Traditionally rolling contact fatigue observed in bearing field applications was subsurface initiated. However, despite the improvement of steel properties, some factors such as downsizing in bearing design, extreme loading of the bearings as well as demanding application conditions (start up-stop cycles) have led to an increase on the cases of surface damage related to surface initiated fatigue, that comes basically from surface distress. Possible causes leading to surface initiated fatigue are: material and surface properties, marginal lubrication and lubricant chemical composition. Lubricants are formulated products composed of base oil, and an additive package designed for a specific application. Extreme-pressure (EP) and antiwear (AW) additives are chemically active additives, they react with the steel surfaces in contact to form a protective additivederived layer, thus reducing friction and controlling wear. However, certain EP/AW additives that increase the performance of other machine elements, such as gears, can be detrimental for the bearings running in the same lubrication environment. In order to identify the plausible mechanisms that govern the detrimental effect of EP/AW additives on bearing performance, it is necessary to study closely the interactions occurring in the system form by the base oil, the additives present and the steel surface, as well as the influence of operating conditions. The focus of the present work is to identify the parameters affecting the additive-derived layer formation, as it is directly related to the additive reactivity towards the surface, and the tribological properties of the layer, that will determine the tribological performance. Zinc dialkyldithiophosphate (ZDDP), and two low viscosity model oils with different polarity were selected. The influence of base oil polarity on the additive performance was studied in the nanoscale using Atomic Force Microscopy and the tribological performance was evaluated using a ball-on-disc test rig under mixed rolling-sliding conditions in the boundary lubrication regime. An in-situ interferometry technique was used to monitor the additive derived reaction layer formation, and the chemical composition, morphology and nanomechanical properties were studies using X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Nanoindentation respectively. It was found that base oil polarity determines the transport of additives to the surface thereby controlling the maximum reaction layer thickness, friction and wear, as well as the morphology and nanomechanical properties of the additive-derived reaction layer. However the reaction layer chemical composition is not determined by the base oil polarity. Among the operating conditions, shear was identified as a fundamental parameter on the activation of additives on rubbing steel surfaces and the properties of the derived reaction layer.

Polymer composites research designed for orthopaedic applications are commonly focused on Ultra high molecular weight polyethylene (UHMWPE) reinforced by a variety of different nanoparticles. However, the high melt viscosity of UHMWPE renders conventional melt mixing techniques impossible for composite manufacturing. Either solvents that are often difficult to extract from the finished composite or addition of high density polyethylene is necessary in order to use conventional melt mixing techniques. Therefore, solid state mixing is convenient option for manufacturing of UHMWPE based nanocomposites.The aim of this work is to optimize manufacturing parameters (rotational speed and mixing time) for CNT and ND reinforced UHMWPE prepared by planetary ball milling. Many reports have previously been presented, where UHMWPE has been reinforced by CNTs through ball milling, but typically, only mixing time is presented as the crucial variable in ball milling and the movement of the vials, size of the balls, ball-to-powder mass ratio, mixing media and even rotational speed are often overlooked.During this work, both multi walled carbon nanotubes (MWCNTs) and nanodiamonds (NDs) as reinforcement in UHMWPE have been studied. Beginning with the optimal speed in a planetary ball mill for CNT reinforcement and continuing to time and mixing media for NDs. Scanning electron microscopy (SEM) has been used to study the dispersion of nanoparticles using an extreme high resolution SEM (XHR-SEM). Differential scanning calorimetry (DSC) was used to study the thermal properties of the nanocomposite and X-ray diffraction (XRD) was used to complement the crystallinity measurements obtained by DSC. The water contact angles were measured using the sessile drop method. The results showed changes in morphology on UHMWPE powder due to ball milling, such as flattening, welding of powder and changes in powder particle size. The ball milling procedure also negatively affected the crystallinity of the powder, however the crystallinity of the sintered material did not show this negative trend for all composites. Furthermore, thermal analysis did not show any changes in melting temperatures, which indicates that any thermal effects on the powder due to ball milling is only temporary. SEM analysis also showed that a higher speed and longer mixing times more effectively distribute and break down nanoparticle clusters, but at the expense of flattening of the powder and reduced powder crystallinity. It was also shown that wet mixing with ethanol was more efficient and less detrimental to powder morphology compared to dry mixing. Water contact angles were overall increased for composites compared to UHMWPE.

