Human civilization is largely based on the transformation of stored chemical energy in the form of fossil fuels into usable work through the use of the combustion engine. In recent years, the awareness of problems such as global warming due to emissions of greenhouse gases and the depletion of fossil fuel reserves has increased. New regulations have been put into place in order to limit these effects and as a result, the efforts made by the automotive industry to limit the emissions and fuel consumption of their products has taken a higher priority. Attempting to minimize power losses is one approach taken, where the knowledge of Tribology is often made use of to lower the fuel consumption. In heavy duty equipment such as wheel loaders and articulated haulers, a large portion of the mechanical power is lost in the transmission. One of the largest contributors to the transmission power losses is the viscous drag in disengaged wet clutches. While the method of reducing these losses are quite clear (e.g. a reduction in clutch size will yield lower losses), the effects of the subsequent increase in power and energy density in clutch engagements are less clear.In this thesis, the effects of increasing the load, i.e. increasing energy input density and power absorption density, on the clutch are investigated. The main concern with increased load on the clutch is the durability of the clutch. In particular, wear of the paper-based friction material lining of the friction discs has been evaluated, as well as the influence of friction material wear on the clutch performance. A new type of test rig has been developed to quantitatively measure the clutch wear throughout a durability test. The wear measured continuously in the test rig correlates well with verification measurements performed while the engagement behavior could be clearly distinguished through the measured torque transfer.At high energy densities and high power levels, a wear phenomenon was observed where the wear rate suddenly changed. During and after the change there was no significant change in engagement characteristics. However, if a clutch which experiences two different wear rates are to be used in a transmission, it is necessary to accurately model the wear so that a compensation can be implemented in the clutch control system. The model was derived in the form of a wear equation where the clutch pack thickness is described in terms of the number of clutch engagements, employing two different wear constants. Total clutch failure due to wear occurs once most of the friction lining has been removed from the friction discs. Up to that point, the clutch is still able to deliver the required torque transfer granted that the increased distance between discs can be compensated for by smart control of the clutch. Heavy duty wet clutches need to be designed in such a way that maximum power and energy density is achieved without reaching the point where the friction lining is totally removed within the required clutch life. The use of the proposed wear model can aid in the design of new wet clutch systems to minimize the clutch's size without compromising the length of its service life.

An important issue in hydropower and other industries is the increasing demand for introduction of environment friendly solutions. Mineral oil based lubricants have long been used in various sliding bearings in hydropower stations. Their use in aqueous environments however raises concerns about the environmental impact if they leak into downstream water. This has necessitated research in replacing mineral oils with more bio-degradable lubricants and the ultimate goal of ‘oil-free’ hydropower generating machines. Replacing oil with water however poses many challenges. Due to considerably lower viscosity of water compared to that of turbine oils, the water lubricated bearings are likely to operate in boundary/mixed lubrication regime for relatively longer period. Therefore choice of the materials and their tribological performance are very important for the bearings operating in the boundary/mixed lubrication regimes. Application of compliant polymers in water lubricated bearings introduces many advantages which cannot be achieved with coatings (DLC, etc.) or ceramics. Most previous tribological studies on polymers have been carried out in dry conditions and only a few studies in presence of water have been reported. This work is thus aimed at investigating the tribological behavior of some selected polymer materials in water lubricated conditions. The results of these studies provide an insight into polymers’ tribological performance in boundary/mixed and hydrodynamic lubrication regime and associated wear mechanisms in presence of water. Polymers also enhance performance of oil lubricated bearings. Application of polymers in the oil lubricated bearings provides a smooth transition from oil lubricated Babbitt bearings to water lubricated polymer bearings. Therefore a part of this thesis is also aimed at investigating the tribological characteristics of several polytetrafluoroethylene based materials at the onset of sliding (break-away friction) at different pressures and temperatures. The results of this study show significantly lower breakaway friction of PTFE materials compared to Babbitt at all pressures and temperatures. SEM investigations revealed wear modes of the PTFE materials and the abrasive nature of hard fillers. Bronze-filled, carbon-filled and pure PTFE were found to provide lower and more stable break-away friction and generally superior properties compared to the other materials.

