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High-performance, reaction sintered lithium disilicate glass–ceramics
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High-performance, reaction sintered lithium disilicate glass–ceramics Ting Zhao, Yi Qin, Pei Zhang, Bo Wang, Jian-Feng Yangn State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, PR China Received 5 March 2014; received in revised form 31 March 2014; accepted 16 April 2014
Abstract A novel method of reaction sintering combined with hot-pressing technology was developed to prepare Li2Si2O5 glass–ceramics with high flexural strength, hardness and fracture toughness. Li2SiO3 crystal, SiO2 and Li2Si2O5 glass were used as the starting powders. The results show that via an interaction between Li2SiO3 crystals and SiO2 glass, rod-like Li2Si2O5 crystals with length of up to tens microns could be obtained, which were several times longer than those directly crystallized from the Li2Si2O5 parent glass. Through adjusting the Li2Si2O5 glass content, Li2Si2O5 glass–ceramics displayed a denser microstructure with the bimodal distribution consisting of large elongated Li2Si2O5 grains formed during the reaction and smaller precipitates of the Li2Si2O5 glass powder. Although the crystallinity decreased with increasing the Li2Si2O5 glass content, the Li2Si2O5 glass–ceramics exhibited high flexural strength (350 7 13 MPa), high hardness (5.92 70.18 GPa) and fracture toughness (3.3 70.14 MPa m1/2). & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Lithium disilicate; Reaction sintering; Elongated rod-like crystals; Flexural strength; Fracture toughness
1. Introduction Li2Si2O5 glass–ceramics are promising candidates for restorative dentistry applications because of their high strength and comparable translucency as natural teeth [1]. To date, several processing techniques have been developed to fabricate Li2Si2O5 glass–ceramics. The most convenient route is casting and crystallization in a variety of systems based on the Li2O– SiO2 composition by adding P2O5, ZrO2 and TiO2 [2–8]. These oxides act as nucleation agents, which can promote nucleation and volume crystallization either by accumulating in a specific microphase of the phase-separated base glass or promoting phase separation. In addition, powder sintering methods including pressureless sintering [9] and hot pressing [9,10] were also successfully adopted to synthesize the Li2Si2O5 glass–ceramics. In the sintering process, the surface crystallization dominates over volume crystallization when the particle size of the crushed glass is smaller than 300 μm [11], or the glass system is free from any nucleating agents [12]. n
Corresponding author. Tel.: þ86 29 82667942 803. E-mail address: [email protected] (J.-F. Yang).
No matter volume crystallization or surface crystallization applied, all the crystalline phases are precipitated from the parent glass, in which plenty of nucleation sites are provided. Therefore, the synthesized Li2Si2O5 glass–ceramics display small grain size usually below 5 μm, and even less than 1 μm [5,9,10,13]. The existence of these small crystals provides a high interfacial area with the glass matrix. Not only does it generate more flaws acting as a cracking source, but also leads to a shorter crack propagation path [14]. As a result, the glass– ceramics are usually characterized by poor fracture toughness. It is well known that the mechanical properties, especially the fracture toughness, could be considerably improved by the interpenetrating network structures formed by the elongated rod-like grains in Si3N4 [15,16], Al2O3 [17], and SiAlON ceramics [18]. These large elongated grains can indeed act as whiskers in ceramic composites [19], which serve to toughen the material via crack deflection, crack bridging and grain pullout within a zone immediately behind the crack tip [20,21]. In order to toughen Li2Si2O5 glass–ceramics without sacrificing other properties, a valid method is needed for obtaining elongated Li2Si2O5 crystals with high aspect ratio. Reaction sintering is a special way for fabricating glass–ceramics, in
http://dx.doi.org/10.1016/j.ceramint.2014.04.096 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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which the main crystalline phases are introduced by the reaction between the preexisted crystals and glass powder rather than precipitation from the parent glass [22–24]. The microstructure of glass–ceramics can be tailored by reaction sintering method. As for the crystallization process of Li2Si2O5 glass–ceramics, Li2SiO3 instead of Li2Si2O5 is first precipitated from the parent glass, and then, Li2SiO3 crystals react with the glassy matrix to form Li2Si2O5 as the following equation [5,25,26]. Li2 SiO3 ðcrystalÞþ SiO2 ðglassÞ ¼ Li2 Si2 O5 ðcrystalÞ
sieved to 20 μm. All the processing was the same as that of the LM glass powder as mentioned above. 2.2. XRF analysis The chemical compositions of LM crystals and LD glass were analyzed by X-ray fluorescence (XRF, S4Pioneer, Bruker, Germany).
ð1Þ
It is indicated that Li2Si2O5 glass–ceramics can be fabricated by the direct reaction between Li2SiO3 crystals and SiO2 glass powders. Compared with spontaneous nucleation, the heterogeneous nucleating sites provided by the Li2SiO3 crystals addition are limited and they can be controlled. The reaction rates between Li2SiO3 and SiO2 during sintering played a decisive role in forming Li2Si2O5 crystals [23,24,27]. On the other hand, Li2Si2O5 crystals grow epitaxially along specific planes (such as (0 2 0), (1 1 1) and (1 3 0) planes) of Li2SiO3 crystals [5]. The limited nucleation density, epitaxial growth mechanism and the preferred growth orientation [1] make the preparation of large elongated rod-like Li2Si2O5 crystals possible, which will contribute to high mechanical properties. However, few reports concerning the new method have been mentioned. In this paper, high-performance Li2Si2O5 glass– ceramics with elongated rod-like Li2Si2O5 grains were fabricated by hot-pressing sintering using pre-synthesized Li2SiO3 crystals, SiO2 and Li2Si2O5 glass as starting materials. The effects of Li2Si2O5 glass contents on the reactive crystallizing behavior, sintering behavior and mechanical properties of Li2Si2O5 glass–ceramics were investigated. 2. Experimental procedure 2.1. Synthesis of reactants Li2CO3 powder (Merck kGaA Corp., Germany) and SiO2 powder (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were used as the starting powders to synthesize Li2SiO3 crystals (written as LM). The molar mass proportion of the two powders was 1:1 according to the stoichiometric composition of Li2SiO3. After uniformly mixing the mixtures, the batches were placed in a Pt crucible and melted in an electric furnace at 1450 1C for 2 h in air. And then, the glass melt was quenched into deionized water to obtain a frit for milling. Subsequently, the glass frit was dried, ball-milled with high purity zirconia balls for 4 h, and then sieved to yield a powder of particle size o 15 μm. After that, the obtained LM glass powder was crystallized to LM crystals at 490 1C for 2 h in an air furnace. Commercially available silica (amorphous, 99% pure, Donghai Chemical plant, Jiangsu, China; mean particle size: 20 μm) was used to react with LM crystals. The chemical composition of Li2Si2O5 (LD) glass was as below: 68.6% SiO2, 28.6% Li2O, 2.0% K2O and 0.8% La2O3 (in molar mass). The powder mixture was mixed, melted, ball-milled and
2.3. DTA study To investigate the reaction process and crystallization temperature of the powders, differential thermal analysis (DTA) testing was conducted in a TG/DTA apparatus (SDTQ600, TA, USA) at a heating rate of 10 K/min from room temperature (RT) to 1100 1C under a continuous flow of nitrogen gas at a rate of 60 mL/min. Measurement was performed twice for each condition and excellent repeatability was obtained. 2.4. Preparation of glass–ceramics According to the reaction (1), if LM crystals react completely with SiO2, the resulting reaction product is pure Li2Si2O5 crystals without any glass phase. However, as a dental restorative glass–ceramic, the glass phase content must be in the range of 30–40 vol% [25,28]. To ensure that, LD glass powder was added based on the mixture consisting of 1 mol LM and 2 mol SiO2 (M1S2), in which excessive SiO2 was used to promote the reaction more completely. In this case, despite being free from any nucleation agents, Li2Si2O5 crystals could also crystallize from the parent LD glass via surface nucleation such that large grains from the reaction and small ones from the crystallization coexisted. This is an important structure for the toughening of ceramics [16,29]. The mixtures of LM crystals powder, SiO2 and LD glass powder with molar ratio of 1:2:0, 1:2:0.5, 1:2:1, 1:2:4 and 1:2:8 were used for the samples preparation. These mixtures were named as M1S2, M1S2D0.5, M1S2D1, M1S2D4 and M1S2D8, respectively. They were wet mixed with ZrO2 balls in anhydrous alcohol for 2 h. After drying, the mixtures were put into a BN-coated graphite die and uniaxially cold-pressed under 5 MPa, and then the samples were hot-pressing sintered at proper temperatures for 1 h under a pressure of 20 MPa in a vacuum furnace. Finally, the samples were cooled down to ambient temperature. The surface layers of the samples were removed for subsequent characterization. 2.5. Characterization 2.5.1. Density The bulk density and porosity of the sintered products were measured by the Archimedes method using distilled water. Three samples were estimated to get the mean value.
