- Email: [email protected]
Sintering and crystallization of akermanite-based glass–ceramics
Materials Letters 60 (2006) 1488 – 1491 www.elsevier.com/locate/matlet
Sintering and crystallization of akermanite-based glass–ceramics J.M.G. Ventura a,b , D.U. Tulyaganov a,c , S. Agathopoulos a , J.M.F. Ferreira a,⁎ a
b
Ceramics and Glass Engineering Department, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal ÁgoraMat, Produção de Materiais Cerâmicos Lda, Incubadora do Beira Atlântico Parque, Sala 2, Piso 1, Rua António José de Almeida, N°278, 3070-304 Mira, Portugal c Scientific Research Institute of Space Engineering, 700128, Tashkent, Uzbekistan Received 30 September 2005; accepted 19 November 2005 Available online 9 December 2005
Abstract Akermanite-based glass–ceramics were successfully produced from the SiO2–Al2O3–B2O3–MgO–CaO–Na2O–F system via sintering and crystallization of glass-powder compacts at low temperatures between 750 and 800 °C. The experimental results indicated that the amount of Al2O3 in the parent glass composition is seemingly a key factor with regard to the potential of this system to crystallize into a mono-mineral akermanite glass–ceramic. The aesthetics and the mechanical, the chemical and the thermal properties of the produced glass–ceramics in conjunction with the evaluation of the economic processing route proposed qualify these glass–ceramics for further investigation as potential materials suitable for applications in restorative dentistry. © 2005 Elsevier B.V. All rights reserved. Keywords: Akermanite; Glasses; Ceramics; Sintering; Heat treatment; Properties
1. Introduction A processing route for obtaining dense glass–ceramics (GCs) is the sintering of glass-powder compacts [1–6], whereby glass-powders with high specific surface area provide uniformly distributed nucleus sites in the bulk of the glass. Coatings of ceramic powder frits applied on metallic substrates are very popular in restorative dentistry [7], such as the clinically used IPS Empress system, which features good biocompatibility, excellent aesthetics to imitate teeth, such as suitable colour, translucency, and reflection to different light sources, sufficient mechanical and corrosion resistance, etc. [1,7,8]. There is currently an increasing demand for new dental restorative materials that can feature low processing temperatures, high aesthetic, and moderate hardness, which would anticipate low abrasive wear of the natural tooth located opposite to the implant. In the frame of this quest, this work presents the synthesis and the characterization of akermanite-based GCs. This topic has received a rather poor documentation in the literature. ⁎ Corresponding author. Tel.: +351 234 370242; fax: +351 234 425300. E-mail address: [email protected] (J.M.F. Ferreira). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.059
Akermanite (Ca2MgSi2O7) and gehlenite form solid solutions, being the end members of melilite, which belongs to the group of sorosilicates [9], having a general formula of X2YZ2O7, where X is Ca or Na, Y is Al or Mg, and Z is Si or Si and Al. X is a large 8-coordinated site, while Y and Z are tetrahedral. Melilites usually occur in igneous and metamorphic rocks and meteorites, but also in the slag of blast furnaces [9,10], which can be considered as an evidence of their high chemical stability at elevated temperatures. In our earlier study [11], we have produced akermanitebased GCs from the multi-component system of SiO2– Al2O3–B2O3–MgO–CaO–Na2O–(P2O5)–F. The GCs produced at relatively low temperatures via sintering of glasspowder compacts exhibited good properties [11]. Nevertheless, the use of alumina crucibles for glass melting in that study caused an inevitable Al2O3-uptake in glass composition (verified also by EDS). In the present study, the influence of Al2O3 content on the crystallization of glasses from SiO2–Al2O3–B2O3–MgO– CaO–Na2O–F system was experimentally investigated with the three compositions shown in Table 1. To eliminate Al2O3uptake, melting took place in Pt crucibles. With respect to composition 1, which was similar to the composition A of our
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
1489
Table 1 Chemical (batch) composition of the investigated glasses Composition 1 2 3
