**5. Ion exchange on strength, reliability, and lifetime of dental porcelains**

In this section, the effects of chemical toughening using ion exchange by the paste method on the strength and lifetime of dental porcelains are shown. In this study, a feldspathic porcelain (Ultropaline Super Transparent, Jen Dental – UST) recommended for metal-ceramic or all ceramic restorations was used. The chemical composition of this porcelain is shown in Table 4.


**Table 4.** Chemical composition (mol%) measured by XRF spectroscopy and calculated glassy matrix composition (in parenthesis) of Ultropaline Super Transparent (UST) dental porcelain (containing 12 vol% leucite particles)

Disc-shaped green specimens were prepared by vibration-condensation method and then sintered at 930°C following the firing schedule recommended by the manufacturer. After firing, the specimens were machined and mirror-polished with diamond suspensions down to 1 μm. For each test condition at least 10 specimens (12.5 mm in diameter and 1 mm in thickness) were prepared.

For the chemical tempering, a paste was prepared by mixing 10 g of KNO3 powder (Merck) with 4 mL of deionized water. Porcelain discs, containing 0.4 g of this paste on the polished surface, were subjected to the ion exchange treatment in an electric furnace (FP-32, Yamato) with a heating rate of 5°C/min at 470°C (or other specified temperature) during 15 min, after an intermediate step at 150°C for 20 min for drying. After the treatment, the paste residue was easily removed with sprayed water.

The porcelain's strength was determined in biaxial flexural mode, which is an adequate loading condition for thin specimens, like the dental restorations. The biaxial flexure strength (σ<sup>f</sup> ) was determined using a piston-on-three-balls loading device in a universal mechanical testing machine (Syntech 5G, MTS) at a constant stress rate of 10 MPa/s (or other specified rate), with the specimen immersed in artificial saliva (Table 5) heated to 37°C (Figure 11). Flexural test performed in artificial saliva at 37°C is more severe (strength tends to be lower) than in usual laboratory environment, but these conditions are more clinically relevant, since they are closer to the oral environmental conditions.


**Table 5.** Composition of artificial saliva [5,54]

**Figure 11.** Images of biaxial flexural device: (a) loading piston and three-ball support; (b) disc specimen positioned in the flexural device and immersed in artificial saliva with heating element; (c) general view of the biaxial flexural load‐ ing device [5]

The biaxial flexural strength (σ<sup>f</sup> ) was calculated using the following equation [5,55]:

$$\sigma\_f = \frac{0.2387 \cdot F}{w^2} \left\{ (1+\nu) \left[ 1 - 2\ln\left(\frac{B}{A}\right) \right] + \left(1-\nu\right) \left[ \left(\frac{A}{C}\right)^2 - \frac{1}{2} \left(\frac{B}{C}\right)^2 \right] \right\} \tag{10}$$

where, *F* is the load at fracture, *w* is the specimen thickness, *A* is the radius of the support circle (4 mm), *B* is the radius of the piston (0.85 mm), *C* is the radius of the specimen, and *ν* is the Poisson's ratio (determined by ultrasonic pulse-echo method [56]).

Figure 12 shows the effects of ion exchange temperature on biaxial flexural strength, *σ<sup>f</sup>* , and fracture toughness, *KIc*, determined by the indentation fracture method. It can be seen that ion exchanged specimens had significantly higher *σ<sup>f</sup>* values (around 130 MPa) compared to the control sample – without chemical tempering (57 MPa). This substantial increase of around 130% in strength was directly related to the increase in material's resistance to crack propa‐ gation, that is, fracture toughness. This property increased from 1.3 MPa.m1/2 in control sample to around 2.8 MPa.m1/2 in chemically tempered samples (relative increase of around 120%) [57]. The ion exchange by the paste method although short in time results in significantly strength‐ ening and toughening of dental porcelains. Other works also showed increases between 20 to 83% in flexural strength by applying this method for feldspathic porcelains [58,59], although there are also reports showing no significant increases in some dental porcelains [60], partic‐ ularly those with high K2O content [61].

