A parametrical finite element analysis for functionally graded material overlay restoration

The main cause of failure in bonded ceramic restorations is material fracture due to excessive stress concentration at the base of the prosthesis. The design of restorative functionally graded materials (FGM) could represent a major advance in dissipating mechanical stresses during occlusal contacts. The aim of this paper is to carry out a complete factorial design of finite element analyses to optimize a multilayer FGM introduced at the bottom of an overlay prosthesis. The number and thickness of layers vary within a spectrum compatible with ceramic shaping processes whereas Young's moduli variations are set in the range of dental tissues. For a 1.5-mm thick prosthesis, the optimal FGM configuration appears to be a 5 layers of 0.2 mm thickness with a linear distribution of Young's modulus from 30 to 70 GPa. This configuration was implemented in a 3D model of a restored tooth with realistic geometry to validate the proof-of-concept.

Recycling of waste glass and incinerated sewage sludge ash in glass-ceramics

Disposal of waste glass and incinerated sewage sludge ash (ISSA) in landfills is a waste of resources and poses significant environmental risks. This work aims to recycle waste glass and ISSA together to form value-added glass-ceramics. The physical and mechanical properties, leaching behaviour, and microstructure of the glass-ceramics produced with different proportions of waste glass powder (WGP) and ISSA were investigated. Thermodynamic calculations were performed to predict the formation of crystalline phases and the phase transformation involved. The results showed the potential of WGP and ISSA as raw materials in glass-ceramics production. WGP effectively densified the microstructure of the glass-ceramics by forming a viscous phase. As WGP content increased, the total porosity of glass-ceramics decreased whereas the density increased, accompanied by the formed anorthite transforming into wollastonite. The incorporation of WGP densified and refined the pore structure of the glass-ceramics, thereby improving the mechanical properties and reducing the water absorption. The glass-ceramics produced with a 50:50 blend of WGP and ISSA exhibited the highest compressive strength of 43.7 MPa and the lowest water absorption of 0.3 %. All fabricated glass-ceramics exhibited innocuous heavy metal leaching. The co-sintering of ISSA and WGP can produce additive-free glass-ceramics, characterized by reduced energy consumption and notable heavy metal immobilization capacity. These materials hold promise for utilization in construction as building materials.

Study on controllable preparation of high performance andradite based glass-ceramics by harmful iron-rich waste slag

Due to containing abundant FeOx and trace heavy metals (Pb, As, Cr, Cd, etc.), the disposal of lead/zinc smelting slag (LSS-ZSS) with ultra-high historical reserves has attracted increased attention. Using LSS-ZSS to prepare glass-ceramics is a good method to solve the problem of LSS-ZSS accumulation and realize heavy metal solidification, whereas there are some technical challenges that are difficult to deal with. A large amount of FeOx component in LSS-ZSS would not only lead to melt overflow, but also cause early crystallization of basic glass. In this work, through the directional modification of LSS-ZSS and the subsequent crystallization kinetics regulation, we successfully prepare high performance glass-ceramics with andradite and hematite as the main crystal phases. In addition, by means of SEM, PXRD, FTIR spectra and XPS, the morphology/phase transformation, fine structure and valence variations of iron components in LSS-ZSS at different temperature zones are systematically studied. The maximum shrinkage rate of resultant basic glasses is 27 %, and the maximum flexural strength and compressive strength of glass-ceramics are 128 MPa and 890 MPa, respectively. This work would not only benefit to solve the problem of resource utilization of harmful LSS-ZSS, but also provide a possible reference for the utilization of iron-rich waste slag in magnetic properties related fields.

Crystallization characteristics, microstructural evolution, and Cr migration mechanism of glass-ceramics synthesized entirely from low-carbon ferrochromium slag and waste glass

The disposal and large-scale reutilization of industrial solid waste in construction materials have been gaining considerable interest. Herein, the synthesis of glass-ceramics from low-carbon ferrochromium slag (FCS) and waste glass is reported. The crystallization characteristics, microstructural evolution, and Cr migration mechanisms of the glass-ceramics were investigated through offline quenching experiments. The physical properties and Cr ion leaching performance were tested and evaluated. As the low-carbon FCS content was increased, glass network depolymerization was promoted and the crystallization ability was effectively improved. When 50 wt% low-carbon FCS was added, the compressive strength and Vickers hardness of the glass-ceramics reached maximum values of 160.26 MPa and 736 Hv, respectively. Simultaneously, the dominant crystalline phase in glass-ceramics was transformed from diopside and wollastonite into gehlenite when the low-carbon FCS content was further increased to 60 wt%, thereby reducing the mechanical properties. Cr₂O₃ primarily existed as a [CrO₆] octahedron and Cr-containing spinel in the parent glass, resulting in composition segregation. On combining the growth orientation of diopside and spinel, the resulting specific crystal coating structure improved the Cr ion leaching performance. Considering the physical properties and risk of Cr leaching, the optimal FCS content in glass-ceramics is 40-50 wt%. These findings indicate that the production of glass-ceramics from low-carbon FCS and waste glass is an effective and environment-friendly method for co-utilizing these two solid wastes.

