STRESS-RELAXATION BEHAVIOR OF DENTAL PORCELAINS AT HIGH-TEMPERATURES

Objectives. The purpose of this study was to measure the stress relaxa tion behavior at elevated temperatures of three experimental opaque po rcelains and three experimental body porcelains. Methods. Feldspathic porcelain formulations covering a range of thermal contraction coeffic ients were supplied by a dental ceramics manufacturer. Six specimens, 11 mm in diameter by 22 mm long, were fabricated for each porcelain. T he specimens were tested in compression at five temperatures controlle d to +/- 1 degrees C in a hot stage furnace attached to a screw-type u ni-axial testing machine. Results. Mean values of relaxation time, tau (u), and the b function were determined by a regression fit to the rel ation: psi(t) = exp [-(t/tau(u))(b)]. Values of b ranged from 0.23 to 0.53 for opaque porcelain and 0.47 to 0.64 for body porcelain. Relaxat ion times ranged from 2.6 s to 4 x 10(4) s for the opaque porcelains a nd 1.5 s to 5.5 x 10(2) s for the body porcelains. A statistically sig nificant variation of b with temperature for three of the experimental porcelains is an indication that these porcelains do not satisfy the theoretical requirements for the porcelains to be classified as thermo rheologically simple. Significance. A knowledge of the relaxation beha vior of dental porcelains is necessary so that dental researchers can identify metal/porcelain combinations that will result in low stress v alues and, therefore, reduce the potential for failure from thermally induced stresses. These properties can be used in the optimization of prosthesis design to reduce the destruction of healthy tissue to accom modate the placement of the dental prosthesis.