Correlation between microfracture indentation and residual compressive stresses in leucite porcelain

Abstract

Statement of the problem: Surface residual stress is an important factor in the comparison of the resistance to fracture of ceramic restorative materials because flaws or cracks may arise in a material or nucleate while in service, and sudden fractures can arise at stresses well below the yield stress. Vicker’s hardness microfracture indentation (IM) has been used as a simple method to measure fracture toughness of ceramics and then applied to the estimation of residual stress. Although the IM method is simple and convenient, it is a destructive process and not appropriate for the examination of finished materials. Objective: the aim of this study was to attempt the estimation of residual stress in dental porcelain by examining the changes of the Raman spectrum. For comparison, the residual stresses in the specimens were also estimated by the indentation micro-fracture method . Methods: In this study, residual stresses were generated in commercial enamel porcelain ( VMK 95, Vita) disks by tempering or ion exchange. In tempering, porcelain discs were reheated at 650, 750, 850, or 950 °C for 5 min each and then removed from the furnace and cooled by convection. For ion exchange, the porcelain discs were coated with a commercial paste ( Ceramicoat, GC, Tokyo, Japan) and reheated at 450 °C for 15,20, and 30 min. Results: The specimens were analyzed using a laser-Raman spectroscopy to estimate the residual stresses . The Raman shift of the largest peak near 500 cm-1 originated from Silica was used as an indicator of the level of residual stress. The residual stress in the specimen was also estimated by the indentation micro-fracture method, the obtained results were compared to that of laser-Raman spectroscopy. The Raman shift of the porcelain increased with increasing residual stress. The increase in the Raman shift corresponded to the increase in the compressive stress. These results revealed