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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:582087 |
| Date | January 2012 |
| Creators | Mohd Tobi, Abdul Latif |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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