By defined structuring of sliding surfaces at dynamic contact seals friction and leakage can be reduced. Compared to macro-structures, micro-structures have the advantage of a quasi-homogeneous influence on the fluid behavior in the sealing gap. The development of suitable microstructures based on prototypes, whose properties are studied on the test bench, is very expensive and time-consuming due to the challenging manufacturing process and measuring technologies, which are necessary to investigate the complex rheological behavior within the sealing gap. A simulation-based development of microstructured sealing surfaces offers a cost- and time-saving alternative. This paper presents a method for simulative design and optimization of microstructured sealing surfaces at the example of a microstructured mechanical face seal.

By defined structuring of sliding surfaces at dynamic contact seals friction and leakage can be reduced. Compared to macro-structures, micro-structures have the advantage of a quasi-homogeneous influence on the fluid behavior in the sealing gap. The development of suitable microstructures based on prototypes, whose properties are studied on the test bench, is very expensive and time-consuming due to the challenging manufacturing process and measuring technologies, which are necessary to investigate the complex rheological behavior within the sealing gap. A simulation-based development of microstructured sealing surfaces offers a cost- and time-saving alternative. This paper presents a method for simulative design and optimization of microstructured sealing surfaces at the example of a microstructured mechanical face seal.

MACHINABILITY ENHANCEMENT OF STAINLESS STEELS THROUGH CONTROL OF BUILT-UP EDGE FORMATION / Demand for parts made from stainless steel is rapidly increasing, especially in the oil and gas industries. Stainless steel provides a number of key advantages, such as high tensile strength, toughness, and excellent corrosion resistance. However, stainless steel cutting faces some serious difficulties. At low cutting speeds, workpiece material and the chips formed during machining tend to adhere to the cutting tool surface, forming a built-up edge (BUE). The BUE is an extremely deformed piece of material which intermittently sticks to the tool at the tool-chip interface throughout the cutting test, affecting tool life and surface integrity. Unstable BUE can cause tool failure and deterioration of the workpiece. However, stable BUE formation can protect the cutting tool from further wear, improving the productivity of stainless steel machining. This thesis presents an in-depth study of machining performance using different coated tools and various coolant conditions to examine the nature of the different tool wear mechanisms present during the turning of stainless steels. Then, different textures are generated on the tool rake face to control the stability of BUE and reduce friction during the machining process. Results show that the BUE can significantly improve the frictional conditions and workpiece surface integrity at low cutting speeds. Finally, square textures on tool rake face were found to control the stability of BUE and minimize the friction at the tool-chip interface. This reduces the average coefficient of friction by 20-24% and flank wear by 41-78% and increases surface finish by 54-68% in comparison to an untextured tool. / Thesis / Doctor of Philosophy (PhD) / Three main objectives are presented in this thesis. The first is a detailed investigation of the performance of stainless steel machining obtained by the use of different coated cutting tools and various cooling conditions. The goal of this research is to assess the reduction of tool service life, productivity, and part quality. The thesis also examines the causes of workpiece material adhesion to the cutting tool during the cutting test and to better explain its effects on tool wear and workpiece surface finish. This phenomenon is known as the "built-up edge" (BUE). Finally, different textures are applied on the cutting tool via a laser to stabilize the BUE formation on the cutting tool, thereby improving the quality of the part.

