Fractal Analysis of Toughening Behavior in 3BaO.5SiO2Glass-Ceramics

The objective of this paper was to analyze the crystal aspect ratio (AR), the fracture toughness, and the fractal dimension of 3BaO^.5SiO₂ glass-ceramics and to relate the topography of fracture surfaces to fractal behavior. These analyses demonstrate that crystal morphology strongly affects the fracture path, the fracture toughness, and the fractal dimension. Fracture toughness increased from 0.7 ± 0.1 MPa^.m^1/2 for the glass to 2.2 ± 0.6 MPa^.m^1/2for the glass-ceramic with an AR of 8.1 while the fractal dimensional increment ( D *) for the glass and the glass-ceramic increased from 0.10 ± 0.01 to 0.25 ± 0.02, respectively. The materials with lower aspect ratios (AR = 1.4 and 3.6) exhibited the predicted relationship between toughness and D * while the glass-ceramic with an aspect ratio of 8.1 did not satisfy the expected relationship because of multiple toughening mechanisms.     

Mechanical Properties of Mica Glass-Ceramics

The mechanical properties of fluorophlogopite mica-based glass-ceramics, such as fracture toughness, flexural strength, and machinability, were investigated. It has been shown that toughening increments in the glass-ceramics occurred by crack deflection and branching by crystals with a high aspect ratio. All the glass-ceramics heat treated at 1000°–1150°C exhibited a higher fracture toughness of 1.2–2.2 MPa-m^1/2 as compared to 0.8 MPa-m^1/2 for the parent glass, and showed average flexural strengths of 140–160 MPa. It has been suggested that ( H/K_Ic )² of the mica glass-ceramic be used to estimate the machinability, because it decreases linearly with machinability.     

Effect of Alumina on the Strength, Fracture Toughness, and Crystal Structure of Fluorcanasite Glass-Ceramics

The influence of alumina content (0-15 wt% Al₂O₃) on the indentation strength, fracture toughness ( K _{I c} ), and crystal structure of fluorcanasite (Al₂O₃-CaO-F-K₂O-Na₂O-SiO₂) glass-ceramics was analyzed. Increasing the Al₂O₃ content from 0 wt% (CAN0) to 8 wt% (CAN8) caused the mean indentation strength and K _{I c} values to decrease from 213 ± 14 MPa and 2.7 ± 0.1 MPa·m^1/2, respectively, for the CAN0 glass-ceramic to 78 ± 16 MPa and 1.3 ± 0.2 MPa·m^1/2, respectively, for the CAN8 glass-ceramic. Increased Al₂O₃ concentrations (0-15 wt%) significantly affected the crystal size, crystal shape, aspect ratio, and crystal aggregation characteristics of the fluorcanasite glass-ceramics. The addition of greaterthan equal to8 wt% of Al₂O₃ to fluorcanasite glass caused a transformation from canasite to leucite.     

Crystallization behavior and mechanical properties of high strength mica-containing glass-ceramics from granite wastes

The high-strength mica-containing glass-ceramics were prepared from granite wastes by bulk crystallization. The influences of SiO₂/Al₂O₃ molar ratio (S/A = 7.72, 9.62, 12.58, 17.82 and 29.67) on the crystallization behavior, microstructure, mechanical properties and machinability of glass-ceramics were investigated. The results demonstrated that the polymerization degree of the glass network decreased with the S/A ratio increasing, which further caused the decrease in glass transition temperature and crystallization temperatures. The increase in the S/A ratio promoted the precipitation of diopside, hectorite, kalsilite and tainiolite in glass-ceramics when the samples were heated at 750 °C, while inhibiting the precipitation of forsterite. For the glass-ceramics crystallized at 800 and 900 °C, the main crystalline phases transformed from diopside, forsterite, and nepheline to diopside, kalsilite, and tainiolite, with the S/A ratio increasing. As the SiO₂ gradually replaced Al₂O₃, the morphology of crystals changed from lamellar to granular, while the mean size of crystals reduced. The Vickers-Hardness values of glass-ceramics crystallized at 800 and 900 °C ascended with S/A ratio rising, and the values were above 6.30 GPa. The bending strength of most glass-ceramics is stable between 90 and 140 MPa, among which the maximum bending strength is 133.28 ± 14.81 MPa. The fracture toughness of the glass-ceramic crystallized at 800 and 900 °C declined, while that at 700 °C increased with a larger S/A ratio. Glass-ceramics after heat-treated at 900 °C with S/A ratio of 9.62 had the largest fracture toughness of 3.28 ± 0.15 MPa m^1/2. In preliminary tests of machinability, glass-ceramic after heat-treated at 900 °C with S/A ratio of 9.62 showed better results.     

