Self-reinforced lithium disilicate (Li 2 Si 2 O 5 ) glass-ceramics were hot pressing sintered by introducing 5 wt% Li 2 Si 2 O 5 crystal seeds into two different glass systems. The results show that the seeds play an important role in the crystallization inducement, microstructural and properties improvement of the glass, especially for the glass powder without the nucleating agent of P 2 O 5 . The microstructure features a wider bimodal grain size distribution with large rod-like crystals epitaxially grown along the seeds and small crystals nucleated from the glass powder itself, contributes to the improvement of the performance especially the fracture toughness. The 4C5S specimen exhibited the best comprehensive properties with a good flexural strength (396 ± 7 MPa), improved fracture toughness (3.31 ± 0.19 MPa·m 1/2 ) and comparable translucency as IPS e.max. This research will provide a new idea and method for the perfect matching of the fracture toughness and the aesthetic appearance of lithium disilicate glass-ceramics, and lay the foundation for its clinical applications.
Research Article
Improved performances of lithium disilicate glass-ceramics by seeds induced crystallization
https://doi.org/10.21203/rs.3.rs-48775/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 03 Mar, 2021
Read the published version in Journal of Advanced Ceramics →
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Lithium disilicate glass-ceramics, as a new kind of dental restorative materials, has many incomparable advantages with the traditional metal materials, macromolecule materials and ceramics, such as high mechanical properties (flexural strength ~300-400 MPa), comparable wear resistance, aesthetical characteristics as natural teeth, good machinability and so on [1-6]. Because of which, it has gained increasing attention in the latest research. However, due to its intrinsic brittleness and low defect tolerance, the fracture toughness of Li2Si2O5 glass-ceramics is far less than that of zirconia, alumina and other traditional ceramics restorations [7-9]. Therefore, how to improve the fracture toughness on the basis of maintaining good aesthetics has become an urgent research topic, which has a very important practical significance for its large-scale application in the field of dental restoration.
However, there are few reports on the toughening of lithium disilicate glass-ceramics. The most common method is to introduce zirconia into the glass composition, by using its transformation toughening effect [10-12], glass-ceramics can be reinforced [13, 14]. Huang et al. [10] successfully prepared t-ZrO2 (3Y-TZP) toughened lithium disilicate glass-ceramic by vacuum hot-pressing sintering, with the highest strength of 340 MPa and fracture toughness of 3.50 MPa·m1/2. However, the addition of zirconia also had some adverse effects such as the difficulty of uniform distribution and the obstacle to densification, making the porosity as high as 1.90%. Also toughening of other second phases such as whisker , SiC nanowire, alumina, carbon fiber were involved to toughen glass-ceramics [15-19]. Although the toughening mechanisms of heterogeneous phase transformation, crack deflection, crack bridging and pull-out effect were analyzed [20], the effects of these toughening measures on the translucency of glass-ceramics were not explicitly mentioned. In addition, the introduction of heterogeneous toughening phases has some problems, such as difficulty in densification and the matrix powder can not filling the fiber skeleton uniformly without damaging the fibers. It will inevitably lead to strong lights scattering and refraction caused by non-uniformly dispersed second phase, heterogeneous interface and mismatch of the refractive index, which will decrease the translucency of lithium disilicate glass-ceramics and make it difficult to match the appearance of natural teeth.
Considering the shortcomings of the toughening by heterogeneous phases mentioned above, and the fact that Li2Si2O5 crystalline phase and its glass phase have nearly the same refractive index [21], many researchers tried to improve the fracture toughness and translucency of Li2Si2O5 glass-ceramics by using different nucleating agents and changing the heat treatment processes (casting, sintering, etc.)[22-27]. However, the effect is not satisfactory. The reason is that no matter the overall crystallization (melting method) or surface crystallization (powder sintering method) applied, all the crystalline phases are from the crystallization of parent glass. The nucleation number is very large so the precipited Li2Si2O5 crystals usually display small grain size with a narrow size distribution, which form more interfacial area with the glass matrix [23, 27-29]. These interfaces not only easily generate more flaws acting as a cracking source, but also shorten the crack propagation path, limiting the improvement of fracture toughness[30]. At the same time, many small interfaces can not effectively reduce the scattering and refraction of visible light, making the translucency poor. Therefore, for the homogeneous toughening of lithium disilicate glass-ceramics, it is necessary to control the number of nucleation sites and the size of crystals, that is, crystallization controllable.
