Effects of nd2o3 on the crystallization and properties of glass ceramic in li2o–k2o–al2o3–sio2– p2o5 system

Journal of Science and Technology 55 (1B) (2017) 238–248 EFFECTS OF Nd2O3 ON THE CRYSTALLIZATION AND PROPERTIES OF GLASS CERAMIC IN Li2O–K2O–Al2O3–SiO2– P2O5 SYSTEM Minh N. H. *, Hung H. T. D., Khoi T. N., Minh Q. D. Faculty of Materials Technology, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam *Email: hnminh@hcmut.edu.vn Received: 30 December 2016; Accepted for publication: 9 March 2017 ABSTRACT Glass ceramics (GCs), which often contain a small amount of rare earth oxides to improve their performance, are ideal for dental restorative applications. The aim of this study was to investigate the various effects of Nd2O3 content (0–1 wt%) on crystallization and properties of GC derived from Li2O–K2O–Al2O3–SiO2–P2O5 system. The glass blocks were formed from the molten at 1450 °C. Based on the DTA results, the glass samples were experienced by two–stage heat–treatment (600 °C/ 90 min + 720 °C/ 30 min) to change to ingots. After that, the ingot samples were fired in a hot pressing furnace EP3000 at 930 °C for 30 min. The results of powder X–ray diffraction (XRD) indicated that the final GCs contained crystals such as lithium disilicate (Li2Si2O5 or LS2), lithium metasilicate (Li2SiO3 or LS) and the traces of lithium phosphate (Li3PO4). With increasing Nd2O3 content, the relative amount of LS phase increased slightly while LS2 phase decreased. However, the final GC containing 0.75 wt% Nd2O3 had the highest bending strength at 293 MPa, the lowest chemical solubility and relative high Vicker hardness. These samples had a high degree of crystallization and the highest relative content of desired LS2 phase. Keywords: glass ceramic, dental ceramic, lithium disilicate, rare earth oxide, neodymium oxide Nd2O3. 1. INTRODUCTION Glass ceramics (GCs) containing lithium disilicate (Li2Si2O5 or LS2) as main crystal phase are ideally suitable for multi options of dental restorative applications: esthetic, layer technique, hot pressing or CAD/CAM all–ceramic restoration. Besides Li2O, SiO2, these materials also contain Al2O3 and K2O to enhance the chemical durability [1, 2], P2O5 as a heterogeneous nucleating agent that promotes volume nucleation of the LS2 phases [3–6] and small amounts of rare earth oxides playing a role as colorants and fluorescent agents [7]. The rare earth elements (La, Ce, Pr, Nd) now are widely used in GCs because there are a large number of fluorescing states and wavelengths to choose among the 4f electron Minh N. H., Hung H. T. D., Khoi T. N., Minh Q. D. 239 configurations. The changes in density, molar volume, hardness, thermal expansion (CTE) and glass transition temperature (Tg) of the glass system Ln–Si–Al–O–N (Ln = Ce, Nd, Sm, Eu, Dy, Ho and Er) were presented by Ramesh et al. [8], the changes of these properties were found to vary linearly with the cationic fields strength or ionic radius of the rare earth modifier. Wang et al. [9] studied the effects of ZrO2, La2O3, CeO2, Yb2O3 and V2O5 on the crystallization kinetics, microstructure and mechanical properties of mica GC. The arrangement in order of subsequence of improving crystallization was ZrO2 > V2O5 > Yb2O3 > CeO2 > La2O3. Bighetti et al. [10] investigated the viability of a silica glass containing rare earth oxides as La2O3, Y2O3, CeO2 (total was 42 wt%), these oxides were used as infiltration agents in different ceramic substrates. The results demonstrated that calculated compressive residual stress (based on CTE of the substrate and glass) enhanced toughness of the glass–infiltrated composites and the application of this glass composition was feasible. Holand and Beall [1] commented that the effects