Preparation and experimental characterization of glass–alumina functionally graded materials

This work aims at investigating the effects of the processing conditions on the final microstructure of glass–alumina functionally graded materials (FGMs). The ingredient materials, i.e. a polycrystalline sintered alumina and a CaO–ZrO₂–SiO₂ glass, were accurately characterized, since their mechanical and thermal properties may deeply influence the fabricating process and the overall FGM behaviour. The functionally graded materials were obtained by means of percolation of the molten glass into the alumina substrate. Two types of samples were considered—the “Bulk” FGMs, produced starting from a glass bulk, and the “Powder” FGMs, produced starting from a glass powder; in both cases four different heating cycles were attempted. The functionally graded materials were analysed using a SEM-EDS and a X-ray diffractometer. Great attention was devoted to the resulting microstructure; moreover the depth of penetration was measured and related to the fabricating parameters, such as time and temperature.     

Investigation of the structure and mechanical properties of a novel dental graded glass/zirconia ceramic

Constructing graded structure is a promising solution to reduce the occurrence of cracking and delamination of bilayered zirconia prosthesis. In this work, a novel graded glass/zirconia ceramic was developed by utilizing the interdiffusion between dense zirconia and a novel lithium disilicate glass in the SiO₂-Li₂O-Al₂O₃ system. Results demonstrated that a graded glass/zirconia structure with a depth of about 300 µm was constructed, which exhibited obviously gradient characteristics in microstructure, glass content and mechanical properties. The hardness (H) and elastic modulus (EM) values at the surface reduced significantly (64% for H and 79% for EM), and increased gradually with depth of graded layer. When the graded layer was subjected to loading force, the biaxial flexural strength increased. The mechanism of the evolution of graded structure and change of strength were also elucidated in detail. This study provides a promising strategy to improve the interface stability of bilayered zirconia restoration.     

Effect of siliconizing temperature on microstructure and phase constitution of Mo–MoSi2 functionally graded materials

Mo–MoSi₂ functionally graded materials were prepared by a liquid phase siliconizing method. The microstructure, phase constitution, cross-section elemental distribution, grains size, and coating thickness of these materials were investigated with scanning electron microscopy (SEM), back scattered electron (BSE), energy dispersive spectroscope (EDS), glow discharge spectrum (GDS) and X–ray diffraction (XRD). The results indicate that the Mo–MoSi₂ functionally graded materials have a dense multi-layer structure, mainly composed of surface layer (Si–MoSi₂ layer, 1–10?µm), intermediate layer (MoSi₂ layer, 22–40?µm), transitional layer (Mo₅Si₃ and Mo₃Si layer, 2–3?µm) and Mo substrate. Moreover, the silicon concentration, grains size, and coating thickness increase gradually with the increasing temperature. The surfaces silicon concentrations are about 68–75?wt%, the average grains sizes of MoSi₂ columnar crystals are about 7.1–9.4?µm, and the coating thicknesses are about 28–35?µm.     

Fabrication of Functionally Graded SiO2‐Mo Material by Centrifugation and Floc‐Casting of Highly Concentrated Slurry

To fabricate functionally graded materials, a highly concentrated slurry of SiO₂‐Mo system was prepared and centrifugal force was applied in an attempt to achieve a graded composition. Subsequently, we formed a homogeneous green body with compositional gradation by floc‐casting at 80°C, which was then fired at 1750°C for 10 min in Ar. The sintered body had compositional ratios of SiO₂ and Mo as well as electrical conductivities that changed gradually along the direction of centrifugal force. The results demonstrate that centrifugation and control of slurry characteristics such as flocculation are effective in fabricating functionally graded SiO₂‐Mo materials.     

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

In this study, first, Gd₂Zr₂O₇/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd₂Zr₂O₇ coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO₂ laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd₂Zr₂O₇ was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.     

Development of functionally graded ZrB2–B4C composites for lightweight ultrahigh-temperature aerospace applications

In the present work, a lightweight three-layer ZrB₂–B₄C functionally graded composite material has been developed by spark plasma sintering route. The functionally graded material (FGM) is free from interlayer defects and displays a smooth transition between the individual layers. The composition of each layer was designed to reduce the overall density without sacrificing the functionality of the material. The density of the FGM is almost 40% lower than monolithic ZrB₂ and 24% lower than ZrB₂–30B₄C rendering it potentially very attractive for high-temperature aerospace applications. A detailed structural characterization of the FGM was carried out to evaluate the elemental distribution in the graded composite, as well as determine the spatial distribution of the crystalline phases. Vickers hardness was measured within each layer and in the interlayer regions to further evaluate the gradient structure and interlayer transitions. Longitudinal elastic constants of the FGM along the thickness and across the layers measured using ultrasound phase spectroscopy showed that despite the gradient structure, the FGM can be treated as a quasi-isotropic solid.     