Effect of crystal orientation on flexural strength of pressable lithium disilicate glass-ceramics

This study examined the crystal orientation of four kinds of pressable lithium disilicate glass-ceramics and evaluated the effect of crystal orientation on flexural strength. Bar-shaped (24 mm in length, 1.2 mm in thickness, 4.0 mm in width), disk-shaped (12 mm in diameter, 0.5 mm in thickness), and crown-shaped (maxillary first molar) specimens were prepared according to the manufacturer's instructions. Three-point and biaxial flexural strengths were measured for bar- and disk-shaped specimens. Microstructure analysis was performed using X-ray diffractometry and scanning electron microscopy. Three-point flexural strength was improved by parallel crystal orientation along the longitudinal direction of the bar-shaped specimen. There was no relationship between two-dimensional crystal orientation and biaxial flexural strength. The results of this study assumed that biaxial flexural strength was improved by the crystal orientation in the cross-sectional direction. Pressed restorations are expected to possess higher strength than milled restorations due to their crystal orientation.

Effect of the region of the CAD/CAM block on the flexural strength and structural reliability of restorative materials

The aim of the study was to evaluate whether the region (internal or external) of the CAD/CAM block influences the mechanical performance of restorative materials. Blocks of different CAD/CAM materials (Composites: KAV - Katana Avencia; LULT - LULT Ultimate; Ceramics: ENAM - Vita Enamic; NICE - N!CE; EMP - IPS Empress CAD; VMII - Vita Mark II; EMAX - IPS e.max CAD) were selected, and direct resin composite blocks (APX - Clearfil AP-X; FSUP - Filtek Supreme) were built using the incremental technique on a mold from one of the CAD/CAM blocks. All blocks were sectioned into bar-shaped specimens (1 × 1 × 14 mm), which were separated into two groups according to the region of the block (inside or outside). 3-point bending tests were performed following ISO 6872:2015 and flexural strength data were analyzed using one-way ANOVA and Bonferroni post-hoc tests (p < 0.05). Weibull module and characteristic strength were also calculated. In general, the mean flexural strength values were not statistically different (p > 0.05) between the inside and outside regions of each material, except for LULT (inside > outside) and EMAX (outside > inside). Comparing the materials, a similar strength (only discrete variations) was observed for inside, outside or merged (inside and outside data assembled together) regions: EMAX > KAV = LULT > NICE = APX > EMP = ENAM = VMII ≥ FSUP (merged condition). Characteristic strength at 63.2% failure probability and at 5% failure probability generally corroborates such observations. There were no statistical differences for Weibull module data (inside, outside, or merged), except for KAV and NICE (outside > inside). Thus, the region from where the restoration is milled within the CAD/CAM block generally does not influence the mechanical performance hereof (flexural strength, failure probability, risk of premature failures, and mechanical reliability) of the material, except for LULT and EMAX.

Effect of thickness on the translucency of resin-based composites and glass-ceramics

This study was aimed to evaluate the effect of thickness (1, 2, 3, and 4 mm) on the translucency of resin-based composites (RBCs) and glass-ceramics, and compare the influence of the thickness of those materials on the translucency parameter (TP) value. The materials were divided into two groups, eight RBCs in Group 1 and five glass-ceramics in Group 2 and TP, ΔL*, Δa*, and Δb* were compared. Statistically significant differences were present in the 2, 3, and 4 mm in the TP, in the 2 and 4 mm in ΔL*, and in all thicknesses in Δa* and Δb* betweent the two groups. The TP of RBCs and glass-ceramics decreased as thickness increased, especially from 1 mm to 2 mm. The TP values of the RBCs were more significantly decreased as the thickness of the material increased from 2 mm to 4 mm than those of the glass-ceramics.

Effect of phosphoric acid and sodium hydroxide on cleaning and bonding of saliva-contaminated feldspar porcelain

Purpose Hydrofluoric acid has been used to remove salivary contamination in dental glass-ceramics before bonding treatment. However, alternative methods are required because hydrofluoric acid is harmful. This study examined the cleaning effects of phosphoric acid and sodium hydroxide on glass-ceramics for bonding pre-treatment.Methods Feldspar porcelain was divided into four groups: (C) cleaned porcelain without any contamination, (S) porcelain contaminated with saliva, (SPA) porcelain cleaned with 37% phosphoric acid after saliva contamination, and (SSH) porcelain cleaned with 10% sodium hydroxide after saliva contamination. Each sample was bonded to the resin cement using a silane-containing primer. They were then subjected to a shear bond strength (SBS) test. Each surface was analyzed by scanning electron microscopy (SEM), contact angle measurements, and Fourier transform infrared spectroscopy (FT-IR).Results The SBS of group SSH was comparable to that of group C but significantly higher than that of groups S and SPA. SEM observations showed that saliva-like structures remained on the samples of groups S and SPA, but not on the SSH group. The contact angles of groups C and SSH were comparable and significantly smaller than those of groups S and SPA, respectively. FT-IR analysis also revealed saliva in groups S and SPA, which was absent in the SSH group.Conclusions The saliva remained on the porcelain even after cleaning with phosphoric acid, and SBS was not restored to the same level as before the contamination. In contrast, sodium hydroxide eliminated saliva and restored SBS to the same level as before contamination.

Antibacterial activity improvement of dental glass-ceramic by incorporation of AgVO3 nanoparticles

OBJECTIVE

This study aimed to investigate the role of the incorporation of an antibacterial nanoceramic (AgVO₃) on the properties of a restorative dental glass-ceramic.

METHOD

A commercially available restorative glass-ceramic, commonly designated as porcelain (IPS d.SIGN) was functionalized with an antibacterial agent (nanostructured β-AgVO₃), synthesized by a hydrothermal route. Both functionalized and pristine samples were processed according to the manufacturer's instructions. All samples were characterized by X-ray diffraction, Rietveld refinement, particle size distribution, Scanning Electron Microscopy, chemical solubility, and Inductively Coupled Plasma Spectroscopy. Their antibacterial potential (Mueller-Hinton test) was analyzed against gram-positive (Staphylococcus aureus) and gram-negative bacteria (Escherichia coli).

