Stress relaxation in titanium–ceramic beams during veneering

Spalling of a ceramic from the metallic substrate is related to stress at the metallo-ceramic interface of crowns. The purpose of this study is to (1) establish a method for recording the bending radius of metallo-ceramic strips during cooling, (2) calculate the stress in the metal adjacent to the metallo-ceramic interface based on the recorded bending and (3) compare this stress with that calculated from the mismatch between measurements of the coefficients of thermal expansion, alpha, of the ceramic and titan. Metallo-ceramic specimens (n = 9) were made by firing a 0.6 mm thick layer of ceramic on (11.3 x 3.2 x 0.6) mm Ti-substrates. The opaque material of three commercial porcelains for titanium were applied: Titankeram (Vita), TiBond (DeTrey/Dentsply) and Duceratin (Ducera). The Ti-side of the specimens rested on two quartz supports of a jig in an apparatus for thermo-mechanical analysis (TMA). The vertical, mid-span movements of a quartz flexure probe resting on the ceramic-side of the metallo-ceramic specimen were continuously recorded during heating and cooling in the temperature range 20-600 degrees C. The cooling rate was 6 degrees C min(-1) from 600 degrees C, to below the glass transition temperature, Tg. The bending radii recorded continuously during cooling were reproducible. The thermal expansions, alpha(cer), lay in the range 23-47 x 10(-6) K(-1) above Tg compared to 7.9-9.8 x 10(-6) K(-1) (alpha(cer, 20-400 degrees C)) below Tg. For Ti, the coefficient of thermal expansion was found to be 9.7 x 10(-6) K(-1) (alpha(Ti, 20-600 degrees C)). The calculated stresses above Tg based on the thermal mismatch were much higher than those calculated from the flexure movement. The observed differences are attributed to stress relaxations in the ceramic above Tg.