Systematic approach to preparing ceramic-glass composites with high translucency for dental restorations.

DENTAL-2586; No. of Pages 10

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Systematic approach to preparing ceramic–glass composites with high translucency for dental restorations Humberto N. Yoshimura a,∗ , Afonso Chimanski a , Paulo F. Cesar b a

Center for Engineering, Modeling and Applied Social Science, Universidade Federal do ABC, Av. dos Estados, 5001, Santo André, SP 09210-580, Brazil b Department of Biomaterials and Oral Biology, School of Dentistry, Universidade de São Paulo, São Paulo, Brazil

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. Ceramic composites are promising materials for dental restorations. However, it

Received 30 June 2014

is difficult to prepare highly translucent composites due to the light scattering that occurs

Received in revised form

in multiphase ceramics. The objective of this work was to verify the effectiveness of a sys-

16 December 2014

tematic approach in designing specific glass compositions with target properties in order to

Accepted 25 June 2015

prepare glass infiltrated ceramic composites with high translucency.

Available online xxx

Methods. First it was necessary to calculate from literature data the viscosity of glass

Keywords:

tion was designed for targeted viscosity and refractive index. The glass of the system

at the infiltration temperature using the SciGlass software. Then, a glass composiCeramic–glass composites

SiO2 –B2 O3 –Al2 O3 –La2 O3 –TiO2 prepared by melting the oxide raw materials was sponta-

Glasses

neously infiltrated into porous alumina preforms at 1200 ◦ C. The optical properties were

Optical properties

evaluated using a refractometer and a spectrophotometer. The absorption and scattering

Infiltration

coefficients were calculated using the Kubelka–Munk model.

Dental ceramics

Results. The light transmittance of prepared composite was significantly higher than a commercial ceramic–glass composite, due to the matching of glass and preform refractive indexes which decreased the scattering, and also to the decrease in absorption coefficient. Significance. The proposed systematic approach was efficient for development of glass infiltrated ceramic composites with high translucency, which benefits include the better aesthetic performance of the final prosthesis. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The most widely used system to produce fixed partial dentures (FPDs) is the metal–ceramic, which is constituted of a

metallic framework veneered with a dental porcelain. High clinical success rates for this system have been reported, varying between 72% and 87% after 10 years [1]. However, the opacity of the metal jeopardizes the final aesthetic outcome of the prosthesis, as it is difficult to mimic the translucent

∗ Corresponding author at: Universidade Federal do ABC (UFABC), Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas (CECS), Av. dos Estados, 5001, Santo André, SP 09210-580, Brazil. Tel.: +55 11 4996 8204. E-mail addresses: [email protected], [email protected] (H.N. Yoshimura).

http://dx.doi.org/10.1016/j.dental.2015.06.015 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yoshimura HN, et al. Systematic approach to preparing ceramic–glass composites with high translucency for dental restorations. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.06.015


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2.

Materials and methods

In this work, alumina (Al2 O3 ) was chosen as the ceramic phase for the composite preform because of its good mechanical properties (421 GPa of Young’s modulus, 4.0 MPa m1/2 of fracture toughness, 397 MPa of four-point flexural strength, and 19.8 GPa of Vickers hardness), higher than spinel (MgAl2 O4 ) ceramic [12,13]. Although the preform of zirconia–toughened alumina results in a stronger composite [14], it is difficult to match the refractive index of the infiltrating glass to both ceramic phases, since they have significantly different refractive indexes (1.76 and 2.19 for alumina and zirconia, respectively [8]).