wt.% mol% wt.% mol% wt.% mol%
SiO2
Al2O3
B2O3
MgO
CaF2
CaO
Na2O
42.51 40.91 42.25 40.91 41.98 40.91
2.00 1.14 3.98 2.28 5.94 3.41
4.10 3.41 2.72 2.27 1.35 1.14
9.51 13.64 9.45 13.64 9.39 13.64
6.14 4.54 6.10 4.54 6.06 4.54
30.86 31.82 30.66 31.82 30.47 31.82
4.87 4.54 4.84 4.54 4.81 4.54
previous study [11], the other two compositions (2 and 3) featured a gradual increase of the amount of Al2O3. 2. Materials and experimental procedure Powders of SiO2, Al2O3, CaCO3 (all with purity N99.5%), and (of reactive grade) H3BO3, 4MgCO3·Mg(OH)2·5H2O, Na2CO3, and CaF2 were used. Homogeneous mixtures of batches (∼100 g), obtained by ball milling, were preheated at 1000 °C for 1 h for decarbonization and then melted in Pt crucibles at 1400 °C for 1 h, in air. Glass frit was obtained by quenching of the melt in cold water. The frit was dried and then milled in a high-speed porcelain mill, resulting in powders with a mean particle size of ∼13 μm (determined by light scattering technique; Coulter LS 230, UK, Fraunhofer optical model). Rectangular bars (3 × 4 × 50 mm3) of glass-powder compacts were prepared by uniaxial pressing (80 MPa). The bars were subjected to heat treatment at 700, 750 and 800 °C for 1 h in air at a slow heating rate of 2–3 K/min to avoid deformation. The evolution of crystalline phases of the samples sintered at different temperatures was determined by X-ray diffraction analysis (XRD, Rigaku Geigerflex D/Mac, C Series, Cu Ka radiation, Japan). Scanning electron microscopy (SEM, Hitachi S-4100, Japan, 25 kV acceleration voltage) was used for microstructure observation at polished and then etched surfaces (immersion in 2 vol.% HF for 15 s). With regards to other properties, shrinkage was calculated from the dimensions of the green and the resultant sintered samples. Dilatometry thermal analysis (Bahr Thermo Analyse DIL 801 L, Germany) was performed up to 800 °C with a heating rate of 3 K/min. Archimedes method by immersion in ethylenoglycol was employed to determine the density. Threepoint bending strength was measured with parallelepiped bars (3 × 4 × 50 mm3) (Shimadzu Autograph AG 25 TA; 0.5 mm/min
Fig. 1. Dilatometry curves of the investigated glasses (see Table 1).
displacement; the presenting results are the average of 15 bars). Vickers microhardness was obtained from 10 indentations for each sample (Shimadzu microhardness tester type M, Japan; load of 9.8 N). Chemical resistance was estimated by immersion of 3 rectangular bars (3 × 4 × 25 mm3) in 4 vol.% acetic acid at 95 °C for 24 h. 3. Results and discussion From the dilatation curves of the three glasses in Fig. 1, the glass transition temperatures (Tg) of the glasses 1 and 2 were evaluated to be 592 and 595 °C, respectively, while that of the glass 3 was 625 °C, slightly higher. The softening points of the glasses 1, 2, and 3 were at 639, 631, and 654 °C, respectively. Typical amorphous halo patterns were recorded in the X-ray diffractograms of the investigated glasses heat treated at 700 °C. Crystallization started at temperatures higher than 700 °C. According to Fig. 2, akermanite was the major crystalline phase after heat treatment of the glasses at 800 °C. The intensity of akermanite X-ray peaks increased from composition 1 to 3. The X-ray spectra of the GC 1 and GC 2 predominantly comprised diffraction lines of akermanite phase, while cuspidine and diopside were also detected. Note that cuspidine (Ca4Si2O7F2) belongs to the group of sorosilicates, representing the same family of silicates with akermanite, because both minerals compose of [Si2O7]− 6 groups. Few negligible small peaks of non-identified phases, marked in the spectrum of GC 3 (Fig. 2), were also registered. It was also observed a perfect matching of the X-ray spectrum of the GC 3 to the spectrum of the GC of composition A prepared in our previous study [11] (the shift was only 0.4 degrees), although there is a
Fig. 2. XRD spectra of the three investigated glass-powder compacts (see Table 1) fired at 800 °C for 1 h. ICDD cards for Akermanite, (Ca2MgSi2O7) 01-0760841 (bold lines), Cuspidine (Ca4Si2O7F2), 00-041-1474 (thin lines and triangles), and Diopside (CaMgSiO6) 01-072-1497. (The intensities have not been normalised; full scale of the intensity axis 10,000 cps).
