Microstructure,residual stresses,and mechanical performance of surface crystallized translucent glass-ceramics

Mechanical properties of glasses can be significantly increased by inducing surface crystallization of a low coefficient of thermal expansion phase. In this work, we produced surface crystallized lithia-alumina-silica glass-ceramics with different crystallized layer thicknesses and analysed the resulting residual stresses and their effect on mechanical properties. The residual stress magnitude was estimated by analytical and experimental methods, as well as numerical modeling. The surface compressive stress reached 390 MPa and 490 MPa, as given by the analytical and experimental determination, respectively. These stresses prevented radial cracking in microhardness and scratch tests. The best glass-ceramic achieved a Vickers hardness of 7.5 GPa and fracture strength of 680 ± 50 MPa in a ball-on-three-ball test. These glass-ceramics are translucent, providing 50–60% transmittance over the visible wavelength spectrum (1.3 mm-thick-sample). This study unveiled the causes of improved mechanical properties and validates the concept that surface crystallization is a valuable technique for developing high strength glass-ceramics.     

Formation of translucent nanostructured zirconia ceramics

In this work the mechanisms that affect the optical transparency of nanostructured translucent ZrO₂ ceramics are studied. The translucent ceramic samples were obtained from a low agglomeration nanosized powder at low pressure and low temperature sintering. Even low pressures cause structural changes and defect creation in the nanocrystals. Annealing was used to study the grain formation, structure and impact of defects. Significant changes in translucency were observed with increase in pore size. In order to further understand the defect creation, the obtained ceramics were doped with Er³⁺ ions and studied optically. Photoluminescence studies revealed a change in the ratio of green to red parts of the spectrum as well as luminescence quenching when samples were pressed into pellets. Additionally, grain and pore size dependence on annealing temperatures was studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy.     

Mechanical properties of solid oxide fuel cell glass-ceramic sealants in the system BaO/SrO-MgO-B2O3-SiO2

Planar solid oxide fuel cells (p-SOFCs) require materials that can satisfy the high mechanical demands related to their utilization in stationary and, especially, in mobile applications. Two suitable glass-ceramic sealants based on the system BaO/SrO-MgO-B₂O₃-SiO₂ have been characterized with respect to their mechanical properties such as hardness, Young’s modulus, flexural strength at room and elevated temperature, fracture toughness as well as creep behavior at relevant operation temperatures (800 °C). Fracture toughness was calculated from crack opening displacements (COD) and the results were compared with fracture toughness measured by bending tests of notched bar samples. The mechanical behavior has been discussed regarding different thermal aging times of the glass-ceramics and their microstructural evolution. The glass-ceramics containing SrO revealed a better mechanical behavior than glass-ceramics with BaO. In particular, several superior properties were found in comparison to previously reported materials for this application.     

Effect of aging process on some properties of conventional and multilayered translucent zirconia for monolithic restorations

The purpose of this study was to evaluate the surface characteristics and optical and mechanical properties of materials through hydrothermal aging using different types of translucent monolithic zirconia.Three conventional materials (LT, KT, and ET) and three multilayered materials (LM, KM, and EM) of translucent monolithic zirconia were investigated. 45 bar-shaped specimens in each group were fabricated for the measurement of surface characteristics and mechanical properties and were randomly divided into 3 subgroups. 10 plate-shaped specimens in each group were fabricated for the measurement of optical properties. All specimens were autoclaved for 0, 5, and 10 h, respectively. The various properties of translucent monolithic zirconia, such as phase distribution, surface roughness, surface texture, elemental analysis, total transmittance (Tt), contrast ratio (CR), three point flexural strength (TFS), nanoindentation hardness (NH), and Young's modulus (YM), were measured through different tests. Statistical analyses were performed with one-way ANOVA, Tukey HSD post hoc test, independent sample t-test, paired sample t-test, and two-way ANOVA (α = 0.05).After aging, the monoclinic phase of all groups increased relative to the tetragonal phase. All groups exhibited an increase in the surface roughness and oxygen levels, but exhibited a decrease in the zirconium and yttrium levels (P < .05). All groups had increased microcracks and grain push-out. Tt increased and CR decreased in the LT, KT, and ET groups. However, Tt decreased and CR increased in the LM, KM, and EM groups (P < .05). The TFS, NH, and YM showed a decreasing tendency, but there were no significant difference in several materials (P > .05).Alterations in the surface characteristics were found on the surfaces of translucent monolithic zirconia after aging. The aging process caused conventional materials to become more translucent, but multilayered materials were more opaque. The various properties of translucent monolithic zirconia were affected by the materials fabricated with different types after aging.     