RESULTS

The commercial glass-ceramic showed leucite (KAlSi₂O₆) as the only detectable crystalline phase, and, for both strains, no antibacterial activity could be detected in the Mueller-Hinton agar plates test. A monophasic, needle-shaped, and nanometric β-AgVO₃ powder was successfully synthesized by a simple hydrothermal route. After thermal treatment, glass-ceramic samples containing different percentages of β-AgVO₃ showed a second crystalline phase of microline [K_0.95(AlSi₃O₈)]. For modified samples, inhibition halos were easily visible on the Mueller-Hinton test, which ranged from 11.1 ± 0.5 mm to 16.6 ± 0.5 mm and 12.7 ± 0.3 mm to 15.5 ± 0.3 mm in the S. aureus and E.coli cultures, respectively, showing that the halos formed were dose-dependent. Also, increasing the percentage of β-AgVO₃ promoted a significant increase in chemical solubility, from 72 µg/cm² (samples with 1 wt% of β-AgVO₃) to 136 µg/cm² (samples with 2 wt% of β-AgVO₃), which was associated with the silver and vanadium ions released from the glass matrix.

SIGNIFICANCE

Our in vitro results indicate that IPS d.SIGN, as most of the dental glass-ceramics, do not exhibit antibacterial activity per se. Nonetheless, in this concept test, we demonstrated that it is possible to modify dental veneering materials giving them antibacterial properties by adding at least 2 wt% of β-AgVO₃, a nanomaterial easily synthesized by a simple route.

On the assignment of quartz-like LiAlSi2O6 - SiO2 solid solutions in dental lithium silicate glass-ceramics: Virgilite, high quartz, low quartz or stuffed quartz derivatives?

OBJECTIVES

Here we aim to provide a background on X-Ray Diffraction analysis of quartz-like crystal structures with varying amounts of Al³⁺ and Li⁺ substitution, existing confusions on their nomenclature and its implications for novel lithium silicate glass-ceramics.

METHODS

We reviewed the literature dealing with modifications of the quartz crystal structure and their stuffed LiAlSi₂O₆ derivates, LiAlSi₂O₆ - SiO₂ solid solutions, the terminology of such phases and criteria used to define the structure known as virgilite. Based on this information, we attempted to allocate the quartz-like phases found in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill in the range of LiAlO₂ - SiO₂ solid solutions. For this purpose, their lattice parameters obtained from Rietveld refinement were compared with the lattice parameters of members of the corresponding solid solutions with defined SiO₂ molar fraction found in the literature.

RESULTS

Based on the lattice parameters available for low quartz, high quartz and its stuffed derivatives, including LiAlSi₂O₆ and the mineral virgilite, a plot of the a- and c-parameters vs. the mol% SiO₂ related to LiAlO₂ was constructed with the literature data and the data found for the three dental lithium silicates. As per the definitions of virgilite as either Li_xAl_xSi_3-xO₆, with 0.5 < x < 1 or especially as members of the LiAlSi₂O₆ - SiO₂ solid-solution series with more than 50 mol% LiAlSi₂O₆, the crystal structures in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill failed to fall within the ranges of mol% SiO₂ confined for virgilite.

SIGNIFICANCE

Based on available literature and definitions, the quartz-like phases found in the three dental lithium silicates should be addressed as stuffed (probably low) quartz solid solutions instead of "virgilite". However determined by mineralogical practices, the term "virgilite" for parts of the LiAlSi₂O₆ - SiO₂ solid solution is ambiguous and can be considered as arbitrary.

Simulation of CAD/CAM milling on lithium disilicate: Mechanical and topographic analyses of surface grinding different protocols

The aim of this study was to evaluate the topography and the fatigue performance of lithium disilicate glass-ceramic after surface grinding through different laboratory protocols used to simulate the Computer-aided design/Computer-aided manufacturing (CAD/CAM) milling. Ceramic discs (IPS e.max CAD, Ø = 13.5 mm × 1.2 mm of thickness) were produced through different methodologies: milling in CAD/CAM system (CAD/CAM group); produced in-lab with a polished surface (POL group); or produced through in-lab methods and randomly distributed into five groups according to different grinding protocols to simulate the CAD/CAM milling [grinding with a CAD/CAM bur coupled to a mandrel (CAD/CAM Bur group); fine diamond bur using oscillatory movements (DBO group); fine diamond bur in x and y axes of the disc (DBXY group); #60-grit silicon carbide sandpaper (SiC group); and #60-grit wood sandpaper (WS group)]. The specimens were fatigue tested (n = 15) according to the step-stress method (initial load: 60 N; step-size: 20 N; 10,000 cycles/step; 20 Hz frequency). A roughness analysis was performed on all specimens, while fractal dimension (FD) and fractography were performed on representative samples. The Kaplan-Meier analysis showed that the POL (293.3 N) group presented better fatigue performance (higher load and number of cycles for failure) (p < 0.05) than the other groups (CAD/CAM = 222.7 N; CAD/CAM Bur = 181.3 N; DBO = 184.0 N; DBXY = 192.0 N; SiC = 182.6 N; WS = 182.6 N). For roughness, only the SiC (Ra = 1.616; Rz = 10.465) and WS (Ra = 1.673; Rz = 10.655) groups produced statistically similar Ra (μm) and Rz (μm) values to the CAD/CAM (Ra = 1.628; Rz = 9.571) group (p > 0.05). The surface created by CAD/CAM milling and POL group exhibited more complexity (FD) higher values than the experimental groups. For the ceramic surface topography images, the CAD/CAM milling visibly produced a uniform surface compared to the other groups; however, the POL group was the smoothest. The DBO, DBXY, SiC, and WS groups resulted in similar characteristics of surface topography. Therefore, although the SiC and WS groups showed similar roughness to the control group (CAD/CAM), no in-lab simulation method was fully capable to mimic the mechanical performance of the CAD/CAM-milled lithium disilicate glass-ceramic.

