Crack propagation in directionally solidified eutectic ceramics

Improving the strength and toughness of directionally solidified eutectic ceramics is essential for practical applications. In this respect, a biaxial disc flexure testing device has been designed. This system, which presents the ring supported/ring loaded test geometry, is essentially used as a means for the nucleation and early propagation of cracks, with the flexure test being stopped as soon as a load drop is detected in the loading curve. The composites under investigation are either binary (Al2O3/Y3Al5O12, Al2O3/Er3Al5O12 and Al2O3/GdAlO3) or ternary (Al2O3/Y3Al5O12/ZrO2, Al2O3/Er3Al5O12/ZrO2 and Al2O3/GdAlO3/ZrO2) eutectics. Examination of cracks is performed in field emission gun-scanning electron microscope on the polished surface of the biaxial bending test specimens, with attention being paid to the possibility of crack deflection in the various phases and in the phase boundaries, a phenomenon which may markedly improve the toughness of these eutectic ceramics. These observations are correlated to internal stress calculations, such as normal stress acting on the phase boundaries.