22 - Crystallization Effect on Biaxial Flexural Strength of Lithium Disilicate

Abstract

Glass-ceramics destined for dental prostheses have evolved over the years allowing reaching ceramics materials with high quality, lithium disilicate is one of the most used materials due to its wide range of indications. Appropriate mechanical analyses of dental ceramics require a suitable biaxial flexural strength. The aim of this work was developed a composition of a lithium disilicate glass and evaluated the effect of different nucleation heat treatments on the biaxial flexural strength. Discs (12 mm diameter x 1.2 mm thickness) of LS2 experimental and IPS e.max CAD (C, control group n=15) were made. LS2 experimental discs were annealed at 380 C/2 h followed by different nucleation heat treatments in which time and temperature were varied composing 4 groups (n=15): T1 (1h30/500 C), T2 (3 h/500 C), T3 (6 h/500 C) and T4 (6 h/480 C). X-ray diffraction (XRD) with Rietveld refinement was performed to analyze the crystalline phases. The characterization of the crystals was performed for all samples using a scanning electron microscope (SEM). Quantification of the crystalline fraction (CF) was performed using the Image J software, obtaining the estimated percentage for the glass and the crystalline phase. The biaxial flexural strength (BFS) was evaluated by the biaxial piston-on-three balls. The XRD, SEM, and CF data were analyzed descriptively. The BFS were statistically analyzed by one-way non-parametric ANOVA (Kruskal-Wallis test) with post hoc all pairwise. (α=0.05). XRD analysis showed peaks of lithium disilicate for all groups. The morphology of crystals was in an acicular and homogeneous format for all groups. The mean values for CF (%) were: T1= 59.31; T2 = 61.73; T3 = 60.13; T4 = 55.51 and C = 57.52. The mean values for BFS (MPa) were: T1= 399.32; T2 = 357.86; T3 = 224.83; T4 = 492.65 and C = 337.34. It was concluded that the time and temperature of nucleation influenced the crystals morphology, crystals size, and the crystalline fraction, impacting the properties assessed; the group was that presented the best result was that received treatment for 6 hours at 480 C.