The effect of a high thermal gradient on sintering and stiffening in the top coat of a thermal barrier coating system |
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Authors: | S A Tsipas I O Golosnoy T W Clyne R Damani |
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Affiliation: | (1) Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, CB2 3QZ Cambridge, UK;(2) Sulzer Innotec, Postfach 65, CH-8404 Winterthur, Switzerland |
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Abstract: | Superalloy substrates coated with plasma-sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats (TCs) have
been subjected to a high heat flux under a controlled atmosphere. The sintering exhibited by the TC under these conditions
has been studied and compared with the behavior observed during isothermal heating. Sintering has been characterized by (a)
microstructural examinations, (b) dilatometry, in both the in-plane and through-thickness directions, and (c) stiffness measurements,
using both cantilever bending and nanoindentation. A numerical model has been used to explore the stress state under isothermal
and thermal gradient conditions. Dilatometry data indicate significant linear contractions during holding at elevated temperatures,
particularly in the through-thickness direction. This is largely attributed to microstructural changes associated with sintering,
with any volume changes due to phase transformations making relatively small contributions. Sintering proceeds faster at higher
temperatures but is retarded by the presence of tensile stresses (from differential thermal expansion between the coating
and substrate) within the TC. Thus, it occurs preferentially near the free surface of the TC under gradient conditions, not
only due to the higher temperature, but also because the in-plane stress is more compressive in that region. |
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Keywords: | high thermal gradient sintering stiffness thermal barrier coatings thermal conductivity |
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