Effects of rare‐earth oxides on microstructure, thermal conductivity, and mechanical properties of silicon nitride

Abstract For the purpose of systematically studying the effects of rare‐earth (RE) oxide additives on the microstructure and properties of silicon nitride ceramics, all the 16 members of the RE family (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, Lu, and Sc) were employed in parallel in the present study. An amount of 2 mol% of each RE oxide, in combination of 5 mol% of MgO, was used as additives for a high purity silicon powder in preparing Si 3 N 4 ceramics by the sintering of reaction‐bonded silicon nitride method. Full nitridation was achieved for all the 16 samples. A post‐sintering at 1900°C for 3 h resulted in complete densification, except for the Eu‐bearing sample whose densification was hindered by the large weight loss occurring during sintering. The effects of the 16 RE oxides on the microstructural characteristics (grain diameter, grain length, and grain aspect ratio) and the materials properties (thermal conductivity, bending strength, and fracture toughness) were examined for the sintered samples. While a clear correlation between the grain growth behavior or properties and the RE ionic size was often not found, a rough trend can occasionally be observed. It was found that some materials achieved remarkable properties. For example, the material doped with Sc 2 O 3 attained a high thermal conductivity of 122 W m −1 K −1 ; while fracture toughness of the material doped with Sm 2 O 3 was as high as 10.36 MPa m 1/2 ; the Si 3 N 4 doped with Tm 2 O 3 showed a good balance between the thermal conductivity, fracture strength and fracture toughness (118 W m −1 K −1 , 748 MPa, and 9.23 MPa m 1/2 , respectively).