Sintering of dental porcelain: effect of time and temperature on appearance and porosity

Abstract

Objective: Mechanical condensation of powder-method dental porcelains can only achieve a limited effect—the majority of consolidation and porosity elimination is achieved by sintering. However, there is a surprising lack of information on the process in the literature, and the effects of the two basic conditions of sintering, time and temperature, are very poorly described despite theoretical expectations. The present study was to investigate the effects of these conditions on porosity and to consider its relationship to the recognizable firing stages of low and high biscuit, and other aspects of appearance, with a view to schedule recommendations. Methods: The variation of appearance, pore form and percentage porosity with sintering time and temperature was studied for five dental dentine porcelains, two aluminous: Alpha (Vita Zahnfabrik, Bad Säckingen, Germany) and Vitadur-N (Vita); and three feldspathic: Omega (Vita), VMK68 (Vita) and Carmen (Esprident, Ispringen, Germany). Disc specimens were sintered for 0–1000 min over 750–1100°C in a systematic search pattern to establish limits of acceptable appearance (high biscuit to not quite slumped). Porosity was measured using an image analyser on specimens fired for sintering times of 24 and 30 s; 1, 3, 6 and 30 min; 1, 5, 10, 15 and 20 h; with sintering temperatures from 750 to 950°C for Carmen and 800–1050°C for the others, all with 50°C increments. Measurements were made on a ground and polished surface (1 μm diamond), five fields on each of five specimens per condition. The percentage porosity, pore count, median pore area and pore size distribution were analyzed. Results: The boundaries of the acceptable appearance areas in maps of sintering temperature vs. sintering time were clearly delineated; analysis showed that they may be related to the activation energy of the diffusive processes occurring during sintering. Minimum porosity was obtained at high temperature and short time, close to but not consistently coincident with the manufacturers’ recommendations. There is also a conductivity-related minimum sintering time at high temperatures. The theoretical reduction on prolonging sintering at high temperature was not a general result, and porosity increased markedly in the feldspathic porcelains, particularly VMK-68. Significance: The reduction of porosity of dentine porcelain is much more sensitive to temperature than to time. It is possible systematically to identify optimum conditions using the present approach. Detailed study of the effects of formulation on the sintering process may be made by reference to the activation energy.