No effects of ion exchange temperature on *σ<sup>f</sup>* and *KIc* were observed when this treatment was carried out between 430 and 510°C (Figure 12), which corresponded to a range between 75 and 89% of the glass transition temperature (Tg) of UST porcelain. This temperature (Tg), deter‐ mined by differential thermal analysis, DTA (404S, Netzsch), at a heating rate of 5°C/min in air, was 575°C [57]. At this temperature range, one could expect a significant increase in the ion exchanged K+ ions with the increase in temperature, since the kinetic of ion exchange by interdiffusion with Na+ ions of the porcelain is exponentially dependent on the temperature (Equations 2–4). In fact, it was determined by XRF spectroscopy that the sodium content in the KNO3 paste residue increased with the increase in temperature of ion exchange treatment (Figure 13). The increase in K2O content and reduction of Na2O content in porcelain were also confirmed by EDS analysis in SEM. In this case, it seems that the increase in ion exchange rate was counterbalanced by the stress relaxation with the increase in temperature (Equation 6), inhibiting further increase in residual compressive stress and increases in toughness and strength. Therefore, an appropriate temperature for making ion exchange in dental porcelains with K+ ions exchanged by Na+ ions by paste method seems to be around 80% of glass transition temperature (Tg), or around 100°C lower than Tg (for UST porcelain at 470°C).

**KH2PO4 (2.5 mM)**

ing device [5]

**Na2HPO4 (2.4 mM)**

180 Ion Exchange - Studies and Applications

**Table 5.** Composition of artificial saliva [5,54]

The biaxial flexural strength (σ<sup>f</sup>

2

exchanged specimens had significantly higher *σ<sup>f</sup>*

ularly those with high K2O content [61].

No effects of ion exchange temperature on *σ<sup>f</sup>*

su

**KHCO3 (1.5 mM)**

**NaCl (1.0 mM) MgCl2 (0.15**

**Figure 11.** Images of biaxial flexural device: (a) loading piston and three-ball support; (b) disc specimen positioned in the flexural device and immersed in artificial saliva with heating element; (c) general view of the biaxial flexural load‐

( ) ( ) 2 2

ì ü é ù <sup>×</sup> ï ï é ù æ ö æ ö æö

 u

*F B AB w A CC*

where, *F* is the load at fracture, *w* is the specimen thickness, *A* is the radius of the support circle (4 mm), *B* is the radius of the piston (0.85 mm), *C* is the radius of the specimen, and *ν* is the

fracture toughness, *KIc*, determined by the indentation fracture method. It can be seen that ion

control sample – without chemical tempering (57 MPa). This substantial increase of around 130% in strength was directly related to the increase in material's resistance to crack propa‐ gation, that is, fracture toughness. This property increased from 1.3 MPa.m1/2 in control sample to around 2.8 MPa.m1/2 in chemically tempered samples (relative increase of around 120%) [57]. The ion exchange by the paste method although short in time results in significantly strength‐ ening and toughening of dental porcelains. Other works also showed increases between 20 to 83% in flexural strength by applying this method for feldspathic porcelains [58,59], although there are also reports showing no significant increases in some dental porcelains [60], partic‐

carried out between 430 and 510°C (Figure 12), which corresponded to a range between 75 and 89% of the glass transition temperature (Tg) of UST porcelain. This temperature (Tg), deter‐ mined by differential thermal analysis, DTA (404S, Netzsch), at a heating rate of 5°C/min in

= + - +- - í ý ê ú ê ú ç ÷ ç ÷ ç÷ ï ï ë û è ø è ø èø ê ú î þ ë û

Figure 12 shows the effects of ion exchange temperature on biaxial flexural strength, *σ<sup>f</sup>*

0.2387 <sup>1</sup> 1 1 2ln 1 2 *<sup>f</sup>*

Poisson's ratio (determined by ultrasonic pulse-echo method [56]).

) was calculated using the following equation [5,55]:

100 mL 100 mL 100 mL 100 mL 100 mL 100 mL 6 mL

**mM)**

**CaCl2 (1.5 mM) Citric acid (0.002 mM)**

(10)

, and

values (around 130 MPa) compared to the

and *KIc* were observed when this treatment was

**Figure 12.** Biaxial flexural strength, *σ<sup>f</sup>* , and fracture toughness, *KIc*, of UST dental porcelain before and after ion ex‐ change (IE) at different temperature. In parenthesis is the temperature of IE relative to the glass transition temperature (Tg = 575°C). Data from [57]

**Figure 13.** Sodium (Na) content in the KNO3 paste residue after ion exchange treatment at different temperature for UST porcelain. Data from [57]

1

22 more sensitive to the size of flaws.

28 distribution is given by [14]:

Small variations in flexural strength with varying ion exchange temperatures were also reported in another work. In general no significant differences were observed in strength when ion exchange with K-containing compound was applied to three dental porcelains between 300 and 600°C (for 30 min), although maximum values were observed at 450°C. Besides, small variations in strength with varying ion exchange time (10 to 90 min at 450°C) were also observed [62].