Tensile fatigue of a laminated carbon-carbon composite at room temperature

The tensile fatigue behavior of a cross-ply carbon-carbon (C/C) laminate was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 213 MPa (93% of the tensile strength), and no fracture was observed at over 10⁴ cycles. The residual tensile strength of specimens that survived fatigue loading was enhanced with increase in fatigue cycles and applied stress. Observations of the fatigue-loaded specimens revealed that the formation of micro-cracks at the fiber-matrix interfaces was facilitated during fatigue loading. These interfacial cracks were concluded to protect the fibers from being damaged by matrix cracks and this behavior was considered to be the governing mechanism of strength enhancement by fatigue loading.     

Influence of heat treatment temperature on the properties of the lithium disilicate-fluorcanasite glass-ceramics

This study aims to investigate the influence of heat treatment temperatures on the mechanical properties and chemical solubility (CS) of lithium disilicate-fluorcanasite glass-ceramics and to develop new dental materials. The glasses and glass-ceramics were prepared using CaF₂-SiO₂-CaO-K₂O-Na₂O-Li₂O-Al₂O₃-P₂O₅-based glass system using a conventional melt quenching method followed by a two-stage crystallization process. This two-stage method involves two heating temperature steps: first at a constant temperature (T_S1) of 600°C and second step at varying temperatures (T_S2) of 650, 700, 750, and 800°C. The crystallization behavior, phase formation, microstructure, translucency characteristic, density, hardness, fracture strength, and CS were investigated. It was found that the lithium disilicate crystal acted as the main crystalline phase, and the crystalline phase of fluorcanasite occurred at the heat treatment temperatures of 750 and 800°C. In addition, it was found that density, hardness, fracture strength, and CS increased while the translucency values decreased with increasing heat treatment temperatures. Furthermore, the CS increased dramatically when the fluorcanasite phases occurred in the glass-ceramic samples. The maximum density values, Vickers hardness, fracture toughness, and flexural strength are 2.56 g/cm³, 6.73 GPa, 3.38 MPa.m^1/2, and 259 MPa, respectively. These results may offer a possibility to design a new material for dental applications based on lithium disilicate-fluorcanasite glass-ceramics.     

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures were investigated in this research. Fatigue behavior tests were performed at 1200℃ and 1000°C at 10 Hz and stress ratio of 0.1 for maximum stresses ranging from 80 to 120 MPa, and the fatigue run-out could be defined as 10⁶ cycles. Evolution of the cumulative displacement and normalized modulus with cycles was analyzed for each fatigue condition. Fatigue run-out was achieved at 80 MPa and 1000°C. It could be found that the cycle lifetimes of the composites decreased sharply with the increasing maximum stress and temperature conditions significantly affected the fatigue performance under matrix cracking stress. The cumulative displacement showed no noticeable increase before 1000 cycles and the modulus of the failed specimens decreased before fracture. The retained properties of composites that achieved fatigue run-out, as well as the microstructures, were characterized in order to understand the fatigue behavior and failure mechanisms. The composites exhibited similar fracture morphology with matrix crack extension and glass phase oxidation formation under different conditions. In general, the high-temperature fatigue damage and failure of composites could be affected by combination of stress damage and oxidative embrittlement.     

