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Air plasma sprayed thermal barrier coating (APS TBC) systems are usually applied to engine components to reduce the temperature of the substrate and increase the efficiency of engines. However, failure of these coatings leads to oxidation and corrosion of the substrate. Therefore, a thorough understanding of the coating failure is necessary to predict the lifetime of coated components. This project has carried out stress analysis and prediction of subsequent failure of APS TBC systems associated with sintering of the TBC, oxidation of the bond coat (BC), substrate geometry, undulations at the coating interfaces and coating fracture toughness. Stress analysis is crucial for predicting TBC failure as stresses in the vicinity of the coating interfaces cause cracks and subsequent coating delamination. The Finite element (FE) method was used for stress analysis of TBC systems at high temperature stage and at cooling stage after operation. Initially, FE model of an axisymmetric unit cell representing the slice of a coated cylinder was used. Different radii for cylinders were used to investigate the significance of substrate curvature on coating stresses. The effect of asperities at the coating interface on residual stresses was observed using 3D models. The models were built based on the actual geometries of asperities, which were extracted from 3D SEM images of the coating interfaces. An Arrhenius approach was utilised to implement changes in mechanical and physical properties of TBC due to sintering. BC oxidation and related changes in its composition were also implemented. The accuracy of assumptions for FE models was validated by comparing the evaluated stresses against experimental results by project partners. Finally, the effects of stresses and fracture toughness of the coatings and coating interfaces on failure of the TBC system were studied, using cohesive surface modelling and extended finite element modelling (XFEM) methods.

This thesis reports on the microstructure of Al-Sn based powders and the development of Al-Sn based coatings for automotive shell bearing applications deposited using the high velocity oxy-liquid fuel (HVOLF) thermal spray technique. The microstructure of the coating and its associated physical and chemical properties, such as microhardness and corrosion resistance, are investigated as a function of the HVOLF thermal spraying parameters. In particular, a detailed microstructural understanding of the thermal sprayed coatings is developed to explain the coating properties. Two alloy systems, Al-12wt. %Sn-1 wt. %Cu and Al-20wt. %Sn-3wt. %Si have been investigated in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. The high resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS) and energy filtered TEM have also been used to examine nanoscale precipitates as supplementary methods. The statistical image analysis of fine scale particulate dispersions has also been used to study the second phase evolution with annealing. The microstructure of the large gas atomised powder particles used in the HVOLF thermal spray process comprise dendritic Al and interdendritic Sn, whilst the small powder particles exhibit fine scale Sn particles distributed within an Al matrix. The as-sprayed coatings comprise a mixture of melted and partially melted splats due to the full and partial melting of the deposited powder. Nanoscale Sn particles distributed in the Al matrix are present in fully melted regions, whilst micron / sub-micron Sn particle distributions and Sn-particle free Al regions delineate partially melted regions. Cu remains in solid solution within the Al matrix of the Al-12wt. %Sn-1 wt. %Cu as-sprayed coatings, whilst Si formed nanoscale particles in the Al-20wt.%Sn-3wt.%Si as-sprayed coatings. The critical cooling rate to form the metastable liquid phase separationh within Al-12wt%Sn alloys is put forward according to calculation. If the cooling rate is lower than the critical cooling rate, dendritic Al and interdendritic Sn are formed, thereby explaining the structure of large gas atomised powder particles. If the cooling rate is higher than the critical cooling rate, a liquid phase separation reaction occurs to form fine scale Sn dispersion. The calculated critical Al-12wt.%Sn powder diameter for liquid phase separation is close to the experimentally observed Al-12wt. %Sn-l wt. %Cu powder diameter. The discrepancy between experiment result and theoretical calculation is attributed to the additional element Cu promoting the liquid phase separation. The nano and sub-micron scale Sn distribution in small gas atomised powder particles and the as-sprayed coatings is attributed to the cooling rate being higher than the critical cooling rate. The dendritic structure of the large Al-Sn-Cu gas atomised powder is due to the cooling rate being lower than the critical value. Heat treatments are applied to the as-sprayed coatings to alter the mechanical and chemical properties, such as, microhardness and corrosion resistance, of the bearing material coatings. Annealing causes the nanoscale and sub-micron Sn particles to coarsen within both Al-12wt.%Sn-1 wt.%Cu and Al-20wt.%Sn-wt.3%Si coatings according to the analysis of SEM and TEM images. The Sn particles coarsen greatly within the Al-12wt.%Sn-1wt.%Cu coatings annealed at 300°C for 5 hours, as compared with coatings annealed for 1 hour. The Ɵ’-phase (CuAl2) also precipitates in the Al-12wt.%Sn- 1 wt.%Cu coatings after annealing at 300°C. Annealing also causes fine scale Si particles to coarsen greatly in the Si containing alloy. The microhardness decreases in the annealed coatings for both alloys and is attributed to a coarsening of Sn particles and the release of residual strain within the as-sprayed coatings. As compared with the as-sprayed coatings and the coatings annealed at 300°C for 1 hour, the corrosion rate in 0.1M NaCI solution of Al-12wt.%Sn-1wt.%Cu coatings annealed at 450°C for 1 hour is very greatly reduced. However, an annealing temperature of ~450°C is not appropriate for these coatings because of the introduction of interlayer cracks and a coating / substrate reaction which might degrade the mechanical properties of the bearing.

Thermal barrier coatings (TBCs) are usually applied on high temperature gas turbine components. They reduce the need for additional cooling of the exposed surfaces and improve the durability of the underlying materials. However, the lack of reliable lifting methods limits their applicability in the design of turbine components and so they are usually employed as additional protection for components that already meet the design requirements. In order to develop failure models and equations of practical interest, the mechanical behaviour and degradation of properties of coatings at elevated temperature needs to be understood. Several phenomena such as the growth of an oxide layer, degradation of bond coats, creep and thermal expansion mismatch between the different layers that compose the TBC contribute in the development of stresses at high temperature. The effect of thermal cycling has been covered in previous research, giving rise to models that explained how accumulated cyclic inelastic strains occurred in the bond coat and oxide layer due to the thermal expansion mismatch. This favoured the wrinkling of the oxide layer and the concentration of stresses, which could eventually cause crack nucleation, growth and failure of the coating. The research contained in this thesis focuses mainly on the development of stress concentrations during high temperature exposure. A coupled micro-structural-mechanical constitutive model was implemented in order to take into account the processes the coatings undergo at high temperature. High tensile stresses, perpendicular to the oxide-top coat interface, which may induce crack nucleation within the oxide layer at high temperature, were obtained.

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