Effect of Firing Temperature of Opaque in Metal/Porcelain Bond Strenght

Objectives: This study evaluated the opaque layer firing temperature and the effect of mechanical- and thermal-cycling on the flexural strength (ISO 9693) of a ceramic fused to commercial cobalt-chromium alloy (Co-Cr). Methods: Metallic frameworks (25x3x0.5 mm) (N=60) were cast in Co-Cr and their central area were air abraded with 150 µm aluminum oxide (8x3 mm); after that the samples were divided into three groups (n=20) according to the opaque layer firing temperature: Gr1 (control) –900°C; Gr2 – 950°C and Gr3 – 1000°C. The opaque ceramic was applied and the glass ceramic (Vita Omega 900, Vita Zahnfabrick, Bad Säckingen, Germany) was fired onto it (thickness: 1 mm). While half of the specimens from each group were randomly tested without aging (water storage for 24 hours at 37ºC only), the other half were mechanically loaded (20,000 cycles under 10 N load, immersed in distilled water at 37ºC) and thermocycled (3,000 cycles, between 5-55ºC, dwell time: 30 seconds). After the flexural strength test, failure types were noted. The data were analyzed using 2 way-ANOVA and Tukeyxs test (a = 0.05). Results: Gr2 (19.41±5.5 N) and Gr3 (20.6±5 N) presented higher flexure strength values than Gr1 (13.3±1.6 N) (p=0.001). The predominant failure type, for Gr1, was adhesive failures at the opaque ceramic/Co-Cr interfacial zone with no presence of ceramic on the metallic surface but with a visible dark oxide layer in all experimental samples. Cohesive failures at the opaque layer were observed in all samples for the other groups (9500C and 1.0000C). Conclusions: Mechanical- and thermal-cycling did not influence significantly the mean flexural strength values (p>0.05) for all the opaque firing temperature/Co-Cr combinations tested. Increasing the opaque layer firing temperature improved the flexure bond strength values significantly (p<0.05).