Abstract:

BACKGROUND: Zirconia ceramic has superior strength and toughness to traditional feldspar porcelain and alumina ceramic; however, the flexural strength relates to the thickness ratio of core to veneer.
OBJECTIVE: To evaluate the effect of different core: veneer thickness ratios to veneer on the flexural strength and failure mode of bilayered dental ceramic sheet specimens.
METHODS: Zirconia ceramic blocks (15 mm×5.0 mm) after sintering were cut into 30 sheets with low speed saw according to the following thickness: 0.5 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm and 2.0 mm. After veneering porcelain was fired, all bilayered ceramic sheets and simple zirconia sheets had same specimen thickness of 2.0 mm and equally divided into six groups of five each according to core: veneer thickness ratio: A group (1:3), B group (2:3), C group (1:1), D group (3:2), E group (3:1) and F group of simple zirconia as control. All specimens were mounted in a custom-made jig (span: 12 mm) and subjected to a 3-point flexural testing in a universal testing machine. Recorded maximum load (N) of each specimen and calculated flexural strength (MPa).
RESULTS AND CONCLUSION: Mean flexural strength (MPa) of 6 groups were as follows: A group (522.17±41.11), B group (664.00±68.44), C group (695.06±31.16), D group (794.10±46.11), E group (843.20±38.22) and F group (926.89±44.46). The strength of bilayered ceramic specimens was smaller than simple zirconia ceramic. There was statistically significant difference between any two of six groups except B and C, D and E. The number of fracture fragments in A and B groups was higher than that in the other groups, and most of specimens presented with delaminations in A and B groups and no in the others. Different core: veneer thickness ratios could significantly influence the flexural strength and failure mode of bilayered zirconia and veneering porcelain materials, and flexural strength could be enhanced with the increase of core:veneer thickness ratio.