Energy based models to determine fracture toughness of thin coated systems by nanoindentation

Abstract

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 <SOOnm) including ITO, Sn02, ZnO, and TiOxNy on glass, which is the case of harder coating on hard (but relatively softer) substrate and they are the main samples investigated in this project; the other is a Ij.lm fullerene-like CNx coating on various ceramic substrates such as SiC, Si, and Ah03, which is the case of hard (but relatively softer) coating on a harder substrate. Some common brittle bulk materials (e.g. Si) were also tested to examine the applicability of the models. Reasonable toughness values for both coated systems and bulk materials have been obtained by the new models developed in this work. To gain more insight into the fracture mechanisms of coated systems, the threshold of fracture is also an important issue to be addressed. The loading rate may influence the critical load for fracture in brittle materials. It was observed that within the penetration rate range used in this study (lO-40nmls) the higher the penetration rate the higher was the threshold for cracking in the optical multilayer coatings on glass indented by a cube comer tip. When analysing the P-J curves at different loading rates, significantly different behaviour was observed for ITO and Sn02 coatings which is possibly due to a pressureinduced phase transformation in these coatings.