Metal-to-metal seals are critical components as their failure can lead to leakage of hazardous fluids to the environment or to fatal failure of the systems they operate on. Most systems are subjected to increasingly more demanding conditions and deeper knowledge about how different parameters affect the leakage is necessary to design seals with the desired performance. Fundamental knowledge can be obtained by means of numerical simulations, since it can provide in-situ information which would be extremely difficult, if not impossible, to obtain by means of physical experiments only. Moreover, in the virtual experiments it is possible isolate the effect of variations in a single parameter. However, no model that can serve as a predictive tool and thus has been tested against experimental results has been found in literature. The reason for this is the complexity in accounting for both the multi-scale nature of surface roughness and its intrinsic randomness. This lack have defined the main objective of this work, i.e., to develop a model for the leakage through metal-to-metal seals, which can output quantitative results that can be used for comparison against experimental work. This has been accomplished by including the stochastic nature of the surface topography explicitly in a two-scale method. The model constructed following this approach fulfills the requirement of giving a quantitative prediction of metal-to-metal seals. Moreover, it also provides new insight on the expected variability in leakage introduced by the stochastic nature of the roughness.A secondary objective has been to investigate the seal behaviour during unloading, i.e., when the applied load is gradually released after having caused significant plastic deformation. The reason for assessing this topic is that metal-to-metal seals subjected to a certain load cycle exhibit, at any given load, a significantly larger leakage during the first loading than it does during the subsequent unloading for the same load. The numerical simulations of the seal behavior during unloading also confirmed the smaller leakage during unloading. Moreover, it was observed that a substantial load release was required before a significant leakage increase could be detected and that the leakage remained nearly constant up to that point. This is an important finding that can be used when designing seals in order to account for stress relaxation during service live.

In the automotive industry, press hardening is usually employed to produce safety orstructural components from advanced high–strength steels. This hot forming process, andthermomechanical forming processes in general, is highly dependent on friction betweentool and workpiece as friction affects and controls the deformation of the workpiece.However, friction is also directly associated with wear of the forming tools. Tool wear isa complex system response depending on contact conditions and is a serious issue whenit comes to process economy as it reduces the service life of the tool. Therefore, it isnecessary to enhance the durability of thermomechanical forming tools by studying theinfluence of parameters such as contact pressure, cyclic thermal loading, repetitive mech-anical loading and others on tool wear. Then, computational mechanics can be utilised tonumerically simulate and optimise the thermomechanical forming process by predictingwear of the tools.Dry sliding tests were carried out on a high temperature reciprocating friction andwear tester. The aim was to identify the occurring wear mechanisms and determine thetribological behaviour of prehardened hot work tool steel when sliding against 22MnB5boron steel. A normal load of 31 N, which corresponds to a contact pressure of 10 MPa, asliding speed of 0.2 ms −1 and temperatures ranging from 40◦Cto800◦ C were employed.It was found that the coefficient of friction and the specific wear rate decreased at elevatedtemperature because of the formation of compacted wear debris layers on the interactingsurfaces.Increasing material and energy expenses, rising demands for process flexibility andstability as well as requirements for minimal trial and error have led to a growing interestin numerical simulation of wear phenomena. Finite element simulations of a strip drawingtest were conducted to explore the possibility of predicting tool wear in press hardening.The focus laid on unveiling the contact conditions on the forming tools through numericalsimulation. The influence of high temperature on wear was studied and the results wereimplemented in Archard’s wear model to introduce temperature dependence. Further-more, another wear model used for warm forging was also considered. It was found thatthe extreme contact conditions occurred at tool radii and that the different wear modelsled to similar wear depth profiles on the radii but with different orders of magnitude.Standard high temperature tribometers allow fundamental tribological studies to becarried out in order to investigate the tribological behaviour of the materials in contact.However, the conditions prevalent during the interaction of the hot workpiece and toolsurfaces in thermomechanical forming are not adequately simulated in these tribometers.A novel high temperature tribometer has been employed in order to more closely simulatethe interaction between tool and workpiece at elevated temperatures during thermomech-anical forming. It was found that a higher load led to a lower and more stable coefficient

Moving machine assemblies are increasingly exposed to extreme operating conditions involving high temperatures owing to demands on higher power densities, high performance/efficiency and extreme environments. The changes in surface and near surface properties of contacting surfaces caused by exposure to high temperature and deformation govern the occurrence of friction, wear and material transfer of the tribological system. However, these changes have not been thoroughly investigated. In order to enable development of new products and processes, there is a need for new knowledge pertaining to tribological phenomena occurring at elevated temperatures.One of the most commonly used engineering materials is steel as it offers a good compromise between performance and cost even at high temperatures. For example, prehardened (quenched and tempered) tool steels are commonly used in hot forming dies can also be employed in other technological applications involving elevated temperatures. Although the research pertaining to hot stamping, and high temperature tribology in general, has significantly grown during the last years there are still knowledge gaps that need to be bridged. Adhesion and abrasion have been identified as the most dominant wear mechanisms in high temperature tribological systems but the detailed understanding of the mechanisms is still inadequate.The objective of this work is therefore to obtain a deeper understanding of the tribological phenomena associated with adhesion and abrasion that takes place at high temperatures. Unidirectional sliding wear tests have been conducted in order to investigate the influence of contact pressure and temperature on the wear and friction characteristics of tool steel and boron steel pair. Tribological studies involving boron steel, tool steels and heat-treated high-Si steels in a three body abrasive environment were also carried out with a view to explore the effect of temperature on the wear rate, wear mechanisms and to correlate this with material properties like hot hardness and toughness.The results from the unidirectional sliding tests showed that the frictional behaviour of tool steel and boron steel is load and temperature dependent. In general the friction coefficient decreases as both temperature and load are increased as a result of the formation of oxide layers. At temperatures above 200 °C, the compaction and sintering of these layers led to the formation of a wear protective glaze layer. Consequently, the wear rate for both materials decreased at elevated temperatures. Additionally, a friction and wear mechanisms map was developed for the investigated materials.In the case of abrasive wear tests, the results showed that the main wear mechanism presented for each material varied with temperature. In general, a transition from micro-ploughing to a combination of micro-cutting and micro-ploughing was present. The tool steels and boron steel showed a decrease in wear rate in the range of 100 to 400 °C compared to that at room temperature. This was attributed to the toughness in case of the tool steel and the formation of a protective tribolayers for the boron steel. Above 400 °C the wear rate increased for these three materials mainly due to the recovery and recrystallization processes. The wear rate of the high-Si steels increased with testing temperature. At 500 °C, these steels had the same hardness and the differences in wear were attributed to the changes in the material toughness.

