Experimental evidence of the impact of rare-earth elements on particle growth and mechanical behaviour of silicon nitride

The impact of various rare-earth and related doping elements (R = Lu,
Sc, Yb, Y, Sin, La) on the grain growth anisotropy and the mechanical
properties of polycrystalline beta-silicon nitride ceramics has been
studied. Model experiments, in which Si3N4 Particles can grow freely in
an R-Si-Mg-oxynitride glass matrix, show that, with increasing ionic
radius of the additive, grain anisotropy increases due to non-linear
growth kinetics. Toughness and strength are affected by the rare-earth
element. Samples of equivalent grain sizes and morphologies yield an
increasing toughness with increasing ion size of the R3+, reflecting an
increasingly intergranular crack path. These samples are also strong
and flaw tolerant, but the trends of strength and toughness do not
exactly match. The choice of the rare-earth is essential to tailor
microstructure, interfacial strength and mechanical properties.
However, somewhat different trends for properties from IIIb and
lanthanide additives indicate that more than the R3(+) size (i.e.,
purely ionic bond strength between R3+ and its neighbours) is
important. The electronic structure of the R-element is responsible for
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the type of dopant adsorption and the properties of the interface.