1490
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
∼3 wt.% difference on their Al2O3 content. This fair matching confirms the occurrence of a significant Al2O3-uptake from the alumina crucibles during melting of composition A [11]. The above experimental results indicate that the amount of Al2O3 may play a key role on the yield of akermanite phase in the investigated system. Hence, a deep understanding of the role of Al2O3 has to be resolved in future thorough studies. However, at the present stage, one should keep in mind that a solid solution between akermanite and gehlenite, with a general formula of 2CaO·(1 − x)MgO·xAl2 O3·(2 − x) SiO2, should form in the investigated GC samples and not exactly pure akermanite phase [9]. The microstructure observation of the GCs revealed well developed prismatic akermanite crystals embedded in glassy phase (Fig. 3). Although the shape of the crystals resembles one to another for the three investigated compositions, GC 1 featured a coarser microstructure comprising octahedral and tetrahedral shaped crystals of 5–7 μm in the length. As far as the appearance is concerned, it is worth noting that completely dense materials were synthesized at relatively low temperatures between 750 and 800 °C, which featured homogenous white color and translucence. Table 2 summarizes the values of other important properties, which all qualify the produced GCs for further investigation as potential materials for restorative dentistry. Among them, it should be noticed the good matching of microhardness with that of natural teeth enamel [12] and the fair matching of the linear thermal expansion (CTE) with other materials used in biomedicine, such as ZrO2, Ti, and hydroxyapatite [13–15]. The values of bending strength vary at the range of the commercial leucite-based GC system [1,7,8], for instance 112 (± 10) MPa for the IPS Empress [1]. Note that the mechanical properties might be further improved by using hot pressing technique. The results of chemical resistance are also very promising
Fig. 3. Microstructure of the three investigated glass–ceramics (see Table 1) fired at 800 °C: (a) 1, (b) 2, (c) 3.
Table 2 Properties of samples made of the three investigated glass-powder compacts (see Table 1) heat treated at different temperatures for 1 h Property
Shrinkage (%)
Composition Temperature
1 2 3 Density (g/cm3) 1 2 3 Bending strength 1 (MPa) 2 3 Microhardness (MPa) 1 2 3 CTE × 106 1 2 (100–600 °C) 3 (K− 1) Softening point (°C) 1 2 3 Chemical resistance 1 (mg/cm2) 2 3
700 °C
750 °C
800 °C
13.85 ± 0.26 12.95 ± 0.67 14.85 ± 0.22 2.87 ± 0.01 2.87 ± 0.01 2.87 ± 0.01 85.2 ± 9.2 73.1 ± 9.8 70.8 ± 24.0 4499 ± 300 4782 ± 154 5160 ± 89 10.3 10.3 11.3 641 648 652 21.9 ± 0.3 21.0 ± 0.3 19.3 ± 0.2
13.54 ± 0.24 13.16 ± 0.36 14.41 ± 0.36 2.84 ± 0.01 2.85 ± 0.01 2.87 ± 0.01 114.0 ± 5.9 93.0 ± 8.8 116.0 ± 11.1 2856 ± 159 3479 ± 61 5258 ± 114 10.3 10.3 9.71 862 836 682 15.5 ± 0.1 18.4 ± 0.1 18.5 ± 0.2
13.13 ± 0.20 12.72 ± 0.43 14.38 ± 0.13 2.82 ± 0.01 2.84 ± 0.01 2.84 ± 0.01 104.0 ± 10.7 88.0 ± 6.9 98.0 ± 12.6 3086 ± 300 3735 ± 779 3538 ± 66 10.3 10.4 10.1 809 812 937 18.2 ± 0.1 19.9 ± 0.2 17.6 ± 0.1
considering that the values of the IPS Empress GC (with the ISO testing) range between 100 and 200 g/cm2. With regard to the processing of the investigated GCs, densification is seemingly complete at 700 °C, but it safely occurs up to 800 °C. It has been often observed that, when densification has been almost completed, secondary porosity (i.e. the development of new pores) and enlargement of the existing ones can occur, likely due to the crystallization of phases that are denser than the parent glass [16,17]. For the phases of the present study, akermanite is slightly denser (2.98 g/cm3 [18]) than a glass with similar composition (2.92 g/cm3 calculated by Appen's method [19]), and diopside is denser in the crystalline state (3.27 g/cm3) than in glassy phase (2.75 g/cm3). The slight reduction of density and shrinkage of the samples heat treated at 800 °C, comparing to the values of these properties after heat treatment at 700 and 750 °C (Table 2), should be attributed to the effect of secondary porosity. Further increase of firing temperatures above 850 °C caused pronounced decrease of density and shrinkage and the development of visible bubbles (results are not shown), due to bloating of liquid phase. These materials feature a great potential for processing in common dental laboratories. The overall evaluation of the proposed processing route indicates that it is more economic than the commercial ones because of (a) the inexpensive raw materials used, (b) the low melting temperatures applied (1400 °C for investigated glasses and N 1500 °C for commercial dental GCs [1]), and (c) the possibility of reducing the pressing temperature, as suggested by the relatively low dilatometric softening points (810–940 °C, Table 2) at which the glass matrix demonstrates viscous flow (viscosity is ∼1010 Pa·s [20,21]), whereas the IPS Empress system starts to flow at temperatures between 1000 and 1200 °C [1]. Tests for biocompatibility evaluation of these materials are underway. However, the results of our previous study [22] might be considered as positive evidence of biocompatibility of the new materials, whereby GCs with similar compositions have been successfully qualified from in vitro tests with osteoblasts cell cultures.