Internal combustion (IC) engines used in road transport applications employ pistons to convert gas pressure into mechanical work. Frictional losses abound within IC engines, where only 38- 51% of available fuel energy results in useful mechanical work. Piston-bore and ring-bore conjunctions are fairly equally responsible for circa 30% of all engine friction - equivalent to 1.6% of the input fuel each. Therefore, reduction in piston assembly friction would have a direct impact on specific performance and / or fuel consumption. In motorsport, power outputs and duty cycles greatly exceed road applications. Consequently, these engines have a shorter useful life and a high premium is placed on measures which would increase the output power without further reducing engine life. Reduction of friction offers such an opportunity, which may be achieved by improved tribological design in terms of reduced contact area or enhanced lubrication or both. However, the developments in the motorsport sector are typically reactive due to a lack of relative performance or an ad-hoc reliance, based upon a limited number of actual engine tests in order to determine if any improvement can be achieved as the result of some predetermined action. A representative scientific model generally does not exist and as such, investigated parameters are often driven by the supply chain with the promise of improvement. In cylinder investigations are usually limited to bore surface finish, bore and piston geometrical form, piston skirt coatings and the lubricant employed. Of these investigated areas newly emerging surface coatings are arguably seen as predominate. This thesis highlights a scientific approach which has been developed to optimise piston-bore performance. Pre-existing methods of screening and benchmarking alterations have been retained such as engine testing. However, this has been placed in the context of validation of scientifically driven development. A multi-physics numerical model is developed, which combines piston inertial dynamics, as well as thermo-structural strains within a thermoelastohydrodynamic tribological framework. Experimental tests were performed to validate the findings of numerical models. These tests include film thickness measurement and incylinder friction measurement, as well as the numerically-indicated beneficial surface modifications. Experimental testing was performed on an in-house motored engine at Capricorn Automotive, a dynamometer mounted single-cylinder 'fired' engine at Loughborough University, as well as on other engines belonging to third party clients of Capricorn. The diversity of tests was to ascertain the generic nature of any findings. The multi-physics multi-scale combined numerical-experimental investigation is the main contribution of this thesis to knowledge. One major finding of the thesis is the significant role that bulk thermo-structural deformation makes on the contact conformity of piston skirt to cylinder liner contact, thus advising piston skirt design. Another key finding is the beneficial role of textured surfaces in the retention of reservoirs of lubricant, thus reducing friction.

A thorough investigation on the existing hydrodynamic lubrication theories and the reverse pumping theories for the conventional lip seal is conducted. On that basis, the algorithms and the methods used in the numerical modeling of the conventional lip seal are modified and applied to the study of the lip seal running against surface textured shafts. For each step of the study, the numerical model is benchmarked against the experimental results. Important physical mechanisms which explain the reverse pumping ability of the triangular surface structures are revealed. Meanwhile, the accuracy of the numerical model is tested. In general, the numerical simulation results match the experimental observation well. However, there are several important discrepancies. For each discrepancy the possible causes are discussed, which benefits the further attempts of the modeling work on the lip seal running against surface textured shafts. The conclusions of this study themselves can be used as a guidance to the design of the surface textured shafts for the lip seal applications. Finally the limitation of the current theories and the modeling methods are discussed and reasonable improvements which can be done are proposed for the future work.

