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Study of Bond Coats for Thermal Barrier Coating Applications
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
Bond coats used in thermal barrier coatings (TBCs) for gas-turbine engine applications are studied in this thesis, with a focus on oxidation behaviour, surface rumpling and stress evolution. Bond coats made of γ/γ’ Ni-Al-Pt alloys have been widely used in TBCs and it has been found that addition of platinum greatly improves the oxidation resistance of the coatings. The mechanisms behind this benefit, however, are not well understood. For this reason, the oxidation behaviour of four γ/γ’ Ni-20Al-xPt (x= 0, 5, 10 and 15 at. %) alloys at 1150 °C is studied and compared in terms of oxide spallation, oxide microstructure and growth, residual stress in the oxide scale and oxide/alloy interface morphology. The progressive increase of platinum addition into the alloys results in (1) greater resistance to oxide spallation, (2) reduction in oxidation of nickel, (3) lower stresses in the α-Al2O3 scale and (4) more planar oxide/alloy interfaces. It is found that the selective oxidation of aluminium promoted by platinum plays a central role in the evolution of the oxidation behaviour of the alloys.Surface rumpling of a NiCoCrAlY bond coat deposited on a Ni-base superalloy during cyclic oxidation at 1150 °C is studied. The extent of rumpling is found to depend on thermal history, coating thickness and exposure atmosphere. While the coating surface progressively roughens with cyclic oxidation, bulk NiCoCrAlY alloys with the same nominal composition show a much less tendency to rumple under the same thermal cycling condition. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation and exhaustion of yttrium) induced by oxidation and coating/substrate interdiffusion during thermal exposure. The observations together suggest that rumpling is driven by the lateral growth of the thermally grown oxide (TGO) and the coating deforms in compliance with the TGO. While the dependence of rumpling development on experimental conditions is generally in agreement with the prediction of the existing model, it is suggested that the degradation of the NiCoCrAlY coating and its dependence on coating thickness need to be taken into consideration when predicting the rumpling development of NiCoCrAlY coatings.The residual stresses in a NiCoCrAlY bond coat deposited on a Ni-base superalloy are studied by X-ray diffraction using the sin2Ψ technique. The stresses at room temperature are found to be tensile; they first increase and then decrease with oxidation time. The stress develops and builds up upon cooling, predominantly within the temperature range from 1150 °C to 600 °C. Due to the limited penetration depth into the bond coat, the X-ray only probes the stress in a thin surface layer consisting of a single γ-phase formed through aluminium depletion during oxidation. Above 600 °C, the volume fraction of the β-phase in the bond coat increases with decreasing temperature. The mechanisms of stress generation in the coating are examined and discussed based on experiments designed to isolate the contribution of possible stress generation factors. It is found that the measured bond coat stresses are mainly induced by the volume change of the bond coat associated with the precipitation of the β-phase upon cooling.