Remaining Fatigue Life Assessment of Plasma Sprayed Thermal Barrier Coatings
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Philippe Robin, Franc¸ois Gitzhofer, Pierre Fauchais, and Maher Boulos (Submitted September 15, 2009; in revised form February 18, 2010) Ceramic functional coatings are frequently applied to structural materials, covering a wide range of thermomechanical and electrochemical applications. The main limiting feature is their reliability when subjected to cyclic transient thermal stresses. The study described in this article is a continuation of earlier research study focused on acoustic emission (AE) monitoring of the thermomechanical aging effects in ceramic coatings. Here, emphasis is placed on the usefulness of combining AE short-term monitoring with finite element modeling (FEM) to predict the performance of such coatings when subjected to cyclic thermal loads. The FEM study presented in this article is based on a three-dimensional, time-dependent approach, of the stress fields that developed within the coatings during the postdeposition cooling step and the thermal cycling. Experiments were conducted using yttrium-stabilized zirconia (YSZ) and Alumina (Al2O3) ceramic coatings combined with a NiCr-based intermetallic bond coat.
Keywords
acoustic emission, alumina, finite element modelling, thermal barrier coating (TBC), thermal cycling, yttrium-stabilized zirconia (YSZ)
1. Introduction Oxide ceramic materials commonly show specific properties of relatively high hardness, high passive resistance to chemical and thermo-chemical aggressions, and low thermal conductivity. Therefore, oxide ceramics are widely used as protective coatings applied to various underlying alloys. Among them, thermal barrier coatings (TBCs) applied to the hot sections of gas turbines, aero-engines, or diesel engines, are extensively used. TBCs are generally subjected to cyclic thermal shocks, which mainly consist in a transient thermal stress, resulting from a transient thermal gradient, applied to the material. In general, the worst conditions develop during engine cooling, either when the power is reduced or the engine is shut down. Since most coated components are cooled from their backside during operation of the engine, a thermal gradient applies previously to any transient conditions (Ref 1). A recent study, through presenting the delaminating mechanisms in coatings that experience thermal
Philippe Robin, Centro de Nanociencias y Nanotecnologı´a (CNyN), Universidad Nacional Auto´noma de Me´xico, Ensenada, Mexico; Franc¸ois Gitzhofer, Centre de Recherche en Energie Plasma et Electrochimie (CREPE), Universite´ de Sherbrooke, Sherbrooke, Canada; Pierre Fauchais, Sciences des Proce´de´s Ce´ramiques et Traitements de Surface (SPCTS), Universite´ de Limoges, Limoges, France; and Maher Boulos, Tekna Plasma Systems Inc., Sherbrooke, Canada. Contact e-mail: philippe.c. [email protected] and [email protected].
Journal of Thermal Spray Technology
gradients, has shown that delaminating within the oxide layer can be explained by the presence of significant stress gradient within the coating. It has also been shown in this article that bo
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