On the Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High-Purity Y₂O₃-Al ₂O₃-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 Si ₃N ₄ processed using two-step gas-pressure-HIP sintering. Varying the grain-boundary 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 grain-boundary films exceeds a lower limit, which is ∼ 0.87 equiv% oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues.