Influence of various rare-earth oxide additives on microstructure and mechanical properties of silicon nitride based nanocomposites

Influence of rare-earth oxide additives (La2O3, Nd2O3, Y2O3, Yb2O3 and Lu2O3) on the microstructure and mechanical properties of hot-pressed silicon nitride/silicon carbide micro/nanocomposites has been investigated. The results were consequently compared to those obtained on the reference monolithic Si3N4 sintered with the same additives. The composites exhibited finer microstructure compared to monolithic materials and the aspect ratio of β-Si3N4 grains increased with a decreasing ionic radius of rare-earth elements both in monolithic Si3N4 and in Si3N4–SiC nanocomposites. The hardness of both kinds of material increased with a decreasing ionic radius of rare-earth element. Fracture toughness and four-point bending strength also increased with a decreasing ionic radius of rare-earth oxide additives. The fracture toughness of monoliths was higher in comparison with composites due to the coarser microstructures which provide beneficial toughening mechanisms during crack propagation. Ceramics containing smaller size of rare-earth cations exhibited crack deflection more frequently compared to the materials doped with larger ones because of higher aspect ratio of β-Si3N4 grains. The positive influence of finer microstructure of the composites on the strength was not observed due to the present strength degrading processing flaws in their microstructures in the form of agglomerates of SiC and non-reacted carbon areas.