The Design of the Interface Phase for Obtaining Thermal Shock Resistance in Silicon Nitride

Abstract

Guidelines for the microstructure design of silicon nitride are developed under the assumption that high temperature structural materials should have zero tolerance for thermal shock induced microcrack damage. The residual stresses resulting from anisotropic thermal expansion coefficient are analyzed and it is shown that the magnitude of these stresses can be reduced by employing graded interfaces. It is proposed that an elongated grain structure of silicon nitride embedded in an interconnected intergranular phase, having a volume fraction greater than 8%, offers the possibility of precluding microcrack damage. The nanostructure of this intergranular phase is important in the creep properties of silicon nitride. A crystalline intergranular phase having an elastic modulus that is nearly equal to the modulus of silicon nitride crystal is proposed as the optimum choice for reducing residual stresses and enhancing resistance to creep cavitation.