In our previous studies [31, 32], the coexistence of large-sized rod-like Li2Si2O5 and small-sized LD grains formed by solid state reaction of LM crystal and SiO2 glass can effectively improve mechanics performance, especially the fracture toughness of Li2Si2O5 glass-ceramics. Seeds toughening is one of the most effective methods to obtain this kind of microstructure and it has been widely used in traditional ceramic materials such as Si3N4 [33, 34], Al2O3 [35] and SiAlON [36]. It is known that the so-called in-situ growth is to make some grains grow into elongated grains with high aspect ratio by controlling the sintering process, thus obtaining the microstructure similar to that of fiber or whisker reinforced composites [37]. This microstructure with a bimodal grain size distribution can play a good role in deflection and bridging cracks to improve the fracture toughness and strength of materials, while basically not affecting other properties [38, 39].
However, few reports concerning the seeds toughening in glass-ceramics have been mentioned. In this paper, Li2Si2O5 seeds were added to different glass systems to induce crystallization and help crystals epitaxially grow to large-sized rod-like grains, while the parent glass powders with high surface energy precipitate small-sized rod-like crystals through spontaneous surface nucleation, thus forming the coexistence of multi-scale crystals, as shown in Fig. 1. The effect of Li2Si2O5 seeds on the microstructure, mechanical properties and translucency characteristics of lithium disilicate glass-ceramics was also discussed. We believe that marrying the very good aesthetics with good mechanical properties may help open the area of restorative application of Li2Si2O5 glass-ceramics, and provides a new way for preparing toughened glass-ceramics.
2.1. Synthesis of rod-like Li2Si2O5 seeds
99.9% purity Li2CO3 powder and SiO2 powder were used as the starting materials, and Li2Si2O5 seeds were solid-state reaction synthesized according to the following reaction: Li2CO3 + 2SiO2 = Li2Si2O5 + CO2. The mixture of Li2CO3 and SiO2 in molar ratio of 1:2 were ball-milled for 5 h and dried by using a rotatory evaporator. Then the powders were placed in a corundum crucible and underwent a two-step heat treatment in an electric furnace. The mixture was firstly heated at 710 ºC for 2 h, and then the temperature was raised to 980 ºC for 4 h at a heating rate of 3 °C∙min-1. After that, the heated powders were milled, dispersed and acid washed at 1 vol.% HF to remove the residual glass phase.
2.2. Preparation of Li2Si2O5 glass-ceramics
In order to explore the controlled crystallization effect of seeds on different glass systems, two Li2Si2O5 glass powders were prepared and the compositions are shown in Table 1. They have the same molar ratio of SiO2:Li2O = 2.39:1 to ensure that the main crystalline phase is Li2Si2O5 [40, 41]. Besides that, 4C LD glass only contains K2O to promote the glass melting process [42] and La2O3 to decrease the viscosity of the glass [43], while removing the nucleating agent of P2O5 compared with 7C LD glass. Reagent grade powders of Li2CO3, SiO2, NH4H2PO4, ZrO2, Al2O3, K2CO3 and La2O3 were used as the raw materials. After mixing, melting (at 1450 ºC for 2 h), quenching and ball-milling (10 h, with high purity zirconia balls in an ethanol environment), the glass powders with D50 values of ~ 5 μm were obtained. The detailed process is the same as our previous studies [31, 32].
Table 1 The compositions of Li2Si2O5 glass (mol.%)
|
|
Li2O |
SiO2 |
P2O5 |
ZrO2 |
Al2O3 |
K2O |
La2O3 |
|
7C LD |
27.6 |
66.0 |
0.8 |
1.4 |
1.8 |
1.8 |
0.6 |
|
4C LD |
28.6 |
68.6 |
-- |
-- |
-- |
2.0 |
0.8 |
The starting powder mixtures containing 4C or 7C LD glass and 5 wt.% Li2Si2O5 seeds were wet mixed with ZrO2 balls in anhydrous alcohol for 2 h. The slurry was dried, uniaxially pressed (30 MPa in a hardened steel die, Φ30 mm × 10 mm), and presintered in a furnace (SN33810297, DeguDent GmbH, Germany) at 800 °C for 20 min. After that, the middle compacts were hot-pressing sintered in a vacuum furnace at 850 °C for 1 h under a pressure of 20 MPa. Finally, after cooling down to ambient temperature, the surface layers of the samples were removed.
