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Glass–ceramics of LAS (Li2O–Al2O3–SiO2) system enhanced by ion-exchange in KNO3 salt bath
Journal of Non-Crystalline Solids 428 (2015) 90–97
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Glass–ceramics of LAS (Li2O–Al2O3–SiO2) system enhanced by ion-exchange in KNO3 salt bath Karolina Łączka a, Katarzyna Cholewa-Kowalska a,⁎, Marcin Środa a, Jakub Rysz b, Mateusz M. Marzec c, Maria Łączka a a b c
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Glass Technology and Amorphous Coatings, al. Mickiewicza 30, 30-059 Kraków, Poland M. Smoluchowski Institute of Physics, Jagiellonian University, ul. Łojasiewicza 11, 30-348 Kraków, Poland AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, al. Mickiewicza 30, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 1 June 2015 Received in revised form 1 July 2015 Accepted 2 August 2015 Available online xxxx Keywords: LAS glass–ceramics; Ion-exchange; Flexural strength; Decrystallization
a b s t r a c t The paper describes a low-temperature ion-exchange process on specially designed Li2O–Al2O3–SiO2 glass–ceramics (LAS). Due to thermal treatment, two main crystalline phases: lithium di-silicate and lithium aluminasilicate were obtained in glass–ceramic materials. As a result of proper time/temperature profile treatment of precursor glasses, flexural strength of final materials increased up to 300–450 MPa. Crystallized samples were immersed in KNO3 molten salt, at temperatures of 400–420 °C for 9 or 16 h. The final materials achieved almost 100% greater mechanical resistance than glass–ceramics before ion-exchange. We claim that the process of ion-exchange improved flexural strength significantly. Furthermore, it resulted in some changes of the surface layer structure. We noticed that chemical tempering process leads to decrystallization of the multiphase LAS glass–ceramic surface layer consisting of lithium di-silicate and lithium alumina-silicate phases. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Glass is transparent, which is one of its biggest advantages. Due to its nature, unfortunately, it is a brittle material with not very high mechanical strength. For many years, scientists have attempted to find a way to improve its strength, maintaining the transparency at the same time. One of these methods turned out to be controlled crystallization. It uses glasses that are chemically designed in a proper way to create glass–ceramics as final materials. Glass–ceramics are polycrystalline solids obtained upon controlled crystallization from glasses. This procedure usually leads to mechanical reinforcement when compared to the initial glass strength. Because of crystals grown up in the glass–ceramics, some of the transparency is lost [1,2]. Nevertheless, thanks to a proper thermal treatment, it is possible to create small crystals (b50 nm) in final material, obtaining the required transparency [3]. Glass–ceramics obtained in this way are produced and used as cooker top panels, telescope mirrors, high temperature resistant windows, tile glaze, and so on [3]. Reinforced glass–ceramics of esthetic appearance are also successfully applied in dentistry as dental crowns [4]. Another successful way to improve mechanical properties of glass and glass–ceramic materials is low-temperature ion-exchange. This method induces compressive stress in the material surface layer by substituting small ions in the glass or glass–ceramics with larger ions ⁎ Corresponding author. E-mail address: [email protected] (K. Cholewa-Kowalska).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.08.003 0022-3093/© 2015 Elsevier B.V. All rights reserved.
diffusing from an appropriate molten salt [5,6]. This process results in an increase of mechanical resistance without losing material's transmittance. A special group of aesthetic materials is glass–ceramics based on Li2O–SiO2 system with lithium di-silicate (LS) as a main crystalline phase. They have been available for dental applications since 1998 (IPS Empress 2, Ivoclar-Vivadent, Schaan, Liechtenstein) [7,8]. Chemical compositions of basic glasses for LS glass–ceramic production are characterized by the ca. 15% content of Li2O and lack or a low content of Al2O3 (usually 1–3%). Compared to starting LS glass (flexural strength about 150 MPa), the crystallization of lithium di-silicate reinforces the final material up to about 400 MPa or even more. In addition, lithium di-silicate glass–ceramics may also be reinforced by low-temperature ion-exchange. Fisher H. et al. [9] studied the lithium di-silicate glass–ceramic material IPS Empress 2. Samples of glass–ceramics were ionexchanged in batch 1: KNO3 at 400 °C for 11 h; batch 2: KNO3 + LiNO3 at 350 °C for 11 h; and batch 3: KNO3 at 400 °C for 11 h, and also in NaNO3 + KNO3 at 400 °C for 1 h. With respect to a characteristic strength value, flexural strength of untreated material was 265.5 MPa, while after ion-exchange a 25% increase was observed for batch 1 and batch 3 (331.4 MPa and 320.6 MPa, respectively). Experimental results of the ion-exchange study conducted for LS glass–ceramics indicate the amorphization of material surface layer after a bath in molten salts: NaNO3 and KNO3 [10,11]. Tagantsev [11] observed a phenomenon, where crystalline phase of lithium metasilicate LiSiO3 disappeared slightly upon the Li+ ↔ Na+ exchange, and
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he was the first to call it using a term “decrystallization”. He discussed this process basing on thermodynamics and some proposed interpretation “in the terms of the phase equilibrium theory in the heterogeneous liquid solution. The vitreous portion of the glass–ceramics is considered as a solvent and the crystals as a precipitates, which are dissolving when altering the solvent composition through ion-exchange”. Chokchai Yatongchai [10] also examined the ion-exchange for LS transparent glass–ceramics using NaNO3 and KNO3 molten baths under various temperatures and durations of treatment to determine optimum process conditions. His results showed, that the maximum flexural strength of glass–ceramics of 487 ± 40 MPa could be obtained by the Li+ ↔ K+ exchange at 500 °C for 5 h. Destruction and disappearance of lithium di-silicate crystals caused by Li+ ↔ Na+ ion-exchange in NaNO3 molten salt baths were observed. The other important group of glass–ceramics having a high commercial value is Li2O–Al2O3–SiO2 (LAS) glass-crystalline materials based mainly on the crystallization of a solid-solution of β-spodumene (Li– Al-silicates) and silica SiO2, with rather no Li-di-silicate crystallization. A basis for the production of glass–ceramics of this type is low-lithium glasses (a few wt% of Li2O) with a larger amount of Al2O3 (Li2O/Al2O3 clearly less than 1). These LAS glass–ceramics exhibit rather low to moderate mechanical properties. Their flexural strength and fracture toughness are in the range of 100–250 MPa and 1–1.5 MPa ∗ m1/2, respectively [3,12,13]. However, these materials are characterized by their low thermal expansion, combined with refractoriness up to 600 °C and great thermal shock resistance [14–16]. Soares et al. [17] applied the ion-exchange treatment to the LAS glass–ceramic low thermal expansion to minimize surface crystallization and thus improve the sintering compacts made of glass particles during the forming process of glass–ceramic product. The subject of our previous study was also LAS glass–ceramics; however, that study focused on a new chemical composition of starting glasses where the ratio of Li2O to Al2O3 was approx. 1, and the content of these oxides was approx. 10 wt.% for each component. At this ratio (Li2O/Al2O3), simultaneous crystallization of lithium alumina-silicates and lithium di-silicate was successfully induced [18–20]. That resulted in strengthening the glass by creating lithium di-silicate crystallization in the final material. We received the final glass–ceramics with flexural strength about 400 MPa. What is more, this material had a high aesthetic appearance (white-yellowish color, translucency), and could be applied in dentistry. This paper is a continuation of our previous study on the new LAS glass–ceramics enhanced due to lithium di-silicate crystallization [18–20]. In this study, reinforcement upon the low-temperature ionexchange of our LAS glass–ceramics is presented. The paper describes to what extent the strength of this material can be enhanced. What is more, we studied the changes in surface structure upon low thermal ion-exchange process as those observed for LS materials [10,11].
