Strength, toughness, microstructure, and atomic adsorption arrangement in silicon nitrides with MgO and RE2O3 additions (RE = La, Gd, Y, Lu) were examined. Mechanical properties were high for La, Gd, and equal La-Lu additions, but surprisingly were progressively lower for Y- and Lu-doped samples. The lower strength and toughness were associated with fewer visible crack deflections and grain bridges. Detailed microstructural analysis of the Lu-doped material revealed a complex intergranular nanostructure with variable Lu content and Si3N4 nanocrystals. Furthermore, the Lu-rich areas showed an extra Lu-adsorption site on the Si3N4 prismatic planes not previously observed in other studies. This inhomogeneous structure was attributed to grain growth impingement and higher viscosity of the Lu-doped oxynitride glass that slows homogenization. The Y-doped material with nearly identical glass viscosity demonstrates intermediate behavior. Finally, substituting half of the Lu2O3 with La2O3 resulted in a homogenous intergranular structure, attributed to a lower viscosity of the oxynitride glass phase, and high mechanical properties. Overall, care must be taken when adapting Si3N4 processing parameters for the smaller ionic radius rare earth dopants such as Lu and Y.