2.3. Characterization
To evaluate the densification degree, the bulk density was measured by using archimedes method and from that the relative density was calculated. Three samples were estimated to get the mean value. Crystalline phases were identified by X-ray diffraction analysis (XRD, D/MAX-RA, Rigaku, Japan) with Cu Kα radiation and scanning from 10º to 70º. The sampling interval was 0.02º 2θ. The crystallinity was calculated by the locations of the diffraction peaks, their number and relative intensity in the XRD pattern according to Equation (1) [28, 44].
(1)
Electron backscatter diffraction (EBSD, Phenom Pro-X, Netherland) and field emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan) were used to characterize microstructures and phases of the unetched and etched samples (5 vol. % HF for 1 min), respectively. The grain size was counted and analyzed. More than 200 grains were chosen for each sample. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) investigations were performed using a JEM 200CX microscope with 120 kV electron beam. For the preparation of TEM specimen, the disc specimens were first ground to about 20 μm thick, then polished and followed by ion beam thinning (Gatan Model 691 PIPS).
Rectangular bars (3 mm × 4 mm × 20 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). 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. For the determination of optical properties, the required fine surface and uniform thickness (0.8 mm in thickness) of the specimens were obtained by polishing with wet silicon carbide papers. Translucent characteristics of the final glass-ceramics were evaluated in term of contrast ratio (CR) [21], which is defined as the ratio of illuminance of the material when it is placed over a black background to the illuminance of the same material when it is placed over a white background.Illuminance values were tested using a colorimeter and CR values were calculated, which is 0.0 for a transparent material and 1.0 for a totally opaque material.
3.1. Characterization of Li2Si2O5 Seeds
Fig. 2 shows the SEM images and XRD patterns of the Li2Si2O5 seeds synthesized by solid-state reaction. In Fig. 2(a), the seeds are rod-like crytsals with a length of 1.2-6.5 µm and a width of 0.1-1.0 µm. The average length is ~2 µm and the aspect ratio of the seeds is 3~6. The XRD analysis in Fig. 2(b) indicated the existence of Li2Si2O5 crystalline phase and no other secondary phase, confirming the reaction of Li2CO3 and SiO2 is complete. And most of the glass phases were removed after acid rinse treatments. Thus, the synthesized high-quality Li2Si2O5 seeds provide the development of the self-toughed Li2Si2O5 glass-ceramics microstructure with elongated grains.
3.2. Phase formation
Fig. 3 shows the XRD patterns and crystallinity of the Li2Si2O5 glass-ceramics from 4C and 7C glass powders without and with 5 wt.% Li2Si2O5 seeds. In Fig. 3(a), it can be found despite the difference in composition, the main crystalline phase of all the samples was only Li2Si2O5 (ICCD card 01-040-0376). The intensity of the Li2Si2O5 diffraction peaks varied slightly, indicating the crystallinity changed. In Fig. 3(b), it can be seen that for the two kinds of glass powders with different composition, by adding 5 wt.% Li2Si2O5 seeds helped the crystallinities of the samples increased by ~10%.
3.3. Morphology
Fig. 4 shows the SEM morphologies of the etched Li2Si2O5 glass-ceramics with different compositions. All the specimens showed a closely packed and multi-directionally interlocked microstructure of numerous rod-like Li2Si2O5 crystals protruding from the glass matrix. However, the crystals morphology especially the grain size are quite different, and the statistical results are clearly shown in Table 2. It is found that the Li2Si2O5 crystals in 7C and 7C5S samples have smaller grain size but larger aspect ratio, making them typical long rod-like crystals. Another interesting phenomenon is that the grain size of 4C and 7C samples are more uniform and no larger grain is found. While in the seeds addition samples of 4C5S and 7C5S, some larger grains with high aspect ratio embedded in a smaller grained matrix was observed. It is consistent with the microstructural design (in Fig. 1) that large rod-like crystals was believed to result from epitaxy growth of the Li2Si2O5 seeds while the small grains are grown from the spontaneous nucleation of the glass powders. The bimodal distribution morphology is more obvious in 4C5S so that the average grain size increased.
Table 2 The average length (
), width (
) and aspect ratio (
) of the Li2Si2O5 glass-ceramics with different compositions.
|
Specimens |
4C |
4C5S |
7C |
7C5S |
|
|
2.94 |
3.34 |
1.88 |
2.29 |
|
|
0.86 |
0.87 |
0.32 |
0.43 |
|
3.41 |
3.83 |
5.88 |
5.37 |
To further explore the variation of the grain size caused by the seeds addition in different glass powders, the grain size distribution in length and width directions of the samples is statistical analysized, as shown in Fig. 5. Compared to 7C, the grain size of 7C5S sample increased slightly both in the direction of length and width and apparently a small number of large grains appeared. Nevertheless, the overall grain size distribution of 7C5S was still concentrated. By contrast, the grain size of 4C5S sample shows a much wider bimodal distribution with the length covers from 2 to 8 μm and the width in the range of 0.5-1.5 μm. And the aspect ratio is also larger than that of 4C.
