Effect of rising R-curve behavior on the high-temperature delayed-failure resistance of Si3N4 ceramics under elastic slow-crack-growth regime

The effect of a rising R-curve behavior on the high-temperature lifetime and daw-tolerance of Si3N4 ceramics has been systematically investigated. Extensive mechanical testing was pursued at 1400 degrees C, typical temperature at which the selected materials fractured under the elastic slow-crack-growth (SCG) regime (i.e., with negligible plastic strain). Hot-isostatically pressed (HIP) Si3N4, its WC-platelet, SiC-platelet and SiC particulate composites were selected for this basic investigation. All the composites investigated contained the same volume fraction (i.e., 25%) of reinforcing phase and were fully dense after the HIP process without external addition of densification agents. This latter important processing circumstance has made available materials which, besides having the same grain-boundary structure and, hence, the same intrinsic thermo-mechanical response of the grain boundaries at high temperature, showed different R-curve behaviors. It was systematically found that the rising R-curve, especially pronounced in composites reinforced with high-aspect-ratio platelets, provided a positive effect on lifetime and flaw-tolerance. From the theoretical side, both the lifetime elongation and the improved flaw-tolerance were well explained by an algorithm incorporating into the mechanical driving force acting on the crack tip, the closure-field contribution due to the rising R-curve behavior of the material.