Densification Behavior, Microstructural Development and Mechanical Properties of Spark Plasma Sintered α/β-Silicon Nitride Ceramics with Different Rare Earth Oxide Additives

Monolithic Si3N4 ceramics were fabricated by spark plasma sintering at 1750 degrees C for 15 min under 50 M Pa applied pressure using alpha-Si3N4 powder with 2 wt% Y2O3, 2 wt% Dy2O3, 2 wt% Eu2O3 and 8 wt% Y2O3 as single rare earth oxide additives. It aimed to investigate the effects of the composition and content of rare earth oxide additives on densification, microstructural development, and mechanical properties of alpha/beta-silicon nitride ceramics via XRD and SEM analyses, temperature and displacement curves obtained through spark plasma sintering and mechanical tests. Addition to 2 wt% Y2O3 as an additive into composition resulted in high relative density (98%), hardness (14.9 GPa), and fracture toughness (9.8 M Pa.m(1/2)). The use of Dy2O3 instead of Y2O3 decreased beta-Si3N4 phase ratio and densification rate. This behavior continued further with Eu2O3 addition. The increment of Y2O3 content to 8 wt% provided the same relative density and fracture toughness (98% and 9.8 M Pa.m(1/2)), whereas hardness was reduced to 12.6 GPa. As a result, densification was achieved only by Y2O3 addition regardless of additive content, and both high hardness and fracture toughness were obtained using only 2 wt% Y2O3 as a single additive.