The lead removal evolution from hazardous waste cathode ray tube funnel glass under enhancement of red mud melting and synthesizing value-added glass-ceramics via reutilization of silicate resources

Waste CRT funnel glass (FG) is a typical hazardous waste produced by the electronics industry that contains toxic lead oxide, red mud (RM) is the first waste produced during alumina production. Both of these are extremely difficult to reuse. Here, we report a method to control FG waste, in which RM was used to enhance the removal of Pb from FG via a vacuum thermal process. The removed residual glass was utilized to create glass-ceramics. The results showed that RM can enhance the lead removal from waste CRT funnel glass by the vacuum thermal process. When 30% RM was added, the removal rate reached 98.54%. A significant mechanism of enhancing delead is investigated by a Fourier transform infrared (FTIR) spectrometer and X-ray photoelectron spectroscopy (XPS). The results showed that the -Pb-O-Si-O- network structure was broken by the free calcium ions of RM. Afterward, valuable glass-ceramics with tetragonal-KAlSi₂O₆ and triclinic-CaSiO₃ crystals were synthesized using the residual glass. The Pb, Ba, Cr, and Cu leaching concentrations of the glass-ceramics were well below the regulatory limit (5 mg/L) of the CA-EPA, as measured by the toxicity characteristic leaching procedure (TCLP) test. Overall, the results indicated that RM enhanced the removal of lead during the vacuum thermal process. The synthesis of value-added glass-ceramics reutilized silicate resources from waste cathode ray tube (CRT) funnel glass and RM.

Biocompatibility, mechanical and microstructural properties after crystallization.: Microstructure and Properties of Glass Ceramics for CAD/CAM Applications

OBJECTIVES

The objective of this study was to define the impact of heating rate on the crystal growth, the mechanical properties, and the biocompatibility of three different kinds of CAD/CAM glass-ceramics treated with a conventional furnace.

METHODS

Lithium disilicate (IPS EMax-CAD, Ivoclar Vivadent) (LS₂) and two zirconia reinforced lithium silicate (ZLS) ceramics (Vita Suprinity PC, VITA Zahnfabrik; Celtra Duo, Dentsply Sirona) (ZLSS; ZLSC) were used. The mechanical properties and the crystal growth were evaluated on 42 specimens (n=14 per group). The thermal treatments recommended by the manufacturers were carried out. All groups were tested for fracture toughness (Ft) and Vickers hardness (Hv). Scanning electron microscope (SEM) images were taken after a slight surface etching with hydrofluoric acid solution (1% for 20 sec). Differential Thermal Analysis (DTA) was performed and cellular adhesion with human periodontal ligament stem cells (hPDLSCs) culture was qualitatively assayed. Data were analysed with Repeated Measurements ANOVA and ANOVA followed by Tukey post hoc test.

RESULTS

The crystals' mean size (±SD) after heat treatment was 1650.0 (±340.0) nm for LS₂, 854.5 (±155.0) nm for ZLSS and 759.9 (±118.4) nm for ZLSC (p<0.05 among the groups). As consequence of crystallization, the Hv was 6.1±0.3 GPa for LS₂, 7.6±0.7 GPa for ZLSS and 7.1±0.5 GPa for ZLSC (p<0.05 for LS₂ vs ZLSS and ZLSC), while the Ft was 2.2±0.1 MPa m^1/2 for LS₂, 4.7±0.8 MPa m^1/2 for ZLSS and 3.8±0.6 MPa m^1/2 for ZLSC (p<0.05 among the groups). The DTA curves showed a crystallization process for LS₂, ZLSS and ZLSC at a temperature range 810°C to 840°C. The amount of adherent hPDLSCs was superior on LS₂ than on ZLS.

CONCLUSIONS

All the CAD/CAM materials can be properly crystallized if heat treated following the manufacturers' instructions. The crystallization process highly depends on temperature. ZLS glass ceramics show significantly inferior crystals dimensions and higher fracture toughness and Vickers hardness than LS₂ ceramic. hPDLSCs cultured on LS₂ have a superior adhesion than those cultured on ZLS.

CLINICAL SIGNIFICANCE

The clinical interest of this study relies on the demonstration that a proper heat-treatment of CAD/CAM lithium disilicate and ZLS glass ceramics generates products that are suitable for clinical service. The differences highlightable in mechanical properties and biocompatibility behaviour do not affect their successful clinical application.

Multi-tip indenter tool scratch behavior of glass-ceramics

To reduce the surface flaws and subsurface damages of glass-ceramics as clinical substitute and repair materials, it is necessary to clarify the material removal behaviors and crack propagation modes in the grinding process. In this paper, a multi-tip indenter tool was fabricated and assembled on a Nanomechanical testing system. The experiments of ramp load scratching with the single-tip/multi-tip indenter tools were conducted on glass-ceramics to compare the material removal behavior under different scratch methods. The surface morphology, subsurface cracks propagation, and the brittle-ductile transition region were observed and investigated. Based on the scratch experiments, the finite element simulation model was established to analyze the stress characteristics in the glass-ceramics. The result shows that in multi-tip scratches, as the depth of scratch increases, the growth of surface cracks mainly takes three forms. Compared with non-instant multi-scratch, instant multi-scratch makes the propagation path of lateral cracks closer to specimen surface, suppresses the propagation severity of lateral and median cracks, and increases the depth of brittle-ductile transition region. This paper's results provide a fundamental understanding of multiple abrasive grains instant interaction on the material removal mechanism, and help to improve the clinical performance of glass-ceramics as biomedical materials.

