MICROSTRUCTURAL EVOLUTION AND MECHANICAL-PROPERTIES OF SI3N4 WITH YB2O3 AS A SINTERING ADDITIVE

Ytterbium oxide (Yb2O3) was used as a sintering aid to enhance the mec hanical properties of silicon nitride (Si3N4) ceramics, The amount of Yb2O3 had significant effects on microstructural evolution and the com position of secondary phases at the grain boundary. When the Yb2O3 add ed was less than 8 wt%, small homogeneous grains mere formed, At the g rain boundary, crystalline Yb2Si2O7 was Formed along with a glassy pha se, As the amounts of Yb2O3 mere increased to higher than 8 wt%, large elongated grains mere developed in the fine matrix. In those cases, t he grain boundary crystalline phase was changed from Yb3Si2O7 to Yb4Si 2O7N2. Mechanical properties were influenced by these changes in micro structure and grain boundary phase, The fracture toughness increased w ith the Yb2O3 content up to 8 wt% and decreased slightly thereafter Th e increase in fracture toughness tvas apparently due to the formation of the large elongated grains. When more than 5 wt% of Yb2O3 mas added , interfacial debonding energy between the elongated grains and grain boundary phase became too large, resulting in a decrease in the fractu re toughness, The room-temperature flexural strength aas not significa ntly affected by the Yb2O3 content or the microstructure, other than i re the case of 2 wt% addition. The high-temperature strength in nitrog en, however, increased steadily with Yb2O3 content. The highest streng th, 870 MPa at 1400 degrees C, was observed when 16 wt% of Yb2O3 was a dded. The increase in the high-temperature strength with Yb2O3 content was attributed to the formation of crystalline Yb4Si2O7N2 phase at th e grain boundary.