Effect of silicon nitride addition on the thermal and mechanical properties of magnesium silicon nitride ceramics

The effect of silicon nitride (Si3N4) addition on the sintering of magnesium silicon nitride (MgSiN2) powder has been examined using a hot-pressing technique. The MgSiN2 powder compacts with 1, 4, and 9 mol% of Si3N4 addition [sintering aid: 1 mol% ytterbium oxide (Yb2O3)] were hot-pressed at a temperature between 1550 and 1800 °C for 90 min in a nitrogen (N2) atmosphere under the pressure of 75 MPa. The relative density of a MgSiN2 compact with 4 mol% of Si3N4 addition hot-pressed at 1600 °C for 90 min showed a maximum (98.6%), although the relative density was reduced by the addition of Si3N4 at other hot-pressing temperatures. The maximum fracture toughness (6.6 MPa·m1/2) was achieved for the MgSiN2 specimen with 4 and 9 mol% of Si3N4 addition hot-pressed at 1600 °C for 90 min. This value was 2.5 times higher than that of MgSiN2 specimen without Si3N4 addition (2.6 MPa·m1/2). The improvement of the fracture toughness appeared to be due to the elongation of the Si3N4 grains, which had occurred in the presence of a liquid phase during the hot pressing. The thermal conductivity of the MgSiN2 specimen with 9 mol% of Si3N4 addition hot-pressed at 1750 °C for 90 min increased to 34.1 W m-1 K-1, which is to our knowledge, the highest among the values so far reported for MgSiN2 ceramics.