Effect of different surface treatments on the biaxial flexure strength, Weibull characteristics, roughness, and surface topography of bonded CAD/CAM silica-based ceramics

OBJECTIVE

To investigate the influence of different surface treatments on biaxial flexure strength, roughness, and surface topography of lithium silicate/disilicate-based ceramics.

METHODS

225 discs (∅: 12 mm; 1.2 mm - ISO 6872) were made from three ceramics: IPS e.max CAD (LD - Ivoclar Vivadent), Suprinity (LSS - Vita) and Celtra Duo (LSC - Dentsply). The samples were randomly divided into 5 groups (n = 15): no treatment (C); 10% hydrofluoric acid + silane (HF); sandblasting Al₂O₃ + silane (SB); silicatization + silane (SC); and self-etching ceramic primer (SEP). After surface treatment, a resin cement layer was applied to the disc surface (RelyX U200, 3M ESPE), mechanical cycled (1.2 × 10⁶ cycles, 50 N, 3.8 Hz) and submitted to biaxial flexural strength test (1 mm/min, 1000 Kgf). Roughness, EDS and SEM were also performed. Data were analyzed by one-way ANOVA, Tukey test (5%) and Weibull.

RESULTS

ANOVA revealed that the "surface treatment" factor was significant for all ceramics (p < 0.05). The groups LD-HF (289.30 ± 40) LD-SEP (298. 87 ± 53.29), LSC-HF (195.51 ± 42.12), LSS-HF (269.58 ± 27.07) and LSS-SEP (207.45 ± 28.63) presented significantly higher biaxial flexure strength than respective control groups, except for the LSC-SEP (165.41 ± 33.86), which was statistically similar to the control. The Weibull modulus was significantly higher for the LD-SB, LSC-SC groups. Additionally, the LD-SB, LSC-SC and LSS-HF groups showed higher roughness compared to the other treatments.

SIGNIFICANCE

HF etching followed by silanization and self-etching ceramic primer are the most suitable surface treatments for lithium silicate/disilicate-based glass-ceramics.

Improved reliability of mechanical behavior for a thermal tempered lithium disilicate glass-ceramic by regulating the cooling rate

Traditional thermal tempering was applied to lithium disilicate (LD) glass-ceramic specimens with bar-like and disc-like shapes. The tempering process was conducted by heating the specimens to a temperature below the dynamic softening point, and then rapid cooling in silicone oil with different temperatures ranging from room-temperature to 300 °C to regulate the cooling rate. Effect of the oil-temperature on mechanical behavior of the tempered glass-ceramic was investigated. For the tempering at the lower oil-temperature (e.g., at room-temperature), it was found that the LD glass-ceramic specimens with both the bar-like and disc-like shapes could be remarkably strengthen and toughen, however, obvious anisotropy in fracture toughness was displayed by the specimens with the bar-like shape. With increasing the oil-temperature up to 250 °C, the mechanical anisotropy of the bar-like specimens could be significantly alleviated without much loss of the strengthening effect. The results can provide references for improving reliability of mechanical behavior for the tempered LD glass-ceramic by regulating the cooling condition according to specimen geometry.

Optimization of mechanical and tribological properties of a dental SiO2-Al2O3-K2O-CaO-P2O5 glass-ceramic

Dental glass-ceramics with main crystal of fluorapatite are usually employed as veneering porcelain. However, failure of the porcelain happens clinically in the form of fracture or excessive wear. The aims of the study were to evaluate the influence of microstructures of a SiO₂-Al₂O₃-K₂O-CaO-P₂O₅ fluorapatite glass-ceramic on its mechanical properties and tribological behaviors, and to improve the comprehensive performance by adjusting content and sintering process. The glass-ceramics were fabricated by sintering method with different CaO contents and heat treatment regimes. Phase compositions and crystal length of specimens were characterized by X-ray diffractometer and scanning electron microscope. Combined with mechanical properties, friction and wear behaviors in both dry and artificial saliva lubrication conditions were investigated. The results show that different content of CaO and heat treatment temperature could change crystallinity of main fluorapatite crystal. Larger crystallinity improves the mechanical properties, significantly influencing friction and wear behaviors. The specimens with 6.0 wt % CaO and sintered at 1100 °C have the best comprehensive performance, which show excellent mechanical properties and wear resistance.

Microstructural development during heat treatment of a commercially available dental-grade lithium disilicate glass-ceramic

OBJECTIVE

To elucidate the microstructural evolution of a commercial dental-grade lithium disilicate glass-ceramic using a wide battery of in-situ and ex-situ characterization techniques.

METHODS

In-situ X-ray thermo-diffractometry experiments were conducted on a commercially available dental-grade lithium disilicate glass-ceramic under both non-isothermal and isothermal heat treatments in air. These analyses were complemented by experiments of ex-situ X-ray diffractometry, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, differential scanning calorimetry, and field-emission scanning electron thermo-microscopy.

