Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High Purity Y2O3-Al2O3-Containing Silicon Nitride Ceramic: Role of Oxygen

摘要

The effects of grain-boundary chemistry on the mechanical properties were investigated on high-purity silicon nitride ceramics, specifically involving the role of oxygen. Varying the grainboundary oxygen content, 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 that appears to affect fracture by weakening the interface, facilitating debonding and crack advance along the boundaries, and consequently toughening by grain bridging. It is concluded that if the oxygen content in the thin grain-boundary films exceeds a lower limit, which is approximately 0.87 equiv % oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues.