Influence of the rare-earth element on the mechanical properties of RE-Mg-bearing silicon nitride

Silicon nitride bulk ceramics with varying compositions of the grain
boundary phase but similar grain sizes were developed, which allows to
analyze the influence of the grain boundary chemistry on mechanical
properties. Micrographs of the crack path reveal a much weaker
interface when the rare-earth element in the grain boundary phase
changes from a small to a large rare-earth cation (RE3+). Room
temperature measurements of toughness and bending strength show that
weaker grain boundaries result in a higher toughness, but in a
decreased strength of the bulk material. This investigation
demonstrates that the mechanical behavior of silicon nitride can be
readily tuned by chemical composition changes of the grain boundary
phase, but that a compromise between toughness and strength has to be
found when designing the material.