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This thesis presents a combination of experimental study and finite element (FE) based fretting wear simulations of a coated substrate to assist in development of methodologies for coating selection in fretting applications. The methods provide explanations of coating failure mechanisms and the stresses associated with them under fretting conditions. Parameter studies of coating properties highlight the key parameters in choosing the optimum coating for a fretting application. A methodology has also been presented to predict coating life under fretting conditions. Comparisons made with the behaviour of uncoated substrates highlighted the beneficial effect of coating application under fretting conditions. Work concerning the fretting of uncoated SCMV (a high strength steel) and Ti-6Al-4V (a titanium alloy) highlighted the role of plasticity in a high friction contact. An elastic-plastic, FE-based, incremental wear simulation approach was employed using a linear kinematic hardening (continuum) plasticity model to represent the cyclic plasticity behaviour in Ti-6AI-4V case. For Ti-6AI-4V under partial slip conditions, the evolution of stick and slip zone sizes and surface tractions are shown to be influenced by plasticity. This leads to plastic ratchetting at the evolving stick-slip interface region, with the associated plastic damage accumulation. It is also shown that the plasticity due to shear yielding at the surface promotes wear across the contact region for the gross sliding case, and at the stick-slip interface for the partial slip case. An elastic-only response was observed in SCMV due to its low coefficient of friction and low wear rate. Experimental testing of a thin (~ 4 urn), hard diamond-like carbon coating deposited on SCMV and Ti-6AI-4V substrates has been employed to identify the tribological behaviour of the coating. It is found that with high normal loads, failure occurs by brittle fracture of the coating in the early cycles followed by an increase in coefficient of friction and partial-slip induced cracking in the substrate. The effect of fretting wear of a single layer coating on SCMV substrate on the stress evolution is simulated using a FE-based method under gross sliding conditions. The effects of coating stiffness, thickness and coefficient of friction (COF) are studied. It is found that the risk of surface and interface tensile fracture and coating (buckling-type) delamination at the interface are predicted to reduce asymptotically with wear while the risk of shear-driven delamination (at the coating-substrate interface) is generally predicted to increase with wear. Thicker coatings are found to be more resistant to tensile fracture, interfacial buckling-delamination and shear- delamination. Stiffer coatings are more resistant to tensile fracture and buckling- delamination but less resistant to shear-type delamination. It is also found that increasing COF leads to an increase of all the stress components. A parameter consisting of normal load applied and total sliding distance can be used to predict the coating wear life based on worn coating thickness.

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The study of fluid film lubrication in mechanical components is fundamental to the analysis of their dynamic behaviour as well as determining friction losses between moving parts. In several cases, gases present in the system can interact with the oil changing lubrication characteristics - by their dissolution and release from the oil, forming bubbles or even producing foam. Additionally, in low pressure regions the lubricant loses the capacity to flow as a continuous film, and cavitation occurs. Generally cavitation is treated in numerical models via boundary conditions for the pressure equation, which is then used to solve the pr<;>blem only in the full film region instead of the whole solution domain. Several criteria are available, and the results are often sensitive to the chosen conditions. The present work proposes the study of cavitation considering the changes suffered by the lubricant as it flows through the lubricated component, considering the release of gas from the liquid and the existence of a two-phase flow. The numerical model treats the liquid-gas mixture as a homogeneous fluid, whose properties are calculated as weighted averages of the properties from the constituent phases. The model then solves the lubrication equation numerically using the finite volume methodology, considering variable fluid properties and without using any intermediate boundary conditions. The lubricant is considered a mixture of oil and refrigerant, widely studied due to its importance in lubrication and heat transfer problems in refrigeration systems. To advance in the understanding of the two-phase flow model, it was initially applied to simplified geometries, such as that of a partial journal bearing, and gradually the geometrical assumptions were relaxed, applying the model to a full journal bearing, where a . discussion against experimental results was possible. Finally, the problem of the reciprocating motion of the piston in a refrigeration compressor was considered. From the results, it is shown that the model can automatically predict three regions for the lubricated component,the positive pressure region, cavitation and pressure recovery. When the model is compared to solutions considering Reynolds boundary conditions, good agreement is observed for moderate and heavy loads. Studying the main operational parameters and the mixture behaviour, the discussion focuses on the fluid properties, which affect particularly density and as a result the cavitation region - or even the full bearing if bubbles are considered in all the bearing extent. Finally, in the light of the two-phase solution for piston lubrication, existing cavitation criteria for the problem seem inappropriate.

