**3. Dental porcelains**

The feldspathic porcelain is a predominantly glassy material with variable crystalline content. Its basic structure has a network of silica with potassium, sodium, and other ions as network modifiers. In order to reduce the glass softening temperature and increase fluidity, metal oxide fluxes are added (CaO, K2O, Na2O), which decrease the softening temperature by reducing the amount of cross linking in the porcelain structure. The addition of alumina (between 8 and 20 wt%) is used for controlling the viscosity and decreasing the flow at high temperatures. When added, B2O3 in concentrations below 12 wt% also acts as a flux to form a less stable network of BO4/SiO4 [30–33].

Silica, soda, potash, and alumina are the constituents of mineral feldspar (Na2O/ K2O.Al2O3.6SiO2), the main raw material used in the manufacturing of dental porcelain [30]. Silica and alumina account for most of the feldspar, about 70 and 17 wt%, respectively. Feldspar porcelains are relatively pure and colorless, so pigments are added to produce shades of natural tooth [34]. In the process of obtaining porcelain, feldspar is mixed with fluxes, then heated to temperatures between 1150 and 1530°C, and rapidly cooled in water. With the thermal shock, the glass breaks into fragments called frits. Opacifiers (TiO2, ZrO2) and pigments (Cr2O3, Fe2O3) are also added to this glass [34]. Heating feldspar to high temperatures leads to an incongruent melting resulting in the formation of leucite and a liquid glass. The leucite crystal is a mineral (potassium aluminum silicate) with high coefficient of thermal expansion compared to feldspathic glasses [30]. The porcelain structure after incongruent melting of feldspar has leucite (K2O.Al2O3.4SiO2) crystals involved in an aluminosilicate glassy matrix [32,33,35]. In general, the leucite ratio is governed by the K2O content of the frit and the time and temperature of the heat treatment; thus, the desired leucite content can be achieved by controlling the appropriate time and the crystallization temperature range [36].

residual compressive stress [27]. Stress relaxation occurs by viscous flow of the glass and can

*G t*

h

where, *σ* is the remaining stress at time *t*, *σ*0 is initial stress, and *G* and *η* are shear modulus and viscosity of glass, respectively. Since *η* decreases strongly with temperature, the rate of

The chemical tempering has been applied to strengthen cockpit windows for aircrafts, high speed train windshields, photocopier scanner glass, display windows in mobile personal electronic devices, compact disks for portable hard drives, high-end ophthalmic glasses, and glass items for drug delivery [22,26]. Advantageous characteristics of chemical tempering include: possibility to strengthen complex geometries and thin components (thickness of up to around 100 μm), which are difficult in thermal tempering; higher compressive stress level on the surface compared to thermal tempering; low level of internal residual tensile stress, with less fragmentation and explosion-like fracture propagation; and did not cause optical distortion. Disadvantageous characteristics include: limited to alkali-containing glasses; shallow depth of residual compressive stress layer (case depth); generation of corrosive alkalicontaining salt residue; and high cost when long time of ion exchange is applied [22,26].

The feldspathic porcelain is a predominantly glassy material with variable crystalline content. Its basic structure has a network of silica with potassium, sodium, and other ions as network modifiers. In order to reduce the glass softening temperature and increase fluidity, metal oxide fluxes are added (CaO, K2O, Na2O), which decrease the softening temperature by reducing the amount of cross linking in the porcelain structure. The addition of alumina (between 8 and 20 wt%) is used for controlling the viscosity and decreasing the flow at high temperatures. When added, B2O3 in concentrations below 12 wt% also acts as a flux to form a less stable network

Silica, soda, potash, and alumina are the constituents of mineral feldspar (Na2O/ K2O.Al2O3.6SiO2), the main raw material used in the manufacturing of dental porcelain [30]. Silica and alumina account for most of the feldspar, about 70 and 17 wt%, respectively. Feldspar porcelains are relatively pure and colorless, so pigments are added to produce shades of natural tooth [34]. In the process of obtaining porcelain, feldspar is mixed with fluxes, then heated to temperatures between 1150 and 1530°C, and rapidly cooled in water. With the thermal shock, the glass breaks into fragments called frits. Opacifiers (TiO2, ZrO2) and pigments (Cr2O3, Fe2O3) are also added to this glass [34]. Heating feldspar to high temperatures leads to an incongruent melting resulting in the formation of leucite and a liquid glass. The

è ø (6)

<sup>0</sup> exp

æ ö <sup>×</sup> <sup>=</sup> ç ÷

s s

stress relaxation is more rapid with the increase in temperature.