2.1. Simulation of glass properties and compositional design The software SciGlass 7.7 (Glass Property Information System, Lhasa) was used to calculate the viscosity and refractive index of the glass from its chemical composition. In order to design the glass compositions, it was necessary to know the viscosity of the glass at the infiltration temperature, in order to restrict the maximum temperature to 1200 ◦ C, which is the capacity of commercial furnaces for this process. Therefore, it was necessary to calculate this viscosity based on the data provided by the literature. Fig. 1 shows the calculated viscosities from the chemistry of some glasses as a function of the temperature used for the infiltration process reported in Refs. [15–17]. It could be observed that the viscosities of the glasses at the infiltration temperature ranged from around 102 to 103 dPa s. Therefore, in this work it was established that the glass should have viscosity lower than 102 dPa s at 1200 ◦ C for the design of glass composition. The basic glass composition chosen was 45% SiO2 , 25% Al2 O3 , 15% La2 O3 , and 15% TiO2 (mol%), since a similar composition was reported [9] to result in glass with refractive index close to that of alumina. The simulation of this composition with software SciGlass 7.7 indicated viscosity of 102 dPa s at 1422 ◦ C for this glass, but this temperature was too high for the scope of this work. In this way, the addition of alkaline oxides (Li2 O, Na2 O, K2 O, Rb2 O, and Cs2 O) was simulated in order to replace silica in this glass, however instead of decreasing the temperature, this change resulted in an increase in the temperature at the viscosity of 102 dPa s (Fig. 2a). The simulations showed temperature dependence of viscosity that was similar for all alkaline oxides up to the addition of 25 mol% (Fig. 2b). The addition of most of the alkaline-earth oxides (MgO, CaO, SrO, and BaO) to the basic glass composition also increased the temperature at the viscosity of 102 dPa s (Fig. 2c), except for the addition of beryllium oxide which lowered this temperature to 1319 ◦ C for 25 mol% addition (Fig. 2d), but not sufficiently to the target temperature (99.5% H3 BO3 , PA ACS, Vetec); Al2 O3 (99.9%, UA5105, Showa Denko); La2 O3 (99.9%, PA,

12

Log η (dPa.s)

0 mol% 5 mol% 10 mol% 15 mol% 20 mol% 25 mol%

B2 O3

10 8 6 4 2 0 -2 1000

1500

2000

2500

Temperature (°C) Fig. 3 – Effects of addition of boron oxide on the viscosity of glass 45SiO2 –25Al2 O3 –15La2 O3 –15TiO2 calculated with software SciGlass 7.7. The arrow indicates the increase of added oxide content.

Vetec); and TiO2 (>99%, 1001, Kronos). The raw materials were weighted to prepare a mixture of 100 g to achieve the following composition: 20% SiO2 , 25% B2 O3 , 25% Al2 O3 , 15% La2 O3 , and 15% TiO2 (mol%), considering the theoretical mass loss of water from the boric acid. After manual mixing, the powder mixture was melted in a 100 mL Pt–5%Au crucible at 1500 ◦ C for 1 h using an electrical furnace (FE-1700, Fortelab). Then, part of the melt was poured into the distilled water to prepare the frit and the remaining melt in a graphite mold to prepare the disc-shaped samples (diameter of 20 mm). The frit was milled in a ball mill with plastic jar and alumina balls to prepare a powder finer than 100 mesh (150 ␮m). The discs were annealed in an electric muffle (1612, Jung) at 810 ◦ C (above the glass transition temperature, Tg = 740 ◦ C, calculated using SciGlass 7.7) for 1 h and cooled slowly at the rate of 0.5 ◦ C/min to relieve the internal residual stresses.

2.3.

Preparation of the composite by infiltration

The alumina preforms were prepared from commercial porous presintered alumina blocks with 25.5% porosity (CA-12, InCeram Alumina, Vita Zahnfabrik) by cutting with diamond saw aiming at dimensions of 10.5 × 12.5 × 1.5 mm3 . The glass powder mixed with a small amount of water was placed on the surface of the porous alumina preforms and afterwards taken into an electric furnace (Kerampress, Kota). The thermal cycle for spontaneous infiltration was carried out under vacuum with heating rate of 60 ◦ C/min up to 1200 ◦ C and maintained at this temperature for 60 min; then, the furnace was switched-off.


