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 engine1408 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 dura￾bility 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 non￾dense 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 struc￾ture 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) resis￾tant to inter-diffusion with the substrate, (ii) minimal strain misfit with the substrate (based on thermal expansion, phase transfor￾mations and minimal inter-diffusion-induced swelling) and (iii) 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. 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 oxida￾tion, 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 transforma￾tion 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 expan￾sion 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 funda￾mentally 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 repro￾duced 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 remain￾ing 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 pre￾sented 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