Impact of thermal shock cycles on mechanical properties and microstructure of lithium disilicate dental glass-ceramic

In the present paper, we report results on the impact of thermal shock cycles on mechanical properties and microstructure of lithium disilicate dental glass-ceramic relevant to the production process. A distinct reduction of the bending strength is observed after suffering more than five thermal shock cycles. The crystal, morphological and atomic structural characteristics before and after the thermal shock were detected by X-ray diffraction, infrared spectroscopy and field-emission scanning electron microscopy. Furthermore, theoretical calculations, based on density functional theory, were performed on models for the distorted and undistorted lithium disilicate crystal lattices. We show that the morphological and mechanical changes can be attributed primarily to the distortion of the Si-O-Si structural unit of the lithium disilicate glass-ceramic.     

Controlling the microstructure and properties of lithium disilicate glass-ceramics by adjusting the content of MgO

In order to obtain lithium disilicate glass-ceramics for dental restoration with both high strength and high translucency, lithium disilicate glass-ceramics with different MgO contents were prepared by melt-casting and heat treatment method. The effects of MgO content on the crystallization temperature, microstructure and flexural strength of lithium disilicate glass-ceramics were investigated. The results indicate that Mg²⁺ exists in the form of [MgO₄] in the network of lithium disilicate glass-ceramics when the MgO content is 0.56 mol% (M0.56), which is beneficial to increasing the homogeneity and thermal stability of the glass system, and short rod-like lithium disilicate crystals can be formed after heat treatment at 840°C. Thus, the obtained lithium disilicate glass-ceramics exhibit excellent comprehensive performance, with the flexural strength being 312 ± 23 MPa, and the average transmittance of visible light being 37.3% (d = 1.62 mm). Especially, the glass-ceramic sample shows better translucency than the commercially available products. The research results are of great significance for developing high performance lithium disilicate glass ceramics and promoting its broad application in the field of dental restoration.     

Etching efficacy and bonding performance of resin to lithium disilicate ceramic using self-etching primer with different reaction times

The purpose of this study was to evaluate the effect of self-etching primer on glass-ceramic at different reaction durations. Fifty specimens of lithium disilicate ceramic were divided into five groups based on the surface treatment (n?=?10); Group A (control): no treatment, Group B: treated with 5%-hydrofluoric acid (HF) followed by a primer, Group C: treated with one-bottle system self-etching primer; (Monobond Etch & Prime (MEP)) for 20?s plus 40?s reaction time, Group D: MEP for 20?s plus 80?s reaction time, Group E: MEP for 20?s plus 120?s reaction time. The outcomes of shear bond strength (SBS), failure mode and surface topography were assessed. Statistical analyses were performed using ANOVA, Chi-squared test and Tukey’s test at a significance level of α?=?0.05. The control group (A) demonstrated the lowest SBS value (0.11MPa, SD:0.022) which was statistically significant, compared to other groups (P?<?0.0001). However, the mean SBS values were not statistically significant among the other four experimental groups. Group B yielded the highest SBS value (13.79MPa, SD:3.57), although was not statistically significant when compared to group C (11.60MPa, SD:1.23), group D (11.94MPa, SD:1.24), and group E (11.67MPa, SD:0.64), P?>?0.05. Scanning electron microscope (SEM) showed increased surface roughness in HF/primer specimens compared to MEP. Stereomicroscopy analyses showed that the main mode of failure was adhesive failure in group A, while was mixed failure in the other experimental groups, with no statistical significance between the groups. The self-etching system can be considered as an alternative for the glass-ceramic surface treatment without compromising the bond strength. The time required for conditioning seems to be sufficient with 40?seconds reaction time.     