Metals have excellent properties, such as high strength, ductility and toughness, which make them the material of choice for many biomedical applications. However, the main drawback of metals is their general tendency to corrode, which is an important factor when they are used as biomaterials due to the corrosive nature of the human body.Titanium and titanium alloys are widely used in biomedical devices due to their excellent corrosion resistance and good biocompatibility. However, one of the disadvantages of titanium is its low wear resistance. Hafnium is a passive metal with a number of interesting properties, such as high ductility and strength, as well as resistance to corrosion and mechanical damage. Previous studies have shown that hafnium has good biocompatibility and osteogenesis. However, the behaviour of hafnium in biological environment has not been studied in great depth. Furthermore, little is known about the resistance of the passive layer under wear-corrosion conditions and the effect of proteins on its corrosion and tribocorrosion behaviour. The overall goal of this study is to assess the potential of hafnium for use in biomedical applications. The aim of this work is to investigate the corrosion resistance of hafnium in simulated body fluids as well as its behaviour in wear corrosion and fretting corrosion conditions.The results showed that hafnium presents a passive state in the presence of proteins and its oxide layer provides high protection to corrosion. In addition, although the passive layer could be disrupted due to wear and fretting, increasing the corrosion of the metal, it was rapidly rebuilt when the damaging ceased. On the other hand, the main drawback of hafnium was its tendency to suffer from localised corrosion. Although the formation of corrosion pits was retarded in the presence of proteins, it was drastically increased when hafnium was scratched or subjected to fretting.

Modelling and simulation of friction is a research issue that still requires an extensive amount of input from the scientific community. In a lubricated system, the dissipation of energy is connected to the direct contact between the surfaces, or more precisely the tribofilms, as well as of the shearing of the lubricant film. Elastohydrodynamic lubrication (EHL) is a lubrication regime which is characteristic for contacts found in machine components such as in roller bearings, gears and cam mechanisms. These contacts have in common that they carry load on a very small/concentrated area and exhibit elastic deformations that are much greater than the thickness of the hydrodynamically formed film. There are a vast number of parameters that affect the friction in EHL contacts and it is a challenge to include other than the most basic ones in the model. The most advanced and sophisticated models are very complex with millions of degrees of freedom and are, therefore, not yet feasible to conduct parametric studies with. The extreme conditions associated with EHL, i.e., nm thin films, with phase transition from liquid to solid, GPa pressure, temperature increase with considerable implications on lubricant flow and surface chemistry, etc., makes it even more difficult to model these systems. The shape of the contacting parts further complicates modeling of EHL. More precisely, an EHL contact can geometrically be either of a line, circular, elliptic or truncated contact type. Since the line contact appears between two cylindrical shaped bodies of infinite length, it permits a 2D-model for the flow and there are analytical solutions, in the most elementary cases. The circular and the elliptic contacts more are complicated. The case when the surfaces are fully separated by the lubricant film has, however, been addressed by many researchers, who also have presented numerical predictions validated by experimental data. The finite line contact appears to be the most challenging type, but it is also the only physically reasonable model for EHL contacts where the edge effects cannot be neglected. In this work, both steady state and time dependent, fully deterministic models are utilized and further developed to enable the study edge effects under variable operating conditions in cam and roller follower systems. The numerical investigations were specified so that generic knowledge about friction in these systems would be generated and also to provide validation data for the development of a semi-analytical, low degree freedom model, for rapid estimation of friction. The main objective was to design such a low degree of freedom model so that it can be employed in a multibody dynamic model, requiring friction estimation in milliseconds. The semi-analytical low degree of freedom model developed in this work, takes thermal effects into account and is built on an advanced and well-characterized rheological model, including lubricant shear thinning, in order to estimate the viscosity and volume of the lubricant. The model was utilized to perform friction prediction covering a range of operating conditions, which were also run in an experimental investigation using a ball-on-disk test device. The results turned out to compare well, suggesting that it constitutes a suitable foundation for further developments.

Tribology is the science of friction, lubrication and wear. A tribological interface is composed of two contacting surfaces in relative motion. The radial piston hydraulic motor includes a lot of tribological interfaces. The interfaces are made up of the contacts between the moving parts in the motor that transform energy in the form of hydraulic flow into motion of the output shaft. Many of them are highly loaded due to the large forces that must be transmitted to produce the required torque. The performance in view of efficiency of the hydraulic motor depends strongly on the tribological properties in these interfaces. In industry today, there is a general strive towards better efficiency to minimize impact on the environment by lowering energy consumption. This puts a lot of focus on tribology, especially in applications like the hydraulic motor whose purpose is to convert energy into some desired motion. Indeed, minimizing the losses/friction in the tribological interfaces leads to less energy needed to produce the desired motion. Another aspect of tribology is wear in the interfaces which can affect durability or toughness of the motor. At present, design and optimization of tribological interfaces is mostly done by trial and error experimental work and testing. This will always be needed but if more of the testing can be done using computer based computational models a more cost effective and faster design process could be achieved. A computational model would also lead to better understanding of tribological processes present in the simulated application due to the possibility to investigate properties that are very hard to measure. The work in this thesis involves development of computer based computational models for simulation of the tribology in hydraulic motors. The model is validated through comparison with experiments. Furthermore it is shown that modelling can be used to analyze tribology in the hydraulic motor and consequently can be used as a design tool for improving efficiency.

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.