G. Evans et al. Joumal of the European Ceramic Sociery 28(2008)1405-1419 misfit differs for each of the layers. It is least important for the external oxide because this layer need not be dense: it serves only to insulate the underlying alloy and does not provide oxi- dation protection. It is designed with a microstructure having spatially configured porosity that affords low in-plane stiffness and strain tolerance 40-44 This strategy cannot be used for either the tGo or the bond coat because to serve their functions both need to be dense(minimal porosity). The TGO misfit can- can be managed by limiting its thickness. The misfits between the bond coat and substrate are more nuanced: they occur not only from thermal expansion, but also phase transformations and swelling. 45 Understanding these misfits, ascertaining their importance to system durability, and finding means to control them, has been an important research focus from engines(Fig 4). Small diameter spalls can be tolerated, because backside cooling and boundary layer effects still allow the exposed surface to be protected by the(surrounding) intact oxide. Degradation only becomes a concern after an appreciable Fig. 1. A schematic of an airfoil and a magnified view of a surface zone with area fraction of the coating has been removed. Actual spall for- the TBC and bond coat layers identified. The thermal conditions are defined. mation is preceded by smaller cracks that extend and coalesce atures of interest(900-1150oC), with correspondingly small or at the interface between the TGO and the bond coz de layer along delamination planes located either within the oxid The ensuing article highlights the roles of the oxide con- counter-diffusion of the metallic elements. (ii) The bond coat stituents. It is organized as follows. The constituent materials should have sufficient thermo-chemical compatibility with the and their salient thermo-mechanical prope rues are o structural alloy that the basic composition, microstructure and The spectrum of mechanisms governing the performance and usability of hot section components are described, thereby singular solution is an alloy thatforms a-Al2O3 upon oxidation. illuminating the oxide functionalities. With reference to these To achieve this, near its surface, the alloy must contain sufi- mechanisms. the dominant characteristics of the oxides are cient Al that the primary oxidation product is, indeed, a-Al2O discussed, with associated mechanistic understanding. In turn and, moreover, acts as a reservoir for re-formation of a-Al203 these mechanisms reveal opportunities for new research on should spallation occur. The common choices are alloys based oxides that might further enhance the fuel efficiency on Ni(Al) with various additions(such as Cr, Co, Pt, Y and hf Other requirements are more nuanced. They dictate competi 2. The constituents and their thermo-mechanical tive advantage, through key aspects of system performance and properties durability. In practice, three categories of bond coat have been implemented, differentiated by the phases present and the alloy additions.(a)One category consists of a single B-phase usually The requirements imposed on each layer( Fig. 2)dictate th constituent property attributes. In current implementations, the made by inter-diffusing Al and Pt with Ni adjacent to the surface structure and composition of the substrate and the insulating of the superalloy. 4.(b)A second consists of a two-phase y/p- oxide are largely fixed. Options exist for the bond coat, which EB-PVD 29-31(c)The third is a two-phase yhy alloy made by affect the formation of the ensuing TGO infusing Pt(and Hf) into the substrate 32.33 Systems made using 2.1. Insulating oxide ese bond coats perform differently with durability governed The thermal expansion coefficient of this layer, atbc, is appreciably lower than that for the substrate, asub: the dif- 1. 2. Performance and durability ference is about.athe-asub≡△abc≈-3ppm/K. To prevent spontaneous delamination due to this misfit, the in-plane mod- o survive extreme thermal cycling the misfit st rains between ulus of the layer, Etbe, must be controlled, as illustrated by arise due to differences in thermal expansion coefficient, as well creep in the Ysz causes it to become stress-free. Subsequer as phase transformations and inter-diffusion. They cause resid- cooling induces residual stress through the thermal expan ual stresses upon temperature cycling, which activate inelastic sion misfit. If the Ysz were fully dense(Etbe=200 GPa, mechanisms that, in turn, limit durability. The importance of the Tbc N0. 2), for a typical value of the average temperature1406 A.