Determination of creep behavior of thermal barrier coatings under laser imposed high thermal and stress gradient conditi
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Determination of creep behavior of thermal barrier coatings under laser imposed high thermal and stress gradient conditions Dongming Zhu and Robert A. Miller National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44135 (Received 19 November 1997; accepted 15 March 1998)
A laser sintering/creep technique has been established to determine the creep behavior of thermal barrier coatings under steady-state high heat flux conditions. For a plasma sprayed zirconia–8 wt. % yttria coating, a significant primary creep strain and a low apparent creep activation energy were observed. Possible creep mechanisms involved include stress induced mechanical sliding and temperature and stress enhanced cation diffusion through the splat and grain boundaries. The elastic modulus evolution, stress response, and total accumulated creep strain variation across the ceramic coating are simulated using a finite difference approach. The modeled creep response is consistent with experimental observations.
I. INTRODUCTION
Plasma-sprayed ceramic thermal barrier coatings have been developed for advanced gas turbine and diesel engine components to improve engine durability and efficiency.1–5 However, the reliability of the coating systems remains a crucial issue under high temperature thermal cycling conditions. The coating failure mechanisms, which strongly depend on coating systems and operating conditions, are complex. Coating delamination and spallation might occur due to thermal expansion mismatch and bond coat oxidation in the coating system. Ceramic sintering and creep at high temperatures, resulting in coating shrinkage and through-thickness cracking during cooling,6–8 will further accelerate the coating failure process.9 The ceramic coating creep properties and creep related coating durability issues have long been recognized.10–14 In addition, changes in thermomechanical and thermophysical properties as a result of coating sintering, such as the increase in coating elastic modulus and thermal conductivity, are also detrimental to coating fatigue resistance and performance. Therefore, determination of the sintering and creep behavior of plasma-sprayed thermal barrier coatings is of great importance. Also, future development of advanced sintering and creep resistant ceramic coatings will greatly benefit from reliable test methods that can efficiently and accurately evaluate coating sintering and creep properties under simulated engine temperature and stress conditions. In this paper, a laser sintering and creep technique having a high strain measurement sensitivity is developed and applied to thermal barrier coating configurations. Because of the laser imposed temperature and stress gradients across the coating system, the sintering and creep behavior of 146
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J. Mater. Res., Vol. 14, No. 1, Jan 1999
Downloaded: 11 Mar 2015
the ceramic coating is assessed from a set of integrated temperature and stress gradient experiments that ar
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