Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1405-1419 www.elsevier.comlocate/jeurceramsoc The influence of oxides on the performance of advanced gas turbines A G. Evans D R. Clarke. C G. Lev Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106-5050, USA Available online 28 January 2008 Zirconia and alumina have been successfully incorporated into turbines used for propulsion and power generation. They exert a crucial influence on the fuel efficiency. The roles of these oxides within the overall system are described, relative to those for the other constituents, and their most important properties are outlined. The mechanisms that govern their properties are presented ar hes for adjusting them in desirable directions are discussed. Opportunities for new materials with potential for superior performance are 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Alumina; Zirconia; Interfaces: Thermal properties 1. The motivation excess of 1200C)within an oxidizing environment. A single material would be incapable of satisfying these requir emen Oxides are present in turbines used for propulsion and power The viable solution is an oxide/metal multilayer(Fig. 2). The generation. Their benefits are manifest in a substantial outer oxide imparts thermal protection: while the metallic layer in the longevity of various hot section components (bond coat) affords oxidation protection through the formation technology demonstrates how oxides can be used to of a second oxide, as well as plastic accommodation of strain. -8 structural members that experience environmental extremes At the technology inception, the preferred insulating oxide Documenting the principles that underlie this success facilitates was determined to be yttria-stabilized zirconia (YSZ), chosen dissemination to other systems. The technology involves choices because of its low, temperature-invariant, thermal conductivity of materials and spatial configurations, as well as survivability(Fig 3a). The most desirable phase was ascertained by conduct upon extreme temperature cycling without loss of functionality. ing laboratory-based thermal cycle tests to seek the composition durability: that is, the lar ber of l.I. Materials and configurations cycles before the coating spalls(Fig. 3b). The outcome w 7wt. o yttria-stabilized zirconia(7-YSZ). This composition The following considerations have motivated the choice of still used. &e 1-20 It remains the material of choice because e the discovery of lower thermal conductiv materials and their spatial configurations. The thermal require- Ity opt ig.1).By directing air through other properties(especially toughness2-2)are also crucial channels, the structural alloy is internally cooled: with heat On rotating components, the layer thickness is important. It i transfer coefficient determined by the flow rate and the chan a compromise between having sufficient thickness to achieve nel geometry. Subject to a combustion temperature, Tgas, and the desired temperature drop, yet thin enough to avert exces- an external heat transfer coefficient, superposing an external sive inertial loads, due to the extra mass. The outcome is insulting oxide allows Tgas to be raised while retaining the thickness in the range 100= Htbc 3250 umOn stationary com- alloy at an allowable maximum temperature. Remarkably, insu- ponents, such as shrouds and combustors, the mass is less critical lating oxides deposited onto geometrically complex structural and much thicker layers can be used. The choice is typically, components, such as airfoils, remain attached for extended peri- 500um sTbc I mm. ods despite cycling through an enormous temperature range (in gov materials for oxidation protec tion are straightforwar with nuanced implementation (i)A thermally grown GO) forms at the bond coat sur- orresponding author. face by reaction with the combustion gas. The preferred TGO E-mail address: agevans @engineering. ucsb.edu(A G. Evans) should have the lowest possible oxygen ingress at the temper 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc 2007 12.023
Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1405–1419 The influence of oxides on the performance of advanced gas turbines A.G. Evans ∗, D.R. Clarke, C.G. Levi Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106-5050, USA Available online 28 January 2008 Abstract Zirconia and alumina have been successfully incorporated into turbines used for propulsion and power generation. They exert a crucial influence on the fuel efficiency. The roles of these oxides within the overall system are described, relative to those for the other constituents, and their most important properties are outlined. The mechanisms that govern their properties are presented and approaches for adjusting them in desirable directions are discussed. Opportunities for new materials with potential for superior performance are assessed. © 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Alumina; Zirconia; Interfaces; Thermal properties 1. The motivation Oxides are present in turbines used for propulsion and power generation. Their benefits are manifest in a substantial increase in the longevity of various hot section components.1–8 The technology demonstrates how oxides can be used to protect structural members that experience environmental extremes. Documenting the principles that underlie this success facilitates dissemination to other systems. The technology involves choices of materials and spatial configurations, as well as survivability upon extreme temperature cycling without loss of functionality. 