To further understand the microstructure with a bimodal grain size distribution caused by the heterogeneous nucleation of Li2Si2O5 crystal seeds, a specimen of 4C5S was subjected to TEM investigation. The bright field TEM micrographs and their related electron diffraction patterns are shown in Fig. 6. In Fig. 6(a), it shows a large crystal with the length up to ~ 6 μm and some smaller grains embedded in the glass matrix. The corresponding diffraction pattern of this large crystal in Fig. 6(b) reveals that the crystal was oriented with the [110] plane parallel to the beam direction. And based on that, the calibrated diffraction pattern matches that of a single crystalline Li2Si2O5 lattice. Thus, the TEM analysis confirmed that crystalline phase in sample 4C5S is lithium disilicate. The diffraction pattern in Fig. 6(c) indicated that the matrix was amorphous. More often than not, the thinnest part collapsed dramatically if the focused electron beam was on (the region marked in red shown in Fig. 6(d)), indicating that the local heat was so high that the specimen was rapidly perforated. So the epitaxial growth mode of large crystals along the seeds has not been observed yet in the TEM investigation.
3.4. Mechanical properties
Fig. 7 shows the relative density, three-point flexural strength and fracture toughness of the Li2Si2O5 glass-ceramics with different compositions. It can be seen that the relative densities are all above 98.80%, indicating the densification degree is very high. The relative density decreased with the addition of seeds and the highest relative density up to 99.72 ± 0.09% was obtained in the 4C sample. The flexure strength of 4C5S, 7C and 7C5S samples are very close to each other, all of which are approximately 400 MPa. The lowest flexural strength of 356 ± 11 MPa was recorded for the 4C specimen. It can be concluded that by adding 5 wt.% seeds, the strength of 4C glass system increased by 11% while that of 7C glass system was not improved. As for the fracture toughness, the values of the both two glass systems increased with the addition of Li2Si2O5 seeds. The toughness of 7C5S sample increased by 15% compared with 7C, reaching the maximum value of 3.48 ± 0.17 MPa·m1/2, and that of 4C5S samples increased by 10% compared with 4C, reaching 3.31 ±0.19 MPa·m1/2.
3.5. Translucency characteristics
For dental restorative materials, in addition to the good mechanical properties, they also need to have a certain degree of translucency to simulate the gloss of natural teeth. Fig. 8 compares the contrast ratio and appearances of the sintered samples with the commercially used IPS e.max (Ivoclar-Vivadent, casting-crystallization treatment at 850 ºC for 1 h). The contrast ratio of the specimens in this study was in a range of 0.47-0.68. All the samples except the 7C5S specimen showed clearly underlying black-and-white backing, indicating a high visual degree of translucency.
Regardless of the differnent glass systems with or without the seeds addition, Li2Si2O5 was detected as the only crystalline phase in Fig. 2. This is because the composition of the original glass powders are very close to that of the added seeds, which satisfies the requirements of Li2O:SiO2≈1:2.4 to easily form Li2Si2O5 crystals without other impurities. The added Li2Si2O5 seeds could act as nuclei sites, so the simultaneous effect of homogeneous nucleation from the glass powders and nucleation induced by the seeds increased the degree of crystallization. In addition, P2O5 in the 7C LD glass powder can also promote phase separation nucleation [23] so the crystallinities of 7C and 7C5S were higher than that of 4C and 4C5S, respectively. It is worth noting that the crystallinity of 4C5S is 64.0% is higher than that of 7C (60.2%), which means that the seeds can completely replace the nucleating agent of P2O5 in promoting crystallization of the glass powder without nucleating agent, and the effect is even better.
