Tough Ceramics Mimics mother of Pearls: Ceramics that wont shatter

A crack must zig-zag its way through the stacked platelets in the new ceramic.

Thanks to a little inspiration from nature, new ceramics could be made from materials that make them stronger and tougher, researchers have discovered.

The new ceramics are inspired by a material called nacre, also known as mother-of-pearl. Nacre is much stronger and tougher than common ceramics and is derived from the nacre of abalone, a small, single-shelled marine mollusk.

Biomimicry in bulk: The Berkeley researchers made large pieces of the tough ceramic, while other scientists mimicking tough natural materials have been able to make only thin films. A tough ceramic’s structure mimics that of abalone shells. This scanning electron microscope image (bottom), taken during a stress test, shows one source of the material’s toughness: damage is widely distributed in small, contained cracks.

Ceramics, as the flagship brittle materials, have been the centre of focus on toughness improvements. One of the first solution found was to copy the architecture of long fibre polymer matrix composite and use only ceramic components. In ceramic matrix composites (CMCs) , cracks are delayed by strong fibres that bridge the crack first and then provide pull-out mechanisms once broken . These mechanisms are potent enough to reach up to 1% plastic strains for in-plane woven architecture but at the cost of long and complex fabrication processes. After 40 years of development, CMCs are so tough and heat resistant that they are used in civil plane engines now . From this research stemmed a body of work on short fibres and elongated grain ceramics that are simpler to process and also provide some toughening mechanisms. The most studied example is the one of silicon nitride, once looked after to build ceramic car engines . Then came the discovery of transformation toughening in zirconia based ceramics [14]. Zirconia present a phase transformation from tetragonal to monoclinic that also triggers a 4% volume increase. If the zirconia tetragonal is stabilized down to room temperature using dopants, then this volume increase can be used to block crack from propagating . In the most recent work, some researchers managed to get to toughness of 10 MPa m1/2 and strength of 570 MPa with a strain at failure of almost 0.6% .

A more recent approach has been to use natural structures as blueprint to make tougher, stronger, and more lightweight composites. Through millions of years of evolution, natural materials evolved as intricate and hierarchical structures that solved the same problem we are now facing. One of such delicate structure is natural nacre, also called mother of pearl, a constituent of mollusc’s shells. It is one of the toughest, strongest, and stiffest natural material studied so far, and it is made of 95 vol% CaCO3 and 5 vol% of protein.

At first glance, the microstructure resembles a brick and mortar structure, with bricks of 7 lm in diameter and 500 nm in thickness perfectly stacked together [Fig. 1(b)]. On closer observation of the interface between the bricks, it appears that the protein forms a film a few nanometres thick, whereas the bricks present a rough surface, with 5–10% of the area bridged by mineral nanoparticles . The bricks also present a surface waviness that could promote interlocking during failure . During failure, instead of having a crack growing, the microstructure adapts, and a collective movement of the bricks is observed [Fig. 1(c)], allowing a macroscopic deformation.

Figure 1: Mechanical properties and microstructure of natural nacre. (a) Stress–strain curve of nacre in bending compared with an aragonite crystal. (b) Scanning Electron Microscope image of nacre. (c) SEM image of nacre showing the collective platelets movement under tensile stresses.

An extended body of work can now be found on both processes and materials to make nacre-inspired composite . Ice templating , laser engraving , heatassisted slip casting spray forming , coextrusion , sedimentation, or lamination has been used to produce brick and mortar structures at the hundreds to tens of micron scale with alumina, silicon carbides , or hydroxyapatite , whereas another branch focused on using 2D materials and paper-making process . Because of Griffith scaling law, smaller reinforcement sizes usually mean stronger composites, so the next development was to use micron-sized bricks, with alumina platelets , glass flakes , or brushite platelets Several secondary phases have been used in these composites, from polymers to metal and even graphene or metallic glasses . The controlled dewetting of TiO2 nanolayer allows in addition the careful study of the mineral bridges’ effect on the mechanical properties .

To keep the temperature and corrosion resistance, the hardness and in general the assets of ceramic material intact, the idea emerged to use purely mineral constituents to form a nacre-like aluminas (NLAs) . The structure, fabricated by ice-templating and field assisted sintering technique (FAST) sintering first, used micron-sized Al2O3 platelets as bricks and an amorphous SiO2 1 CaO glass as mortar, with Al2O3 nanoparticles as nano-asperities and bridges. NLAs have been later fabricated through magnetically assisted slip casting (MASC) and hot pressing . With MASC, the NLAs have been first produced with a SiO2 1 CaO glass secondary phase and more recently using a transient liquid phase (TLP) sintering resulting in an aluminium borate secondary phase. With the right composition, these NLAs presented strength and toughness better than the best technical alumina, but additionally present stable crack propagation. The combination of these properties put them in a region of strength and toughness that traditional tough ceramics could not achieve . Because only minerals were used, NLAs could additionally be used at temperature up to 1200 °C .