Fabrication of functionally graded nano‐TiO2‐reinforced epoxy matrix composites

Functionally graded nano‐TiO₂ epoxy matrix composites were successfully fabricated using a centrifugal method. In the preparation of the composite, the aggregation of nano‐TiO₂ occurred during curing, which had a negative effect on the composite performance. To solve this problem, we introduced a silane coupling agent to modify the surface of the nano‐TiO₂, thereby improving the performance and mechanical properties simultaneously. The modified nano‐TiO₂ (s‐TiO₂) had better dispersion in the epoxy resin, making it possible to produce depth gradients of the mechanical properties of functionally graded materials (FGMs). The s‐TiO₂ was characterized with respect to functional groups, morphology, and chemical elements using transmission electron microscopy, X‐ray photoelectron spectroscopy, and Fourier‐transform infrared spectroscopy. The results show that a silane layer was successfully coated on the surface. Also, the gradients of the mechanical and permittivity properties of the FGM indicated that by modifying the surface of the nano‐filler, it is possible to fabricate nano‐filler‐reinforced epoxy matrix FGMs using a centrifugal method. POLYM. COMPOS., 35:557–563, 2014. © 2013 Society of Plastics Engineers     

Investigation of shrinkage control in Ni–Al2O3 (Metal–Ceramic) functionally graded materials

Nickel–alumina (metal–ceramic) graded composites were fabricated based on empirical relationships that were used to predict shrinkage expected during the sintering process. Compositions ranged from pure Al₂O₃ to nickel. Compositions with different sintering behaviors were composed of Ni powders having 3-μm particles and Al₂O₃ powders having 0.16-μm particles that were mixed in specific volume percentages. The green and sintered densities were measured for each layer of the multi-layered samples. Based on these measurements, an empirical formulation for shrinkage was derived to express the relationship between green and sintered densities. Shrinkage was predicted from the observed decrease in porosity during sintering. This was used to establish a relationship between shrinkage and composition. Dramatic shrinkage, which can lead to cracking of the sample, was resolved by minimizing shrinkage differences within the functionally graded materials using the derived empirical formulation for shrinkage; internal cracking was significantly reduced by maintaining a consistent shrinkage gradient.     

Ceramic layers formed on metals by reactive plasma processing

Thick layers of ceramics mainly composed of TaC, ZrO₂ and Ti₂AlN were successfully synthesized on the surfaces of Ta, Zr and TiAl by means of novel plasma dry processing with or without 20 MeV-electron beam irradiation as the pretreatment. The thickness of these ceramic layers reached 10–100 μm under appropriate process conditions. Vickers hardness of the layers including TaC, ZrO₂ and Ti₂AlN were estimated to be approximately 3000, 1300 and 3500 Hv[kg/mm²], respectively. The cross-sectional profiles of constituent elements graded gradually in the modified surface regions, suggesting that these ceramics must have some properties of functionally graded materials.     

Epoxy resin/polyurethane functionally graded material prepared by microwave irradiation

The glass‐transition temperature (T_g) and modulus (E) of graded material in a plastic–elastomer system prepared with layer‐by‐layer casting in connection with microwave curing were studied. Epoxy (EP) resin and polyurethane (PU) were selected as the plastic and elastomer components, respectively. The structure of the functionally graded material (FGM) was such that EP (E = 3.2 GPa, and T_g = 162°C) and PU (E = 0.069 GPa and T_g = ?54°C) were both surfaces, with a stepwise gradient in EP and PU content existing between the two over a thickness of 9 mm. Fourier transform infrared spectroscopy and scanning electron microscopy were used to investigate the PU content and the morphologies of the FGMs separately. Finite element analysis (FEA) was used to simulate the temperature and thermal stress distribution along the graded direction under a steady‐state, nonuniform temperature field. The results of FEA showed that the temperature and thermal stress distribution decreased along the graded direction. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 994–999, 2004     

Unusual Indentation Behavior of Alkali Metaphosphate Glass Above Glass Transition Temperature

The mixed alkali metaphosphate glass 12.5Li₂O–12.5Na₂O–12.5K₂O–12.5Cs₂O–50P₂O₅ (mol%) compressed under uniaxial stress shows unusually large recovery of its shape during heating above its glass transition temperature (T_g). To clarify the mechanism, the viscous, elastic, and plastic behavior of the metaphosphate glass was investigated with the depth‐sensing indentation method. From the load‐displacement (P–h) curves of the glass, a large recovery of the depth displacement was found during unloading above T_g. By analyzing the P–h curves with a viscous–elastic–plastic model, the large recovery was concluded to be due to larger elastic deformation than viscous deformation even above T_g. The phenomenon can be attributed to relaxation to the randomly oriented ‐P–O–P– chain structure from the oriented chain structure formed under stress in the metaphosphate glass .     

