Microstructural Evolution and Mechanical Properties of Si3N4 with Yb2O3 as a Sintering Additive

Abstract

Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mechanical properties of silicon nitride (Si3N4) ceramics. The amount of Yb2O3 had significant effects on microstructural evolution and the composition of secondary phases at the grain boundary. When the Yb2O3 added was less than 8 wt%, small homogeneous grains were formed. At the grain boundary, crystalline Yb2Si2O7 was formed along with a glassy phase. As the amounts of Yb2O3 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 Yb2Si2O7 to Yb4Si2O7N2. Mechanical properties were influenced by these changes in microstructure and grain boundary phase. The fracture toughness increased with the Yb2O3 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 Yb2O3 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 Yb2O3 content or the microstructure, other than in the case of 2 wt% addition. The high‐temperature strength in nitrogen, however, increased steadily with Yb2O3 content. The highest strength, 870 MPa at 1400°C, was observed when 16 wt% of Yb2O3 was added. The increase in the high‐temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at the grain boundary.