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Lehrstuhl für Struktur- und Funktionskeramik


Layered ceramics have been proposed as an alternative choice for the design of structural ceramics with improved fracture toughness and mechanical reliability. The use of energy release mechanisms such as crack branching, crack deflection and/or crack bifurcation can improve the crack growth resistance of the material (Fig. 1). The use of tailored residual compressive stresses, either at the surface or in the internal layers, may also improve the strength as well as the crack resistance of the material during crack growth. In this regard, layered architectures designed with high compressive stresses can show higher strength and enhanced mechanical reliability, yielding in some particular cases a minimum mechanical resistance ( threshold strength ), below which the material does not fail.

Fig. 1: Step-wise fracture of a laminate.
Fig. 2: (Left) ECS-Laminates; (Right) ICS-Laminates.
Fig. 3: Crack arrest in the compressive layers of an ICS-Laminate.

In this project design parameters to optimize the strength and toughness of layered ceramics are sought, based on Fracture Mechanics analyses and experiments. Two multilayer ceramics based on the alumina-zirconia system, designed with external (ECS-laminates) and internal (ICS-laminates) compressive stresses (Fig. 2), have been investigated. An optimal architecture that maximizes material toughness and strength has been found for each design as a function of geometry and material properties.

From a flaw tolerant viewpoint, ECS-laminates are suitable for ceramic components with small cracks or flaws which are embedded in or near the potential tensile surface of the piece. On the other hand, the existence of large cracks or defects suggests the use of ICS-laminates to attain a more reliable response. This analysis can be extended to other laminates which hold such energy release mechanisms.

A minimum strength has been found for ICS-Laminates ( threshold strength), leading to crack arrest in the compressive layers (see Fig. 3). This behaviour can be explained using a Weight Function analysis, showing that the residual compressive stresses are responsible for the toughening of the material, where the compressive layers can act as a barrier to crack propagation.