Flexural strength of lithium disilicate-based dental glass-ceramics
The database is developed aimed at validating the hypothesis: (i) Polishing does not have a significant impact on the flexural strength of glass-ceramics; (ii) chamfering does not have a significant impact on the flexural strength of glass-ceramics; (iii) applying adhesive tape on the compressive side of the specimen has no impact on the flexural strength of glass-ceramics. According to the flexural strenght shwon in this database, the first hypothesis can be accpted that the polishing leads to a statistically significant impact on the three-point flexural strength of the lithium disilicate-based glass-ceramics; while the rest two are rejected that the impact of edge chamfering and applying an adhesive tape on the compressive surface of the specimen seems to be statistically insignificant. The starting glass with the composition 15.44 Li2O–3.26 K2O–3.55 Al2O3 –3.38 P2O5–74.37 SiO2 (mol.%) was synthesized by melting at 1450 °C for 1 h in a Pt–Rh crucible, followed by annealing at 425 °C for 1 h. The obtained glasses were crystallized following a multi-step heat treatment: (i)Room temperature to 500 °C; (ii) dwell at 500 °C for 10 min; (iii) 500 – 700 °C; (iv) dwell at 700 °C for 10 min; (v) 700 – 850 °C; (vi) dwell at 850 °C for 1 h. The resulting white-colored glass-ceramics were converted into rectangular bars with the dimensions of 22 × 5 × 2 mm3 by hot-pressing under vacuum (700 – 965 °C at 60 °C/min). The bars were then divided into five groups comprising 25 bars each. The specimens in Group I are as received unpolished ones. The bars in Group II were polished, and the ones in Group II were both polished and edge chamfered following the specifications described in ISO 6872:2015. Group IV and Group V specimens were silimiar to Group II samples except that a 0.04 mm thick adhesive tape (ScotchTM MagicTM Tape) was applied in Group IV and a 1.6 mm thick compliant adhesive urethane foam tape was applied in Group V to be pasted on the compressive side of the specimens. The three-point flexural strength measurements were performed using a universal mechanical testing machine (MTS QTest Elite 25). The span length (distance between two support rollers) was 14.8 mm and the load was applied at the midpoint between the suppor rollers using a third roller with a diameter of 1.5 mm. The crosshead speed was kept at 1 mm/min. Failure loads were recorded, and the flexural strength values were calculated using Equation: σ=3PL/(2wb^2 ) where P is the applied load at failure, L is the length of the outer (total) span, w is the specimen’s width, and b is the effective thickness of the specimen.