Bioinspiration to Improve Structural Ceramics and Composites

While most of the structural materials we encounter every day have to withstand mechanical stress at near-ambient temperatures, high-performance engine parts or prostheses have to withstand environmental stress as well. Unfortunately, materials capable of withstanding these extreme conditions are just as fragile. Ceramics are the best example of compositions that are resistant to high temperatures, wear and contact with the human body, but at the same time have a very fragile mechanical behavior. Since the 1970s, microstructures derived from long-fibre composites have been used to make ceramics tougher. Thanks to these advances, ceramic parts are now used in civil and military aircraft engines. More recently, the study of natural materials has opened up another avenue for making ceramics tougher, with the discovery of microstructures that, over millennia of evolution, have solved this very problem.

The main function of certain natural materials is to protect against predators and impact, as in the case of mollusc shells, or to act as a framework for the body, as in the case of vertebrate bones. To resist mechanical stress, these materials generally feature a very high fraction of brittle ceramics in their structures, but thanks to the shape and way in which these ceramic grains are arranged, they exhibit mechanical behavior that is anything but brittle. The idea of bio-inspiration is neither new nor limited to structural properties. Indeed, it's a field of research in its own right, tasked with deciphering the link between the sometimes remarkable structural or functional performance of natural materials and their microstructures. Spurred on by these discoveries, materials researchers began by understanding the origin of these materials' efficiency, so as to be able to draw inspiration from them. The next step was to adapt or even invent processes capable of reproducing certain aspects of these natural structures, in order to increase the toughness of ceramics and ceramic-based composites.

As far as the very notion of ceramics is concerned, it should be made clear from the outset what we're talking about: a ceramic has always undergone, during its manufacture, a firing stage or a passage at high temperature, which obviously never exists in "biological" materials. Hence, in the remainder of this article, we will mainly consider the ceramic nature of the composition and microstructure of the materials under consideration, and not their production process. For the time being, it is impossible to use processes similar to those used naturally to form these organisms: firstly, they are extremely slow by industrial standards, and secondly, they are limited to compositions that can crystallize in water at near-ambient temperatures. Once these bio-inspired materials have...