RESULTS

It was found that the non-fired blue block consists of ∼40 vol % crystals embedded in a glass matrix. The crystals are mainly lithium metasilicate (Li₂SiO₃) along with small amounts of lithium orthophosphate (Li₃PO₄) and lithium disilicate (Li₂Si₂O₅). Upon heating, the glassy matrix in the as-received block first crystallizes partially as SiO₂ (i.e., cristobalite) at ∼660 °C. Then, the SiO₂ crystals react with the original Li₂SiO₃ crystals at ∼735 °C, forming the desired Li₂Si₂O₅ crystals by a solid-state reaction in equimolar concentration (SiO₂ + Li₂SiO₃ → Li₂Si₂O₅). Precipitation of added colourant Ce ions in the form of CeO₂ appears at ∼775 °C. These events result in a glass-ceramic material with the aesthetic quality and mechanical integrity required for dental restorations. It also has a microstructure consisting essentially of elongated Li₂Si₂O₅ grains in a glassy matrix plus small cubic CeO₂ grains at the outermost part of the surface.

SIGNIFICANCE

It was found that by judiciously controlling the heat treatment parameters, it is possible to tailor the microstructure of the resulting glass-ceramics and thus optimizing their performance and lifespan as dental restorations.

Contemporary Dental Ceramic Materials, A Review: Chemical Composition, Physical and Mechanical Properties, Indications for Use

BACKGROUND

The high esthetic expectations from the prosthodontic restorations have directed the qualitative development of the materials towards the all-ceramic materials that are capable of replacing porcelain-fused-to-metal systems.

AIM

This article reviews the literature covering the contemporary all-ceramic materials and systems with a focus on the chemical composition and materials' properties; also it provides clinical recommendations for their use.

RESULTS

The glass-matrix ceramics and polycrystalline ceramics are presented, as well as recently introduced machinable materials, all-zirconia and resin-matrix ceramics. The specific properties of zirconia, such as transformation toughening, stabilisation of the crystallographic structure, low-temperature degradation and factors affecting the zirconia's ageing, are emphasised.

CONCLUSION

The favourable properties of the resin-matrix ceramics, such as modulus of elasticity similar to dentin, shock-absorbing characteristics and high resilience and fracture resistance, are also covered in this article.

Upgrading the glassy slag from waste disposal by thermal plasma treatment

The paper reports on the experimental results obtained from the production of glassy slag by the plasma smelting of a mixture of two different wastes. The combination of two wastes with different chemical compositions is a promising way to optimise the energy consumption in the disposal process. Asbestos-cement roof tiles (ACRTs) and fly ash from fluidised-bed boilers (with a weight ratio of 1:1) were used for the preparation of glassy (vitrified) slag. The thermal process facilitated a 14.4% reduction of the weight of the original mixture and a 72% volume reduction of the waste. The glassy slag is then adopted as a raw material in the production of porous materials intended for various architectural applications, thus eliminating the necessity for its further disposal. The formation of a porous glass-ceramic matrix, using the vitrified slag containing CaSO₄ as the pore-forming agent, is described in detail. A glass-ceramic foam with 66% porosity is formed by the rapid heating of the mixture of glassy slag and a 1 wt% of CaSO₄, consisting of crystallised calcium aluminosilicate (Ca₂Al₂SiO₇, Ca_0.88Al_1.77Si_2.23O₈). The thermal conductivity of the prepared porous material, measured by a laser flash thermal analysis, is 0.22 W·m^-1·K^-1.

Dental Ceramics for Restoration and Metal Veneering

A survey of the development of dental ceramics is presented to provide a better understanding of the rationale behind the development and clinical indications of each class of ceramic material. Knowledge of the composition, microstructure, and properties of a material is critical for selecting the right material for specific applications. The key to successful ceramic restorations rests on material selection, manufacturing technique, and restoration design, including the balancing of several factors such as residual stresses, tooth contact conditions, tooth size and shape, elastic modulus of the adhesives and tooth structure, and surface state.

Multifunctional bioactive glass and glass-ceramic biomaterials with antibacterial properties for repair and regeneration of bone tissue

Bioactive glasses (BGs) and related glass-ceramic biomaterials have been used in bone tissue repair for over 30years. Previous work in this field was comprehensively reviewed including by their inventor Larry Hench, and the key features and properties of BGs are well understood. More recently, attention has focused on their modification to further enhance the osteogenic behaviour, or further compositional changes that may introduce additional properties, such as antimicrobial activity. Evidence is emerging that BGs and related glass-ceramics may be modified in such a way as to simultaneously introduce more than one desirable property. The aim of this review is therefore to consider the evidence that these more recent inorganic modifications to glass and glass-ceramic biomaterials are effective, and whether or not these new compositions represent sufficiently versatile systems to underpin the development of a new generation of truly multifunctional biomaterials to address pressing clinical needs in orthopaedic and dental surgery. Indeed, a number of classical glass compositions exhibited antimicrobial activity, however the structural design and the addition of specific ions, i.e. Ag⁺, Cu⁺, and Sr²⁺, are able to impart a multifunctional character to these systems, through the combination of, for example, bioactivity with bactericidal activity.

STATEMENT OF SIGNIFICANCE

In this review we demonstrate the multifunctional potential of bioactive glasses and related glass-ceramics as biomaterials for orthopaedic and craniofacial/dental applications. Therefore, it considers the evidence that the more recent inorganic modifications to glass and glass-ceramic biomaterials are able to impart antimicrobial properties alongside the more classical bone bonding and osteoconduction. These properties are attracting a special attention nowadays that bacterial infections are an increasing challenge in orthopaedics. We also focus the manuscript on the versatility of these systems as a basis to underpin the development of a new generation of truly multifunctional biomaterials to address pressing clinical needs in orthopaedic, craniofacial and dental surgery.

