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.

该数据库旨在验证以下假设：(i) 磨光对玻璃陶瓷的弯曲强度没有显著影响；(ii) 倒角对玻璃陶瓷的弯曲强度没有显著影响；(iii) 在试样的压缩侧施加粘合胶带对玻璃陶瓷的弯曲强度没有影响。根据本数据库中显示的弯曲强度数据，可以接受第一个假设，即磨光对基于锂玻璃陶瓷的三点弯曲强度有显著的统计学影响；而其余两个假设被拒绝，即边缘倒角和在试样压缩表面施加粘合胶带的影响在统计学上似乎并不显著。

起始玻璃的组成为 15.44 Li2O–3.26 K2O–3.55 Al2O3 –3.38 P2O5–74.37 SiO2 (摩尔百分比)，通过在 1450 °C 下熔融 1 小时，并在 Pt–Rh 熔舟中退火 425 °C 1 小时进行合成。获得的玻璃随后经过多步热处理过程结晶：(i) 室温至 500 °C；(ii) 在 500 °C 下停留 10 分钟；(iii) 500 – 700 °C；(iv) 在 700 °C 下停留 10 分钟；(v) 700 – 850 °C；(vi) 在 850 °C 下停留 1 小时。所得的白色玻璃陶瓷被转换为 22 × 5 × 2 mm³ 的矩形棒，通过在真空中（700 – 965 °C，升温速率 60 °C/min）进行热压而成。棒材被分为五组，每组 25 根。第一组的试样为未经过磨光的原始试样。第二组的棒材经过磨光，而第二组的棒材则按照 ISO 6872:2015 的规定进行了磨光和边缘倒角。第四组和第五组的试样与第二组相似，但第四组在试样压缩侧施加了 0.04 mm 厚的粘合胶带（ScotchTM MagicTM Tape），第五组则施加了 1.6 mm 厚的柔性粘合聚氨酯泡沫胶带粘贴在试样压缩侧。

三点弯曲强度测量使用的是通用机械测试机（MTS QTest Elite 25）。跨距长度（两个支撑滚筒之间的距离）为 14.8 mm，负载通过直径为 1.5 mm 的第三滚筒施加在支撑滚筒之间的中点。十字头速度保持在 1 mm/min。记录了破坏载荷，并使用以下公式计算弯曲强度值：
σ=3PL/(2wb^2 )
其中 P 是破坏时的施加载荷，L 是外跨距（总跨距）的长度，w 是试样的宽度，b 是试样的有效厚度。