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 Fig. 1. A schematic of an airfoil and a magnified view of a surface zone with the TBC and bond coat layers identified. The thermal conditions are defined. atures of interest (900–1150 ◦C), with correspondingly small counter-diffusion of the metallic elements. (ii) The bond coat should have sufficient thermo-chemical compatibility with the structural alloy that the basic composition, microstructure and properties are retained for the expected life of the system. The singular solution is an alloy that forms -Al2O3 upon oxidation. To achieve this, near its surface, the alloy must contain suffi- cient Al that the primary oxidation product is, indeed, -Al2O3 and, moreover, acts as a reservoir for re-formation of -Al2O3 should spallation occur. The common choices are alloys based on Ni(Al) with various additions (such as Cr, Co, Pt, Y and Hf). Other requirements are more nuanced. They dictate competi￾tive advantage, through key aspects of system performance and durability. In practice, three categories of bond coat have been implemented, differentiated by the phases present and the alloy additions. (a) One category consists of a single -phase usually made by inter-diffusing Al and Pt with Ni adjacent to the surface of the superalloy.26–28 (b) A second consists of a two-phase, /- alloy, usually deposited onto the substrate by plasma spraying or EB-PVD.29–31 (c) The third is a two-phase / alloy made by infusing Pt (and Hf) into the substrate.32,33 Systems made using these bond coats perform differently with durability governed by different mechanisms. 1.2. Performance and durability To survive extreme thermal cycling the misfit strains between the layers must be understood and managed.34–39 These strains arise due to differences in thermal expansion coefficient, as well as phase transformations and inter-diffusion. They cause resid￾ual stresses upon temperature cycling, which activate inelastic mechanisms that, in turn, limit durability. The importance of the misfit differs for each of the layers. It is least important for the external oxide because this layer need not be dense: it serves only to insulate the underlying alloy and does not provide oxi￾dation protection. It is designed with a microstructure having spatially configured porosity that affords low in-plane stiffness and strain tolerance.40–44 This strategy cannot be used for either the TGO or the bond coat: because, to serve their functions, both need to be dense (minimal porosity). The TGO misfit can￾not be independently controlled, but its adverse consequences can be managed by limiting its thickness. The misfits between the bond coat and substrate are more nuanced: they occur not only from thermal expansion, but also phase transformations39 and swelling.45 Understanding these misfits, ascertaining their importance to system durability, and finding means to control them, has been an important research focus. Ultimately the durability is governed by spalling of the exter￾nal insulating oxide, as deduced from components removed from engines (Fig. 4). Small diameter spalls can be tolerated, because backside cooling and boundary layer effects still allow the exposed surface to be protected by the (surrounding) intact oxide. Degradation only becomes a concern after an appreciable area fraction of the coating has been removed. Actual spall for￾mation is preceded by smaller cracks that extend and coalesce along delamination planes located either within the oxide layer or at the interface between the TGO and the bond coat. The ensuing article highlights the roles of the oxide con￾stituents. It is organized as follows. The constituent materials and their salient thermo-mechanical properties are outlined. The spectrum of mechanisms governing the performance and durability of hot section components are described, thereby illuminating the oxide functionalities. With reference to these mechanisms, the dominant characteristics of the oxides are discussed, with associated mechanistic understanding. In turn, these mechanisms reveal opportunities for new research on oxides that might further enhance the fuel efficiency. 2. The constituents and their thermo-mechanical properties The requirements imposed on each layer (Fig. 2) dictate the constituent property attributes. In current implementations, the structure and composition of the substrate and the insulating oxide are largely fixed. Options exist for the bond coat, which affect the formation of the ensuing TGO. 2.1. Insulating oxide The thermal expansion coefficient of this layer, αtbc, is appreciably lower than that for the substrate, αsub: the dif￾ference is about, αtbc − αsub ≡ αtbc ≈ −3 ppm/K. To prevent spontaneous delamination due to this misfit, the in-plane mod￾ulus of the layer, Etbc, must be controlled, as illustrated by the following simple argument. At the highest temperature, creep in the YSZ causes it to become stress-free. Subsequent cooling induces residual stress through the thermal expan￾sion misfit. If the YSZ were fully dense (Etbc = 200 GPa, vtbc ≈ 0.2), for a typical value of the average temperature