Effects of multiple firing processes on the mechanical properties of lithium disilicate glass-ceramics produced by two different production techniques

Statement of problem. Repeated firings cause materials to be exposed to additional heat treatments. The effect of these additional heat treatments on the mechanical properties of lithium disilicate glass-ceramics is not fully known. Purpose. The purpose of this in vitro study was to determine the effects of repeated firing on the mechanical properties of lithium disilicate glass-ceramics produced by 2 different techniques, press and computer-aided design and computer-aided manufacturing (CAD-CAM). Material and methods. Eighty rectangular (25x4x2 mm) lithium disilicate glass-ceramic specimens were used in this study, 40 produced by heat pressing and 40 by milling, and divided into 4 groups (n=10) with a different number of veneer porcelain firings (1 to 4). After firing, the Vickers hardness, flexural strength (3-point bend test), and fracture toughness were determined, and the specimens were analyzed with an environmental scanning electron micrograph. Data were analyzed with analysis of variance (ANOVA) (alpha=.05). Results. The repeat firing processes did not affect the flexural strength of the specimens in either group (P>.05), while the surface hardness and fracture toughness were significantly changed (P<.05). Conclusions. Increasing the number of firings adversely affected the mechanical properties of lithium disilicate glass-ceramics.

Automated summary

Repeated firing can have a negative impact on the mechanical properties of lithium disilicate glass-ceramics, particularly on surface hardness and fracture toughness.