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2.5.2. Crystalline phases Crystalline phases were identified by X-ray diffraction analysis (XRD, D/MAX-RA, Rigaku, Japan) with Cu Kα radiation and scanning from 101 to 701. The sampling interval was 0.021 2θ. The crystallinity was calculated by the locations of the diffraction peaks, their number and relative intensity in the XRD pattern according to Eq. (2) [5], Xc ¼
Σ i I ci 100% Σ i I ci þ KI a
ð2Þ
where Xc is the crystallinity, Ia is the integrated intensity of amorphous phases, Ic is the integrated intensity of the crystalline phase and K is a constant related to the measurement condition and glass compositions. According to the previous experimental results [5], K ¼ 0.963. 2.5.3. Microstructures Electron backscatter diffraction (EBSD, Phenom Pro-X, Netherland) was used to characterize microstructures and phases of the samples. Moreover, microstructures of the etched samples (5 vol% HF for 1 min) were observed by field emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan). The grain size was counted and analyzed. More than 200 grains were chosen for each sample. 2.5.4. Mechanical properties Rectangular bars (3 mm 4 mm 30 mm, after polishing) were used to determine the three-point flexural strength with a span of 16 mm at a cross-head speed of 0.5 mm/min by the universal testing machine (INSTRON1195, Britain). In order to analyze the hardness, several specimens were indented with a Vickers diamond pyramid indenter (HVC-10A1, China) at a load of 4.9 N. The radial cracks after the indentation were measured by using an optical microscope of the microindentation system, and then the Vickers hardness was calculated [30]. Six indentations were measured to get the mean value. For the fracture toughness measurement, the samples (3 mm 4 mm 20 mm) with a V-notch depth of 0.8 mm to 1.2 mm and the single edge V-notched beam (SEVNB) testing method were used. The fracture toughness was calculated according to ISO 6872 (International Standard). The average flexural strength and fracture toughness were obtained from five samples and three specimens, respectively. 3. Results and discussion 3.1. XRF results Table 1 shows the chemical compositions of the LM crystals and LD glass. It can be seen that in the LM crystals, besides the main components of Li (could not be detected), O and Si, little Al was detected due to the introduction of alumina from stirring rods while other impurities were very few. The chemical composition of the LD glass was almost the same besides the composed elements La and K.
3
3.2. DTA analysis Fig. 1a shows the DTA results of the mixtures with different LD contents. All the plots displayed well-defined features comprising endothermic and exothermic peaks. The glass transition point (Tg), the first and second exothermic peak temperatures (Tp1 and Tp2) as well as the melting temperature (Tm) could be determined. As can be seen from Fig. 1b, all the temperatures shifted to lower values with increasing the LD glass content. It has been stated that Tg, depending on the polymerization extent of the silicate network, was closely related to the numbers of non-bridging oxygen per tetrahedron (NBO/T) [6]. The NBO/T of LD glass was higher than that of the SiO2 glass, in which all the oxygen ions acted as the bridge. So low values of Tg appeared for the LD rich samples, suggesting a lower degree of depolymerization in the glass network. The crystallization temperature of Li2SiO3 (Tp1) was not detected in M1S2, whereas it was distinctly visible in the mixtures with LD additions. It indicated that the crystallization of LD glass and the growth of Li2Si2O5 crystals took place at the consumption of Li2SiO3. At temperature Tp2, Li2Si2O5 precipitated as the main phase, including the reacted crystals and the self-crystallized ones. The surface nucleation sites increased with the addition of LD glass. The greater the number of nuclei the lower the temperature at which the heat of crystallization becomes detectable and the higher the crystallization peak [31]. As a result, the Tp1 and Tp2 tended to decrease while the height of the crystallization peaks increased with the increasing LD glass. The appropriate sintering temperatures for each composition differed according to the DTA results, so several sintering temperatures around them were tested. In the following text, the individual optimized sintering condition for the each composition was used for discussion of the crystalline phases, microstructures and mechanical properties of the samples. 3.3. Phase formation Fig. 2 shows the XRD pattern of LM glass powder after crystallization at 490 1C for 2 h. It can be seen that Li2SiO3 (ICCD card 01-029-0828) was developed as the major crystalline phase, indicating that LM glass powder crystallized completely. The XRD patterns of the reaction sintered specimens with different LD contents are shown in Fig. 3. For the sample without LD glass addition (M1S2), the main crystalline phases included Li2Si2O5 (ICCD card 01-040-0376) and cristobalite (ICCD card 01-039-1425) as a precipitation of extra SiO2. This indicated that the complete reaction between LM crystals and SiO2 could take place at a higher temperature of 890 1C corresponding to the Tp2 in Fig. 1a. Minor lithium aluminum silicate (Li0.6Al0.6Si2.4O6, ICCD card 01-021-0503) also appeared due to the reaction between the LM crystals containing the impurity of Al (Table 1) and SiO2. Only Li2Si2O5 and Li0.6Al0.6Si2.4O6 phases were detected in the M1S2Dx (x 40) samples. With increasing the LD glass contents, the intensity of the major diffraction peaks of Li2Si2O5 crystals shifted
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Table 1 The chemical compositions of LM crystals and LD glass analyzed by XRF. Sample
LM LD
Composition (wt%) O
Si
La
K
Al
Cr
Fe
Zr
Ca
58.57 57.8
40.34 32.9
— 4.62
— 3.51
0.95 1.03
0.0526 0.0474
0.04 0.0441
0.0059 0.0152
0.0063 0.0172
Fig. 1. Thermal behavior of different mixtures: (a) DTA curves; (b) evolution of Tg, Tp1, Tp2 and Tm with the amount of LD glass added to the parent composition.