Increasing demand for improved fuel economy 'and environmental protection has meant that there has been increased focus on the implementation of non-ferrous materials and coatings which have excellent tribological properties for automotive applications. Since all . . commercially. available lubricants are typically optimised for ferrous materials, it is therefore expected that lubricants will not perform in an optimised way on alternative surfaces: It is therefore crucial to understand how the existing lubricants interact with non- . . ferrous coatings to be able to sensibly approach the design of new additives for non-ferrous surfaces. In this study, CrN and DLC coatings which have excellent generic tribological properties, chosen for a full evaluation of their tribochemistry using both model and fully formulated oils. Tribological tests were performed using three test rigs, namely pin-on-plate, min~ traction machine (MTM) and cam/follower rig. The pin-on-plate tests were performed to evaluate friction, wear and fturability of CrN and DLC coatings under boundary lubrication .. conditions. Based on the results obtained from pin-on-plate tests, a' few coatings were chosen for cam/follower tests. In addition, MTM tests were performed to investigate the effect of slidelroll ratio and lubrication regime on the tribologicalltribochemical performance of a particular DLC coating. Based on the MTM results, the link between the results obtained from pin-on-plate and cam/follower rigs were assessed. Using the state-of-the-art surface analysis techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS), both physical and chemical characterisation of tribofilms formed by lubricant additives on CrN and DLC coatings have been characterised. Finally, they are correlated with the tribological results and also compared with those observed in conventional ferrous systems. This study has shown that the tribological performance of CrN and DLC coatings are oil dependent. The physical and chemical analyses of tribofilms suggest that the CrN coating responds in a similar manner to steel surfaces to the conventional additives. Optimisation 'of lubricant additives has been found to be immensely important, especially for the effective lubrication of110n/low-hydrogenated DLC coatings.

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The design of a typical steam generator involves contact between tubes and supporting plates within the shell of the generator. Flow-induced vibrations can cause tubes to impact and move against these supports and results in tube fretting-wear. The overall damage involves mechanical and corrosive wear and synergistic effects between those two are generally observed. Most of the work on fretting damage of Inconel-600 is directed towards predicting and preventing fretting wear alone, without much consideration of the corrosion factor. This necessitates more research to be done to understand the fretting-corrosion wear behaviour of this alloy in wide range of industrial applications. In this research project, the aspect of corrosion of two common materials of steam generator components has been investigated; Inconel-600 was coupled with stainless steel UN8 843100. Three in-situ electrochemical measurement techniques have been employed during fretting to investigate the synergistic effects between mechanical and chemical damage; Monitoring of the free corrosion potential, corrosion current density. Conducting of linear polarisation during fretting to study the role of various fretting parameters and test environments on the mechanical and electrochemical aspects of surface damage in three environments. Increasing the applied normal load has influenced the wear volume, Increasing the fretting period and displacement amplitude have found to increase the wear volume of Inconel-600 in all test environments. Lowering of the corrosion potential and increasing corrosion current was dependent on the applied normal load and vibration's amplitude within the contact area. Increasing the fretting period has been also found to increase the wear volume. Increasing the fretting period and the normal applied load were found to force the fretting regime to move from slip to stick conditions. The electrochemical response has shown dependency of the corrosion evolution on the fretting regime. In this work, the rates and mechanisms of fretting-corrosion wear of Incone-600 were discussed. Wyko surface profile meter and 8EM have been employed to assess the effect of varying fretting parameters on the mechanical and electrochemical aspects of the surface damage. And any suggested future work will be acknowledged.

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