Thermal-Barrier Coatings for Advanced Gas-Turbine Engines
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Coatings for Advanced GasTurbine Engines
Dongming Zhu and Robert A. Miller Introduction Ceramic thermal-barrier coatings (TBCs) have received increasing attention for gasturbine engine applications. The advantages of using TBCs include increased fuel efficiency by allowing higher gas temperatures and improved durability and reliability from lower component temperatures. As illustrated in Figure 1, TBCs can provide effective heat insulation to engine components, thus allowing higher operating temperatures and reduced cooling requirements. A typical two-layer TBC system consists of a porous ZrO2-Y2O3 ceramic top coat and an oxidation-resistant metallic bond coat. These TBC systems can be applied to the metal substrate either by plasma spray or by electron-beam physical vapor deposition (EB-PVD) techniques. Durability issues for TBCs under hightemperature, cyclic conditions are still of major concern, especially as future engine temperatures increase. Coatingdelamination failure is closely related to the thermal stresses in the coating systems as well as the oxidation of the bond coat and substrate. Coating failure may be further accelerated by shrinkage-cracking and elastic-modulus increase from ceramic sintering and creep with an associated density increase at high temperatures. In general, coating delamination failure can occur when the failure driving force is greater than the resistance (Figure 2). Note that in a TBC system, the driving force for coating delamination increases while the resistance decreases with time, due to time- and temperature-dependent processes. In order to fully utilize the potential capabilities of TBCs, advanced coatingdesign tools are essential. It is important to investigate the thermophysical and thermomechanical properties of coatings under MRS BULLETIN/JULY 2000
near-realistic temperature and stressgradient conditions. The dynamic properties of the coating and the failure information will become a basis for establishing coating-life prediction models. The purpose of this article is to address some of the critical issues surrounding TBCs, such as ceramic sintering and creep, bond-coat oxidation, the effects of thermal cycling, and their relevance to coating-life prediction. Experimental testing techniques have been developed to characterize these TBC properties and to investigate the fail-
ure mechanisms of the coatings. Emphasis is placed on the dynamic changes of the coating thermal conductivity and elastic modulus, fatigue and creep interactions, and resulting failure mechanisms during simulated engine tests.
Life Prediction Issues and Property Testing of Thermal-Barrier Coatings Sintering and Creep Ceramic-coating sintering and creep at high temperatures are among the most important issues for the development of advanced TBCs. It has long been recognized that high-temperature coating sintering and creep effects are profound, and they are detrimental to coating performance.1–8 Not only can sintering and creep result in considerable coating thermal conductivity and elastic-modul
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