Effect of Y2O3 content on the microstructural characteristics and the mechanical and thermal properties of Yb-doped SiAlON ceramics

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RE_xSi_12-(m+n)Al_m+nO_nN_16-n (m = 0.4, n = 1.0) composition. Yb₂O₃, the RE oxide, is the main sintering additive. For RE_x composition design with (Yb_1-y + Y_y)_x, Yb₂O₃ is replaced by Y₂O₃ (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb₂O₃ has excellent high-temperature stability, it is limited by its microstructural characteristics, that is, the β-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y₂O₃ additive contents substituting for Yb₂O₃. The average grain width decreases, and the equiaxed β-SiAlON grains are elongated with increasing Y₂O₃ content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y₂O₃ content; however, the fracture toughness exhibits a significant increase (～53.6%) from 4.6 to 7.0 MPa?m^1/2. Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y₂O₃ substitution increases the α/β-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y₂O₃ content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.