The purpose of the study was to investigate densification, phase composition, microstructure and properties of mullite–ZrO2 composite ceramics with the Si3N4 additive, sintered by various methods: conventional sintering, spark-plasma sintering, and sintering in both microwave and solar furnaces. The strength properties (compressive strength and elastic modulus) of the ceramic samples sintered by the conventional method and by spark-plasma sintering were compared. Some reasons for the formation of defects in the samples after sintering in microwave and solar furnaces are indicated