**6. Two-step tempering processes**

There are some two-step tempering methods proposed to overcome some limitations of the conventional (one step) ion exchange process. It was demonstrated that the application of ion exchange by the paste method after a thermal tempering treatment in a feldspathic porcelain resulted in an increase in the Weibull modulus (*m* = 14.6) compared to only thermal tempered condition (*m* = 8.7), with small decrease in average strength value [84]. Similar results were observed after applying a thermal tempering followed by a short-time chemical tempering in a silica-soda-lime glass dielectric, in which the reliability of thermal shock resistance was enhanced, with no change in the strength [85].

In order to increase the case depth, a two-step ion exchange was applied to a feldspathic porcelain. The specimens coated with a slurry containing 10 mol% LiCl and 90 mol% NaCl were heat treated at 750°C for 30 min and then cooled and heat treated at 450°C for 30 min. The first step was conducted above the melting temperature of chloride mixture and the second step below the glass transition temperature (Tg) of porcelain. The intent of first step was to exchange Na+ ions and some K+ ions in porcelain by smaller Li+ ions from chloride mixture, considering that the high diffusivity at 750°C could result in a deeper exchanged layer. In the second step, some of Li+ ions would be re-exchanged by bigger Na+ ions, introducing a deep compressive layer. The determined thickness of ion exchanged layer was at least 140 μm [86].

Using the results of the dynamic fatigue test, it is possible to extrapolate the strength decrease after long lifetimes, as shown in Figure 17b. For both conditions, the average strength decreases over time, and after 10 years the expected remaining strength drops to around 30 MPa for the non-treated porcelain, but it still remains high (around 90 MPa) for the ion exchanged porcelain [71]. The increase in the stress corrosion coefficient, *n*, is a significant effect, since the difference in strength between ion exchanged porcelain and non-treated one increases over time. Therefore, besides increasing the strength the compressive layer generated by ion exchange process also decreases the rate of strength degradation by slow crack growth phenomenon. Using the results of Weibull distribution (Figure 15) and dynamic fatigue test (Figure 17a) it is possible to construct the strength-probability-time (SPT) diagram [79,82,83], as shown in Figure 18. This diagram makes possible the estimation of a fracture stress at any time during the lifetime of a dental restoration at any fracture probability level. For example, it is possible to verify that UST porcelain after ion exchange has at least twice the fracture stress than nontreated porcelain even at a fracture probability as low as 1% during long lifetimes (e.g., 100 years). Note that the difference in fracture stress increases over time, at any level of fracture

**Figure 18.** SPT (strength-probability-time) diagram for 1 day (1 d), 1 year (1 y), and 100 years (100 y) for UST dental

There are some two-step tempering methods proposed to overcome some limitations of the conventional (one step) ion exchange process. It was demonstrated that the application of ion exchange by the paste method after a thermal tempering treatment in a feldspathic porcelain resulted in an increase in the Weibull modulus (*m* = 14.6) compared to only thermal tempered condition (*m* = 8.7), with small decrease in average strength value [84]. Similar results were observed after applying a thermal tempering followed by a short-time chemical tempering in a silica-soda-lime glass dielectric, in which the reliability of thermal shock resistance was

porcelain, with and without ion exchange. Data from [63,71]

enhanced, with no change in the strength [85].

**6. Two-step tempering processes**

probability.

186 Ion Exchange - Studies and Applications

Another two-step ion exchange method was proposed to introduce an engineered stress profile (ESP), in which a designed ion exchange stress profile with steep increase of the compression stress from the surface achieves a maximum at a predetermined depth and then decreases towards the interior of the glass. In the ESP glass, surface cracks growth in a stable manner up to the maximum compressive stress and are subsequently arrested, resulting in an uncommon surface crack pattern with a set of arrested cracks, before unstable fracture occurs. This behavior results in significant lower variability of fracture stress. The ESP is generated by a short second ion exchange process carried out for partial removal of the stuffing ion introduced in the first extended treatment [87,88]. This method was applied to a leucite-reinforced glassceramic prepared by heat-pressing method (Empress), using a KNO3 bath in first step during 11 h at 450°C followed by a second step with a bath of 70 mol% KNO3 and 30 mol% NaNO3 at 400°C for 30 min (both temperature lower than Tg of Empress). The two-step method resulted in increase in Weibull modulus (*m* = 11.7) in relation to the single-step ion exchange (*m* = 5.1) [89], and very high slow crack growth (SCG) susceptibility coefficient (*n* = 107) [90]. Figure 19 shows schematically the residual surface stress profiles from different ion exchange methods.

**Figure 19.** Residual surface stress profiles expected from different ion exchange (IE) methods: conventional extended (one-step) IE; two-step IE; and paste method (short IE treatment). Vertical dotted lines represent the depths of small and large surface crack. Data from [71]
