ENHANCED CHEMICAL STRENGTHENING OF FELDSPATHIC DENTAL PORCELAIN

Previous studies on ion exchange of dental ceramics have shown that the biaxial flexural strength can be improved by exchanging potassium for sodium ions at temperatures below the strain point. The rubidium ion is bigger than the potassium ion and can also be considered as a candidate for replacing smaller ions, i.e., sodium or potassium, although it has not been used for dental ceramics. The double-step method uses the exchange of a small ion for a large ion (Li for Na) above the strain point, and the further exchange of a large ion for a small ion (K for Li) below the strain point. The purpose of this study was to compare the effect of rubidium-for potassium ion exchange with that of potassium-for-sodium exchange (Tuf-Coat, G-C International Corp., Japan) on the flexural strength of a feldspathic dental porcelain and to test the hypothesis that a double-step ion exchange can lead to greater strengthening than potassium ion exchange alone. Weight measurements were performed before and after treatment. Qualitative chemical analyses allowed the rubidium, potassium, and sodium concentration profiles to be determined along cross-sections of the specimens. The maximum biaxial stresses were calculated after specimens were fractured in water on a ball-on-ring fixture at 0.5 mm/min. Relative to the untreated control group, the flexural strength of the potassium-exchanged groups was significantly increased, except for those treated at 400 or 500-degrees-C. All the groups treated with RbNO3 exhibited a significant increase in flexural strength, with a maximum for the group treated at 450-degrees-C. The group submitted to a double-step exchange showed a statistically significant increase in the mean flexural strength compared with the group treated with Tuf-Coat at 450-degrees-C.