Joint replacements have considerably improved the quality of life of patients with joints damaged by disease or trauma. However, problems associated with wear particles generated due to the relative motion between the components of the bearing are still present and can lead to the eventual failure of the implant. Ultra high molecular weight polyethylene (UHMWPE) has been extensively used as a bearing surface in total joint replacements. Although in the short- to medium term UHMWPE provides excellent clinical performance, in the longer term, problems associated with its high wear characteristics and biological responses to polyethylene wear particles leads to the failure of the implants.The first part of the thesis focuses on the current status of total joint replacements (hard-on-soft and hard-on-hard bearings), with particular attention on implant wear debris and the biological response to wear debris, as well as on the tribological behaviour of the potential materials currently under investigation. The aim of the second part of the thesis consists of an analysis of the wear rate and the size and volume distributions, morphology and biocompatibility of the wear debris generated from a multiwalled carbon nanotube (MWCNT) reinforced polyethylene material compared with conventional UHMWPE. The results showed that MWCNT’s can improve the characteristics of UHMWPE, in terms of both wear rate and biocompatibility. UHMWPE-MWCNT composite material was shown to generate low wear rates and a reduced osteolytic and cytotoxic potential compared to conventional virgin polyethylene of the same grade.The final part of the thesis focuses on the possibilities of graphene oxide (GO) as reinforcement of UHMWPE. The aim of this work is to investigate the manufacturing procedure to prepare a homogeneous UHMWPE/GO composite under optimised conditions that might improve the performance of UHMWPE in artificial joints. In this study, composites prepared under different mixing conditions were thermally and morphologically characterised and compared with conventional UHMWPE. The results showed that, under optimized manufacturing conditions, GO has the ability to improve the performance of conventional UHMWPE. This thesis has provided an insight into the potential of carbon based composites as an alternative to conventional UHMWPE for use in total joint replacements and further work concerning the influence of graphene oxide on the tribological performance of UHMWPE/GO composites is currently under investigation.

Lubricating grease is commonly applied to lubricate e.g. rolling bearings, seals and gears. Grease has some clear advantages over lubricating oil. It is a semi-solid material, which prevents it from flowing/leaking out from the lubricated system and gives it sealing properties, protecting the system against contaminants. Unlike oil, grease has a much more complicated rheology, which makes it more difficult to model and understand grease flow. Grease acts as a lubricant reservoir, and understanding grease flow is essential in order to model and predict how grease is transported within e.g., a rolling element bearing housing, a sealing arrangement or replenishment of a gear mesh. Three greases with different rheological behaviors (NLGI 2 grease, NLGI1 grease and NLGI00 grease) have been used in two kinds of test rigs: a straight channel with different restrictions and a rotating shaft with two narrow gap sealing-like restrictions.In the first test rig two types of flow restrictions were applied into a straight channel in order to simulate flow of grease near a sealing pocket. In the case of a single restriction, the distance required for the velocity profile to fully develop when going from a wide to a narrow gap is approximately the same as the initial height of the channel. In the corner pocket before and after the restriction, the velocity is very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving since the yield stress of the grease is not exceeded. This condition particularly applies to the cases with a low-pressure gradient and where high consistency grease is used. In practice this means that grease is not replaced in such “pockets” and that some aged/contaminated grease will remain there. A test rig comprising of a rotating shaft with two narrow gap sealing-like restrictions (a so called Double Restriction Seal, DRS) was designed to simulate the a labyrinth type of seal. Two different gap heights in the DRS have been designed to compare grease flow. It is shown that partially yielded grease is detected in the large gap geometry and fully yielded grease in the small gap geometry. Grease shear thinning behavior and wall slip effects have been detected and discussed. For the small gap geometry, it is shown that three distinct grease flow regions are present: a slip layer close to the stationary wall, a bulk flow layer, and a slip layer near the rotating shaft.

Reducing losses in transmissions has become a high priority in the automotive market during recent years, mainly due to environmental concerns leading to regulations placed on the automotive industry to drive the development of vehicles with lower fuel consumption and CO2 emissions. Rising fuel prices and increasing environmental concerns have also made customers more prone to purchase more fuel efficient vehicles. In addition to the fuel savings that could be achieved by increased efficiency of transmissions there are other benefits as well. A more efficient transmission will in general generate less heat, and experience less wear. This will lead to fewer failures, longer service life of components, and possibly longer service intervals. Furthermore this implies a possibility to reduce coolant components, thus reducing the total weight of the system, leading to a further decrease in consumption and a lower impact on the environment due to a reduction of material usage. A low weight design is also beneficial for vehicle dynamics and handling. In addition to the automotive market, gears are extensively used in many other fields, such as wind power and industry. In some cases a substantial part of the losses in a gear transmission is attributed to gear contact friction due to sliding and rolling between the gear teeth. To better understand the contact friction phenomena in gears an experimental apparatus capable of running under similar conditions to gears is chosen. By using a ball on disc test device the contact friction can be measured in a broad range of speeds and slide to roll ratios (SRR). The results are presented as a 3D friction map which can be divided into four different regions; Linear, Non-linear, mixed and thermal. In each of these regions different mechanisms are influencing the coefficient of friction. Several tests have been conducted with different lubricants, EP- additive packages, operating temperatures, surface roughness and coatings. The method gives a good overview, a system fingerprint, of the frictional behaviour for a specific system in a broad operating range. By observing results for different systems, it is possible to identify how different changes will influence the coefficient of friction in different regimes, and therefore optimize the system depending on operating conditions. Among other things the tests have shown that reducing base oil viscosity increases contact friction in most operating conditions, introducing an earlier transition from full film to mixed lubrication, and increasing full film friction in many cases with high sliding speeds. An increase in operating temperature could both increase, and decrease the coefficient of friction depending on running conditions. Introducing smoother surfaces reduces the coefficient of friction at lower entrainment speeds since thinner lubricant films are required to avoid asperity collitions. By applying a DLC coating on one or both surfaces in a EHL contact, the friction coefficient is shown to decrease, even in the full film regime.

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.