**4. Hardness and toughness of chemically strengthened dental porcelains**

13 in fracture toughness, KIc [42,46]. Besides the quantity, the size, and distribution of the

15 **4. Hardness and toughness of chemically strengthened dental porcelains** 

14 leucite particles also influences the mechanical properties of the porcelain [18,41].

In this section, the effects of chemical strengthening using ion exchange by paste method on the hardness and fracture toughness of dental porcelains with different microstructures are shown. Table 1 shows descriptions of five porcelain powders used in this study, two recom‐ mended to be used as veneering materials for alumina cores (V and Cb) and three recom‐ mended for porcelain fused-to-metal restorations (C, D, and B), containing wide variation of leucite fraction (0 to 22 vol%). 16 In this section, the effects of chemical strengthening using ion exchange by paste method on 17 the hardness and fracture toughness of dental porcelains with different microstructures are 18 shown. Table 1 shows descriptions of five porcelain powders used in this study, two 19 recommended to be used as veneering materials for alumina cores (V and Cb) and three 20 recommended for porcelain fused-to-metal restorations (C, D, and B), containing wide 21 variation of leucite fraction (0 to 22 vol%).

Table 2 shows the chemical compositions of the porcelains measured by X-ray fluorescence spectroscopy (XRF 1500, Shimadzu), showing that all porcelains had aluminosilicate compo‐ sitions with alkali and alkaline-earth metal oxides, besides other minor oxides. For porcelains containing leucite particles (C, D, and B), parts of SiO2, Al2O3, and K2O contents composed these particles. Considering the fraction and stoichiometry of leucite (KAlSi2O6 = K2O.Al2O3.4SiO2), the compositions of glassy matrix of these porcelains were calculated and are also shown in Table 2. Note that all porcelains had in the glassy matrix an initial K2O content, and also potentially exchangeable Na+ ions by K+ ions from an external source. 22 Table 2 shows the chemical compositions of the porcelains measured by X-ray fluorescence 23 spectroscopy (XRF 1500, Shimadzu), showing that all porcelains had aluminosilicate 24 compositions with alkali and alkaline-earth metal oxides, besides other minor oxides. For 25 porcelains containing leucite particles (C, D, and B), parts of SiO2, Al2O3, and K2O contents 26 composed these particles. Considering the fraction and stoichiometry of leucite (KAlSi2O6 = 27 K2O.Al2O3.4SiO2), the compositions of glassy matrix of these porcelains were calculated and 28 are also shown in Table 2. Note that all porcelains had in the glassy matrix an initial K2O content, and also potentially exchangeable Na+ ions by K+ 29 ions from an external source.

Green specimens with bar shape (5 × 6 × 40 mm) were prepared by the vibration-condensation method, mixing the porcelain powder with distilled water and using a steel mold. Then, the specimens were vacuum sintered in a dental porcelain furnace (Keramat I, Knebel), following the firing schedules recommended by the manufacturers (sintering temperatures indicated in Table 1). After firing, the specimens were machined following the guidelines in ASTM C 1161 to the dimensions of 3 × 4 × 30 mm, and one of larger surfaces was mirror-polished using a Porce lain Manufacturer / Brand Name Manufacturer's Description Leucite fraction (vol%) V VITA Zahnfabrik/Vitadur Alpha Porcelain used with alumina frameworks. Sintering temperature: 960°C 0

semi-automatic polishing machine (Ecomet 3, Buehler), with diamond suspensions of 45, 15, 6, and 1 μm. For each material, 10 specimens were prepared.


**Table 1.** Description of five dental porcelains. Data from [47]


**Table 2.** Overall chemical composition (mol%) of dental porcelains. The calculated compositions of glassy matrix for porcelains containing leucite particles are given in parenthesis. Data from [48]

Later, the polished surfaces were subjected to an ion exchange treatment. First, the surface of the specimen was coated with a layer of a paste composed of distilled water mixed with potassium nitrate (KNO3). The amount of paste placed on each specimen was controlled by mass measurement. Then, the specimens coated with the paste were heat treated in an electric furnace (FP32, Yamato). The ion exchange cycle was conducted at a heating rate of 5°C/min with a first step of drying at 150°C for 20 min to remove the water from the paste, followed by a step of melting of KNO3 and ion exchange at 450°C for 30 min, and then cooled inside the furnace to room temperature. The KNO3 paste residue was easily detached and removed with a wet piece of cotton and the ion exchange treatment did not affect the superficial appearance of the specimens.

semi-automatic polishing machine (Ecomet 3, Buehler), with diamond suspensions of 45, 15,

