Nacre (mother of pearl), a natural ceramic composite found in some molluscs, is the gold standard material for combined high strength and high toughness. This is achieved through a bricks-and-mortar-like structure of mineral (aragonite) platelets separated by an organic polymer [1]. Nacre’s toughening mechanisms arise from its hierarchical structure; on the micro-scale, platelet pull-out and interlocking, and on the nano-scale bridge fracture and asperity interactions [2]. Combined high strength and toughness is desirable in demanding environments for energy generation and transport applications. It is now possible to produce composites which mimic nacre’s hierarchical structure though infiltration of freeze cast aerogels [3]. Whilst the structure of nacre is replicated, the aim of the present research is to determine whether they function as seen in nacre, and which toughening mechanisms occur in these bioinspired materials.

Composites were produced by freeze-casting of MXene aerogel, followed by infiltration with epoxy during compression (to 0%, 30% or 60% strain), producing tailored microstructures. Samples were tested in single edged notched bending (as per ASTM D5045) during in situ X-ray micro computed tomography (µCT; both in the laboratory using a Versa 520 and and at beamline ID19 at the European Synchrotron Research Facility) allowing observation of the crack path.

µCT imaging revealed crack deviation in the aerogel-epoxy composite, which resulted in an increased crack length. A nacre-like, stepped fracture surface was observed (Figure 1a) suggesting platelet pull-out, in contrast with the smooth fracture surface seen in epoxy-only samples (Figure 1b). µCT also revealed delamination, oriented with the aerogel sheets within the composite (Figure 1a).

In conclusion, a nacre-like structure is achieved in the aerogel-epoxy samples, with the bricks-and-mortar like structure acting to deflect cracks. µCT data shows evidence of platelet pull-out, crack deviation and delamination. Further analysis will reveal the effects of structural variation of the epoxy-aerogel composites.