Fig. 2. The XRD pattern of the LM glass powder after crystallization at 490 1C for 2 h. (Li2SiO3: ICCD card 01-029-0828).
slightly due to the change of crystallinity degree and grain size, whereas the intensity of Li0.6Al0.6Si2.4O6 diffraction peaks decreased and almost disappeared in the sample of M1S2D8. For the solid phase reaction process, adequate contact of reactant particles was essential to sustain a fast reaction process [32]. Too much addition of the LD glass powder in this study could decrease the contact between the LM crystals
Fig. 3. The XRD patterns of glass–ceramics with different compositions. (Li2Si2O5: ICCD card 01-040-0376; Li0.6Al0.6Si2.4O6: ICCD card 01-0210503; Cristobalite: ICCD card 01-039-1425;) The sintering conditions were as follows: M1S2, 890 1C/1 h; M1S2D0.5, 855 1C/1 h; M1S2D1, 840 1C/1 h; M1S2D4, 820 1C/1 h; M1S2D8, 820 1C/1 h.
and SiO2 so the reaction (1) was restrained. Consequently, most of the Li2Si2O5 crystals were formed through crystallization rather than the reaction. The resulted Li2Si2O5 crystalline phase accounted for the vast majority so that minor SiO2 and Li0.6Al0.6Si2.4O6 phases could not be detected in the LD richer compositions, such as in the sample of M1S2D8.
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Fig. 4. EBSD micrographs (left panel) of Li2Si2O5 glass–ceramics and SEM morphologies with etched samples (right panel) with different compositions after reaction sintering, (a) M1S2, 890 1C/1 h; (b) M1S2D0.5, 855 1C/1 h; (c) M1S2D1, 840 1C/1 h; (d) M1S2D4, 820 1C/1 h; (e) M1S2D8, 820 1C/1 h.
3.4. Morphology Fig. 4 shows the typical EBSD micrographs of the Li2Si2O5 glass–ceramics and SEM morphologies of the etched samples with different LD contents. The crystalline phases (PC) shown in dark color and the glass phases (PG) in white color could be
easily distinguished from the EBSD micrographs (left panel). For the M1S2 specimen, it was clearly observed that the Li2Si2O5 crystals were rod-like with an average length of 10.18 μm and width of 1.66 μm. The average aspect ratio of the grains was 6.1 (as shown in Table 2 and Fig. 4a). This morphology was due to the full reaction of LM crystals and
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SiO2 powder. Some pores appeared because the SiO2 glass could not completely fill in the skeleton formed by the large grains. The porosity was as high as 6.02% (Table 2). As shown in the right panel of Fig. 4a, after etching with 5% vol HF for 1 min, the SiO2 glass in the samples dissolved and Li2Si2O5 grains with a loose structure were exposed outside. This further indicated that the SiO2 glass phase was insufficient to provide a binding in the M1S2. Compared to the M1S2 specimen, although Li2Si2O5 crystals in the M1S2Dx (x4 0) samples remained elongated shape, the grain size, glass phase content and density were quiet different. It could be clearly seen from the EBSD micrographs that the proportion of PG in white color increased with the increasing LD content and the porosity decreased (as shown in Table 2). The reason for these changes was that with the addition of LD glass powder, the viscosity of the whole reaction system reduced as stated above. This was proved in DTA curves where Tg decreased (Fig. 1b). At higher temperatures, the glass phase easily penetrated into the Li2Si2O5 crystal skeleton, so that dense microstructures with Li2Si2O5 crystals embedded in the glassy matrix were achieved. What is more, all the samples showed a bimodal grain size distribution in which both large elongated Li2Si2O5 grains and fine grains existed. The large rod-like Li2Si2O5 grains were achieved by the reaction between LM and SiO2, and the small grains were directly crystallized from the LD glass. Specifically, when the content of the LD glass was less than 1 mol, the reaction sintered elongated Li2Si2O5 crystals were dominant (see Fig. 4a–c). With further increasing the LD contents to 4 and 8 mol, large amounts of small Li2Si2O5 crystals crystallized Table 2 Length (maximum value of Lmax and average value of L), width (maximum value of Wmax and average value of W ), aspect ratio (R) and porosity of the reaction sintered specimens with different LD content. Specimens
Lmax (μm)
L (μm)
Wmax (μm)
W (μm)
R
Porosity (%)
M1S2 M1S2D0.5 M1S2D1 M1S2D4 M1S2D8
16.71 20.54 9.50 13.38 6.71
10.08 7.05 4.50 5.51 3.68
3.09 3.00 2.06 3.81 1.87
1.66 1.60 1.20 1.61 1.12
6.1 2.4 3.8 3.4 3.3
6.02 1.41 1.20 0.82 0.46
from the LD glass (as shown in Fig. 4d and e). The specific grain size and aspect ratio data are summarized in Table 2. These results showed that the average length and width of Li2Si2O5 crystals decreased from 7.05 μm to 3.68 μm and 1.6 μm to 1.12 μm, respectively, as the LD content increased from 0.5 mol to 8 mol. This was because the large amounts of the LD glass addition enhanced the self-crystallization in the LD glass powder while it restrained the contact between the LM crystals and SiO2. Thus, elongation of the Li2Si2O5 crystals from the reaction was prohibited. Hot-pressing usually results in a textured microstructure also with elongated crystals [33–35]. However, there was little difference between the morphology of the pressed plane and the plane parallel to the pressed direction of the specimen as shown in Fig. 5. It was seen that a large amount of elongated grains with high aspect ratio and small crystals were uniformly distributed in the glass matrix in both directions. Owing to the lower temperature of Tg as well as the applied axial pressure in the present study, almost complete densification could be achieved by viscous flow before the crystallization at a relatively low temperature during hot-pressing process [36]. In this case, the thermal deformation was weak. It could be concluded that the textured microstructure with elongated crystals was resulted from the reaction sintering rather than hot-pressing. The hot-pressing process was mainly contributed to the densification. The essence of the reactive crystallization mechanism could be understood through the different samples with and without LD glass contents as compared in Fig. 6. In specimen M1S2, heterogeneous Li2Si2O5 nuclei probably formed on the LM crystals by the reaction between the SiO2 glass phase and the LM crystal phase. Then Li2Si2O5 crystals grew up epitaxially at the expense of consuming the two reactants according to the reaction (1). However, the nucleation sites were limited due to restricted contact and the reaction between LM and SiO2. So Li2Si2O5 crystals could grow sufficiently to become elongated rod-like crystals with large size. Meanwhile, part of the excess SiO2 precipitated as the form of cristobalite, which was verified by XRD analysis. In the case of M1S2Dx (x4 0), besides the heterogeneous nucleation, surface nucleation occurred on the LD glass powder. Because the nucleation density in the LD glass was
Fig. 5. EBSD micrographs of Li2Si2O5 glass–ceramics sintered from M1S2D0.5 at 855 1C for 1 h, (a) the pressed plane; (b) the plane parallel to the pressed direction.
Please cite this article as: T. Zhao, et al., High-performance, reaction sintered lithium disilicate glass–ceramics, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.04.096
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Fig. 6. Crystallization models of the reaction sintered lithium disilicate glass–ceramics with and without LD glass contents. Li2Si2O5 nuclei; Cristobalite; Li2Si2O5 crystal.
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Li2SiO3 crystal;
SiO2 glass;
high, the linear growth speed of Li2Si2O5 crystals precipitated from LD glass decreased. On the other hand, like the M1S2, Li2Si2O5 crystals from heterogeneous nucleation and the reaction exhibited the large grain size. The microstructure consisting of crystals with bimodal grain size distributions could cause crack blunting or branching, which was advantageous in enhancing the mechanical properties [37]. For the reaction process, considering partial SiO2 crystallized during the sintering [10], the reaction (1) can be written as, LMðcrystalÞ þ 2SiO2 ðglassÞ-Li2 Si2 O5 ðcrystalÞ þ SiO2 ðglassÞþ SiO2 ðcrystalÞ
ð3Þ Thus, the whole reaction sintering process of M1S2Dx (x4 0) samples can be deduced as following LMðcrystalÞ þ 2SiO2 ðglassÞ þ xLDðglassÞ-Li2 Si2 O5 ðelongated crystalÞ þ SiO2 ðglassÞþ SiO2 ðcrystalÞþ LDðsmall crystalÞ þ LDðglassÞ
Fig. 7. Three-point flexural strength and fracture toughness of lithium disilicate glass–ceramics with different LD contents.
