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This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.
The digital dentistry, requires materials with wo opposite properties of machining ability and also enough hardness. The main objective of this experimental study was to investigate the fabrication feasibility of the lithium metasilicate glass-ceramic in partially crystalized stated using the spark plasma sintering (SPS) method.
In this study, SPS for the first time was used to fabricate primary lithium metasilicate glass-ceramic (LMGC) blocks. The raw materials were mixed and melted and then quenched in water and the resulted frits were grinded. The resulting powder was sintered by SPS at 660, 680, and 700°C.
Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and Vicker's microhardness assay were used to evaluate the properties of samples. Statistical comparison of the obtained data was performed by ANOVA, followed by the post hoc test of Duncan. Microstructural studies by SEM and XRD showed that all samples were composed of lithium metasilicate phase in a glassy matrix. With increasing the sintering temperature, the number and size of lithium metasilicate particles increased and higher mechanical properties have been achieved. However, the sintered sample at 700°C has less processing ability than the samples sintered at 660 and 680°C.
The optimum sintering temperature for glass frit consolidation was determined by SPS at 680°C.
Lithium disilicate glass ceramics (LDGCs) are good candidates for computer-aided design and computer-aided manufacturing (CAD/CAM) of dental restoration.
Ye et al.
SPS is an advanced technique of powder metallurgy which uses electric current and pressure for the consolidation and compaction of materials. In comparison with conventional powder metallurgy methods, SPS can heat the sample rapidly because the heat is generated by the electrothermal effect and distributed only within the die-punch system rather than throughout the whole furnace chamber. SPS is a relatively new powder metallurgy method for the rapid fabrication of advanced materials, and this technique can endow materials with outstanding properties.
To the best of our knowledge, fewer studies were performed for the fabrication of lithium metasilicate in the PC state, and in the most of research, the lithium disilicate was processed directly to the FC state.
In the case of chemical composition, some constituents are added to the parent glass to improve the properties of the final parts. For example, the P
2O
5acts as a nucleating agent, and its increasing, shifts the crystallization temperatures toward higher values.
In this research, the blocks of partially crystallized lithium metasilicate glass-ceramic were fabricated using high-pressure SPS for the first time. To avoid lithium disilicate formation (which have high hardness and have not machining ability), we have to try sintering at fewer temperatures. Hence, SPS at high pressure has been employed for the first time. The SPS method is used to consolidate the powders at fewer temperatures than conventional powder metallurgy routes or melting technology. Furthermore, while the powders are in compaction pressure during sintering, so fewer residual micro pores will form in the matrix.
Glass fabrication
In this experimental study, we have tried to fabricate LMGC in PC state to have machining ability,
The glass including the molar composition of 65.0 SiO
2-27.5 Li
2O-2.0 P
2O
5-2.0 K
2O-2.0 Al
2O
3-1.5 ZrO
2was fabricated by melting raw materials of SiO
2, Li
2CO
3, Al (PO
3)
3, K
2CO
3, Al
2O
3, and ZrO
2(which all of them were purchased from Merck company) in a platinum crucible at 1450°C for 1 h. The molten glass was poured in distilled water for rapid cooling and glass frit formation.
Glass-ceramic block fabrication
SPS technique was used for glass powder densification and partially crystallization. The frit powder was poured in a graphite mold (cylinder shape with diameter and height of 10 mm) and placed in SPS machine (Chakad Sanat Spadan, Iran) at an initial pressure of 20 MPa. Sintering of precompacts was performed by increasing the temperature at a heating rate of 100°C/min, up to 660, 680, and 700°C under vacuum conditions (1.33 Pa), while the final temperature and final pressure of 200 MPa were applied for 5 min. While the thermal shock can affect the mechanical properties of the samples, so, the cooling rate was set on 3°C/min to room temperature to prevent thermal shock.
Sample characterization
Differential scanning calorimetry
A calorimeter (Netzsch DSC 404F1 Pegasus) was used to study the thermal properties of the glass frit. The sample was heated from room temperature to 1000°C with a heating rate of 5°C/min to study the thermal behavior and detect any phase transformation phenomenon of lithium disilicate glass.
Phases analysis
X-ray diffraction (XRD: Philips X'Pert-MPD, Netherlands) was measured to identify the phases present in the glass frit and the sintered samples. X-rays were generated using the Cu-Kα lamp with the wavelength of 1.54060 A ◦in the range of 20°–80°. Quantitative analyses (Rietveld method) were used to determine the weight percent of phases (crystallinity) and also the Delf model of Rietveld was used to calculate the crystallite size of the crystalline phase using MAUD software.
Microstructural study
After polishing one surface of sintered samples using SiC abrasive paper to grit number of 2500, samples were etched using HF 3 wt.% solution (to dissolve the glassy matrix
Mechanical properties
The indentation test was performed using a Vicker's hardness tester (FM-700, Future-Tech Corp, Kanagawa, Japan) using a square-based pyramidal shape with a tip angle of 136° diamond indenter and a load of 1 kg and 10 s dwell time, in accordance with the ASTM E384-17 standard, to calculate the hardness and fracture toughness of samples (Equations 1 and 2). Each test was repeated five times and the average values were reported.