Hertzian-Crack Suppression in Ceramics with Elastic-Modulus-Graded Surfaces

Hertzian (spherical) indentation experiments were carried out in a graded alumina-glass composite whose Young's modulus increased with depth beneath the indented surface. An in situ processing method involving impregnation of a dense, fine-grained alumina by an aluminosilicate glass was employed to fabricate such a composite. With this technique, a monotonic, unidirectional variation in Young's modulus of as much as 50% was introduced over a distance of approximately 2 mm, while keeping the coefficient of thermal expansion and the Poisson ratio for the glass and the alumina nearly the same. The macroscopically graded, elastic composite so produced with nearly full density has essentially no macroscopic, long-range residual stresses following processing. The unidirectional variation in Young's modulus under the indenter is shown to fully suppress the formation of Hertzian cone cracks. Without these elastic-modulus gradients, cone-crack formation was observed in bulk glass and alumina. Finite-element analyses of spherical indentation on elastically graded substrates were also performed to develop a quantitative understanding of the experimental trends. It is reasoned that the present innovations, involving functionally graded surfaces and their in situ processing, provide new possibilities for enhancing certain contact-damage resistance characteristics in various ceramic materials for a broad range of engineering applications. Furthermore, this contact-damage-resistance phenomenon in functionally graded ceramics is elastic in nature, and is, therefore, likely to be immune to mechanical fatigue within the elastic limit.     

Barium Borosilicate Sealing Glasses Synthesized by a Sol–Gel Process: Chemical Interactions with a Stainless Steel and Gas‐Tightness of a SOFC

Several glasses synthesized by sol–gel route and based on the BaO–B₂O₃–X–Al₂O₃–SiO₂ (X = CaO, MgO) glass system have been investigated to evaluate their applicability as sealant for solid oxide fuel cell (SOFC). Chemical interactions with K41X stainless steel and hydrogen‐tightness of these materials were evaluated after operations at high temperatures over 1,000 h in air atmosphere. Formation of a new phase at the steel–glass interface and formation of porosity in the glass were observed and determined as critical problems over mid‐term operations. The role of MgO is important to obtain a gas‐tight sealing. Application of the glass paste without binder addition was performed in order to avoid possible residual porosity related problems. The best glass was finally used as sealant between anodic and cathodic compartments in complete SOFCs operated at 760 and at 800 °C. Open circuit voltages and power densities of the cells were recorded during the first hours of operation.     

Microstructure-based modelling and experimental investigation of crack propagation in glass–alumina functionally graded materials

The aim of the present work was the determination of the fracture mechanisms in glass–alumina functionally graded materials (FGMs). The investigation was performed by means of a combined approach based on microscale computational simulations, which provided for an accurate modelling of the actual FGM microstructure, and experimental analysis. The numerical results proved that microstructural defects, such as pores, deeply influenced the damage evolution. On the contrary, the minimization of the mismatch in the coefficients of thermal expansion of the ingredient materials allowed to obtain low thermal residual stresses, which did not relevantly affect the crack propagation. In order to support the numerical model, microindentation tests were performed on the cross-section of FGM specimens and the experimentally observed crack paths were compared to the computationally predicted ones.     

Fabrication strategy of complicated Al2O3-Si3N4 functionally graded materials by stereolithography 3D printing

For multi-ceramic materials based on the stereolithography (SL) principle, a 3D printing strategy was developed, and then an Al₂O₃-Si₃N₄ functionally graded material (FGM) ceramic part was fabricated using this strategy. Six groups of mixtures, with a Si₃N₄ content gradient of 20 vol% and a certain bimodal particle size distribution, were prepared using UV-curable pastes. A modified formula was proposed to evaluate the relationship between the actual minimum voidage of mixtures and the viscosities of their corresponding pastes. The viscosity of each paste was controlled using the prediction formula and optimization of dispersants. To design theprinting layer thickness, a mathematical relationship was established between Si₃N₄ content and curing depth of paste. The Al₂O₃-Si₃N₄ green body without deformation was printed using optimized parameters such as a layer thickness of 40 μm and a paste viscosity of ∼13,000 mPa·s. Finally, using debinding and sintering, denseparts having a complicated shape were obtained.     