Synergistic effect of glass fibre and Al powder on the mechanical properties of glass-ceramics

To explore the synergistic effect of glass fibre and Al powder on the mechanical properties of glass-ceramics, blast furnace slag was chosen as the main material, and glass fibre and Al powder as reinforcement materials. The phase compositions, microstructures, compressive properties, and apparent density of the glass-ceramics with varying quantities of glass fibre and Al powder were investigated. The experimental results indicated that Al powder could exist as a simple substance in glass-ceramics and form a dense net coating on the surface of blast furnace slag to improve the plasticity of the glass-ceramic. The glass fibre had better reinforcement effect than Al powder because of its extremely high mechanical strength. The plasticity of glass-ceramics, however, severely decreased; the glass-ceramics exhibited brittle failure during compression. A slight increase in the content of CaSi₂ and SiO₂ in the glass-ceramics was closely related to the addition of glass fibre. Considering safety and economy, glass-ceramics with 6% Al and 14% glass fibre (S4) have the best mechanical properties. The compressive strength, strain at maximum force, and apparent density were 40?MPa, 19% and 1.974?g/cm³, respectively.     

Influence of Loading Frequency on the Room-Temperature Fatigue of a Carbon-Fiber/SiC-Matrix Composite

The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio (σ_min/σ_max) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the postfatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 × 10⁶ cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1- and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1 × 10⁶ fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0° and 90° fiber bundles.     

Effect of notches and glass fiber reinforcement on fatigue behavior of polycarbonate

In order to investigate the effect of a notch on the tensile properties of polycarbonate and 30% glass fiber-reinforced polycarbonate, two types of notched specimens were prepared. These notches were a sharp 60° notch and a dull notch with rounded tip 1.5 mm in radius at the base of the 60° notch. The notches decreased the tensile strength of polycarbonate. The sharp notch reduced tensile strength more effectively than the dull notch. In 30% glass fiber-reinforced polycarbonate, even the dull notch decreased the tensile strength considerably. Unnotched polycarbonate was subjected to cyclic tensile loading of 10⁴ cycles at 10 Hz, with varying cyclic stress. It was found that the elongation at break decreased rapidly with increase in cyclic stress. The notches considerably decreased the tensile fatigue strengths of polycarbonate and glass fiber-reinforced polycarbonate in 10⁴ cycles at 10 Hz.     

Strength evolution of cyclic loaded LSI-based C/C-SiC composites

An experimental investigation was performed to study the influence of fatigue damage introduced by different loading cycles on the residual tensile strength (RTS) of plain-weave reinforced C_f/C-SiC composites (2D C/C-SiC). The specimens were subjected to the fatigue stress of 57 MPa for the preselected numbers of cycles as follows: 10², 10⁴ and 10⁵, respectively, before the static tensile test. The microstructures and fractured surfaces after the tensile test were examined by optical and scanning electron microscopy, respectively. The results showed that the RTS of the specimens after the preselected fatigue cycles numbers of 10², 10⁴ and 10⁵ increase to 89.8, 94.1 and 82.4 MPa, respectively, which are somewhat higher compared to the virgin samples (79.7 MPa). Additionally, we found that the linear part of the tensile stress-strain curve is independent on the fatigue cycles. Finally, the increased fatigue damage in C/C-SiC composites could determine a reduction of elastic modulus in all cases of fatigue tests.     