mixed using a mechanical stirrer and then inserted into a 1 dm3 Pt/Rh crucible. The batches were melted in an electric furnace at the maximum temperature 1480 °C during 5 h. Melts were homogenized by stirring during melting at 1460 °C with the SiO2 rod and also by bubbling. Homogeneous melted glasses were cast on a steel plate and annealed at 500 °C. The specimens of parent glasses for further examinations were made in the form of bars: 4 × 1.5 × 17 mm and plates: 13 × 13 × 1 mm. The specimen surface was ground and polished. The samples were prepared by wet grinding (Struers Tegramin 30 polishing machine) with a sequence of 240, 400, 600, 800 mesh SiC abrasive papers, followed by final polishing with polishing cloth (Fedo 1; METKON) and CeO2 polishing powder (Bohle). For such prepared glass samples, thermal treatment leading to control crystallization was performed in the electric furnace as per the evaluated profiles. In order to determine the temperature/time profile of nucleation and crystallization of parent glasses, the differential thermal analysis (DSC) was used as the basic method (Fig. 1). The DSC measurements were performed with Thermal Analyzers NETSH using a Pt crucible of 90 mg capacity. Glass particles of sizes between 425 and 500 μm were prepared for these measurements. DSC was performed at a heating rate 10 K/min in a flowing atmosphere (50 cm3/min) of dry air. Base on our previous study [18,20] as well as the performed DTA analysis, parameters of thermal treatment for both parent glasses were evaluated. After thermal treatment types of crystalline phases as well as their volumetric content in materials were characterized by powder X-ray diffraction (PANalytical B.V. Spectris plc., The Netherlands) using CuKα radiation (Table 1). The diffractometer resolution was 0.026° 2Θ measured on the line d100 LaB6 (NIST SRM660a). XRD patterns were recorded at room temperature with a step size of 0.033° in a 15°–60° 2Θ on powdered glass–ceramic samples. The crystalline phase content was determined by the Rietveld method as suggested in works by Taylor and Matulis. The vitreous phase was determined also using the Rietveld method [21,22]. In order to recognize suspected decrystallization of lithium disilicate and lithium alumina-silicate upon ion-exchanged glass–ceramics, the plate samples of GAM7 and GBM10 glass–ceramics before and after ion-exchange were measured with X-ray diffraction (Fig. 6). Prepared glass–ceramic samples, were immersed in a molten salt KNO3 (p.a. quality, Merck, Germany) at the temperature range 400– 420 °C, upon particular durations: 9 or 16 h. After ion-exchange process samples were pulled out and washed in warm water. Both glass–ceramic samples and glass–ceramics after ion-exchange were tested for flexural strength by three-point bending in a universal
2. Experiment Studies were carried out for glasses from Li2O–Al2O3–SiO2 system with the following chemical compositions determined by chemical analysis (wt.%): GAM7: SiO2 66; Al2O3 11.0; Li2O 11, CaO 1.5; K2O 0.5, Na2O 3, P2O5 4.5, ZrO2 0.5, CeO2 1.5, V2O5 0.5; GBM10: SiO2 69; Al2O3 10.5; Li2O 10.5, CaO 1.5; K2O 0.50, Na2O 1.5, P2O5 4, ZrO2 0.5, CeO2 1.5, V2O5 0.5. The chemical compositions of glasses were determined using X-ray fluorescence spectroscopy and analytical chemical methods. The parent glasses were prepared according to the melt-quenching way. The following compounds were used as raw materials (p.a.; Sigma Aldrich Co., POCh): SiO2; P2O5; ZrO2; CeO2; V2O5; Al(OH)3; Li2CO3; CaCO3; K2CO3; and Na2CO3. About 1 kg of components was
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Fig. 1. DSC curves for not-pre-heated parent glasses GAM7 and GBM10.
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Table 1 Relations between heat treatment programs, crystallization/amorphous phase content and flexural strength of GAM7 and GBM10 glass–ceramics. Materials
Heating profile
GAM7 GAM7/GC GAM7/GC GBM10 GBM/10/GC GBM/10/GC
0 A B 0 A B
Heating profile (nucleation, crystallization) — temp.(°C)/time (min) 530/600; 660/30; 730/30 530/600; 660/30; 830/90 550/600; 710/30; 550/600; 710/30; 870/90
Quantitative phase analysis LS/LAS/Am.Ph [%]
Flexural strength [MPa] mean value with SD
Characteristic flexural strength [MPa] (δ0)
Weibull modulus (m)
Optical properties
0/0/100 18.1/23.7/47.5 17.6/31.9/38.8 0/0/100 20.9/39.4/35.7 21.4/50.2/20.2
209 ± 35 314 ± 50 402 ± 26 159 ± 31 376 ± 71 406 ± 40
220 327 433 171 400 462
6.92 13.2 8.65 5.02 6.12 7.52
Transparent Well translucent Less translucent Transparent Well transparent Less translucent
LS — lithium di-silicate; LAS — lithium alumina-silicates; Am.Ph — amorphous phase; nucleation temperature/time is marked by underline.
testing machine (ZwickRoell Z020, Ulm, Germany) (Tables 1, 2). Each measurement of bending strength was conducted for series of 10–30 samples. Results are expressed as a characteristic flexural strength value δ0 and a calculated Weibull parameter m. The characteristic strength value represents the value at which 63.21% of the test specimens have fractured. The Weibull modulus describes the relative spread of strength values in the asymmetrical distribution [23,24]. Dynamic secondary ion mass spectrometry (SIMS) technique was used to investigate the glass composition in the vicinity of the surface. It allowed defining the approximate profiles of the exchanged alkaline ions Na+, K+, and Li+ in the surface layer. TOF-SIMS 5 (ION-TOF GmbH, Germany) system working in a non-interlaced dual beam mode was used to obtain composition versus depth profiles. Samples were sputtered with an O2 ion beam (2 keV, 600 nA) rastered over 350 × 350 μm, and analyzed with a high energy (30 keV) pulsed bismuth beam rastered over 100 ×100 μm. Masses of secondary ions induced by a Bi+ beam were analyzed in a time-of-flight mass spectrometer. The DektakXT stylus profilometer (Bruker) was used to measure the depth of sputtered craters (Figs. 2, 3). Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES OPTIMA 7300DV Perkin Elmer) was performed additionally in order to identify the Li, Na and K elements content in the representative KNO3 salt solution (saturated in 20 °C) used for ion-exchange process (15 series performed in 240 cm3 of salt; examined samples: 10 bars and 1 plate for 1 series, ca. 130 cm2). For the sake of comparison, the untreated KNO3 salt solution underwent this examination as well (Table 3). The structure of material surface layer before and after ion-exchange was characterized by Raman spectroscopy (Confocal Raman microscope LabRAM HR UV–Visible-NIR 200–1600 nm, Laser Ar+ 532 nm.). Moreover, the depth profile of Raman spectra (ca. 10 μm) was determined for glass–ceramics after ion-exchange (Figs. 4, 5). Raman spectroscopy is a very sensitive method to determine structural disorders, which is why this method, apart from XRD, was chosen to determine a possible amorphization of surface layer after ion-exchange. 3. Results Chemical compositions of parent glasses slightly differ from each other in the content of silica SiO2 (66 wt % for GAM7, and 69 wt.% for GBM10) and sodium oxide Na2O (3% for GAM7, and 1% for GBM10). The content of other components was similar while maintaining the characteristic for the new LAS glass–ceramic Al2O3/Li2O ratio about 1.
The DSC plots of parent glasses GAM7 and GBM10 (Fig. 1) allowed setting their nucleation and crystallization parameters. The glass transition temperature for GAM7 was Tg = 484 °C, so the nucleation temperature was established at Tn = 530 °C. For GBM10, Tg was 503 °C, while nucleation was set at Tn = 550 °C. The DSC plots of parent glasses GAM7 and GBM10 differ from each other due to the number of crystallization effects as well as their temperature ranges. For GAM7 glass, two strong exothermic effects in the range 600–750 °C are observed, whereas for GBM10 glass only one effect occurs in this range. For both glasses very weak effects in the ranges of higher temperatures (above 800 °C) were observed. We established detailed crystallization programs of GAM7 and GBM10 glasses on the basis of DSC results, also taking into account our previous study [18,20], as follows: - A. heating in the range of main crystallization peaks (625–730 °C), - B. heating in the range of main crystallization peaks (625–730 °C), and additional heating in the temperatures 830–870 °C.
Table 1 shows representative programs of non-isothermal heat treatments (heating rate: 10 °C/min), resulting in the best mechanical properties of glass–ceramics. After crystallization a quantitative XRD phase analysis of glass–ceramics was made. The results related to the main crystalline phases (lithium di-silicate; lithium alumina-silicates) as well as amorphous phase are given in Table 1. Crystallization in the range of main DSC peaks (temperature 660–730 °C) leads to obtaining translucent glass– ceramics which were composed mainly of lithium di-silicate and lithium alumina-silicates crystals; the ratio of these phase was about 1:1.3 for glass–ceramics based on GAM7 glass (GAM7/GC), and close to 1:2 for glass–ceramics based on GBM10 glass (GBM10/GC) (Table 1). Crystallization resulting from a thermal treatment (Table 1) according to the A program resulted in strengthening of both GAM7 and GBM10 glasses up to the characteristic flexural strength δ0 about 327 MPa (GAM7GC) and 400 MPa (GBM10/GC); (about 50% reinforcement for GAM7/GC, and more than 100% reinforcement for GBM10/GC in relation to characteristic flexural strength of basic glasses GAM7 and GBM10: 220 MPa and 171 MPa, respectively). As a result of the thermal treatment according to the B program, higher crystallization degree was reached with a significant increase of lithium aluminasilicates in the samples. This resulted in the increase of characteristic flexural strength up to about 430–460 MPa.
Table 2 The effect of ion exchange IE conditions of GAM7 and GBM10 glass–ceramics on material flexural strength. Materials
Heating profile [temp (°C)/time (min)]
IE conditions [temp (°C)/time (h)]
Flexural strength before/after IE [MPa]; mean value with SD
Characteristic flexural strength before/after IE [MPa] (δ0)
Weibull modulus (m) before/after IE
GAM7 GAM7 GBM10 GBM10
530/600; 660/30; 730/30 530/600; 660/30; 830/90 550/600; 710/30; 550/600; 710/30; 870/90
400/9 400/16 410/16 420/16
314 ± 50/725 ± 80 402 ± 26/595 ± 52 376 ± 71/584 ± 71 406 ± 40/837 ± 81
327/755 433/627 400/640 462/870
13.2/10.32 8.65/12.16 6.12/9.89 7.52/12.58
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Fig. 2. Secondary ion intensity profiles of the exchanged ions in the surface layer of a specimen determined by secondary ion mass spectrometry (SIMS) for GAM7 glass–ceramics (heat-treatment: 530 °C/600 min; 660 °C/30 min; 730 °C/30 min a) before and b) after 400 °C/9 h ion-exchange).
