IADR Abstract Archives
Effect of Polishing on the Strength of Bilayered Dental Ceramics
Objectives: Objectives: Bilayered systems are popular due to their better esthetics and mechanical behavior. This study aimed to investigate the effect of polishing of the core material on the strength of bilayered glass-ceramics.
Methods: Methods: Rectangular beams (1.8 x 4 x ~29 mm3) were sectioned from lithium disilicate blocks (Li2S2O5, e.max CAD, Ivoclar Vivadent, n=13). A layer of nano-fluoroapatite powder mixed with de-ionized water (Ceram, e.max Ceram, Ivoclar Vivadent) was built on the lithium disilicate beams. The bilayered beam specimens were fired as per manufacturer’s guideline and grounded to a final dimension of 3 x 4 x ~29 mm3. A controlled flaw was created using a Knoop indentation (25 N) on the Li2S2O5 side followed by polishing to remove residual stresses (n=9). The specimens were subjected to four-point flexure at a rate of 1.25 mm/min until failure with the controlled flaw side under tension in de-ionized water. Traditional fractography was performed to confirm the origin of failure, and the failure strength was calculated using the composite beam theory.
Results: Results: Fractography showed that the specimens failed either from the interface between the two layers (n=6) or from the controlled flaw (n=7). The mean (standard deviation) of maximum strength calculated is shown in the table. The strength ratio of flaws without residual stress (polished, 387.6 MPa) to those with residual stress (unpolished, 297.1 MPa) was 1.3. The standard deviation of the maximum strength in the Li2S2O5 layer increased by 5-10 folds for the polished specimens as compared to the unpolished ones.
Conclusions: Conclusions: The strength ratio for the polished to the unpolished specimens follows the theoretical value from the ratio of geometric shape factor. The polishing of the core material improved the fracture strength of the bilayered system.
Methods: Methods: Rectangular beams (1.8 x 4 x ~29 mm3) were sectioned from lithium disilicate blocks (Li2S2O5, e.max CAD, Ivoclar Vivadent, n=13). A layer of nano-fluoroapatite powder mixed with de-ionized water (Ceram, e.max Ceram, Ivoclar Vivadent) was built on the lithium disilicate beams. The bilayered beam specimens were fired as per manufacturer’s guideline and grounded to a final dimension of 3 x 4 x ~29 mm3. A controlled flaw was created using a Knoop indentation (25 N) on the Li2S2O5 side followed by polishing to remove residual stresses (n=9). The specimens were subjected to four-point flexure at a rate of 1.25 mm/min until failure with the controlled flaw side under tension in de-ionized water. Traditional fractography was performed to confirm the origin of failure, and the failure strength was calculated using the composite beam theory.
Results: Results: Fractography showed that the specimens failed either from the interface between the two layers (n=6) or from the controlled flaw (n=7). The mean (standard deviation) of maximum strength calculated is shown in the table. The strength ratio of flaws without residual stress (polished, 387.6 MPa) to those with residual stress (unpolished, 297.1 MPa) was 1.3. The standard deviation of the maximum strength in the Li2S2O5 layer increased by 5-10 folds for the polished specimens as compared to the unpolished ones.
Conclusions: Conclusions: The strength ratio for the polished to the unpolished specimens follows the theoretical value from the ratio of geometric shape factor. The polishing of the core material improved the fracture strength of the bilayered system.