Effect of heat treatment on the roughness and mechanical properties of dental lithium disilicate glass-ceramics

In dental clinics, it is common to perform small fitting adjustments in dentures using a micro-grinding tool after testing them in the patient's mouth. This procedure increases local roughness and can lead to formation of microcracks on the prosthesis surface. This study aimed to investigate the benefits of a post-finishing heat treatment to surface roughness and crack healing and its effect on the flexural strength of lithium disilicate (LD) dental glass-ceramics. Commercially available lithium metasilicate, Li₂SiO₃, samples were heat treated at 840 °C for 7 min to induce the phase transformation into LD, Li₂Si₂O₅. The LD samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, Vickers hardness, Young’s modulus, and fracture toughness. One of the surfaces of the LD samples was sanded aiming to simulate the denture fitting adjustments performed in the dentist’s laboratory, generating a rough surface, Group 1. Half of the LD samples had their biaxial flexural strength evaluated by the piston-on-three-ball test (P–3B) and the other half were submitted to a second short-term heat treatment (840 °C - 5 min), Group 2, and later assessed by the P–3B. Roughness parameters in both groups were measured by 3D optical profilometry. After the crystallization heat treatment, formation of elongated LD crystals, Li₂Si₂O₅, 35% amorphous phase, and residual Li₃PO₄ was observed. In addition, the following mechanical property values were obtained: Vickers hardness = 5.8 ± 0.1 GPa, fracture toughness = 2.2 ± 0.1 MPa m^1/2, and Young’s modulus = 100.3 ± 0.3 GPa. The samples in Group 1 showed bending strength of 206 ± 30 MPa and the following roughness parameters: Ra = 0.45 ± 0.16 μm, Rz = 22.7 ± 6.7 μm, and PV = 27.7 ± 7.1 μm. In the samples in Group 2, the Ra, Rz and PV roughness parameters were 0.31 ± 0.12 μm, 5.2 ± 2.5 μm, and 9.2 ± 4.7 μm, respectively. With this decrease in roughness, the bending strength increased by 62%, with a mean value of 331 ± 59 MPa. In the need for machine finishing of LD-based glass-ceramic dental prostheses, the use of a second short-term heat treatment at 840 °C for 5 min generates considerable gains in bending strength, increasing the lifecycle of the prosthesis as a result of reduced surface roughness caused by softening of the remaining amorphous phase in the glass-ceramic. These conditions can be adapted to each chemical and crystallographic composition of the glass-ceramic under study.     

Effect of the Etching Duration and Ultrasonic Cleaning on Microtensile Bond Strength Between Feldspathic Ceramic and Resin Cement

This study assessed the effect of different etching durations of feldspathic ceramic with hydrofluoric acid (HF) and ultrasonic cleaning of the etched ceramic surface on the microtensile bond strength stability of resin to a feldspathic ceramic. The research hypotheses investigated were: (1) different etching times would not affect the adhesion resistance and (2) ultrasonic cleaning would improve the adhesion. Ceramic blocks (6 × 6 × 5 mm) (N = 48) were obtained. The cementations surfaces were duplicated in resin composite. The six study groups (n = 8) were: G1—Etching with 10% aqueous HF (30 s) + silane; G2—10% HF (1 min) + silane; G3—10% HF (2 min) + silane; G4—10% HF (30 s) + ultrasonic cleaning (4 min) in distilled water +silane; G5—10% HF (1 min) + ultrasonic cleaning + silane; G6—10% HF (2 min) ultrasonic cleaning + silane. The cemented blocks were sectioned into microbars for the microtensile test. The etching duration did not create significant difference among the groups (p = .156) but significant influence of ultrasonic cleaning was observed (p = .001) (Two-way ANOVA and Tukey's test, p > 0.05). All the groups after ultrasonic cleaning presented higher bond strength (19.38–20.08 MPa) when compared with the groups without ultrasonic cleaning (16.21–17.75 MPa). The bond strength between feldspathic ceramic and resin cement was not affected by different etching durations using HF. Ultrasonic cleaning increased the bond strength between ceramic surface and resin cement, regardless of the etching duration.     

