Rare-earth doped Si3N4/SiC micro/nano composites – microstrucure and mechanical properties

Abstract:

Influence of different rare-earth oxide sintering additives (La2O3, Nd2O3, Y2O3, Yb2O3 and Lu2O3) on the microstructure and mechanical properties of hot-pressed silicon nitride/silicon carbide micro/nano-composites has been investigated. The results were subsequently compared to those obtained on the Si3N4 monoliths sintered with the same rare-earth oxide additives. The bimodal character of microstructures was observed in all studied materials where elongated β-Si3N4 grains were embedded in the matrix of much finer Si3N4 grains. The fracture toughness values increased with decreasing ionic radius of rare-earth elements in both monolithic and composite systems. Similarly, the hardness and bending strength values increased with decreasing ionic radius of rare-earth elements. The composite materials exibited lower fracture toughness due to their finer microstructure. The positive influence of finer microstructure of the composites on strength was not observed due to the present defects in the form of SiC clusters and non-reacted carbon zones. Wear resistance at room temperature also increased with decreasing ionic radius of rare-earth element in both kind of materials. Mechanical wear (micro-fracture) and tribochemical reaction were found as the main wear mechanisms in all investigated materials. Significantly improved creep and oxidation resistance were observed in the case of composite materials and/or materials with smaller radius of RE3+.

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