Effect of porcelain veneering technique in bilayered zirconia on bond strength and residual stress distribution

The use of all porcelain materials in dentistry has significantly increased in recent years. However, chipping has remained a common problem that affects bilayered zirconia restorations. Bonding between porcelain and the underlying zirconia framework is crucial to the success of the restoration. The bond strength may be affected by such factors as residual thermal stress and the veneering technique. This research focuses on investigating the potential and constraints of materials through an examination of the porcelain veneering technique, particularly hand-layering and heat-pressing. Forty-two cylindrical disc samples of zirconia (n = 7/group) were fabricated in the dimensions of 10 x 1.2 mm (diameter [D] x height [H]). The zirconia specimens were milled from IPS e. max (R) ZirCad [Z] block and Luxen Zr [L] block (n = 21/zirconia). The zirconia cores were layered with IPS e. max (R) Zirliner and heat-pressed with IPS e.max (R) ZirPress to produce a final veneer dimension of 5 x 3 mm (D x H). Conventional layering was performed for the rest of the zirconia cores using IPS e.max (R) Ceram and Shofu Vintage Zr. The final study groups were Luxen-Vintage (LV), Luxen-Ceram (LC), Luxen Zirpress (LP), ZirCadVintage (ZV), ZirCad-Ceram (ZC) and ZirCad-Zirpress (ZP). Five samples were subjected to shear bond testing (SBS) with a universal testing machine with a 5 kN load cell and 0.5 mm/min crosshead speed (n = 5/group). A sample underwent nanoindentation, and another was sectioned using Isomet machine to study the bonding interface. One-way ANOVA was used to run the statistical analyses of the SBS test. Statistical differences were found between ZV with LC and LP (p < 0.05). Residual stress is estimated to be higher in the middle of the porcelain compared with that on the surface and the interface. FESEM imaging reveals portions of visible bare zirconia on Luxen zirconia, whilst crack propagation occurred through voids in all hand-layered groups. Heatpressed veneering showed comparable but not superior results to conventional hand-layered veneering. Heatpressed veneering produced similar stress distribution profiles compared with hand-layered veneering.