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 RExSi12-(m+n)Alm+nOnN16-n (m = 0.4, n = 1.0) composition. Yb2O3, the RE oxide, is the main sintering additive. For REx composition design with (Yb1-y + Yy)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⋅m1/2. 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.