Machinability Evaluation of Dental Restorative Materials

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Ceramic materials are ideal candidates for dental restorative applications for their color, texture, and mechanical properties which closely resemble those of the human enamel. However, due to the inherent brittleness of ceramic material, material processing, especially machining, poses a variety of difficulties. Research efforts of this thesis are directed to the development of a critical guideline for evaluating the machinability of ceramic materials, where human enamel is used as a reference material for comparison.

Using a systems engineering approach, a computer-based surface integrity assessment methodology is formulated. It combines the most recently developed image processing technology with computer graphics while incorporating the principles of fracture mechanics. Microhardness testing is used to study material properties related to machining. Four types of material selected are human enamel, Dicor-MGC, HCC Dentine, and HCC Enamel. Three- dimensional visualization of the surface impressions is achieved using an environmental scanning electron microscope and an atomic force microscope. Machining experiments are conducted to study the surface integrity, including surface finish, micro- cracking, and edge chipping. Analytical investigation correlates these surface responses to the machining parameters, such as spindle speed, feed rate, and the depth of cut, to seek a parametric region in which quality of machined ceramic components can be ensured. Surface integrity performance indices such as surface roughness, cavity density, and chip aspect ratio are proposed to quantify such evaluations.

Major contributions of this thesis research include the development of the combined SEM- AFM stereophotography method. The high resolution achieved with this method ensures coverage of rich information on the surface texture formed during machining. Specific findings of this thesis research include the identification of micro-mechanics of fracture occurred during the material removal process, and a good understanding of possible influences of the microstructures on the machining performance.