of dopant d or f ions on the color of transparent GC depend on the degree of their partitioning into the crystals as opposed to remaining in the residual glassy phase. In fact, there have been some crystallization studies on LS2 GC that contain a certain amount of rare earth oxides in the compositions [11–13]. Nevertheless, the influences of those oxides’ content were not found in these papers. In this study, the effects on crystallization and the properties of GC derived from the base glass of Li2O–K2O–Al2O3–SiO2–P2O5 system with different Nd2O3 content (0–1 wt%) were investigated. 2. MATERIALS AND METHODS 2.1. Melting base glass Glass batches were prepared by mixing powder of SiO2 (Precipitated Silica, Toxolux- Korea); Li2CO3, Al(OH)3, KH2PO4, K2CO3 (Guangdong Guanghua Chemical Factory Co., Ltd. - China) and Nd2O3 (Institute for Technology of Radioactive and Rare Elements (ITRRE) - Vietnam Atomic Energy Institute (VINATOM)). The chemical composition of the base glass was (wt%): Li2O 17.04, K2O 3.12, Al2O3 3.41, SiO2 74.05, P2O5 2.39. Nd2O3 was added in base glass with the content of 0, 0.25, 0.50, 0.75 and 1.00 wt%, The samples were denoted as N–0, N–0.25, N–0.50, N–0.75 and N–1.00. The glass batches were melted at 1450 °C for 90 minutes in platinum crucibles and casted into preheated stainless steel molds to form transparent glass blocks. The glass blocks were annealed inside a furnace at 450 °C for an hour, then cooled slowly to room temperature to decrease the internal stress in glass. 2.2. Heat treatment The base glass blocks were milled and sieved through a mesh 325 (45 µm). The fine glass powders were heated from 30 °C to 1050 °C with a heating rate of 10 °C/min in differential thermal analyzer DTA (Perkin Elmer, DTA7, Massachusetts, USA, at Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Thailand). α–Al2O3 powder was used as the reference material. From the results of DTA, the two–stage heat treatment process was conducted at 600 °C for 90 minutes and 720 °C for 30 minutes to crystal nucleation and crystal growth of the glass Effects of Nd2O3 on the crystallization and properties of glass ceramic in 240 blocks to form GC ingots. Subsequently, these ingots were sintered at 930 °C or hot pressed at 965 °C using EP3000 pressing furnace to produce final GCs. 2.3. Characterization techniques The phases of glasses and GCs were characterized by powder X–ray diffraction XRD (Bruker D8-Advance, at Institute of Applied Materials Science – Vietnam Academy of Science and Technology (VAST)) with CuKα radiation and 2θ scanning from 10° to 70° at step size 0.01°, mean time per step was 16.38 s. The integrated intensity (I) of the diffraction lines from any phase in a mixture is proportional to the mass of phase present in the sample [14]. Therefore, the semi quantitative analysis of crystallinity and the crystalline phases can be calculated and assessed by the number, position and intensity of peaks by using a powdery X–ray diffraction pattern general analysis software X’Pert HighScore Plus. Degree of crystallinity was estimated by the ratio [15]: % ࡯࢙࢚࢘࢟ࢇ࢒࢒࢏࢔࢏࢚࢟ ൌ ࡿ࢛࢓ ࢕ࢌ ࢔ࢋ࢚ ࢇ࢘ࢋࢇ ሺࡿ࢛࢓ ࢕ࢌ ࢔ࢋ࢚ ࢇ࢘ࢋࢇሻାሺࡿ࢛࢓ ࢕ࢌ ࢈ࢇࢉ࢑ࢍ࢘࢕࢛࢔ࢊ ࢇ࢘ࢋࢇሻ ൈ ૚૙૙% (1) Crystallite sizes (nm) of LS2 phase can be calculated via Scherrer’s equation, using the maximum peak at hkl [111] (2θ ≈ 24.8°): ࡯࢙࢚࢘࢟ࢇ࢒࢒࢏࢚ࢋ ࢙࢏ࢠࢋ ൌ ࡷࣅሺࡲࢃࡴࡹሻ ܋ܗܛࣂ (2) where K is the Scherrer constant and is 0.9 [16]; λ is the X–ray diffraction wavelength and is 0.154 nm; FWHM is the full width at half its maximum intensity (an angular width, in terms of 2θ); θ is the Bragg angle (in radians). Each group consisted of ten final GC rods was ground and polished by SiC papers (240, 600, 1200 and 2500–grit) to create smooth and parallel faces, then cleaned in