ð4Þ
The reaction (4) clearly presented that the fabrication process of Li2Si2O5 glass–ceramics involved in this paper, including reaction (1), (3) and crystallization of the LD glass powder itself. 3.5. Mechanical properties For the glass–ceramics, improvement of the mechanical properties can be obtained with the lower porosity, higher volume of crystalline phases and the appropriate morphology of precipitated crystals [25]. In this study, although the highest crystallinity of 68.58% was obtained for the specimens of M1S2, the lowest flexural strength and fracture toughness was seen (Fig. 7). It could be attributed to the high porosity of 6.02% resulted from the insufficient glass phase. The flexural strength and fracture toughness of the Li2Si2O5 glass–ceramics with the LD glass addition was strongly improved (Fig. 7). With increasing the LD content, the flexural strength increased
then decreased after its peak value. The highest flexural strength (350 7 13 MPa) was recorded for the M1S2D1 specimens, which was high when compared to that of Li2Si2O5 glass–ceramics obtained by powder sintering [9,10]. The improvement of strength was possibly attributed to the decreased porosity, more suitable crystallinity and high average aspect ratio of Li2Si2O5 grains, which was consistent with Cramer's research [3] that glass–ceramics with elongated grains of aspect ratios 2–6 showed excellent mechanical properties. The variation of fracture toughness with increasing the LD content was quite similar to that of flexural strength, while the highest value (3.3 7 0.14 MPa m1/2) was obtained for the samples of M1S2D0.5. Compared with M1S2, the toughness improvement in specimens with LD addition was resulted from the denser microstructure. For the reaction sintered samples in this study, the key factor for the fracture toughness was the high content of the elongated rod-like Li2Si2O5 crystals formed by reaction (3). It was similar to that
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4. Conclusions
Fig. 8. Vickers hardness of lithium disilicate glass–ceramics with different LD contents.
of self-toughening Si3N4 ceramics, in which the fracture toughness was positively correlated to the diameter and volume percentage of elongated rod-like Si3N4 crystals [29]. Moreover, the interlocking microstructure of coexisted elongated Li2Si2O5 crystals with small ones also played an important role in hindering crack propagation and toughening, providing Li2Si2O5 glass–ceramics in the present study with better fracture toughness than other powder sintered ones [9,10]. As seen in Fig. 4b, the M1S2D0.5 sample had a large amount of abnormal enlarged Li2Si2O5 grains (Lmax of 20.54 μm, and Wmax of 3.0 μm), which contributed to the highest toughness. However, with the continued addition of LD glass, the number of large elongated crystals decreased and the grains were refined (see Fig. 4), which was caused by the inhibited reaction between the LM crystals and SiO2 along with the increased small crystals directly precipitated from the LD glass. This resulted in the fracture toughness decrease. It also explicated that Li2Si2O5 glass–ceramics without any toughening phase always has poor fracture toughness and the method in this study provides the possibility for toughening with elongated rod-like crystals. Hardness is also considered an important property when judging the abrasiveness of restorative materials. It is a measure of the resistance to permanent surface indentation or penetration [38]. In the present study, we also investigated the Vickers hardness of the reaction sintered Li2Si2O5 glass– ceramics as shown in Fig. 8. It can be seen that with increasing the LD content, the hardness increased then decreased and the highest value (5.92 7 0.18 GPa) was recorded for the M1S2D0.5 specimens. The tested hardness of bulk Li2Si2O5 glass ( 500 MPa) was much lower than that of Li2Si2O5 crystalline phase (several GPa). Consequently, the hardness variation of Li2Si2O5 glass–ceramics is mainly attributed to the different crystallinity, while the lower hardness in M1S2 is caused by the high porosity. The hardness values ranged from 5.48 GPa to 5.92 GPa in this study are in good agreement with results found in previous literatures [38,39].
Dense Li2Si2O5 glass–ceramics with bimodal microstructures in which some large elongated Li2Si2O5 grains with high aspect ratio and fine grains embedded in the glass matrix was successfully fabricated by hot-pressing sintering. The elongated rod-like Li2Si2O5 crystals with large grain size, heterogonous nucleation mechanism controlled, were obtained from the reaction between LM crystals and SiO2 glass. And the fine grains were directly crystallized from the LD parent glass. With increasing the LD content from 0 mol to 8 mol, the temperature of Tg, Tp1, Tp2 and Tm decreased, the intensity of the diffraction peaks of Li2Si2O5 varied slightly and the average grain size decreased. Although the crystallinity decreased with increasing the LD amount, the Li2Si2O5 glass–ceramic exhibited high flexural strength (350713 MPa, M1S2D1), high hardness and fracture toughness (5.9270.18 GPa and 3.370.14 MPa m1/2, M1S2D0.5). The interlocking microstructure contributed to high strength, the high crystallinity resulted in good hardness and the reaction sintered elongated Li2Si2O5 crystals led to high toughness. A technique such as the one presented here offers a novel route for preparing dental restorative materials. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 51272205, 51072157, 51302208, and A3 Foresight Program-51161140399), by the High-Tech R & D Program of China (863, no. 2011AA030102), and by Doctoral Fund of Ministry of Education of China (Grant no. 20100201110036 and 20130201120003), and by Science. References [1] W. Höland, G.H. Beall, Glass–Ceramic Technology, second ed., John Wiley & Sons, Inc., New Jersey, America, 2012. [2] D. Holland, Y. Iqbal, P. James, B. Lee, Early stages of crystallisation of lithium disilicate glasses containing P2O5–An NMR study, J. Non-Cryst. Solids 232-234 (1998) 140–146. [3] S.C. von Clausbruch, M. Schweiger, W. Höland, V. Rheinberger, The effect of P2O5 on the crystallization and microstructure of glass–ceramics in the SiO2–Li2O–K2O–ZnO–P2O5 system, J. Non-Cryst. Solids 263 (2000) 388–394. [4] E. Apel, C. van't Hoen, V. Rheinberger, W. Höland, Influence of ZrO2 on the crystallization and properties of lithium disilicate glass–ceramics derived from a multi-component system, J. Eur. Ceram. Soc. 27 (2007) 1571–1577. [5] X. Zheng, G. Wen, L. Song, X.X. Huang, Effects of P2O5 and heat treatment on crystallization and microstructure in lithium disilicate glass ceramics, Acta Mater. 56 (2008) 549–558. [6] H.R. Fernandes, D.U. Tulyaganov, A. Goel, M.J. Ribeiro, M.J. Pascual, J.M.F. Ferreira, Effect of Al2O3 and K2O content on structure, properties and devitrification of glasses in the Li2O–SiO2 system, J. Eur. Ceram. Soc. 30 (2010) 2017–2030. [7] H.R. Fernandes, D.U. Tulyaganov, J.M.F. Ferreira, The role of P2O5, TiO2 and ZrO2 as nucleating agents on microstructure and crystallization behaviour of lithium disilicate-based glass, J. Mater. Sci. 48 (2013) 765–773. [8] D.H. Xiong, J.S. Cheng, H. Li, Composition and crystallization kinetics of R2O–Al2O3–SiO2 glass–ceramics, J. Alloys Compd. 498 (2010) 162–167.
