The Effect Of Core:veneer Thickness Ratio On The Flexural Strength Of Layered Zirconia And Porceplain Dental Ceramic Materials

Objective: The aim of this study was to evaluate the effect of core:veneer thickness ratio on the flexural strength and failure mode of bilayered dental ceramic sheet specimens.Materials and methods: Zirconia ceramic blocks (15 mm×5 mm) after sintering were cut into 30 sheets with low speed saw according to following thickness: 0.5 mm,0.8 mm,1.0 mm,1.2 mm,1.5 mm and 2 mm. After veneering porcelain was fired, all bilayered ceramic sheets and simple zirconia sheets were polished with wet silicon carbide papers of 400 to 1000 grit size to obtain same specimen thickness of 2.0 mm and divided into following six equal 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 an universal testing machine. A continuously increasing compressive load was applied to porcelain surface of specimen at a crosshead speed of 0.5 mm/min until specimen fractured. Recorded maximum load (N) of each specimen and calculated flexural strength (MPa). The data obtained were statistically analyzed by one-way ANOVA and SNK test. The failure mode was recorded and classified after a careful examination of each fractured specimen with a microscope atх20 magnification.Results: 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. One-way ANOVA and SNK test showed that 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 in the others, and most of specimens occurred delaminations in A and B groups and no in the others.Conclusion: Core:veneer thickness ratio could significantly influenced the flexural strength and failure mode of bilayered zirconia and veneering porcelain materials, and flexural strength will be enhanced with the increase of core:veneer thickness ratio.