In order to predict and optimize energy efficiency, fuel consumption and service life, friction and wear need to be predetermined with higher accuracy than what is possible today. This prediction and optimization is crucial for the development of sustainable mechanical components and systems with excellent environmental performance.Better and more reliable models for predicting friction, wear and scuffing risk in boundary lubricated contacts will be developed in this project. This includes a model for asperity-asperity collision with components of contact mechanics, thermodynamics and physics.In the boundary lubricated contact, loads are mostly carried by asperities. This makes the real area of contact is so different from the nominal contact area, a small fraction of the nominal contact area supporting the load will cause high contact stress and large deformation. Surfaces of machine components operating under high stress in long period can easily cause damage. Therefore, an elastoplastic analysis of asperity collision was conducted with the Finite Element Method. The contact area and contact stress were studied based on the change of parameters as adhesive friction coefficient, interference and collision velocity. The plastically deformed area and residual stress after collision were also depicted in figures.Friction will generate heat in the sliding contact, and eventually cause a temperature rise. Due to the heat is generated at asperities, heat flux is not continuous and the temperature both increase to a relatively high value and decrease to a small value in very short time. This kind of temperature is often called flahtemperature, and it is important to study because it can affect the viscosity of the lubricant, the formation of tribolayer and in turn it will affect the mechanical properites of the surface. The flash temperature was analyzed based on the previous study of the elastoplastic asperity collision, the times for flash temperature to reach maximum value were given and thermal expansion was also included.The FEM model can conduct a study regardless of the geometry and material properties of the surface asperity, but due to the very fine mesh required at the interface, it is not suitable to carry out an analysis of the rough surface contact. Therefore the Boundary Element Method was adopted to have a thorough study of the rough surface contact. The features of the analysis coudcuted in the FEM model, such as strain hardening and friction, should be replicable in the BEM model. In the end, an Engineering tool for the rough surface contact will be developed.

With today’s striving towards reduction of fuel consumption it is moreimportant than ever to understand how different components in theinternal combustion engine function. There is a need for tools that canbe used to investigate and predict the result of specific design changesmade on the components. In this work, the mechanics and the tribologyof the power cylinder unit and more specifically the operation of thetwin land oil control ring (TLOCR) is investigated. In heavy duty dieselengines (HDDE), TLOCR are typically used. The TLOCR plays a veryimportant role in the engine since it is supposed to distribute the correctamount of oil on the liner to lubricate the other rings. It is importantthat the TLOCR does not leave too much oil on the liner for the twotop rings since it could lead too high oil consumption. In a HDDE thepiston assembly is the largest contributor to frictional losses where thepiston ring pack accounts for the major part of this. The oil control ringhas the largest contribution to frictional losses in the piston ring packtherefore making it very interesting to study from a fuel consumptionperspective. The objective of this thesis is to develop a simulation toolthat can be used to quantify design changes to the TLOCR. Such as thedimensions of the ring itself but also ring tension, running land profileand out of roundness of the cylinder liner.The model developed in this work accounts for the tribological interfaceof the TLOCR against the cylinder liner and piston ring groove aswell as the elastic deformation of the ring and the ring dynamics withinthe piston ring groove. The actual ring cross section is modelled in orderto account for the full three dimensional elastic deformation of the ring.By solving all of these problems as a coupled system it is believed thatthe entire operation of the oil control ring could be understood in a betterway than earlier and open up new optimisation possibilities for theTLOCR. The full ring is modelled in order to account for out of roundcylinder liners. Since the cylinder liner in an engine will always havesome deviation from perfectly round this is important. The model cantherefore be used to investigate the effect on oil distribution by reducedring tension which will affect the frictional losses of the system.

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.