The addition of P2O5 in 7C glass as the nucleating agent can promote the nucleation and increase the nucleation number, but cause a decrease in the linear growth rate of crystals [23]. Therefore, the grain size of 7C and 7C5S samples decreased. While in the 4C glass powder without P2O5, the mechanism of phase separation nucleation is eliminated so that the nucleation rate decreased resulting in an increase in the grain size of Li2Si2O5 crystals, especially in the direction of width. Actually Li2Si2O5 crystal is layered silicate structure, due to its higher growth rate along the c-axis direction, ‘long flake’ grains piled up layer by layer so that the anisotropic rod-shaped morphology was formed [45]. In the 4C and 4C5S samples, the number of layers in the width direction increased. Consequently, the lamellar morphology was more obvious and the aspect ratio decreased. While for the 7C glass already containing P2O5, by adding seeds increased the nuclei number and the nucleis will grow in competition with each other, which is not conducive to the formation of large size grains. Therefore it can be concluded that the addition of Li2Si2O5 seeds is more effective for improving the microstructure of the glass without the nucleating agent. The TEM observation of 4C5S in Fig. 6 is consistent with the EBSD observation on the microstructure (the inset of Fig. 5(b)), and it is believed that such a microstructure consisted with large crystals induced by seeds embeded in small crystals may contribute to the enhancement of mechanical properties especially the fracture toughness.
The high density in powder sintered glass-ceramics played an important role in the improvement of mechanical properties and translucency. For the sintering of glass-ceramics, both the crystallization and the densification processes tend to meet the simultaneous motivation to decrease the free energy of glass powder [23]. The relative density decreased with the addition of seeds, which is due to the fact that on one hand, the crystalline phase increases the viscosity of the glass system so that hinders the flow of liquid phase in the sintering process to make the pore difficult to drain out. On the other hand, seeds promoted the nucleation and increased the crystallinities as shown in Fig. 3(b), leading to a high viscosity hinders densification. Moreover, the relative density of 4C and 4C5S were higher than that of 7C and 7C5S, respectively. It is because the 7C glass system has the nucleating agent of P2O5, which could promote phase separation nucleation and increase the crystallinities. Consequently, it can be concluded that the variation of relative density was closely related with the crystallinity, the higher crystallinity and the lower relative density.
Besides the higher relative density, improvement of the mechanical properties of the glass-ceramics can also be obtained with the higher crystallinity and the appropriate morphology of precipitated crystals [4, 46, 47]. From the above we can conclude that the mechanical properties are improved even when the density decreased with the addition of seeds, indicating the unfavorable influence of seeds on the densification can be minimized and made up by microstructure adjustment. For the 4C LD glass, because of its dependence only on surface nucleation, its crystallinity is not high. Moreover, the grain size distribution is concentrative and the aspect ratio is small (see Fig. 4 and Fig. 5), which can not effectively prevent the crack propagation. The effect of these factors is that despite having a high relative density, the strength and fracture toughness of 4C specimen are not ideal. By contrast, the increased strength of 4C5S specimen was mainly due to its increased crystallinity and higher aspect ratio of crystals. Because the thermal expansion coefficient of Li2Si2O5 crystalline phase is larger than that of the glass matrix, and thereby the crystals could develop a significant residual compressive stress in the surface of the glass during the cooling process, which strengthens the glass-ceramics [45]. The higher the crystallinity, the more obvious the strengthening effect is. The increased fracture toughness of 4C5S sample might be related to the bimodal grain size distribution and large rod-like grains, which was ascribed to grain bridging and crack deflection. As for the 7C specimen, the high crystallinity, small grain size and large aspect ratio contributed to the excellent mechanical properties. When seeds are added, the density decreased but the crystallinity increased, making the flexure strength remains unchanged. And appearance of few large grains can effectively improve the fracture toughness.
It is reported that the translucency of glass-ceramics is highly dependent on light scattering, and is closely related to the type and the size of the precipitated crystals, the crystallinity and the porosity [4]. For Li2Si2O5 glass-ceramics, the refractive index of the crystalline phase and the parent glass are 1.55 and 1.50 [21], respectively, which are very close to each other. The grains size of the samples obtained in this study are all larger than 1 μm, it is beyond the range of strong light scattering (0.39-0.78 μm) so the scattering effect of the type and size of crystals could be negligible [4]. Therefore, the porosity and crystallinity are the main factors affecting the translucency. The 7C5S sample shows a low relative density (98.92 ± 0.22%) and a high crystallinity up to 72.08 ± 2.50%, which means the pores become the strong light scattering centers and the glass phase does not meet the requirement (30-40 vol.%) for dental restorative glass-ceramics [4, 21]. Thus, its translucency is poor. By contrast, owing to the higher relative density (＞99.10%) and lower crystallinity (＜64%), the other three groups of samples show a high translucency and their appearance are very close to the IPS e.max. From them we can clearly see the black and white background, and also they have a certain shading ability, similar to many dental restorative materials reported in the literature [7, 21].
