Crack propagation and stress distribution in binary and ternary directionally solidified eutectic ceramics

The development of directionally solidified eutectic (DSE) ceramics for gas turbine applications necessitates improving their strength and toughness. The early stage of crack propagation is investigated in either binary (Al₂O₃/Y₃Al₅O₁₂, Al₂O₃/GdAlO₃ and Al₂O₃/Er₃Al₅O₁₂) or ternary (Al₂O₃/Y₃Al₅O₁₂/ZrO₂, Al₂O₃/GdAlO₃/ZrO₂ and Al₂O₃/Er₃Al₅O₁₂/ZrO₂) DSE ceramics. Post-mortem scanning electron microscopy (SEM) examination of biaxial flexure induced cracks revealed crack deflection and branching in the various phases and in the phase boundaries. These observations are correlated to analytical and finite element (FE) internal stress calculations, FE determination of the axial shear stress component in the interfaces in the vicinity of the specimen surface (free-edge effect) and FE calculations of the stress distribution resulting from an applied loading. Results from ruby (Cr³⁺) fluorescence piezo-spectroscopy measurements are analyzed, taking into account the hydrostatic and plane stress hypotheses. Moreover, transmission electron microscopy (TEM) examinations have confirmed the role of interfaces in the crack nucleation and propagation modes.