**Porcelain Manufacturer / Brand Name Manufacturer's Description Leucite fraction**

temperature: 960°C

temperature: 960°C

Cb Noritake/Cerabien Porcelain used with alumina frameworks. Sintering

C Dentsply/Ceramco Finesse Leucite-based porcelain, used for metal-ceramic or

D Ivoclar/d.Sign Leucite-based porcelain, used for metal-ceramic or

875°C

B Dentsply/Ceramco II Leucite-based porcelain, used for metal-ceramic or

**Oxide V Cb C D B**

SiO2 75.9 82.9 70.1 (69.9) 67.6 (66.7) 72.0 (71.9) Al2O3 10.0 5.6 6.4 (5.7) 9.0 (7.6) 9.5 (7.5) K2O 7.1 4.6 8.7 (8.6) 8.1 (7.7) 9.2 (8.8) Na2O 3.6 3.6 5.5 (5.8) 4.9 (5.8) 3.8 (4.9) CaO 3.0 1.1 3.8 (4.0) 3.7 (4.4) 3.9 (5.0)

> 5.0 (5.3) MgO 0.4 (0.5) Tb4O7

Fe, Ni, Zr, Rb, Sr, Re,

**Table 2.** Overall chemical composition (mol%) of dental porcelains. The calculated compositions of glassy matrix for

Cl

**Table 1.** Description of five dental porcelains. Data from [47]

0.7 MgO 0.3 CeO

Hf, V, I

Fe, Ni, Zn, Ti, Cr,

porcelains containing leucite particles are given in parenthesis. Data from [48]

Others 0.3 ZrO2 0.8 ZrO2

Fe, Ni, Ti, Rb, Sr, Pb

Traces (<0.2%) Porcelain used with alumina frameworks. Sintering

all ceramic restorations, containing fine-grained leucite particles. Sintering temperature: 800°C

all ceramic restorations, containing leucite particles and crystals of fluorapatite. Sintering temperature:

all ceramic restorations, containing equiaxial leucite

3.0 (3.5) ZnO 1.3 (1.6) ZrO2 1.2 (1.4) BaO 0.6 (0.7) TiO2 0.4 (0.5) P2O2

particles. Sintering temperature: 1000°C

**(vol%)**

0

0

6

15

22

0.7 (0.9) BaO 0.6 (0.7) CeO

Fe, Ni, Cr, Hf Fe, Ni, Rb, Sr, Cs, Tb, Cl

6, and 1 μm. For each material, 10 specimens were prepared.

V VITA Zahnfabrik/Vitadur Alpha

174 Ion Exchange - Studies and Applications

The contents of K2O and Na2O on the polished surfaces before and after ion exchange were determined by energy dispersive spectroscopy (EDS, Noram) coupled in a scanning electron microscopy (SEM, JSM 6300, Jeol). This analysis showed that the KNO3 paste method caused the decrease of Na2O content with the increase of K2O content in all porcelains (Figure 6), indicating that the Na+ ions from the glassy matrix were successfully exchanged by K+ ions from the paste. The porcelains Cb and B (with 0 and 22% of leucite, respectively) had the highest relative increase in K2O content of around 35%.

**Figure 6.** K2O content on the surface of dental porcelains before and after ion exchange process (value in parenthesis is the volume fraction of leucite crystal). Data from [47]

Vickers hardness and fracture toughness by indentation fracture (IF) method were evaluated on the polished surface before and after ion exchange. These properties were measured using a Vickers microhardness tester (MVK-H-3, Mitutoyo) with load of 9.8 N and dwell time of 20 s. The diagonal of hardness impression and the length of radial crack emanated from the corner of hardness impression were measured using an optical microscope (Zeiss) under magnifica‐ tion of 200 times, within 30 s after indentation to minimize the slow crack growth phenomenon [16]. Vickers hardness, HV, and fracture toughness, KIc, were calculated according to the following equations [16,49]:

$$HV = \frac{1.8544 \cdot P}{\left(2a\right)^2} \tag{7}$$

$$K\_{\rm lc} = 0.016 \left( \frac{E}{H} \right)^{\rm ly} \left( \frac{P}{c^{\rm gl}} \right) \tag{8}$$

where, *P* is the indentation load, *a* and *c* is the half-size of diagonal of the indentation or radial/ median crack, respectively (Figure 7), *E* is the elastic modulus, and *H* is the material's hardness [defined as *H* = P/(2a2 )]. The elastic modulus of each porcelain was determined by the ultrasonic pulse-echo method [50].

**Figure 7.** Optical micrograph of a Vickers hardness impression and the radial/median cracks generated on the corners of impression on the polished surface of a dental porcelain (a), and schematic image showing the dimensions a and c used to calculate the hardness, HV, and fracture toughness, *KIc*, by indentation fracture (IF) method

Figure 8a shows that ion exchange process increased significantly the fracture toughness, KIc, of most of the tested dental porcelains, with the increase in this property achieving up to around 150% (variation from 0.61 to 1.56 MPa.m1/2 in porcelain C). However, there was also one porcelain (B with highest leucite content) that had no positive response to this toughening treatment. In general, the increase in *KIc* was higher for the porcelains with lower leucite content. The variation in Vickers hardness, *HV*, followed similar tendency as *KIc* (Figure 8b), but with lower relative increases that achieved up to around 70% (variation from 7.6 to 12.7 GPa in porcelain Cb).

