Effect of residual microstresses at crystalline multigrain junctions on the toughness of silicon nitride

Two Si3N4 materials with either tensile or compressive residual microstresses localized at triple-grain junctions have been analyzed with respect to their fracture behavior and microstructural characteristics. Residual tensile stresses at triple pockets were obtained, upon addition of Sc2O3, by initiating the crystallization of Sc2Si2O7 with a negative volume change. On the other hand, compressive microstresses localized at triple-grain junctions were induced by adding fine ZrO2 particles to the Si3N4 material, which underwent martensitic transformation upon cooling with a positive volume change. The presence of these highly localized stress fields has been shown to actually be the critical factor in determining the fracture mode of Si3N4 materials and, accordingly, their respective fracture toughness. In particular, tensile stress fields at triple-grain pockets can trigger debonding and splitting elf the crack tip at the interface and, therefore, may provide a precursor effect for elastic bridging in the crack wake. (C) 1999 Elsevier Science Limited. All rights reserved.