Bond strength and thermal relaxation of 3Y-TZP and dental porcelains

Objectives: To measure and model the interfacial thermal stresses induced by veneering zirconia with porcelain and to investigate phase instability in zirconia under these stresses. Method: The ISO 9693 standard bond strengths of three porcelains (VT: Vita Titankeramik, CZ: Cerabien Zirconia and CS: Cercon Smart Ceramics) veneered on a slip cast zirconia core material were determined by the Schwickerath method. The interfacial stress developed during heating and cooling of bi-material strips (11 x 2,6 x (0,6 + 0,6)) mm was monitored in a thermo-mechanical analyser via the deflection of the strips during uniform heating to the dynamic softening point of the porcelain and subsequent cooling. Candidate visco-elastic models for each porcelain were tested against thermal expansion data measured over the same thermal cycle. Results: The standard bond strengths were [MPa] VT: 39 ± 6, CZ: 24 ± 3 and CS: 35 ± 8. The porcelains' glass transition temperatures, Tg / dynamic softening points, Ts, were determined to be [°C] VT: 540 / 638, CZ: 570 / 677 and CS: 490 / 535. Below Tg, sudden changes in bend radii, indicating interfacial instability, occurred for the CZ and CS veneers. Conclusion: The shear component of the stress may induce instability at the veneer-core interface. The thermal expansion of porcelain is described by a two-regime visco-elastic model.