The overall goal of the proposed research is to determine the mechanisms responsible for the thermally induced changes in coefficient of thermal expansion that are experienced by dental porcelains and to use the understanding thus gained to develop porcelains more resistant to such changes. The development of such thermally stable porcelains would greatly reduce the tendency for cracking and checking of the dental porcelains during cooling of porcelain-fused-to-metal restorations, with consequent reductions in cost, inconvenience, and re-treatment trauma to the dental patient receiving this type of restoration. The first specific aim is to complete the measurement of the effects of multiple firings, isothermal heat treatments, and different cooling rates on the coefficients of thermal expansion for a variety of dental porcelains. These measurements will be accomplished in a conventional differential dilatometer and in a laser dilatometer developed in the Dental Materials Laboratory at the Medical College of Georgia. The second specific aim is to determine the mechanisms responsible for porcelain expansion changes during various heat treatments. The methods which will be used to discriminate among the possible mechanisms are: quantitative x-ray diffractometry, high-temperature x-ray diffractometry, hot-stage scannign electron microscopy, and measurement of the mid-span deflection of bimaterial porcelain-metal strips in a high rate infrared bending beam viscometer developed at the Dental Materials Laboratory at the Medical College of Georgia. During the present grant period, quantitative x-ray diffractometry was shown to be a suitable tool for the measurement of the thermal expansion coefficients of the crystalline components of dental porcelains in situ in the glassy porcelain matrix. This technique will greatly aid in the determination of the mechanisms involved in the thermal instability of dental porcelains. Possible mechanisms for thermal instability of dental porcelains involve the crystallization or dissolution of leucite, the conversion of leucite to sanidine, the retention of leucite particles in the glass matrix via microcracking and sintering, and trapping of various levels of excess volume in the glass matrix owing to different cooling rates. The development of methods for improving the thermal stability of porcelain frits is the third specific aim of the proposed work. The strategies for developing improved porcelain frits involve reduction or elimination of leucite coupling/decoupling, stabilization of the leucite fraction, avoidance of metastable cubic leucite retention upon cooling from the porcelain firing temperature, and minimization of the effects due to structural relaxation of the glass matrix.
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