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Title: Energy based models to determine fracture toughness of thin coated systems by nanoindentation
Author: Chen, Jinju
Awarding Body: Newcastle University
Current Institution: University of Newcastle upon Tyne
Date of Award: 2006
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This project alms to determine the fracture toughness of thin coated systems by nanoindentation. The development of techniques for the assessment of coating toughness lags behind the determination of Young's modulus and hardness of thin coatings. No universal model or technique has been agreed to estimate coating toughness. With the development of complex coating stacks (e.g. multilayered coated systems) and the presence of variable crack patterns, the difficulty of generating a solution by stress analysis based models is dramatically increased. Therefore, there is an urgent need for the development of models to deal with complex coatings and varied cracking patterns. The most successful models in this respect are energy-based. In this thesis the existing models and techniques to assess coating toughness and adhesion have been critically reviewed. The stress analysis based models usually require empirical fitting parameters and they only deal with specific crack patterns. In contrast, the energy based models can deal with different cracking patterns without empirical constants; but they usually require that the crack propagates during loading cycle only, whilst, stress analysis based models do not have such a restriction. Several new models have been developed to assess coating toughness in this work. Two of them are based on excursions in load-displacement (P-c5) curves resulting from fracture during nanoindentation. The first model (Wcdp method) is based on extrapolating the plot of total work during indentation versus displacement. Compared to a literature model based on extrapolating P-c5 curve, this approach removes the influence on fracture dissipated energy from plastic deformation of the substrate. The second is a modified model to estimate the limiting value of coating toughness which could equally give the upper and lower boundary for toughness from nanoindentation performed under load control and displacement control, thus improving on the initial boundary model by Toonder et al which could only provide an upper boundary of coating toughness for nanoindentation under displacement control. However, it is often observed that fracture does not result in an excursion in the P-c5 curve, which requires a different modelling approach. The third model (Wirr-W p model) developed addresses this problem. All the previous models address through-thickness fracture which is widely observed in nanoindentation testing of hard coatings. In addition, another energy based method is proposed to estimate the adhesion of coatings by analysing the extra linear recovery of unloading curve associated with the rebound of the coating during unloading. Models were validated by experiments carried out by a range of nanoindentation techniques. The low load tests were performed by a Hysitron Triboindenter fitted with a sharp cube comer tip and a Berkovich tip. The maximum penetration was in the range of 40-400nm. Higher load tests were performed using a Nanoindenter II ™ fitted with a Berkovich tip in the range of lOmN-SOOmN. Atomic force microscopy (AFM) , highresolution scanning electron microscopy (SEM) and reflected light microscopy have been employed to investigate the fracture behaviour. To examme the models developed in this work, two different coated systems were investigated: one is multilayer optical coatings (total thickness
Supervisor: Not available Sponsor: Overseas Research Students Award Scheme (ORSAS), Newcastle University
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available