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d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx

Characterization of glass and composite

S=

Rmed = r1 +

r1 =

(n − 1)

2

(n + 1)

2

r2 = 1 −

(1 − r1 )(1 − r2 )R∗ 1 − r2 R∗

(1)

1 − rd n2

a=

1 2

K = S(a − 1)

(8)

where, n is the refraction index of the material, R is reflectance and subscript med, w, b, sw, and sb are measured, white background, black background, specimen with white background and specimen with black background, respectively. In order to determine the biaxial flexural strength, the samples were tested immersed in artificial saliva at 37 ◦ C using a piston-on-three-balls fixture in a universal testing machine (3366, Instron) at the stress rate of 1 MPa/s. The fracture stress was calculated with the equation given in ASTM standard [21]. The fracture toughness, KIc , was determined by the indentation fracture method using a Vickers hardness tester (Macrovickers 5112, Buehler) with an indentation load of 49 N, applying the Anstis et al. equation [22]. Further details regarding the parameters of mechanical tests can be found elsewhere [3,23,24]. Ten samples of each material were tested in the tests.

3.

Results

The XRD pattern of the prepared 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 glass (Fig. 4a) showed only two broad bands (around 2 of 29◦ and 43◦ ), confirming an amorphous structure without the presence of any crystalline phase. The measured refractive index, n, of the prepared glass was 1.743. This value was only 1.2% lower than that predicted by the

(a)

30

40

50

b=

a2 − 1

70

80

1.85

n − 1 n+1

∗ ∗ ∗ Rsw Rw Rsb

+

∗ Rb − Rsb ∗ Rb Rsw

∗ ∗ − Rw Rsb Rb ∗ ∗ − Rw Rsb

− Rw +

(4)

∗ ∗ Rsw Rsb Rb

+

∗ Rsw

(5)

1.80

(b)

1.75 1.70 1.65 1.60 1.55 1.50 Refractometer

60

2θ (°)

2n3 (n2 + 2n − 1) 8n4 (n4 + 1) ln(n) + 2 4 2 2 (n + 1)(n − 1) (n + 1)(n4 − 1)

−

20

(3)

2

∗ ∗ Rw Rb Rsw

(7)

∗ − bR∗ bRw sw

(2)

(n − 1)(3n + 1) 1 n2 (n2 − 1) rd = + + ln 2 2 (n2 + 1) 6(n + 1) −

aR∗ + aR∗ − R∗ R∗ − 1 w sw sw w

Intensity

The prepared glass powder was analyzed by X-ray diffraction (XRD) analysis using a diffractometer (D8 Focus, Bruker) and Cu-K␣ radiation with scan step of 0.05◦ and 2 s of counting per step in order to confirm its amorphous structure. The refractive index of the glass was determined on the disc sample with polished surfaces using a refractometer (2010/M Prism Coupler, Metricon) at the wavelength of 633 nm. The microstructure of the prepared composite was analyzed using a scanning electron microscope (SEM, Quanta 600 FEG, FEI) coupled to an energy dispersive spectrometer (EDS, Xflash Quanta 400, Bruker). The crystalline phases were identified by XRD analysis. The composite samples with polished surfaces and thickness of 1.0 mm were analyzed in a spectrophotometer (CM-3700d, Konica Minolta) to measure the total transmittance and the diffuse reflectance, using white and black backgrounds with glycerol as coupling liquid, in the visible light region (wavelength from 360 to 740 nm) with 10 nm measurement interval. For comparison, samples of a commercial glass infiltrated alumina composite (InCeram Alumina, Vita Zahnfabrik), prepared with the same porous alumina preform used in this study and the commercial glass powder (AL 2 In-Ceram Alumina Glass Powder, Vita Zahnfabrik), and a commercial lithium disilicate reinforced glass-ceramic (IPS Emax Press, Ivoclar Vivadent), prepared by the heat-pressing technique, with polished surfaces and thickness of 1.0 mm were also analyzed. The contrast ratio, which is a measure of the opacity of a material, was determined by the ratio of the reflectance values with black and white backgrounds [18]. The scattering (S) and absorption (K) coefficients were determined by Kubelka–Munk (K–M) model using the following equations [19,20]:

1 coth−1 bX

Refractive index, n

2.4.

(6)

SciGlass

Fig. 4 – XRD pattern of the prepared glass (a) and its refractive index measured in a refractometer (at 633 nm) and calculated with software SciGlass 7.7 (at 589 nm) (b).