ultra–sonic bath for 5 minutes and dried before properties testing. The dimensions of the bar–shaped samples after polishing were about 40 x 8 x 4 (mm). Three–point bending strength was tested by Testometric M350–10CT equipment (England) at Faculty of Materials Technology (HCMUT–VNUHCM), referred to the ISO 6872–2008 [17]. Vicker hardness was tested by Highwood–HWMMT–X3 equipment (Japan) at Materials Technology Laboratory (HCMUT–VNUHCM) with the 4 x 8 x 15 (mm) bars, referred to ASTM C1327–99 [18]. The chemical solubility test method was referred to the ISO 6872–2008 (the mass loss in µg/cm2 after immersed in 4 vol% acetic acid solution at 80 ± 3 °C for 16 h). Color measurements were made by CR–300 Chroma Meter (Konica Minolta – Japan) at Faculty of Chemical Engineering (HCMUT–VNUHCM). The colorimetric effects of Nd2O3 additions in the range used on the CIE L*a*b* color parameters of GC. The CIE L*a*b* color difference, ΔE*, was calculated [19] between the blank sample N–0 and the Nd2O3–containing samples. ∆ࡱ∗ ൌ ሾሺ∆ࡸ∗ሻ૛ ൅ ሺ∆ࢇ∗ሻ૛ ൅ ሺ∆࢈∗ሻ૛ሿ૚/૛ (3) where: ΔE*: the CIE unit of color difference, ܮ௖ ∗ , ܽ௖∗, ܾ௖ ∗ : the CIE color parameters of the Nd2O3–containing specimens. ∆ࡸ∗ ൌ ࡸࢉ ∗ െ ࡸ࢔∗ (4) ∆ࢇ∗ ൌ ࢇࢉ ∗ െ ࢇ࢔∗ (5) Minh N. H., Hung H. T. D., Khoi T. N., Minh Q. D. 241 ∆࢈∗ ൌ ࢈ࢉ ∗ െ ࢈࢔∗ (6) ܮ௡ ∗ , ܽ௡ ∗ , ܾ௡ ∗ : the CIE color parameters of the samples without Nd2O3 (N–0). The microstructure of GCs was analyzed by scanning electron microscope (SEM– Philips XL30, at Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Thailand). Fracture surfaces of samples were not etched, polished surfaces were etched in 10 vol% HF solution for 10 seconds and then Au sputtered. 3. RESULTS AND DISCUSSION 3.1. The results of DTA The DTA curves of the glasses are demonstrated in Figure 1. The glass transition temperature is denoted by Tg, at which the sample changes from solid to liquid behaviour. The clear exothermic peak Tc is determined for crystallization and the endothermic peak Tm showed the melting point. Regarding to the use of Nd2O3 in raw mixture, they show small variation of characteristic temperatures of glass powders N–0, N–0.25, N–0.50, N–0.75, N–1.00 (Table 1). Figure 1. DTA analysis of basic glasses. Table 1. Characteristic temperature of glass powders N–0, N–0.25, N–0.50, N–0.75, N–1.00 analyzed by DTA. Sample Tg (°C) Tc (°C) Tm (°C) N–0 482 664 982 N–0.25 487 653 974 N–0.50 474 657 980 N–0.75 480 659 980 N–1.00 462 657 981 3.2. The results of XRD analysis The XRD patterns of glass group N–0 are represented in Figure 2. The degree of crystallinity, calculated by equation (1), and characteristic peaks of N–0 in different states are shown in Table 2. It is indicated that, the LS and LS2 crystals were crystallized from the samples Effects of Nd2O3 on the crystallization and properties of glass ceramic in 242 undergoing the first preheat treatment at 600 °C. After the second stage and sintering step, there had been the rise in the intensity of LS2 peaks as opposed to the decline of LS peaks, this demonstrates that the LS crystals continuously react with silica through a solid–state reaction to form LS2 crystals. Li3PO4 crystals can be detected after being fired in hot pressing furnace at 930 °C. Figure 2. XRD patterns of N–0 in different states. Table 2. Degree of crystallinity and characteristic peaks of N–0 in different states. State of N–0 Degree of crystallinity (%) Intensities of main XRD peaks (cts) LS2 LS Li3PO4 Glass – – 600 24 1670 820 – GC ingot 28 1963 902 144 Final GC 37 4351 333 137 Consequently, the crystallization process of this GC system can be inferred, as follow: )(3222 crystalSiOLiSiOOLi ationcrystalliz ⎯⎯⎯ →⎯+ (7) )()( 522232 crystalOSiLiSiOcrystalSiOLi ⎯→+ (8) )(23 43252 crystalPOLiOLiOP