Browsing by Author "Rickerby, David S."

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    Methods to reduce the thermal conductivity of EB-PVD TBCs.
    (Elsevier Science B.V., Amsterdam., 2002-03-01T00:00:00Z) Nicholls, John R.; Lawson, K. J.; Johnstone, A.; Rickerby, David S.
    This paper reviews the advantages and disadvantages of various thermal barrier coating (TBC) systems, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity. Methods of heat transfer within zirconia based ceramics are discussed, including the influence of coating microstructure and ceramic composition. It is shown the addition of dopant atoms (colouring) is effective in reducing â  phononâ  transport and that layered microstructures are effective in reducing â  photonâ  transport. Advanced processing, using EB-PVD coating methods has allowed both coloured and layered ceramic coatings to be produced. Measured thermal conductivities of 1.0 W mKâ  1 have been achieved using these methods, much lower than current commercial EB- PVD coatings at 1.5âÂ
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    A new spallation mechanism of thermal barrier coatings and a generalized mechanical model
    (Elsevier, 2019-08-16) Yuan, Bo; Harvey, Christopher M.; Thomson, Rachel C.; Critchlow, Gary W.; Rickerby, David S.; Wang, Simon
    Multilayer thermal barrier coating (TBC) systems typically consist of three layers of materials: A thermal barrier top coat (TC), a thermally-grown oxide (TGO), and a bond coat (BC) in addition to the substrate. Local strain energy concentrations, called ‘pockets of energy concentration (PECs)’ in this work, often occur around the interface between the TGO and the BC. They have various causes, including local phase changes, and non-uniform creep and plastic relaxation. It is discovered that both PECs and buckling drive the spallation of a TBC in a new spallation mechanism. A PEC-based mechanical model is developed that describes, explains and predicts how blisters nucleate in a TBC under constant biaxial compressive residual stress, steadily and then unsteadily grow, and finally spall off. Two conditions are established for the occurrence of TBC spallation, which depend on the compressive residual strain energy density in the TC and the TGO, and the interface fracture toughness. Experimental validation of the model was performed using aircraft jet engine turbine blades with electron beam physical vapor deposition (EBPVD) TBCs. The predictions from the developed PEC-based mechanical model for the radii of spallation in the TBC are in a good agreement with experiment results.
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    Spontaneous formation and morphology of telephone cord blisters in thin films: the Ω formulae
    (Elsevier, 2019-06-01) Yuan, Bo; Harvey, Christopher M.; Thomson, Rachel C.; Critchlow, Gary W.; Rickerby, David S.; Wang, Simon
    Telephone cord blisters (TCBs) are frequently observed in film/substrate material systems. They nucleate and propagate forward with wavy boundaries between the film and the substrate. The current study views the problem from a completely new angle: It is discovered that the spontaneous formation and morphology of TCBs in thin films under biaxial compressive residual stresses can be accurately explained and determined by assuming the existence of a pocket of energy concentration (PEC) instead of the existence of a separation of critical size. For the first time, completely-analytical formulae—the ‘Ω formulae’—are derived for the two local morphology parameters of TCBs of any shape, that is, width and height, and for the two global morphology parameters of TCBs of sinusoidal shape, that is, the wavelength and transverse amplitude. Mechanical conditions are also given for the first time for the formation of TCBs. Predictions for the four morphology parameters of the developed theory agree very well with extensive experimental results. In addition, by reversing the calculation, the residual stress and the film/substrate interface fracture toughness are also accurately determined from measurements of the TCB morphology parameters.
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    Swelling-induced telephone cord blisters in hydrogel films
    (Elsevier, 2021-11-08) Yuan, Bo; Harvey, Christopher M.; Shen, Ke; Thomson, Rachel; Critchlow, Gary; Rickerby, David S.; Yu, Suyuan; Wang, Simon
    Polymeric hydrogels can undergo dramatic shape and volumetric change when immersed into an appropriate solvent due to swelling or shrinking. Experimental studies have observed a variety of instability patterns in hydrogels. The telephone cord blister (TCB) with large deformability is one intriguing instability pattern but the assessment of its global morphology parameters, that is, the wavelength and transverse amplitude are still of inadequate appreciation. The present paper considers swelling-induced TCBs in a hydrogel-based film on a rigid substrate. Based on a previously developed theoretical framework for TCBs under small deformation, typically in a hard thin film, the theoretical derivations for the two global morphological parameters are furthermore developed for TCBs under large deformation in a soft thin film. Predictions for the morphology parameters of the developed theory agree very well with extensive experimental results. The critical mechanical conditions associated with the material-specific parameters such as the cross-linking density and swelling ratio are revealed. In addition, by reversing the calculation, the swelling-induced compressive stress in the un-delaminated film and the interfacial adhesion toughness are also accurately determined from measurements of the hydrogel TCBs. The present work provides an insight to design the microfluidics by controlling the morphology parameters with high precision.