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
High-performance, reaction sintered lithium disilicate glass–ceramics Ting Zhao, Yi Qin, Pei Zhang, Bo Wang, Jian-Feng Yangn State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, PR China Received 5 March 2014; received in revised form 31 March 2014; accepted 16 April 2014
Abstract A novel method of reaction sintering combined with hot-pressing technology was developed to prepare Li2Si2O5 glass–ceramics with high flexural strength, hardness and fracture toughness. Li2SiO3 crystal, SiO2 and Li2Si2O5 glass were used as the starting powders. The results show that via an interaction between Li2SiO3 crystals and SiO2 glass, rod-like Li2Si2O5 crystals with length of up to tens microns could be obtained, which were several times longer than those directly crystallized from the Li2Si2O5 parent glass. Through adjusting the Li2Si2O5 glass content, Li2Si2O5 glass–ceramics displayed a denser microstructure with the bimodal distribution consisting of large elongated Li2Si2O5 grains formed during the reaction and smaller precipitates of the Li2Si2O5 glass powder. Although the crystallinity decreased with increasing the Li2Si2O5 glass content, the Li2Si2O5 glass–ceramics exhibited high flexural strength (350 7 13 MPa), high hardness (5.92 70.18 GPa) and fracture toughness (3.3 70.14 MPa m1/2). & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Lithium disilicate; Reaction sintering; Elongated rod-like crystals; Flexural strength; Fracture toughness
1. Introduction Li2Si2O5 glass–ceramics are promising candidates for restorative dentistry applications because of their high strength and comparable translucency as natural teeth [1]. To date, several processing techniques have been developed to fabricate Li2Si2O5 glass–ceramics. The most convenient route is casting and crystallization in a variety of systems based on the Li2O– SiO2 composition by adding P2O5, ZrO2 and TiO2 [2–8]. These oxides act as nucleation agents, which can promote nucleation and volume crystallization either by accumulating in a specific microphase of the phase-separated base glass or promoting phase separation. In addition, powder sintering methods including pressureless sintering [9] and hot pressing [9,10] were also successfully adopted to synthesize the Li2Si2O5 glass–ceramics. In the sintering process, the surface crystallization dominates over volume crystallization when the particle size of the crushed glass is smaller than 300 μm [11], or the glass system is free from any nucleating agents [12]. n
Corresponding author. Tel.: þ86 29 82667942 803. E-mail address: [email protected] (J.-F. Yang).
No matter volume crystallization or surface crystallization applied, all the crystalline phases are precipitated from the parent glass, in which plenty of nucleation sites are provided. Therefore, the synthesized Li2Si2O5 glass–ceramics display small grain size usually below 5 μm, and even less than 1 μm [5,9,10,13]. The existence of these small crystals provides a high interfacial area with the glass matrix. Not only does it generate more flaws acting as a cracking source, but also leads to a shorter crack propagation path [14]. As a result, the glass– ceramics are usually characterized by poor fracture toughness. It is well known that the mechanical properties, especially the fracture toughness, could be considerably improved by the interpenetrating network structures formed by the elongated rod-like grains in Si3N4 [15,16], Al2O3 [17], and SiAlON ceramics [18]. These large elongated grains can indeed act as whiskers in ceramic composites [19], which serve to toughen the material via crack deflection, crack bridging and grain pullout within a zone immediately behind the crack tip [20,21]. In order to toughen Li2Si2O5 glass–ceramics without sacrificing other properties, a valid method is needed for obtaining elongated Li2Si2O5 crystals with high aspect ratio. Reaction sintering is a special way for fabricating glass–ceramics, in
http://dx.doi.org/10.1016/j.ceramint.2014.04.096 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: T. Zhao, et al., High-performance, reaction sintered lithium disilicate glass–ceramics, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.04.096
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which the main crystalline phases are introduced by the reaction between the preexisted crystals and glass powder rather than precipitation from the parent glass [22–24]. The microstructure of glass–ceramics can be tailored by reaction sintering method. As for the crystallization process of Li2Si2O5 glass–ceramics, Li2SiO3 instead of Li2Si2O5 is first precipitated from the parent glass, and then, Li2SiO3 crystals react with the glassy matrix to form Li2Si2O5 as the following equation [5,25,26]. Li2 SiO3 ðcrystalÞþ SiO2 ðglassÞ ¼ Li2 Si2 O5 ðcrystalÞ
sieved to 20 μm. All the processing was the same as that of the LM glass powder as mentioned above. 2.2. XRF analysis The chemical compositions of LM crystals and LD glass were analyzed by X-ray fluorescence (XRF, S4Pioneer, Bruker, Germany).
ð1Þ
It is indicated that Li2Si2O5 glass–ceramics can be fabricated by the direct reaction between Li2SiO3 crystals and SiO2 glass powders. Compared with spontaneous nucleation, the heterogeneous nucleating sites provided by the Li2SiO3 crystals addition are limited and they can be controlled. The reaction rates between Li2SiO3 and SiO2 during sintering played a decisive role in forming Li2Si2O5 crystals [23,24,27]. On the other hand, Li2Si2O5 crystals grow epitaxially along specific planes (such as (0 2 0), (1 1 1) and (1 3 0) planes) of Li2SiO3 crystals [5]. The limited nucleation density, epitaxial growth mechanism and the preferred growth orientation [1] make the preparation of large elongated rod-like Li2Si2O5 crystals possible, which will contribute to high mechanical properties. However, few reports concerning the new method have been mentioned. In this paper, high-performance Li2Si2O5 glass– ceramics with elongated rod-like Li2Si2O5 grains were fabricated by hot-pressing sintering using pre-synthesized Li2SiO3 crystals, SiO2 and Li2Si2O5 glass as starting materials. The effects of Li2Si2O5 glass contents on the reactive crystallizing behavior, sintering behavior and mechanical properties of Li2Si2O5 glass–ceramics were investigated. 2. Experimental procedure 2.1. Synthesis of reactants Li2CO3 powder (Merck kGaA Corp., Germany) and SiO2 powder (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were used as the starting powders to synthesize Li2SiO3 crystals (written as LM). The molar mass proportion of the two powders was 1:1 according to the stoichiometric composition of Li2SiO3. After uniformly mixing the mixtures, the batches were placed in a Pt crucible and melted in an electric furnace at 1450 1C for 2 h in air. And then, the glass melt was quenched into deionized water to obtain a frit for milling. Subsequently, the glass frit was dried, ball-milled with high purity zirconia balls for 4 h, and then sieved to yield a powder of particle size o 15 μm. After that, the obtained LM glass powder was crystallized to LM crystals at 490 1C for 2 h in an air furnace. Commercially available silica (amorphous, 99% pure, Donghai Chemical plant, Jiangsu, China; mean particle size: 20 μm) was used to react with LM crystals. The chemical composition of Li2Si2O5 (LD) glass was as below: 68.6% SiO2, 28.6% Li2O, 2.0% K2O and 0.8% La2O3 (in molar mass). The powder mixture was mixed, melted, ball-milled and
2.3. DTA study To investigate the reaction process and crystallization temperature of the powders, differential thermal analysis (DTA) testing was conducted in a TG/DTA apparatus (SDTQ600, TA, USA) at a heating rate of 10 K/min from room temperature (RT) to 1100 1C under a continuous flow of nitrogen gas at a rate of 60 mL/min. Measurement was performed twice for each condition and excellent repeatability was obtained. 2.4. Preparation of glass–ceramics According to the reaction (1), if LM crystals react completely with SiO2, the resulting reaction product is pure Li2Si2O5 crystals without any glass phase. However, as a dental restorative glass–ceramic, the glass phase content must be in the range of 30–40 vol% [25,28]. To ensure that, LD glass powder was added based on the mixture consisting of 1 mol LM and 2 mol SiO2 (M1S2), in which excessive SiO2 was used to promote the reaction more completely. In this case, despite being free from any nucleation agents, Li2Si2O5 crystals could also crystallize from the parent LD glass via surface nucleation such that large grains from the reaction and small ones from the crystallization coexisted. This is an important structure for the toughening of ceramics [16,29]. The mixtures of LM crystals powder, SiO2 and LD glass powder with molar ratio of 1:2:0, 1:2:0.5, 1:2:1, 1:2:4 and 1:2:8 were used for the samples preparation. These mixtures were named as M1S2, M1S2D0.5, M1S2D1, M1S2D4 and M1S2D8, respectively. They were wet mixed with ZrO2 balls in anhydrous alcohol for 2 h. After drying, the mixtures were put into a BN-coated graphite die and uniaxially cold-pressed under 5 MPa, and then the samples were hot-pressing sintered at proper temperatures for 1 h under a pressure of 20 MPa in a vacuum furnace. Finally, the samples were cooled down to ambient temperature. The surface layers of the samples were removed for subsequent characterization. 2.5. Characterization 2.5.1. Density The bulk density and porosity of the sintered products were measured by the Archimedes method using distilled water. Three samples were estimated to get the mean value.