A comparative study of progressive wear of four dental monolithic, veneered glass-ceramics

OBJECTIVES

This study evaluated the wear performance and wear mechanisms of four dental glass-ceramics, based on the microstructure and mechanical properties in the progressive wear process.

METHODS

Bar (N = 40, n = 10) and disk (N = 32, n = 8) specimens were prepared from (A) lithium disilicate glass-ceramic (LD), (B) leucite reinforced glass-ceramic (LEU), (C) feldspathic glass-ceramic (FEL), and (D) fluorapatite glass-ceramic (FLU). The bar specimens were tested for three-point flexural strength, hardness, fracture toughness and elastic modulus. The disk specimens paired with steatite antagonists were tested in a pin-on-disk tribometer with 10N up to 1000,000 wear cycles. The wear analysis of glass-ceramics was performed using a 3D profilometer after every 200,000 wear cycles. Wear loss of steatite antagonists was calculated by measuring the weight and density using sensitive balance and Archimedes' method. Wear morphologies and microstructures were analyzed by scanning electron microscopy (SEM). The crystalline phase compositions were determined using X-ray diffraction (XRD). One-way analysis of variance (ANOVA) was used to analyze the data. Multiple pair-wise comparison of means was performed by Tukey's post-hoc test.

RESULTS

LD showed the highest fracture toughness, flexural strength, elastic modulus and crystallinity, followed by LEU and FEL, and FLU showed the lowest. However, the hardness of LD was lower than all the other three types of ceramics. For steatite antagonists, LD produced the least wear loss of antagonist, followed by LEU and FEL, and FLU had the most wear loss. For glass-ceramic materials, LD exhibited similar wear loss as LEU, but more than FLU and FEL did. Moreover, fracture occurred on the wear surface of FLU.

CONCLUSIONS

In the progressive wear process, veneering porcelains showed better wear resistance but fluorapatite veneering porcelains appeared fracture surface. Monolithic lithium disilicate glass-ceramics with higher mechanical properties showed more wear loss, however, they did not fracture and produced less wear loss of antagonists.

Three-dimensional characterization and distribution of fabrication defects in bilayered lithium disilicate glass-ceramic molar crowns

OBJECTIVE

To investigate and characterize the distribution of fabrication defects in bilayered lithium disilicate glass-ceramic (LDG) crowns using micro-CT and 3D reconstruction.

METHODS

Ten standardized molar crowns (IPS e.max Press; Ivoclar Vivadent) were fabricated by heat-pressing on a core and subsequent manual veneering. All crowns were scanned by micro-CT and 3D reconstructed. Volume, position and sphericity of each defect was measured in every crown. Each crown was divided into four regions-central fossa (CF), occlusal fossa (OF), cusp (C) and axial wall (AW). Porosity and number density of each region were calculated. Statistical analyses were performed using Welch two sample t-test, Friedman one-way rank sum test and Nemenyi post-hoc test. The defect volume distribution type was determined based on Akaike information criterion (AIC).

RESULTS

The core ceramic contained fewer defects (p<0.001) than the veneer layer. The size of smaller defects, which were 95% of the total, obeyed a logarithmic normal distribution. Region CF showed higher porosity (p<0.001) than the other regions. Defect number density of region CF was higher than region C (p<0.001) and region AW (p=0.029), but no difference was found between region CF and OF (p>0.05). Four of ten specimens contained the largest pores in region CF, while for the remaining six specimens the largest pore was in region OF.

SIGNIFICANCE

LDG core ceramic contained fewer defects than the veneer ceramic. LDG strength estimated from pore size was comparable to literature values. Large defects were more likely to appear at the core-veneer interface of occlusal fossa, while small defects also distributed in every region of the crowns but tended to aggregate in the central fossa region. Size distribution of small defects in veneer obeyed a logarithmic normal distribution.

Vitrification of incinerated tannery sludge in silicate matrices for chromium stabilization

The vitrification process was applied for the stabilization and solidification of a rich in chromium ash that was the by-product of incineration of tannery sludge. Six different batch compositions were produced, based on silica as the glass former and sodium and calcium oxides as flux agents. As-vitrified products (monoliths) were either composed of silicate matrices with separated from the melt Eskolaite (Cr₂O₃) crystallites or were homogeneous glasses (in one case). All as-vitrified products were thermally treated in order to transform them to partially crystallized, i.e. devitrified products. Devitrification is an important part of the work since studying the transformation of the initial as-vitrified products into glass-ceramics with better properties could result to stabilized products with potential added value. The devitrified products were diversified by the effective crystallization mode and separated crystal phase composition. These variations originated from differences in: (a) batch composition of the initial as-vitrified products and (b) thermal treatment conditions. In devitrified products crystallization led to the separation of Devitrite (Na₂Ca₃Si₆O₁₆), Combeite (Na₄Ca₄Si₆O₁₈) and Wollastonite (CaSiO₃) crystalline phases, while Eskolaite crystallites were not affected by thermal treatment. Leaching test results revealed that chromium was successfully stabilized inside the as-vitrified monoliths. Devitrification impairs chromium stabilization, only in the case where the initial as-vitrified product was a homogeneous glass. In all other cases, devitrification did not affect successful chromium stabilization.