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
4. Conclusions Akermanite-based glass–ceramics were produced from the SiO2–Al2O3–B2O3–MgO–CaO–Na2O–F system via sintering and crystallization of glass-powder compacts at low temperatures between 750 and 800 °C. Increasing amount of Al2O3 in the parent glass suppressed the formation of secondary phases, such as diopside. The aesthetics and the mechanical, the chemical, and the thermal properties of the produced glass–ceramics meet the requirements of materials used in restorative dentistry. Inexpensive raw materials, low melting temperatures, and the possibility of reducing the pressing temperature feature the proposed processing route with regard to its potential application in dental laboratories. Acknowledgements This work was supported by CICECO and the Portuguese Foundation of Science and Technology. References [1] W. Höland, G. Beall, Glass–Ceramic Technology, The American Ceramic Society, Westerville, Ohio, 2002, pp. 287–309. [2] C. Lira, A.P.N. Oliveira, O.E. Alarcon, Glass Technol. 42 (2001) 91. [3] C. Siligardi, M.C. D'Arrigo, C. Leonelli, Am. Ceram. Soc. Bull. 9 (2000) 88. [4] T. Toya, Y. Tamura, Y. Kameshima, K. Okada, Ceram. Int. 30 (2004) 983.
1491
[5] T. Toya, Y. Mameshima, A. Yasumori, K. Okada, J. Eur. Ceram. Soc. 24 (2004) 2367. [6] A.R. Boccaccini, J. Schawohl, H. Kern, B. Schunck, J.M. Rincon, M. Romero, Glass Technol. 41 (2000) 99. [7] M.J. Cattell, T.C. Chadwick, J.C. Knowles, R.L. Clarke, E. Lynch, Dent. Mater. 17 (2001) 21. [8] C.M. Gorman, W.E. McDevitt, R.G. Hill, Dent. Mater. 16 (2000) 389. [9] H. Yang, R.M. Haze, R.T. Downs, L.W. Finger, Phys. Chem. Miner. 24 (1997) 510. [10] C. Fredericci, E.D. Zanotto, E.C. Ziemath, J. Non-Cryst. Solids 273 (2000) 64. [11] D.U. Tulyaganov, S.Agathopoulos, H.R. Fernandes, J.M.F. Ferreira. Ceram. Int. (in press). [12] S. Agathopoulos, P. Nikolopoulos, A. Salomoni, A. Tucci, I. Stamenkovic, J. Mater. Sci., Mater. Med. 7 (1996) 629. [13] L.L. Hench, J. Wilson, An Introduction to Bioceramics, World Scientific, Singapore, 1993. [14] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94. [15] S. Agathopoulos, S. Pina, R.N. Correia, Ceram. Trans. 138 (2003) 135. [16] A. Karamanov, L. Arriza, I. Matekovits, M. Pelino, Ceram. Int. 30 (2004) 2119. [17] D.U. Tulyaganov, S. Agathopoulos, J.M. Ventura, M.A. Karakassides, O. Fabrichnaya, J.M.F. Ferreira. J. Eur. Ceram. Soc. (in press). [18] ICDD card of akermanite-synthetic 01-087-0052. [19] A.A. Appen, The Chemistry of Glasses, Himiq, Leningrad, 1974 (in Russian). [20] N.M. Pavlushkin, Principals of Glass Ceramics Technology, 2nd edition, Stroiizdat, Moscow, 1979 (in Russian). [21] W. Vogel, Chemistry of Glasses, The American Ceramic Society, Westerwille-Ohio, 1985. [22] S. Agathopoulos, D.U. Tulyaganov, P. Valério, J.M.F. Ferreira, Biomaterials 26 (2005) 2255.