1.1. Materials and configurations The following considerations have motivated the choice of materials and their spatial configurations. The thermal requirements are straightforward (Fig. 1). By directing air through channels, the structural alloy is internally cooled: with heat transfer coefficient determined by the flow rate and the channel geometry. Subject to a combustion temperature, Tgas, and an external heat transfer coefficient, superposing an external insulting oxide allows Tgas to be raised while retaining the alloy at an allowable maximum temperature. Remarkably, insulating oxides deposited onto geometrically complex structural components, such as airfoils, remain attached for extended periods despite cycling through an enormous temperature range (in ∗ Corresponding author. E-mail address: agevans@engineering.ucsb.edu (A.G. Evans). excess of 1200 ◦C) within an oxidizing environment. A single material would be incapable of satisfying these requirements. The viable solution is an oxide/metal multilayer (Fig. 2). The outer oxide imparts thermal protection: while the metallic layer (bond coat) affords oxidation protection through the formation of a second oxide, as well as plastic accommodation of strain.1–8 At the technology inception, the preferred insulating oxide was determined to be yttria-stabilized zirconia (YSZ), chosen because of its low, temperature-invariant, thermal conductivity9 (Fig. 3a). The most desirable phase was ascertained by conducting laboratory-based thermal cycle tests to seek the composition affording greatest durability: that is, the largest number of cycles before the coating spalls (Fig. 3b).16 The outcome was 7 wt.% yttria-stabilized zirconia (7-YSZ). This composition is still used, despite the discovery of lower thermal conductivity options.4,17–20 It remains the material of choice because other properties (especially toughness21–25) are also crucial. On rotating components, the layer thickness is important. It is a compromise between having sufficient thickness to achieve the desired temperature drop, yet thin enough to avert excessive inertial loads, due to the extra mass. The outcome is thickness in the range 100 ≤ Htbc ≤ 250m. On stationary components, such as shrouds and combustors, the mass is less critical and much thicker layers can be used. The choice is typically, 500m ≤ Htbc ≤ 1 mm. The principles governing the materials for oxidation protection are straightforward: albeit with nuanced implementation. (i) A thermally grown oxide (TGO) forms at the bond coat surface by reaction with the combustion gas. The preferred TGO should have the lowest possible oxygen ingress at the temper- 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.12.023
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 temperature
1406 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 competitive 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 residual 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 oxidation 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 cannot 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 external 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 formation 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 constituents. 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 difference is about, αtbc − αsub ≡ αtbc ≈ −3 ppm/K. To prevent spontaneous delamination due to this misfit, the in-plane modulus 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 expansion misfit. If the YSZ were fully dense (Etbc = 200 GPa, vtbc ≈ 0.2), for a typical value of the average temperature
A.G. Evans et al. Journal of the European Ceramic Society 28(2008)1405-1419 1407 EXPLODED DESIGNATION REQUIREMENTS VIEW T, >T THERMAL Low Thermal Conductivity. OXIDE Microstructural Stability Chemical Compatibility MAy30MAYMAT2 Scale Change -A2O3 THERMALLY Minimum Thickness GROWN OXIDE TGO) Controlled defects Adherent With BC TGO Chemically Homogeneous orms o-A22O Devoid Of Segregants Creep Resistant. (b) TBC ond Coat Superalloy 50u on zone 20 um Fig. 2. An exploded view of the tri-layer thermal barrier system indicating the functionalities of each of the layers. Cross sections of actual systems are included
