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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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.
Keywordshigh thermal gradient sintering stiffness thermal barrier coatings thermal conductivity
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- 4.R.A. Handoko, J.L. Beuth, G.H. Meier, F.S. Pettit, and M.J. Stiger: “Mechanisms for Interfacial Toughness Loss in Thermal Barrier Coating Systems,” Key Eng. Mater., 2001, 197, pp. 165–83.Google Scholar
- 11.D. Basu, C. Funke, and R.W. Steinbrech: “Effect of Heat Treatment on Elastic Properties of Separated Thermal Barrier Coatings,” J. Mater. Res., 1999, 14, pp. 4643–50.Google Scholar
- 16.R.A. Miller, J.L. Smialek, and R.G. Garlick: “Phase Stability in Plasma-Sprayed, Partially Stabilized Zirconia-Yttria” in Proceedings of First International Conference on the Science and Technology of Zirconia, A.H. Heuer and L.W. Hobbs, ed., American Ceramic Society, Westerville, OH, 1980, pp. 241–53.Google Scholar
- 22.J.A. Thompson and T.W. Clyne: “The Stiffness of Plasma Sprayed Zirconia Top Coats in TBCs,” in Proceedings of United Thermal Spray Conference, E. Lugscheider and P.A. Kammer, ed., DVS, Dusseldorf, Germany, 1999, pp. 835–40.Google Scholar
- 24.F. Cardarelli: Materials Handbook: A Concise Desktop Reference, Springer-Verlag, London, UK, 2000.Google Scholar
- 25.J. Malzbender and R.W. Steinbrech: “Mechanical Methods to Determine Layer Compliances Within Multilayered Composites,” J. Mater. Res., 2003, 18, pp. 1374–82.Google Scholar
- 26.S.C. Gill and T.W. Clyne: “Stress Distributions and Material Response in Thermal Spraying of Metallic and Ceramic Deposits,” Met. Trans., 1990, 21B, pp. 377–85.Google Scholar
- 27.T.W. Clyne and S.C. Gill: “Heat Flow and Thermal Contraction During Plasma Spray Deposition” in Heat Transfer in Manufacturing and Processing of New Materials, I. Tanasawa, ed., Hemisphere, New York, NY, 1991, pp. 33–48.Google Scholar
- 28.T.W. Clyne and S.C. Gill: “Residual Stresses in Thermally Sprayed Coatings and Their Effect on Interfacial Adhesion: A Review of Recent Work,” J. Thermal Spray Technol., 1996, 5, pp. 1–18.Google Scholar
- 29.Y.C. Tsui, J.A. Thompson, and T.W. Clyne: “The Effect of Bond Coat Creep on Residual Stresses and Debonding in Plasma Sprayed Thermal Barrier Systems” in Proceedings of Thermal Spray: Meeting the Challenges of the 21st Century. Proceedings of the 15th International Thermal Spray Conference, C. Coddet, ed., ASM International, Materials Park, OH, 1998, pp. 1565–70.Google Scholar
- 30.J.A. Thompson, J. Matejicek, and T.W. Clyne: “Modelling and Neutron Diffraction Measurement of Stresses in Sprayed TBCs” in Proceedings of Superalloys 2000, T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S.L. Olsen, and J.J. Schirra, ed., TMS, Warrendale, PA, 2000, pp. 639–47.Google Scholar