Effects of Al 2 O 3 –Y 2 O 3 /Yb 2 O 3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot‐pressing sintering

Silicon nitride (Si3N4) ceramics doped with two different sintering additive systems (Al2O3–Y2O3 and Al2O3–Yb2O3) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si3N4 ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si3N4 ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si3N4 to β-Si3N4. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al2O3–Yb2O3 sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si3N4 grains. In addition, the density, relative density, and flexural strength of the as-prepared Si3N4 ceramics doped with Al2O3–Y2O3 were 3.25 g/cm3, 99.9%, and 1053 ± 53 MPa, respectively. When Al2O3–Yb2O3 additives were introduced, the above properties reached 3.33 g/cm3, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si3N4 ceramics are feasible by tailoring sintering additives.