This thesis is based on two different projects, one more focused on applied research and one on more basic research. The first project examines the po- tential of nitriding as an alternative to case hardening in tribologically loaded components while the second project involves micro scale studies of the relations between roughness, transfer and friction between metals. The first project consists of an evaluation of the tribological properties of nitrided steels. The aim is to increase the understanding of the wear and fric- tion behavior of different nitrided steels in relation to the choice of steel grade, microstructure, thickness and composition of the compound layer, among other parameters. This study is a part of a bigger project called Surf- Nit, which primary objective is to optimize the nitriding process in order to increase the use of nitrided steels for applications like high stress compo- nents. Today case hardening is the standard heat treatment for these applica- tions but nitriding is both more environmentally friendly and less time con- suming. In the present study, the steel grade with the highest content of ni- tride-forming elements and highest hardness showed the best wear re- sistance, regardless of the composition of the compound layer. Further, steels of a given grade but with different phase compositions of the compound layer showed differences in their wear behaviour. It was also shown that nitrided steels with ε-phase in the compound layer acted more brittle than those containing Υ’. The goal of the more basic project is to increase the understanding of the mechanism behind sliding friction. The main focus has been the relation between friction and material transfer. A better understanding can be of help when developing new tribological materials, for example wear resistant components that can operate without lubrication. It could also enable specif- ic recommendations for surface finishes to avoid material transfer and be of help when trying to make more realistic tribological models. Scratch tests have been performed on samples with different surface roughness and dif- ferent surface composition. It was shown that nano scale topography had a bigger impact on both material transfer and friction compared to micro scale topography. Experiments both in air and in situ in a SEM have been per- formed to determine the effect of presence of air on the friction and material transfer. The experiments in air resulted in more material transfer and higher friction than those performed in vacuum. Both lubricated and unlubricated contacts have been studied in order to see how surfaces otherwise separated by boundary lubrication will be affected if the lubrication fails.

The wear of a machine, whether it is due to fatigue or abrasive wear, will add contaminants, in the form of particulates, to the system in question. Since a total breakdown of the machine can be rather costly, one wants to be able to foresee breakdowns and increase the machine life. Follow-up checks of machines are often performed to enable one to detect an increase in wear, and thereby replace the machine or remove it for service. This licentiate thesis mainly deals with the problems associated with contamination control and sample withdrawal. A survey of where and how to take a representative sample is performed using Stokes' law and the migration of spheres in a channel. Some different techniques to measure the contamination are also presented, together with their advantages and disadvantages. Sampling routines for proper sample withdrawal are included. The thesis also includes some field aspects concerning the influence of particles and the wear of grease-lubricated rolling element bearings. / Godkänd; 2001; 20070316 (ysko)

In the field of tribology, there are numerous theoretical models that may be described mathematically in the form of integro-differential systems of equations. Some of these systems of equations are sufficiently well posed to allow for numerical solutions to be carried out resulting in accurate predictions. This work has focused on the contact between rough surfaces with or without a separating lubricant film. The objective was to investigate how surface topography influences contact conditions. For this purpose two different numerical methods were developed and used. For the lubricated contact between rough surfaces the Reynolds equation were used as a basis. This equation is derived under the assumptions of thin fluid film and creeping flow. In highly loaded, lubricated, non- conformal contacts of surfaces after running-in, the load concentration no longer results in plastic deformations, however large elastic deformations will be apparent. It is the interaction between the hydrodynamic action of the lubricant and the elastic deformations of the surfaces that, in certain applications, enable the lubricant film to fully separate the surfaces. This is commonly referred to as full film elastohydrodynamic (EHD) lubrication. Typical machine elements that operates in the full film EHD lubrication (FL) regime include rolling element bearings, cams and gears. Unfortunately, a cost effective way of machining engineering surfaces seldom results in a surface topography that influence contact conditions in the same way as a surface after running-in. Such topographies may prevent the lubricant from fully separating the surfaces because of deteriorated hydrodynamic action. In this case the applied load is carried in part by the lubricant and in part by surface asperities and/or surface active lubricant additives. This could also be the case in lubricant starved contacts, which is a common situation in not only grease lubricated contacts but also in many liquid lubricated contacts, such as high speed operating rolling element bearings. The load sharing between the highly compressed lubricant and the surface and/or surface active lubricant additives is the reason why this lubrication regime is most commonly referred to as mixed EHD lubrication (ML). Machine elements that while running operate in the FL regime may experience a transition into the ML regime at stops or due to altered operating conditions. It is not possible to simulate direct contact between the surfaces using a numerical method based on Reynolds equation. A parameter study, of elementary surface features passing each other inside the EHD lubricated conjunction, was performed. The results obtained, even though no direct contact could be simulated, does indicate that a transition from the FL to the ML regime would occur for certain combinations of the varied parameters. At start-ups, the contact in a rolling element bearing could be both starved and drained from lubricant. In this case the hydrodynamic action becomes negligible in terms of load carrying capacity. The load is carried exclusively by surface asperities and/or surface active lubricant additives. This regime is referred to as boundary lubrication (BL). Operation conditions could also make both FL and ML impossible to achieve, for example, in the case in a low rpm operating rolling element bearing. The BL regime is in this work modeled as the unlubricated frictionless contact between rough surfaces, i.e., a dry contact approach. A variational principle was used in which the real area of contact and contact pressure distribution are those which minimize the total complementary energy. A linear elastic-perfectly plastic deformation model in which energy dissipation due to plastic deformation is accounted for was used. The dry contact method was applied to the contact between four different profiles and a plane. The variation in the real area of contact, the plasticity index and some surface roughness parameters due to applied load were investigated. The surface roughness parameters of the profiles differed significantly. / Godkänd; 2004; 20070128 (ysko)