Please cite this article as: T. Zhao, et al., High-performance, reaction sintered lithium disilicate glass–ceramics, Ceramics International (2014), http://dx.doi.org/ 10.1016/j.ceramint.2014.04.096
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2.5.2. Crystalline phases Crystalline phases were identified by X-ray diffraction analysis (XRD, D/MAX-RA, Rigaku, Japan) with Cu Kα radiation and scanning from 101 to 701. The sampling interval was 0.021 2θ. The crystallinity was calculated by the locations of the diffraction peaks, their number and relative intensity in the XRD pattern according to Eq. (2) [5], Xc ¼
Σ i I ci 100% Σ i I ci þ KI a
ð2Þ
where Xc is the crystallinity, Ia is the integrated intensity of amorphous phases, Ic is the integrated intensity of the crystalline phase and K is a constant related to the measurement condition and glass compositions. According to the previous experimental results [5], K ¼ 0.963. 2.5.3. Microstructures Electron backscatter diffraction (EBSD, Phenom Pro-X, Netherland) was used to characterize microstructures and phases of the samples. Moreover, microstructures of the etched samples (5 vol% HF for 1 min) were observed by field emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan). The grain size was counted and analyzed. More than 200 grains were chosen for each sample. 2.5.4. Mechanical properties Rectangular bars (3 mm 4 mm 30 mm, after polishing) were used to determine the three-point flexural strength with a span of 16 mm at a cross-head speed of 0.5 mm/min by the universal testing machine (INSTRON1195, Britain). In order to analyze the hardness, several specimens were indented with a Vickers diamond pyramid indenter (HVC-10A1, China) at a load of 4.9 N. The radial cracks after the indentation were measured by using an optical microscope of the microindentation system, and then the Vickers hardness was calculated [30]. Six indentations were measured to get the mean value. For the fracture toughness measurement, the samples (3 mm 4 mm 20 mm) with a V-notch depth of 0.8 mm to 1.2 mm and the single edge V-notched beam (SEVNB) testing method were used. The fracture toughness was calculated according to ISO 6872 (International Standard). The average flexural strength and fracture toughness were obtained from five samples and three specimens, respectively. 3. Results and discussion 3.1. XRF results Table 1 shows the chemical compositions of the LM crystals and LD glass. It can be seen that in the LM crystals, besides the main components of Li (could not be detected), O and Si, little Al was detected due to the introduction of alumina from stirring rods while other impurities were very few. The chemical composition of the LD glass was almost the same besides the composed elements La and K.
3
3.2. DTA analysis Fig. 1a shows the DTA results of the mixtures with different LD contents. All the plots displayed well-defined features comprising endothermic and exothermic peaks. The glass transition point (Tg), the first and second exothermic peak temperatures (Tp1 and Tp2) as well as the melting temperature (Tm) could be determined. As can be seen from Fig. 1b, all the temperatures shifted to lower values with increasing the LD glass content. It has been stated that Tg, depending on the polymerization extent of the silicate network, was closely related to the numbers of non-bridging oxygen per tetrahedron (NBO/T) [6]. The NBO/T of LD glass was higher than that of the SiO2 glass, in which all the oxygen ions acted as the bridge. So low values of Tg appeared for the LD rich samples, suggesting a lower degree of depolymerization in the glass network. The crystallization temperature of Li2SiO3 (Tp1) was not detected in M1S2, whereas it was distinctly visible in the mixtures with LD additions. It indicated that the crystallization of LD glass and the growth of Li2Si2O5 crystals took place at the consumption of Li2SiO3. At temperature Tp2, Li2Si2O5 precipitated as the main phase, including the reacted crystals and the self-crystallized ones. The surface nucleation sites increased with the addition of LD glass. The greater the number of nuclei the lower the temperature at which the heat of crystallization becomes detectable and the higher the crystallization peak [31]. As a result, the Tp1 and Tp2 tended to decrease while the height of the crystallization peaks increased with the increasing LD glass. The appropriate sintering temperatures for each composition differed according to the DTA results, so several sintering temperatures around them were tested. In the following text, the individual optimized sintering condition for the each composition was used for discussion of the crystalline phases, microstructures and mechanical properties of the samples. 3.3. Phase formation Fig. 2 shows the XRD pattern of LM glass powder after crystallization at 490 1C for 2 h. It can be seen that Li2SiO3 (ICCD card 01-029-0828) was developed as the major crystalline phase, indicating that LM glass powder crystallized completely. The XRD patterns of the reaction sintered specimens with different LD contents are shown in Fig. 3. For the sample without LD glass addition (M1S2), the main crystalline phases included Li2Si2O5 (ICCD card 01-040-0376) and cristobalite (ICCD card 01-039-1425) as a precipitation of extra SiO2. This indicated that the complete reaction between LM crystals and SiO2 could take place at a higher temperature of 890 1C corresponding to the Tp2 in Fig. 1a. Minor lithium aluminum silicate (Li0.6Al0.6Si2.4O6, ICCD card 01-021-0503) also appeared due to the reaction between the LM crystals containing the impurity of Al (Table 1) and SiO2. Only Li2Si2O5 and Li0.6Al0.6Si2.4O6 phases were detected in the M1S2Dx (x 40) samples. With increasing the LD glass contents, the intensity of the major diffraction peaks of Li2Si2O5 crystals shifted
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Table 1 The chemical compositions of LM crystals and LD glass analyzed by XRF. Sample
LM LD
Composition (wt%) O
Si
La
K
Al
Cr
Fe
Zr
Ca
58.57 57.8
40.34 32.9
— 4.62
— 3.51
0.95 1.03
0.0526 0.0474
0.04 0.0441
0.0059 0.0152
0.0063 0.0172
Fig. 1. Thermal behavior of different mixtures: (a) DTA curves; (b) evolution of Tg, Tp1, Tp2 and Tm with the amount of LD glass added to the parent composition.
