Application of Laser-Raman Spectroscopy to the Analysis of Residual Compressive Stresses in Leucite Porcelain

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. 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 minutes. 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 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 that the increase of residual stress in porcelain clearly appears on the Raman spectrum as an increase in the Raman shift. It is concluded that laser-Raman spectroscopy is a useful tool for the estimation of residual compressive stresses in dental porcelain.