Influence of additive composition on thermal and mechanical properties of delta -Si_3N_4 ceramics

摘要

Dense p-Si_3N_4 ceramics were fabricated from a-Si_3N_4 raw powder by gas-pressure sintering at 1900 deg C for 12 h under a nitrogen pressure of 1 MPa, using four different kinds of additive compositions: Yb_2O_3-MgO, Yb_2O_3-MgSiN_2, Y_2O_3-MgO, and Y_2O_3-MgSiN_2. The effects of additive composition on the microstructure and thermal and mechanical properties of delta-Si_3N_4 ceramics were investigated. It was found that the replacement of Yb_2O_3 by Y_2O_3 has no significant effect on the thermal conductivity and fracture toughness, but the replacement of MgO by MgSiN, leads to an increase in thermal conductivity from 97 to 113 Wm~(-1)K~(-1) and fracture toughness from 8 to 10 MPa m~(1/2), respectively. The enhanced thermal conductivity of the MgSiN_2-doped materials is attributed to the purification of delta-Si_3N_4 grain and increase of Si_3N_4-Si_3N_4 contiguity, resulting from the enhanced growth of large elongated grains. The improved fracture toughness of the MgSiN_2-doped materials is attributed to the increase of grain size and fraction of large elongated grains. However, the same thermal conductivity between the Yb_2O_3- and Y_2O_3-doped materials is related to not only their similar microstructures, but also the similar abilities of removing oxygen impurity in Si_3N_4 lattice between Yb_2O_3 and Y_2O_3. The same fracture toughness between the Yb_2O_3- and Y_2O_3-doped materials is consistent with their similar microstructures. This work implies that MgSiN_2 is an effective sintering aid for developing not only high thermal conductivity (>110 Wm~(-1)K~(-1)) but also high fracture toughness (>10 MPa m~(1/2)) of Si_3N_4 ceramics.