Effect of veneering technique, heat rate, holding time and zirconia thickness on the mechanical properties of porcelain veneers

Abstract

OBJECTIVES: Evaluate the effect of different veneering techniques, Y-TZP core thicknesses, and firing cycles with different heat rates and holding times on the mechanical properties of veneering porcelain. MATERIALS AND METHODS: Biaxial flexural strength and Thermal shock resistance tests were conducted to evaluate the mechanical properties of veneer porcelain. Core material, Y-TZP and two porcelain veneers, IPS e.max Ceram and VITA VM9, were used in this study. Vita YZ zirconia blocks were sectioned and sintered to provide slabs of 1.65, 3.25, and 6.50mm in thickness. Two techniques were used to fabricate VITA VM9 and e.max Ceram porcelain veneer porcelain discs, Hand Layered, mixing powder with manufacturer’s molding liquid, condensed in a mold, and Pressed, pressing powder uniaxially in a mold. A layer of carbon paint was applied to the zirconia to allow removal of the porcelain discs. For thermal shock test the veneering porcelain fired on the Y-TZP core with a wash layer. After the veneer discs were fabricated, they were sintered: (1) According to the manufacturer’s instructions; (2) Two different cycles with slower heat rate and longer holding than the recommended value. Biaxial flexural strength was determined using a universal mechanical tester. Thermal shock tests were performed using a Pober thermal shock device. Statistical analysis was conducted for all tests using ANOVA and Tukey post hoc test at p= 0.05.

RESULTS: There was significant effect from changes of YZ Core thickness, firing cycle, and veneer technique on the mechanical properties of porcelain veneer. The veneering technique had the largest impact on the mechanical properties of veneer porcelain followed by firing cycle, followed by YZ core thickness. In using different firing cycles with different heat rates and holding times, slowing the heating rate had more influence on the mechanical properties of veneer porcelain. In measuring the surface temperature of the Y-TZP/VITA VM9 assembly, the specimen surface temperature was coincidental with the furnace programmed firing cycle using a slower heating rate firing cycle. CONCLUSIONS Veneering technique, slower heat rate firing cycle, and Y-TZP core thickness have significant impact on veneering porcelain mechanical properties.