First-principles study of rare-earth effects on grain growth and microstructure in beta-Si3N4 ceramics

Rare earth (RE) and group III oxide additions are frequently used to
optimize densification during the processing of ceramics. Silicon
nitride ceramics frequently serve as model cases, and in these systems
the effects of rare earths are important. Additions often determine the
morphology of beta-Si3N4 crystallites that grow in the multiphase
ceramic, thereby affecting the microstructure and mechanical toughness
of the ceramic. The influence of different rare earths has recently
been experimentally characterized in terms of their effects on grain
growth aspect ratios. In the study reported here, a new energy
parameter is introduced that provides a first-principles based
understanding of these effects. Grain growth aspect ratios measured for
various RE additions in silicon nitride correlate well with
corresponding differential binding energies (DBE) calculated within the
partial wave self-consistent field atomic cluster model. The DBE
provides a second-difference measure of relative site stabilities of RE
vs Si atoms in regions of variable O/N content. The physical mechanism
that underlies anisotropic grain growth is found to originate from the
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site competition between REs and Si for bonding at beta-Si3N4
interfaces and within the O-rich glass. The different segregation
strengths exhibited by rare earth elements in oxynitride glasses are
simply a reflection of their different local chemistries in O, N
environments. Elements that segregate to the prism planes of the
embedded beta-Si3N4 grains impede the attachment of Si-based silicon
nitride growth units, and the extent of this limitation leads to the
observed grain growth anisotropy.