ationcrystalliz ⎯⎯⎯⎯ →⎯+ (9) Figure 3 is the results of a semi–quantitative analysis by the intensity values of main peaks of XRD of the LS2 (2θ ≈ 24.8°, hkl = [111]), LS (2θ ≈ 27°, hkl = [111]) and Li3PO4 (2θ ≈ 22.3°, hkl = [110]), it was noticed that the change of the crystalline phase formed after heat treatment steps. The calculated degree of crystallinity increased after each heat treatment step, the cry sig 4b dir 4a h – fou in in in the fin pe stallinity of nificantly fr Figure 3. Figure 4 i ) with diffe ectly propor , b, we can d 720 °C/0.5 nd in the fin these sample N–0.75 was Table 3 and presence o al GCs, but ak at hkl [11 N–0 final G om the trans Illustration o Figure 4 s used to co rent Nd2O3 tional to the educe that L h) was dec al GCs (aft s, except fo higher than Figure 5. Th f Nd2O3 ten increase the 1] by equati (a) Glass c C was 37%. parent glasse f the change o . XRD pattern mpare XRD contents. B amount of t S2 in the ing reased when er being fire r the sample N–0). The s e result sho ded to reduc crystallite on (2)). eramic ingots Min As a result, s to transluc f the crystall s of samples patterns of ecause the he phase in ot samples ( the content d in hot pres s named N– emi–quantit wed that wh e the crysta size of LS2 . h N. H., Hun the appeara ence GCs. ine phase form with different GC ingots ( intensity of the mixture, after the 2–s of Nd2O3 i sing furnace 0.75 (the int ative phase a en being he llinity and from 63.6 n (b) Fin g H. T. D., K nce of the m ed after heat Nd2O3 conte Figure 4a) a an X–ray d according to tage heat tre ncreased. Si at 930°C) a ensity of ma nalysis of fi at–treated at LS2 crystalli m to 91.8 nm al glass ceram hoi T. N., M aterials coul treatment ste nt. nd final GC iffraction p the results atment at 60 milar effect nd LS still in XRD pea nal GCs is p the same co ne phase fo (calculate ic. inh Q. D. 243 d change ps. s (Figure attern is in Figure 0 °C/1.5 was also remained k of LS2 resented nditions, rming of d for the Effects of Nd2O3 on the crystallization and properties of glass ceramic in 244 Figure 5. Dependence of crystal phases formed in final GC on Nd2O3 content (determined by intensities of main XRD peaks). Table 3. Degree of crystallinity (C) and characteristic peaks of final GC containing different Nd2O3 content. Sample C (%) Intensities of main XRD peaks (cts) Crystallite size of LS2 at hkl [111] (nm) LS2 LS Li3PO4 N–0 37 4351 119 144 63.6 N–0.25 39 2670 57 175 56.5 N–0.50 29 2834 83 121 63.6 N–0.75 38 4802 69 131 75.2 N–1.00 28 2558 76 128 91.8 3.3. Mechanical and chemical properties Table 4. The bending strength, Vicker hardness and chemical sobility of final GC. Sample 3–point bending strength (MPa) Vicker hardness (MPa) Chemical solubility (µg/cm2) N–0 213±6 4738±24 27.0±0.7 N–0.25 208±4 6306±75 20.0± 0.7 N–0.50 197±5 4337±20 21.7± 0.4 N–0.75 293±7 5958±23 3.7±0.4 N–1.00 204±7 5349±8 20.3±0.4 The values of three–point bending strength, Vicker hardness and chemical solubility of final GCs are shown in Table 4 and Figure 6. The bending strength, the chemical solubility of the samples was decreased, the Vicker hardness was increased slightly versus the rising of the Nd2O3 content. Nevertheless, the N–0.75 samples were very special with the highest bending strength, the lowest chemical solubility and relative high Vicker hardness. Minh N. H., Hung H. T. D., Khoi T. N., Minh Q. D. 245 Figure 6. The properties of final GC with different Nd2O3 content. 