5

Fig. 5 – XRD pattern of the prepared glass-infiltrated ceramic composite. All sharp peaks were identified as ␣-Al2 O3 (JCPDS card no. 42-1468).

software SciGlass 7.7 (Fig. 4b), indicating the good predictability of n values by this software for glasses of the system SiO2 –B2 O3 –Al2 O3 –La2 O3 –TiO2 . After the infiltration process, it was observed that the glass was capable to infiltrate through the whole thickness of the alumina preform (1.5 mm). This finding indicates that the glass 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 has adequate wetting behavior and good flowability, in accordance with the predicted viscosity calculated by the software SciGlass 7.7 (Fig. 3). The XRD pattern of the prepared composite had only diffraction peaks of ␣-Al2 O3 as crystalline phase and broad bands related to the glass (Fig. 5), indicating that no devitrification occurred in the infiltrated glass. Fig. 6a shows a low magnification SEM micrograph of the prepared composite. The microstructure consisted of alumina particles with broad grain size (∼0.5 to 5 ␮m) surrounded by the glass phase. The degree of dispersion of the ceramic particles is related to the structure of alumina preform, since this framework was pre-sintered to form a rigid skeleton through the formation of strong chemical bonds (necking) between alumina particles [24,25]. Infiltration of the glass into the alumina preform resulted in a co-continuous interpenetrating phase composite [15,26]. It was observed that even narrow spaces between the particles of the preform were filled by the glass, indicating good infiltration capability of the developed glass. However, few residual pores that were not infiltrated by the glass could also be noted. At higher magnification, it was possible to observe clear interfaces between alumina particles and the glassy matrix (Fig. 6b). The EDS analysis indicated the presence of only aluminum and oxygen in the alumina particles (Fig. 7a) and silicon, aluminum, lanthanum, titanium, and oxygen in the glass matrix (boron was not identified because of the limitation of EDS analysis to detect elements with low atomic number). A small peak of sodium was also detected in the glass matrix, however its source (raw material or processing) could not be identified (Fig. 7b). Fig. 8 shows images of elemental dot map for the main elements identified by EDS analysis (Al, Si, La, and Ti). It was observed that the highest concentration of aluminum was

Fig. 6 – Backscattered electron images (BEI-SEM) of the prepared glass-infiltrated ceramic composite.

located in the particles and that silicon, lanthanum, and titanium were homogeneously distributed in the glass matrix. These images showed that no reaction or formation of new phases occurred between the alumina particles and the infiltrated glass. Fig. 9a shows the reflectance spectra of the composite measured with white and black backgrounds in the visible region. The difference between these curves is related to the translucency of the material. The tested composite showed higher light transmission at higher wavelengths; but translucency decreased gradually with the decrease in wavelength down to around 450 nm, and below this value the translucency decreased rapidly. Fig. 9b shows the scattering (S) and absorption (K) coefficients calculated from the reflectance values using the Kubelka–Munk model. Both S and K coefficients increased gradually with the decrease in wavelength down to around 450 nm. Fig. 10a shows the transmittance spectrum of the composite prepared in this work compared with a commercial composite (InCeram Alumina, Vita Zahnfabrik) and a commercial lithium disilicate reinforced glass–ceramic (IPS


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cps/eV

cps/eV


450 Al 400 350 300 250 200 150 O 100 50 C Pt 0 0 2

90 O 80 70 Al 60 50 Si 40 30 20 C La Na Pt 10 0 0 2

Pt 4

6

keV

8

10

(a) 12

La Ti La Ti La La La

4

6

Pt

8

10

(b) 12

keV Fig. 7 – EDS spectrum of alumina particles (a) and glass matrix (b). The sample was coated with platinum.

