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The purpose of this research is to evaluate the lubrication properties of a synthetic synovial fluid in combination with biocompatible polyurethanes, versus materials commonly used in biomedical implants. This combination is found in endurance testing of meniscal implants made from polyurethane. Two different polyurethanes were used for friction measurements, applying a synthetic lubricant, containing a Ringer's solution, hyaluronic acid and bovine serum albumin. The results were compared with friction measurements, using a polyurethane sphere against bovine cartilage, lubricated with bovine synovial fluid. The influence of the lubricants was tested by comparing water, synthetic- and bovine synovial fluids with the various material combinations, found in existing knee implants. From the measurements it was shown that the friction pairs including metal surfaces did not show the common Stribeck behaviour, with respect to transitions from the boundary regime to full film lubrication, and friction remained relatively constant over the whole velocity range. Friction pairs including the polymer counter surfaces and the water lubricated contacts, showed the expected transitions from boundary to mixed lubrication. From this it was concluded that protein adsorption mainly defined the frictional behaviour when using metal surfaces, leading to a coefficient of friction (COF)≈0.2 using synthetic synovial fluid, and COF≈0.15 when using bovine synovial fluid. PEEK samples showed higher values in the boundary lubrication region, which decreased to values of COF≈0.1 at higher velocities. Polyethylene samples showed higher friction results, which was attributed to the surface roughness. From the observed friction results and wear tracks it was concluded that a synthetic synovial lubricant performs very well with all material combinations, when more attention is paid to the polyethylene surface finish.

Two water-soluble N-containing additives without sulfur and phosphorus, hexanamide with tetraethylenepentamine (EEHA) and benzotriazole amide with diethylenetriamine (BTAA), were synthesized and used as additives in the oil-in-water emulsion to investigate the effect of amino and benzotriazole ring on the tribological properties of the emulsion. The results showed that EEHA/BTAA exhibited wear resistance under all tested conditions. This may be attributed to the basic amino-functional groups in the additives which reduce the corrosive wear of the base emulsion. BTAA-emulsion showed the best friction-reducing and anti-wear performance under the 3 N loading condition, which may be attributed to its benzotriazole ring, which cannot be easily decomposed and could form self-assembled layers through π–π stacking. This π–π stacking may effectively compensate for the low chemical reactivity of the benzotriazole ring, and serve as a better protective film to exert tribological properties.

The use of synthetic materials for outdoor and indoor sport fields has increased over the last decades. Artificial turfs, commonly used on football fields, are basically infilled with recycled crumb rubber derived from old tires, the main of which are NBR (acrylonitrile butadiene rubber) and SBR (styrene-butadiene rubber) with fibers usually made of Polyethylene (PE) or Nylon. The use of these polymers for artificial turf designing purposes has caused controversy to whether their impact in human health, especially in skin abrasions during players sliding on the artificial turf. We have studied the tribological performance of different artificial human skin and real human skin against NBR, PE and Nylon 6.6 at different environmental conditions: normal conditions (25 ˚C and 50% of relative humidity) and high humidity conditions (37 ˚C and 80% of relative humidity) and forces of 2 and 4 N to achieve the association between friction and skin damage. The applied forces correspond to a range of pressures of 121 to 175 kPa and the experiments were conducted at 50 mm/s. The friction coefficient was obtained for different artificial skin samples: Lorica, Silicone L7350, pure PDMS, Cutinova and ESE (an epidermal skin equivalent developed by us), and these results were compared to those obtained from excised human skin samples obtained from healthy people after a surgery carried out at the Radboud Hospital of Nijmegen. Later on the human skin samples were analyzed with confocal microscopy and histological images to study the case of the surface properties of skin and determine any possible damage on the Stratum Corneum related to the tribological tests.

The application of tribology to sheet metal forming processes (SMF) contributes to a general industrial aim i.e., to make products of high quality at an increasingly competitive way, by enhancing the tool life and maintaining a constant level of friction during forming. Both aspects are served by the development of models, able to predict friction and wear. This thesis provides such a model, restricted to lubricated SMF-operations and focused on prediction and control of galling (initiation). The main hypothesis of the work - galling initiation in lubricated sheet metal forming processes occurs at the asperity level as a result of the fact that the lubricant’s critical temperature is exceeded, due to frictional heating -, is validated by a combination of modelling, experimental work and industrial trials.