Another work also observed beneficial effects of applying ion exchange (with K-containing compound at 450°C for 30 min) to increase the fracture toughness of dental porcelains. For eight dental porcelains, *KIc* increased between 39% and 116%, but no significant differences in hardness values were observed [51].

10 Book Title

1 where, *P* is the indentation load, *a* and *c* is the half-size of diagonal of the indentation or 2 radial/median crack, respectively (Figure 7), *E* is the elastic modulus, and *H* is the material's hardness [defined as *H* = P/(2a2 3 )]. The elastic modulus of each porcelain was determined by

6 **Fig. 7.** Optical micrograph of a Vickers hardness impression and the radial/median cracks 7 generated on the corners of impression on the polished surface of a dental porcelain (a), and 8 schematic image showing the dimensions a and c used to calculate the hardness, HV, and

10 Figure 8a shows that ion exchange process increased significantly the fracture toughness, 11 KIc, of most of the tested dental porcelains, with the increase in this property achieving up to around 150% (variation from 0.61 to 1.56 MPa.m1/2 12 in porcelain C). However, there was also 13 one porcelain (B with highest leucite content) that had no positive response to this 14 toughening treatment. In general, the increase in *KIc* was higher for the porcelains with 15 lower leucite content. The variation in Vickers hardness, *HV*, followed similar tendency as 16 *KIc* (Figure 8b), but with lower relative increases that achieved up to around 70% (variation

18 Another work also observed beneficial effects of applying ion exchange (with K-containing

4 the ultrasonic pulse-echo method [50].

17 from 7.6 to 12.7 GPa in porcelain Cb).

21 in hardness values were observed [51].

9 fracture toughness, *KIc*, by indentation fracture (IF) method

5

22

( ) 2 1.8544 2 *<sup>P</sup> HV a*

3 2 0.016 *Ic E P <sup>K</sup>*

[defined as *H* = P/(2a2

GPa in porcelain Cb).

hardness values were observed [51].

pulse-echo method [50].

176 Ion Exchange - Studies and Applications

1 2

*H c* æ öæ ö <sup>=</sup> ç ÷ç ÷ è øè ø

where, *P* is the indentation load, *a* and *c* is the half-size of diagonal of the indentation or radial/ median crack, respectively (Figure 7), *E* is the elastic modulus, and *H* is the material's hardness

**Figure 7.** Optical micrograph of a Vickers hardness impression and the radial/median cracks generated on the corners of impression on the polished surface of a dental porcelain (a), and schematic image showing the dimensions a and c

Figure 8a shows that ion exchange process increased significantly the fracture toughness, KIc, of most of the tested dental porcelains, with the increase in this property achieving up to around 150% (variation from 0.61 to 1.56 MPa.m1/2 in porcelain C). However, there was also one porcelain (B with highest leucite content) that had no positive response to this toughening treatment. In general, the increase in *KIc* was higher for the porcelains with lower leucite content. The variation in Vickers hardness, *HV*, followed similar tendency as *KIc* (Figure 8b), but with lower relative increases that achieved up to around 70% (variation from 7.6 to 12.7

Another work also observed beneficial effects of applying ion exchange (with K-containing compound at 450°C for 30 min) to increase the fracture toughness of dental porcelains. For eight dental porcelains, *KIc* increased between 39% and 116%, but no significant differences in

used to calculate the hardness, HV, and fracture toughness, *KIc*, by indentation fracture (IF) method

)]. The elastic modulus of each porcelain was determined by the ultrasonic

<sup>×</sup> <sup>=</sup> (7)

(8)

**Figure 8.** Fracture toughness, *KIc* (a), and Vickers hardness, *HV* (b), of dental porcelains before and after ion exchange (IE) process (in parenthesis the vol% of leucite crystal). Data from [47]

The increase in *KIc* value is indicative of the operation of ion stuffing mechanism by ion exchange of smaller Na+ by larger K+ in the glassy matrix, which introduced residual com‐ pressive stress fields on the surface region of the porcelain. The compressive stresses hinder the radial/median crack propagation generated by the Vickers indentation (Figure 9), decreas‐ ing the ratio c/a (ratio between the sizes of radial/median crack and indentation diagonal) and increasing fracture toughness. This behavior is highly desirable since the compressive surface layer may decrease or even inhibit the generation of large and deep cracks on the surface of a dental porcelain restoration during mastication. Since surface cracks are deleterious to the mechanical strength of porcelains, decreasing its size results in lower strength degradation.

