THERMALLY INDUCED CHANGES IN DENTAL PORCELAIN EXPANSION

Project: Research project

Project Details

Description

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.
StatusNot started