Joining dense Si3N4 to porous Si3N4 using a novel glass-ceramic interlayer with precipitated β-LiAlSi2O6/Mg2SiO4

In this work, we developed a new β-LiAlSi₂O₆/Mg₂SiO₄ glass-ceramic interlayer to join dense Si₃N₄ and porous Si₃N₄ ceramics for high-temperature applications. The microstructure and phases of the joint were investigated using systematic analysis methods and the mechanism for the interfacial bonding was proposed. The double crystalline phases of β-LiAlSi₂O₆ and Mg₂SiO₄ precipitated in the joint seam during the joining process, forming a novel glass-ceramic interlayer with high crystallinity. Due to the good wettability of liquid glass on porous Si₃N₄, a gradient and dense infiltration layer was formed in porous Si₃N₄ and reinforced it, which improved the mechanical strength of the whole joint. In this case, the shear strength of the joint reached 78 MPa, and it could still maintain 74 MPa up to 850 °C, which fully demonstrated the high-temperature resistance of the joint.     

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.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Mechanical and dielectric properties of porous Si2N2O–Si3N4 in situ composites

Mechanical and dielectric properties of porous Si₂N₂O–Si₃N₄ in situ composites fabricated for use as radome by gel-casting process were investigated. The flexural strength of the Si₂N₂O–Si₃N₄ ceramics is 230.46 ± 13.24 MPa, the complex permittivity of the composites varies from 4.34 to 4.59 and the dissipation factor varies from 0.00053 to 0.00092 from room temperature to elevated temperature (1150 °C) at the X-band. In the porous regions, some Si₂N₂O fibers (50–100 nm in diameter) are observed which may improve the materials properties.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

Effect of ZrO2 addition on in-vitro bioactivity and mechanical properties of SiO2–Na2O–CaO–P2O5 bioactive glass-ceramic

Herein, a multicomponent bioactive glass (0Z, 46SiO₂–30CaO–18Na₂O–6P₂O₅, wt.%) is prepared via melting. ZrO₂ is introduced into the bioglass using two different methods, and then three types of glass-ceramic bulks are manufactured using low-cost pressureless sintering. The effect of ZrO₂ addition on the bioactivity and mechanical properties of the bioactive glass-ceramic is assessed. The results indicate that the main crystalline precipitate from the bioactive glass-ceramic is Na₂Ca(Si₂O₆). The crystallisation ability of the 5Z glass-ceramic, bioactive glass-ceramic with ZrO₂ added during melting at high temperature, is reduced because ZrO₂ participated in the reconstruction of the glass network. Further, the ZrO₂ addition led to a low rate of cation release when soaked in simulated body fluid, indicating a decreased bioactivity. At the same time, the 5Y bioactive glass-ceramic, prepared by mixing YSZ particles with 0Z using ball-milling, possesses not only the highest mechanical strength (about twice the strength of 0Z) but also a high bioactivity. This study presents a promising method for the production of excellent bioactive glass-ceramic and a candidate (5Y) for the clinical applications where load bearing is required.     

Effect of spray process on dielectric properties of APS-deposited CaO–B2O3–SiO2 glass-ceramic coatings

Low-dielectric properties are highly desirable for successful realization of thermal spray coatings in electromagnetic wave absorption. Herein, CaO–B₂O₃–SiO₂ (CBS) glass-ceramic coatings are prepared via high-enthalpy atmospheric plasma spraying (HE-APS) method, and the influence of spraying power on physical and dielectric properties of APS-deposited CBS coatings is systematically investigated. Under high-power conditions, the increase in liquid phase hinders the discharge of gases and leads to an increase in the porosity of CBS coatings. The experimental results reveal that the coating density decreases and coating porosity increases with the increase of spraying power. Based on the crystallization behavior of CBS coatings, an excellent low-dielectric crystalline phase (β-CaSiO₃) was obtained after heat treatment at 800 °C. According to the dielectric mixing rule of composite materials, the density and permittivity exhibit the same trend and a minimum permittivity of 5.74 is obtained.     

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel-base superalloy joints

Si₃N₄/nickel-base superalloy (Inconel-625) and Si₃N₄/Si₃N₄ joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti-active braze Cu-ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si₃N₄/Mo/W/Inconel and Si₃N₄/Mo/W/Si₃N₄. The joints exhibited Ti segregation at the Si₃N₄/Cu-ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si₃N₄/Si₃N₄ joints both with and without W and Mo layers was ∼142 MPa but the strength of Si₃N₄/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.     

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.     

Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Phase evolution and microwave dielectric properties of SrTiO3 added ZnAl2O4–Zn2SiO4–SiO2 ceramics

Phase evolution and microwave dielectric properties of SrTiO₃ added ZnAl₂O₄–3Zn₂SiO₄–2SiO₂ ceramics system were investigated. With the addition of SrTiO₃, the sintering temperature for dense ceramic is reduced from 1320 °C to 1180–1200 °C. According to the nominal composition ZnAl₂O₄–3Zn₂SiO₄–2SiO₂-ySrTiO₃, phase evolution is revealed by XRD patterns and Back Scattering Electron images: Zn₂SiO₄, ZnAl₂O₄ and SiO₂ phases coexist at y = 0; SrTiO₃ reacts with ZnAl₂O₄ and SiO₂ to form SrAl₂Si₂O₈, TiO₂ and Zn₂SiO₄ at y = 0.2 to 0.8, and SiO₂ phase disappears at y = 0.8; new phase of Zn₂TiO₄ is obtained at y = 1. The existence of TiO₂ has important effect on the dielectric properties. The optimized microwave dielectric properties are obtained at y = 0.6 and the ceramics show low dielectric constant (7.16), high-quality factor (57, 837 GHz), and low temperature coefficient of resonant frequency (−30 ppm °C⁻¹).     

