Effects of interfacial variables on ceramic adherence to cast and machined commercially pure titanium.

STATEMENT OF THE PROBLEM

Titanium-ceramic bonding is less optimal than conventional metal-ceramic bonding, due to excessive oxidation of titanium during porcelain firing.

PURPOSE

This in vitro study evaluated the effects of porcelain firing atmosphere and gold sputter coating on titanium surfaces on porcelain bonding to machined and as-cast titanium substrates. Material and methods Eight groups of ASTM grade 2 commercially pure (CP) titanium specimens (13 mm x13 mm x1 mm) were prepared (n=10). A conventional Au-Pd-In metal-ceramic alloy (Orion) and an ultra low-fusing porcelain (Finesse) served as the control (n=10). Forty machined titanium specimens were prepared from 1.00-mm thick titanium sheets with a diamond band saw. Forty titanium specimens were produced in a centrifugal dental titanium casting machine. All titanium specimens were airborne particle abraded with 110-microm alumina particles, whereas the control specimens were airborne particle abraded with 50-microm alumina particles. Forty titanium specimens (20 specimens each of as-cast and machined titanium) were randomly selected for gold sputter coating before ceramic firing. An ultra low-fusing porcelain (Vita Titankeramik) was fused on the central 6-mm diameter circular area on each titanium specimen. Porcelain firing environments for the titanium specimens consisted of vacuum and a reduced argon atmosphere. Porcelain was debonded by a biaxial flexure, constant strain test at a cross-head speed of 0.25 mm/min. Specimens were analyzed by standardized SEM/EDS analysis 3 times throughout the study to determine the silicon atomic percentage (Si at %): (1) after airborne particle abrasion, before porcelain application; (2) after the application of the first layer of porcelain; and (3) after the fracture of porcelain from the metal substrate. The titanium-ceramic adhesion was characterized by determining the area fraction of adherent porcelain (AFAP). Results were analyzed by analysis of variance and the Student-Newman-Keuls test (alpha=.05).

RESULTS

Statistical analysis showed a significant difference in the AFAP values among all the groups. AFAP value of the control group was significantly higher (135.35 +/- 23.68) than those of the experimental groups (P<.001). For the machined titanium, AFAP value of gold sputter-coated/argon group (91.38 +/- 7.93) was significantly higher than the rest of the groups (P<.001). For the as-cast titanium fired in vacuum, significantly lower AFAP values (P<.001) were found in the gold sputter-coated group (50.2 +/- 11.26 vs 66.15 +/- 10.41). AFAP values between the argon groups with or without the gold coating were not significantly different (P=.303); however, both argon groups (93.83 +/- 4.65 and 98.09 +/- 6.35) showed significantly higher AFAP values compared with the vacuum groups (P<.001).

CONCLUSION

Firing porcelain in a reduced argon atmosphere significantly improved titanium-ceramic bonding for machined and as-cast titanium. The sputter-coated gold layer on titanium provided improved titanium-ceramic bonding only when combined with firing porcelain in reduced argon atmosphere. When porcelain was fired in vacuum in the presence of the gold layer, the titanium-ceramic bonding was weakened in as-cast titanium and was not affected in machined titanium. Conventional noble metal-ceramic bonding was superior to titanium-ceramic bonding regardless of the interfacial variables examined in this study.