Sintering and crystallization of akermanite-based glass–ceramics J.M.G. Ventura a,b , D.U. Tulyaganov a,c , S. Agathopoulos a , J.M.F. Ferreira a,⁎ a
b
Ceramics and Glass Engineering Department, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal ÁgoraMat, Produção de Materiais Cerâmicos Lda, Incubadora do Beira Atlântico Parque, Sala 2, Piso 1, Rua António José de Almeida, N°278, 3070-304 Mira, Portugal c Scientific Research Institute of Space Engineering, 700128, Tashkent, Uzbekistan Received 30 September 2005; accepted 19 November 2005 Available online 9 December 2005
Abstract Akermanite-based glass–ceramics were successfully produced from the SiO2–Al2O3–B2O3–MgO–CaO–Na2O–F system via sintering and crystallization of glass-powder compacts at low temperatures between 750 and 800 °C. The experimental results indicated that the amount of Al2O3 in the parent glass composition is seemingly a key factor with regard to the potential of this system to crystallize into a mono-mineral akermanite glass–ceramic. The aesthetics and the mechanical, the chemical and the thermal properties of the produced glass–ceramics in conjunction with the evaluation of the economic processing route proposed qualify these glass–ceramics for further investigation as potential materials suitable for applications in restorative dentistry. © 2005 Elsevier B.V. All rights reserved. Keywords: Akermanite; Glasses; Ceramics; Sintering; Heat treatment; Properties
1. Introduction A processing route for obtaining dense glass–ceramics (GCs) is the sintering of glass-powder compacts [1–6], whereby glass-powders with high specific surface area provide uniformly distributed nucleus sites in the bulk of the glass. Coatings of ceramic powder frits applied on metallic substrates are very popular in restorative dentistry [7], such as the clinically used IPS Empress system, which features good biocompatibility, excellent aesthetics to imitate teeth, such as suitable colour, translucency, and reflection to different light sources, sufficient mechanical and corrosion resistance, etc. [1,7,8]. There is currently an increasing demand for new dental restorative materials that can feature low processing temperatures, high aesthetic, and moderate hardness, which would anticipate low abrasive wear of the natural tooth located opposite to the implant. In the frame of this quest, this work presents the synthesis and the characterization of akermanite-based GCs. This topic has received a rather poor documentation in the literature. ⁎ Corresponding author. Tel.: +351 234 370242; fax: +351 234 425300. E-mail address: [email protected] (J.M.F. Ferreira). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.059
Akermanite (Ca2MgSi2O7) and gehlenite form solid solutions, being the end members of melilite, which belongs to the group of sorosilicates [9], having a general formula of X2YZ2O7, where X is Ca or Na, Y is Al or Mg, and Z is Si or Si and Al. X is a large 8-coordinated site, while Y and Z are tetrahedral. Melilites usually occur in igneous and metamorphic rocks and meteorites, but also in the slag of blast furnaces [9,10], which can be considered as an evidence of their high chemical stability at elevated temperatures. In our earlier study [11], we have produced akermanitebased GCs from the multi-component system of SiO2– Al2O3–B2O3–MgO–CaO–Na2O–(P2O5)–F. The GCs produced at relatively low temperatures via sintering of glasspowder compacts exhibited good properties [11]. Nevertheless, the use of alumina crucibles for glass melting in that study caused an inevitable Al2O3-uptake in glass composition (verified also by EDS). In the present study, the influence of Al2O3 content on the crystallization of glasses from SiO2–Al2O3–B2O3–MgO– CaO–Na2O–F system was experimentally investigated with the three compositions shown in Table 1. To eliminate Al2O3uptake, melting took place in Pt crucibles. With respect to composition 1, which was similar to the composition A of our
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
1489
Table 1 Chemical (batch) composition of the investigated glasses Composition 1 2 3
wt.% mol% wt.% mol% wt.% mol%
SiO2
Al2O3
B2O3
MgO
CaF2
CaO
Na2O