Microstructure evolution during air bonding of Al2O3 to Al2O3 joints using bismuth–borate–zinc glass

Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi₂O₃–40B₂O₃–20ZnO (mol.%). The wettability of the glass on the Al₂O₃ substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi₂₄B₂O₃₉ was detected to be the product in the joints sealed at 530–580 °C, while ZnAl₂O₄ was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al₂O₃ substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl₂O₄ increased with increasing holding time.     

Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.     

Brazing of porous Si3N4 ceramic to Invar alloy with a novel Cu–Ti filler alloy: Microstructure and mechanical properties

Porous Si₃N₄ (P–Si₃N₄) ceramic was successfully joined to Invar alloy using a Cu–Ti active brazing alloy for the first time. The interfacial reactions between the Cu–Ti filler and two dissimilar substrates were studied. The influence of brazing process on the microstructure evolution of the joint was revealed, along with the formation of an infiltration layer that permitted the bonding of P–Si₃N₄ substrate. Ti reacted with Si₃N₄ to form TiN and Ti₅Si₃ compounds, resulting in the decomposition of Si₃N₄. In addition, the reaction and diffusion dual-layer formed at the Cu–Ti/Invar interface, which was attributed to the interaction of alloy elements between the braze filler and the Invar alloy. Fe–Ti and Ni–Ti intermetallics together with Cu solid solution (s,s) constituted the microstructure of the brazing seam. In addition, the optimal shear strength of the brazed joint was 20 MPa and the fracture propagation occurred in the P–Si₃N₄ ceramic substrate adjacent to the infiltration layer during the shearing tests.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Tailoring bioactive and mechanical properties in polycrystalline CaO–SiO2–P2O5 glass-ceramics

Biological functions and mechanical properties are vital factors for artificial bone materials, with great clinic demand for bone injuries and defects. This study highlights mechanical strengths, in vitro and in vivo biological behaviors of the bioactive CaO–SiO₂–P₂O₅ glass-ceramics tailored by the nucleating agent P₂O₅. A high mechanical flexural strength of ～170 MPa and hardness of ～720 HV were achieved, owing to strengthened Si–O bonds in the network. In vitro cell tests demonstrate remarkable viability (using L929 cells) and bioactivity (using bone marrow-derived mesenchymal stromal cells), associated with the release of Ca²⁺ ions in the solution due to weakened Ca–O bonds in the glass-ceramic network. The assay in the simulated body ?uid revealed a formation of the hydroxyapatite-phase layer, which may act as a bridge to facilitate bioactivity on the CaO–SiO₂–P₂O₅ glass-ceramics. In vivo assay shows a significant bone-ingrowth capability on the CaO–SiO₂–P₂O₅ glass-ceramic. This work paves a promising route to utilize P₂O₅–nucleated CaO–SiO₂–P₂O₅ glass-ceramics for load-bearing bone replacement.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Mechanical properties of Si3N4–Al2O3 FGM joints with 15 layers for high-temperature applications

“Crack-free” alumina-silicon nitride joints, comprised of 15 layers of gradually differing compositions of Al₂O₃/Si₃N₄, have been fabricated using sialon polytypoids as functionally graded materials (FGM) bonding layers for high-temperature applications. Using flexural strength tests conducted both at room and at elevated temperatures, the average fracture strength at room temperature was found to be 437 MPa; significantly, this value was unchanged at temperatures up to 1000 °C. Scanning electron microscopy (SEM) observations of fracture surfaces indicated the absence of any glassy phase at the triple points. This result was quite contrary to the previously reported 20-layer Al₂O₃/Si₃N₄ FGM samples where three-point bend testing revealed a severe strength degradation at high temperatures. Consequently, we believe that the joining of alumina to silicon nitride using polytypoidally functional gradients can markedly improve the suitability of these joints for high-temperature applications.     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

Camphene-based freeze casting technique was adopted to fabricate ZrB₂–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB₂–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB₂–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.     

Effects of Sr and Mg dopants on biological and mechanical properties of SiO2–CaO–P2O5 bioactive glass

In the present study, the effects of Sr and Mg were investigated on mechanical and biological properties of 58S bioactive glass (BG). SiO₂-P₂O₅-CaO BG with different contents of Sr and Mg were synthesized via the sol-gel method and immersed in simulated body fluid (SBF) for several days to explore their biocompatibility. Precise analyses of the BG using X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy showed that the Mg-doped BG containing 8 wt % MgO possessed better biocompatibility. It was also found that mechanical properties of the BG could be improved by increasing the amounts of MgO and SrO. Both 5Sr-BG and 8Mg-BG samples did not exhibit any cytotoxicity while showing high alkaline phosphatase activity in comparison with control specimens. However, the Sr-doped BG sample including 5 wt % SrO demonstrated enhanced bioactivity and biocompatibility.