Improvements in Plasma Sprayed Thermal Barrier Coatings for Use in Advanced Gas Turbines
T.W.Clyne, C.J.Humphreys
Department of Materials Science & Metallurgy University of Cambridge
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, whic...Moreh 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. It increases as the roughness is raised, but there are clear indications that the value reaches a plateau, so there is little point in generating very rough surfaces. The reason for this appears to be that the interfacial fracture path switches to being within the top coat, just above the interface, when the roughness becomes high. Over the (relatively narrow) range of improved purity (notably lower silica content) which can be achieved without serious economic penalty, it was found that this had very little effect on the sintering characteristics A model has been developed for in-flight heat and momentum transfer during spraying. This model has been used to simulate co-spraying, and to explore how porous microstructures could be formed which might be resistant to sintering The in-flight model has also been used to explain how hollow particles can be produced by melting and re-solidification, and the material properties which favour this happening. Work has been done on the laser drilling of TBCs (which is carried out to create cooling channels). The effect of laser drilling on the likelihood of top coat spallation has been investigated, and the nature of the damage resulting from this operation has been studied. The formation of overhanging top coat material was found to inhibit melt ejection from the substrate. A numerical process model was developed (in a parallel EPSRC project), which explained this effect. Fourteen publications have already arisen from the project, with several more likely to appear shortly. There has been extensive and fruitful collaboration between Cambridge and both Sulzer Metco and Rolls Royce (partly via the UTC based in the Materials Science Department). DSTL have also been strongly involved, via Prof. Richard Jones.
keywords:residual; stiffness; TBCs; Thermal Barrier; top coat; plasma spraying; vacuum plasma; Gas Turbines; Improvements in Plasma; Use in Advanced
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