Consumption of food products containing polyphenols has resulted in a feeling of dryness, roughness and puckering of the oral epithelium which is known as astringency. At high concentrations, astringent food compounds are perceived by consumers as unpleasant and has led food scientist to initiate multiple studies with the aim of reducing the unwanted effects of astringency. This project proposes a tongue equivalent design that closely mimics the properties of oral surfaces which are relevant for assessing changes to oral lubrication resulting from astringency. The nature of interactions occurring at oral interfaces containing astringents point towards a system dependence of astringency and shows the importance of adopting a tribosystems approach. By adopting a systems engineering approach, this project has been able to identify the main stakeholders and the influence they possess on the system under design. This was followed by the identification of needs from the main stakeholders which were translated into system requirements. Design concepts with the aim of satisfying the system’s requirements were proposed alongside some planned evaluation methods, performance indicators and expectations from the project main stakeholders. A final design concept was chosen based on the conceptual designs proposed to the stakeholders. The final design considers the topography, mechanical properties and operating conditions of the human tongue during oral assessment. Experiments have been used to verify the properties of several aspects of the design including Direct Mechanical Analysis, Dynamic Light Scattering and Tribological work. This thesis concludes by making recommendations on possible applications and future developments to the proposed design. Based on the investigation into the mechanism of astringency, it is recommended that current test protocols must undergo further development, making them capable of investigating the tribochemical basis of astringency.

Implantation of a hip prosthesis (HP) can have an extremely positive impact on quality of life, however, a number of medical problems that could limit the functioning of the prosthesis, including primary dislocation (impingement) and secondary dislocation (wear on the liner surface). A new dual mobility (DM) principle is presented based on a review of engineering strategies to reduce the risk of impingement. The DM principle was adapted to suit the Indonesian demands with respect to production, hip sizes and specific extreme movements. The final design provides a solution to overcome the limitations of the RoM within the Indonesian context. The aim of this research was to study the tribological behaviour of the new DM HP in relation to impingement based on (cross-linked) UHMWPE material and SS316L, which are available on the domestic Indonesian market, and which have passed biocompatibility tests. Furthermore, the relation between impingement was studied numerically, and a wear model was derived analytically. The result of wear model was in agreement with experimental work described in other literature. For the variation in specific wear rate and head size, the results show a large effect on the linear wear and increasing the head diameter will decrease the linear wear, respectively. The contact stress by using finite element (FE) was used to predict wear in the DM model on the contact between the head vs liner and the outer liner vs cup. For variations in the cup size, the results show that the maximum contact stress is predicted to occur for the thinner liner. The FE simulation was conducted to determine the effect of impingement on liner damage for the conventional model. The results showed that the wear on the liner surface was related to the damage of liner lip. An experimental procedure to determine the specific wear rate in a pre-clinical setting for the HP design was developed based on a pin-on-disk setup and was based on a newly developed hip-simulator setup. The results showed that the specific wear rate was above the threshold for non-cross-linked UHMWPE under dry conditions, while for bovine serum lubricated conditions was contrary.

