**Strengthening Dental Porcelains by Ion Exchange Process**

Humberto Naoyuki Yoshimura and Paulo Francisco Cesar

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/60617

#### **Abstract**

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 weaken‐ ing 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

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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.

**Keywords:** Bioceramics, dental porcelain, ion exchange, chemical tempering, strength, toughening, lifetime

### **1. Introduction**

Dental porcelains have been used in dental restorations due to their good qualities, including high color stability, high resistance to stain, good biocompatibility, low thermal conductivity, high wear resistance, and capacity to mimic dental structures [1–3]. Notwithstanding, disadvantages of these restorations include high susceptibility to fracture, risk of debonding, and microleakage [4–6]. For feldspathic porcelain onlays placed in posterior teeth after 6 years, the observed cumulative survival rate was ~60%, with bulk fracture in 16% of the restorations [7]. The reported clinical success rate for maxillary anterior porcelain veneers after 10 years was 64%, and main reasons for failure were fracture (11%) and large marginal defects (20%) [2]. Similar behavior was also observed for posterior feldspathic porcelain inlays, and marginal defects and fracture were 22% and 11% of the restorations, respectively, after an 8-year period of clinical assessment [8].

The high susceptibility to fracture of porcelain restorations is caused by their brittle nature, that is, their low capability to absorb strain energy due to an external loading before fast crack propagation occurs. The resistance to crack propagation can be quantified by the fracture toughness (*KIc*) which is given by [9,10]:

$$K\_{lc} = \mathbf{Y} \cdot \boldsymbol{\sigma}\_f \cdot \mathbf{\check{\mathbf{c}}} \tag{1}$$

where, *Y* is a geometrical constant, σ<sup>f</sup> is fracture stress, and c is crack size that results in fracture. From Griffith's energy failure criterion, the term*σ f.c*1/2 is constant, which implies that strength (*σf* ) is not constant and varies inversely with the square root of critical flaw size (*c*1/2). Further‐ more, Equation 1 also shows that the porcelain strength is directly related with its fracture toughness, *KIc*.

Dental porcelains have relatively low values of *KIc* (around 0.6 to 1.2 MPa.m1/2) [11,12], especially when compared to metals, which have *KIc* higher than around 30 MPa.m1/2 [13]. As a consequence, porcelains have low strength values usually in the range of 40 to 120 MPa [14, 15]. Moreover, the strength of porcelain restorations can decrease during their lifetimes in the oral environment, since a weakening effect known as subcritical crack growth causes the flaw sizes to increase slowly over time [16,17]. Therefore, it is important to develop processing methods that can enhance the overall resistance to crack propagation, particularly strength and fracture toughness, in order to increase the lifetime of porcelain restorations.

Different methods have been proposed to strengthen dental porcelains, including addition of reinforcing phases, like ceramic fibers or phase transformable tetragonal zirconia particles, and incorporating a compressive surface layer, which can be achieved by thermal tempering, glazing with a glassy material having lower thermal expansion coefficient, or chemical tempering [18–20]. Among these, chemical strengthening by ion exchange is a promising method to significantly enhance the mechanical behavior of dental porcelain restorations.
