Shear Bond Strength Comparison between Conventional Dental Nano Zirconia Combinations and New Functionally Graded Nano Zirconia Combinations after Thermal-Mechanical Cycling

The aim of the study was to evaluate the core-veneer bond strength of graded zirconia combinations and compared those to conventional zirconia combinations after thermal and mechanical tests. Conventional zirconia-veneer combinations and graded zirconia-veneer combinations were made into cylinders. Prior to shear bond testing, half of each group was subjected to thermal and mechanical cycling testing at three conditional levels. All specimens were thereafter subjected to a shear force. The fractured surfaces were visually analyzed with scanning electron microscopy. The shear bond strength values of functionally graded zirconia combinations were significantly higher than zirconia combinations, irrespective of the fatigue conditions (P < 0.05). The shear bond strength values of graded zirconia combinations and zirconia combinations before thermal-mechanical cycling were higher than after (P < 0.05). The shear bond strength values of graded zirconia combinations was $40.99 ± 2.22$ MPa, $39.44 ± 2.36$ MPa, $37.45 ± 2.06$ MPa, and $36.87 ± 2.18$ MPa for conditions 0 to 3, respectively. The shear bond strength values were $26.75 ± 2.16$ MPa, $23.95 ± 2.16$ MPa, $21.65 ± 2.14$ MPa, and $20.49 ± 2.16$ MPa for zirconia combinations from conditions 0 to 3, respectively. The functionally graded zirconia combinations exhibited greater shear bond strength than zirconia combinations, irrespective of the fatigue conditions. Thermal-mechanical cycling had an impact on the shear bond strength of both graded zirconia combinations and conventional zirconia combinations.