For the lifetime assessment of surfaces affected by droplet impingement erosion, only droplet impact test facilities were available at the start of this research to evaluate and rank materials and coatings. In this thesis, a fatigue-based analytical model for the prediction of the droplet impingement erosion incubation period (timespan before erosion by detachment of wear particles starts) has been developed with the intention of creating a science-based alternative that allows for the selection and development of materials and/or material properties for an enhanced resistance to droplet impingement erosion. The impact of a water drop on a solid surface at high velocity results in a high water pressure in the contact area due to the local compression of water in a part of the drop (water hammer pressure). This pressure depends partly on the physical properties of the solid material. Upon drop impact, three different waves start travelling in the solid material. The Rayleigh surface wave has been identified as producing the highest stress cycles in the region around the drop impact contact area. Depending on the type of material and material properties, different types of wear modes have been identified: brittle fracture, surface fatigue and a mixture of the two. Surface fatigue, without any local brittle fracture, can be considered as the wear mode with the highest erosion lifetime. Thus, in the predictive model, surface fatigue has been considered as the wear mechanism, provided that the fracture toughness of all microstructural phases is above a certain threshold value. This depends, however, on the drop impact velocity and the drop size. As the maximum stress cycle at the surface follows from the Rayleigh surface wave, fatigue properties of the material as given by standard S-N curves, and fatigue damage accumulation based on the Palmgren-Miner, theory have been used for this predictive model for the droplet impingement erosion incubation period. For an accurate model validation, the predictive model has first been applied to the thermoplastic material PBT, produced by injection moulding and compression moulding, and compared with drop impact erosion results on the same PBTs. For both PBTs a good similarity between test results and model predictions for the injection moulded PBT was found (deviation of 29%). However, the absolute value of the incubation period predicted by the model for the compression moulded PBT differed by a factor of 15. In a second validation, the analytical model for the prediction of the incubation period of metal surfaces was compared with a wide range of liquid droplet erosion incubation period tests. The model was extended for the use of S-N curves for non-ferrous and ferrous metals –aluminium and stainless steel respectively – by including the effects of additional surface hardening and residual compressive stress at the surface due to a water drop peening effect. Model predictions were performed for stainless steel AISI 316 and aluminium 6061-T6, using S-N fatigue curves from different literature sources, and including the defined additional surface hardening and a residual compressive stress state at the surface due to “water drop peening effect”. For the droplet impact velocity range of 140 to 400 m/s they showed excellent agreement with the multi-regression equation as determined from an ASTM interlaboratory test program. Nearly all incubation period predictions were within the 95% confidence limits of the aforementioned multi-regression equation. In this research, a strongly enhanced understanding of the relationship between the physical and mechanical properties and the drop impact erosion incubation period of metals, thermoplastics and elastomers has been obtained. The physical and metallurgical mechanisms resulting in the degradation process of the metal surface during the incubation period were identified. These consisted of 1) surface plastic deformation and formation of dents; 2) surface hardening and residual compressive stress as a result of these surface plastic deformations; 3) fatigue crack initiation; and 4) fatigue crack growth. The selected wave properties (dynamic impedance) and fatigue properties of the metals, thermoplastics and elastomers used in the presented analytical model were identified with respect to developing guidelines for enhanced droplet impingement erosion incubation life. The relative impact pressure (pwh/vd) can be used to identify to what extent certain material classes reduce the water hammer pressure and corresponding maximum stress due to the Rayleigh wave.

In everyday life, people interact with numerous products through touch. A massive amount of information is generated through such tactile contact with a product surface. Surfaces thus influence the perception of the product physical properties, grip performance and forces applied during object handling and manipulation. The main aim of the current work is to control and enhance tactile perception through modification of the frictional behaviour by surface topography design. Following a systematic approach, an asperity and texture design was selected for this research. A contact model was developed to predict finger pad contact area on the micro- and macro- scales. The model was applied to understand the role of component parts in a tactile tribological system – namely to estimate skin elastic modulus, characterise skin deformation and determine the role of varying surface texture dimensions. A significant influence of the skin contact state on tactile friction was shown experimentally. Deterministic surfaces, which remained in asperity contact, showed a considerable reduction of friction coefficient. A bidirectional frictional behaviour was achieved with ellipsoidal texture design and was correlated to the feature geometry and material properties. The friction measurements performed with a group of volunteers, with the aim of normalisation, show that the reference sample can be used to normalise and compare values between individuals. A foundation for a texture design map is developed, towards establishing a connection between the texture dimensions and effects attributed to tactile friction. The design map can be used as a reference for the geometrical boundaries in surface texture design with the aim to control and predict frictional behaviour and to enhance tactile perception.

Wear and wear-induced debris is a significant factor in causing failure in implants. Reducing contact pressure by using a textured surface between the femoral head and acetabular cup is crucial to improving the implant’s life. This study presented the effect of surface texturing as dimples on the wear evolution of total hip arthroplasty. It was implemented by developing finite element analysis from the prediction model without dimples and with bottom profile dimples of flat, drill, and ball types. Simulations were carried out by performing 3D physiological loading of the hip joint under normal walking conditions. A geometry update was initiated based on the patient’s daily routine activities. Our results showed that the addition of dimples reduced contact pressure and wear. The bottom profile dimples of the ball type had the best ability to reduce wear relative to the other types, reducing cumulative linear wear by 24.3% and cumulative volumetric wear by 31% compared to no dimples. The findings demonstrated that surface texturing with appropriate dimple bottom geometry on a bearing surface is able to extend the lifetime of hip implants Refereed/Peer-reviewed

