The Effect of the Pre-Crystallisation Defect Size on the Strength Limitation in Lithium Silicate Glass Ceramics

Abstract

The objective of this study was to explore how the size of pre-crystallisation defects affects the strength of a lithium disilicate glass ceramic. A total of seven groups of lithium disilicate glass-ceramic (IPS e.max ® CAD) disc shape specimens (thickness 1.1 ± 0.1mm) were fabricated (n = 15). Each group corresponded to the varying severity of controlled surface defects made by a Vickers hardness indenter in a partially crystallised state. All the discs followed the manufacturer specified crystallisation process. The controlled defects were analysed with the use of optical microscopy and atomic force microscopy in both the partially crystallised phase and after crystallisation. The bi-axial flexural strength (BFS) was measured using a ball-on-ring configuration after crystallisation. A one-way ANOVA revealed a significant difference (p = 0.028). The post-hoc Tukey test revealed a significant difference (p < 0.01) existed between the 0.2 kg and 2 kg indentation groups, with no other pairwise differences. The survivability plots highlight the low BFS outliers occurring in the 1.0 and 2.0 kg static load groups. The Atomic Force Microscopy shows apparent differences before and after crystallisation. The crystallisation process helps to mitigate the strength limiting defects, but it also has a limit. Therefore, it is necessary to minimise the surface defects generated through the pre-crystallisation manufacturing procedures.