Structural integrity of alumina-zirconia multilayered ceramics
- Autores: Raul Bermejo Moratinos
- Directores de la Tesis: Luis Llanes Pitarch (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2006
- Idioma: español
- Tribunal Calificador de la Tesis: Marcos Juan Anglada Gomila (presid.), Pavol Hvizdos (secret.), Juan Bautista Carda Castelló (voc.)
- Materias:
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- Resumen
In this doctoral thesis the fracture, fatigue and thermal shock behaviour of multilayered ceramics of alumina-zirconia, designed with compressive stresses in the internal layers, has been studied. An introductory Chapter gives an overview of the problematic of using ceramic components and the solutions arising from the latest investigations, one of them being the proposed use of layered ceramics as for structural applications. The second Chapter comprises the design and fabrication of various alumina/zirconia- containing multilayered systems with different thickness layer ratios and their corresponding monoliths with the same composition of the constituent layers by means of sequential slip casting of colloidal suspensions. The multilayered structures under consideration consist of nine alternated layers cast one on top of the other; five thick layers of 95% vol. of alumina and 5% vol. of tetragonal zirconia, and four thinner layers composed of 70% vol. of alumina with 30% vol. of monoclinic zirconia. Depending on the particular architecture, the thickness of the layers range between 540 µm and 650 µm for the thick layers and 60 µm and 140 µm for the thin ones. A microstructural characterization of the monoliths and laminates is performed. Grain size and content of the different phases are determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, as well as the chemical composition of the compounds. In the third Chapter the physical and mechanical properties of monoliths and laminates are studied. Density, thermal expansion coefficients, Young's modulus, and Vickers Hardness, are evaluated in the monoliths.
Mechanical strength of monoliths and laminates is assessed by 4-point bending tests on prismatic bars. Chapter four is dedicated to the evaluation of the residual stresses developed in the layers of the different architectures investigated during sintering. In doing so, several approaches are undertaken based on analytical equations, experimental indentation techniques and a 3D finite element model. Several phenomena accompanying these residual stresses are discussed in terms of stress magnitude and distribution throughout the samples. The evaluation of the fracture behaviour of the materials investigated is presented in Chapter five. A fractographic analysis is accomplished on the fractured specimens to find both the origin and the type and size of defects causing the failure. Several experimental methods are employed to evaluate the fracture toughness of the monoliths, such as IM, IS and SEVNB. For the case of the layered architectures apparent fracture toughness, Kapt, is estimated by the SEPB method. In addition, the R-curve developed in these layered structures is assessed by the fracture mechanics weight function approach. Within this context, flaw tolerance capability of the multilayers is investigated by flexural tests conducted on specimens with different critical indentation flaw sizes, to demonstrate the existence of a threshold strength. Additionally, crack propagation studies are performed on notched samples by means of crack opening displacement tests to evaluate the fracture energy and the role of the different layers impeding the unstable crack propagation through the material. The influence of the layer thickness ratio and the loading configuration with respect to the layer plane is discussed in terms of strength, fracture toughness and work of fracture. Moreover, the threshold strength concept is extrapolated to the study of the behaviour of these laminates under cyclic and static fatigue loading conditions.
Furthermore, thermal shock experiments are performed on the multilayered structure to be compared with the ATZ monolith response. Results are discussed in terms of thermal shock cracks pattern. The role of the thin compressive layers avoiding thermal shock crack propagation is qualitatively investigated. Finally, the last Chapter presents a summary of the more significant conclusions and a discussion of the achieved results in terms of the mechanical response of the layered ceramics investigated and their future application as structural ceramic components.