42.51 40.91 42.25 40.91 41.98 40.91
2.00 1.14 3.98 2.28 5.94 3.41
4.10 3.41 2.72 2.27 1.35 1.14
9.51 13.64 9.45 13.64 9.39 13.64
6.14 4.54 6.10 4.54 6.06 4.54
30.86 31.82 30.66 31.82 30.47 31.82
4.87 4.54 4.84 4.54 4.81 4.54
previous study [11], the other two compositions (2 and 3) featured a gradual increase of the amount of Al2O3. 2. Materials and experimental procedure Powders of SiO2, Al2O3, CaCO3 (all with purity N99.5%), and (of reactive grade) H3BO3, 4MgCO3·Mg(OH)2·5H2O, Na2CO3, and CaF2 were used. Homogeneous mixtures of batches (∼100 g), obtained by ball milling, were preheated at 1000 °C for 1 h for decarbonization and then melted in Pt crucibles at 1400 °C for 1 h, in air. Glass frit was obtained by quenching of the melt in cold water. The frit was dried and then milled in a high-speed porcelain mill, resulting in powders with a mean particle size of ∼13 μm (determined by light scattering technique; Coulter LS 230, UK, Fraunhofer optical model). Rectangular bars (3 × 4 × 50 mm3) of glass-powder compacts were prepared by uniaxial pressing (80 MPa). The bars were subjected to heat treatment at 700, 750 and 800 °C for 1 h in air at a slow heating rate of 2–3 K/min to avoid deformation. The evolution of crystalline phases of the samples sintered at different temperatures was determined by X-ray diffraction analysis (XRD, Rigaku Geigerflex D/Mac, C Series, Cu Ka radiation, Japan). Scanning electron microscopy (SEM, Hitachi S-4100, Japan, 25 kV acceleration voltage) was used for microstructure observation at polished and then etched surfaces (immersion in 2 vol.% HF for 15 s). With regards to other properties, shrinkage was calculated from the dimensions of the green and the resultant sintered samples. Dilatometry thermal analysis (Bahr Thermo Analyse DIL 801 L, Germany) was performed up to 800 °C with a heating rate of 3 K/min. Archimedes method by immersion in ethylenoglycol was employed to determine the density. Threepoint bending strength was measured with parallelepiped bars (3 × 4 × 50 mm3) (Shimadzu Autograph AG 25 TA; 0.5 mm/min
Fig. 1. Dilatometry curves of the investigated glasses (see Table 1).
displacement; the presenting results are the average of 15 bars). Vickers microhardness was obtained from 10 indentations for each sample (Shimadzu microhardness tester type M, Japan; load of 9.8 N). Chemical resistance was estimated by immersion of 3 rectangular bars (3 × 4 × 25 mm3) in 4 vol.% acetic acid at 95 °C for 24 h. 3. Results and discussion From the dilatation curves of the three glasses in Fig. 1, the glass transition temperatures (Tg) of the glasses 1 and 2 were evaluated to be 592 and 595 °C, respectively, while that of the glass 3 was 625 °C, slightly higher. The softening points of the glasses 1, 2, and 3 were at 639, 631, and 654 °C, respectively. Typical amorphous halo patterns were recorded in the X-ray diffractograms of the investigated glasses heat treated at 700 °C. Crystallization started at temperatures higher than 700 °C. According to Fig. 2, akermanite was the major crystalline phase after heat treatment of the glasses at 800 °C. The intensity of akermanite X-ray peaks increased from composition 1 to 3. The X-ray spectra of the GC 1 and GC 2 predominantly comprised diffraction lines of akermanite phase, while cuspidine and diopside were also detected. Note that cuspidine (Ca4Si2O7F2) belongs to the group of sorosilicates, representing the same family of silicates with akermanite, because both minerals compose of [Si2O7]− 6 groups. Few negligible small peaks of non-identified phases, marked in the spectrum of GC 3 (Fig. 2), were also registered. It was also observed a perfect matching of the X-ray spectrum of the GC 3 to the spectrum of the GC of composition A prepared in our previous study [11] (the shift was only 0.4 degrees), although there is a
Fig. 2. XRD spectra of the three investigated glass-powder compacts (see Table 1) fired at 800 °C for 1 h. ICDD cards for Akermanite, (Ca2MgSi2O7) 01-0760841 (bold lines), Cuspidine (Ca4Si2O7F2), 00-041-1474 (thin lines and triangles), and Diopside (CaMgSiO6) 01-072-1497. (The intensities have not been normalised; full scale of the intensity axis 10,000 cps).
