Toughening of ceramics and ceramic composites through microstructure engineering: A review

We conventionally assume that ceramics are brittle. The risks induced by this structural behaviour do not cancel the undeniable need for ceramics in applications with extreme environmental demands. These constraints could be a high working temperature or wear, where metals cannot be used, or in contact with the human body, where the inherent chemical resistance of ceramics makes them more biocompatible and durable. While the first statement on brittleness still holds true for most ceramics, solutions have been found for some: damage-resistant Ceramic Matrix Composites (CMC) are now part of plane engines, and tough zirconia-based ceramics are routinely used in orthopaedic and dental implants. Because of the intrinsic brittleness of ceramics, the required toughness increase must be introduced by tailoring their microstructure. A lot of ground has been covered since the first descriptions of the toughening mechanisms in ceramics in the 1970s. The goal of this review is thus three-fold: summarise the necessary background in fracture mechanics to discuss complex fracture behaviours, provide a picture of the recent development on all the strategies to toughen ceramics, and finally extract lessons on the efficiency of these various toughening mechanisms. In terms of materials, this review will cover CMCs, zirconia-based ceramics, and bulk ceramics using microstructural engineering, which includes the combination of elongated grains with grain boundary engineering, using carbon allotropes, MAX phases, and bioinspired microstructures. For each we will discuss both the core toughening mechanisms and the most recent studies on it. By gathering all these research strands, we will provide in the final part a reflection on the link between toughness and pseudo-ductile behaviour and how it can help to hopefully reach uncharted territory in terms of structural properties in the future.