Ion-exchange in a molten potassium salt (KNO3) was carried out for both well translucent glass–ceramics (A program) and the less translucent, more crystallized materials (B program). The best results of material strengthening by ion-exchange are presented in Table 2. The conditions were as follows: - temperature 400 °C and holding time 9 h or 16 h for GAM7/GC; - temperature 410–420 °C and holding time 16 h for GBM10/GC.
After such performed ion-exchange, spectacular mechanical strengthening of translucent GAM7/GC was observed (the characteristic flexural strength value δ0 above 700 MPa) (Table 2). As for the translucent GBM10/GC, the effect was weaker (δ0 about 600 MPa), but mechanical strengthening was still observed. In both cases, the optical properties of material did not change after ion-exchange, and glass–ceramic was still highly translucent (Photo 1). The ion-exchange process conducted for glass–ceramics after a multi-step crystallization (660\\730 °C; 830\\870 °C) also lead to the visible strengthening of both GAM7/GC and GBM10/GC materials. In the case of GAM7/GC, the best results of characteristic flexural strength were achieved at 627 MPa, while for GBM10/GC this effect was stronger (870 MPa) (Table 2). Higher Weibull modulus values m were obtained for the
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Fig. 3. Secondary ion intensity profiles of the exchanged ions in the surface layer of a specimen determined by secondary ion mass spectrometry (SIMS) for GAM7 glass–ceramics (heat treatment: 530 °C/600 min; 660 °C/30 min; 830 °C/90 min a) before and b) after 400 °C/16 h ion-exchange).
samples after ion-exchange, which indicates that after ion-exchange the samples were more uniform and more reliable than the non treated ones. To evaluate changes of the alkali ions concentration in the surface layer after ion-exchange, the profiles of the exchanged ions in the surface layers were approximately determined by SIMS mapping. The ion-exchange treatment resulted in change of the K+, Na+ and Li+ ion concentrations in the specimen surface layer. The penetration depth of incorporated additional potassium ions was about 15 μm for both glass–ceramics GAM7/GC: well translucent (after heat-treatment: 530 °C/600 min; 660 °C/30 min; 730 °C/90 min and ion-exchange 400 °C/9 h) and less translucent GAM7/GC (heat treatment 530 °C/ 600 min; 660 °C/30 min; 830 °C/90 min and ion-exchange 400 °C/16 h) (Figs. 2(b) and 3(b)). The analysis of sodium ions showed a decrease of their presence in the material up to 12–15 μm deep. The SIMS analysis Table 3 Elements K, Na, Li concentration in salt solution KNO3 (saturated 20 °C) before and after ion exchange. Elements concentration [mg/dm3]
K
Na
Li
Salt KNO3 Salt KNO3 after IE
57,802.2 ± 2104.5 10,236.9 ± 157.8
1.9 ± 0.1 39.3 ± 3.1
b0.01 0.88 ± 0.06
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Fig. 4. Raman spectra of surface layer of GBM10 glass–ceramics (heat-treatment 550 °C/ 600 min; 710 °C/30 min; 870 °C/90 min) before and after ion exchange in molten KNO3:420 °C/16 h.
revealed a slight decrease of lithium in the surface layer after the ionexchange of specimens of both glass–ceramics GAM7/GC after different heat treatments. A complementary method of KNO3 salt analysis before and after fifteen cycles of the ion-exchange of glass–ceramics was performed (Table 3). The obtained results confirmed that lithium ions also participate in ion-exchange because their amount clearly increased in the molten salt.
In order to determine possible structural changes occurring in the surface layer of glass–ceramics after ion-exchange, two methods: XRD and Raman spectroscopies, were applied. The results of these examinations are presented in Figs. 4–6. The XRD analysis proves that for both GAM7/GC and GBM10/GC materials the intensity of peaks derived from crystalline phases: lithium disilicate (at 2θ = 23.84°; 24.42°; 24.85°) and lithium alumina-silicates (2θ = 22.26°; 25.9°) clearly decreases after the ion-exchange. We present the results obtained for both GAM7/GC and GBM10/GC materials (Fig. 6). The decrease in intensity of characteristic peaks resulting from both phases crystallized in glass–ceramics indicates the amorphization of surface layer of these materials as caused by the ionexchange. A structural study using Raman spectroscopy of the glass–ceramic surface layer before and after ion-exchange showed significant changes similar for both GAM7/GC and GBM10/GC materials. We present results for GBM10/GC (Fig. 4). These changes are related to the bands located at [25–27]: - 470 cm−1 derived from Si–O–Si polymerized groups (Q4 units); - 490 cm− 1 derived from the four-fold silicate D1 rings normally observed in vitreous silica; - 950 cm−1 derived probably from SiO− groups (Q2 units).
Ion-exchange caused a decrease in the intensity of the band 467–465 cm−1 obtained from the bending vibration of Si–O–Si groups in Q4 units [23–25] indicating de-polymerization of the network. On the other hand, an increase in the intensity of the band 491–497 cm−1
Fig. 5. Raman spectra depth profile of surface layer of GBM10 glass–ceramics (heat-treatment 550 °C/600 min; 710 °C/30 min; 870 °C/90 min) after ion exchange in molten KNO3:420 °C/ 16 h.
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the surface layer. That indicates its amorphization by ion-exchange. The spectroscopy results are consistent with those resulting from XRD. 4. Discussion
Fig. 6. The XRD patterns of a) GAM7 and b) GBM10 glass–ceramics before (black) and after (red) ion exchange. The XRD patterns show decrease in the peaks intensity of lithium disilicate (Li2Si2O5, ICDD, 4-9-8780) and lithium alumina-silicate (LiAlSi2O6, ICDD, 37-794) crystalline phases. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
originating from 4-membered rings of SiO4 tetrahedra was observed, which also indicates an increase of defects in the silicate network after ion-exchange. Moreover, after ion-exchange, the increase of the band at 948 cm−1 derived from de-polymerized Q2 units in the silicate network (two bridging oxygens) occurred [26,27]. For disordered amorphous phase, these results show an increase of characteristic groups in
Photo 1. Macroscopic observation of GAM7 and GBM10 base glasses and glass ceramics after thermal treatment at the temperatures listed in Table 1.
Commercial LAS glass–ceramic is based on crystalline phases with highly anisotropic thermal expansion composition (TEC), such as a solid solution of β-quartz type, where crystallization of lithium alumina-silicates usually occurs (β-spodumene; β-eucryptite and others) [21–23]. The slightly positive TEC of the residual glassy phase combined with the negative TEC of the crystalline phase results in ultra-low thermal expansion of the glass–ceramics. However, these glass–ceramics show moderate or low mechanical properties. The newly developed LAS glass–ceramics represented by GAM7/GC and GBM10/GC materials have a different chemical composition in comparison to commercials. These composition changes allow simultaneous crystallization of two phases: lithium di-silicate and lithium aluminasilicates to grow. Thanks to this, lithium di-silicate reinforces LAS glass–ceramics, which in turn results in an increase of flexural strength up to about 400 MPa [18–20]. For the examined GAM7 and GBM10 parent glasses, chemical composition differ from each other in relation to both the content of silica and alkali oxides. A lower silica concentration and a higher alkali oxides content in the GAM7 glass affect the transformation temperature, 484 °C, which is lower than for the GBM10 glass — 503 °C. This also results in a lower nucleation temperature of the GAM7 glass as compared to the GBM10 glass (530 °C/550 °C, respectively). Controlled crystallization of GAM7 glass at 660–730 °C led to the glass–ceramics with about 50% crystallization, and similar content of both lithium di-silicate and lithium alumina-silicates phases (with a slightly higher content of the latter), where translucency was well manifested. Translucent glass–ceramics were obtained also on the base of GBM10 glass, as a result of thermal treatment at 710 °C. Unlike the GAM7 glass–ceramics, the GBM10 was characterized by a higher crystallization degree (about 65%) as well as a higher content of lithium alumina-silicates (about 40%). Strengthening translucent glass–ceramics is obvious, compared to GAM7 and GBM10 basic glasses. Applying a high-temperature thermal treatment in the range of 830–870 °C resulted in a higher crystallization degree of material as well as a decrease of its translucency. At the same time, flexural strength of both GAM7/GC and GBM10/GC materials increased even above 400 MPa, which is an unusual value as compared to classical LAS glass–ceramics with only lithium alumina-silicates crystallization. This confirmed our previously mentioned thesis that additional crystallization of lithium di-silicate evidently strengthens this type of materials [18–20]. The aim of our study was to strengthen already obtained LAS glass– ceramics further by an ion-exchange in the bath salt of KNO3. This process was performed for both translucent and opaque glass-ceramics. Due to the presence of crystalline phase/phases near amorphous, the ion-exchange process for glass–ceramics is more complicated than the process occurring in glass. We believe that during the ion-exchange process performed in glass–ceramics, the movement of alkali ions from residual glassy phase should be easier than the one from ordered crystalline phases. Reinforcement of LAS glass–ceramics by low temperature ionexchange can occur only by replacement of smaller ions with larger ones. Here, it may appear as an exchange of Na+, and Li+ ions into K+ ions. The concentration of Na2O in basic glasses is small (1.5–3.0 wt.%), in comparison to the Li 2 O content, (about 10 wt.%), however, it all remains in residual glassy phase of glass–ceramics (sodium does not participate in crystallization unlike to lithium). Furthermore, alkali content in the surface layer is usually higher than the one in the glass interior. Probably this also refers to sodium Na and lithium Li. We can draw conclusions about a mechanism of ion-exchange in the examined glass–ceramics only on the basis of alkali ions profile near the surface (SIMS examinations). The results of this study show an evident loss of