In order to evaluate the effects of the size of surface flaws on the strength of ion exchanged porcelain, the polished surface of specimens was indented with Vickers impression with increasing load from 2 to 49 N. In Figure 14a, it can be seen that the biaxial flexural strength, σf , of UST porcelain, without (control) and with ion exchange treatment (KNO3, 470°C, 15 min), decreased with the increase in indentation load, since this increase causes the increase in radial/ median crack size, *c* (Figure 7b). However, the decrease in σ<sup>f</sup> was more accentuated in the ion exchanged specimens. These results showed that the beneficial effects of ion exchange are more pronounced for small surface cracks, and less effective for larger and deeper flaws. This behavior is related to the gradient of K+ ion content introduced in the surface region, which results in a gradual decrease of residual compressive stress to the interior of porcelain. The positive effects of ion exchange disappear for flaws deeper than the thickness of compressive stress layer.

16 Book Title

2 **Fig. 14.** Biaxial flexural strength, f, after a Vickers indentation at different loads (a) and 3 fracture resistance, KR, as a function of crack extension, Δc (b), for UST dental porcelain with 4 and without ion exchanges. Data from [63,64] **Figure 14.** Biaxial flexural strength, σ<sup>f</sup> , after a Vickers indentation at different loads (a) and fracture resistance, KR, as a function of crack extension, Δc (b), for UST dental porcelain with and without ion exchanges. Data from [63,64]

5 The fracture resistance, *KR*, or *KIc* as a function of crack extension, Δ*c*, can be evaluated using 6 the following equation [64–66]: The fracture resistance, *KR*, or *KIc* as a function of crack extension, Δ*c*, can be evaluated using the following equation [64–66]:

$$\mathbf{K}\_{\mathbb{R}} = \mathbf{k} \cdot \left(\Delta \mathbf{c}\right)^{\circ} \tag{11}$$

9 the results of biaxial flexural strength, f, as a function of indentation load, P, in Figure 14a. 10 The calculated fracture resistance, *KR*, values as a function of crack extension, *Δc*, are shown Where, the parameters *k* and *q* are determined using the data obtained by power law fits on the results of biaxial flexural strength, σ<sup>f</sup> , as a function of indentation load, P, in Figure 14a.

8 Where, the parameters *k* and *q* are determined using the data obtained by power law fits on

11 in Figure 14b. For the non-treated porcelain, *KR* increased with the increase in crack size, the 12 so-called rising R-curve behavior (crack growth resistance curve). This behavior is observed 13 in leucite-containing porcelains and is caused by the friction between rough crack surfaces 14 caused by crack deflection around leucite agglomerates [11]. Since this mechanical grip acts 15 in the crack wake, the shielding effect at the crack tip is intensified with the increase in crack 16 extension [67]. Rising R-curve effect is a desirable material behavior since it is necessary 17 additional energy to propagate the crack, besides that needed at the crack tip [65,66]. For the 18 ion exchanged porcelain, an opposite result was observed, with the decrease in KR as the 19 crack size increased. The decreasing residual compressive stress from the ion exchanged 20 surface annulled the rising R-curve effect of porcelain microstructure and more, resulted in 21 a decreasing R-curve effect. This behavior makes the strength of ion exchanged porcelain

23 The variability of strength in ceramic materials is closely related to its flaw population, since 24 fracture is a probabilistic event due to a random-like distribution of the strength-limiting 25 flaws [68]. The strength variability of ceramic materials can be evaluated using Weibull 26 statistic, which is based on the weakest-link theory, where the more severe flaw results in 27 fracture propagation and determine the strength [69]. The Weibull two-parameter The calculated fracture resistance, *KR*, values as a function of crack extension, *Δc*, are shown in Figure 14b. For the non-treated porcelain, *KR* increased with the increase in crack size, the so-called rising R-curve behavior (crack growth resistance curve). This behavior is observed in leucite-containing porcelains and is caused by the friction between rough crack surfaces caused by crack deflection around leucite agglomerates [11]. Since this mechanical grip acts in the crack wake, the shielding effect at the crack tip is intensified with the increase in crack extension [67]. Rising R-curve effect is a desirable material behavior since it is necessary additional energy to propagate the crack, besides that needed at the crack tip [65,66]. For the ion exchanged porcelain, an opposite result was observed, with the decrease in KR as the crack size increased. The decreasing residual compressive stress from the ion exchanged surface cancelled the rising R-curve effect of porcelain microstructure and in addition, resulted in a decreasing R-curve effect. This behavior makes the strength of ion exchanged porcelain more sensitive to the size of flaws.