Strengthening of Al2O3 porous composites with a glass-ceramic binder doped with nanocopper

Glass ceramics are the result of controlled crystallisation of one or more crystalline phases embedded in the glass matrix, which usually results in better mechanical properties than conventional glasses. Crystalline phases precipitated in the vitrified bond improve mechanical strength and provide new functions that are not present in the original glass. The most widely encountered issue in standard composites is the presence of inclusions, generated during the process of thermal treatment, of finely dispersed crystalline phases, which are spread in the amorphous phase.Another way of increasing the mechanical properties of composites is via surface crystallisation of the binder. The mechanical strength can be significantly and permanently increased by inducing compressive stress at the glass surface. In the case of the binder designed here, the generated crystalline phase is willemite (Zn₂SiO₄), which is characterised by a low coefficient of thermal expansion (CTE). Zn₂SiO₄ usually crystallises at the glass surface, which results in the development of favourable compressive stresses in the glass-crystalline binder.Transition metal oxides significantly affect the crystallisation characteristics, thus influencing the thermal behaviour of glass ceramics. The majority of current publications focus on studies of the influence of copper on certain properties, and mainly the optical and photonic properties, and the influence of copper on the mechanical properties of silicate systems is still poorly recognised. This article investigates the mechanical and physical properties of porous Al₂O₃ composites with a glass-ceramic bond doped with nanocopper.     

Laser induced crystallization in lithium borate-bismuth tungstate glass-ceramic for photocatalytic dye degradation

This research presents the first study on laser-induced crystallization to prepare glass-ceramics for water-cleaning applications. In previous reports, glass crystallization was accomplished through heat treatment at a specific temperature determined by performing differential scanning calorimetry (DSC). The new method proposed in this study offers advantages in terms of speed, control, less damage, less contamination, and miniaturization for the crystallization of glass-ceramics. Lithium borate-bismuth tungstate glasses were prepared using the conventional melt-quench technique and then crystallized using a laser power source. The resulting glass-ceramic was analyzed using X-ray diffraction (XRD) and Raman spectroscopy to determine its structure and phase, while field-emission scanning electron microscopy (FE-SEM) was used to examine its morphology. The band gap of the glass-ceramic was determined to be 2.8 eV using UV–vis spectroscopy. The photocatalytic degradation of methylene blue dye (MB) was used as a model, achieving 65% degradation within 240 min of visible light irradiation.     

Optical and photostimulated luminescence properties of Eu:BaFBr translucent ceramics synthesized by SPS

Eu:BaFBr translucent ceramics were synthesized by the spark plasma sintering method, and then the crystal structure, optical and photostimulated luminescence (PSL) properties were investigated. The synthesized 0.05–0.5% Eu:BaFBr are translucent, and the optical transmittance is 30–50%. The photoluminescence and PSL spectra consist of an emission band peaking around 390 nm due to the presence of Eu²⁺ ion. The photoluminescence quantum yield and the sensitivity of PSL to X-rays are the highest when the doping concentration is 0.5%, and they are higher than those of the commercial Eu:BaFBr imaging plate (IP). In addition, the spatial resolution of an X-ray image obtained by using the 0.5% Eu:BaFBr translucent ceramic as an imaging plate is higher than the one obtained by using the commercial IP.     

Anorthite-based transparent glass-ceramic glaze for ceramic tiles: Preparation and crystallization mechanism

A series of high hardness anorthite-based transparent glass-ceramic glaze was successfully developed for ceramic tiles. These glazes were prepared by mixing a high-calcium frit (denoted as HCa) that can crystallize anorthite with other fluxes. The effect of HCa content on the crystalline phase, microstructure and properties of the glaze was studied. In addition, the phase transformation, microstructure evolution and crystallization kinetics of anorthite from frit glaze were investigated in detail. The results revealed that when the HCa content was 45–55 wt%, the glazes had excellent transparency with a hardness of 6.4–7.3 GPa, which was higher than conventional glazes. The XRD and SEM tests showed the crystallization of single-phase lamellar-shaped anorthite crystals up to 20–40 μm in length. Anorthite was found to crystallize directly from the frit glaze at 850 °C. The laminated anorthite was composed of thinner lamellae, and cracks were observed inside the large crystals. The activation energy of anorthite crystallization was about 420 kJ/mol, and the Avrami index (n) was 2.03?2.36. The n value indicates that the crystal is grown in two dimensions, which explains the formation of lamellar-shaped anorthite in SEM.     

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.     