**Figure 9.** Schematic images of radial/median cracks generated at the corners of Vickers impression: (a) before ion ex‐ change, without residual stresses and (b) after ion exchange, indicating the shortening of the cracks due to the residual compressive stress fields (indicated by arrows)

The residual stress (MPa) introduced by ion exchange was calculated according to the following equation [47]:

$$
\sigma\_r = \frac{K\_{lc,b} - K\_{lc,a}}{2\left(\frac{c}{\pi}\right)^{4^2}} \tag{9}
$$

where, *KIc,b* and *KIc,a* are the fracture toughness measured before and after ion exchange, respectively. Figure 10 shows the calculated residual compressive stress values. It can be seen that ion exchange by paste method can generate significant compressive stresses, up to around 90 MPa, on the surface of dental porcelains.

**Figure 10.** Residual compressive stress generated on the surface of dental porcelains by ion exchange process. Data from [47]

For porcelain B, which had the highest leucite fraction (22%), however, there was no intro‐ duction of residual compressive stress, although significant increase in K2O content after ion exchange have been detected (Figure 6). A possible explanation could be the occurrence of stress relaxation caused by the viscoplasticity of glassy matrix during the heat treatment of ion exchange process. The temperature used for ion exchange (450°C), however, seemed to be sufficiently lower than the glass transition temperatures, Tg, for all porcelains, as indicated by their annealing point (Table 3). This point is defined as the temperature at which the glass viscosity is 1014 Pa.s and most of the internal stresses are reduced within about 15 min [52]. The annealing point was determined by calculating the viscosity curve as a function of temperature from the chemical composition using the program SciGlass (SciGlass v.7.7, MDL Information Systems) [53].


**Table 3.** Calculated annealing point (temperature) of dental porcelains

The overall and glassy matrix chemical compositions (Table 2) of porcelain B, compared to the other porcelains, did not justify the ineffectiveness of ion exchange to improve the mechanical properties in the porcelain B. The tendency that the relative increases in fracture toughness and hardness decreases with the increase in leucite content (Figure 8) suggests that the beneficial effects of ion exchange are counterbalanced by the toughening effect of leucite particles. It is possible that the ion exchanged K+ ions could preferentially occupy the sites under the tensile residual stressed regions around leucite particles and agglomerates, which could be energetically more favorable causing less increase in residual stresses. The chemistry of the glassy matrix also affected the response to ion exchange treatment, as can be seen from the results of both completely glassy porcelains (V and Cb, Figure 8). Both porcelains had the same initial Na2O content (3.6 mol%, Table 2), but in porcelain Cb higher residual compressive stress was generated (Figure 10). It is difficult to predict the interactive effects of different ions present in the glassy matrix in the ion exchange process between Na+ and K+ ions, since even low concentrations of some elements can have strong effects [26].

where, *KIc,b* and *KIc,a* are the fracture toughness measured before and after ion exchange, respectively. Figure 10 shows the calculated residual compressive stress values. It can be seen that ion exchange by paste method can generate significant compressive stresses, up to around

**Figure 10.** Residual compressive stress generated on the surface of dental porcelains by ion exchange process. Data

For porcelain B, which had the highest leucite fraction (22%), however, there was no intro‐ duction of residual compressive stress, although significant increase in K2O content after ion exchange have been detected (Figure 6). A possible explanation could be the occurrence of stress relaxation caused by the viscoplasticity of glassy matrix during the heat treatment of ion exchange process. The temperature used for ion exchange (450°C), however, seemed to be sufficiently lower than the glass transition temperatures, Tg, for all porcelains, as indicated by their annealing point (Table 3). This point is defined as the temperature at which the glass viscosity is 1014 Pa.s and most of the internal stresses are reduced within about 15 min [52]. The annealing point was determined by calculating the viscosity curve as a function of temperature from the chemical composition using the program SciGlass (SciGlass v.7.7, MDL

**Porcelain V Cb C D B** T (°C) at η = 1014 Pa.s 708 673 564 641 659

The overall and glassy matrix chemical compositions (Table 2) of porcelain B, compared to the other porcelains, did not justify the ineffectiveness of ion exchange to improve the mechanical properties in the porcelain B. The tendency that the relative increases in fracture toughness and hardness decreases with the increase in leucite content (Figure 8) suggests that the beneficial effects of ion exchange are counterbalanced by the toughening effect of leucite

ions could preferentially occupy the sites

**Table 3.** Calculated annealing point (temperature) of dental porcelains

particles. It is possible that the ion exchanged K+

90 MPa, on the surface of dental porcelains.

178 Ion Exchange - Studies and Applications

from [47]

Information Systems) [53].