The increased demand in stringent requirements on engineered surfaces in the aerospace and manufacturing industries drove the need for developing advanced surface engineering techniques such as chemical etching, plasma etching, corona discharge and laser surface texturing in order to alter material surface physical and chemical properties. Among these techniques, laser surface texturing has been identified as one of the most efficient and effective surface treatment/ texturing techniques which utilizes laser ablation to meet the demand of practical engineering requirements. This thesis details three practical engineering challenges in the field of paint adhesion, dust adhesion and tribology performance of SPF sheet forming dies in which case the problems and motivation for development came from projects by industrial partner collaborations with BAe Systems, Rolls-Royce and Dyson Ltd. The proposed solutions to these challenges are formulated around laser surface texturing techniques using excimer and femtosecond lasers on three engineering materials which are CFRP, ABS polymer and HR4 nickel alloy. As ablation is the main mechanism used in laser surface texturing techniques in achieving surface property changes, the understanding of laser beam interaction with materials is crucial. The fundamental understanding of laser beam interaction with different materials has been researched since the use of lasers in practical engineering applications by which laser material interaction parameters such as ablation threshold, incubation coefficient and optical penetration depth are of primary interest in addition to the ablation rate. Currently the published literatures are either material specific or laser specific with minimal or zero comparison between different types of lasers and materials as such which limits the understanding of laser beam interaction with materials. In addition, laser beam interaction with polymers has always been done using ultraviolet wavelength lasers. In this thesis, the interaction between two types of lasers and three types of materials which includes metals and polymers are presented through comparison and discussion between different interactions. It has been discovered that the ablation threshold value is lower for ABSinteraction with excimer laser as compared to the interaction with femtosecond laser due to the difference in the ablation mechanism. The optical penetration depth value is higher for ABS interaction with the excimer laser as compared to the interaction with the femtosecond laser due to differences in the photon energy of the laser beam of different wavelength. Two ablation rate curves were identified on ABS interaction with infrared wavelength femtosecond laser beams which has not been reported before. Chemical composition of the laser treated layer changes through chain scission process, creating free radical carbons that reacted with oxygen, nitrogen and water vapour in air creating oxygen and nitrogen rich functional groups which increased with increasing laser fluence and number of pulses. Laser ablation is known for its capability of altering surface morphology and surface chemistry of materials through excitation of electrons causing bond scission or melt where materials are vaporised, ejected or undergo chemical compositional changes. In the case of polymers, addition of oxygen and nitrogen rich functional groups are identified whereas in the case of metals, changes in crystallographic, orientation and oxidation states are identified. Such changes are deemed ideal for applications such as adhesion where it is mainly used for bonding and joining of similar or dissimilar materials. In this research, excimer laser surface treatment showed improvement in CFRP paint adhesion where a better adhesion is achieved than sand-papered surfaces. Paint adhesion of CFRP surfaces is affected by surface contaminants, surface chemical composition and surface roughness where the degree of inuence is in the respective order. In addition, excimer and femtosecond laser surface treated ABS also improved dust adhesion. The main factors that affect the ABS surface dust adhesion performance are surface roughness and surface chemical composition. The increase in surface roughness increases the surface area available for dust to adhere to. In addition, it also increases the drag coefficient of the air flow results in a higher removal force exerted by the air flow onto the dust particles through changes in the localised aerodynamic flow. The increase in polar functional groups increases the adhesion of the dust particles onto the surface due to an induced dipole moment by the charged dust particles. Laser micro-dimpled surfaces have been reported to be effective in reducing friction coefficient and wear rate of surfaces under oil lubricated conformal contact conditions where the dimples act as reservoirs to store lubricant and wear particles. But such surfaces have not been extensively researched under non-conformal contact conditions for different lubricated environments. In addition, there are contradictory results found between published literatures which observe under similar wear environments and conditions but with different dimple geometry. Hence a detailed investigation on dimpled surfaces under non-conformal contact conditions is being carried out. Laser surface texturing of 100 μm size dimples shows a reduction in nickel alloy wear rate under dry and oil lubricated environments. A higher dimple area ratio reduces the wear rate under dry condition with abrasive wear as the main wear mechanism. Under oil lubricated environments, the friction coefficient is dependent on the surface contact pressure, sliding speed and the viscosity of the lubricant and the wear rate is dependent on the film thickness which correlates to the friction coefficient. The wear rate of a dimpled surface is dependent on three factors which are the dimple diameter to contact area diameter ratio, depth of the dimple produced and the density of the dimples. Positive results are obtained in all three engineering applications indicating the feasibility of laser surface texturing techniques in providing suitable material surface properties for these applications.

Les émissions de polluants dans l’atmosphère représentent l’un des objectifs majeurs à l'heure actuelle. Dans le domaine automobile, la réduction des émissions de CO2 repose en partie sur l’amélioration du rendement moteur. Pour ce faire, ces travaux de thèse sur la texturation de surface à micro et nano échelle proposent de diminuer les pertes par frottements dans les moteurs thermiques alternatifs à combustion interne. Le poste moteur retenu dans cette thèse est le segment/piston/chemise car près de 40 % des pertes par frottement y sont générées. On s’intéresse à la texturation par LASER femtoseconde de cavités sur la surface du segment coup de feu. Le comportement tribologique de surfaces texturées est étudié en régime ElastoHydroDynamique (EHD) et Limite. En régime EHD, la capacité de formation d’un film lubrifiant et le contrôle du frottement en surface texturée indique un comportement tribologique dépendant du couplage entre les paramètres expérimentaux - cinématique des surfaces, pression de contact, taux de rétention en huile et temps de résidence des textures dans le contact et les paramètres géométriques - diamètre, profondeur, densité de textures. En régime Limite, l’influence des textures sur le comportement tribologique a permis de confirmer l’hypothèse sur le rôle de piégeage des débris par les cavités et de mettre en évidence des configurations de réseaux réduisant le coefficient de frottement pour des profondeurs faibles. / Nowadays pollutant emissions in the atmosphere are at stake. In the field of automotive industry, the reduction of CO2 emissions lies mostly on improving engine efficiency. This study about textured surface on micro- and nano-scale aims to diminish the friction losses in internal combustion engine.The main element is the piston ring pack because of the creation of 40 percent in friction losses. This analysis consists in LASER texturing with cavities on the surface of the first piston ring. The tribological behavior of textured surfaces is lead in Boundary and ElastoHydroDynamic (EHD) regime. In EHD regime the impact on load capacity of lubricant and frictional behavior in textured surface show linking between experimental parameters as surfaces cinematic, contact pressure, retention volume of oil, the dwelling time of cavities and the geometrical parameters as diameter, cavity depth and cavity density. In Boundary lubrication, the effect of cavities on tribological behavior has led to confirm the hypothesis of trapped debris in the cavities and to make in evidence swallow network of cavities reducing friction coefficient.