Fatigue behavior of filament‐wound glass fiber reinforced epoxy composite tubes under tension/torsion biaxial loading

A study of filament‐wound glass fiber/epoxy composite tubes under biaxial fatigue loading is presented. The focus is placed on fatigue lives of tubular specimens under tension/torsion biaxial loading at low cycle up to 100,000 cycles. Filament‐wound glass‐fiber/epoxy tubular specimens with three different lay‐up configurations, namely [±35°]_n, [±55°]_n, and [±70°]_n lay‐ups, are subjected to in‐phase proportional biaxial cyclic loading conditions. The effects of winding angle and biaxiality ratio on the multiaxial fatigue performance of composites are discussed. Specimens are also tested under two cyclic stress ratio: R = 0 and R = −1. The experimental results reveal that both tensile and compressive loading have an influence on the multiaxial fatigue strength, especially for [±35°]_n specimens. A damage model proposed in the literature is applied to predict multiaxial fatigue life of filament‐wound composites and the predictions are compared with the experimental results. It is shown that the model is unsuitable for describing the multiaxial fatigue life under different cyclic stress ratios. POLYM. COMPOS. 28:116–123, 2007. © 2007 Society of Plastics Engineers     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Characterisation of fluorine containing glasses and glass-ceramics by 19F magic angle spinning nuclear magnetic resonance spectroscopy

¹⁹F magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was used to characterise the local environment of fluorine in three types of fluorine containing glass-ceramics. The glass-ceramic compositions studied included four that crystallised to fluorcanasite, one which crystallised to barium fluorphlogopite and one which crystallised to fluorrichterite. In the fluorcanasite glasses, prior to crystallisation, the fluorine was present solely as an F–Ca(n) species whilst following crystallisation it was also present as an F–Ca(n) species in the fluorcanasite phase and in those glasses containing AlPO₄ it was also present as an F–Ca(n) species in fluorapatite.In the fluorrichterite and fluorphlogopite glasses the fluorine was present predominantly as F–Mg(3) and following crystallisation it was also present as F–Mg(3) in the fluorrichterite and fluorphlogopite phases. In all these glass-ceramics fluorine appears to be preferentially associated with the cations of highest charge to size ratio and the local environment of fluorine in the glass and the crystal phase is almost identical.     

Preparation and characterization of alkali-free glass substrates with enhanced properties for TFT–LCDs applications

To obtain an alkali-free glass substrate with enhanced properties for thin-film transistor–liquid crystal displays (TFT–LCDs) applications, we chose a base glass composed of 3B₂O₃-15Al₂O₃-58SiO₂-22MgO-0.5SrO-1.5MgF₂ (mol%) for nucleation–crystallization. The results show that when the nucleation–crystallization processes of the base glass are 810 °C/6 h + 880 °C/6–9 h, the prepared GC/6–GC/9 glass-ceramics exhibit enhanced properties because of the precipitation of nano-sized cordierite. The transmittances in the visible range of the GC/6–GC/9 glass-ceramics exceed 85%, the densities are 2.564–2.567 g/cm³, thermal expansion coefficients are 2.934–3.059 × 10^-6/°C (25–300 °C), compressive strengths are 417–589 MPa, bending strengths are 141–259 MPa, Vickers hardnesses are 6.8–7.8 GPa, and strain points are approximately 735 °C. Considering these properties, the prepared GC/6–GC/9 glass-ceramics have good potential as candidate materials for alkali-free glass substrates. Additionally, these results demonstrate that it is feasible to improve the properties of alkali-free glass substrates by nucleation–crystallization.     

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.     

Eu2+-doped oxyfluoride glass-ceramic with nepheline as an efficient 400 nm UV-LED color converter

SiO₂-Na₂O-Al₂O₃-LaF₃ glass-ceramics doped with Eu²⁺ were synthesized as an efficient inorganic color converter for 400 nm UV-LED. When Eu²⁺ formed within the glass matrix, the obtained glass showed cyan emission under 400 nm excitation, but its emission peak drastically shifted to greenish yellow upon heat treatment. With heat treatment the glass-ceramic also showed highly increased emission intensity, and the color coordinate of the glass-ceramic shifted to yellow. When it was mounted on top of a 400 nm UV-LED, it demonstrated high color conversion efficiency and practical feasibility as an UV-LED color converter. To vary the color coordination the heat-treatment conditions and the thickness of the glass-ceramic were adjusted. The resulting ceramic showed a high quantum yield of up to 78%, which is comparable to conventional ceramic phosphors. The spectral change in the glass-ceramic is attributed to the formation of nepheline and LaF₃ crystalline phases. X-ray diffractometer (XRD), transmission electron microscope with energy dispervise spectroscopy (TEM-EDS) and cathode luminescence (CL) were used to investigate the mechanism of Eu²⁺-doped nepheline crystal formation, and its effect on the spectral change with heat treatment.     

