Process-structure-properties relationship in bioinspired highly textured ceramic composites: Influence of composition, grain morphology and self-assembly on the fracture behaviour
File(s)
Author(s)
Vilchez, Victoria
Type
Thesis
Abstract
Unlike monolithic ceramics which are brittle and prone to catastrophic failure, ceramic-based biological materials have found ways to combine both stiffness and toughness. Nacre, for example, is composed of 95 vol% brittle calcium carbonate, yet exhibits a fracture energy 3 orders of magnitude higher than calcium carbonate. This toughness amplification stems from nacre’s brick-and-mortar architecture which provides multiple toughening mechanisms acting at different length scales. Taking inspiration from nacre, many processes and compositions have been developed to produce tough ceramic composites. Fracture testing in nacre-inspired composites has shown complex fracture geometries with branching and deflection that resist crack opening, although the highly delocalised damage found in natural nacre has yet to be observed in synthetic composites. Using the current standards of fracture mechanics, it has been proven that nacre-like composites exhibit higher toughness and improved crack-resistance curves compared to bulk ceramics. However, characterising fracture in materials that exhibit high degrees of deflection and branching remains a challenge, and all the mechanisms accounting for improved damage resistance in nacre-like composites have not been characterised or explained. During this PhD, we have worked on characterising fracture propagation in ceramic-based nacre-like composites, with the aim of determining the role of microstructure and composition on the mechanical response. We have studied an analytical method validated by finite element analysis to measure crack resistance curves at the tip of deflected and branched cracks. Quantifying the local stress states has allowed us to look at the direct interaction between the crack tip and the highly textured microstructure, which we used to reveal the influence of specific parameters on the fracture response of the composite. The parameters studied were either related to the composition, such as the choice of interphase or the addition of residual stresses, but also to the microstructure. We showed that the fracture geometry appears to be driven more by the grain morphology than by the choice of interphase. Combining this observation with the lack of delocalised damage in synthetic composites has led us to question the role of the microstructural order on the fracture behaviour. We have studied this hypothesis by developing a new system with controlled packing of reinforcement through which we produce pseudo-crystalline colloidal structures with high packing fraction and orientational control over macroscopic volumes. This new system has allowed us to study the role of packing order on the fracture response, and showed that it could be the key to observing delocalised damage in highly textured bioinspired composites.
Version
Open Access
Date Issued
2023-06-18
Date Awarded
01/12/2023
License URL
Advisor
Bouville, Florian
Saiz, Eduardo
Sponsor
UK Research and Innovation
Grant Number
EPSRC - DTP 2018/19
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)