Hierarchically Structured Biomimetic Alumina/Epoxy Composite with Enhanced Strength and Toughness

Abstract

Abalone nacre, also known as mother-of-pearl, is a natural composite known for its exceptional combination of strength, toughness, and resistance to impact. This remarkable performance is attributed to its hierarchical ‘brick-and-mortar’ architecture, consisting of hard aragonite (CaCO₃) platelets embedded in a thin biopolymer matrix. Drawing inspiration from this structural motif, the present study focuses on the fabrication and mechanical property evaluation of nacre-mimetic alumina/epoxy composites; which were produced by adopting 3-step processing approach comprising freeze-casting, uniaxial pressing and vacuum-assisted epoxy infiltration. Initially porous lamellar ceramic scaffolds were produced by freeze casting of aqueous suspensions containing 15 vol% α-alumina platelets and silica-calcia liquid phase sintering aid (0 to 7.5 wt% of alumina platelets). Level-1 hierarchy, representing large-scale alignment of the platelets, was achieved by uniaxial pressing of porous alumina scaffolds orthogonal to the freezing direction, followed by sintering at 1500 °C/4 h. To introduce level-2 hierarchy analogous to nacre’s biopolymer mortar, the sintered structures of aligned brick layers (i.e., alumina platelets) were infiltrated with epoxy resin under high vacuum conditions. Mechanical characterizations through 3-point bending and single-edge notched beam (SENB) tests revealed significant improvements in structural performance. The composite with 7.5 wt.% sintering additive achieved flexural strength of 450 ± 14 MPa and fracture toughness of 7.7 ± 0.2 MPa√m, exceeding both natural nacre and many conventional engineering materials. Ashby plot of strength vs toughness further underscore the superior position of our biomimetic hybrid composites with amalgamation of both high strength and toughness. This work demonstrates the potential of bioinspired hierarchical design in engineering high-performance ceramic-polymer composites, thereby opening avenues for their application in aerospace, protective systems and structural domains where simultaneous strength and damage tolerance are critical.