**3. Dental porcelains**

of BO4/SiO4 [30–33].

be described by the Maxwell's model given by [28,29]:

170 Ion Exchange - Studies and Applications

The frits based on leucite are used in Dentistry since the early 60s [31]. However, feldspar is not essential as a precursor for the formation of leucite, and many dental porcelains do not use feldspar as the raw material. These materials are called feldspar-free porcelain and are synthesized in the laboratory by controlled addition of leucite instead of mineral processing. It has been suggested that porcelains with a large amount of leucite dispersed in the glass matrix are called leucite-based instead of feldspathic porcelains [30].

Feldspathic porcelains are usually presented as a liquid-powder system, in which the liquid (water with dispersant) is mixed with porcelain powder to form a slurry or paste that is applied to the refractory die or metal framework. After production of the green body, it is taken into an electric furnace for the firing cycle (Figure 4a) [3,37]. Chemical reactions between the porcelain powder components are completed during the process of obtaining the frits. Therefore, the main purpose of firing is sintering of particles, although some chemical reactions may occur during prolonged firing times or during multiple firing [30]. According to the firing (sintering) temperature, the porcelain used for restorations and bridges can be classified as being of high fusion (850 to 1100°C) and low fusion (below 850°C) [30]. The firing procedure involves high heating rates (around 60°C/min) under vacuum and few minutes at the maxi‐ mum temperature. During this procedure, sintering process transforms the porous green body (Figure 4b) in a translucent and dense solid, almost pore free (Figure 4c), by means of densi‐ fication mechanisms with mass transport by viscous flow [3,37]. Vacuum firing is a resource used to reduce the porosity of these materials [31]. At the end of the firing procedure, the porcelain can have three distinct phases: a crystalline phase (leucite), the glass matrix, and pores.

Most dental porcelain developed for metal-ceramic restorations contain leucite (KAlSi2O6) as a main crystalline phase [38]. Leucite was first introduced so that the porcelain could reach a linear thermal expansion coefficient close to that of alloys used in metal-ceramic restorations. In this way, metal and porcelain have similar behavior when cooled together during the firing process, preventing the appearance of cracks in the porcelain [39]. Leucite is also the main crystalline constituent of most generations of porcelain for all-ceramic restorations [40]. In this case, leucite is not added with the aim of achieving thermal compatibility, but to increase the strength and fracture toughness of the material [32,41]. The amount, average crystal size, and crystal structure of leucite directly affect the thermal, optical, and mechanical properties of the final restoration [42]. One of the advantages having leucite as the crystalline phase is that the translucency of the porcelain is not lost, since the refractive index of this crystal is similar to that of the glassy matrix [43,44].

**Figure 4.** Dental porcelain furnace (a), green (b), and sintered (c) porcelain body [5]

The leucite crystal has a crystallographic polymorphic transformation (without change of composition) from tetragonal to cubic during heating. This transformation is displacive (martensitic) and accompanied by a marked change in the parameters of crystalline lattice with an increase of 1.2% by volume of the unit cell [38]. When a porcelain containing leucite is cooled from the firing temperature to room temperature, residual stresses arise in the material as a result of the large difference between the linear thermal expansion coefficients of the glass matrix (8.6 ppm °C-1) and tetragonal leucite (22.3 ppm °C-1). After cooling, tangential com‐ pressive stresses and radial tensile stresses appear in the glass matrix around the leucite particle and opposite stress fields in the tetragonal crystals [38,43]. The compressive stresses have a beneficial effect on the porcelains as they function as a mechanism for increasing the toughness, as opposed to the tensile stresses, which drive the cracks forward [45].

Figure 5 shows typical micrographs of dental porcelains containing leucite crystals. For porcelains with high leucite contents, the distribution of crystals usually is not homogeneous in the glassy matrix (Figure 5a), forming agglomerates of leucite particles (Figure 5b). In these micrographs, it is possible to see some circumferential cracks in the glass matrix surrounding leucite agglomerates, which reveal the radial tensile stresses generated during cooling [11]. The residual stress fields associated with the leucite crystals have significant effects on the fracture behavior of porcelain, since they change the propagation trajectory of a crack, driving it through the glassy matrix region with radial tensile stresses. The result is that a crack propagates bowing around leucite particles and agglomerates (Figure 5c). This effect is called crack deflection and is the main toughening mechanism caused by leucite crystals in dental porcelains. In fact, it has been observed that fracture toughness, *KIc*, increases with the increase of volume fraction of leucite [11].