Pressure ulcers develop as a result of sustained mechanical loading and are commonly observed in patients with reduced mobility and reduced tissue viability as well as in patients that have undergone an amputation. Shear loading is known to be a risk factor that accelerates the onset of tissue damage. In this thesis it is hypothesized that pressure ulcer prevalence can be reduced by decreasing the shear load acting on the skin. The susceptibility of developing pressure ulcers whilst applying a range of shear loads for a prolonged period is demonstrated by means of cytokine measurements performed on volunteers. The expression of the IL-1α cytokine, which triggers the inflammatory response, was found to have a shear stress threshold value; at low levels of applied shear loading no increase was observed, whereas an increased expression was observed when the shear load was raised. The exact level of the threshold varies from one person to another. Considering the critical threshold for cytokine release, people who exhibit a high coefficient of friction might be more prone to developing pressure ulcers as it is more likely that a high shear load might be acting on the skin. Although previous researchers have established that skin hydration might be responsible for causing interpersonal differences in friction behaviour, correlation was only obtained within individuals, whereas this was not obtained for groups. In this thesis, Fourier transfer infrared spectroscopy (FTIR) has been identified as a useful tool for identifying skin characteristics that may be linked to interpersonal differences in friction behaviour. A strong correlation was obtained between the coefficient of friction and FTIR peaks relating to the hydration of the skin and the a measure for the viscosity of the sebum layer, meaning that a direct relationship between FTIR spectrum and an individual's friction behaviour has been obtained. To reduce the friction coefficient between skin and various counter surfaces, a selection of parameters can be altered to change the frictional behaviour. Results from friction experiments using smooth counter surfaces showed that the interfacial shear strength depends primarily on the environmental conditions while the material of the counter surface is of secondary importance. It was shown that the microenvironment has a major influence on the coefficient of friction. Both temperature and relative humidity had strong effects on the frictional behaviour. This work suggested that, from a tribological point of view, reducing the humidity should be the primary focus. The effect of surface roughness and hardness was studied using silicone counter surfaces which are commonly used for prosthetic liners. Employing surface roughness and varying the compliance of the silicone compound allowed the coefficient of friction to be altered. The results obtained are combined into two design maps: one for optimising the micro-environment in the skin-object interface, and one relating to surfaces in contact with skin exhibiting controlled levels of friction, with the objective of optimizing conditions for preventing pressure injury.

Surgical sutures are essential for the re-approaching of divided tissues, for the ligation of the cut ends of vessels, and play a significant role in wound repair by providing support to healing tissues. Frictional behaviour is one important part of the physical and handling characteristics and knot security of surgical suture. High friction between surgical sutures and tissues may cause inflammation and pain to the person, leading to a longer recovery time. Therefore, it is essential to investigate the frictional behaviour of surgical suture sliding through tissue. Based on the literature review of the frictional behaviour of surgical suture, little is actually understood about the frictional behaviours of surgical suture in sliding contact with tissue and about the simulation of friction of surgical suture during stitching. The aim of this thesis is to investigate the frictional behaviour of surgical suture penetrating through skin substitute by means of experimental and modelling work, in order to generate guidelines for the development of surgical suture with desirable tribological performance. In line with this, firstly, the frictional behaviour of three commercialized surgical sutures interacting with skin substitute is investigated in this thesis, with respect to surgical suture material and structure, by means of a capstan experiment approach and a contact area model. The results indicated that structure and surface topography of the surgical suture had a pronounced effect on the tribological interactions. In the stitching process, it is well known that the needle penetrates the tissue and creates a freshly formed counter surface with a damaged tissue. The needle is slightly larger in diameter than the surgical suture, which results in the spring back and subsequent normal force over the circumference of the surgical suture. Hence, from the tribological point of view, the sliding contact between the surgical suture with tissue is not restricted to surface phenomena, but might also lead to deformations beneath the surface layer. The capstan method could not simulate the frictional conditions of the surgical suture in the stitching process. Hence, a new penetration friction apparatus (PFA) was developed that allowed for the evaluation of the frictional behaviours of various surgical needles and surgical sutures during the suturing process, under similar contact conditions. It considered the deformation of tissue and could realize the puncture force measurements of surgical needles as well as the friction force of surgical sutures. The developed PFA could accurately evaluate the penetration friction behaviour of surgical suture - tissue under the simulating clinical conditions. The developed test method provides a new way to investigate the frictional behaviour of surgical suture. Additionally, based on the new test method, the relationship between the friction force and the normal force was established when the surgical suture penetrated through skin substitute. The friction force was measured by the PFA. The normal force that acted on the surgical suture was estimated based on a uniaxial deformation model, a Hertzian contact model and a finite element model (FEM), respectively. Furthermore, it is expected that the structure of the suture significantly affects the friction during sliding and possibly influences the amount of tissue abrasion. As a result, the influence of monofilament and multifilament structures on the frictional behaviour of the surgical suture was evaluated. Meanwhile, the difference of abrasion at the pull-in boundary and pull-out boundary of skin substitute was discussed. Lastly, based on the above research, two kinds of biopolymeric composite films were developed on the surgical suture, and their influence on the tribological performance of the surgical suture was explored.

