Microstructural Aspects of Creep Deformation in Two Phase Ceramics

Abstract

Two phase ceramics in which one phase is crystalline and the other non-crystalline constitute an important class of high temperature structural materials. They are of interest not only because they include the silicon nitride based alloys and certain glass-ceramics but because their behavior is expected to simulate that of the multi-phase refractories containing an intergranular phase. The creep behavior of these two phase ceramics is already known to be complex involving homogeneous phenomena such as diffusional flow and inhomogeneous phenomena such as cavitation and sub-critical crack growth. In order to gain an insight into the mechanisms occurring the microstructures of two types of ceramic alloy deformed in compressional creep have been examined for characteristic features. The two ceramics are silicon nitride alloys hot pressed with MgO and an alumina/glass material. The first is an example of a material containing a very thin intergranular glass phase, typically 8–15 Å thick at the two grain interfaces, whereas the second contains a relatively thicker, 50–300Å, intergranular phase. Also, the volume fraction and composition of the glass in the silicon nitride alloys is unknown whereas both are known for the alumina/glass ceramic.