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sodium in the glass–ceramic surface layer up to approx. 15 μm depth after the ion-exchange (Figs. 2–3). Thus, we suppose that an exchange of Na+ ions with K+ ions is mainly responsible for material strengthening. We believe that also an exchange of Li+ ions with K+ ions affects the strengthening process since the Li loss from material was shown. Nevertheless, the lithium loss was smaller, and on a smaller depth than the one relating to sodium; therefore, we claim that Na was primarily responsible for material reinforcement. This suggests a possibility of another mechanism of ion-exchange in LAS glass–ceramics containing sodium oxide addition, compared with Fisher H. results [9] for sodium-free LS glass–ceramics. There, the exchange Li+ ↔ K+ only is responsible for material reinforcement. Hence, our studies confirmed that an evident increase of flexural strength observed in developed by us LAS glass–ceramics is possible by the ion-exchange in KNO3 molten salt. In addition, reinforced glass–ceramics by ion-exchange showed no further loss of its translucency. At the same time, the calculated Weibull modulus m shows that after the ion-exchange, an increase of material reliability was observed. As already mentioned, ion-exchange in glass–ceramics (GC) should mainly be applied between molten salt and residual glassy phase present in GC. However, for examined LAS GC, a clear relation between the content of residual glassy phase and strengthening of material by ion-exchange was not observed. A very high reinforcement, up to about 700 MPa, for high translucent GAM7/GC material containing about 50% of glassy phase occurred, although a very high flexural strength above 800 MPa was also observed for GAM10/GC containing 20% amorphous phase only. This confirms that phenomena occurring in glass–ceramics during ion-exchange are more complex, and more factors had an influence on the material properties, as compared with glasses. The XRD and the Raman spectroscopy examinations gave us some information about the structural changes occurring in the glass–ceramic surface layer after the ion-exchange. The Raman spectra showed that a replacement of smaller alkaline ions with the larger ones led to de-polymerization, and also increased the defects in the surface layer. Probably more de-polymerized Q2 units of silicon were created as well as four-member rings composed of [SiO4] tetrahedral form. This could indicate a destruction of the crystalline phases occurring in the examined materials confirming the results of Ch. Yatongchai for LS glass–ceramics [10]. Similarly, our results are consistent with a study by Soares W. O. et al., conducted on commercial LAS glass–ceramics, which showed a decrease of the surface crystallization as a result of the ionexchange, improving the sintering conditions of LAS glass–ceramics [17]. The Raman spectroscopy results are consistent with the XRD study. The changes in XRD patterns of examined LAS glass–ceramics before and after ion-exchange are seen as diminishing of the crystalline phases effects, which indicates the amorphization of the surface layer. That phenomenon is in agreement with Tagantsev [11] and Ch. Yatongchai's [10] studies for lithium di-silicate glass–ceramics. We have shown, for the first time, that decrystallization also occurs in LAS glass–ceramics where lithium di-silicate and lithium alumina-silicates crystallize simultaneously, and this refers to both phases. 5. Conclusions • The LAS glass–ceramics with simultaneous crystallization of lithium di-silicate and lithium alumina-silicates phases can be produced base on the Li2O–Al2O3–SiO2 system. The glass compositions (GAM7 and GBM10) are characterized by the content of about 10 wt.% each of Al2O3 and Li2O. They differ from each other in the silica content as compared to the content of alkali. • Crystallization of lithium di-silicate in combination with the lithium alumina-silicates causes the strengthening of materials up to 400 MPa of flexural strength. Translucency of the glass–ceramics can
•
•
•
•
be controlled by the thermal treatment conditions. Both of these properties could be moderated depending on needs. Additional enhancement of glass–ceramics can be achieved by ionexchange of smaller ions (Na, Li) with larger ones (K) using batch salts of KNO3. Strengthening of materials depends on temperature/ time conditions of ion-exchange as well as on the initial chemical composition of starting glasses (the content of silica and alkali). The maximum strengthening (flexural strength about 700–800 MPa) was achieved for highly translucent glass–ceramics obtained by thermal treatment of GAM7 glass and less translucent glass–ceramics produced on the base of GBM10 glass. Ion-exchange induces changes in the surface layer at least to the depth of about 15–20 μm, and involves a formation of defects, which probably leads to a destruction of crystalline phases. The X-ray patterns indicate a decrease in the content of crystalline phases (lithium di-silicate and lithium alumina-silicate) in the surface layers of GAM7 and GBM10 glass–ceramics after the ion-exchange. The way of reinforcement of glass–ceramic that is shown here can be used to produce glass–ceramic materials for various applications, e.g. dentistry, cooker top panels, telescope mirrors, high temperature windows, protective cover for portable electronic devices (smartphones, tablets, etc.) and so on.
Acknowledgments This study was supported financially by the National Science Centre Grant No. 2012/07/N/ST8/02174 and by the Polish Ministry for Science and Higher Education, AGH University of Science and Technology, Faculty of Material Science and Ceramics Grant No. 11.11.160.365. The authors would like also to thank A.B. Kounga PhD and Institute Straumann AG, Basel, Switzerland for scientific and finacial support. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). References [1] Glass–Ceramics, P.W. McMillan, Academic Press, New York, 1979. [2] Glass–Ceramic Technology, W. Höland& G. Beall, The American Ceramic Society, Westerville, OH, 2002. [3] E.D. Zanotto, A bright future for glass–ceramics, Am. Ceram. Soc. Bull. 89 (8) (2010) 19–27. [4] W. Höland, V. Rheinberger, E. Apel, C. van 't Hoen, M. Höland, A. Dommann, M. Obrecht, C. Mauth, U. Graf-Hausner, Clinical applications of glass–ceramics in dentistry, J. Mater. 17 (11) (2006) 1037–1042. [5] F. Rehouma, K.E. Aiadi, Glasses for ion-exchange technology, Int. J. Commun. 4 (2008) 148–155 (10). [6] S. Karlsson, Modification of float glass surfaces by ion exchange, Dissertations, No 89/2012 Linnaeus University, 2012. [7] W. Holand, M. Schweiger, V.M. Rheinberger, H. Kappert, Bioceramics and their application for dental restoration, Adv. Appl. Ceram. 108 (2009) 373–380. [8] W. Höland, E. Apel, C. van'tHoen, V. Rheinberger, Studies of crystal phase formations in high strength lithium disilicate glass–ceramics, J. Non-Cryst. Solids (2006) 4041–4050. [9] H. Fischer, et al., Chemical strengthening of dental lithium disilicate glass–ceramics materials, J. Biomed. Mater. Res. 87A (3) (2008) 582–587. [10] Chokchai Yatongchai, Strengthening of Li2O–SiO2 Transparent Glass–ceramics by Ion-exchange(Thesis) School of Ceramic Engineering, Suranaree University of Technology, 2005, ISBN 974-533-528-2. [11] D.K. Tagantsev, Decrystallization of glass–ceramics under ion exchange diffusion, J. Eur. Ceram. Soc. 19 (1999) 1555–1558. [12] G.H. Beall, B.R. Karstetter, H.L. Rittlert, Crystallization and chemical strengthening of stuffed β-Qaurtz Glass–Ceramics, J. Am. Ceram. Soc. 50 (4) (1967) 181–190. [13] H. Anmin, L. Ming, M. Dali, Preparation of whiskers β-spodumene glass–ceramics, J. Am. Ceram. Soc. 89 (1) (2006) 358–360. [14] W. Pannhorts, Overview in: Low Thermal Expansion Glass Ceramics, SpringerVerlag Berlin, New York 1995, pp. 1–12. [15] G. Muller, Structure, Composition, Stability, and Thermal Expansion of High-Quartz and Keatite-type alumino-silicates; Wolfgang Pannhorts: Glass–Ceramics Based on Lithium-Alumino-Silicates Solid Solution in Low Thermal Expansion Glass–Ceramics, 1995 13–24 (39–50).
