Chapters authored
Strengthening Dental Porcelains by Ion Exchange Process
Porcelains have been used in dentistry for many decades because of their excellent aesthetic qualities, besides other favorable characteristics. Despite these desirable characteristics, porcelain restorations may fail in the oral environment due to fracture. Studies on the clinical success rate of porcelain onlays, inlays and veneers have shown that their fracture rate is relatively high and is among the main reasons for failure of these restorations. The fracture of dental porcelains is a consequence of its brittle nature and low fracture toughness. Porcelains are also highly susceptible to weakening during their lifetime in the oral environment, because the sizes of defects tend to increase by the slow crack growth phenomenon. Therefore, in order to increase the lifetime of porcelain restorations, it is necessary to enhance their overall resistance to crack propagation. Among the methods proposed to strengthen glasses and ceramics, a potential method to improve the mechanical properties of dental porcelains is the chemical strengthening or tempering by the ion exchange process. In this chapter, the effects of chemical tempering on mechanical behavior of dental porcelains are reviewed. Dental porcelains are based on alkali-containing aluminosilicate glass compositions and can have leucite (KAlSi2O6) crystalline particles dispersed in the glassy matrix. The ion exchange process can be carried out by the paste method using KNO3 salt at a temperature that is 80% of glass transition temperature (Tg) of porcelain during a short time (15 to 30 min). In this treatment, the small Na+ ions in the glassy matrix are exchanged by larger K+ ions from the salt, resulting in a K+ concentration profile that results in a steep gradient of residual compressive stress by the ion stuffing effect at the surface region of the porcelain. No significant variations in strengthening have been observed when temperature and time varied around the above indicated values, since the increase in these parameters enhanced the stress relaxation process, which hinders the effect of higher ion interdiffusion. Although few porcelains with high leucite content have no strengthening response to ion exchange process, most dental porcelains can be strengthened and significant increases in fracture toughness (up to around 150%) have been reported. The same level of increase in flexural strength has been observed, but the variability of fracture stress also increases due to the relative small thickness of compressive layer and the decreasing resistance curve effect. The lower reliability is counterbalanced by significant increases of the resistance to slow crack growth phenomenon, leading to higher strength retention after long lifetimes even at low levels of fracture probability. Therefore, it is expected that the application of chemical tempering (strengthening by ion exchange) can improve the lifetime of dental porcelain restorations.
Part of the book: Ion Exchange