Crack propagation in directionally solidified eutectic ceramics

Potential of innovative ceramics for turbine applications

“…The Al 2 O 3 -YAG-ZrO 2 ternary eutectic displays a fine microstructure with curved smooth interfaces ( Figure 3 right), instead of the coarser microstructure with large planar interfaces and sharp angles observed in the Al 2 O 3 -YAG binary eutectic [28][31]. In the Al 2 O 3 -YAG-ZrO 2 ternary eutectic, the cubic zirconia phase grows essentially at the interface between Al 2 O 3 and YAG, but also in the alumina phase (more clearly evidenced at a higher magnification, in Figure 4 (right), reporting crack propagation modes in the ternary eutectic).…”

“…In this respect, a biaxial disc flexure testing device has been designed and built at Onera [28]. In the directionally solidified eutectics [28][31] investigated, the essential propagation mode is transgranular crack propagation (Figure 4). A zigzag crack growth with multiple branches is observed in most cases.…”

“…Crack branching is observed, not only in one of the various phases such as ZrO 2 (split arrow in Figure 4 right), but also in the interfaces where crack deflection through debonding has occurred (split arrows in Figure 4 left). In most cases, these bifurcation mechanisms lead to stopped cracks [28][31]. Figure 4 right may be represented by a 1 µm in diameter ZrO 2 bar, bonded into a 1.5 µm thick Al 2 O 3 sleeve, surrounded by a 0.5 µm thick ZrO 2 sleeve, these three concentric cylinders being embedded into an equivalent homogeneous medium (EHM) having the diameter of the specimen under investigation and the macroscopic thermo-mechanical properties of the bulk Al 2 O 3 -YAG-ZrO 2 eutectic.…”

“…Consequently, due to the presence of a two-or three-phase 3-D interconnected microstructure, a highly detrimental crack propagation mode such as transgranular crack propagation in the brittle ceramic phases is drastically limited by energy dissipative crack deflection modes resulting from thermal mismatch stresses, the effects of Young's modulus ratios and the nature of the interfaces between the various phases [28] Ceramic materials prepared by unidirectional solidification of oxides from the melt are under investigation at Onera for applications in the aerospace field and in particular for gas turbine blades. Studies to control the microstructure of these eutectic ceramics have been performed.…”

Polarized micro‐Raman study of Al2O3‐based directionally solidified oxide eutectics containing GdAlO3 perovskite, Er3Al5O12 garnet and cubic ZrO2

“…The laser was focused on the sample through a microscope and the backscattered Table 1. The directionally solidified eutectic samples of the study [8,33] light was analyzed ('micro-Raman spectroscopy'). The laser spot diameter was limited to about 1 µm with a 100× magnification objective, which was about the size of the domains in the AG and AGZ sample and smaller than the domains in the AE sample (Fig.…”