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Title: Characterisation of armour ceramics at the microstructural scale for modelling of impact
Author: Jiang, Jiawei
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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The present DPhil thesis aims to provide new insights into the deformation and failure mechanisms of armour ceramics by carrying out experimental investigations at the microstructural scale, which can be integrated into numerical modelling to enhance its predictive capability for ceramic armour designs. Grain size distributions of polycrystalline alumina with varying microstructures were characterised precisely by employing the method of digitising grain contours from SEM images. The experimental results were incorporated to develop an innovative algorithm for generating representative numerical microstructures, and to validate the numerical models via a novel method of virtual cross sectioning. The effect of shear stresses on brittle failure of grain boundaries was investigated by measuring the strengths of grain boundaries of alumina in combined normal and shear loading. The contribution of the shear component was found to be significant, which overthrew known theories usually adopted in micromechanical modelling that assume otherwise. Progress towards making reliable toughness measurements of individual microstructural components was made by allowing for, or avoiding the effect of focused ion beam (FIB) damage at notch roots through the following methods. (a) Enabling stable crack growth allowed the stress intensity factor for crack propagation Kp to be measured reliably in Al2O3 and SiC. Kp measured in vacuum represented Kc, giving values of 2.8 MPa∙√m for a-plane sapphire. Kp measured in air was 1.8 MPa∙√m for both a- and m-plane sapphire, and 2.2 MPa∙√m for (1̅100) plane SiC. The value was reduced substantially by moisture-assisted subcritical crack growth (SCG) and was close to K0, with crack velocities on the scale of 10-8 m/s. Additionally, cyclic loading tests in vacuum demonstrated the self-healing of interfaces in alumina. (b) Clarification of the extent of the effect of the implantation damage. First, a microstructural investigation of Ga+- and Xe+-FIB damage induced in Al2O3 and SiC exhibited the formation of amorphous and crystalline damaged layers, which were determined by the combination of ion species, target material and FIB settings. GaAlO3 particles and Xe bubbles were found in Al2O3 implanted by Ga+ and Xe+, respectively. Second, Ga+ and Xe+ implantation stresses (IS) were quantified in Al2O3, SiC and Si using a beam-curvature method, and were mostly on the scale of GPa. With increasing ion dose, while the Ga+ IS in Al2O3 and SiC remained compressive, Ga+ IS in Si and Xe+ IS in Al2O3 and SiC transited from compressive to tensile. For fracture toughness measurement, Ga+-FIB was recommended for notching all three materials investigated.
Supervisor: Petrinic, Nik ; Todd, Richard Sponsor: Defence Science and Technology Laboratory (Dstl ; UK Ministry of Defence)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering science ; Materials science