Institute for Systems Research

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    Hardness Assessment of Human Enamel
    (1997) Zhang, G.; Ng, S.J.; Le, Dung T.; Young, D.; ISR
    This paper presents results from investigating indentation impressions on human enamel under micro-hardness tests. The experiments of hardness testing were performed on a microhardness indentation machine under different loading conditions. Images of indentation impressions were obtained using an environmental scanning electron microscope. Geometrical shapes of hardness indentations were visualized in three-dimentional space using computer graphics. Quantitative information was obtained through atomic force measurements to characterize ﲰile-up , ﲳink-in , and elastic recovery of enamel. Special efforts have been made to study the microstructual effect of the calcified rods orientations on the fracture patterns formed during the hardness tests. Significant finding include that the occlusal surface demonstrates much stronger resistance to the indentation force than does the buccal surface and shows 40% elastic recovery after indentation. A new formula to determine hardness value has been proposed. By incorporating the reversible deformation into the evaluation, a normalized hardness measurement can be made to form a basis for comparison and other investigations where hardness has its unique role to play.
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    Microstructural Effects on the Machining Performance of Dental Ceramics
    (1997) Le, Dung T.; Qi, L.; Zhang, G.; ISR
    Constraints of mechanical, thermal, and chemical properties are making ceramics the material choice for industrial and dental applications. The quality of a machined surface of ceramics is fundamentally dependent on the response of the material to the machining process. This paper presents a combined analytical and experimental study with focus on optimizing the machining performance of dental ceramics -- DICOR/MGC -- with three distinguished microstructures. The study starts from analyzing the microstructural characteristics to searching for the machining conditions that provide satisfactory performance in terms of acceptable flexural strength. Evidence gained from the cutting force measurements and evaluation of fracture strength degradation indicates that the control of micro-scale fracture formed on the machined surface, with microstructural characteristics being considered, is the key factor which dominates the machining performance.
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    Characterization of Indentation Impressions on Human Enamel For Hardness Measurement
    (1997) Zhang, G.; Le, Dung T.; Tucker, S.R.; Ng, S.J.; ISR
    This paper presents results from investigating indentation impressions on human enamel under micro-hardness tests. The experiments of hardness testing were performed on a microhardness indentation machine under different loading conditions. Images of indentation impressions were obtained using an environmental scanning electron microscope. Geometrical shapes of hardness indentations were visualized in three-dimensional space using computer graphics. Quantitative information was obtained through atomic force measurements to characterize "pile-up", "sink-in", and elastic recovery of enamel. Special efforts have been made to study the microstrucutual effect of the calcified rods orientations on the fracture patterns formed during the hardness tests. Significant findings include that the occlusal surface demonstrates much stronger resistance to the indentation force than does the buccal surface and shows 40% elastic recovery after indentation. A new formula to determine hardness value has been proposed. By incorporating the reversible deformation into the evluation, a normalized hardness measurement can be made to form a basis for comparison and other investigations where hardness has its unique role to play.

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    Control of Machining Induces Edge Chipping on Glass Ceramics
    (1996) Ng, S.J.; Le, Dung T.; Tucker, S.R.; Zhang, G.M.; ISR
    Edge chipping is a phenomenon commonly observed during the machining of ceramic material. Characterization of edge chipping, both in macro and in micro scale, and correlating its formation to machining parameters form a basis for developing new and innovative technologies for controlling in machining induced damage. An experimental-based study using glass ceramic material is performed. Three types of edge chipping are identified. The SEM-sterephotography method and the finite element method are used to evaluate the edge chipping effect under a set of machining conditions. Significant findings are obtained and guidelines for controlling edge chipping during machining are suggested.