Nanosized cordierite–sapphirine–spinel glass-ceramics from natural raw materials

Inexpensive nanosized sintered cordierite glass-ceramic was prepared from quartz sand, kaolin, and magnesite. The addition of nucleation catalysts, such as TiO_2, Cr₂O_3, and admixed TiO₂–Cr₂O₃, was tested in the cordierite base glass. Cordierite, sapphirine, spinel, magnesium aluminium silicate, and cristobalite were developed using the crystallisation process. These glass-ceramics have ultra-fine grain sizes with nanorounded crystals measuring less than 200 nm, particularly in the Cr₂O₃-containing samples. Due to its different crystalline phases, the new glass-ceramics varied in hardness from 6374 to 8139 MPa and had coefficients of thermal expansion (CTE) from 0.83 to 6.89×10⁻⁶ °C⁻¹. In glass-ceramic samples, the spinel and sapphirine imparted high CTE (from 6.89 to 5.31×10⁻⁶ °C⁻¹) and hardness values (from 8139 to 7894 MPa), whereas cordierite provided lower CTE (from 0.83 to 2.66×10⁻⁶ °C⁻¹) and hardness values (from 7453 to 6374 MPa).     

High-strength,translucent glass-ceramics in the system MgO-ZnO-Al2O3-SiO2-ZrO2

In a MgO/Al₂O₃/SiO₂/ZrO₂ glass-ceramic MgO was substituted by equimolar ZnO concentrations. The effect of this substitution on the crystallization behavior, the microstructure and the mechanical properties of the glass-ceramics was studied. The crystal phases and the microstructure were analysed by X-ray diffraction and scanning electron microscopy. Tetragonal ZrO₂, a high-/low-quartz solid solution (high-/low-QSS) and spinel/gahnite were observed in the entire bulk of the glass-ceramics. Additionally, indialite or cristobalite are detected at the surface of some glass-ceramics. The substitution of small ZnO concentrations induces an increasing low-QSS concentration and hence higher microhardness and Young’s modulus. By contrast, higher ZnO concentrations lead to a liquid/liquid phase separation in the glass. Moreover, spinel/gahnite is the main crystal phase and the concentration of the low-QSS is smaller in theses glass-ceramics which also do not show as good mechanical properties. However, the biaxial flexural strength of the glass-ceramics is not notably affected by the ZnO concentration.     

Microstructure,cytocompatibility, and chemical durability of chemically strengthened LAS (Li2O-Al2O3-SiO2) glass-ceramic materials

The K⁺-Na⁺ ion exchange was used to strengthen LAS glass-ceramic materials prepared by hot-pressing sintering. The microstructure, cytocompatibility, and chemical durability of the chemically strengthened LAS glass-ceramics were characterized. The XRD results showed that the K⁺-Na⁺ ion exchange mainly occurred between the glass phase of the LAS glass-ceramics and molten salt baths. The ion-exchange process was mainly responsible for the improved chemical durability of the LAS glass-ceramics. The dissolution in acetic acid was significantly reduced from 72 to 15 μg·cm^?2 after the ion-exchange treatment, which was attributed to residual compressive stress and increased contents of Q³ and Q⁴ structural units in the surface region of the LAS glass-ceramics. In addition, the chemically strengthened LAS glass-ceramic samples exhibited good biocompatibility determined by the CCK-8 process using the L929 cell line, having a promising potential as dental restorative materials.