Professor TW Clyne, Department of Materials Science & Metallurgy, Cambridge

Thermo-mechanical Stability of Plasma Sprayed Thermal Barrier Coatings in Gas Turbines
When Nov 04, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865 283302
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Thermal barrier coatings (TBCs) are key elements in the development of improved gas turbine efficiency.  A brief summary will be given of the background to this.  Their stability, and particularly their resistance to spallation (debonding), is critical to wider and more effective usage.  A central problem is that they tend to undergo sintering at service temperatures (~1200-1400˚C), giving enhanced stiffness, which leads to higher stresses, and hence a greater driving force for spallation, during differential thermal contraction on cooling [1].  These changes can be accelerated by the presence of impurities that segregate to the grain boundaries, where they enhance the solid state diffusivity [2] or, at sufficiently high concentrations, produce a vitreous phase that can dramatically accelerate sintering.  The impurities that are most likely to have such effects are often termed CMAS (calcia-magnesia-alumina-silica), which are ingested into the engine in the form of particulates and may be deposited on the coating.  Environmental deposits, such as volcanic ash, with low melting points compared to typical gas turbine entry temperatures may pose a particular threat in this respect [3].  An outline will be given of the modelling and experimental work carried out to investigate these effects, and of the implications for creation of coatings with improved stability.  The experimental work relates mostly to plasma sprayed coatings, but the main conclusions are equally applicable to those produced using the other commonly-employed technique, which is physical vapour deposition.

References

[1]        Shinozaki, M and Clyne, TW, A methodology, based on sintering-induced stiffening, for prediction of the spallation lifetime of plasma-sprayed coatings, Acta Materialia, 61 (2012) p.579-588.

[2]        Cipitria, A, Golosnoy, IO, and Clyne, TW, A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings, Acta Materialia, 57 (2009) p.980-992.

[3]        Shinozaki, M and Clyne, TW, The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings, Surface and Coatings Technology, 216 (2013) p.172-177.