Effects of processing method and additive composition on microstructure and thermal conductivity of Si3N4 ceramics

A comparative investigation of sintered reaction-bonded silicon nitride (SRBSN) and sintered silicon nitride (SSN) was carried out by using two additive compositions of Y2O3–MgO and Yb2O3–MgO. The effects of additive composition on the processing, microstructure and thermal conductivity of both SRBSN and SSN materials were evaluated. It is shown that the replacement of Yb2O3 by Y2O3 has no significant effect on the densification, microstructure and thermal conductivity of SSN materials, but it has a large effect on the processing, microstructure and thermal conductivity of SRBSN materials. The replacement of Yb2O3 by Y2O3 results in highly bimodal microstructural development during post-sintering and this may be attributed to the formation of large rodlike β-nuclei during nitridation. Although the replacement of Yb2O3 by Y2O3 leads to the production of fully dense SRBSN materials, but it leads to a decrease in thermal conductivity, which is consistent with microstructural difference. More importantly, the present work implies that SRBSN materials with comparable thermal conductivity to SSN materials could be achieved.