A.G. Evans et al. Journal of the European Ceramic Society 28(2008)1405-1419 1413 um 1 um REO Fig. 9. Illustration of the same area of an alumina TGO on re-oxidation after smoothly polishing the TGO formed in the first oxidatio New oxide forms the grain boundaries of t ally formed TGO and the amount increases with further oxidation. Schematic of the counter-diffusion of O and Al along the grain boundaries leading to a thickening of the oxide above and below the oxide on either side of the boundary. In between grain boundaries, the oxide is signifi inner as indicated by the arrows. ferred compositions within this range are those at the lower dense 7-YSZ) are representative: consistent with a microstruc- end, pre-eminently exemplified by 7-YSZ. Typically, the mode ture comprising columns bonded at periodic attachment sites. I toughness is in the range, 40<I<50J/m223-25,76: sufficient Yielding. The ability of the layer to yield when impacted by for to prevent spalling after manufacturing and to suppress large- eign objects in the engine is a significant attribute. The plastic scale delamination when exposed to thermal gradients. Such response upon impact at high temperature is reflected in the rel- compositions are ferro-elastic 77-79 Upon crack extension, dissi- atively low yield strength of 7-YSZ above 900C(Fig. 18a) pation occurs through the formation(or switching)of nano-scale The associated plastic dissipation serves to absorb much of the domains, resulting in toughening that scales with the tetrago- kinetic energy from the impact and thereby, diminish the ampli nality, c/a, and the coercive stress 2324. The concept that tude of the elastic waves that propagate through the layer2 Also the tetragonality governs the toughness of tetragonal oxides has of interest is the role of yielding in the development of kink bands led to the development of new compositions with appreciably in systems having columnar microstructure(Fig. 18b). The plas- higher toughness than7-YSZ24 25.80 The most prominent exam- tic bending of the columns is apparent, as well as the incidence ple resides within the ZrO2-YO1 5-TiO2 ternary phase field of cracks wherever the bending induces large tensile strains.68. 76 ig. 16), having toughnes 24 As engine temperatures continue to increase, two factors lus. Given that the in-plane modulus of the layer is so important, will inevitably limit the capability of 7-YSZ and tetragonal it is remarkable that measurements are still sparse. The only compositions derived from it. One limitation arises from the comprehensive results are those published by Johnson et al. 40 metastable nature of non-transformable t'phases, since the Values in the range 20<E<40 GPa(compared with 200 GPa for amount of solute required for non-transformability typicallyA.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 1413 Fig. 9. Illustration of the same area of an alumina TGO on re-oxidation after smoothly polishing the TGO formed in the first oxidation step. New oxide forms along the grain boundaries of the initially formed TGO and the amount increases with further oxidation. Schematic of the counter-diffusion of O and Al along the TGO grain boundaries leading to a thickening of the oxide above and below the oxide on either side of the boundary. In between grain boundaries, the oxide is significantly thinner as indicated by the arrows. ferred compositions within this range are those at the lower end, pre-eminently exemplified by 7-YSZ. Typically, the mode I toughness is in the range, 40 < Γ < 50 J/m2 23–25,76: sufficient to prevent spalling after manufacturing and to suppress large￾scale delamination when exposed to thermal gradients. Such compositions are ferro-elastic.77–79 Upon crack extension, dissi￾pation occurs through the formation (or switching) of nano-scale domains, resulting in toughening that scales with the tetrago￾nality, c/a, and the coercive stress.23,24,77–79 The concept that the tetragonality governs the toughness of tetragonal oxides has led to the development of new compositions with appreciably higher toughness than 7-YSZ.24,25,80 The most prominent exam￾ple resides within the ZrO2–YO1.5–TiO2 ternary phase field (Fig. 16), having toughness, Γ ≈ 90 J/m2 24 (Fig. 17). Modu￾lus. Given that the in-plane modulus of the layer is so important, it is remarkable that measurements are still sparse. The only comprehensive results are those published by Johnson et al.40 Values in the range 20 < E < 40 GPa (compared with 200 GPa for dense 7-YSZ) are representative: consistent with a microstruc￾ture comprising columns bonded at periodic attachment sites. Yielding. The ability of the layer to yield when impacted by for￾eign objects in the engine is a significant attribute. The plastic response upon impact at high temperature is reflected in the rel￾atively low yield strength of 7-YSZ above 900 ◦C (Fig. 18a). The associated plastic dissipation serves to absorb much of the kinetic energy from the impact and thereby, diminish the ampli￾tude of the elastic waves that propagate through the layer.21 Also of interest is the role of yielding in the development of kink bands in systems having columnar microstructure (Fig. 18b). The plas￾tic bending of the columns is apparent, as well as the incidence of cracks wherever the bending induces large tensile strains.68,76 As engine temperatures continue to increase, two factors will inevitably limit the capability of 7-YSZ and tetragonal compositions derived from it. One limitation arises from the metastable nature of non-transformable t’ phases, since the amount of solute required for non-transformability typically