Synthesis and properties of lithium disilicate glass-ceramics in the system SiO2–Al2O3–K2O–Li2O

The purpose of this study was the synthesis of lithium disilicate glass-ceramics in the system SiO₂–Al₂O₃–K₂O–Li₂O. A total of 8 compositions from three series were prepared. The starting glass compositions 1 and 2 were selected in the leucite–lithium disilicate system with leucite/lithium disilicate weight ratio of 50/50 and 25/75, respectively. Then, production of lithium disilicate glass-ceramics was attempted via solid-state reaction between Li₂SiO₃ (which was the main crystalline phase in compositions 1 and 2) and SiO₂. In the second series of compositions, silica was added to fine glass powders of the compositions 1 and 2 (in weight ratio of 20/100 and 30/100) resulting in the modified compositions 1–20, 1–30, 2–20, and 2–30. In the third series of compositions, excess of silica, in the amount of 30 wt.% and 20 wt.% with respect to the parent compositions 1 and 2, was introduced directly into the glass batch. Specimens, sintered at 800 °C, 850 °C and 900 °C, were tested for density (Archimedes’ method), Vickers hardness (H_V), flexural strength (3-point bending tests), and chemical durability. Field emission scanning electron microscopy and X-ray diffraction were employed for crystalline phase analysis of the glass-ceramics. Lithium disilicate precipitated as dominant crystalline phase in the crystallized modified compositions containing colloidal silica as well as in the glass-ceramics 3 and 4 after sintering at 850 °C and 900 °C. Self-glazed effect was observed in the glass-ceramics with compositions 3 and 4, whose 3-point bending strength and microhardness values were 165.3 (25.6) MPa and 201.4 (14.0) MPa, 5.27 (0.48) GPa and 5.34 (0.40) GPa, respectively.     

Adhesion of metal brackets to glassy matrix and hybrid CAD/CAM materials after different physico-chemical surface conditioning

This study compared the adhesion of metal brackets bonded to different CAD/CAM materials after various surface conditioning methods. CAD/CAM blocks (N = 204, n = 17 per group) of (a) VITA Mark II (VM), (b) IPS e.max CAD (IP), (c) Lava Ultimate (LU), and (d) VITA ENAMIC (VE) were conditioned with one of the following methods: C-Control: (fine diamond bur); CJ: (fine diamond bur + air-abrasion with 30 μm SiO₂ + silane), and HF: (fine diamond bur +9.5% hydrofluoric acid + silane). Metal brackets were bonded to the conditioned surfaces of the specimens, stored in artificial saliva for 24 h at 37 °C and thermocycled (×1000). Subsequently, the brackets were debonded under shear in a Universal Testing Machine (1 mm/min). Failure types were analyzed under scanning electron microscope. Data were analyzed using two-way ANOVA and Tukey’s tests (α = 0.05). Two-parameter Weibull distribution values, including the Weibull modulus, scale, and shape, were calculated. Mean bond strength (MPa) values were significantly affected by the surface conditioning method (p < 0.001) but not the CAD/CAM material type (p = 0.052). Bond strengths for all CJ and HF-conditioned specimens were twofold higher (11.83 ± 1.95 ? 9.44 ± 1.63) than those for control specimens with all materials (4.73 ± 0.93 ? 6.02 ± 0.69). Lower mean values were obtained in LU-CJ (9.78 ± 1.61) and LU-HF (9.44 ± 1.63) than those for other groups (11.83 ± 1.95 ? 10.93 ± 1.33) groups (p < 0.05). Weibull distribution showed higher shape values for VM-CJ (11.26) and VM-HF (8.87) than those for other groups (0.82–1.83). In control groups, exclusively adhesive and after conditioning mainly mixed failures were observed. Chairside silica coating or HF conditioning significantly improved metal bracket adhesion to both glassy matrix and hybrid CAD/CAM materials tested, with the most reliable adhesion being observed for feldspathic ceramic.