A.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 1407 Fig. 2. An exploded view of the tri-layer thermal barrier system indicating the functionalities of each of the layers. Cross sections of actual systems are included
A G. Evans et al Joumal of the European Ceramic Sociery 28(2008)1405-1419 lay with ideal"bond coat would have the following attributes: (i)resis- tant to inter-diffusion with the substrate, (ii) minimal strain misfit YSZ [101 with the substrate(based on thermal expansion, phase transfor- mations and minimal inter-diffusion-induced swelling) and (iii) EB-PVD YSZ [11 high creep strength with adequate ductility. All of these pref erences cannot be realized simultaneously. The challenge has been to identify those attributes having the greatest importance Gdzr207[11 La2zr2o7 2.3. Thermally grown oxide EB-PVD Gd2Zr2O7 [11] The characteristics of the TGO are controlled largely by the nd coat microstructure and microchemistry, but modulated 00 by impurities, water vapor and dopants. Upon initial oxida tion, transient phases of alumina generally form. Later, these 2500 convert into a-Al2O3. -s The bond coats used in practice develop this phase at a relatively early stage within the cyclic life, minimizing adverse influences of the phase transforma- tion on durability. As the a-Al2O3 layer grows, it develops a small(but significant) compressive stress. 54.5 Upon cooling. the compression increases dramatically, due to thermal expan sion misfit with the substrate: atgo -asub Aatgo -7 ppm/K, 1500 such that igo N-4 GPa at ambient 56-58 Consequently, even though the TGo may be relatively thin at the end of the cyclic life (htgo 6 um), the energy stored/area is quite large. Utgo=tgohtgo/2Etgo 80J/m- and contributes substantiall to the potential for delamination at the tgo/bond coat interface (Fig. 5) 2.4. Inte 70 While interfaces between metals and oxides involve funda mentally strong(covalent and ionic)bonds, 9-6I their adhesion mole fraction YO,5 can be compromised by minor impurities(S is especially detrimental). To inhibit such degradation, there has been a (a)The thermal conductivity of several insulating, ternary oxides as a long history in the industry of systematically lowering the S of temperature. ( b)A binary phase diagram for the ZrOz-YO1s level in superalloys, as well as using selected alloy additions(Y, nowing the phases expected. A line representative of the cyclic dura- Pt, Hf, etc. ) to tie-up remnant S drop(ATN1100C), the residual stress at ambient would 3. Mechanisms limiting the durability of hot section be,oR≈Etc△the△T(l-h)≈-8GPa. For thickness, components hc100μm,. the stored energy/area,Ubc≡哏hh/2Ebc2 160J/m", would substantially exceed the mode I toughness (rtbe 45 J/m2 for 7-YSZ2), rendering the system prone systems was the difficulty in realizing laboratory tests that repro- to spontaneous delamination. To obviate this problem, duced the conditions that arise in an operating turbine. Furnace deposition methods have been developed that create a non- cycle and burner rig tests were widely used, but the spalling dense microstructure with appreciably lower in-plane modulus, mechanisms were not always representative of those found in Ebc<50 GPa. 40,4 In this modulus range, the stored energy airfoils, shrouds or combustors removed from actual engine becomes of order the toughness(typically, Utbc 45 J/m2 for service. As the body of information acquired on component Htbe= 150 um), enabling implementation. The columnar struc- accumulated, this concern became less problematic. A remain- ture developed by EB-PvD is especially effective. 42 ing issue is the merit of purported failure mechanisms presented in the literature, obtained on specimens tested in a laboratory 2.2 Bond coat setting. To eliminate the concern, each of the mechanisms pre- sented below has been carefully scrutinized and correlated with The relationships between the properties of the bond coat and engine experience. Namely, the mechanisms are those that the ystem durability are much more nuanced, because of the highly authors deem reproducible and verifiable, on the basis of engine
1408 A.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 Fig. 3. (a) The thermal conductivity of several insulating, ternary oxides as a function of temperature.10–14(b) A binary phase diagram for the ZrO2–YO1.5 system showing the phases expected.15 A line representative of the cyclic durability is superposed.16 drop (T ≈ 1100 ◦C), the residual stress at ambient would be, σR ≈ EtbcαtbcT/(1 − vtbc) ≈ −0.8 GPa. For thickness, Htbc ≥ 100m, the stored energy/area, Utbc ≡ σ2 RHtbc/2Etbc ≥ 160 J/m2, would substantially exceed the mode I toughness (Γ tbc ≈ 45 J/m2 for 7-YSZ23), rendering the system prone to spontaneous delamination.22 To obviate this problem, deposition methods have been developed that create a nondense microstructure with appreciably lower in-plane modulus, Etbc ≤ 50 GPa.40,41 In this modulus range, the stored energy becomes of order the toughness (typically, Utbc ≈ 45 J/m2 for Htbc = 150m), enabling implementation. The columnar structure developed by EB-PVD is especially effective.42 2.2. Bond coat The relationships between the properties of the bond coat and system durability are much more nuanced, because of the highly non-linear interplay with the substrate and the TGO.36,37 The “ideal” bond coat would have the following attributes: (i) resistant to inter-diffusion with the substrate, (ii) minimal strain misfit with the substrate (based on thermal expansion, phase transformations and minimal inter-diffusion-induced swelling) and (iii) high creep strength with adequate ductility. All of these preferences cannot be realized simultaneously. The challenge has been to identify those attributes having the greatest importance. 2.3. Thermally grown oxide The characteristics of the TGO are controlled largely by the bond coat microstructure and microchemistry, but modulated by impurities, water vapor and dopants. Upon initial oxidation, transient phases of alumina generally form. Later, these convert into -Al2O3. 46–53 The bond coats used in practice develop this phase at a relatively early stage within the cyclic life, minimizing adverse influences of the phase transformation on durability. As the -Al2O3 layer grows, it develops a small (but significant) compressive stress.54,55 Upon cooling, the compression increases dramatically, due to thermal expansion misfit with the substrate: αtgo − αsub ≡ αtgo ≈ −7 ppm/K, such that σtgo ≈ −4 GPa at ambient.56–58 Consequently, even though the TGO may be relatively thin at the end of the cyclic life (htgo ≈ 6m), the energy stored/area is quite large, Utgo = σ2 tgohtgo/2Etgo ≈ 80 J/m2 and contributes substantially to the potential for delamination at the TGO/bond coat interface (Fig. 5). 2.4. Interfaces While interfaces between metals and oxides involve fundamentally strong (covalent and ionic) bonds,59–61 their adhesion can be compromised by minor impurities (S is especially detrimental).61 To inhibit such degradation, there has been a long history in the industry of systematically lowering the S level in superalloys, as well as using selected alloy additions (Y, Pt, Hf, etc.) to tie-up remnant S. 3. Mechanisms limiting the durability of hot section components An early challenge in the implementation of thermal barrier systems was the difficulty in realizing laboratory tests that reproduced the conditions that arise in an operating turbine. Furnace cycle and burner rig tests were widely used, but the spalling mechanisms were not always representative of those found in airfoils, shrouds or combustors removed from actual engine service. As the body of information acquired on components accumulated, this concern became less problematic. A remaining issue is the merit of purported failure mechanisms presented in the literature, obtained on specimens tested in a laboratory setting. To eliminate the concern, each of the mechanisms presented below has been carefully scrutinized and correlated with engine experience. Namely, the mechanisms are those that the authors deem reproducible and verifiable, on the basis of engine
A.G. Evans et al. Journal of the European Ceramic Society 28(2008)1405-1419 1409 NT verging Cracks m N RUMPLING/RATCHETING EDGE-DELAMINATIONVOID FORMATION RINS Bond Coat .Delamination IMPACT DAMAGE Substrate MOLTEN DEPOSITS Fig 4. A summary of the various mechanisms that can cause spalling of the TBC on turbine airfoils. The intrinsic mechanisms are governed by strain misfits between the constituent layers upon thermal cycling. The extrinsic mechanisms are determined by external factors. Also shown at the left is an airfoil removed from engine ervice that contains various spalled regions. experience. They reside within two basic categories: intrinsic ings, three different intrinsic mechanisms have been identified, and extrinsic(Fig. 4). Those in the intrinsic category are not differentiated in terms of the surface exposed by the spall (i)Or especially sensitive to the presence of a thermal gradient in the mechanism exposes zirconia and some alumina on both delani- component and vice versa. The intrinsic category is character- nation surfaces. Cross sectioning indicates that it is accompanied ized by a group of mechanisms that arise because of the strain by rumpling (or ratcheting) of the TGO, manifest as undt misfits associated with the constituent materials. These mecha- tions that, locally, penetrate into the bond coat(Fig. 6). 