| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 96 === Aluminum oxide (Al2O3) is a hard refractory ceramic, which has been investigated for high temperature structural and substrate applications because of its good strength and low thermal expansion coefficient. Nevertheless, like other monolithic ceramics, Al2O3 is apt to suffer from low ductility and low fracture toughness. Therefore, metals (e.g. aluminum, cobalt, and niobium) or alloys are added to ceramics to improve their toughness.
This study aims at investigating the physical, mechanical properties and fracture behaviors, and internal residual stresses in metal reinforced ceramic matrix composites (CMCs). A356, 6061 and 1050 aluminum alloys were infiltrated into the aluminum oxide (Al2O3) preforms in order to fabricate Al2O3/A356, Al2O3/6061 and Al2O3/1050 composites, respectively, with different volumes of aluminum alloy content using the pressure infiltration technique of squeeze casting. The contents of aluminum alloy in the composites were 10 to 40 percent by volume. For all different Al alloy composites, the hardness decreased dramatically, the four-points bending strength increased, the fracture toughness increased, and the resistivity decreased dramatically with increasing Al alloy content in the composites, respectively. From SEM microstructural analysis and TEM bright field images, the porous ratio and the relative density of the composites were the most important factors that affected the physical and mechanical properties, and there are four different toughening mechanisms affected the toughness of the composites, i.e. metal phase increased, crack bridging, crack deflection, and crack branching in the composites.
Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. The residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic / metal interface of the Al2O3/A356 composites. The relationship between residual stresses and the contact area of the ceramic / metal interface are also discussed.
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