To combat the intrinsic brittleness of ceramics, recent research has taken inspiration from biological materials found in nature. Nacre, also known as mother of pearl and found in molluscs, has a ‘brick-and-mortar’ structure that facilitates toughness far beyond that of its brittle components. Natural nacre comprises 95 vol% calcium carbonate (and 5 vol% biopolymer) yet exhibits a fracture energy 3 orders of magnitude higher than CaCO3. Experimental and modelling work on nacre showed that the key to this toughness amplification seems to originate from a collective movement of the bricks under load that spreads the damage over a millimetre size zone. Consequently, researchers have been working on mimicking this structure in the hope of obtaining a similar behaviour and toughness amplification in nacre-like composites.

From this research endeavour, nacre-like alumina (“NLA”) mimics nacre structure using alumina platelets as ‘bricks’ with a thin ‘mortar’ material. While multiple chemistries have been used for the mortar up to now, ranging from polymer, metals to ceramics, no bioinspired composites have shown this collective movement of bricks and thus high toughness amplification yet. The premise of our research is that to unlock this elusive toughening mechanisms present in natural nacre, one must have a continuous mortar that allows brick movement during loading. We have thus developed a selection of NLA ceramics that use alumina bricks pre-coated in oxides. Using pre-coated bricks allow us to ensure that the bricks are fully embedded in the mortar; it also increases the mortar content up to 30 vol% in some cases.

NLA samples were created from green bodies comprising alumina platelets pre-coated in a 40nm TiO2 continuous layer and then sintered using Spark Plasma Sintering. Depending on their size, some platelets were aligned prior to sintering using magnetically-assisted slip casting in order to ensure better packing during sintering. Mechanical characterisation consisted of 4-point flexural and 3-point SENB tests at ambient temperature, as well as 3-point flexural tests at a range of temperatures up to 1300°C. High spatial resolution optical characterisation was used to track strain and fracture behaviour within the samples for both conditions. The results show that unusual mechanical behaviour can occur in ceramics with a continuous oxide mortar at high temperature.