Biomimetic Alumina/Epoxy Composite with Hierarchical Structure and Improved Mechanical Properties

Abstract

Abalone Nacre (i.e. mother of pearl) is one of the natural biological composite which have outstanding ability to integrate a multitude of properties such as stiffness, strength, toughness and impact resistance. It is composed of hard aragonite platelets (CaCO3) glued with thin biopolymer matrix, in a ‘brick-and-mortar’ arrangement. The present work comprises of producing and evaluating the fracture properties of natural nacre like alumina/epoxy composites by taking a combinatorial approach of unidirectional freeze casting, consolidation and subsequent infiltration. At first, porous lamellar ceramic preforms were prepared using freeze casting of 15 vol% of α-alumina platelets, along with varying concentration of silicacalcia (0 to 7.5 wt.% of platelets) liquid phase sintering additives. The level-1 hierarchy, i.e. large-scale aligned brick layers, was achieved by uniaxial pressing (perpendicular to the freezing direction) of porous freeze-cast green alumina scaffolds, followed by sintering at 1500°C/4 h. The level-2 hierarchy (mortar phase) was achieved by infiltrating epoxy into the aligned brick (i.e. alumina platelets) layers under high vacuum conditions. Flexural and single edge notched beam (SENB) tests revealed maximum flexural strength of 451.4 MPa and a fracture toughness of 8 MPa√m (for 7.5 wt.% of sintering additive). The current research delivers a useful insight into the optimization of structural hierarchy of our synthetic nacre results in superior mechanical performance, as compared to natural nacre and other engineering materials (illustrated in the form of Ashby map).