Ultrasound freeze-casting of a biomimetic layered microstructure in epoxy-ceramic composite materials to increase strength and hardness

Some natural materials, such as the dactyl club of the mantis shrimp, have impressive mechanical properties (e.g. strength) due to their microstructure that consists of periodic layers of high and low density material, which prevent crack propagation. Although such layered structures have the potential to increase the strength of engineered epoxy-ceramic composites relative to their constituents, synthetically replicating this class of layered structures in engineered materials has been challenging to date. To overcome this challenge, ultrasound freeze casting (UFC) was used to manufacture macroscale specimens of epoxy-ceramic composite materials with periodic layers of high and low density that mimic the structure of natural materials. The critical operating parameter of the UFC technique, the ultrasound operating frequency, was related to the resulting hardness, porosity, and flexural strength of the resultant epoxy-ceramic composite materials. Scanning electron microscopy and micro X-ray CT was used to visualize the microstructure of the specimens and connect it to the mechanical properties. The ultrasound operating frequency controlled the spacing of the layers as well as the local hardness of the epoxy-ceramic composite, which increased by up to 18%. The flexural strength of the epoxy-ceramic composite was also related to the ultrasound operating frequency, with a maximum increase of 52%.