[en] Mixed additions of Sm₂O₃ and Y₂O₃ were used in the formation of zirconia-containing O'-sialon composites, where Sm₂O₃ was used for the purpose of densification and Y₂O₃ for zirconia stabilization. Dense ZrO₂/O' -sialon composites were produced at 1500 deg C. by sintering in nitrogen for 4 hours. Tetragonal zirconia remained in the product and t→m transformation was observed when the sample was ground into powder. However, the improvement in fracture toughness was not significant and this was attributed to the weakened transformability of the tetragonal zirconia phase in the nitrogen-based materials. 18 refs., 4 tabs., 2 figs