Study of Static Flexural Strength Anisotropy of Yttrium Calcium Oxyborate Crystals

As a promising optical and piezoelectric crystal, yttrium calcium oxyborate (YCOB) has been criticized for its easy fracture during both the cooling and machining processes. Herein, in order to evaluate the mechanical energy (G) required for crack propagation on one crystal plane, we measured the bending strength of the YCOB samples with different orientations via employing a three-point bending test. The surface topography of the polished sample was measured, and the average surface roughness is limited to 10 nm. Fracture surfaces were also observed and analyzed. The results showed that the bending strengths of Y-cut and Z-cut samples were higher than other cut directions in the XZ principal plane, showing obvious anisotropy. It can be found that the cleavage planes (2¯01) and (101) were always observed in fractured samples in multiple directions in the XZ principal plane. The contribution of the cleavage planes (2¯01) and (101) to the fracture of the specimens was discussed in detail by the theorem of energy minimum. Finally, based on the obtained scanning electron microscopy results, we propose the mechanism for crack advance in YCOB crystals. A perfectly brittle crack in a crystal prefers cleavage planes with low surface energy. It is believed that these findings may provide fresh ideas to overcome the cleavage fracture of larger size YCOB crystals.