Emax Press, Ivoclar Vivadent). It is evident that the prepared composite was significantly more translucent and the transmittance decreased more slowly with the decrease in wavelength than the commercial composite, although still less translucent than the glass-ceramic. In the visible light region (wavelength from 360 to 740 nm) the average transmittance was 20.1% for the developed composite against 8.9% for the commercial composite and 31.6% for the glass–ceramic. Fig. 10b shows the results of contrast ratio (CR) of these materials, calculated from the reflectance results, which confirmed the higher opacity (lower translucency) of the commercial composite and lower opacity for the glass–ceramic. The CR value was 93.9% and 51.6% for commercial composite and glass–ceramic, respectively, against 84.8% for composite prepared with the designed glass 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 . The results of evaluated mechanical properties are shown in Fig. 11. The composite prepared in this work had similar results of biaxial flexural strength, f , and fracture toughness, KIc , compared to the results of commercial composite (InCeram Alumina) and higher than the results of glass–ceramic (IPS Emax Press). The measured values of f and KIc were, respectively: 223 MPa and 4.06 MPa m1/2 for composite developed in this work; 204 MPa and 3.84 MPa m1/2 for commercial composite; and 171 MPa and 1.68 MPa m1/2 for glass–ceramic.

4.

Discussion

The results showed the effectiveness of the proposed systematic approach in designing a glass composition with target

Fig. 8 – Images of X-ray area scanning (SEM) of the same region of Fig. 6b showing elemental dot maps of: (a) aluminum; (b) silicon; (c) lanthanum; and (d) titanium. Please cite this article in press as: Yoshimura HN, et al. Systematic approach to preparing ceramic–glass composites with high translucency for dental restorations. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.06.015

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1.00

0.45 0.40

Transmittance

Reflectance

0.80

0.60

White background

0.40

IA

0.35

EP

0.30 0.25 0.20 0.15 0.05 0.00

0.20

350

350

This work

0.10

Black background

(a)

(a)

450

550

650

450

750

Wavelength (nm)

S (mm-1)

K

0.4

8

0.3

7

0.2

6

0.1

(b)

0.90 0.80 0.70 0.60 0.50 0.40

5

0.0 350

750

(b)

K (mm-1)

S

9

650

1.00

0.5

CR - Contrast Ratio

10

550

Wavelength (nm)

450

550

650

750

Wavelength (nm) Fig. 9 – Results of reflectance spectrum (a) and coefficients of scattering, S, and absorption, K, (b) of the composite alumina infiltrated with 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 glass.

properties and for the preparation of a glass infiltrated alumina composite with high translucency. The method used in the current investigation was successful in predicting the infiltration processing parameters for glasses using a specific software based on glass properties (SciGlass 7.7). This software also helped to design the glass compositions with target viscosity and specific refractive index values. After defining the glass viscosity (102 dPa s, Fig. 1) at the infiltration temperature, it was possible to verify the effects of different oxide additives in the viscosity of glasses of the system SiO2 –Al2 O3 –La2 O3 –TiO2 (Fig. 2). Compositional simulations indicated that boron oxide was the compound that could strongly reduce viscosity at the infiltration temperature (Fig. 3). The new designed glass with composition of 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 also had a high refractive index, as predicted by the software (Fig. 4b), and it was close to that of the alumina preform (n = 1.76). The experimental setup confirmed the good infiltration performance of the designed glass and its high refractive index value. The prepared glass-infiltrated ceramic composite showed good microstructural features (Fig. 6) and higher translucency than commercial composite (Fig. 10) due to the matching refractive indexes of the glass and the ceramic preform. The scattering coefficients found in the present investigation (Fig. 9b) were relatively low because the refractive index, n, of the prepared glass (1.743, Fig. 4b) was very close

This work

IA

EP

Fig. 10 – Results of transmittance spectrum (a) and contrast ratio (b) of the composite prepared in this work, a commercial composite (InCeram Alumina) and a commercial glass–ceramic (Emax Press – EP).

