Nacre-inspired ceramic-based composite materials via bi-directional freeze casting

Abstract

Natural nacre is well known for its significant improvements in strength and toughness compared to its monolithic constituents because of its unique layered architecture. The toughening mechanisms of nacre have drawn considerable research interest. To synthesize materials with good mechanical properties of high strength and toughness, nacre-like materials were developed to replicate the architecture of nacre. In this work, manufacturing techniques based on bi-directional freeze casting and densification process were employed to prepare nacre-like ceramic (alumina, zirconia and hydroxyapatite) scaffolds. Two densification methods, namely one-step and two-step method, were investigated to produce nacre-like ceramic scaffolds with micro-layered (μL) and brick-and-mortar (BM) architecture, respectively. Afterwards, nacre-like ceramic scaffolds were infiltrated with different polymers (acrylates, epoxy, polyurethane) or metals (aluminium, magnesium) as the compliant phase to generate nacre-like ceramic/polymer or ceramic/metal composites. By tuning of processing parameters such as sintering temperature, binder content, cooling rate, compressive distance and solid loading, the resulting ceramic scaffolds exhibited various architecture and microstructure which were retained in the nacre-like ceramic/polymer or ceramic/metal composites. To understand the strengthening and toughening mechanisms in these nacre-like composites, mechanical characterization was systematically conducted on composites with different processing methods and parameters. Nacre-like composites with relatively lower ceramic fractions (<85%) revealed various intrinsic toughening (plasticity) and extrinsic toughening mechanisms (crack deflection, pull-out, frictional sliding, and ceramic bridges breakage). The difference in mechanical performance of composites manufactured from different processing parameters were attributed to their microstructure and strengthening/toughening mechanisms. The μL composite exhibited superior mechanical properties to those of the BM composite due to its better stress-transfer and crack deflection effects. The manufacturing method of one-step densification also had advantages of shorter processing time and more flexible control in microstructure. The novel μL composites with different combinations of ceramics and polymers or metals could potentially be used in many engineering and biomedical applications.

Date of Award	4 Mar 2021
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Bo Su (Supervisor)