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Glass ceramics for dental crown prosthesis were prepared by crystallization of CaO-MgO-SiO2- glasses. Their crystallization behaviors have been investigated as a function of heattreatment temperature, holding time and chemical composition in relation to mechinical properties. Crystallization peak temperatures were determined by differential thermal analysis(DTA). Crystalline phases and mircostructures of heat-treated sample were determined by the means of powder X-ray diffraction(XRD) and scanning electron microscopy(SEM). The final crystalline phase assemblages and the microstructures of the samples were found to be dependent on glass compositions, heattreatment temperature, and holding time. 1st crystallization peak temperature(TP), affected strongly by apatite, was found to be increased or decreased. From the experiment, the following results were obtained : 1. The crystallization peak temperature() formed by apatite increased until adding up to 9wt% to base glass composition, then decreased above that. 2. Apatite(), whitlockite(), -wollastonite(), magnesium tianate() and diopside(CaO-MgO-) crystal phase were precipitated in MgO-CaO- glass system containing 9wt% and 11wt% of 3. Vickers hardness of samples increased with increasing heat-treatment temperature and Vickers hardness of S415T9 samples heat-treated at 1075 was approxi-mately 813Kg as maximum value. 4. Vickers hardness of samples increased due to precipitation of apatite, whitlockite, -wollastonite, magnesium titanate, and diopside crystal phases within glass matrix.
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