to the n value of the alumina (difference of 1%). Therefore, it was possible to prove that matching the refractive index of both phases (alumina and glass) significantly decreases light scattering at the interfaces between these phases. The light transmittance of the prepared composite infiltrated with designed glass 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 was at least double of that presented by the commercial composite (Fig. 10a). For the designed composite at wavelengths lower than 450 nm, the absorption coefficient increased abruptly (Fig. 9b), indicating that the decrease in transmittance at the low wavelength region (Fig. 10a) was caused by the light absorption within the material. In fact, the rapid decrease of transmittance value with the decrease of wavelength in almost all visible light region for the commercial composite (Fig. 10a) showed the importance of reducing both absorption and scattering coefficients for developing highly translucent composites. Because of the strong light absorption at low wavelengths, mainly in the regions of violet and indigo, the commercial composite had brownish color, while the designed composite had whitish color, close to the white color of alumina. Besides the matching refractive indexes of the glass and ceramic phases, other factors were responsible for the increased translucency of the composite, such as the good quality of the microstructure with low residual porosity and without presence of new phases, which could cause light scattering and absorption [27,28]. The microstructural


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Fracture strength (MPa)

300

(a) 250 200 150 100 50

This work

IA

EP

IA

EP

5.0

KIc (MPa.m1/2)

(b) 4.0 3.0 2.0 1.0 0.0

This work

Fig. 11 – Results of biaxial flexural strength (a) and fracture toughness, KIc (b), of the composite prepared in this work, a commercial composite (InCeram Alumina – IA) and a commercial glass–ceramic (Emax Press – EP).

analysis indicated that the glass 20SiO2 –25B2 O3 –25Al2 O3 –15La2 O3 –15TiO2 was very effective in infiltrating the alumina preform as it was capable of filling almost all pores (Fig. 6). Another important feature revealed by this analysis was the good chemical compatibility between the glass and the alumina phases (Fig. 8). Previous works reported that when the infiltrating glass had low alumina content, excessive dissolution of alumina particles occurred, increasing the viscosity of the glass and inhibiting the progress of infiltration into the preform [29]. The surface roughness, phase contents, grain size and particle size distribution can also affect light scattering and transmittance through the body [6,27]. Both composites tested in this investigation (experimental and commercial) were prepared with polished surfaces and with the same alumina preform, that is, with no difference in particle size distribution and crystalline phase content; only with different infiltrating glasses. These results highlighted the importance of developing glasses with high refractive indexes in order to match that of the ceramic preform. It is important to note that the benefits of preparing an alumina-glass interpenetrated phase composite with higher translucency are related to better aesthetic performance of the final prosthesis, which will more easily mimic the dental tissues, especially in the anterior region of the oral cavity [30–32]. This can increase the use of ceramic-glass composites to produce dental prostheses. In fact, ceramics recommended for bridges in the posterior region, like glass-infiltrated alumina-zirconia

composite (InCeram Zirconia, Vita Zahnfabrik), are claimed to be as opaque as metal–ceramic systems, with contrast ratio of 100% (at the thickness of 0.5 mm) [18]. comparison to the lithium disilicate-based In glass–ceramic (IPS Emax Press), the designed composite had lower translucency, mainly because of the higher transmittance above the wavelength of 450 nm in glass–ceramic (Fig. 10). However, the mechanical properties (flexural strength and fracture toughness, KIc ) of glass–ceramic were lower than the designed glass-infiltrated alumina composite (Fig. 11). Thus, there is a window of opportunity to design new composites with even higher translucency, but it is also important to keep the high mechanical properties. The oral environment is aggressive for dental ceramics, as besides the chewing stresses, the water molecules in saliva can cause stress corrosion at the crack tip enhancing the slow crack growth (SCG) phenomenon, which degrade the material’s strength during their lifetime [3,23]. In this work, the flexural test was performed with samples immersed in artificial saliva at 37 ◦ C at a stress rate of 1 MPa/s, which tends to weaken the ceramic materials, and is closer to the clinical situation in oral environment. In similar test conditions, compared to the results of this work (Fig. 11a), it was reported similar strength values for the glass–ceramic IPS Emax Press (181 MPa) and higher strength values for the composite InCeram Alumina with the preform prepared by slip casting technique (386 MPa) [24]. This difference seems to be related to the technique of preform preparation, since it has been reported significantly higher strength for this composite prepared with slip casted preform than using dry-pressed presintered block, due to the presence of large elongated alumina particles in slip casted preform, which are absent in presintered blocks and are responsible for strengthening the composite [33]. Thus, optimizing the particle shape and size in the preform can result in composites with higher mechanical strength. Besides, the fracture toughness, KIc , of the designed composite was more than twice that of the glass–ceramic (Fig. 11b), therefore decreasing its brittleness [25]. Although the results of this work showed significant improvements in the preparation of highly translucent multiphase ceramic, it is still necessary to advance even more in the development of composites with high light transmittance. This can be achieved by reducing the residual porosity of glass–infiltrated ceramic, which will result in increased transmittance, since light scattering by pores strongly affects light transmission due to the significant difference between the refractive indexes of ceramic and vacuum [6,28]. Given that it is difficult to avoid the formation of residual pores by spontaneous infiltration (even by applying vacuum [3,33]), an alternative could be to carry out the glass infiltration by applying an external pressure, which can assist with the penetration of the glass into the preform. Pressure-assisted infiltration has been successfully applied to infiltrate molten metal into ceramic preforms in order to prepare metal matrix composites [26,34]. In the heat-pressing method, used to prepare pressable dental ceramics, a glass-ceramic ingot is heated and pressed into a cavity of refractory mold by a thermal resistant alumina plunger attached to the furnace [35,36]. This method is not used to prepare composites, but it could be adapted to make pressure-assisted infiltration of a glass ingot into a




