Deformation and failure of layered zirconia materials on different length scales under quasi-static and cyclic loading

Many natural materials have exceptional mechanical properties, which are difficult to reproduce in materials engineering. One example is teeth which are astoundingly resistant to the harsh fatigue and environmental loading conditions in the mouth and possess potent strengthening and toughening mechanisms to hinder crack formation and propagation. Zirconia is a commonly used material for dental restorations, because of its toughness, biocompatibility, and aesthetic. However, unlike teeth, it is prone to brittle fracture even with today’s inbuilt transformation toughening mechanism, which is the transformation from the tetragonal phase into the monoclinic phase. Past work has shown that the implementation of a layered structure could increase the toughness of zirconia-based ceramic constructs. The mechanisms behind this characteristic, however, is not fully understood. Therefore, this work investigates the mechanical properties of layered yttria-stabilised zirconia materials with different crystal structures produced by electrophoretic deposition (EPD). At the macro level, the crack propagation depending on the number and sequence of layers and at the submicron level, the mechanical properties and fatigue behaviour of the different layers were investigated. The production of layered ceramic structures with EPD is state of the art. However, only very small samples have been produced so far with edge lengths in the order of a few millimetres. Due to their size, it has not yet been possible to carry out macro level tests with these samples to analyse the crack propagation within the material. In this thesis a new hardware setup is introduced which overcomes this restriction. This enables the production of large enough layered ceramic samples which could be used for the very first time for investigating crack propagation in these structures by 3-point bending tests on the macro level. The fractured samples were analysed by dye penetrant testing, light microscopy, and scanning microscopy. The fatigue behaviour of the layered material was investigated using cyclic nanoindentation. Nanoindentation was performed repeatedly at the same location up to a maximum number of 100 000 cycles. The influence of the number of cycles and the frequency of the cyclic loading on the microstructure was investigated. Also, the microstructure and the transformation from the tetragonal to the monoclinic phase on the surface and underneath the nanoindent were analysed by Raman spectroscopy and scanning precession electron diffraction (SPED). The result obtained in this thesis show that the layers within the structure improve the crack propagation behaviour and increase the toughness of zirconia constructs on a macro level. Hereby, the sequence of the layers plays an important role and determines how a crack is deflected within the structure. The cyclic nanoindentation results demonstrate that mechanical properties and microstructure changes depend on crystal size and orientation. It also revealed that the monoclinic transformation is affected by the cycle number and the loading frequency. The main contributions of this thesis are a new electrode design to produce larger layered EPD samples suitable for bending tests, new insights into the crack propagation behaviour and the influence of the layer sequence in EPD produced materials as well as the investigation of the fatigue behaviour of ceramic materials, which could be studied for the first time at a submicron level for high number of cycles.