Does glaze firing affect the strength of advanced lithium disilicate after simulated defects?

OBJECTIVE

To study the influence of glazing on strength repair of lithium disilicate glass-ceramics after defect incorporation in different production processing phases.

MATERIALS AND METHODS

Bar-shaped specimens (1 × 1 × 12 mm, n = 280; 20/group) made from different lithium disilicate ceramics (IPS e.max CAD, Ivoclar, "LD" or advanced lithium disilicate CEREC Tessera, Dentsply Sirona, "ALD") were exposed to 7 different protocols: crystallized without (c) and with glaze layer (cg), with a defect incorporated before crystallization without (ic) and with glaze layer (icg), with a defect after crystallization without (ci) or with glaze layer (cig), and defect incorporated after the glaze layer (cgi). The flexural strength was determined using the three-point bending test. Analysis of indented areas and fractured specimens was performed by scanning electron microscopy. Flexural strength data were evaluated by two-way ANOVA followed by Tukey tests (α = 5%).

RESULTS

Two-way ANOVA revealed a significant influence of ceramic (p < 0.001; F = 55.45), protocol (p < 0.001; F = 56.94), and the interaction protocol*ceramic (p < 0.001; F = 13.86). Regardless of ceramics, defect incorporation as final step resulted in the worst strength, while defects introduced before crystallization did not reduce strength. Glaze firing after defect incorporation led to strength repair for ALD, whereas such an effect was not evident for LD.

CONCLUSIONS

The advanced lithium disilicate must receive a glaze layer to achieve its highest strength. Defects incorporated in the pre-crystallized stage can be healed during crystallization. Defects should not be incorporated after glazing.

CLINICAL RELEVANCE

Clinical adjustments should be performed on pre-crystallized or crystalized restorations that receive a glazer layer afterwards.

Pre-cementation treatment of glass-ceramics with vacuum impregnated resin coatings

OBJECTIVE

The study aimed to investigate the effectiveness of a vacuum impregnation process to eliminate the porosity at the ceramic-resin interface to optimize the reinforcement of a glass-ceramic by resin cementation.

METHODS

100 leucite glass-ceramic disks (1.0 ± 0.1 mm thickness) were air-abraded, etched with 9.6 % HF acid, and silanated. Specimens were randomly allocated to 5 groups (n = 20). Group A received no further treatment (uncoated control). Groups B and D were resin-coated under atmospheric pressure, whereas groups C and E were resin-coated using vacuum impregnation. The polymerized resin-coating surfaces of specimens in groups B and C were polished to achieve a resin thickness of 100 ± 10 µm, while in groups D and E no resin-coating modification was performed prior to bi-axial flexure strength (BFS) determination. Optical microscopy was undertaken on the fracture fragments to identify the failure mode and origin. Comparisons of BFS group means were made by a one-way analysis of variance (ANOVA) and post-hoc Tukey test at α = 0.05.

RESULTS

All resin-coated sample groups (B-E) showed a statistically significant increase in mean BFS compared with the uncoated control (p < 0.01). There was a significant difference in BFS between the ambient and vacuum impregnated unpolished groups (D and E) (p < 0.01), with the greatest strengthening achieved using a vacuum impregnation technique.

SIGNIFICANCE

Results highlight the opportunity to further develop processes to apply thin conformal resin coatings, applied as a pre-cementation step to strengthen dental glass-ceramics.

Solidification/stabilization of chromite ore processing residue via co-sintering with hazardous waste incineration residue

In order to realize the harmless and resource disposal of hazardous waste incineration residue (HWIR) and chromite ore processing residue (COPR), this paper prepares glass-ceramics by HWIR. The COPR was co-sintered with the base glass of HWIR to realize the solidification and stabilization of COPR. The results shown that the single-stage sintering method has a simple process and low energy consumption, while the two-stage sintering method has better mechanical properties. Chromium in COPR may be solidified/stabilized by physical encapsulation and chemical fixation. When the content of COPR reaches 50%, the leaching concentration of Cr and Cr(VI) in the solidified body of HWIR solidified COPR (IRSC) is less than 5 mg/L, which satisfies the US EPA and CN GB5085.3 standard limits. This study achieves waste control by waste and prepares solidified bodies (IRSC) with good mechanical properties, chemical corrosion resistance, and low leaching concentration of heavy metals, which provides feasibility for its engineering application.

Biaxial flexure strength and physicochemical characterization of a CAD/CAM lithium disilicate ceramic: effect of etching time, silane, and adhesive applications

OBJECTIVE

To evaluate the effect of different acid etching time and bonding agent (silane and/or adhesive system) on biaxial flexural strength and physico-chemical properties of a lithium disilicate ceramic.

