the vibration and acoustic emission monitoring for two different types of greases were investigated. It was found that a simple RMS analysis of the vibration and acoustic emission signals enables the detection of improvements of contaminated contacts by lubrication. Both vibration and acoustic emission from the investigated bearings were reduced by adding extreme pressure additives (EP) to the contaminated contacts.Monitoring of the lubrication condition is necessary to generate information about the current performance of the tribological contact. However, in order to improve the performance of tribological contacts by changes of the lubricant or additives, the effect of such additives on the lubrication condition and the performance of the tribological contact need to be studied more in detail. The presented running-in tests in this work showed that EP additives are only favourable in the very first stage of running-in. The advantage of EP additives for running-in increases with increased surface roughness and increased slide to roll ratio.Another advantage of EP additives was observed during the tests with iron oxide contaminants. The use of EP additives reduced the acoustic emissions of the tribological contact by 70% and reduced the increase of surface roughness of the raceways by as much as 60%. Furthermore the tests indicate a lower wear rate for contaminated EHL contacts lubricated with greases containing EP additives in comparison to plain grease without EP additives, in case of iron oxide contaminated EHL contacts.

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The effect of greenhouse gas emissions contributing to the global warming is today becoming an increasingly important problem worldwide and has led to increased efforts being made on improving tribological performance of interacting surfaces in mechanical systems. Due to increasingly stringent CO2regulations, a reduced fuel consumption has become a key area of interest for the automotive industry where low cost, low emission solutions are continuously developed and where low friction alternatives to machine elements currently in use are evaluated. Crankshaft roller bearings have been shown to reduce the mechanical friction in internal combustion engines compared to the plain (sliding) bearings used today, further leading to a reduced fuel consumption and thereby reduced CO2emissions. However, the transition from plain (sliding) bearings into crankshaft roller bearings means new challenges with e.g. increased noise, vibration and harshness (NVH) levels and reduced durability of the bearings. Therefore, in order to optimise the crankshaft roller bearings that operate under the highly transient conditions in the engine, an increased understanding of the tribological system is required. Research related to elastohydrodynamic lubrication (EHL) has led to the possibility to improve friction performance and durability of machine elements where lubricated non-conformal contact geometries interact. Traditionally, simplifications of the contacting geometries and the assumption of steady-state conditions have often been applied to the EHL analysis. The purpose of this work has thus been to develop a simulation model based on previous work done in the field and further utilise the model to simulate the contact on a detailed level, incorporating transient effects and the influence of oil behaviour using state-of-the-art modelling. The influence of the piezoviscous response and the compressibility-pressure behaviour of the lubricant on the sub-surface stress field were studied, showing that stiff lubricants may lead to increased stress concentrations in the vicinity of the surface, which may further influence the durability of the bearing. It was also seen that highly transient loading conditions applied to the contact initiate oscillations in the lubricated system, affecting pressure, film thickness and sub-surface stresses over time. These findings further elucidate the importance of including non-steady behaviour while analysing highly transient lubricating conditions of EHL contacts. By considering and optimising the aforementioned effects during design of crankshaft roller bearings, an improved NVH performance and an increased durability of the crankshaft roller bearing may be achieved.