1490
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
∼3 wt.% difference on their Al2O3 content. This fair matching confirms the occurrence of a significant Al2O3-uptake from the alumina crucibles during melting of composition A [11]. The above experimental results indicate that the amount of Al2O3 may play a key role on the yield of akermanite phase in the investigated system. Hence, a deep understanding of the role of Al2O3 has to be resolved in future thorough studies. However, at the present stage, one should keep in mind that a solid solution between akermanite and gehlenite, with a general formula of 2CaO·(1 − x)MgO·xAl2 O3·(2 − x) SiO2, should form in the investigated GC samples and not exactly pure akermanite phase [9]. The microstructure observation of the GCs revealed well developed prismatic akermanite crystals embedded in glassy phase (Fig. 3). Although the shape of the crystals resembles one to another for the three investigated compositions, GC 1 featured a coarser microstructure comprising octahedral and tetrahedral shaped crystals of 5–7 μm in the length. As far as the appearance is concerned, it is worth noting that completely dense materials were synthesized at relatively low temperatures between 750 and 800 °C, which featured homogenous white color and translucence. Table 2 summarizes the values of other important properties, which all qualify the produced GCs for further investigation as potential materials for restorative dentistry. Among them, it should be noticed the good matching of microhardness with that of natural teeth enamel [12] and the fair matching of the linear thermal expansion (CTE) with other materials used in biomedicine, such as ZrO2, Ti, and hydroxyapatite [13–15]. The values of bending strength vary at the range of the commercial leucite-based GC system [1,7,8], for instance 112 (± 10) MPa for the IPS Empress [1]. Note that the mechanical properties might be further improved by using hot pressing technique. The results of chemical resistance are also very promising
Fig. 3. Microstructure of the three investigated glass–ceramics (see Table 1) fired at 800 °C: (a) 1, (b) 2, (c) 3.
Table 2 Properties of samples made of the three investigated glass-powder compacts (see Table 1) heat treated at different temperatures for 1 h Property
Shrinkage (%)
Composition Temperature
1 2 3 Density (g/cm3) 1 2 3 Bending strength 1 (MPa) 2 3 Microhardness (MPa) 1 2 3 CTE × 106 1 2 (100–600 °C) 3 (K− 1) Softening point (°C) 1 2 3 Chemical resistance 1 (mg/cm2) 2 3
700 °C
750 °C
800 °C
13.85 ± 0.26 12.95 ± 0.67 14.85 ± 0.22 2.87 ± 0.01 2.87 ± 0.01 2.87 ± 0.01 85.2 ± 9.2 73.1 ± 9.8 70.8 ± 24.0 4499 ± 300 4782 ± 154 5160 ± 89 10.3 10.3 11.3 641 648 652 21.9 ± 0.3 21.0 ± 0.3 19.3 ± 0.2
13.54 ± 0.24 13.16 ± 0.36 14.41 ± 0.36 2.84 ± 0.01 2.85 ± 0.01 2.87 ± 0.01 114.0 ± 5.9 93.0 ± 8.8 116.0 ± 11.1 2856 ± 159 3479 ± 61 5258 ± 114 10.3 10.3 9.71 862 836 682 15.5 ± 0.1 18.4 ± 0.1 18.5 ± 0.2
13.13 ± 0.20 12.72 ± 0.43 14.38 ± 0.13 2.82 ± 0.01 2.84 ± 0.01 2.84 ± 0.01 104.0 ± 10.7 88.0 ± 6.9 98.0 ± 12.6 3086 ± 300 3735 ± 779 3538 ± 66 10.3 10.4 10.1 809 812 937 18.2 ± 0.1 19.9 ± 0.2 17.6 ± 0.1
considering that the values of the IPS Empress GC (with the ISO testing) range between 100 and 200 g/cm2. With regard to the processing of the investigated GCs, densification is seemingly complete at 700 °C, but it safely occurs up to 800 °C. It has been often observed that, when densification has been almost completed, secondary porosity (i.e. the development of new pores) and enlargement of the existing ones can occur, likely due to the crystallization of phases that are denser than the parent glass [16,17]. For the phases of the present study, akermanite is slightly denser (2.98 g/cm3 [18]) than a glass with similar composition (2.92 g/cm3 calculated by Appen's method [19]), and diopside is denser in the crystalline state (3.27 g/cm3) than in glassy phase (2.75 g/cm3). The slight reduction of density and shrinkage of the samples heat treated at 800 °C, comparing to the values of these properties after heat treatment at 700 and 750 °C (Table 2), should be attributed to the effect of secondary porosity. Further increase of firing temperatures above 850 °C caused pronounced decrease of density and shrinkage and the development of visible bubbles (results are not shown), due to bloating of liquid phase. These materials feature a great potential for processing in common dental laboratories. The overall evaluation of the proposed processing route indicates that it is more economic than the commercial ones because of (a) the inexpensive raw materials used, (b) the low melting temperatures applied (1400 °C for investigated glasses and N 1500 °C for commercial dental GCs [1]), and (c) the possibility of reducing the pressing temperature, as suggested by the relatively low dilatometric softening points (810–940 °C, Table 2) at which the glass matrix demonstrates viscous flow (viscosity is ∼1010 Pa·s [20,21]), whereas the IPS Empress system starts to flow at temperatures between 1000 and 1200 °C [1]. Tests for biocompatibility evaluation of these materials are underway. However, the results of our previous study [22] might be considered as positive evidence of biocompatibility of the new materials, whereby GCs with similar compositions have been successfully qualified from in vitro tests with osteoblasts cell cultures.