This thesis focuses on the effect of additives on boundary lubrication of bearing steel for water-based lubrication systems. The oil-in-water (O/W) emulsion and the water-glycol based liquid were selected as the base fluids for research. Sulfur compounds, nitrogen heterocycles and graphene derivatives were synthesized and used as additives. The tribological properties and corrosion resistance of the bearing steel friction couple in water-based lubricants are investigated. Based on the results of tribological tests and the analysis of friction surfaces, various tribochemical mechanisms are suggested to explain the friction processes, as well as the formation of a boundary lubricating film. In addition, the design of water-based additives and the effects of additives and working conditions on the tribological properties of water-based lubricants are described. Firstly, the effect of the interfacial film on the lubrication performance of O/W emulsion was researched. For that, graphene oxide (GO) underwent asymmetric chemical modification with myristyltrimethylammonium bromide (TTAB) to get modified graphene oxide (MGO). This MGO was used as an emulsifier and additive in oil-in-water emulsion. The emulsifying tests showed that MGO greatly improved the stability of base emulsion and decreased its droplet size. Tribological test results showed that under the boundary lubrication condition with MGO emulsion, the friction coefficient (COF) and the wear rate of the steel ball decreased by about 18% and 48% respectively in comparison with base emulsion lubrication. The tribological mechanism of MGO emulsion could be explained by the strong film-forming ability on the metal surface, the high lubricity of its small droplets, and the formed adsorption film, transfer film and tribofilm between the contact surfaces. Secondly, the effect of the oil phase on lubrication performance of O/W emulsion was researched. For that, graphene oxide (GO) was single and double-sided modified with octadecylamine (ODA) to get MGO1 and MGO2 respectively. MGO1 and MGO2 were incorporated into the O/W base emulsion to change the composition of the interface and the oil phase. It was found that the tribological properties of MGO1-containing emulsion were better than those of MGO2-containing emulsion. Chemical composition analysis of the worn surfaces suggests that MGO1 nanosheets might adsorb more easily onto the metal surface and react better with metal surfaces to produce C-N-O-Fe-containing compounds than do MGO2 nanosheets, which means MGO1 nanosheets are more actively involved in the shear process that occurs in the contact area. Nanoindentation indicated that there was a non-uniform softer MGO1 tribofilm present on the metal substrate. Thirdly, the effect of the water phase on lubrication performance of O/W emulsion was researched. For that, two nitrogen-containing water-soluble additives are synthesized and used as an additive in O/W emulsion. The friction tests results showed that the two water-soluble additives might be more rapidly adsorbed to the metal surface than the emulsion droplets could be “plated out” on the metal surface, thereby changing the friction-reducing and anti-wear performance of base emulsion. In addition, it was found that the tribological properties of these two additives were related not only to nitrogen content but also to the functional groups formed by the nitrogen atoms. Fourthly, the effects of sulfur element and applied load on the tribological performance of water-glycol fluid were studied. For that, two triazine derivatives, STB and STC were synthesized and used as additives in water-glycol base fluid (mass ratio 1:1). Water solubility tests indicated that STB and STC met the solubility prerequisite for a water-soluble additive. The tribological tests using a four-ball tribometer showed that STB and STC both improved greatly the PB value, anti-wear and friction-reducing capacities of the base fluid. Based on friction test results and wear scar analysis, it was concluded that the differences in tribological performances of STB and STC samples were due mainly to the differences of the sulfur content and the sulfur activity in additives. XPS found that STB-tribofilm mainly contained sulfate, but STC tribofilm contained sulfide as well. Finally, the effect of molecular chain length, additive concentration and sliding velocity on the tribological performance of water-glycol fluid was investigated. For that, three novel xanthate-containing water-soluble triazine derivatives, EXT, BXT and HXT, were synthesized and used as additives in water-glycol base fluid. It was found that the additive-containing samples had no corrosive effect on the cast iron and copper surface within 2.5 wt.% additive. The tribological test results using a four-ball tribometer showed that the three additives greatly improved the PB value, friction-reducing and anti-wear performances of water-glycol. XPS results indicated that the additives had reacted with the contact metal surfaces and formed complex tribofilms composed of iron oxide, iron sulfide and iron sulfate, which may contribute to the reduction of friction coefficient and wear rate of the friction system. In summary, for O/W emulsion lubrication, if the droplets can be quickly adsorbed onto metal surface, forming an oily layer between the rubbing surfaces, the COF will be reduced. If coupled with the formation of an effective adsorption film and tribofilm by additives, the COF will be further reduced. For water-glycol lubrication, the differences in the tribological performances are due mainly to the differences of the formed adsorption film and the tribofilm on the rubbing surface, including the forming velocity, strength and thickness and the composition. These change not only with the additives but also with the working conditions.

