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 damageA.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 character￾ized by a group of mechanisms that arise because of the strain misfits associated with the constituent materials. These mecha￾nisms 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 coat￾Fig. 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 delami￾nation surfaces. Cross sectioning indicates that it is accompanied by rumpling (or ratcheting) of the TGO, manifest as undula￾tions 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 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￾silicate (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 ero￾sion: also in the vicinity of the leading edges. CMAS damage