J.M.G. Ventura et al. / Materials Letters 60 (2006) 1488–1491
4. Conclusions Akermanite-based glass–ceramics were produced from the SiO2–Al2O3–B2O3–MgO–CaO–Na2O–F system via sintering and crystallization of glass-powder compacts at low temperatures between 750 and 800 °C. Increasing amount of Al2O3 in the parent glass suppressed the formation of secondary phases, such as diopside. The aesthetics and the mechanical, the chemical, and the thermal properties of the produced glass–ceramics meet the requirements of materials used in restorative dentistry. Inexpensive raw materials, low melting temperatures, and the possibility of reducing the pressing temperature feature the proposed processing route with regard to its potential application in dental laboratories. Acknowledgements This work was supported by CICECO and the Portuguese Foundation of Science and Technology. References [1] W. Höland, G. Beall, Glass–Ceramic Technology, The American Ceramic Society, Westerville, Ohio, 2002, pp. 287–309. [2] C. Lira, A.P.N. Oliveira, O.E. Alarcon, Glass Technol. 42 (2001) 91. [3] C. Siligardi, M.C. D'Arrigo, C. Leonelli, Am. Ceram. Soc. Bull. 9 (2000) 88. [4] T. Toya, Y. Tamura, Y. Kameshima, K. Okada, Ceram. Int. 30 (2004) 983.
1491
[5] T. Toya, Y. Mameshima, A. Yasumori, K. Okada, J. Eur. Ceram. Soc. 24 (2004) 2367. [6] A.R. Boccaccini, J. Schawohl, H. Kern, B. Schunck, J.M. Rincon, M. Romero, Glass Technol. 41 (2000) 99. [7] M.J. Cattell, T.C. Chadwick, J.C. Knowles, R.L. Clarke, E. Lynch, Dent. Mater. 17 (2001) 21. [8] C.M. Gorman, W.E. McDevitt, R.G. Hill, Dent. Mater. 16 (2000) 389. [9] H. Yang, R.M. Haze, R.T. Downs, L.W. Finger, Phys. Chem. Miner. 24 (1997) 510. [10] C. Fredericci, E.D. Zanotto, E.C. Ziemath, J. Non-Cryst. Solids 273 (2000) 64. [11] D.U. Tulyaganov, S.Agathopoulos, H.R. Fernandes, J.M.F. Ferreira. Ceram. Int. (in press). [12] S. Agathopoulos, P. Nikolopoulos, A. Salomoni, A. Tucci, I. Stamenkovic, J. Mater. Sci., Mater. Med. 7 (1996) 629. [13] L.L. Hench, J. Wilson, An Introduction to Bioceramics, World Scientific, Singapore, 1993. [14] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94. [15] S. Agathopoulos, S. Pina, R.N. Correia, Ceram. Trans. 138 (2003) 135. [16] A. Karamanov, L. Arriza, I. Matekovits, M. Pelino, Ceram. Int. 30 (2004) 2119. [17] D.U. Tulyaganov, S. Agathopoulos, J.M. Ventura, M.A. Karakassides, O. Fabrichnaya, J.M.F. Ferreira. J. Eur. Ceram. Soc. (in press). [18] ICDD card of akermanite-synthetic 01-087-0052. [19] A.A. Appen, The Chemistry of Glasses, Himiq, Leningrad, 1974 (in Russian). [20] N.M. Pavlushkin, Principals of Glass Ceramics Technology, 2nd edition, Stroiizdat, Moscow, 1979 (in Russian). [21] W. Vogel, Chemistry of Glasses, The American Ceramic Society, Westerwille-Ohio, 1985. [22] S. Agathopoulos, D.U. Tulyaganov, P. Valério, J.M.F. Ferreira, Biomaterials 26 (2005) 2255.