A model for viscous flow creep in ceramics containing secondary crystalline phases

Abstract

Many structural ceramics (e.g., Si{sub 3}N{sub 4}) contain an intergranular amorphous phase which results from liquid phase sintering. This amorphous phase is present both at the multigrain junctions and as thin films at two-grain boundaries. High temperature creep deformation of these materials may occur by viscous flow of the amorphous phase from grain boundaries in compression to those in tension. Several models have been developed to describe this process, based on the assumption of either square or hexagonal grains. All these models assumed that the grain size and grain boundary film thickness were uniform throughout the material. In polycrystalline material, however, a large range of grain size distribution is usually observed and secondary crystalline phases (SP) may also exist. The effect of grain size distribution on the viscous flow process has been analyzed recently by Dey et al. They argued that the presence of large grains can cause very high local stress concentrations. Since a residual glass film always remains along matrix/SP grain boundaries (heterophase boundaries) and is generally 2 or 3 times thicker than that along homophase boundaries (between matrix grains), a new model considering this heterogeneity is required to describe viscous flow creep in ceramics containing secondary crystallinemore » phases.« less