There are many tribological interfaces that are exposed to elevated temperatures. Typical examples are the interfaces of various moving assemblies, for examples in aerospace industry, power generation and metalworking processes. The exposure of materials to elevated temperatures results in highly complex interfaces due to changes in morphology, microstructure and mechanical properties coupled with the occurrence of oxidation and diffusion. All of these changes will influence the tribological behaviour of materials at elevated temperatures. Another major concern is lubrication at elevated temperatures since conventional lubricants do not perform at temperatures above ~300ºC. High strength steels are commonly used as structural reinforcements or energy absorbing systems in automobile applications due to their favourable strength to weight ratios. The high strength of these steels leads to several problems during forming such as poor formability, increased spring back, and tendency to work-harden. In view of these difficulties, high strength steels are usually formed at elevated temperatures with a view to facilitating forming and simultaneous hardening by quenching of complex shaped parts. A review of published literature has revealed that only a few studies pertaining to high temperature tribology (including those of hot metalworking) have been carried out so far. The understanding of the high temperature tribological behaviour of high strength steels and tool steel pairs is also highly inadequate. The aim of this work is therefore to obtain a better understanding of the friction and wear mechanisms of tool steel and high strength boron steel tribological pairs at elevated temperatures. The experimental studies were carried out by using a high temperature version of the Optimol SRV reciprocating friction and wear test machine. The tribological studies were performed at temperatures ranging from 40ºC to 800ºC. The experimental materials were tool steels of three different alloying compositions (with and without nitriding) and high strength boron steel (unhardened, hardened, with and without Al-Si coating). The results have shown that both friction and wear of tool steel and high strength steel pairs are temperature dependant. An increase in temperature has resulted in lower friction for all the material pairs. Tool wear increased when the temperature increased from 40 to 400ºC during sliding against uncoated high strength steel but remained unchanged when the temperature increased further to 800ºC. When sliding against Al-Si coated high strength steel, tool wear increased with increasing temperature. Plasma nitriding of tool steels has been effective in reducing friction as well as in providing protection against severe adhesive wear. The Al-Si coating on the high strength steel has resulted in high friction at low temperatures and low friction at elevated temperatures. It has also shown an increased wear resistance at elevated temperatures. The coating undergoes significant surface morphological changes when exposed to elevated temperatures which are likely to influence its tribological behaviour. Hardening of the high strength steel has resulted in decreased friction at all temperatures. It led to higher tool wear at low temperatures and lower tool wear at elevated temperatures / Godkänd; 2007; 20070820 (pafi)