Microstructural development during crystallization firing of a dental-grade nanostructured lithia-zirconia glass-ceramic

The microstructural development during crystallization firing of a commercially-available dental-grade nanostructured lithia-zirconia glass-ceramic (Vita Suprinity® PC) was unraveled using a wide battery of ex-situ and in-situ characterization techniques. It was found that the milling blocks are slightly crystallized glass-ceramics, with a complex chemical composition and consisting of partially de-polymerized glass plus lithium silicate (Li₂SiO₃) nanocrystals. It was also found that during crystallization firing the glassy matrix first reacts with part of the Li₂SiO₃ to form lithium disilicate (Li₂Si₂O₅) at ～810?820 °C, and then lithium orthophosphate (Li₃PO₄) precipitates from the glass. This results in glass-ceramics with abundant nanocrystals embedded in a sparse zirconosilicate glass matrix (containing many other cations subsumed) that, due to its high viscosity, inhibited crystal growth. Therefore, these dental glass-ceramics are not reinforced with zirconia (ZrO₂) crystals unless over-fired above ～890 °C and at the expense of its singular nanostructure. Finally, this study opens doors for optimizing the clinical performance of these dental glass-ceramics via microstructural tailoring.     

Improved ferroelectric,piezoelectric, and dielectric properties in pure KNN translucent ceramics by optimizing the normal sintering method

In this study, it is reported that various properties can be selectively derived in a pure (K_0.5Na_0.5)NbO₃, KNN ceramics through optimizing the sintering temperature by the conventional sintering method. High piezoelectric, ferroelectric, and dielectric properties such as d₃₃ = 127 pC/N, P_r = 31 μC/cm², and ε_r = 767 are obtained at the sintering temperature of 1100 °C. On the contrary, the specimen sintered at 1130 °C does not show high piezoelectric and ferroelectric properties, but it is translucent with a transmittance of 22% and 57% at the wavelength of 800 and 1600 nm respectively and shows a very high dielectric constant ε_r of 881. The origin of the high piezoelectric constant owes to large remanent polarization and dielectric constant, and dense microstructure with uniform distribution of large grains with the conjunction of relatively large crystal anisotropy. On the other hand, dense microstructure with almost no porosity, highly compacted grain boundaries, uniform distribution of grains, and relatively low crystalline anisotropy are responsible for the translucency and large dielectric constant of the ceramic specimens. This study demonstrates that the lead-free KNN ceramic has the potential to show multiple noteworthy properties such as piezoelectric, ferroelectric, dielectric, and transparent properties. This work provides a pure KNN ceramic simultaneously with high piezoelectric and transparent characteristics prepared only by using the conventional sintering method at a moderate sintering temperature for the first time in the literature.     

Thermal expansion investigation of iron rich glass-ceramic

A theoretical procedure was developed with the aim of obtaining an estimation of the crystallisation fraction in glass-ceramics by means of thermal expansion measurements. The theoretical model was applied to the crystallisation of an iron rich glass composition yielding magnetite and pyroxene as crystalline phases.The crystallisation mechanism was investigated by means of DTA, XRD, FTIR, TEM; the thermal expansion by Differential Dilatometer. A relationship between the weight fraction of crystal phase and the linear thermal expansion coefficient was obtained. The values of the fraction of crystal phase, evaluated by means of dilatometric measurements, showed a good agreement with the results obtained by means of density variation and XRD analyses.The crystallisation kinetic of the iron rich glass, at 630 and 700 °C, was also investigated by dilatometry and the variation of CTE as a function of the crystallisation time explained in terms of increasing of the apparent viscosity of the residual glass.     

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.     

A novel low-clay translucent whiteware based on anorthite

This study was carried out to characterize the properties of a novel low-clay translucent whiteware suitable for daily use. The low-clay whiteware is produced from coarsely and finely milled prefired materials of the same composition plus a small amount of clay. It consists of anorthite (CaO·Al₂O₃·2SiO₂) and mullite (3Al₂O₃·2SiO₂) crystalline phases and a glassy phase with high crystalline to glassy phase ratio. The development of needle shaped long mullite crystals that were forming three dimensional interlocking network had significant effect on the elimination of pyroplastic deformation during glaze firing. Typical flexural strength and fracture toughness values were ∼110 MPa and ∼1.85 MPam^1/2, respectively. The low-clay whiteware had relatively low (4.6 × 10⁻⁶/°C) thermal expansion coefficient which made possible to glaze the whiteware with a typical hard porcelain glaze. A continuous interface layer was produced between the whiteware and the glaze and no crack was present through layer because of expansion mismatch.     

Pressure filtration assisted gel casting in translucent alumina ceramics fabrication

High density and homogeneous green body is important for fabricating transparent ceramics. Inspired by the improved samples’ homogeneity in slip casting, we introduced low pressure filtration to gel casting to further increase the relative density and homogeneity of the green body and to shorten the drying time. The effect of pressure filtration on drying characteristics, bulk density and porosity of the pre-sintered bodies, and the microstructure and optical transmittance of the resultant ceramics were intensively investigated. The results showed that pressure filtration can reduce the drying time by 27.3%, improve the density of the pre-sintered bodies and reduce the residual pores of the sintered ceramics. The in-line transmittance of the translucent alumina prepared by the pressure filtration assisted gel casting was 32.7% at 600?nm with a thickness of 1?mm, which is about 6% higher than that without pressure filtration. In principle, this process can be applied to fabricate any other high-performance ceramic.     

