Fracture of ceramics in the SEM

Abstract

The progress in the development of advanced structural ceramics is characterized by a remarkable increase in their strength and fracture toughness. Further improvement of these essential mechanical properties is still desirable and requires a fundamental understanding of the crack propagation process. In the new types of ceramic microstructures, e. g. in whisker-, particle-, or platelet-reinforced ceramics the relevant crack growth processes are not adequately understood. These processes are responsible, for example, for the R-curve behaviour which may be due to the formation of a process zone around the crack tip, or by a wake effect of interacting forces between the crack surfaces. Furthermore, fatigue in ceramics is a problem in toughened ceramics in particular and relies directly on the various mechanisms of increased crack resistance.In order to understand the formation and extension of the relevant micro- and macrocracks, SEM is a powerful tool because of its ample range of magnification and large depth of focus.A complete description of the bending device used is given elsewhere, so that only the main features will be mentioned here.Using a very stiff frame and a position controlled piezo translator it was possible to obtain controlled fracture of brittle ceramic materials. This means that a crack is not spontaneously propagated through the specimen, but slowly advances micrometer for micrometer. The fracture experiments are recorded by a video recorder directly connected to the SEM.