Institute for Systems Research

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Author: search.filters.author.Zhang, G.Subject: search.filters.subject.measurements

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    A Comparative Machinability Study of Dental Materials
    (1997) Zhang, G.; Rekow, D.; Thompson, V.; ISR
    This paper presents results obtained from a comparative machinability study of newly invented ceramic materials for dental restorations. With the microstructure being the dominant factor on crack initiation and propagation during the fabrication process, the objective of this study is to identify the relationship between the microstructural characteristics and damage created during machining which could compromise the reliability of ceramic-made dental restorations. Glass ceramic material with tailored characteristics of microstructure is used in this study. The machining platform is milling operations where proper tool geometry and machining parameters are selected. Empirical models to correlate the cutting force and degradation of flexure strength with machining parameters are established. The aims of the research are to reduce surface cracking to yield improved surface finish (smooth < 2 mm) and to lessen strength degradation after machining. The study consists of four steps, including characterization of microstructure, identification of machining parameters, on-line monitoring of the machining operations, and evaluation of flexure strength degradation. Contributions of this study are the establishment of a procedure for carrying out the machinability assessment, and the establishment of empirical models relating the cutting force and fracture strength degradation to the cutting conditions.
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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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    Assessment of Non-Linear Dynamics of Material Removal on Surface Integrity
    (1997) Zhang, G.; Ng, S.J.; Le, Dung T.; ISR
    In this chapter, we present our research on the study of non- linear dynamics observed during the machining of ceramic materials. We will focus on both innovation in machining technology and development of non-destructive evaluation methods to assess machining performance. Three aspects of our work are presented in the following sections. They are, 1) Submerged Precision Machining, and 2) Scanning Electron Microscopy Analysis. Aspect 1 relates to an innovative approach to machining, while aspects 2 relates to evaluation methodologies for surface characterization. These methods and techniques have been developed to achieve cost-efficient machining as well as high-quality surface finish in ceramic material.
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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.