Hv = 0.1891F/d 2(1)
K 1C= 0.016 × (E/Hv) 0.5 × (F/C1.5) (2)
where, Hv is the Vickers hardness, F the applied load (N), d the half-diagonal length left by the indenter (mm), K
1
Cthe fracture toughness (MPa. m1/2), E the modulus of elasticity (GPa), and C the mean half-length of the radial cracks from the impression center (mm).
Statistical analysis
Statistical comparison of the obtained data was performed by ANOVA, followed by the post hoc test of Duncan. For the performed tests, the number of repetitions was considered three, and the results were reported as the mean ± standard deviation (SD).
The quenched frits were transparent, like glass. It is evident that the samples are glassy (noncrystalline). However, after sintering, they became opaque, may be because of crystalline phase formation
The images of (a) glassy frit, (b) partially crystallized sample using SPS at 680 C.
Thermal analysis
DSC curve of the quenched frit. DSC: Differential scanning calorimetry.
Base on the result of DSC and previous literature,
Phases analysis
The XRD pattern of the frit (quenched molten glass in water) and sintered samples at 660, 680, and 700°C are presented in
XRD patterns for lithium silicate frit and sintered samples at 660, 680, and 700°C. XRD: X-ray diffraction.
SEM observation
SEM micrographs for sintered samples at (a) 700°C, (b) 680°C, and (c) 660°C.
Mechanical properties
The Vickers hardness of sintered samples at 660, 680, and 700°C were measured; and the mean values and SDs are 645 ± 8, 684 ± 7, and 691 ± 10, respectively. Furthermore, the fracture toughness of sintered samples at 660, 680, and 700°C are calculated (using equation 2) to be 2.23 ± 0.12, 2.45 ± 0.13, and 2.97 ± 0.15, respectively.
Bai et al.
It seems quenching the molten glass in water, inhibited any crystallization during cooling, and the resulted a glassy frit formation. It is evident that the cooling rate was so high enough, to inhibit Li 2SiO 3and Li 2Si 2O 5nucleation and growth during quenching the molten glass from 1450°C into the water.
The diffraction patterns of all sintered samples using SPS at 660, 680, and 700°C included the presence of peaks related to lithium metasilicate (Li 2SiO 3: PDF No.: 01-072-1140) and there were no extra detectable phases. However, the height of sharp peaks of lithium metasilicate related to the broad and wide peak (2 θ = 20°–30°) is increased by increasing the sintering temperature. Actually, by increasing the sintering temperature, more portion of the glassy matrix is transformed to the crystalline lithium metasilicate phase. The height of sharp peaks of lithium metasilicate was increased by increasing the sintering temperature, and also, their wideness was decreased by increasing the sintering temperature. This phenomenon can be because of increasing the crystalline size of lithium metasilicate particles at the glassy matrix. The quantitative assessment of XRD graphs using the Rietveld method shows that the glassy matrix of sintered samples at 660, 680, and 700°C was 46, 62, and 63 wt.%, respectively. It seems that with increasing the sintering temperature, more portion of the glassy matrix is transformed to crystalline phase.
Ortiz et al.
The size of rod-like crystals was almost the same for all samples. The length of rod-like crystals was about 3–5 μm in all samples. However, it is evident that the volume percent of porosity decreased by increasing the sintering temperature. The image processing of this figure using ImageJ software indicates that the volume percent of porosity is about 0.7 ± 0.1, 1.2 ± 0.2, and 1.6% ± 0.3% for sintered samples at 700, 680, and 660°C, respectively. It is evident that more powder densification happened, by increasing the sintering temperature. Furthermore, it is evident that LMS crystals are distributed randomly at the glassy matrix in all samples.
While the mechanical properties of ceramic dental materials are important for the clinical success of clinical restorations,
According to
The hardness and fracture toughness of sintered samples at 700°C, 680°C and 660°C.
Meng et al.
It is evident that by increasing the sintering temperature, the hardness of samples is increased significantly. This phenomenon can be because of porosity decrement at the glassy matrix and also better densification and powder particle bonding during sintering.
Partially crystallized lithium disilicate blocks are fabricated using SPS at higher compaction pressures than the powder metallurgy technique. The fabricated blocks had enough mechanical properties for handling and machining. The microstructure and mechanical properties of partially crystallized lithium disilicate blocks can be tailored by changing the sintering temperature. The fabricated partially crystallized lithium disilicate blocks are promising candidates for dental restoration fabrication using CAD/CAM.
Acknowledgment
This study was approved by the Ethics Committee of Isfahan University of Medical Sciences (IR. MUI. RESEARCH. REC. 1397.227).
Financial support and sponsorship
This study is supported by Dental Materials Research Center, Dental Research Institute of Isfahan University of Medical Sciences, Grant # 297073.
Conflicts of interest
The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.