On the effect of local grain-boundary chemistry on the macroscopic mechanical properties of a high-purity Y2O3-Al2O3-containing silicon nitride ceramic: Role of oxygen

The effects of grain-boundary chemistry on the mechanical properties of
high-purity silicon nitride ceramics have been investigated,
specifically involving the role of oxygen, present along the grain
boundaries, in influencing the fracture behavior. To avoid
complications from inadvertently introduced impurities, studies were
performed on a high-purity Si3N4 processed using two-step
gas-pressure-MP sintering. Varying the grainboundary oxygen content,
which was achieved by control of oxidizing heat treatments and
sintering additives, was found to result in a transition in fracture
mechanism, from transgranular to intergranular fracture, with an
associated increase in fracture toughness. This phenomenon is
correlated to an oxygen-induced change in grain-boundary chemistry and
possibly to a concomitant structural transformation along the
interface. The incorporation of oxygen appears to affect fracture by
"weakening" the interface, thus facilitating debonding and crack
advance along the boundaries, and hence to toughening by grain
bridging. It is concluded that if the oxygen content in the thin
grainboundary films exceeds a lower limit, which is similar to 0.87
... mehr
equiv% oxygen content, then the interfacial structure and bonding
characteristics favor intergranular debonding during crack propagation;
otherwise, transgranular fracture ensues.