The development of an appropriate substitute to simulate the frictional performance of human skin at different conditions is required for the design and optimization of products in contact with the skin. With this purpose, the composition, structure and mechanical properties of the skin need to be analyse to produce a suitable surrogate. In addition, the frictional performance of human skin is subjected to the influence of several factors, such as the surface condition, hydration level or the operational conditions. Thus, the influence of these factors in the frictional performance of an artificial skin substitute must be evaluated and the results compared to in vivo and/or ex vivo skin results. Therefore, this thesis presents a skin substitute with a layered structure and similar tribo-mechanical performance as the human skin. The mechanical model is composed of two layers with different composition and properties. The top layer is a thin film of Poly (vinyl alcohol) mixed with rapeseed oil and crosslinked with Glutaraldehyde which exhibits similar roughness and adhesive properties as the human skin. The bottom layer is made of a water solution of PVA crosslinked by freezing and thawing cycles which displays similar viscoelastic properties as the human dermis. The validation of the mechanical model was done in comparison to ex vivo skin results from abdominal skin and other results on in vivo skin from the literature. Additionally, the influence of the layered structure of the skin and the effect of the capillary forces in the frictional behavior of the skin were introduced as an input in the classic Hertz's model and the modified JKR model. The comparison of the theoretical and the experimental results on ex vivo skin indicated two different frictional regimes due to the relative influence of the adhesive and the elastic properties of the skin depending on the applied force. Moreover, the effect of the capillary forces introduced slight changes at wet conditions which effect on skin friction, especially at low forces, should be analysed further.

Tactile perception a concept with mechanical, physiological and psychological perspective, is of particular concern to the industrial companies and research area. The hedonic attributes of tactile perception are influential to our daily life like wearing clothes, using personal care products, holding tool handles or in domestic appliances. A skin contact model was modified based on Hertzian and Westergaard model to take into account the influence of surface texture and orientation effect. This two-term friction model enables the prediction of the contact area, sliding friction between the skin and counter-surface. Furthermore, the tactile perception involved textures were designed and investigated in both perception experiments and skin friction measurements. The thesis focusses on the relationships among tactile perception, sub-micron surface roughness features and friction at light touch conditions, with the general aim to discover design principles that enhance tactile perception. For the perception experiments, a panel test was conducted and the subjective ratings from 0 to 10 were graded by each participant to describe the level of perceived roughness, perceived stickiness and perceived comfort. For the skin friction measurements, a multi-axis force/torque transducer was used to measure the dynamic friction between skin and counter-surface in vivo. The correlation of the perception experiment and friction measurement provides design tools for texture design of future product surfaces. With the presented tactile friction model, a strategy was extracted for optimizing surfaces with respect to tactility. The pivotal element is minimizing the adhesion term of friction by minimizing the contact area by designing a surface texture with minimal contact area in sliding motion. In addition, the orientation effect is another key factor for the texture design. The results of the study can be beneficial to understand the tactility related research and product developments in the future.