K. Łączka et al. / Journal of Non-Crystalline Solids 428 (2015) 90–97 [16] A. Sakamoto, et al., Spodumene glass–ceramics with anomalous low thermal expansion, Adv. Mater. Res. 39–40 (2008) 381–386. [17] V.O. Soares, et al., Effect of ion exchange on the sinter-crystallization of low expansion Li2O–Al2O3–SiO2 glass–ceramics, Glass Technol. 52 (2) (2011) 1–5. [18] M. Laczka, K. Laczka, K. Cholewa-Kowalska, A.B. Kounga, Ch. Appert, Mechanical properties of a lithium disilicate strengthened lithium aluminosilicate glass–ceramic, J. Am. Ceram. Soc. 97 (2) (2013) 361–364. [19] Borczuch-Łączka M., Cholewa-Kowalska K., Łączka K. — Int.Cl.: C03C 3/083 {(2006.01)}. — World Intellectual Property Organization.; WO 2012/143137 A1. — Nor PCT/EP 2012/001709, 2012–04-20; published: 2012–10-26. — full text: WO2012143137A1.pdf. 2012. [20] K. Laczka, et al., Thermal and spectroscopic characterization of glasses and glass–ceramics of Li2O–Al2O3–SiO2 (LAS) system, J. Mol. Struct. 1068 (2014) 275–282. [21] J.C. Taylor, C.E. Matulis, A new method for Rietveld clay analysis, part 1. Use of a universal measured standard profile for rietveld quantification of montmorillonites, Powder Diffract. 9 (1994) 119–123. [22] C.G. Brandt, A.J. Kinneging, X-ray Powder Diffraction, A Practical Guide to Quantitative Phase Analysis, The PANalytical X-ray Company, 2007, ISBN 90-9017878-3.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Glass–ceramics of LAS (Li2O–Al2O3–SiO2) system enhanced by ion-exchange in KNO3 salt bath Karolina Łączka a, Katarzyna Cholewa-Kowalska a,⁎, Marcin Środa a, Jakub Rysz b, Mateusz M. Marzec c, Maria Łączka a a b c
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Glass Technology and Amorphous Coatings, al. Mickiewicza 30, 30-059 Kraków, Poland M. Smoluchowski Institute of Physics, Jagiellonian University, ul. Łojasiewicza 11, 30-348 Kraków, Poland AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, al. Mickiewicza 30, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 1 June 2015 Received in revised form 1 July 2015 Accepted 2 August 2015 Available online xxxx Keywords: LAS glass–ceramics; Ion-exchange; Flexural strength; Decrystallization
a b s t r a c t The paper describes a low-temperature ion-exchange process on specially designed Li2O–Al2O3–SiO2 glass–ceramics (LAS). Due to thermal treatment, two main crystalline phases: lithium di-silicate and lithium aluminasilicate were obtained in glass–ceramic materials. As a result of proper time/temperature profile treatment of precursor glasses, flexural strength of final materials increased up to 300–450 MPa. Crystallized samples were immersed in KNO3 molten salt, at temperatures of 400–420 °C for 9 or 16 h. The final materials achieved almost 100% greater mechanical resistance than glass–ceramics before ion-exchange. We claim that the process of ion-exchange improved flexural strength significantly. Furthermore, it resulted in some changes of the surface layer structure. We noticed that chemical tempering process leads to decrystallization of the multiphase LAS glass–ceramic surface layer consisting of lithium di-silicate and lithium alumina-silicate phases. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Glass is transparent, which is one of its biggest advantages. Due to its nature, unfortunately, it is a brittle material with not very high mechanical strength. For many years, scientists have attempted to find a way to improve its strength, maintaining the transparency at the same time. One of these methods turned out to be controlled crystallization. It uses glasses that are chemically designed in a proper way to create glass–ceramics as final materials. Glass–ceramics are polycrystalline solids obtained upon controlled crystallization from glasses. This procedure usually leads to mechanical reinforcement when compared to the initial glass strength. Because of crystals grown up in the glass–ceramics, some of the transparency is lost [1,2]. Nevertheless, thanks to a proper thermal treatment, it is possible to create small crystals (b50 nm) in final material, obtaining the required transparency [3]. Glass–ceramics obtained in this way are produced and used as cooker top panels, telescope mirrors, high temperature resistant windows, tile glaze, and so on [3]. Reinforced glass–ceramics of esthetic appearance are also successfully applied in dentistry as dental crowns [4]. Another successful way to improve mechanical properties of glass and glass–ceramic materials is low-temperature ion-exchange. This method induces compressive stress in the material surface layer by substituting small ions in the glass or glass–ceramics with larger ions ⁎ Corresponding author. E-mail address: [email protected] (K. Cholewa-Kowalska).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.08.003 0022-3093/© 2015 Elsevier B.V. All rights reserved.
diffusing from an appropriate molten salt [5,6]. This process results in an increase of mechanical resistance without losing material's transmittance. A special group of aesthetic materials is glass–ceramics based on Li2O–SiO2 system with lithium di-silicate (LS) as a main crystalline phase. They have been available for dental applications since 1998 (IPS Empress 2, Ivoclar-Vivadent, Schaan, Liechtenstein) [7,8]. Chemical compositions of basic glasses for LS glass–ceramic production are characterized by the ca. 15% content of Li2O and lack or a low content of Al2O3 (usually 1–3%). Compared to starting LS glass (flexural strength about 150 MPa), the crystallization of lithium di-silicate reinforces the final material up to about 400 MPa or even more. In addition, lithium di-silicate glass–ceramics may also be reinforced by low-temperature ion-exchange. Fisher H. et al. [9] studied the lithium di-silicate glass–ceramic material IPS Empress 2. Samples of glass–ceramics were ionexchanged in batch 1: KNO3 at 400 °C for 11 h; batch 2: KNO3 + LiNO3 at 350 °C for 11 h; and batch 3: KNO3 at 400 °C for 11 h, and also in NaNO3 + KNO3 at 400 °C for 1 h. With respect to a characteristic strength value, flexural strength of untreated material was 265.5 MPa, while after ion-exchange a 25% increase was observed for batch 1 and batch 3 (331.4 MPa and 320.6 MPa, respectively). Experimental results of the ion-exchange study conducted for LS glass–ceramics indicate the amorphization of material surface layer after a bath in molten salts: NaNO3 and KNO3 [10,11]. Tagantsev [11] observed a phenomenon, where crystalline phase of lithium metasilicate LiSiO3 disappeared slightly upon the Li+ ↔ Na+ exchange, and
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he was the first to call it using a term “decrystallization”. He discussed this process basing on thermodynamics and some proposed interpretation “in the terms of the phase equilibrium theory in the heterogeneous liquid solution. The vitreous portion of the glass–ceramics is considered as a solvent and the crystals as a precipitates, which are dissolving when altering the solvent composition through ion-exchange”. Chokchai Yatongchai [10] also examined the ion-exchange for LS transparent glass–ceramics using NaNO3 and KNO3 molten baths under various temperatures and durations of treatment to determine optimum process conditions. His results showed, that the maximum flexural strength of glass–ceramics of 487 ± 40 MPa could be obtained by the Li+ ↔ K+ exchange at 500 °C for 5 h. Destruction and disappearance of lithium di-silicate crystals caused by Li+ ↔ Na+ ion-exchange in NaNO3 molten salt baths were observed. The other important group of glass–ceramics having a high commercial value is Li2O–Al2O3–SiO2 (LAS) glass-crystalline materials based mainly on the crystallization of a solid-solution of β-spodumene (Li– Al-silicates) and silica SiO2, with rather no Li-di-silicate crystallization. A basis for the production of glass–ceramics of this type is low-lithium glasses (a few wt% of Li2O) with a larger amount of Al2O3 (Li2O/Al2O3 clearly less than 1). These LAS glass–ceramics exhibit rather low to moderate mechanical properties. Their flexural strength and fracture toughness are in the range of 100–250 MPa and 1–1.5 MPa ∗ m1/2, respectively [3,12,13]. However, these materials are characterized by their low thermal expansion, combined with refractoriness up to 600 °C and great thermal shock resistance [14–16]. Soares et al. [17] applied the ion-exchange treatment to the LAS glass–ceramic low thermal expansion to minimize surface crystallization and thus improve the sintering compacts made of glass particles during the forming process of glass–ceramic product. The subject of our previous study was also LAS glass–ceramics; however, that study focused on a new chemical composition of starting glasses where the ratio of Li2O to Al2O3 was approx. 1, and the content of these oxides was approx. 10 wt.% for each component. At this ratio (Li2O/Al2O3), simultaneous crystallization of lithium alumina-silicates and lithium di-silicate was successfully induced [18–20]. That resulted in strengthening the glass by creating lithium di-silicate crystallization in the final material. We received the final glass–ceramics with flexural strength about 400 MPa. What is more, this material had a high aesthetic appearance (white-yellowish color, translucency), and could be applied in dentistry. This paper is a continuation of our previous study on the new LAS glass–ceramics enhanced due to lithium di-silicate crystallization [18–20]. In this study, reinforcement upon the low-temperature ionexchange of our LAS glass–ceramics is presented. The paper describes to what extent the strength of this material can be enhanced. What is more, we studied the changes in surface structure upon low thermal ion-exchange process as those observed for LS materials [10,11].