Bi-layer glass-ceramic sealant for solid oxide fuel cells

A bi-layered concept of glass-ceramic (GC) sealant is proposed to overcome the challenges being faced by solid oxide fuel cells’ (SOFCs). Two separated layers composed of glasses (Gd-0.3 and Sr-0.3) were prepared and deposited onto interconnect materials using a tape casting approach. After heat treating the bi-layered structure at 850 °C for 1–100 h, smooth and void free interfaces over the entire cross-section of joint were obtained. Micro-Raman analysis confirmed the presence of a higher amount of residual glassy phase in Gd-0.3 in comparison to Sr-0.3. The bi-layered GC showed good wetting and bonding ability to the Crofer22APU metallic plate. Slight increase of electrical conductivity with increasing annealing time was observed due to partial crystallization of the glass, but the overall conductivity levels of GC bi-layers were low enough to grant good electrical insulation. This set of relevant properties makes the investigated bi-layered sealants suitable for SOFC applications.     

The impact of nano-quartz on the structure of glass-ceramic glazes from the SiO2–Al2O3–CaO–MgO–Na2O–K2O system

The present study reports the impact of the introduction of nano-grained quartz (SA = 325 m²/g) into the composition on the structure and properties of the ceramic glaze of the CMAS-Na₂O–K₂O system. The results were compared to the glaze which had an identical oxide composition, with a difference that quartz was introduced in the form of quartz powder with a much smaller specific surface area SA = 1.41 m²/g. Both glazes are characterized by a large part of the glassy phase, above 90% by volume. The results obtained show a higher arrangement of the continuous glassy phase structure in the glaze with the addition of nano-quartz. This glaze also shows significantly higher values for all measured mechanical properties. It seems that if, in the near future, new cheaper methods for the production of nano-quartz are developed, it will be a new interesting direction of research aimed at improving the parameters of glazes and glass-ceramic materials of CMAS type.     

Fabrication and properties of dense α-cordierite glass-ceramic coating on porous BN/Si2N2O ceramic

α-Cordierite glass-ceramic coating was fabricated on the porous BN/Si₂N₂O ceramic by glass-ceramic method. The effect of the heating temperature on the phase composition, microstructure, mechanical properties, water resistance and dielectric properties of the coatings was investigated. A large amount of α-cordierite precipitated from the glass phase when the heating temperature was 1050?°C and the content of α-cordierite in the coating increased with increasing the heating temperature. The resulting α-cordierite glass-ceramic coatings had a good wettability and adhesion with the porous ceramic substrate. The coating/substrate interface was continuous without defects. When the heating temperature was 1050–1200?°C, the resulting coatings possessed positive mechanical properties and good water resistance due to the high densification. And the dielectric constant and loss tangent of the coated samples prepared at 1050–1200?°C were 4.1–4.3 and 0.005–0.01 respectively in the frequency of 21–36?GHz.     

Scintillation and dosimeter properties of CaF2 translucent ceramic produced by SPS

We have developed CaF₂ translucent ceramics by spark plasma sintering (SPS) and investigated the scintillation and dosimeter properties, in comparison with a CaF₂ single crystal. Under X-ray irradiation, light emission was observed at 270 and 300 nm, due to self-trapped exciton (STE). The scintillation light yield of CaF₂ translucent ceramic was measured to be about 6000 photons/MeV under ¹³⁷Cs irradiation, and this value was smaller than that of the single crystal by a factor of approximately 2. The afterglow level, after X-ray irradiation, of the ceramic sample was lower than that of the single crystal. Thermally stimulated luminescence (TSL) glow peaks were observed at 100 and 150 °C in both ceramic and single crystal samples, however the TSL intensity was much stronger than that of the single crystal sample.     

Improvement in structural,dielectric and energy-storage properties of lead-free niobate glass-ceramic with Sm2O3

Phase evolution, microstructure, dielectric performance, polarization, breakdown strength as well as energy-storage behaviors for the lead-free niobates glass-ceramics with Sm₂O₃ were systematically investigated. Two crystallographic structures of tetragonal tungsten bronze and orthorhombic perovskite complex phases were obtained and Sm³⁺ entered into the crystalline phases. The optimal microstructure of the glass-ceramic was obtained with Sm₂O₃ of 2 mol%. Both dielectric constant and polarizability were enhanced with increasing Sm₂O₃. The breakdown strength and energy-storage behaviors of the glass-ceramics were also improved by increased Sm₂O₃. The highest breakdown field of 21.2 kV/mm, the highest charged (0.74 J/cm³) and discharged energy density (0.45 J/cm³) were obtained in the glass-ceramic with 2 mol% Sm₂O₃. It is due to the reduced interfacial polarization in this particular composition.