Isothermal phase transformations of a dental porcelain

The purpose of this investigation was to determine if the change in the leucite weight fraction during an isothermal heat treatment could be estimated by observing the deformation of PFM strips in a high-heating-rate, computer-controlled bending beam viscometer (BBV). Methods Specimens of a commercial body porcelain were fired according to the manufacturerʹs instructions—50 disk specimens for quantitative X-ray diffraction (XRD) and 100 bimaterial strip specimens for BBV. The XRD specimens were annealed at temperatures between 650 and 1000 °C, and leucite weight fraction was measured using an alumina internal standard. The BBV specimens were annealed in the BBV using time–temperature schedules designed to elucidate the leucite crystallization behavior between 700 and 1000 °C. Timoshenkoʹs equation for a bimaterial thermostat was used to estimate the change in the thermal expansion of the porcelain near room temperature. Changes in leucite weight fraction were determined from these thermal expansion changes. Results The means and SDs were compared to values obtained by quantitative XRD. Good agreement was obtained between values of leucite weight fraction derived from beam deformation and those determined by quantitative XRD (p≥0.45). The anneal sequence showed that the increase in leucite weight fraction at 800 or 900 °C is reversible by an anneal at 1000 °C. Significance The BBV technique yields comparable results to quantitative XRD and provides the opportunity to efficiently monitor porcelain leucite changes nondestructively over multiple heat treatments. This technique could prove useful for testing firing schedules designed to stabilize the leucite content in dental porcelain.