Microstructures of Microwave‐Sintered Silicon Nitride with Zirconia as Secondary Particulates

<jats:p> <jats:bold> A microwave‐sintered Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> –Y <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> –MgO system containing ZrO <jats:sub>2</jats:sub> particulates was characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although the starting powder was α‐Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> with Y <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> , MgO, and ZrO <jats:sub>2</jats:sub> dopants, the sintered crystalline phases, analyzed by XRD, are mainly β‐Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> with minor N‐Melilites (Y <jats:sub>2</jats:sub> Si <jats:sub>3</jats:sub> O <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> ). TEM micrographs show that the N‐Melilites are distributed at the multigrain junctions, suggesting they are formed by crystallization from a liquid phase. SEM analysis shows that Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> grains are interlocked with each other. A large amount of secondary phase (13 vol%), containing Si–Y–Mg–O–N elements as identified by energy‐dispersive X‐ray spectrometer is present in the multigrain junctions and grain boundaries. </jats:bold> </jats:p> <jats:p> <jats:bold> TEM analysis found that small ZrO <jats:sub>2</jats:sub> grains ( <jats:italic>d</jats:italic> <200 nm) are frequently embedded in Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> grains, while others remain as dispersoids and grains dispersed in the matrix. The addition of ZrO <jats:sub>2</jats:sub> as a susceptor for microwave sintering increases the heating rate. However, there appeared to be very minor dissolution of ZrO <jats:sub>2</jats:sub> in the oxynitride liquid during sintering. Both tetragonal and cubic Zr‐rich phases were identified by selected‐area electron diffraction, which supports the existence of N‐stabilizing ZrO <jats:sub>2</jats:sub> . Microwave sintering favored the formation of the Si–Mg–Y oxynitride liquid phase and thus enhanced the dissolution of α‐Si <jats:sub>3</jats:sub> N <jats:sub>4</jats:sub> . </jats:bold> </jats:p>