Analysis of tempering stresses in metal-ceramic disks.

Previous studies showed that residual compressive stresses induced by thermal tempering retarded the growth of surface cracks in bilayered porcelain disks. The objectives of the present study were: (1) to determine whether thermal tempering by air blasting reduces the length of cracks induced by microhardness indentation in metal-ceramic disks, and (2) to use visco-elastic finite element analyses to calculate transient and residual stresses in metal-ceramic disks. Ni-Cr-Be disks, 16 mm in diameter and 0.3 mm in thickness, were prepared with a 0.5-mm-thick layer of opaque porcelain and a 1.5-mm-thick layer of body porcelain. Metal-porcelain combinations were selected to provide a range of thermal contraction mismatch values. The disks were fired to the maturing temperature of body porcelain and then were subjected to three cooling procedures: (1) slow cooling in a furnace (SC), (2) cooling in air (FC), and (3) air tempering (T) by blasting the surface of the body porcelain with compressed air. The lengths of cracks induced in the surface of the body porcelain by a microhardness indenter were measured immediately after indentation at 20 points along diametral lines. The results of Tukey's multiple-contrast analyses indicated that the mean crack lengths of air-tempered specimens were significantly smaller (p < or = 0.05) than the crack lengths of the fast-cooled and slow-cooled groups. Except for one case, there were no statistically significant differences in the mean crack lengths between FC and SC specimens independent of thermal contraction mismatch. Residual tensile stresses were calculated for SC and FC specimens for all thermal contraction mismatch cases, with the largest values being associated with combinations containing the body porcelain with the smaller contraction coefficient. Calculations by use of the model confirmed that tempering induces large residual compressive stresses in the surface of body porcelain for all of the thermal contraction mismatch cases included in this study.

Stress relaxation behavior of dental porcelains at high temperatures.

OBJECTIVES: The purpose of this study was to measure the stress relaxation behavior at elevated temperatures of three experimental opaque porcelains and three experimental body porcelains. METHODS: Feldspathic porcelain formulations covering a range of thermal contraction coefficients were supplied by a dental ceramics manufacturer. Six specimens, 11 mm in diameter by 22 mm long, were fabricated for each porcelain. The specimens were tested in compression at five temperatures controlled to +/- 1 degree C in a hot stage furnace attached to a screw-type uni-axial testing machine. RESULTS: Mean values of relaxation time, tau u, and the b function were determined by a regression fit to the relation: 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. Relaxation times ranged from 2.6 s to 4 x 10(4) s for the opaque porcelains and 1.5 s to 5.5 x 10(2) s for the body porcelains. A statistically significant 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 thermorheologically simple. SIGNIFICANCE: A knowledge of the relaxation behavior of dental porcelains is necessary so that dental researchers can identify metal/porcelain combinations that will result in low stress values 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 accommodate the placement of the dental prosthesis.