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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:493334 |
| Date | January 2008 |
| Creators | Hermosilla, Unai |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/12155/ |
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