mixed using a mechanical stirrer and then inserted into a 1 dm3 Pt/Rh crucible. The batches were melted in an electric furnace at the maximum temperature 1480 °C during 5 h. Melts were homogenized by stirring during melting at 1460 °C with the SiO2 rod and also by bubbling. Homogeneous melted glasses were cast on a steel plate and annealed at 500 °C. The specimens of parent glasses for further examinations were made in the form of bars: 4 × 1.5 × 17 mm and plates: 13 × 13 × 1 mm. The specimen surface was ground and polished. The samples were prepared by wet grinding (Struers Tegramin 30 polishing machine) with a sequence of 240, 400, 600, 800 mesh SiC abrasive papers, followed by final polishing with polishing cloth (Fedo 1; METKON) and CeO2 polishing powder (Bohle). For such prepared glass samples, thermal treatment leading to control crystallization was performed in the electric furnace as per the evaluated profiles. In order to determine the temperature/time profile of nucleation and crystallization of parent glasses, the differential thermal analysis (DSC) was used as the basic method (Fig. 1). The DSC measurements were performed with Thermal Analyzers NETSH using a Pt crucible of 90 mg capacity. Glass particles of sizes between 425 and 500 μm were prepared for these measurements. DSC was performed at a heating rate 10 K/min in a flowing atmosphere (50 cm3/min) of dry air. Base on our previous study [18,20] as well as the performed DTA analysis, parameters of thermal treatment for both parent glasses were evaluated. After thermal treatment types of crystalline phases as well as their volumetric content in materials were characterized by powder X-ray diffraction (PANalytical B.V. Spectris plc., The Netherlands) using CuKα radiation (Table 1). The diffractometer resolution was 0.026° 2Θ measured on the line d100 LaB6 (NIST SRM660a). XRD patterns were recorded at room temperature with a step size of 0.033° in a 15°–60° 2Θ on powdered glass–ceramic samples. The crystalline phase content was determined by the Rietveld method as suggested in works by Taylor and Matulis. The vitreous phase was determined also using the Rietveld method [21,22]. In order to recognize suspected decrystallization of lithium disilicate and lithium alumina-silicate upon ion-exchanged glass–ceramics, the plate samples of GAM7 and GBM10 glass–ceramics before and after ion-exchange were measured with X-ray diffraction (Fig. 6). Prepared glass–ceramic samples, were immersed in a molten salt KNO3 (p.a. quality, Merck, Germany) at the temperature range 400– 420 °C, upon particular durations: 9 or 16 h. After ion-exchange process samples were pulled out and washed in warm water. Both glass–ceramic samples and glass–ceramics after ion-exchange were tested for flexural strength by three-point bending in a universal
2. Experiment Studies were carried out for glasses from Li2O–Al2O3–SiO2 system with the following chemical compositions determined by chemical analysis (wt.%): GAM7: SiO2 66; Al2O3 11.0; Li2O 11, CaO 1.5; K2O 0.5, Na2O 3, P2O5 4.5, ZrO2 0.5, CeO2 1.5, V2O5 0.5; GBM10: SiO2 69; Al2O3 10.5; Li2O 10.5, CaO 1.5; K2O 0.50, Na2O 1.5, P2O5 4, ZrO2 0.5, CeO2 1.5, V2O5 0.5. The chemical compositions of glasses were determined using X-ray fluorescence spectroscopy and analytical chemical methods. The parent glasses were prepared according to the melt-quenching way. The following compounds were used as raw materials (p.a.; Sigma Aldrich Co., POCh): SiO2; P2O5; ZrO2; CeO2; V2O5; Al(OH)3; Li2CO3; CaCO3; K2CO3; and Na2CO3. About 1 kg of components was
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Fig. 1. DSC curves for not-pre-heated parent glasses GAM7 and GBM10.
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Table 1 Relations between heat treatment programs, crystallization/amorphous phase content and flexural strength of GAM7 and GBM10 glass–ceramics. Materials
Heating profile
GAM7 GAM7/GC GAM7/GC GBM10 GBM/10/GC GBM/10/GC
0 A B 0 A B
Heating profile (nucleation, crystallization) — temp.(°C)/time (min) 530/600; 660/30; 730/30 530/600; 660/30; 830/90 550/600; 710/30; 550/600; 710/30; 870/90
Quantitative phase analysis LS/LAS/Am.Ph [%]
Flexural strength [MPa] mean value with SD
Characteristic flexural strength [MPa] (δ0)
Weibull modulus (m)
Optical properties
0/0/100 18.1/23.7/47.5 17.6/31.9/38.8 0/0/100 20.9/39.4/35.7 21.4/50.2/20.2
209 ± 35 314 ± 50 402 ± 26 159 ± 31 376 ± 71 406 ± 40
220 327 433 171 400 462
6.92 13.2 8.65 5.02 6.12 7.52
Transparent Well translucent Less translucent Transparent Well transparent Less translucent
LS — lithium di-silicate; LAS — lithium alumina-silicates; Am.Ph — amorphous phase; nucleation temperature/time is marked by underline.
testing machine (ZwickRoell Z020, Ulm, Germany) (Tables 1, 2). Each measurement of bending strength was conducted for series of 10–30 samples. Results are expressed as a characteristic flexural strength value δ0 and a calculated Weibull parameter m. The characteristic strength value represents the value at which 63.21% of the test specimens have fractured. The Weibull modulus describes the relative spread of strength values in the asymmetrical distribution [23,24]. Dynamic secondary ion mass spectrometry (SIMS) technique was used to investigate the glass composition in the vicinity of the surface. It allowed defining the approximate profiles of the exchanged alkaline ions Na+, K+, and Li+ in the surface layer. TOF-SIMS 5 (ION-TOF GmbH, Germany) system working in a non-interlaced dual beam mode was used to obtain composition versus depth profiles. Samples were sputtered with an O2 ion beam (2 keV, 600 nA) rastered over 350 × 350 μm, and analyzed with a high energy (30 keV) pulsed bismuth beam rastered over 100 ×100 μm. Masses of secondary ions induced by a Bi+ beam were analyzed in a time-of-flight mass spectrometer. The DektakXT stylus profilometer (Bruker) was used to measure the depth of sputtered craters (Figs. 2, 3). Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES OPTIMA 7300DV Perkin Elmer) was performed additionally in order to identify the Li, Na and K elements content in the representative KNO3 salt solution (saturated in 20 °C) used for ion-exchange process (15 series performed in 240 cm3 of salt; examined samples: 10 bars and 1 plate for 1 series, ca. 130 cm2). For the sake of comparison, the untreated KNO3 salt solution underwent this examination as well (Table 3). The structure of material surface layer before and after ion-exchange was characterized by Raman spectroscopy (Confocal Raman microscope LabRAM HR UV–Visible-NIR 200–1600 nm, Laser Ar+ 532 nm.). Moreover, the depth profile of Raman spectra (ca. 10 μm) was determined for glass–ceramics after ion-exchange (Figs. 4, 5). Raman spectroscopy is a very sensitive method to determine structural disorders, which is why this method, apart from XRD, was chosen to determine a possible amorphization of surface layer after ion-exchange. 3. Results Chemical compositions of parent glasses slightly differ from each other in the content of silica SiO2 (66 wt % for GAM7, and 69 wt.% for GBM10) and sodium oxide Na2O (3% for GAM7, and 1% for GBM10). The content of other components was similar while maintaining the characteristic for the new LAS glass–ceramic Al2O3/Li2O ratio about 1.
The DSC plots of parent glasses GAM7 and GBM10 (Fig. 1) allowed setting their nucleation and crystallization parameters. The glass transition temperature for GAM7 was Tg = 484 °C, so the nucleation temperature was established at Tn = 530 °C. For GBM10, Tg was 503 °C, while nucleation was set at Tn = 550 °C. The DSC plots of parent glasses GAM7 and GBM10 differ from each other due to the number of crystallization effects as well as their temperature ranges. For GAM7 glass, two strong exothermic effects in the range 600–750 °C are observed, whereas for GBM10 glass only one effect occurs in this range. For both glasses very weak effects in the ranges of higher temperatures (above 800 °C) were observed. We established detailed crystallization programs of GAM7 and GBM10 glasses on the basis of DSC results, also taking into account our previous study [18,20], as follows: - A. heating in the range of main crystallization peaks (625–730 °C), - B. heating in the range of main crystallization peaks (625–730 °C), and additional heating in the temperatures 830–870 °C.
Table 1 shows representative programs of non-isothermal heat treatments (heating rate: 10 °C/min), resulting in the best mechanical properties of glass–ceramics. After crystallization a quantitative XRD phase analysis of glass–ceramics was made. The results related to the main crystalline phases (lithium di-silicate; lithium alumina-silicates) as well as amorphous phase are given in Table 1. Crystallization in the range of main DSC peaks (temperature 660–730 °C) leads to obtaining translucent glass– ceramics which were composed mainly of lithium di-silicate and lithium alumina-silicates crystals; the ratio of these phase was about 1:1.3 for glass–ceramics based on GAM7 glass (GAM7/GC), and close to 1:2 for glass–ceramics based on GBM10 glass (GBM10/GC) (Table 1). Crystallization resulting from a thermal treatment (Table 1) according to the A program resulted in strengthening of both GAM7 and GBM10 glasses up to the characteristic flexural strength δ0 about 327 MPa (GAM7GC) and 400 MPa (GBM10/GC); (about 50% reinforcement for GAM7/GC, and more than 100% reinforcement for GBM10/GC in relation to characteristic flexural strength of basic glasses GAM7 and GBM10: 220 MPa and 171 MPa, respectively). As a result of the thermal treatment according to the B program, higher crystallization degree was reached with a significant increase of lithium aluminasilicates in the samples. This resulted in the increase of characteristic flexural strength up to about 430–460 MPa.