porous ceramic preform inserted in the mold cavity, in order to eliminate or reduce the residual porosity and increase transmittance. Decreasing light absorption through the composite can also contribute to increase translucency. The commercial glass powder (AL 2 In-Ceram Alumina Glass Powder, Vita Zahnfabrik) contains some oxides, like CaO, CeO2 , MnO, and Fe2 O3 (besides SiO2 , B2 O3 , Al2 O3 , La2 O3 , and TiO2 ) [3], that can result in staining of the glass. Therefore, it is important to understand the effects of each oxide as a coloring agent in order to decrease the absorption and increase the material transmittance. Another factor that can contribute to the reduction of transmittance is the presence of small particles, with dimensions close to the wavelength of the incident light, which cause Mie-type (particle size dependent) scattering [6,37]. Therefore, optimizing particle size distribution of the ceramic preform can also result in increase of composite transmittance.

[6]

[7]

[8] [9]

[10] [11]

[12]

5.

Conclusions

The current investigation demonstrated that the proposed systematic approach for development of glass infiltrated ceramic composites with high translucency was efficient. The approach involved the following steps: (1) analysis of literature data using SciGlass software for determination of the processing parameters (viscosity and infiltration temperature); (2) design of glass compositions with target properties (viscosity and refractive index) using SciGlass software; (3) preparation of experimental glasses; and (4) infiltration of glasses into ceramic preforms. The analysis of reflectance measurements by Kubelka–Munk model showed that the superior translucency of the prepared composite was due to matching of the glass and alumina preform refractive indexes, which decreased light scattering at the interfaces between these phases, and also to the decrease in absorption coefficient.

Acknowledgments The authors acknowledge the Brazilian agencies FAPESP (grant # 2010/01003-5), CAPES, and CNPq (grant # 481743/2012-0) for the financial support of the present research.

[13] [14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

references [22] [1] Holm C, Tidehag P, Tillberg A, Molin M. Longevity and quality of FPDs: a retrospective study of restorations 30, 20, and 10 years after insertion. Int J Prosthodont 2003;16(3):283–9. [2] Antonson SA, Anusavice KJ. Contrast ratio of veneering and core ceramics as a function of thickness. Int J Prosthod 2001;14(4):316–20. [3] Gonzaga CC, Yoshimura HN, Cesar PF, Miranda Jr WG. Subcritical crack growth in porcelains, glass–ceramics and glass–infiltrated alumina composite for dental restorations. J Mater Sci: Mater Med 2009;20(5):1017–24. [4] Rosa V, Yoshimura HN, Pinto MM, Fredericci C, Cesar PF. Effect of ion exchange on strength and slow crack growth of a dental porcelain. Dent Mater 2009;25(6):736–43. [5] Yoshimura HN, Pinto MM, Lima E, Cesar PF. Optical properties of dental bioceramics evaluated by

[23]

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