Influence of Sintering Temperature On Hierarchical Structure and Mechanical Properties of Bio-Inspired Alumina-PMMA Composite

Abstract

Natural nacre consists of a hierarchical brick-and-mortar microstructure that imparts superior fracture toughness, damage resistance, high specific strength, and lightweight properties, which mark it an inspiring model for aerospace-grade composite materials operating in harsh and tough environments. The challenge of fabricating bulk materials with such intricate microstructure in a cost-effective and low-energy-intensive way has persisted for half a decade. In present investigation, a scalable, bottom-up fabrication route was implemented to fabricate bulk nacre-like alumina-based composites using anisotropic α-alumina platelets (brick phase) with a compliant polymer poly-methyl-methacrylate (PMMA) (mortar phase). The platelets were directionally aligned via vacuum-assisted forced sedimentation process to emulate the lamellar architecture (nacre-like). To densify the ceramic preforms, 4 wt% TiO2 was used as sintering aid, and the preforms were heat-treated at 1200°C, 1350°C, and 1500°C for 4 h. The ceramic fraction improved with increasing temperature, attaining a maximum of 81% (when sintered at 1500°C). The partially dense ceramic preforms were then impregnated with PMMA via in-situ polymerization using 1 wt% AIBN, yielding 98-99% dense composites. All compositions exhibited nacre-like large-scale alignment and brick-and-mortar microstructure. Flexural and single-edged notched beam (SENB) test revealed significant enhancement in mechanical performance of our composite. Firing the ceramic preforms beyond 1350oC enhanced the flexural strength, with trivial reduction in fracture toughness. A maximum flexural strength of 320 MPa (at 1500°C) and fracture toughness of 8.64 MPa√m (at 1350 °C) were achieved. The combination of alignment of ceramic anisotropic particles, interfacial energy dissipation, and hierarchical toughening mechanisms makes this composite considerably fit for high to moderate strain rate applications like aerospace, defence, and other strategic fields.