Table 3 shows a comparison of the reported properties of lithium disilicate glass-ceramics and the results of the current study. It can be seen that different preparation methods lead to different results. The comprehensive properties in this study was comparable to the heat pressing IPS Empress 2 reported by Hölland et al.[48], but better than other listed materials especially the hot-pressing sintered ones. The point is that compared with other methods, it is easier to add pigments to the glass powder during sintering, which can impart the end product with tooth-like optical properties. In addition, from the energy saving point of view, it is helpful to reduce the melting temperature of the glass system without adding the refractory nucleating agents. Consequently, the seeds was demonstrated to have great potential for improving the properties of glass-ceramics, and the sintered 4C5S specimen features a high potential as dental restorative materials.
Table 3
Comparison of the properties of lithium disilicate glass-ceramics between the published papers and the current study
|
Authors |
Processing |
Flexural strength (MPa) |
Fracture toughness (MPa·m1/2) |
Translucency |
|
our work |
Conventional hot-pressing technology |
σ = 396 ± 7 (three-point) |
KIC = 3.31 ± 0.19 (SEVNB) |
CR = 0.47 ± 0.02 (0.8 mm thickness) |
|
Ref. [7] |
IPS e.max |
σ = 462 ± 15 (four-point) |
KIC = 3.1 ± 0.2 (Double-torsion technique) |
CR = 0.28 ± 0.01 (0.5 mm thickness) |
|
Ref. [10] |
Conventional hot-pressing technology |
σ = 340 ± 38 (three-point) |
KIC = 3.5 ± 0.3 (SENB) |
— |
|
Ref. [5] |
Melting and two-stage heat treatment |
σ = 562 ± 107 (three-point) |
KIC = 3.5 ± 0.1 (SENB) |
— |
|
Ref. [23] |
Conventional hot-pressing technology |
σ = 290 ± 10 (three-point) |
KIC = 3.3 ± 0.1 (SENB) |
— |
|
Ref. [27] |
Heat-pressed + two stage heat treatment |
σ = 308 ± 25 (three-point) |
— |
TP = 21.2 ± 0.4 (0.5 mm thickness) |
|
Ref. [17] |
Heat pressing LD GCs |
σ = 303 ± 49 (three-point) |
KIC = 3.0 ± 0.65 (Indentation method) |
— |
|
Ref. [48] |
Heat pressing IPS Empress 2 |
σ = 400 ± 40 (three-point) |
KIC = 3.3 ± 0.3 (SENB) |
CR = 0.55 (1 mm thickness) |
|
Ref. [4] |
Melting and two-stage heat treatment |
σ = 310 (three-point) |
— |
CR = 0.51 (0.5 mm thickness) |
SENB: Single edge notched beam.
TP: Translucency parameter.
CR: Contrast ratio.
A high-performance lithium disilicate glass-ceramics was successfully hot-pressed by adding Li2Si2O5 crystal seeds to different two glass systems. After adding seeds, the crystallinity increased, the relative density decreased, and the samples show a bimodal microstructure in which some large elongated Li2Si2O5 grains with high aspect ratio epitaxial growed along the seeds, and fine grains directly crystallized from the parent glass powder. The role of heterogeneous nucleation by seeds was more effective in the 4C glass powder without the nucleating agent of P2O5, so the grain size distribution of 4C5S sample was more dispersed. Accordingly, although the 4C5S specimen did not possess the highest relative density (99.13 ± 0.07%), it demonstrated the best comprehensive properties with a good flexural strength (396 ± 7 MPa), improved fracture toughness (3.31 ± 0.19 MPa·m1/2) and comparable translucency as IPS e.max. The high crystallinity and interlocking microstructure contributed to high strength, the epitaxial growed elongated Li2Si2O5 crystals along the seeds led to high fracture toughness, and the proper content of glass phase helped gain a good translucency. The high-performance glass-ceramics with seeds is promising for dental restorative applications especially three-unit posterior bridges, and offers a novel route for preparing toughened glass-ceramics without sacrifice other properties.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51702193, 51502165), the General Project in Industrial Area of Shaanxi Province (Grant No. 2020GY-281), the Natural Science Foundation of Shaanxi Provincial Department of Education (Grant No. 20JK0525), the Shaanxi Provincial Education Department serves Local Scientific Research Plan (Grant No. 20JC008) and the Scientific Research Fund of Shaanxi University of Science & Technology (Grant No. BJ16-20, BJ16-21).
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