The increase in porcelain's strength with the increase in leucite content has been observed experimentally. However, there is a tendency of this increase to achieve a maximum, because

Running Title

7 reveal leucite particles)

1

2 **Fig. 5.** Micrographs (a – optical, b,c – scanning electron microscopy) of dental porcelains 3 showing: (a) leucite (KAlSi2O6) crystals dispersed in glassy matrix; (b) agglomerate of leucite 4 particles; (c) radial crack emanated from Vickers indentation corner (blue arrow) deflecting 5 among leucite particles (green arrow). In (a,b), red arrow indicates circumferential crack 6 around leucite agglomerate generated by chemical etching with HF solution (also used to **Figure 5.** Micrographs (a – optical, b,c – scanning electron microscopy) of dental porcelains showing: (a) leucite (KAl‐ Si2O6) crystals dispersed in glassy matrix; (b) agglomerate of leucite particles; (c) radial crack emanated from Vickers indentation corner (blue arrow) deflecting among leucite particles (green arrow). In (a,b), red arrow indicates circum‐ ferential crack around leucite agglomerate generated by chemical etching with HF solution (also used to reveal leucite particles)

for porcelains with high leucite volume fraction spontaneous microcracks can be generated around big leucite agglomerates during cooling after sintering. These flaws have large size (c value in Equation 1) and limit the porcelain's strength, despite the increase in fracture toughness, KIc [42,46]. Besides the quantity, the size, and distribution of the leucite particles also influences the mechanical properties of the porcelain [18,41]. 8 The increase in porcelain's strength with the increase in leucite content has been observed 9 experimentally. However, there is a tendency to this increase to achieve a maximum, 10 because for porcelains with high leucite volume fraction spontaneous microcracks can be 11 generated around big leucite agglomerates during the cooling after sintering. These flaws 12 have large size (c value in Equation 1) and limit the porcelain's strength, despite the increase

**Figure 4.** Dental porcelain furnace (a), green (b), and sintered (c) porcelain body [5]

of volume fraction of leucite [11].

172 Ion Exchange - Studies and Applications

The leucite crystal has a crystallographic polymorphic transformation (without change of composition) from tetragonal to cubic during heating. This transformation is displacive (martensitic) and accompanied by a marked change in the parameters of crystalline lattice with an increase of 1.2% by volume of the unit cell [38]. When a porcelain containing leucite is cooled from the firing temperature to room temperature, residual stresses arise in the material as a result of the large difference between the linear thermal expansion coefficients of the glass matrix (8.6 ppm °C-1) and tetragonal leucite (22.3 ppm °C-1). After cooling, tangential com‐ pressive stresses and radial tensile stresses appear in the glass matrix around the leucite particle and opposite stress fields in the tetragonal crystals [38,43]. The compressive stresses have a beneficial effect on the porcelains as they function as a mechanism for increasing the

toughness, as opposed to the tensile stresses, which drive the cracks forward [45].

Figure 5 shows typical micrographs of dental porcelains containing leucite crystals. For porcelains with high leucite contents, the distribution of crystals usually is not homogeneous in the glassy matrix (Figure 5a), forming agglomerates of leucite particles (Figure 5b). In these micrographs, it is possible to see some circumferential cracks in the glass matrix surrounding leucite agglomerates, which reveal the radial tensile stresses generated during cooling [11]. The residual stress fields associated with the leucite crystals have significant effects on the fracture behavior of porcelain, since they change the propagation trajectory of a crack, driving it through the glassy matrix region with radial tensile stresses. The result is that a crack propagates bowing around leucite particles and agglomerates (Figure 5c). This effect is called crack deflection and is the main toughening mechanism caused by leucite crystals in dental porcelains. In fact, it has been observed that fracture toughness, *KIc*, increases with the increase

The increase in porcelain's strength with the increase in leucite content has been observed experimentally. However, there is a tendency of this increase to achieve a maximum, because