Small variations in flexural strength with varying ion exchange temperatures were also reported in another work. In general no significant differences were observed in strength when ion exchange with K-containing compound was applied to three dental porcelains between 300 and 600°C (for 30 min), although maximum values were observed at 450°C. Besides, small variations in strength with varying ion exchange time (10 to 90 min at 450°C) were also

In order to evaluate the effects of the size of surface flaws on the strength of ion exchanged porcelain, the polished surface of specimens was indented with Vickers impression with increasing load from 2 to 49 N. In Figure 14a, it can be seen that the biaxial flexural strength,

, of UST porcelain, without (control) and with ion exchange treatment (KNO3, 470°C, 15 min), decreased with the increase in indentation load, since this increase causes the increase in radial/ median crack size, *c* (Figure 7b). However, the decrease in σ<sup>f</sup> was more accentuated in the ion exchanged specimens. These results showed that the beneficial effects of ion exchange are more pronounced for small surface cracks, and less effective for larger and deeper flaws. This

results in a gradual decrease of residual compressive stress to the interior of porcelain. The positive effects of ion exchange disappear for flaws deeper than the thickness of compressive

16 Book Title

2 **Fig. 14.** Biaxial flexural strength, f, after a Vickers indentation at different loads (a) and 3 fracture resistance, KR, as a function of crack extension, Δc (b), for UST dental porcelain with

function of crack extension, Δc (b), for UST dental porcelain with and without ion exchanges. Data from [63,64]

5 The fracture resistance, *KR*, or *KIc* as a function of crack extension, Δ*c*, can be evaluated using

The fracture resistance, *KR*, or *KIc* as a function of crack extension, Δ*c*, can be evaluated using

*<sup>R</sup>* 7 *K k c* (11)

( ) *q*

Where, the parameters *k* and *q* are determined using the data obtained by power law fits on

8 Where, the parameters *k* and *q* are determined using the data obtained by power law fits on 9 the results of biaxial flexural strength, f, as a function of indentation load, P, in Figure 14a. 10 The calculated fracture resistance, *KR*, values as a function of crack extension, *Δc*, are shown 11 in Figure 14b. For the non-treated porcelain, *KR* increased with the increase in crack size, the 12 so-called rising R-curve behavior (crack growth resistance curve). This behavior is observed 13 in leucite-containing porcelains and is caused by the friction between rough crack surfaces 14 caused by crack deflection around leucite agglomerates [11]. Since this mechanical grip acts 15 in the crack wake, the shielding effect at the crack tip is intensified with the increase in crack 16 extension [67]. Rising R-curve effect is a desirable material behavior since it is necessary 17 additional energy to propagate the crack, besides that needed at the crack tip [65,66]. For the 18 ion exchanged porcelain, an opposite result was observed, with the decrease in KR as the 19 crack size increased. The decreasing residual compressive stress from the ion exchanged 20 surface annulled the rising R-curve effect of porcelain microstructure and more, resulted in 21 a decreasing R-curve effect. This behavior makes the strength of ion exchanged porcelain

23 The variability of strength in ceramic materials is closely related to its flaw population, since 24 fracture is a probabilistic event due to a random-like distribution of the strength-limiting 25 flaws [68]. The strength variability of ceramic materials can be evaluated using Weibull 26 statistic, which is based on the weakest-link theory, where the more severe flaw results in 27 fracture propagation and determine the strength [69]. The Weibull two-parameter

*<sup>q</sup>*

, after a Vickers indentation at different loads (a) and fracture resistance, KR, as a

*Kkc <sup>R</sup>* =×D (11)

, as a function of indentation load, P, in Figure 14a.

ion content introduced in the surface region, which

observed [62].

182 Ion Exchange - Studies and Applications

stress layer.

behavior is related to the gradient of K+

4 and without ion exchanges. Data from [63,64]

the results of biaxial flexural strength, σ<sup>f</sup>

6 the following equation [64–66]:

the following equation [64–66]:

**Figure 14.** Biaxial flexural strength, σ<sup>f</sup>

22 more sensitive to the size of flaws.

