The Effect Of Core Thickness Changes On The Flexural Strength Of Layered Zirconia And Porceplain Dental Ceramics Materials

Objective: The aim of this study was to evaluate the effect of core thicknesschanges on the flexural strength and failure mode of bilayered dental ceramic sheetspecimens.Materials and methods:Sintered zirconia ceramic blocks(15mm×5mm) were cutinto25sheets with low speed saw, according thickness as follows:0.3mm,0.4mm,0.6mm,0.8mm and1.0mm, and every sheet was handled with sand blasting andultrasonic cleaning. After veneering porcelain was fired, all samples were polished withwet silicon carbide papers of400to1000grit size in order to obtain the same thicknessof veneering porcelain(1.0mm). Finally, the thickness of samples were added up tofollowing1.3mm,1.4mm,1.6mm,1.8mm and2.0mm. Samples were divided into5groups according to thickness of core ceramics: group A(0.3mm), B(0.4mm), C(0.6mm),D(0.8mm) and E(1.0mm), and the thickness of veneering porcelain was1.0mm. Allsheets were mounted in a custom-made jig(span:12mm) and subjected to a3-pointflexural testing in an universal testing machine. A continuously increasing compressiveload was applied to porcelain surface of sheets at a crosshead speed of0.5mm/min tillsheets fractured. Recorded maximum load(N) of each sheet and calculated flexuralstrength(MPa). The data obtained were analyzed by one-way ANOVA and SNK test. The failure mode was recorded and classified after acareful examination of each fractured sheet with a microscope at x20magnification.Results: Mean flexural strength(MPa) of5groups were as follows: group A(352.31±10.80), group B (370.93±12.05), group C(480.19±19.14), groupD(681.73±6.05) and group E(699.06±29.22). One-way ANOVA and SNK test showedthat there was statistically significant difference between any two of five groups exceptA and B, C and D. The number of fracture fragments in group A was higher than in others, and most of sheets occurred delaminations in group A and no in others.Conclusion: Core thickness changes could significantly influenced the flexuralstrength and failure mode of bilayered zirconia and veneering porcelain materials, andflexural strength will be enhanced with the increase of core thickness.