Effects of Initial Temperature and Tempering Medium on Thermal Tempering of Dental Porcelains

Abstract

The objective of this study was to test the hypothesis that quenching of porcelain in silicone oil rather than in compressed air will significantly increase the flexure strength by reducing the potential for crack formation during transient cooling. A secondary hypothesis to be tested is that the initial tempering temperature can be reduced significantly below the porcelain maturing temperature of 982°C but well above Tg without a decrease in strength. Opaque-body porcelain disks, 16 mm in diameter and 2 mm in thickness, with a thermal contraction mismatch (Δα) of -1.5,0, and +3.2 ppm/°C were tempered from initial temperatures of 650, 750, 850, and 982°C in silicone oil with kinematic viscosities of 50,1000, and 5000 centistokes. Porcelain disks were also subjected to three cooling procedures in air: slow cooling in a furnace (SC), free convective cooling in a laboratory bench (FC), and tempering (T) by blasting the surface of body porcelain with air.

The crack size induced by a Vickers microhardness indenter was measured within one minute after crack development. For determination of the influence of initial cooling temperature on biaxial flexure strength, six body porcelain disks (Δα = 0) were tempered in air from initial temperatures of 650, 750, 850, and 982°C. The mean crack size of specimens tempered in oil was significantly smaller (p ≤ 0.001) than that of specimens that were slowly-cooled or fast-cooled in air for all thermal contraction mismatch cases. For the same initial temperature, the compatible system (Δα = 0) yielded a smaller crack size compared with the other mismatch levels. For specimens tempered in air, an increase in initial tempering temperature from 650°C to 850°C produced a 34% increase in flexure strength (80.2 MPa to 107.6 MPa). However, increasing the initial tempering temperature from 850°C to 982°C resulted in only a 7% increase in the mean flexure strength. The results of this study indicate that tempering of dental porcelains in air or in silicone oil from an initial temperature ranging from 650 to 850°C improved the resistance to crack initiation and failure. However, compared with oil quenching, tempering in air was associated with a smaller risk of thermal shock failure.