1414 A G. Evans et al Joumal of the European Ceramic Sociery 28(2008)1405-1419 EXPLODED Al2O3 GROWTH New 50μm Fig. 10. Illustration of preferred nucleation of alpha-alumina islands on hetero- eneous sites, such as scratches, on the alloy surface. Optical micrographs. △D/D exceeds the solubility limit for most viable stabilizers. The Fig. 11. A schematic indicating the elastic compression of the TGO grains implication is that all these compositions have a durability ulti- needed to accommodate the formation of alumina at the internal grain mately limited by the partitioning of the supersaturated t' into boundaries. the equilibrium phases, and the potential for undesirable trans- formation of the depleted tetragonal phase into monoclinic on ooling. 81-87 Small windows of opportunity exist in ternary systems. The composition exhibiting highest toughness resides in the ZrO2-YO15-TiO2 system having a depleted tetragonal phase that is non-transformable upon cooling. 4 An interest ing opportunity is presented by the ZrO2-YO1.5-TaO25 system trigonal field Growth and exhibits a regime in which compositions are both immune toughness comparable to(or slightly higher than)7-YSZ 25 This s, to phase separation(up to at least 1500C)and also exhibit system has yet to be synthesized in coating form and evaluated for durability Heatin compositions)is the degradation by CMAS penetration. 7288 a Rare earth zirconates, notably Gd2Zr2O7, offer a solution as they react with the CMAs melt and induce its crystallization at tem peratures well above the melting point of the original deposit. 89 Because the reaction and crystallization kinetics are competitive with that for infiltration, the process effectively seals the surface Time at Oxidation Temperature(min) against further penetration. However, RE Zirconates are typl- Fig. 12. An illustration of the stress in the TGO measured in situ in the sy cally cubic ( pyrochlore or d-phase)and exhibit the same low chrotron during a single thermal cycle from ambient to 1125%C and back to toughness as cubic zirconia compositions. Moreover, they are ambient.1414 A.G. Evans et al. / Journal of the European Ceramic Society 28 (2008) 1405–1419 Fig. 10. Illustration of preferred nucleation of alpha-alumina islands on hetero￾geneous sites, such as scratches, on the alloy surface. Optical micrographs. exceeds the solubility limit for most viable stabilizers.4 The implication is that all these compositions have a durability ulti￾mately limited by the partitioning of the supersaturated t’ into the equilibrium phases, and the potential for undesirable trans￾formation of the depleted tetragonal phase into monoclinic on cooling.81–87 Small windows of opportunity exist in ternary systems. The composition exhibiting highest toughness resides in the ZrO2–YO1.5–TiO2 system having a depleted tetragonal phase that is non-transformable upon cooling.24 An interest￾ing opportunity is presented by the ZrO2–YO1.5–TaO2.5 system wherein the tetragonal field penetrates deeply into the ternary and exhibits a regime in which compositions are both immune to phase separation (up to at least 1500 ◦C) and also exhibit toughness comparable to (or slightly higher than) 7-YSZ.25 This system has yet to be synthesized in coating form and evaluated for durability. A second major limitation of 7-YSZ (and its derivative t’ compositions) is the degradation by CMAS penetration.71,72,88 Rare earth zirconates, notably Gd2Zr2O7, offer a solution as they react with the CMAS melt and induce its crystallization at tem￾peratures well above the melting point of the original deposit.89 Because the reaction and crystallization kinetics are competitive with that for infiltration, the process effectively seals the surface against further penetration. However, RE zirconates are typi￾cally cubic (pyrochlore or -phase) and exhibit the same low toughness as cubic zirconia compositions. Moreover, they are Fig. 11. A schematic indicating the elastic compression of the TGO grains needed to accommodate the formation of new alumina at the internal grain boundaries. Fig. 12. An illustration of the stress in the TGO measured in situ in the syn￾chrotron during a single thermal cycle from ambient to 1125 ◦C and back to ambient