Improvements in Plasma Sprayed Thermal Barrier Coatings for Use in Advanced Gas Turbines
T.W.Clyne, C.J.Humphreys
EPSRC Final Report
abstract:This project was focussed on the development of improved thermal barrier coatings (TBCs), produced by plasma spraying. Such coatings are in extensive industrial use for aeroengine and industrial power plant gas turbines. They commonly fail by spallation, exposing the underlying metallic substrates to high temperatures and leading to component failure. The project involved upgrading the vacuum plasma spray facility used for coating production. The project provided one post-doctoral position, which was filled consecutively by three research workers, partial support for a technician post and some support for one PhD studentship. The following technical conclusions can be drawn from the work. Sintering of the top coat at temperatures commonly encountered during service has been shown to result in substantial increases in stiffness, and hence in the driving forces for spallation generated during thermal cycling. Modelling of the associated residual stress levels has indicated that the changes induced might well be sufficient to cause spallation in practice. Study of the associated microstructural changes has highlighted the importance of microcrack healing and improved inter-splat bonding. These are promoted by diffusion at high temperature, but are inhibited by the presence of in-plane tensile stresses from differential thermal expansion. The sintering has been shown by dilatometry studies to be quite significantly anisotropic. Contraction (shrinkage) in the through-thickness direction is greater than in the in-plane directions. This technique is much more sensitive to the microstructural changes induced during sintering than is the porosity level, which does not change very dramatically. Dilatometry data are currently being used in the development of a model for the sintering process in plasma sprayed top coats. The toughness (fracture energy) of the top coat bond coat interface has been found to be dependent on the interfacial roughness.
keywords:residual; stiffness; TBCs; Thermal Barrier; top coat; plasma spraying; vacuum plasma; Gas Turbines
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