Evaluating of color changes and mechanical characteristics of different thicknesses of indirect restorations which produced by monolithic lithium disilicate glass ceramic blocks due to different firing times for glaze.

Aim: The aim of this study is to evaluate the changes in the color, the changes in the surface roughness and the changes in the microhardness that may occur as a result of repeated restorations of monolithic lithium disilicate glass ceramic blocks as a result of repeated firings. Material and Method: In our study, IPS e.max CAD blocks (Ivoclar Vivadent AG, Schaan, LICHTENSTEIN) were used as lithium disilicate glass ceramic material. The samples that obtained from IPS e.max CAD blocks were divided into 3 groups as 1 mm, 1.5 mm and 2 mm according to their thickness. There were 30 samples in each group (n). For mechanical evaluations, 48 samples were used after each firing process, 144 samples were used for total mechanical analysis. A total of 234 samples obtained from IPS e.max CAD blocks were used in the whole study. Results: The color and mechanical properties changes were evaluated after baking the samples 1, 2, 3 and 4 times. The results were evaluated statistically. Sample surfaces were examined by SEM analysis. The effect of repeated firing on surface roughness was evaluated with reference to Ra value in AFM analysis results. XRD analysis was performed to evaluate the change of the restoration thickness and the number of firing at the atomic level. Vickers microhardness analysis was used to valuate the effect of repeated firing on hardness. In order to measure microhardness, 300 gr load was applied to the sample for 15 seconds. Conclusion: In our study, samples obtained from IPS e.max CAD blocks 1 mm and 2 mm samples showed the color difference after the 4th firing. In SEM and AFM analysis, it was seen that the main change after the 3rd firing process and the surface of the porcelain changed very much after the 4th firing. As a result of XRD analysis, it was found that the 4th firing causes phase transformation. It was seen that this result was supported by SEM and AFM analyzes.