Fig. 2. The XRD pattern of the LM glass powder after crystallization at 490 1C for 2 h. (Li2SiO3: ICCD card 01-029-0828).
slightly due to the change of crystallinity degree and grain size, whereas the intensity of Li0.6Al0.6Si2.4O6 diffraction peaks decreased and almost disappeared in the sample of M1S2D8. For the solid phase reaction process, adequate contact of reactant particles was essential to sustain a fast reaction process [32]. Too much addition of the LD glass powder in this study could decrease the contact between the LM crystals
Fig. 3. The XRD patterns of glass–ceramics with different compositions. (Li2Si2O5: ICCD card 01-040-0376; Li0.6Al0.6Si2.4O6: ICCD card 01-0210503; Cristobalite: ICCD card 01-039-1425;) The sintering conditions were as follows: M1S2, 890 1C/1 h; M1S2D0.5, 855 1C/1 h; M1S2D1, 840 1C/1 h; M1S2D4, 820 1C/1 h; M1S2D8, 820 1C/1 h.
and SiO2 so the reaction (1) was restrained. Consequently, most of the Li2Si2O5 crystals were formed through crystallization rather than the reaction. The resulted Li2Si2O5 crystalline phase accounted for the vast majority so that minor SiO2 and Li0.6Al0.6Si2.4O6 phases could not be detected in the LD richer compositions, such as in the sample of M1S2D8.
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Fig. 4. EBSD micrographs (left panel) of Li2Si2O5 glass–ceramics and SEM morphologies with etched samples (right panel) with different compositions after reaction sintering, (a) M1S2, 890 1C/1 h; (b) M1S2D0.5, 855 1C/1 h; (c) M1S2D1, 840 1C/1 h; (d) M1S2D4, 820 1C/1 h; (e) M1S2D8, 820 1C/1 h.
3.4. Morphology Fig. 4 shows the typical EBSD micrographs of the Li2Si2O5 glass–ceramics and SEM morphologies of the etched samples with different LD contents. The crystalline phases (PC) shown in dark color and the glass phases (PG) in white color could be
easily distinguished from the EBSD micrographs (left panel). For the M1S2 specimen, it was clearly observed that the Li2Si2O5 crystals were rod-like with an average length of 10.18 μm and width of 1.66 μm. The average aspect ratio of the grains was 6.1 (as shown in Table 2 and Fig. 4a). This morphology was due to the full reaction of LM crystals and
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SiO2 powder. Some pores appeared because the SiO2 glass could not completely fill in the skeleton formed by the large grains. The porosity was as high as 6.02% (Table 2). As shown in the right panel of Fig. 4a, after etching with 5% vol HF for 1 min, the SiO2 glass in the samples dissolved and Li2Si2O5 grains with a loose structure were exposed outside. This further indicated that the SiO2 glass phase was insufficient to provide a binding in the M1S2. Compared to the M1S2 specimen, although Li2Si2O5 crystals in the M1S2Dx (x4 0) samples remained elongated shape, the grain size, glass phase content and density were quiet different. It could be clearly seen from the EBSD micrographs that the proportion of PG in white color increased with the increasing LD content and the porosity decreased (as shown in Table 2). The reason for these changes was that with the addition of LD glass powder, the viscosity of the whole reaction system reduced as stated above. This was proved in DTA curves where Tg decreased (Fig. 1b). At higher temperatures, the glass phase easily penetrated into the Li2Si2O5 crystal skeleton, so that dense microstructures with Li2Si2O5 crystals embedded in the glassy matrix were achieved. What is more, all the samples showed a bimodal grain size distribution in which both large elongated Li2Si2O5 grains and fine grains existed. The large rod-like Li2Si2O5 grains were achieved by the reaction between LM and SiO2, and the small grains were directly crystallized from the LD glass. Specifically, when the content of the LD glass was less than 1 mol, the reaction sintered elongated Li2Si2O5 crystals were dominant (see Fig. 4a–c). With further increasing the LD contents to 4 and 8 mol, large amounts of small Li2Si2O5 crystals crystallized Table 2 Length (maximum value of Lmax and average value of L), width (maximum value of Wmax and average value of W ), aspect ratio (R) and porosity of the reaction sintered specimens with different LD content. Specimens
Lmax (μm)
L (μm)
Wmax (μm)
W (μm)
R
Porosity (%)
M1S2 M1S2D0.5 M1S2D1 M1S2D4 M1S2D8
16.71 20.54 9.50 13.38 6.71
10.08 7.05 4.50 5.51 3.68
3.09 3.00 2.06 3.81 1.87
1.66 1.60 1.20 1.61 1.12
6.1 2.4 3.8 3.4 3.3
6.02 1.41 1.20 0.82 0.46
from the LD glass (as shown in Fig. 4d and e). The specific grain size and aspect ratio data are summarized in Table 2. These results showed that the average length and width of Li2Si2O5 crystals decreased from 7.05 μm to 3.68 μm and 1.6 μm to 1.12 μm, respectively, as the LD content increased from 0.5 mol to 8 mol. This was because the large amounts of the LD glass addition enhanced the self-crystallization in the LD glass powder while it restrained the contact between the LM crystals and SiO2. Thus, elongation of the Li2Si2O5 crystals from the reaction was prohibited. Hot-pressing usually results in a textured microstructure also with elongated crystals [33–35]. However, there was little difference between the morphology of the pressed plane and the plane parallel to the pressed direction of the specimen as shown in Fig. 5. It was seen that a large amount of elongated grains with high aspect ratio and small crystals were uniformly distributed in the glass matrix in both directions. Owing to the lower temperature of Tg as well as the applied axial pressure in the present study, almost complete densification could be achieved by viscous flow before the crystallization at a relatively low temperature during hot-pressing process [36]. In this case, the thermal deformation was weak. It could be concluded that the textured microstructure with elongated crystals was resulted from the reaction sintering rather than hot-pressing. The hot-pressing process was mainly contributed to the densification. The essence of the reactive crystallization mechanism could be understood through the different samples with and without LD glass contents as compared in Fig. 6. In specimen M1S2, heterogeneous Li2Si2O5 nuclei probably formed on the LM crystals by the reaction between the SiO2 glass phase and the LM crystal phase. Then Li2Si2O5 crystals grew up epitaxially at the expense of consuming the two reactants according to the reaction (1). However, the nucleation sites were limited due to restricted contact and the reaction between LM and SiO2. So Li2Si2O5 crystals could grow sufficiently to become elongated rod-like crystals with large size. Meanwhile, part of the excess SiO2 precipitated as the form of cristobalite, which was verified by XRD analysis. In the case of M1S2Dx (x4 0), besides the heterogeneous nucleation, surface nucleation occurred on the LD glass powder. Because the nucleation density in the LD glass was
Fig. 5. EBSD micrographs of Li2Si2O5 glass–ceramics sintered from M1S2D0.5 at 855 1C for 1 h, (a) the pressed plane; (b) the plane parallel to the pressed direction.
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Fig. 6. Crystallization models of the reaction sintered lithium disilicate glass–ceramics with and without LD glass contents. Li2Si2O5 nuclei; Cristobalite; Li2Si2O5 crystal.
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Li2SiO3 crystal;
SiO2 glass;
high, the linear growth speed of Li2Si2O5 crystals precipitated from LD glass decreased. On the other hand, like the M1S2, Li2Si2O5 crystals from heterogeneous nucleation and the reaction exhibited the large grain size. The microstructure consisting of crystals with bimodal grain size distributions could cause crack blunting or branching, which was advantageous in enhancing the mechanical properties [37]. For the reaction process, considering partial SiO2 crystallized during the sintering [10], the reaction (1) can be written as, LMðcrystalÞ þ 2SiO2 ðglassÞ-Li2 Si2 O5 ðcrystalÞ þ SiO2 ðglassÞþ SiO2 ðcrystalÞ
ð3Þ Thus, the whole reaction sintering process of M1S2Dx (x4 0) samples can be deduced as following LMðcrystalÞ þ 2SiO2 ðglassÞ þ xLDðglassÞ-Li2 Si2 O5 ðelongated crystalÞ þ SiO2 ðglassÞþ SiO2 ðcrystalÞþ LDðsmall crystalÞ þ LDðglassÞ
Fig. 7. Three-point flexural strength and fracture toughness of lithium disilicate glass–ceramics with different LD contents.
