Cost-effective fabrication of bio-inspired nacre-like composite materials with high strength and toughness

A cost-effective one-step densification process based on bi-directional freeze casting was investigated to produce nacre-like alumina/poly(methyl methacrylate) (PMMA) composites with a unique micro-layered (µL) architecture. This method has the advantage of shorter processing time, as it requires only sintering once instead of twice as in the fabrication of conventional brick-and-mortar (BM) composites via freeze casting. By tuning the processing parameters, composites with different ceramic content and layer thickness were obtained. The resultant mechanical properties of µL composites showed that ceramic content and wall thickness affected mechanical properties significantly. The µL composite with fine ceramic walls (8 µm) and relatively high ceramic fraction (72 vol.%) exhibited an exceptional combination of high flexural strength (178 MPa) and fracture toughness (12.5 MPa m1/2). The µL composites were also compared with the conventional BM composites. Although the fracture behaviour of both composites exhibited similar extrinsic toughening mechanisms, the µL composites with longer ceramic walls displayed superior mechanical properties in terms of strength and fracture toughness in comparison with the BM composites comprising short ceramic walls (i.e. bricks), due to the effectiveness of stress transfer of load-bearing ceramic phase within the composites.