Intergranular Nanostructure Effects on Strength and Toughness of Si3N4.

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. [ABSTRACT FROM AUTHOR]