Table 2 The effect of ion exchange IE conditions of GAM7 and GBM10 glass–ceramics on material flexural strength. Materials
Heating profile [temp (°C)/time (min)]
IE conditions [temp (°C)/time (h)]
Flexural strength before/after IE [MPa]; mean value with SD
Characteristic flexural strength before/after IE [MPa] (δ0)
Weibull modulus (m) before/after IE
GAM7 GAM7 GBM10 GBM10
530/600; 660/30; 730/30 530/600; 660/30; 830/90 550/600; 710/30; 550/600; 710/30; 870/90
400/9 400/16 410/16 420/16
314 ± 50/725 ± 80 402 ± 26/595 ± 52 376 ± 71/584 ± 71 406 ± 40/837 ± 81
327/755 433/627 400/640 462/870
13.2/10.32 8.65/12.16 6.12/9.89 7.52/12.58
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Fig. 2. Secondary ion intensity profiles of the exchanged ions in the surface layer of a specimen determined by secondary ion mass spectrometry (SIMS) for GAM7 glass–ceramics (heat-treatment: 530 °C/600 min; 660 °C/30 min; 730 °C/30 min a) before and b) after 400 °C/9 h ion-exchange).
Ion-exchange in a molten potassium salt (KNO3) was carried out for both well translucent glass–ceramics (A program) and the less translucent, more crystallized materials (B program). The best results of material strengthening by ion-exchange are presented in Table 2. The conditions were as follows: - temperature 400 °C and holding time 9 h or 16 h for GAM7/GC; - temperature 410–420 °C and holding time 16 h for GBM10/GC.
After such performed ion-exchange, spectacular mechanical strengthening of translucent GAM7/GC was observed (the characteristic flexural strength value δ0 above 700 MPa) (Table 2). As for the translucent GBM10/GC, the effect was weaker (δ0 about 600 MPa), but mechanical strengthening was still observed. In both cases, the optical properties of material did not change after ion-exchange, and glass–ceramic was still highly translucent (Photo 1). The ion-exchange process conducted for glass–ceramics after a multi-step crystallization (660\\730 °C; 830\\870 °C) also lead to the visible strengthening of both GAM7/GC and GBM10/GC materials. In the case of GAM7/GC, the best results of characteristic flexural strength were achieved at 627 MPa, while for GBM10/GC this effect was stronger (870 MPa) (Table 2). Higher Weibull modulus values m were obtained for the
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Fig. 3. Secondary ion intensity profiles of the exchanged ions in the surface layer of a specimen determined by secondary ion mass spectrometry (SIMS) for GAM7 glass–ceramics (heat treatment: 530 °C/600 min; 660 °C/30 min; 830 °C/90 min a) before and b) after 400 °C/16 h ion-exchange).
samples after ion-exchange, which indicates that after ion-exchange the samples were more uniform and more reliable than the non treated ones. To evaluate changes of the alkali ions concentration in the surface layer after ion-exchange, the profiles of the exchanged ions in the surface layers were approximately determined by SIMS mapping. The ion-exchange treatment resulted in change of the K+, Na+ and Li+ ion concentrations in the specimen surface layer. The penetration depth of incorporated additional potassium ions was about 15 μm for both glass–ceramics GAM7/GC: well translucent (after heat-treatment: 530 °C/600 min; 660 °C/30 min; 730 °C/90 min and ion-exchange 400 °C/9 h) and less translucent GAM7/GC (heat treatment 530 °C/ 600 min; 660 °C/30 min; 830 °C/90 min and ion-exchange 400 °C/16 h) (Figs. 2(b) and 3(b)). The analysis of sodium ions showed a decrease of their presence in the material up to 12–15 μm deep. The SIMS analysis Table 3 Elements K, Na, Li concentration in salt solution KNO3 (saturated 20 °C) before and after ion exchange. Elements concentration [mg/dm3]
K
Na
Li
Salt KNO3 Salt KNO3 after IE
57,802.2 ± 2104.5 10,236.9 ± 157.8
1.9 ± 0.1 39.3 ± 3.1
b0.01 0.88 ± 0.06
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Fig. 4. Raman spectra of surface layer of GBM10 glass–ceramics (heat-treatment 550 °C/ 600 min; 710 °C/30 min; 870 °C/90 min) before and after ion exchange in molten KNO3:420 °C/16 h.
revealed a slight decrease of lithium in the surface layer after the ionexchange of specimens of both glass–ceramics GAM7/GC after different heat treatments. A complementary method of KNO3 salt analysis before and after fifteen cycles of the ion-exchange of glass–ceramics was performed (Table 3). The obtained results confirmed that lithium ions also participate in ion-exchange because their amount clearly increased in the molten salt.
In order to determine possible structural changes occurring in the surface layer of glass–ceramics after ion-exchange, two methods: XRD and Raman spectroscopies, were applied. The results of these examinations are presented in Figs. 4–6. The XRD analysis proves that for both GAM7/GC and GBM10/GC materials the intensity of peaks derived from crystalline phases: lithium disilicate (at 2θ = 23.84°; 24.42°; 24.85°) and lithium alumina-silicates (2θ = 22.26°; 25.9°) clearly decreases after the ion-exchange. We present the results obtained for both GAM7/GC and GBM10/GC materials (Fig. 6). The decrease in intensity of characteristic peaks resulting from both phases crystallized in glass–ceramics indicates the amorphization of surface layer of these materials as caused by the ionexchange. A structural study using Raman spectroscopy of the glass–ceramic surface layer before and after ion-exchange showed significant changes similar for both GAM7/GC and GBM10/GC materials. We present results for GBM10/GC (Fig. 4). These changes are related to the bands located at [25–27]: - 470 cm−1 derived from Si–O–Si polymerized groups (Q4 units); - 490 cm− 1 derived from the four-fold silicate D1 rings normally observed in vitreous silica; - 950 cm−1 derived probably from SiO− groups (Q2 units).
Ion-exchange caused a decrease in the intensity of the band 467–465 cm−1 obtained from the bending vibration of Si–O–Si groups in Q4 units [23–25] indicating de-polymerization of the network. On the other hand, an increase in the intensity of the band 491–497 cm−1
Fig. 5. Raman spectra depth profile of surface layer of GBM10 glass–ceramics (heat-treatment 550 °C/600 min; 710 °C/30 min; 870 °C/90 min) after ion exchange in molten KNO3:420 °C/ 16 h.
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the surface layer. That indicates its amorphization by ion-exchange. The spectroscopy results are consistent with those resulting from XRD. 4. Discussion
Fig. 6. The XRD patterns of a) GAM7 and b) GBM10 glass–ceramics before (black) and after (red) ion exchange. The XRD patterns show decrease in the peaks intensity of lithium disilicate (Li2Si2O5, ICDD, 4-9-8780) and lithium alumina-silicate (LiAlSi2O6, ICDD, 37-794) crystalline phases. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
originating from 4-membered rings of SiO4 tetrahedra was observed, which also indicates an increase of defects in the silicate network after ion-exchange. Moreover, after ion-exchange, the increase of the band at 948 cm−1 derived from de-polymerized Q2 units in the silicate network (two bridging oxygens) occurred [26,27]. For disordered amorphous phase, these results show an increase of characteristic groups in
Photo 1. Macroscopic observation of GAM7 and GBM10 base glasses and glass ceramics after thermal treatment at the temperatures listed in Table 1.