62-64 nisms can often be reproduced in well-executed furnace cycle This mechanism arises primarily in systems with B-phase bond and burnerrig tests The failures are ultimately manifest as spalls, coats. (ii) A second exposes the bond coat, with periodic islands usually present in hot sections. In systems with EB-PVD coat- of TGO and some entrained zirconia. The bond coat exhibits imprints of the grains in the TGO, suggesting brittle failure by loss of adhesion at the metal/oxide interface. Cross sections affirm that the failure occurs primarily by delamination along the interface, with local extension through thickness heterogeneities in the tGo 1)A third but now with superposed features indicative of voids formed times 65 All intrinsic mechanisms have a characteristic Toughness TGO thickness, herit, at the incidence of spalling. However, hcrit depends on the bond coat composition and microstructure, as well as the thermal cycling history. In itself, it is not an useful metric for characterizing failure across a range of bond coats Delamination and cycling scenarios. The extrinsic category cannot be repro- duced in furnace cycling or conventional burner rig tests. The mechanisms include damage induced by particle impact(ero- sion and foreign object damage),66-69 delaminations enabled by the penetration of deposits of calcium-magnesium-alumino- Delamination silicate(CMAS) formed from the ingress into the engine of sands and dust in the atmosphere-as well as those introduced by thermal gradients. All are dominated by the microstructure and properties of the insulating oxide. The manifestations in TGO Thickness, htgo(um turbine hardware are as follows. Foreign object damage(FOD) Fig. 5. The energy release rates for delamination along either the TGO/bond is apparent as spalls at the leading edges of airfoils. Less severe ce, as a function of TGO thickness, or internally, within the TBC. particle impacts cause the gradual thinning of the TBC,by ero- is an estimate of the mode ll toughness of the interface. sion: also in the vicinity of the leading edges. CMAS damage
A.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 1409 Fig. 4. A summary of the various mechanisms that can cause spalling of the TBC on turbine airfoils. The intrinsic mechanisms are governed by strain misfits between the constituent layers upon thermal cycling. The extrinsic mechanisms are determined by external factors. Also shown at the left is an airfoil removed from engine service that contains various spalled regions. experience. They reside within two basic categories: intrinsic and extrinsic (Fig. 4). Those in the intrinsic category are not especially sensitive to the presence of a thermal gradient in the component and vice versa. The intrinsic category is characterized by a group of mechanisms that arise because of the strain misfits associated with the constituent materials. These mechanisms can often be reproduced in well-executed furnace cycle and burner rig tests. The failures are ultimately manifest as spalls, usually present in hot sections. In systems with EB-PVD coatFig. 5. The energy release rates for delamination along either the TGO/bond coat interface, as a function of TGO thickness, or internally, within the TBC. Also shown is an estimate of the mode II toughness of the interface. ings, three different intrinsic mechanisms have been identified, differentiated in terms of the surface exposed by the spall. (i) One mechanism exposes zirconia and some alumina on both delamination surfaces. Cross sectioning indicates that it is accompanied by rumpling (or ratcheting) of the TGO, manifest as undulations that, locally, penetrate into the bond coat (Fig. 6).62–64 This mechanism arises primarily in systems with -phase bond coats. (ii) A second exposes the bond coat, with periodic islands of TGO and some entrained zirconia. The bond coat exhibits imprints of the grains in the TGO, suggesting brittle failure by loss of adhesion at the metal/oxide interface. Cross sections affirm that the failure occurs primarily by delamination along the interface, with local extension through thickness heterogeneities in the TGO (Fig. 7).31 (iii) A third exposes the bond coat, but now with superposed features indicative of voids formed at longer times.65 All intrinsic mechanisms have a characteristic TGO thickness, hcrit, at the incidence of spalling. However, hcrit depends on the bond coat composition and microstructure, as well as the thermal cycling history. In itself, it is not an useful metric for characterizing failure across a range of bond coats and cycling scenarios. The extrinsic category cannot be reproduced in furnace cycling or conventional burner rig tests. The mechanisms include damage induced by particle impact (erosion and foreign object damage),66–69 delaminations enabled by the penetration of deposits of calcium–magnesium–aluminosilicate (CMAS) formed from the ingress into the engine of sands and dust in the atmosphere70–72 as well as those introduced by thermal gradients.21 All are dominated by the microstructure and properties of the insulating oxide. The manifestations in turbine hardware are as follows. Foreign object damage (FOD) is apparent as spalls at the leading edges of airfoils. Less severe particle impacts cause the gradual thinning of the TBC, by erosion: also in the vicinity of the leading edges. CMAS damage