Objective: This study tested the hypothesis that thermal conditioning of glass-ceramic specimens at and near the glass transition temperature (Tg), will not significantly affect the residual stress distribution measured by micro-indentation. Methods: Twenty porcelain disks, 16 mm dia x 1 mm, (Eris, Ivoclar) were fabricated according the manufacturer's specifications and polished through 1µm abrasive. Specimens were randomly divided into five heat treatment groups (n=4 each): Tg-25°C, Tg-50°C, Tg, Tg+25°C and Tg+50°C. Samples were held for 1 h at designated temperature and bench cooled. Vickers microindentations, thirty per specimen, were made using a 4.9 N load at designated intervals. Both lateral and vertical crack lengths propagating from the indentations were measured. The average was recorded as mean crack length (2c). The magnitude of non-uniform residual stress distribution within the veneer surface was calculated with respect to the specimens heat treated at Tg. This was done using the equation: σ_R = 0.5*K_c*(π/c_HT)^1/2*[1-(c_TG/c_HT)^3/2] where K_c = 0.77 MPa∙m^1/2 for Eris, c_TG is the mean crack length for treatment Tg and c_HT is the mean crack length of the four heat treated groups. Results: The crack lengths (mean±SD) in each of the five groups were Tg-50˚C (54.4±1.6µm), Tg-25˚C (56.2±1.4µm), Tg (54.6±0.3µm), Tg+25˚C (54.5±0.9µm), Tg+50˚C (53.7±0.7µm). ANOVA showed there were no significant differences among groups (p=0.0550). The residual stresses (mean±SD) in each of the five groups were Tg-50˚C (-0.5±4.4MPa), Tg-25˚C (3.9±3.2MPa), Tg (0±0.8MPa), Tg+25˚C (-0.4±2.3MPa), Tg+50˚C (-2.2±1.8MPa). ANOVA showed there was no significant difference among the groups (p=0.0733). Conclusions: These results imply that within this range, variations in temperature were insufficient to cause a significant change in the residual stress of Eris veneer monoliths.

This research was supported by the UF Undergraduate Scholars Program and NIH/NIDCR grand DE06672.