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Nazir, MIan Hammad
Languages: English
Types: Doctoral thesis
Military vehicles are at a constant threat from harsh operational conditions in terms of temperature variations, humidity and exposure to atmospheric or aggressive chemical environments. These conditions are responsible for structural and components’ failure due to various corrosion failure mechanisms. Protective coatings tend to prevent the effect of physical and chemical attack on substrates. However, in some circumstances this attack is promoted, rather than hindered, and this results in the failure of coatings. This thesis presents novel theoretical models for three modes of coating failures i.e. micro-cracks effects, blistering and edge delamination. These models can be used for the analysis of coating failure initiation and propagation, especially useful for coating life assessment. The models follow multidisciplinary approach coupling the concepts of thermodynamics, fracture mechanics and electrochemistry. Novel equations involving inter-related multidisciplinary parameters are developed for stress-assisted diffusion rate, crack driving force and corrosion current density. These novel equations can be used by the manufacturers to design durable systems and can also be utilised for prognostics. Simple criterions i.e. critical/threshold values are identified which exclude the possibility of widespread coating failure propagation. The developed models are based on comprehensive experimental studies, which are also used to validate the theoretical predictions. The fundamental property which often dictates the performance of a coating is its adhesion to the substrate. There are many experimental techniques to measure the adhesion and analytical techniques to predict and optimise the adhesion failures. Many factors influence the adhesion of coating and substrate such as mechanical properties of the coating and substrate, the interface properties, the microstructure of the coating/substrate system, residual stress and coating thickness. Apart from these properties, the environmental conditions in which the coating-substrate system is exposed also affects the adhesion of coating and substrate. There are many modes of failures of coating-substrate system which either result from defects in coatings such as micro-cracks, interfacial defects such as micro-voids or result from stresses which are too large. Interfaces are intrinsic to these systems, and they are susceptible to interface debonding or delamination. The evolution of micro-cracks in coating layer is possible when tensile stresses develop during fabrication. The micro-cracks in the coatings can provide the pathways for the corrosive species to diffuse towards the interface causing interfacial corrosion of substrate. During application, these micro-cracks can open wide in the presence of tensile stress therefore accelerating the interfacial corrosion due to high diffusion rate of corrosive species. On the other hand these micro-cracks can constrict under the effect of compressive stress and therefore reducing the interfacial corrosion due to low diffusivity. However under the effect of compressive stress, blistering can occur. High compressive stress can cause the blister to propagate in a stable axisymmetric circular pattern till it loses its stability and becomes unstable forming tunnel blistering. Another failure mode is the edge delamination, the propagation rate of which depends on the transport of corrosive species through the defect at edge and electrochemical reactions at the interface. Experiments are reported for each failure mode by using a model coating-substrate system chosen to allow the visualisation of the interface and to permit coating failure propagation along the interface. For micro-cracks analysis, the effects of tensile and compressive stresses on micro-cracks behaviour in the presence of diffusing corrosive species are studied. The experiments are reported for coating-substrate system chosen to allow the visualisation of the interface and to permit the tensile and compressive stresses in the film to be generated over a full range of interest by exploiting the thermal expansion mismatch of the system when exposed to the environmental chamber. Similarly for blistering analysis, the coating-substrate system with micro-void (defect) at the interface is subjected to high compressive stresses in the presence of corrosive species. The study bear out the theoretical prediction of a regime of axisymmetric stable circular growth which gives way to non-axisymmetric tunnel blisters after circular blister becomes sufficiently large and unstable. For edge delamination analysis, the delamination propagation as a result of electrochemical reactions due to high diffusivity through artificial defects is studied. The delamination mechanism is also studied for variable environmental conditions during intended application such as uncertain vehicle movement (in-out) and annual weather changes.

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