28 distribution is given by [14]:

σf

1

The variability of strength in ceramic materials is closely related to its flaw population, since fracture is a probabilistic event due to a random-like distribution of the strength-limiting flaws [68]. The strength variability of ceramic materials can be evaluated using Weibull statistic, which is based on the weakest-link theory, where the more severe flaw results in fracture propagation and determine the strength [69]. The Weibull two-parameter distribution is given by [14]:

$$P\_{f,l} = 1 - \exp\left[-\left(\frac{\sigma\_{f,l}}{\sigma\_0}\right)^w\right] \tag{12}$$

where, *Pf,i* is the probability of fracture of ith specimen, *i* is order number of *σf,i* (fracture stress of ith specimen, ranked in ascending order of values), *σ*0 is the characteristic strength (scale factor, defined for *Pf* = 63.2%), and *m* is the Weibull modulus (shape factor; the lower this value, the higher is variability). For this analysis, 30 specimens of UST porcelain for each condition (without and with ion exchange treatment – KNO3, 470°C, 15 min) were tested in biaxial flexural mode at a stress rate of 1 MPa/s (in artificial saliva at 37°C). The Weibull parameters (*σ*0 and *m*) were calculated based on the maximum likelihood method [70] and the results are shown in Figure 15.

It can be seen in Figure 15 that the ion exchange treatment increased more than 100% the characteristic strength, *σ*0, but it also caused larger variability in fracture stress, reducing around 50% the Weibull modulus, *m*. Higher variability means lower reliability in the strength of porcelain, which was caused by the decreasing R-curve behavior in ion exchanged porcelain (Figure 14b), since shallow cracks were significantly toughened by high residual compressive stress level near the surface, but deeper surface cracks were less shielded by the decreasing resistance to crack growth. Therefore, strongest specimens in non-treated porcelain were strengthened more than the weakest ones. Clinically, it is more relevant to consider the fracture stress in an acceptable fracture probability, for example, at *Pf* = 5%. Although strengthening is not so high as compared to the *σ*0 value (*Pf* = 63.2%), even at this low level of *Pf*,5% a significant

**Figure 15.** Weibull plot of biaxial flexural strength data, *σ<sup>f</sup>* , for UST dental porcelain with and without ion exchange. Dotted lines are 95% confidence interval; *m* is Weibull modulus, *σ*0 is characteristic strength, and σ5% is fracture stress at 5% fracture probability. Data from [71]

strengthening effect (at 95% confidence interval) is observed (Figure 15). In practice, the strengthening effect could be even higher than for *Pf* = 5%, since clinical studies of inlays constructed with feldspathic porcelains have shown high fracture rates, up to 48% in evalua‐ tion periods of up to 3 years [72,73]. Therefore, although ion exchange reduces the mechanical reliability of dental porcelain, the strengthening effect is still significant even for low levels of fracture probability.

Another relevant factor related to the lifetime of a porcelain restoration is the mechanical degradation over time. When a ceramic material is subjected to a stress level lower than the fracture stress, the flaws (cracks) can growth slowly in a stable manner up to the time at which loading comes to a halt, reducing the material's strength due to the increase in crack size, or when a flaw achieves a critical size, given by the Griffith-Irwin fracture criterion (Equation 1), that results in (fast) fracture. This phenomenon is known as slow (or subcritical) crack growth, SGC, and silicate glasses, like porcelains, usually are highly susceptible to this type of degra‐ dation [74–76]. SCG occurs mainly by a stress corrosion mechanism (Figure 16), in which water molecules diffuse and are adsorbed at the crack tip, and then cause bonding rupture of glass network yielding Si–OH groups on each fracture surface, resulting in crack growth, by [76,77]:

$$\text{Si}-\text{O}-\text{Si} + \text{H}\_2\text{O} \rightarrow 2\text{Si}-\text{OH} \tag{13}$$

Since the oral environment is aggressive to porcelain restorations and has many characteristics that favor SCG (water from saliva and dentin tubules, masticatory stresses, temperature and

18 Book Title

1 and then cause bonding rupture of glass network yielding Si–OH groups on each fracture

3 *Si O Si H*2*O* 2*Si OH* (13)

2 surface, resulting in crack growth, by [76,77]:

SiO44- 7 network causing increase in crack size

4

21

5 **Fig. 16.** Schematic molecular images of slow crack growth (SCG) phenomenon in silicate 6 glass: (a) diffusion of water molecule to the crack front (dotted line); (b) bonding breakage of **Figure 16.** Schematic molecular images of slow crack growth (SCG) phenomenon in silicate glass: (a) diffusion of water molecule to the crack front (dotted line); (b) bonding breakage of SiO4 4- network causing increase in crack size

pH variations [67,78]), it is important to understand the response of strengthened porcelain to this phenomenon. The main method used to characterize the material's susceptibility to SCG is the dynamic fatigue method, in which specimens are tested in different stress rates and using the following equation [79–81]: 8 Since the oral environment is aggressive to porcelain restorations and has many 9 characteristics that favor SCG (water from saliva and dentin tubules, masticatory stresses, 10 temperature and pH variations [67,78]), it is important to understand the response of 11 strengthened porcelain to this phenomenon. The main method used to characterize the

