歯科用せっこう硬化体の破面学的研究

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概要

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• From the fractographical point of view the relation between tensile strength and microstructure was studied for several dental plasters and stones by means of diametral compression test, scanning electron microscopy and x-ray diffraction. The following results were obtained. 1. Most of the specimens prepared from various dental plasters and stones fractured in the diametral plane containing the applied load axis. This fact shows the diametrical compression may set up a theoretical tensile stress normal to the diametral plane of the specimen. In practice, the plastic deformation of the specimen at the contact area increased with W/P ratio ; the load is distributed over the extended contact area. The fracture producing a "triplecleft" pattern was rarely observed in the present investigation of wet specimens. 2. The values of tensile strength obtained in the present investigation were nearly equal to the values in direct tension test by Nakamura et al. Consequently, it may be considered that the diametral compression test is a valid one for dental gypsum products. 3. In each specimen the longitudinal growth of dihydrate crystals increased and the tensile strength decreased as the W/P ratio increased. Therefore, the behavior of dihydrate crystals is a parameter of tensile strength in each specimen ; the decrease of tensile strength can be caused by the longitudinal growth of dihydrate crystals which brings about the decrease of contact of dihydrate crystals and the increase of the volume of porosity in the set mass. 4. Three crystal habits of hemihydrates were observed ; S type (dense stubby crystals), E type (dense elongated crystals) and A type (porous agglomerate of fine crystals). 5. Hemihydrate crystals were also observed in the fractured surface. These residual hemihydrates were dominantly observed at lower W/P ratio, although their quantity is not equivalent in each of the specimen even at the same W/P ratio according to the x-ray diffraction patterns. The set mass contained the residual E type of hemihydrates was stronger than that contained S type. 6. It is considered that the fundamental structure of set mass is composed of two phases : dihydrate and hemihydrate. Both crystals should be considered as parameters contributive to the phisical properties of set mass.

• 九州歯科学会の論文
• 1974-03-31

著者

• 森 淑子

  九州大学歯学部第一歯科補綴学教室

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• 歯科用せっこう硬化体の破面学的研究

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