Synthesis, characterization and bioactivity of a calcium-phosphate glass-ceramics obtained by the sol-gel processing method

In this work, a calcium-phosphate glass-ceramics was successfully obtained by heat treatment of a mixture of 26.52 in wt.% of fluorapatite (Fap) and 73.48 in wt.% of 77S (77 SiO₂14 CaO9 P₂O₅ in wt.%) gel. The calcium phosphate-glass-ceramics was prepared by sol-gel process with tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), calcium nitrate and fluorapatite. The synthesized powders were characterized by some commonly used tools such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thin-film X-ray diffraction (TF-XRD). The obtained results seemed to confirm the nucleation and growth of hydroxyapatite (Hap) nano-phase in the glass. Moreover, an in-vitro evaluation of the glass-ceramic was performed. In addition, to assess its bioactive capacity, it was soaked in simulated body fluid (SBF) at different time intervals. The SEM, EDS and TF-XRD analyses showed the deposition of hydroxyapatite on the surface of the specimens after three days of immersion in SBF solution. The mechanical properties of the obtained material such as rupture strength, Vickers hardness and elastic modulus were measured. In addition, the friction coefficient of calcium phosphate-glass-ceramics was tested. The values of the composite of rupture strength (24MPa), Vickers hardness (214Hv), Young's modulus (52.3GPa), shear modulus (19GPa) and friction coefficient (0.327) were obtained. This glass-ceramics can have useful applications in dental prostheses. Indeed, this material may have promising applications for implants because of its content of fluorine, the effective protector against dental caries.

Vitroceramic interface deposited on titanium substrate by pulsed laser deposition method

Pulsed laser deposition (PLD) method was used to obtain biovitroceramic thin film coatings on titanium substrates. The composition of the targets was selected from SiO2-CaO-P2O5-(CaF2) systems and the corresponding masses were prepared using the sol-gel method. The depositions were performed in oxygen atmosphere (100mTorr), while the substrates were heated at 400°C. The PLD deposited films were analysed through different experimental techniques: X-ray diffraction, scanning (SEM, EDX) and transmission (HRTEM, SAED) electron microscopy and infra-red spectroscopy coupled with optical microscopy. They were also biologically tested by in vitro cell culture and the contact angle was determined. The bioevaluation results indicate a high biocompatibilty of the obtained materials, demonstrating their potential use for biomedical applications.

Glass-ceramic coated Mg-Ca alloys for biomedical implant applications

Biodegradable metals and alloys are promising candidates for biomedical bone implant applications. However, due to the high rate of their biodegradation in human body environment, they should be coated with less reactive materials, such, for example, as bioactive glasses or glass-ceramics. Fort this scope, RKKP composition glass-ceramic coatings have been deposited on Mg-Ca(1.4wt%) alloy substrates by Pulsed Laser Deposition method, and their properties have been characterized by a number of techniques. The prepared coatings consist of hydroxyapatite and wollastonite phases, having composition close to that of the bulk target material used for depositions. The 100μm thick films are characterized by dense, compact and rough morphology. They are composed of a glassy matrix with various size (from micro- to nano-) granular inclusions. The average surface roughness is about 295±30nm due to the contribution of micrometric aggregates, while the roughness of the fine-texture particulates is approximately 47±4nm. The results of the electrochemical corrosion evaluation tests evidence that the RKKP coating improves the corrosion resistance of the Mg-Ca (1.4wt%) alloy in Simulated Body Fluid.

Computational study of structural, elastic and electronic properties of lithium disilicate (Li(2)Si(2)O(5)) glass-ceramic

The objective of this study is to investigate theoretically the structural, elastic and electronic properties of Lithium Disilicate (LD) crystal (Li2Si2O5), using the pseudo potential method based on Density Functional Theory (DFT) with the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA). The calculated structural properties namely the equilibrium lattice parameters and cell volume are in good agreement with the available experimental results. However, for the LD crystal elastic moduli: Shear modulus G, Young's modulus E and Poisson's ratio ν we have found a discrepancy between our theoretical values and experimental ones reported in polycrystalline sample containing LD crystals. The calculated elastic properties show that LD is more rigid compared with other components. We also investigated the mechanical stability of Li2Si2O5 compound and we have noticed that this compound is stable against elastic deformations. On the basis of shear to bulk modulus ratio analysis, we inferred that Li2Si2O5 compound is brittle in nature. In order to complete the fundamental characteristics of this compound we have measured the elastic anisotropy. Our results for the energy band structure and Density of States (DOS) show that Li2Si2O5 compound has an insulator characteristic.

Low temperature sintering of fluorapatite glass-ceramics

Fluorapatite glass-ceramics have been shown to be excellent candidates as scaffold materials for bone grafts, however, scaffold production by sintering is hindered by concurrent crystallization of the glass. Objective, our goal was to investigate the effect of Ca/Al ratio on the sintering behavior of Nb-doped fluorapatite-based glasses in the SiO2-Al2O3-P2O5-MgO-Na2O-K2O-CaO-CaF2 system. Methods, glass compositions with Ca/Al ratio of 1 (A), 2 (B), 4 (C) and 19 (D) were prepared by twice melting at 1525°C for 3h. Glasses were either cast as cylindrical ingots or ground into powders. Disk-shaped specimens were prepared by either sectioning from the ingots or powder-compacting in a mold, followed by heat treatment at temperatures ranging between 700 and 1050°C for 1h. The density was measured on both sintered specimens and heat treated discs as controls. The degree of sintering was determined from these measurements. Results and Significance XRD showed that fluorapatite crystallized in all glass-ceramics. A high degree of sintering was achieved at 775°C for glass-ceramic D (98.99±0.04%), and 900°C for glass-ceramic C (91.31±0.10). Glass-ceramics A or B were only partially sintered at 1000°C (63.6±0.8% and 74.1±1.5%, respectively). SEM revealed a unique microstructure of micron-sized spherulitic fluorapatite crystals in glass-ceramics C and D. Increasing the Ca/Al ratio promoted low temperature sintering of fluorapatite glass-ceramics, which are traditionally difficult to sinter.