12 material's susceptibility to SCG is the dynamic fatigue method, in which specimens are

14 (14)

$$\log \sigma\_{/} = \frac{1}{n+1} \log \dot{\sigma} + \log \sigma\_{/0} \tag{14}$$

where, *σ<sup>f</sup>* is the flexural strength, *σ*˙ is the stress rate, *σf*0 the scaling parameter, and *n* is the slow crack growth, SCG, susceptibility coefficient (the higher is *n* value the lower is the suscepti‐ bility). Figure 17 shows the results of this test for UST porcelain, non-treated and ion exchanged (KNO3, 470°C, 15 min). It can be seen that ion exchanged porcelain had significantly higher *n* value (relative increase of around 50%), showing more resistance to SCG degradation. This effect is substantial and can impact the lifetime of a restoration. 15 where, *f* is the flexural strength, is the stress rate, *<sup>f</sup>*0 the scaling parameter, and *n* is the 16 slow crack growth, SCG, susceptibility coefficient (the higher is *n* value the lower is the 17 susceptibility). Figure 17 shows the results of this test for UST porcelain, non-treated and ion 18 exchanged (KNO3, 470°C, 15 min). It can be seen that ion exchanged porcelain had 19 significantly higher *n* value (relative increase of around 50%), showing more resistance to 20 SCG degradation. This effect is substantial and can impact the lifetime of a restoration.

<sup>0</sup> log log <sup>1</sup>

strengthening effect (at 95% confidence interval) is observed (Figure 15). In practice, the

Dotted lines are 95% confidence interval; *m* is Weibull modulus, *σ*0 is characteristic strength, and σ5% is fracture stress

constructed with feldspathic porcelains have shown high fracture rates, up to 48% in evalua‐ tion periods of up to 3 years [72,73]. Therefore, although ion exchange reduces the mechanical reliability of dental porcelain, the strengthening effect is still significant even for low levels of

Another relevant factor related to the lifetime of a porcelain restoration is the mechanical degradation over time. When a ceramic material is subjected to a stress level lower than the fracture stress, the flaws (cracks) can growth slowly in a stable manner up to the time at which loading comes to a halt, reducing the material's strength due to the increase in crack size, or when a flaw achieves a critical size, given by the Griffith-Irwin fracture criterion (Equation 1), that results in (fast) fracture. This phenomenon is known as slow (or subcritical) crack growth, SGC, and silicate glasses, like porcelains, usually are highly susceptible to this type of degra‐ dation [74–76]. SCG occurs mainly by a stress corrosion mechanism (Figure 16), in which water molecules diffuse and are adsorbed at the crack tip, and then cause bonding rupture of glass network yielding Si–OH groups on each fracture surface, resulting in crack growth, by [76,77]:

Since the oral environment is aggressive to porcelain restorations and has many characteristics that favor SCG (water from saliva and dentin tubules, masticatory stresses, temperature and

<sup>2</sup> Si O Si H O 2Si OH --+ ® - (13)

= 5%, since clinical studies of inlays

, for UST dental porcelain with and without ion exchange.

strengthening effect could be even higher than for *Pf*

**Figure 15.** Weibull plot of biaxial flexural strength data, *σ<sup>f</sup>*

at 5% fracture probability. Data from [71]

184 Ion Exchange - Studies and Applications

fracture probability.

**Figure 17.** Biaxial flexural strength, *σ<sup>f</sup>* , as a function of stress rate (a) and predicted flexural strength as a function of time to fracture (b) for UST dental porcelain, with and without ion exchange, in artificial saliva at 37°C. In (a),*σ f*0 is the scaling parameter and n is the slow crack growth, SCG, susceptibility coefficient. Inert strength was determined at 100 MPa/s in air with a drop of silicone oil on the tensile surface to inhibit the occurrence of SCG. In (b), the slope of fitted curve is related with *n*. Data from [63,71]

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 probability.

**Figure 18.** SPT (strength-probability-time) diagram for 1 day (1 d), 1 year (1 y), and 100 years (100 y) for UST dental porcelain, with and without ion exchange. Data from [63,71]