3.4. Color measurements Table 5 shows the values of CIE L*, a*, b* and the color difference, calculated by equations (3) to (6) of the final GC bars containing different Nd2O3–content. The color differences, ΔE*, were illustrated in Figure 7. Table 5. The CIE L*a*b* color and the calculated color difference values of final glass ceramic bars containing different Nd2O3–content. Sample L* a* b* Mean ΔL Mean Δa Mean Δb Mean ΔE N–0 88.87 ± 0.35 –7.24 ± 0.12 5.40 ± 0.01 – – – – N–0.25 85.36 ± 0.29 –6.67 ± 0.01 0.86 ± 0.06 –3.51 0.57 –4.53 5.76 N–0.50 82.36 ± 1.75 –4.77 ± 0.04 –0.70 ± 0.66 –6.51 2.47 –6.09 9.25 N–0.75 81.76 ± 2.04 –4.53 ± 0.21 –1.58 ± 0.31 –7.11 2.71 –6.98 10.32 N–1.00 79.68 ± 0.30 –4.48 ± 0.13 –2.95 ± 0.10 –9.19 2.77 –8.35 12.72 Figure 7. Effect of Nd2O3 additions on color difference CIE ΔE* of final GC bars which contained varying amounts of Nd2O3. Eff 24 sig (ne of the sam inc 3.5 mi mi sha sta Fi ects of Nd2O 6 These re nificantly lo gative b* va Nd2O3 can b green color The color ples, have reasing of Δ . Microstru The morp crostructure crostructure The fractu ped crystals ck and overl gure 8. SEM F 3 on the cry sults indica wer than th lues) and gr e observed decreased. differences shown that E* values fr cture hology of (non–etchin s of polished red surface . These crys ap each othe of fractured s igure 9. SEM stallization a ted that th e blank sam een range (n . With incre , ΔE*, betw the addition om 0 to 12.7 system also g, Au sputte and etched –microstruct tals were ho r that streng urface of N–0 of polished (etched with nd propertie e lightness ples (N–0) egative a* asing of the een the bla s of Nd2O3 2. was studi red) of N–0 surfaces of t ure of the f mogeneousl then of the G and N–0.75 surface of N– 10%–HF for s of glass ce of Nd2O3– . The shift values) of th Nd2O3 cont nk samples in the rang ed. Figure and N–0.7 hese sample inal GCs co y dispersed Cs. final glass cer 0 and N–0.75 10 s, Au–sput ramic in containing in reflectanc e spectra w ent, the blue N–0 and th e of 0 to 1 8 shows th 5 final GCs s. nsisted of di in an interlo amics (non–e final glass ce tered). samples, L e in the bl ith different color incre e Nd2O3–co .00 wt% ca e fractured . Figure 9 s spersed sma cking netwo tching, Au–sp ramics *, were ue range amounts ased and ntaining used the surface– hows the ll plate– rk. They uttered). Minh N. H., Hung H. T. D., Khoi T. N., Minh Q. D. 247 After etching, because the glass phase, LS crystal and Li3PO4 crystal were dissolved in HF solution [5], the major phase still remaining was LS2 crystals. The LS2 crystals with the size about of 2–5 µm can be visible. The pores as etched areas represented for LS and glass. The more closely packed and multidirectional interlocking of plate–shaped crystals in N–0.75 final GC microstructure can explain for its high bending strength, high Vicker hardness and low chemical solubility. 4. CONCLUSIONS The effects of Nd2O3 content (0–1 wt%) on crystallization and the properties of GC derived from the system Li2O–K2O–Al2O3–SiO2–P2O5 were discussed. The results showed as follows: 1. The results of DTA showed the small variation of characteristic temperatures of glass powders, especially the melting temperature Tm. It means that the GC system can be still stable under the hot–pressing process for dental restorations. 2. The main crystalline phase of final GCs was LS2. The chemical reaction of LS and SiO2 had occurred to produce LS2. However, the degree of this reaction decreased with the rise of Nd2O3 content. 3. The calculated crystal sizes of LS2 in final GCs containing Nd2O3 (from 63.6 nm to 91.8 nm) were larger than GC without Nd2O3. 4. Nd2O3 made the color of final GC bars decreased green value and lightness, and increased blue value in CIEL*a*b* color space. The color differences ΔE* increased from 0 to 12.72. 5. The samples N–0.75 had high crystallinity, the highest relative amount of LS2 phase and the highest bending strength value. 6. 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