Commercial LAS glass–ceramic is based on crystalline phases with highly anisotropic thermal expansion composition (TEC), such as a solid solution of β-quartz type, where crystallization of lithium alumina-silicates usually occurs (β-spodumene; β-eucryptite and others) [21–23]. The slightly positive TEC of the residual glassy phase combined with the negative TEC of the crystalline phase results in ultra-low thermal expansion of the glass–ceramics. However, these glass–ceramics show moderate or low mechanical properties. The newly developed LAS glass–ceramics represented by GAM7/GC and GBM10/GC materials have a different chemical composition in comparison to commercials. These composition changes allow simultaneous crystallization of two phases: lithium di-silicate and lithium aluminasilicates to grow. Thanks to this, lithium di-silicate reinforces LAS glass–ceramics, which in turn results in an increase of flexural strength up to about 400 MPa [18–20]. For the examined GAM7 and GBM10 parent glasses, chemical composition differ from each other in relation to both the content of silica and alkali oxides. A lower silica concentration and a higher alkali oxides content in the GAM7 glass affect the transformation temperature, 484 °C, which is lower than for the GBM10 glass — 503 °C. This also results in a lower nucleation temperature of the GAM7 glass as compared to the GBM10 glass (530 °C/550 °C, respectively). Controlled crystallization of GAM7 glass at 660–730 °C led to the glass–ceramics with about 50% crystallization, and similar content of both lithium di-silicate and lithium alumina-silicates phases (with a slightly higher content of the latter), where translucency was well manifested. Translucent glass–ceramics were obtained also on the base of GBM10 glass, as a result of thermal treatment at 710 °C. Unlike the GAM7 glass–ceramics, the GBM10 was characterized by a higher crystallization degree (about 65%) as well as a higher content of lithium alumina-silicates (about 40%). Strengthening translucent glass–ceramics is obvious, compared to GAM7 and GBM10 basic glasses. Applying a high-temperature thermal treatment in the range of 830–870 °C resulted in a higher crystallization degree of material as well as a decrease of its translucency. At the same time, flexural strength of both GAM7/GC and GBM10/GC materials increased even above 400 MPa, which is an unusual value as compared to classical LAS glass–ceramics with only lithium alumina-silicates crystallization. This confirmed our previously mentioned thesis that additional crystallization of lithium di-silicate evidently strengthens this type of materials [18–20]. The aim of our study was to strengthen already obtained LAS glass– ceramics further by an ion-exchange in the bath salt of KNO3. This process was performed for both translucent and opaque glass-ceramics. Due to the presence of crystalline phase/phases near amorphous, the ion-exchange process for glass–ceramics is more complicated than the process occurring in glass. We believe that during the ion-exchange process performed in glass–ceramics, the movement of alkali ions from residual glassy phase should be easier than the one from ordered crystalline phases. Reinforcement of LAS glass–ceramics by low temperature ionexchange can occur only by replacement of smaller ions with larger ones. Here, it may appear as an exchange of Na+, and Li+ ions into K+ ions. The concentration of Na2O in basic glasses is small (1.5–3.0 wt.%), in comparison to the Li 2 O content, (about 10 wt.%), however, it all remains in residual glassy phase of glass–ceramics (sodium does not participate in crystallization unlike to lithium). Furthermore, alkali content in the surface layer is usually higher than the one in the glass interior. Probably this also refers to sodium Na and lithium Li. We can draw conclusions about a mechanism of ion-exchange in the examined glass–ceramics only on the basis of alkali ions profile near the surface (SIMS examinations). The results of this study show an evident loss of
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sodium in the glass–ceramic surface layer up to approx. 15 μm depth after the ion-exchange (Figs. 2–3). Thus, we suppose that an exchange of Na+ ions with K+ ions is mainly responsible for material strengthening. We believe that also an exchange of Li+ ions with K+ ions affects the strengthening process since the Li loss from material was shown. Nevertheless, the lithium loss was smaller, and on a smaller depth than the one relating to sodium; therefore, we claim that Na was primarily responsible for material reinforcement. This suggests a possibility of another mechanism of ion-exchange in LAS glass–ceramics containing sodium oxide addition, compared with Fisher H. results [9] for sodium-free LS glass–ceramics. There, the exchange Li+ ↔ K+ only is responsible for material reinforcement. Hence, our studies confirmed that an evident increase of flexural strength observed in developed by us LAS glass–ceramics is possible by the ion-exchange in KNO3 molten salt. In addition, reinforced glass–ceramics by ion-exchange showed no further loss of its translucency. At the same time, the calculated Weibull modulus m shows that after the ion-exchange, an increase of material reliability was observed. As already mentioned, ion-exchange in glass–ceramics (GC) should mainly be applied between molten salt and residual glassy phase present in GC. However, for examined LAS GC, a clear relation between the content of residual glassy phase and strengthening of material by ion-exchange was not observed. A very high reinforcement, up to about 700 MPa, for high translucent GAM7/GC material containing about 50% of glassy phase occurred, although a very high flexural strength above 800 MPa was also observed for GAM10/GC containing 20% amorphous phase only. This confirms that phenomena occurring in glass–ceramics during ion-exchange are more complex, and more factors had an influence on the material properties, as compared with glasses. The XRD and the Raman spectroscopy examinations gave us some information about the structural changes occurring in the glass–ceramic surface layer after the ion-exchange. The Raman spectra showed that a replacement of smaller alkaline ions with the larger ones led to de-polymerization, and also increased the defects in the surface layer. Probably more de-polymerized Q2 units of silicon were created as well as four-member rings composed of [SiO4] tetrahedral form. This could indicate a destruction of the crystalline phases occurring in the examined materials confirming the results of Ch. Yatongchai for LS glass–ceramics [10]. Similarly, our results are consistent with a study by Soares W. O. et al., conducted on commercial LAS glass–ceramics, which showed a decrease of the surface crystallization as a result of the ionexchange, improving the sintering conditions of LAS glass–ceramics [17]. The Raman spectroscopy results are consistent with the XRD study. The changes in XRD patterns of examined LAS glass–ceramics before and after ion-exchange are seen as diminishing of the crystalline phases effects, which indicates the amorphization of the surface layer. That phenomenon is in agreement with Tagantsev [11] and Ch. Yatongchai's [10] studies for lithium di-silicate glass–ceramics. We have shown, for the first time, that decrystallization also occurs in LAS glass–ceramics where lithium di-silicate and lithium alumina-silicates crystallize simultaneously, and this refers to both phases. 5. Conclusions • The LAS glass–ceramics with simultaneous crystallization of lithium di-silicate and lithium alumina-silicates phases can be produced base on the Li2O–Al2O3–SiO2 system. The glass compositions (GAM7 and GBM10) are characterized by the content of about 10 wt.% each of Al2O3 and Li2O. They differ from each other in the silica content as compared to the content of alkali. • Crystallization of lithium di-silicate in combination with the lithium alumina-silicates causes the strengthening of materials up to 400 MPa of flexural strength. Translucency of the glass–ceramics can
•
•
•
•
be controlled by the thermal treatment conditions. Both of these properties could be moderated depending on needs. Additional enhancement of glass–ceramics can be achieved by ionexchange of smaller ions (Na, Li) with larger ones (K) using batch salts of KNO3. Strengthening of materials depends on temperature/ time conditions of ion-exchange as well as on the initial chemical composition of starting glasses (the content of silica and alkali). The maximum strengthening (flexural strength about 700–800 MPa) was achieved for highly translucent glass–ceramics obtained by thermal treatment of GAM7 glass and less translucent glass–ceramics produced on the base of GBM10 glass. Ion-exchange induces changes in the surface layer at least to the depth of about 15–20 μm, and involves a formation of defects, which probably leads to a destruction of crystalline phases. The X-ray patterns indicate a decrease in the content of crystalline phases (lithium di-silicate and lithium alumina-silicate) in the surface layers of GAM7 and GBM10 glass–ceramics after the ion-exchange. The way of reinforcement of glass–ceramic that is shown here can be used to produce glass–ceramic materials for various applications, e.g. dentistry, cooker top panels, telescope mirrors, high temperature windows, protective cover for portable electronic devices (smartphones, tablets, etc.) and so on.
Acknowledgments This study was supported financially by the National Science Centre Grant No. 2012/07/N/ST8/02174 and by the Polish Ministry for Science and Higher Education, AGH University of Science and Technology, Faculty of Material Science and Ceramics Grant No. 11.11.160.365. The authors would like also to thank A.B. Kounga PhD and Institute Straumann AG, Basel, Switzerland for scientific and finacial support. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). References [1] Glass–Ceramics, P.W. McMillan, Academic Press, New York, 1979. [2] Glass–Ceramic Technology, W. Höland& G. Beall, The American Ceramic Society, Westerville, OH, 2002. [3] E.D. Zanotto, A bright future for glass–ceramics, Am. Ceram. Soc. Bull. 89 (8) (2010) 19–27. [4] W. Höland, V. Rheinberger, E. Apel, C. van 't Hoen, M. Höland, A. Dommann, M. Obrecht, C. Mauth, U. Graf-Hausner, Clinical applications of glass–ceramics in dentistry, J. Mater. 17 (11) (2006) 1037–1042. [5] F. Rehouma, K.E. Aiadi, Glasses for ion-exchange technology, Int. J. Commun. 4 (2008) 148–155 (10). [6] S. Karlsson, Modification of float glass surfaces by ion exchange, Dissertations, No 89/2012 Linnaeus University, 2012. [7] W. Holand, M. Schweiger, V.M. Rheinberger, H. Kappert, Bioceramics and their application for dental restoration, Adv. Appl. Ceram. 108 (2009) 373–380. [8] W. Höland, E. Apel, C. van'tHoen, V. Rheinberger, Studies of crystal phase formations in high strength lithium disilicate glass–ceramics, J. Non-Cryst. Solids (2006) 4041–4050. [9] H. Fischer, et al., Chemical strengthening of dental lithium disilicate glass–ceramics materials, J. Biomed. Mater. Res. 87A (3) (2008) 582–587. [10] Chokchai Yatongchai, Strengthening of Li2O–SiO2 Transparent Glass–ceramics by Ion-exchange(Thesis) School of Ceramic Engineering, Suranaree University of Technology, 2005, ISBN 974-533-528-2. [11] D.K. Tagantsev, Decrystallization of glass–ceramics under ion exchange diffusion, J. Eur. Ceram. Soc. 19 (1999) 1555–1558. [12] G.H. Beall, B.R. Karstetter, H.L. Rittlert, Crystallization and chemical strengthening of stuffed β-Qaurtz Glass–Ceramics, J. Am. Ceram. Soc. 50 (4) (1967) 181–190. [13] H. Anmin, L. Ming, M. Dali, Preparation of whiskers β-spodumene glass–ceramics, J. Am. Ceram. Soc. 89 (1) (2006) 358–360. [14] W. Pannhorts, Overview in: Low Thermal Expansion Glass Ceramics, SpringerVerlag Berlin, New York 1995, pp. 1–12. [15] G. Muller, Structure, Composition, Stability, and Thermal Expansion of High-Quartz and Keatite-type alumino-silicates; Wolfgang Pannhorts: Glass–Ceramics Based on Lithium-Alumino-Silicates Solid Solution in Low Thermal Expansion Glass–Ceramics, 1995 13–24 (39–50).
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