This PhD thesis presents the outcome of employing both nanosecond and femtosecond pulsed lasers in order to modify the surface structure of a material at the micro and nano scales. Literature review was carried out on micro/nano fabrication technologies involved in the semiconductor industry, which are the basis of many current micro and nano-manufacturing processes. The first experiments concentrated on direct laser scanning of Si to produce surface microstructures. This type of texturing was very effective at reducing surface reflectivity and can be implemented in photovoltaic devices. It was also found that the ablation efficiency can be improved if laser processing is performed in an argon environment where oxidation can be suppressed. Moreover, a significant relationship between laser-texture characteristics (i.e. topography/morphology and periodicity) and total surface reflectance was demonstrated. Short-circuit modelling of the laser texture showed that electrical performance of the cell can be improved by 41.3% in the 360-1100 spectrum, even in the near-infrared for which Si is a weak absorber. From these experimental results, it was also noticed that the laser-generated micro-structures made the surface significantly wettable; but as the laser fluence was reduced, the contact angle of the surface could be changed. This led to the investigation of the wetting properties of nano-bumps produced on Si at fluences below the ablation threshold. Their wetting behaviour was reported for the first time. An effect named as 'invisible marking' in this thesis was demonstrated: vapour condenses into water drops of different size depending on the lattice arrangement of c-Si or a-Si. Such an interaction at the near-ablation threshold was also explored for another type of material: NiP/Al data storage disks. From this research, elliptical bumps with vertical dimension in the sub-nanometre scale were fabricated with extremely high repeatability (± 0.4 nm). In addition, it was found that elliptical bumps can offer better stiction performance than circular shapes, even at ultra-low flying height. This type of laser texture could be utilised as a means for tribological optimization of surfaces that are in close proximity and relative motion. Following the use of low-fluences by nanosecond pulses, this was also applied to scanning over a microsphere lens array. So far, the research on near-field effects produced at the bottom of transparent particles has focused on how to generate parallel nano-patterns by single pulses. However, the present work has demonstrated that a focused beam with a tight-focus can be used to fabricate single lines or shapes rather than repeated patterns. In this way, a femtosecond laser was introduced to meet such a challenge. Moreover, laser-induced periodic surface structures (LIPSS) by fs pulses were also identified along the near-field generated nano-patterns. The evolution of such a periodic, self-assembly structuring was also investigated, and new optical characteristics of structural colour were found.

This diploma thesis deals with the influence of the surface texturing of non-conformal surfaces at the friction coefficient. In the work were made adjustment of experimental apparatus for measuring friction in the contact between ball and flat disk. Friction coefficient was calculated by the size of the torque on the ball, which was measured by the torque sensor. For recording and processing signal from the sensor was designed software that makes it possible to measure the coefficient of friction for selected conditions (loading of contact, speed of rubbing surfaces and rolling-sliding conditions). Experiments were carried out with smooth and textured ball, glass, and steel disc at different speeds and rolling-sliding conditions. To determine the effect of the influence of the surface texturing at the friction coefficient the size of the coefficient of friction of smooth and textured ball was compared. It was found that the microtexture of the friction coefficient has little effect.

The diploma thesis deals with an engineering design, a realization and a calibration of a apparatus for surface texturing of test samples. Microdents are produced by a diamond tool pressing. Electromagnetic linear actuator is used for pressing. Translation and rotation of the sample are secured by a couple of stepping motors. Thanks to this the apparatus can produce texture with well defined and easily controlled geometry. The diploma thesis contains a summary of surface texturing methods, designs of constructional variations, a electronic device of the apparatus and the calibration are also included.