MATERIAL AND METHODS

One hundred twenty ceramic discs were made and divided into 8 groups (n = 15) according to factors "etching time" (20 and 120 s) with hydrofluoric acid (HF) and "bonding agent" (C, no bonding agent; S, silane, A, adhesive; and SA, silane + adhesive). After surface treatment, a resin cement layer was applied to the surface and all specimens were subjected to biaxial flexural strength (BFS) test with treated surfaces loaded in tension (1 mm/min). The Weibull analyses and complementary analyses were also performed. Statistical analysis was done with 2-way ANOVA and the Tukey test (α = 0.05).

RESULTS

ANOVA revealed that the factors "etching time" (p = 0.0003) and "bonding agent" (p = 0.007) were statistically significant. In the overall analysis, the HF120S group (272.02 ± 35.30A MPa) presented significantly higher BFS than that of HF120C (218.45 ± 17.15CD MPa) and HF20S (228.40 ± 37.83BCDMPa). On the other hand, the HF20A group (208.92 ± 31.16D MPa) had significantly lower BFS than HF120S (272.02 ± 35.30A), HF120A (254.42 ± 26.87ABC) and HF120SA (259.30 ± 36.55AB) groups (Tukey). The Weibull modulus (m) of all groups was significantly different from each other (p = 0.000).

CONCLUSIONS

Regardless of etching time, the application of silane alone is sufficient to increase the flexural strength of glass ceramic, eliminating the need for the application of adhesive systems. Moreover, if only silane or adhesive is applied, 120-s HF application should increase the flexural resistance of the lithium disilicate ceramic.

CLINICAL SIGNIFICANCE

Applications of adhesive systems after silanization can be suppressed from the surface treatment protocol of glass ceramics, since it does not improve their mechanical strength.

Effect of aqueous environment on wear resistance of dental glass-ceramics

Background

Wear resistance affects dental ceramics longevity and the functions of the opposing teeth. However, data for the effect of aqueous environment on wear resistance of dental ceramics are lacking. This study evaluated the effect of aqueous environment on wear resistance of typical dental glass–ceramics.

Methods

Disk specimens were prepared from lithium disilicate glass–ceramics (LD) and leucite reinforced glass–ceramics (LEU). The disk specimens paired with steatite antagonists were tested in a pin-on-disk tribometer under both wet and dry conditions with 10 N up to 500,000 wear cycles. The wear analysis of glass–ceramics was performed using a 3D profilometer after 100,000, 300,000 and 500,000 wear cycles. Wear morphologies were analyzed by employing scanning electron microscopy (SEM). The crystalline compositions of specimens stored in a dry environment and subsequently immersed in distilled water for 40 h were separately determined using X-ray diffraction (XRD). The chemical states of the wear surfaces for LD were analyzed by X-ray photoelectron spectroscopy (XPS). The data analysis and multiple pair-wise comparisons of means were performed by using one-way analysis of variance (ANOVA) and Tukey’s post-hoc test.

Results

LEU in a wet environment exhibited less wear volume loss than that in a dry environment (p < 0.05). The volume loss of LD in a wet environment was higher than that in a dry environment (p < 0.05). The wear volumes of steatite antagonists paired with two glass–ceramics under dry conditions were higher than under wet conditions.

Conclusions

XPS spectra of LD under wet conditions indicated that high wear loss might result from the effect of stress corrosion by water and reaction of water with the ionic-covalent bonds at the crack tip. XPS spectra and SEM images of LD under dry conditions showed a possible formation of tribofilm. Within the limitations of this in vitro study, water was wear-friendly to LEU and all opposing steatites but aggravated wear for LD.

Remedial processing of oil shale fly ash (OSFA) and its value-added conversion into glass-ceramics

Recently, various solid wastes such as sewage sludge, coal fly ash and slag have been recycled into various products such as sintered bricks, ceramics and cement concrete. Application of these recycling approaches is much better and greener than conventional landfills since it can solve the problems of storage of industrial wastes and reduce exploration of natural resources for construction materials to protect the environment. Therefore, in this study, an attempt was made to recycle oil shale fly ash (OSFA), a by-product obtained from the extracting of shale oil in the oil shale industry, into a value-added glass-ceramic material via melting and sintering method. The influence of basicity (CaO/SiO2 ratio) by adding calcium oxide on the performance of glass-ceramics was studied in terms of phase transformation, mechanical properties, chemical resistances and heavy metals leaching tests. Crystallization kinetics results showed that the increase of basicity reduced the activation energies of crystallization but did not change the crystallization mechanism. When increasing the basicity from 0.2 to 0.5, the densification of sintering body was enhanced due to the promotion of viscous flow of glass powders, and therefore the compression strength and bending strength of glass-ceramics were increased. Heavy metals leaching results indicated that the produced OSFA-based glass-ceramics could be taken as non-hazardous materials. The maximum mechanical properties of compression strength of 186 ± 3 MPa, bending strength of 78 ± 6 MPa, good chemical resistances and low heavy metals leaching concentrations showed that it could be used as a substitute material for construction applications. The proposed approach will be one of the potential sustainable solutions in reducing the storage of oil shale fly ash as well as converting it into a value-added product.