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

Two Si3N4 materials with either tensile or compressive residual microstress es localized at triple-grain junctions have been analyzed with respect to t heir fracture behavior and microstructural characteristics. Residual tensil e stresses at triple pockets were obtained, upon addition of Sc2O3, by init iating the crystallization of Sc2Si2O7 with a negative volume change. On th e other hand, compressive microstresses localized at triple-grain junctions were induced by adding fine ZrO2 particles to the Si3N4 material, which un derwent martensitic transformation upon cooling with a positive volume chan ge. The presence of these highly localized stress fields has been shown to actually be the critical factor in determining the fracture mode of Si3N4 m aterials and, accordingly, their respective fracture toughness. In particul ar, 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.