In-situ toughened α-sialon ceramics doped with Lutecia and Ytterbia

Ytterbia and Lutecia were adopted to stabilize α-sialon ceramics during a two-step hot press sintering, respectively. Although Yb³⁺ and Lu³⁺ possess the similar small ion size (0.86 Å and 0.85 Å) as two adjacent rare-earth elements, Yb- and Lu-a-sialon ceramics present very different microstructure and properties. Yb-α-sialon ceramic is fully composed of α-sialon phase without any secondary phase. The ceramic incorporated with Lu₂O₃ also consists of α-sialon phase, but a trace amount of intergranular phase J′ (Lu₄Si_2-xAl_xO_7+xN_2-x) still remains at the boundary. Microstructure observation indicates the fully equiaxial grain morphology of Yb-α-sialon ceramic. However, elongated grains appear in the Lu-α-sialon ceramic, coexisting with small equiaxed grains. Owing to the almost full α-sialon phase assemblage, both ceramics present high hardness with the value of over 21 GPa. Lu-α-sialon ceramic possesses good toughness, which is attributed to the elongated grains assisting the facilitation of toughening mechanism. The equiaxied grain morphology results in a comparatively lower toughness for Yb-α-sialon ceramic.