• Jabr, Abdullah

Abstract

The “bio-inspired” concept of designing ceramics in a layered architecture has proven to be an effective means for overcoming the lack of damage tolerance and enhancing the mechanical properties of ceramics. The strong interface bonding between layers of different materials with different thermal expansions can be utilized to induce in-plane residual stresses upon cooling during the sintering step. Laminates designed with internal compressive residual stresses in embedded layers, which act as a protective barrier against crack propagation, exhibit increased toughness, reduced strength variability and damage-tolerant behaviour. Recent advances have been achieved by tailoring the architectural design and microstructure of internal compressive layers, i.e. texturing, to further enhance the strength and toughness. An important property required for many modern engineering applications is the resistance to contact damage. Despite its importance, the performance of such multilayer ceramic systems under contact loading remains unexplored. This thesis investigates the contact damage resistance of layered alumina composites consisting of compressive textured alumina layers embedded between equiaxed alumina layers. The effect of microstructure is investigated on monolithic samples of each layer material. The study was carried out using Hertzian indentation. In addition, critical forces responsible for damage initiation and progression were detected using an acoustic emission system. It was found that a textured microstructure causes contact damage to occur below the surface by shear-driven, quasi-plastic deformation instead of the classical Hertzian ring and cone cracking observed in equiaxed alumina. Laminates exhibited cone cracking in the surface layer and quasi-plastic deformation in the underlying textured layer. This internal compressive textured layer deflected cone cracks propagating from the surface and restricted their growth even at higher applied loads. The findings of this work indicate damage tolerant behaviour of laminates under contact loading and provide important implications regarding their architectural design for contact applications.

Topics

• impedance spectroscopy
• microstructure
• surface
• polymer
• crack
• strength
• layered
• composite
• thermal expansion
• acoustic emission
• ceramic
• sintering