Objectives. The objective of the present study was to investigate the effects of heat treatment (HT) time on the optical properties, mechanical properties, and microstructure of lithium disilicate (Li2Si2O5) glass-ceramic blocks.
Methods. Samples were prepared by cutting lithium disilicate glass-ceramic blocks ? Amber Mill Prototype(AMP) and IPS e.max CAD(IEC) ? into a disc shape (diameter of 12 ㎜ and thickness of 1.2 ㎜) and evenly polishing the surface. Each sample was heat treated according to the manufacturer’s manual and HT holding time was set to 15, 30, 45, and 60 minutes according to different groups. The samples were tested by color difference analysis using a spectrophotometer, X-ray diffraction analysis, HR FE-SEM observation, Vickers hardness test, fracture toughness test, and biaxial flexural strength test.
Results. The acicular crystals of lithium disilicate became coarser and less transparent with increasing HT time. The results for color difference (ΔE*) compared to VITA A2 standard shade showed that the difference was significantly higher in the AMP groups than the IEC groups (P<0.05). Translucency parameter (TP) values were significantly higher in the AMP groups than the IEC groups (P<0.05). The AMP30 group showed the highest TP value, while all other experimental groups showed the tendency of decrease in TP value with increase in HT time. Moreover, increase in TP value was associated with decrease in contrast ratio. When fracture toughness was measured by indentation fracture method, both AMP and IEC groups showed similar results with 1.77 after HT for 15 minutes. However, as HT time increased, the IEC groups showed significantly higher values than the AMP groups (P<0.05). X-ray diffraction analysis results showed that quartz (SiO2) and cristobalite (SiO2) peaks were observed together with the lithium disilicate (Li2Si2O5) peak in the AMP groups, whereas a lithium phosphate (Li3PO4) peak was observed together with the lithium disilicate (Li2Si2O5) peak in the IEC groups. With respect to biaxial flexural strength in the AMP groups, the AMP15 group (HT for 15 minutes) showed the highest value of 524.09±89.95 ㎫, whereas the AMP60 group (HT for 60 minutes) showed the lowest value of 446.56±76.75 ㎫, with a significant difference between the two groups (P<0.05). In the IEC groups, IEC30 group showed the highest value of 668.51±158.57 ㎫ and the IEC45 group showed the lowest value of 517.37±129.52 ㎫, with a significant difference between the two groups (P<0.05).
Conclusion. Within the limitations of the present study, it is concluded that if increased strength is required when fabricating restorations using lithium disilicate glass-ceramic, then changing the HT time could be considered as long as such change does not significantly alter the color tone.