Air-brazed Al2O3 joint with a novel bismuth glass

Polycrystalline alumina ceramics were air-brazed using bismuth glass with a chemical composition of 50Bi₂O₃–40B₂O₃–10ZnO (mol.%) at a relatively low temperature. ZnAl₂O₄ particles were formed insitu in the joints and the particles grew rapidly with an increase in joining temperature and holding time increasing. In addition, penetration of the alumina substrate by the glass became increasingly serious at higher temperatures and holding durations. The mechanical properties of the joints were investigated and the maximum shear strength was determined to be 50 MPa when brazed at 650 °C for 0 min.     

Joining of Cf/SiC composites and Si3N4 ceramic with Y2O3–Al2O3–SiO2 glass filler for high-temperature applications

C_f/SiC composites and Si₃N₄ ceramics are candidate materials for applications in thermal protection system. This paper investigated the joining of C_f/SiC and Si₃N₄ using Y₂O₃–Al₂O₃–SiO₂ glass. The reliability of joints was evaluated by thermal shock tests. In this present work, the typical joint structure was C_f/SiC-glass-Si₃N₄. The results demonstrate that Direct bonding has been identified as the interfacial bonding mechanism at the SiC/glass and glass/Si₃N₄ interfaces. The maximum shear strength of the C_f/SiC–Si₃N₄ joint was ~34 MPa, which delivered an effective method to achieve strong, reliable bonding between the dissimilar materials. In addition, after thermal shock for 10 cycles, the residual strength remained ~13 MPa. Bubbles instead of microcracks formed in the glass filler, which was the main factor causing the degradation of the joint performance. It is suggested that improving the high temperature resistance of joining materials is the key to realize the application of this joint structure.     

Crystallization of CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> glass prepared by float process

CaO–MgO–Al₂O₃–SiO₂ (CMAS) glass was prepared by float process. The effects of TiO₂ and heat-treatment on properties and crystallization behaviors of float glasses were investigated by atomic force microscope, differential scanning calorimeter, X-ray diffraction, electron probe microanalyzer, field emission scanning electron microscope and viscosity test. The results showed that CMAS parent glasses produced by float process had a high surface flatness (Ra is less than 80.1 ± 0.1 nm) and low tin penetration (14 μm). When the concentration of TiO₂ increased from 3.51 to 5.01 wt %, the glass transition temperature was decreased, and the crystallization temperature was shifted from 913 to 887°C using differential scanning calorimeter. Field emission scanning electron microscope images showed that phase separation was discovered in CMAS parent glass (containing 3.51 wt % TiO₂) treated at 670°C. Diopside as a major crystalline phase was precipitated in CMAS glass-ceramics nucleated at 700°C for 30 min and followed by crystallization at 910°C for 30 min.     

Novel nanometer alumina-silica insulation board with ultra-low thermal conductivity

Advanced nano-porous super thermal insulation materials are widely used in spacecraft, soler-thermal shielding, heat exchangers, photocatalytic carriers due to their low thermal conductivity. In this work, adopting dry preparation technology, nano-Al₂O₃, nano-SiO₂, SiC and glass fibers as raw materials, novel nanometer alumina-silica insulation board (NAIB) were prepared. The preparation process was simple, safe, and reliable. In addition, the NAIB exhibited a high porosity (91.3–92.3%), small pore size (39.83–44.15 nm), low bulk density (0.22–0.26 g/cm³), better volumetric stability, and low thermal conductivity (0.031–0.050 W/(m·K) (200–800 °C)), respectively. The as-prepared NAIB could render them suitable for application as high-temperature thermal insulation materials.     

Raman imaging as a useful tool to describe crystallization of aluminum/iron-containing polyphosphate glasses

Polyphosphate glasses are materials of a wide spectrum of applications in many fields. The subject of the work is polyphosphate glasses containing aluminum and iron. Three compositions of the glasses were obtained and the materials have been characterized in terms of their crystallization. The differences in crystallization behavior between powder and bulk materials were compared. The crystallized materials were analyzed by Raman scattering spectroscopy and X-ray diffraction method. It was evidenced that depending on the glass composition the main crystalline phases were Al(PO₃)₃, AlPO₄, FePO₄, Fe₃(P₂O₇), Fe₄(P₂O₇)₃, FePO₄. The glass crystallization leads to enrichment of the residual glassy phase in P₂O₅ and increases its polymerization. Thus, it was observed the glass inhomogeneities are being increased due to crystallization. The two dimensions spectral maps of the bulk crystallized samples were executed to describe the mechanism and type of crystallization. The depth profiling proves the differences between surface and interior phase composition.