Interacting surfaces are frequently found in mechanical systems and components. A lubricant is often added between the surfaces to separate them from mechanical contact in order to increase life and performance of the contacting surfaces. In this work various aspects of hydrodynamic lubrication are investigated theoretically. This is where interacting surfaces are completely separated by a fluid film which is often the desired operating condition of machine components when wear and friction is to be reduced. Different flow regimes can be identified within the scope of hydrodynamic lubrication. If the surfaces are separated by a thick fluid film the influence from surface asperities is small and the surfaces can be treated as smooth. If the rate of change in film thickness with respect to the spatial directions is significantly large and if the flow velocity or Reynolds number is large, the ordinary fluid mechanical approach treating viscous flow with Computational Fluid Dynamics (CFD) has to be used. CFD is used to investigate influence from the use of an artificial microscopic surface pattern on one of the two interacting surfaces. The influence from the pattern is isolated from any other pressure generating effects by keeping the interacting surfaces parallel. Results are shown for different shapes of the micro-pattern. If the Reynolds number decreases, the system enters a regime called Stokes flow where the inertia effects are neglected. The full CFD approach is compared with the Stokes for various physical and geometrical cases. If the change in film thickness is small in the spatial directions, the thin film approximation is applicable and the full momentum equations describing fluid flow together with the mass continuity equation can be reduced to the Reynolds equation. Depending on boundary conditions, low pressures can occur at location of expanding fluid gap leading to tensile stress applied to the lubricant. However, a real liquid lubricant can only resist small tensile stresses until it cavitates into a mixture of gas and liquid. This often happens close to atmospheric pressure due to contamination and dissolved air into the liquid and occurs at higher pressures than the actual vaporization. To avoid pressures reaching too low levels, a general cavitation algorithm applied to the Reynolds equation is presented that accommodates for an arbitrary density-pressure relation. It is now possible to model the compressibility of the lubricant in such a way that the density-pressure relation is realistic through out the contact. The algorithm preserves mass continuity which is of importance when inter-asperity cavitation of rough surfaces is considered. For small film thicknesses the surface roughness becomes important in the performance of the lubricated contact. Even the smoothest of real surfaces is rough at a microscopic level and will influence the contact condition. The Reynolds equation still applies since the heights of the surface asperities are small compared to the spatial elongation. Treatment of the roughness of a real surface in a deterministic fashion is however beyond the scope of today's computers. Therefore other approaches need to be employed in order to take the surface roughness into account. In this work a homogenization method is used where the governing equation of the flow condition is formulated with a two-scale expansion, the global geometry and the roughness. Solutions are achieved for the limit of the roughness wavelength approaching zero and the method renders a possibility to treat the two scales separately. A method to generate dimensionless flow factors compensating for the surface roughness is developed. The flow factors, once solved for a particular surface, can be used to compensate for the surface roughness in any smooth global problem for any film thickness. / <p>Godkänd; 2005; 20061213 (haneit)</p>

Rail and rail wheel flange wear on the rail track has been a problem of attention for the last 30 years. The problems arise in curves and depend on increased traffic volume, heavier axle load and also higher speed. Axle loads of 22,5-25 ton is common nowadays and the trend is towards heavier axle loads where the next step is 30-35 ton. Flange wear includes both wheel and rail flanges and is therefore a problem for the operating company as well as the infrastructure owner. The flange wear depend mainly on the number of passed axles, type of traffic, speed and curve radius but also the axle load contributes. Flange lubrication on high rail is a well known way to reduce wear since the middle of 70th and a number of techniques to lubricate the rail flange are developed as grease, aerosol of oil and dry stick with solid lubricants. The trackside lubricator can not apply the grease on the rail flange when the climate is during the winter. The infrastructure owner in Sweden was interested to evaluate the effectiveness of the track side lubricator. The investment in trackside lubricators over 20 years was about 75 Mkr (7,6 USD) and also an additional yearly costs to operate 3000 apparatuses. The work to evaluate effectiveness of the lubrication started 1997 there one of the important matter concern the possibility to use environmentally adapted lubricants without hazard the rail. This licentiate thesis concern effectiveness of trackside lubricators to reduce wear in sharp railroad curves. Also the environmentally aspects have been considered and therefore natural esters synthetic esters and additives suited for those kind of lubricants have been evaluated. The research proved that environmentally adapted lubricants could lubricate as good as earlier used greases. Some amount of metal removal is probably healthy for this kind of application. Lubricants as synthetic esters can be designed get those qualities. It was also found significant difference between the seasons concerning flange wear. This difference depends on the problem to apply grease on the rail flange during the winter.

The term "sliding bearing" refers to types of bearing where two conformal surfaces (usually the stationary bearing and a moving shaft) slide relative to one another with load distributed directly across the interface. A suitable lubricant may be employed to reduce the friction between these two surfaces. In "fluid film" bearings, this lubricant builds up a layer of sufficient thickness such that the two surfaces are completely separated. Examples include journal and thrust bearings and shaft bushings. Unlike ball or roller bearings which have a limited lifespan, sliding bearings have, at least in theory, an indefinite operational lifespan. As long as an oil film of sufficient thickness is maintained and contamination is avoided, the bearing may continue to do its job indefinitely. However, more and more onerous demands are being placed on such bearings and their associated methods of lubrication in order to maximise performance in terms of efficiency and load carrying capacity, for example in hydroelectricity generators and turbines. Given the sensitivity of operating any form of mechanical equipment in the vicinity of watercourses, often the source of drinking water for major population centres, the hydropower industry has a duty of responsibility in ensuring that as much as possible is done to minimise the risk of contamination of the water or damage to flora and fauna. Two aims - increased output, decreased environmental impact - are currently the major driving forces behind Research and Development in the hydroelectric power generation industry. Looking at these from a tribological (friction and lubrication) point of view, three areas of particular interest can be identified. These are: 1) How can the load carrying capacity of thrust bearings be increased whilst at the same time improving performance within existing dimensional constraints? 2) What is the influence on bearings of transient events (rapid changes in speed or loading) and what impact do they have on the machinery components? 3) How does the use of Environmentally Adapted Lubricants (as opposed to mineral oils) influence friction and wear etc, specifically with reference to shaft bushings, and what is the impact of fluid ageing (oxidation) on their performance? Each of these three points has been studied in the course of the work carried out for this thesis. / Godkänd; 2005; 20061214 (haneit)