ð4Þ
The reaction (4) clearly presented that the fabrication process of Li2Si2O5 glass–ceramics involved in this paper, including reaction (1), (3) and crystallization of the LD glass powder itself. 3.5. Mechanical properties For the glass–ceramics, improvement of the mechanical properties can be obtained with the lower porosity, higher volume of crystalline phases and the appropriate morphology of precipitated crystals [25]. In this study, although the highest crystallinity of 68.58% was obtained for the specimens of M1S2, the lowest flexural strength and fracture toughness was seen (Fig. 7). It could be attributed to the high porosity of 6.02% resulted from the insufficient glass phase. The flexural strength and fracture toughness of the Li2Si2O5 glass–ceramics with the LD glass addition was strongly improved (Fig. 7). With increasing the LD content, the flexural strength increased
then decreased after its peak value. The highest flexural strength (350 7 13 MPa) was recorded for the M1S2D1 specimens, which was high when compared to that of Li2Si2O5 glass–ceramics obtained by powder sintering [9,10]. The improvement of strength was possibly attributed to the decreased porosity, more suitable crystallinity and high average aspect ratio of Li2Si2O5 grains, which was consistent with Cramer's research [3] that glass–ceramics with elongated grains of aspect ratios 2–6 showed excellent mechanical properties. The variation of fracture toughness with increasing the LD content was quite similar to that of flexural strength, while the highest value (3.3 7 0.14 MPa m1/2) was obtained for the samples of M1S2D0.5. Compared with M1S2, the toughness improvement in specimens with LD addition was resulted from the denser microstructure. For the reaction sintered samples in this study, the key factor for the fracture toughness was the high content of the elongated rod-like Li2Si2O5 crystals formed by reaction (3). It was similar to that
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4. Conclusions
Fig. 8. Vickers hardness of lithium disilicate glass–ceramics with different LD contents.
of self-toughening Si3N4 ceramics, in which the fracture toughness was positively correlated to the diameter and volume percentage of elongated rod-like Si3N4 crystals [29]. Moreover, the interlocking microstructure of coexisted elongated Li2Si2O5 crystals with small ones also played an important role in hindering crack propagation and toughening, providing Li2Si2O5 glass–ceramics in the present study with better fracture toughness than other powder sintered ones [9,10]. As seen in Fig. 4b, the M1S2D0.5 sample had a large amount of abnormal enlarged Li2Si2O5 grains (Lmax of 20.54 μm, and Wmax of 3.0 μm), which contributed to the highest toughness. However, with the continued addition of LD glass, the number of large elongated crystals decreased and the grains were refined (see Fig. 4), which was caused by the inhibited reaction between the LM crystals and SiO2 along with the increased small crystals directly precipitated from the LD glass. This resulted in the fracture toughness decrease. It also explicated that Li2Si2O5 glass–ceramics without any toughening phase always has poor fracture toughness and the method in this study provides the possibility for toughening with elongated rod-like crystals. Hardness is also considered an important property when judging the abrasiveness of restorative materials. It is a measure of the resistance to permanent surface indentation or penetration [38]. In the present study, we also investigated the Vickers hardness of the reaction sintered Li2Si2O5 glass– ceramics as shown in Fig. 8. It can be seen that with increasing the LD content, the hardness increased then decreased and the highest value (5.92 7 0.18 GPa) was recorded for the M1S2D0.5 specimens. The tested hardness of bulk Li2Si2O5 glass ( 500 MPa) was much lower than that of Li2Si2O5 crystalline phase (several GPa). Consequently, the hardness variation of Li2Si2O5 glass–ceramics is mainly attributed to the different crystallinity, while the lower hardness in M1S2 is caused by the high porosity. The hardness values ranged from 5.48 GPa to 5.92 GPa in this study are in good agreement with results found in previous literatures [38,39].
Dense Li2Si2O5 glass–ceramics with bimodal microstructures in which some large elongated Li2Si2O5 grains with high aspect ratio and fine grains embedded in the glass matrix was successfully fabricated by hot-pressing sintering. The elongated rod-like Li2Si2O5 crystals with large grain size, heterogonous nucleation mechanism controlled, were obtained from the reaction between LM crystals and SiO2 glass. And the fine grains were directly crystallized from the LD parent glass. With increasing the LD content from 0 mol to 8 mol, the temperature of Tg, Tp1, Tp2 and Tm decreased, the intensity of the diffraction peaks of Li2Si2O5 varied slightly and the average grain size decreased. Although the crystallinity decreased with increasing the LD amount, the Li2Si2O5 glass–ceramic exhibited high flexural strength (350713 MPa, M1S2D1), high hardness and fracture toughness (5.9270.18 GPa and 3.370.14 MPa m1/2, M1S2D0.5). The interlocking microstructure contributed to high strength, the high crystallinity resulted in good hardness and the reaction sintered elongated Li2Si2O5 crystals led to high toughness. A technique such as the one presented here offers a novel route for preparing dental restorative materials. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 51272205, 51072157, 51302208, and A3 Foresight Program-51161140399), by the High-Tech R & D Program of China (863, no. 2011AA030102), and by Doctoral Fund of Ministry of Education of China (Grant no. 20100201110036 and 20130201120003), and by Science. References [1] W. Höland, G.H. Beall, Glass–Ceramic Technology, second ed., John Wiley & Sons, Inc., New Jersey, America, 2012. [2] D. Holland, Y. Iqbal, P. James, B. Lee, Early stages of crystallisation of lithium disilicate glasses containing P2O5–An NMR study, J. Non-Cryst. Solids 232-234 (1998) 140–146. [3] S.C. von Clausbruch, M. Schweiger, W. Höland, V. Rheinberger, The effect of P2O5 on the crystallization and microstructure of glass–ceramics in the SiO2–Li2O–K2O–ZnO–P2O5 system, J. Non-Cryst. Solids 263 (2000) 388–394. [4] E. Apel, C. van't Hoen, V. Rheinberger, W. Höland, Influence of ZrO2 on the crystallization and properties of lithium disilicate glass–ceramics derived from a multi-component system, J. Eur. Ceram. Soc. 27 (2007) 1571–1577. [5] X. Zheng, G. Wen, L. Song, X.X. Huang, Effects of P2O5 and heat treatment on crystallization and microstructure in lithium disilicate glass ceramics, Acta Mater. 56 (2008) 549–558. [6] H.R. Fernandes, D.U. Tulyaganov, A. Goel, M.J. Ribeiro, M.J. Pascual, J.M.F. Ferreira, Effect of Al2O3 and K2O content on structure, properties and devitrification of glasses in the Li2O–SiO2 system, J. Eur. Ceram. Soc. 30 (2010) 2017–2030. [7] H.R. Fernandes, D.U. Tulyaganov, J.M.F. Ferreira, The role of P2O5, TiO2 and ZrO2 as nucleating agents on microstructure and crystallization behaviour of lithium disilicate-based glass, J. Mater. Sci. 48 (2013) 765–773. [8] D.H. Xiong, J.S. Cheng, H. Li, Composition and crystallization kinetics of R2O–Al2O3–SiO2 glass–ceramics, J. Alloys Compd. 498 (2010) 162–167.
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