Microstructural Evolution and Mechanical Properties of Si 3 N 4 with Yb 2 O 3 as a Sintering Additive

Ytterbium oxide (Yb 2 O 3 ) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si 3 N 4 ) ceramics. The amount of Yb 2 O 3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb 2 O 3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb 2 Si 2 O 7 was formed along with a glassy phase. As the amounts of Yb 2 O 3 were increased to higher than 8 wt%, large elongated grains were developed in the fine matrix. In those cases, the grain boundary crystalline phase was changed from Yb 2 Si 2 O 7 to Yb 4 Si 2 O 7 N 2 . Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb 2 O 3 content up to 8 wt% and decreased slightly thereafter. The increase in fracture toughness was apparently due to the formation of the large elongated grains. When more than 8 wt% of Yb 2 O 3 was added, interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fracture toughness. The room‐temperature flexural strength was not significantly affected by the Yb 2 O 3 content or the microstructure, other than in the case of 2 wt% addition. The high‐temperature strength in nitrogen, however, increased steadily with Yb 2 O 3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb 2 O 3 was added. The increase in the high‐temperature strength with Yb 2 O 3 content was attributed to the formation of crystalline Yb 4 Si 2 O 7 N 2 phase at the grain boundary.