Influence of additive composition on thermal and mechanical properties of β–Si₃N₄ ceramics

Dense β–Si₃N₄ ceramics were fabricated from α–Si₃N₄ raw powder by gas-pressure sintering at 1900 °C for 12 h under a nitrogen pressure of 1 MPa, using four different kinds of additive compositions: Yb₂O₃–MgO, Yb₂O₃–MgSiN₂, Y₂O₃–MgO, and Y₂O₃–MgSiN₂. The effects of additive composition on the microstructure and thermal and mechanical properties of β–Si₃N₄ ceramics were investigated. It was found that the replacement of Yb₂O₃ by Y₂O₃ has no significant effect on the thermal conductivity and fracture toughness, but the replacement of MgO by MgSiN₂ leads to an increase in thermal conductivity from 97 to 113 Wm^-1K^-1and fracture toughness from 8 to 10 MPa m^1/2, respectively. The enhanced thermal conductivity of the MgSiN₂-doped materials is attributed to the purification of β–Si₃N₄ grain and increase of Si₃N₄–Si₃N₄ contiguity, resulting from the enhanced growth of large elongated grains. The improved fracture toughness of the MgSiN₂-doped materials is attributed to the increase of grain size and fraction of large elongated grains. However, the same thermal conductivity between the Yb₂O₃- and Y₂O₃-doped materials is related to not only their similar microstructures, but also the similar abilities of removing oxygen impurity in Si₃N₄ lattice between Yb₂O₃ and Y₂O₃. The same fracture toughness between the Yb₂O₃- and Y₂O₃-doped materials is consistent with their similar microstructures. This work implies that MgSiN₂ is an effective sintering aid for developing not only high thermal conductivity (>110 Wm⁻¹K⁻¹) but also high fracture toughness (>10 MPa m^1/2) of Si₃N₄ ceramics.