Stress relaxation in titanium-ceramic beams during veneering

Spalling of a ceramic from the metallic substrate is related to stress at t he metallo-ceramic interface of crowns. The purpose of this study is to (1) establish a method For recording the bending radius of metallo-ceramic str ips during cooling, (2) calculate the stress in the metal adjacent to the m etallo-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 cera mic on (11.3 x 3.2 x 0.6) mm Ti-substrates. The opaque material of three co mmercial porcelains for titanium were applied: Titankeram (Vila), TiBond (D eTrey/Dentsply) and Duceratin (Ducera). The Ti-side of the specimens rested on two quartz supports of a jig in an apparatus for thermo-mechanical anal ysis (TMA). The vertical, mid-span movements of a quartz flexure probe rest ing on the ceramic-side of the metallo-ceramic specimen were continuously r ecorded during heating and cooling in the temperature range 30-600 degreesC . The cooling rate was 6 degreesC min(-1) from 600 degreesC, to below the g lass transition temperature, T-g. The bending radii recorded continuously d uring cooling were reproducible. The thermal expansions, alpha (cer), lay i n the range 23-47 x 10(-6) K-1 above T-g compared to 7.9-9.8 x 10(-6) K-1 ( alpha (cer,20-40 degreesC)) below T-g. For Ti, the coefficient of thermal e xpansion was found to be 9.7 x 10(-6) K-1 (alpha (Ti, 20-600 degreesC)). Th e calculated stresses above T-g based on the thermal mismatch were much hig her than those calculated From the flexure movement. The observed differenc es are attributed to stress relaxations in the ceramic above T-g. (C) 2001 Elsevier Science Ltd. All rights reserved.