This licentiate thesis deals with the influence of transient loading, at high pressures, on the pressure-dilatation relation and frictional properties of lubricants. A Split-Hopkinson pressure bar and a Ball and Bar apparatus have been used for the experiments. Both methods share the same type of evaluation approach: evaluation by analysis of wave propagation in structures. The advantage with this method is that results are presented as function of time, i.e. every experiment yields a history of the studied event, not just a single value. Different theoretical evaluation methods have been used and refinements have been made by introducing compensation terms in order to enhance accuracy. Several lubricants have been studied and an empirical second-degree polynomial has been suggested, for a naphthenic mineral oil and a synthetic 5P4E oil, describing the pressure-dilatation relation over a wide pressure range. It is found that 5P4E exhibits lower compressibility, i.e. higher stiffness than the naphthenic oil. Curves representing friction coefficient as function of time for different lubricants at different pressures are included. It is evident that the Naphthenic- and Paraffinic mineral oils have the highest friction coefficient followed by Polyglycol, Polyalphaolefin and Rape seed oil. The observed distinctions agree well with the expected outcome on basis of molecular structure and pressure variation and correlates well with other density and friction investigations. A general trend of decreasing friction coefficient with increasing pressure is discovered. / Godkänd; 2000; 20070318 (ysko)

Long and frequent train delays, striking many periodically make headlines. The question is raised: what might be done about it? The on-going deregulation of the Swedish railway, with more actors entering the market, contributes to the question's topicality. In this thesis, problems and symptoms concerning punctuality are described and analysed. Herein, punctuality is defined as the extent to which an event takes place when agreed, for example the agreement between passengers and the company selling the ticket, considering the event that the train arrives at a certain time. The terminology in use on punctuality-related entities is floating. Causes for unpunctuality are related to the infrastructure, locomotives, wagons and personnel. Herein, primarily failures related to infrastructure are discussed. Maintenance, i.e. correcting and preventing failures, together with the track, rolling stock, weather, driver, other personnel and the timetable determine how good the punctuality will be. To the ore and steel transports studied it is more important to limit the lengths of the transport times, rather than having high punctuality, i.e. adherence to timetable. Generally, for freight transports it is difficult to give an estimation of the cost for unpunctuality. To find the root cause of a delay is vital, as is identification of symptoms and triggering events, in order to be able to carry out appropriate actions. Often the information needed to do so is lacking in the Ofelia database for infrastructure failures. Comparisons between regions or over time are complexified by the varying policies on how failure coding is to be performed. To improve punctuality by learning from others is made more difficult by e.g. different countries employing different ways of denoting punctuality. In the transport chain of post in Norway, terms related to punctuality varied even more among stakeholders. They considered being informed on estimated time of arrival as important, but getting this information was, despite this, not among the quantified goals. Few requirements were quantified, despite this often being simple to do. The consequences of failure are important to consider, not only e.g. the number of failures. Studying the variation in transport time on different line sections might help in identifying problems, because coding of delays shorter than five minutes is not carried out in the Swedish railway. Such causes of delay include low contact wire voltage, minor problems with locomotives and slippery rails due to leaves on the track. Disadvantageously, this variation is masked in the case of heavy trains or steep and curvy lines. In a studied maintenance organisation, the indicators of maintenance measured the result of the entire maintenance process. In order to manage each sub process better, it is more advantageous to measure the performance of them individually. Considering the specific sub process that decides which maintenance actions are to be carried out, it only uses expenses for the action itself when deciding, not resulting future costs and punctuality. The indicators focus on measuring past performance, not on facilitating planning for the future. A design of information systems facilitating information usage from different functions of the organisation would facilitate informed decisions. Information concerning technology, traffic and economy need to be integrated. / Godkänd; 2005; 20061218 (haneit)

No description available.