Oil-based lubricants are widely used in many mechanical applications, but they cannot be used for applications with a high risk of polluting the environment or for applications that involve a bio-medical environment. Water-based lubricants can be used as alternative because they can potentially overcome these problems while maintaining the required tribological performance. A water based environment requires interacting surfaces that combine hardness, wear resistance and corrosion resistance. Typically, stainless steel and CoCrMo alloys are used for such tribological applications including food processing equipment as well as implants. In this thesis, a new concept of aqueous lubrication, i.e. lubrication by hydration of surface active polymers combined with graphene oxide from water or an oil-in-water (O/W) emulsion, is presented. Based on the literature review and the lubrication concept, this work started with the interaction of bearing steel, stainless steel and CoCrMo with several newly developed additives. Then the aqueous lubrication by using O/W emulsions incorporating the additives was examined. The friction profile of the O/W emulsion was quite different to that of the oil lubricant. It exhibited three stage frictional behaviour, including running-in, water dominated status and oil dominated status. Secondly, the CoCrMo alloy was coated with surface-active polymers PAA and PEG and graphene oxide layers. Enhanced friction reducing capability was found in water based fluids for the polymeric coatings in combination with a graphene oxide. Finally, an amphiphilic coating, PEG-lactide, was combined with graphene oxide and used with the O/W emulsions for CoCrMo. The tribological performance of PEG-lactide coating in O/W emulsion was enhanced further compared to the performance of the PEG coated surfaces: a clear indication of the advantage of using hydrophilic and lipophilic group containing surface-active polymers for emulsion lubrication. The overall maximum reduction in friction that was achieved for a sliding contact of coated engineering surfaces from CoCrMo at low sliding velocity and moderate contact pressure was of about 63 % compared to uncoated CoCrMo sliding in water at the same operational conditions.

When machines operate under extreme conditions, they often need to perform to maximum capacity. The high demands cause the amount of wear to increase relative to ‘the normal’ situation. Moreover, the extreme conditions are typically variable, making it impossible to define fixed maintenance intervals. When failure of such machines has to be prevented and maintenance is to be performed efficiently, this means that the amount of wear somehow needs to be calculated based on the use and operating conditions of the machines. Certainly in the case of machines that work in sandy environments, sand particles entering in between the machines’ components in sliding contact cause increased amounts of wear. The wear caused by this sliding movement of hard particles through a softer surface is called abrasion and is a prominent wear mechanism decreasing the life time of e.g. gears and bearings. Traditionally, the amount of wear due to abrasion is estimated with a simple model based on Archard’s wear law. Such models, however, are considered too simple and not adequate to calculate abrasive wear rates and predict maintenance intervals. The work presented in this thesis investigates the abrasion mechanism and improves the model for third-body abrasion. One of the main factors influencing third-body abrasion is formed by the properties of the particles causing the wear. According to the literature, the most important particle properties related to abrasion are the size, shape and hardness. Because the exact influence of particle size and shape on abrasion is not yet fully understood, the relations between these properties and wear have been verified both experimentally and numerically. New parameters were derived to describe the shape of particles in general. A numerical finite element model in Abaqus for a tip with a predefined shape sliding through a surface was set up to simulate the wear types associated to abrasion and further establish the relation between abrasive wear and particle shape and size. The results were implemented in a predictive maintenance setup for vehicles operating in sandy environments.

To a large extent, the functionality and comfort experienced during the use of everyday products, such as apparel, household appliances and sports equipment, are determined by the frictional behaviour of contact that occurs with the skin. For product engineers who aim to control and optimize the sensorial properties of a product surface interacting with the skin, it is essential to understand this frictional behaviour. This involves the study of local friction behaviour at the scale of the surface roughness. In this work a mechanistic approach was adopted in which analytical models from contact mechanics theory were used to develop a model which describes the tactile friction behaviour against the human fingerpad as a function of asperity geometry and operational conditions.

The human skin plays an important role in people’s lives. It is in constant interaction with the environment, clothing and consumer products. This thesis discusses one of the parameters in the interaction between the human skin in vivo and other materials: skin friction. The thesis is divided into three parts. The first part is an introduction to skin friction and to current knowledge on skin friction. The second part presents the RevoltST, the tribometer that was specially developed for skin friction research and which meets the objectives described in the thesis. The third part presents the results of the skin friction measurements obtained with the RevoltST.