Fabrication of Si3N4–SiBC composite ceramic and its excellent electromagnetic properties

To obtain better electromagnetic wave absorbing property, it is vitally necessary to develop novel ceramics with not only high dielectric loss but also low dielectric constant. Si₃N₄–SiBC, a composite ceramic with such dielectric properties, was fabricated by infiltrating SiBC into porous Si₃N₄ ceramic via low pressure chemical vapor infiltration. The high dielectric loss and the low dielectric constant are attributed to the unique microstructure of SiBC, which also leads to a very excellent wave-absorbing property of Si₃N₄–SiBC ceramic, attaining a minimal reflection coefficient of ?28 dB. Besides, the Si₃N₄–SiBC ceramic also shows a high mechanical property. Therefore, the Si₃N₄–SiBC ceramic exhibits great potential as an excellent functional and structural ceramic.     

Improved oxidation resistance properties of Si3N4/O′–SiAlON composite ceramics by a repeated sintering method

Si₃N₄/O′–SiAlON composite ceramics with superior oxidation resistance properties were fabricated by a repeated sintering method. The effects of sintering time on the phase evolution, microstructure, and oxidation resistance properties of the Si₃N₄/O′–SiAlON composite ceramics were investigated. The results indicated that the content of the O′–SiAlON phase and the densification of Si₃N₄/O′–SiAlON composite ceramics increased after two-time sintering. Furthermore, the thickness of the oxide layer of the Si₃N₄/O′–SiAlON composite ceramics after oxidation at 1100–1500°C for 30 h was not significant. Compared to the oxidation weight gain after the one-time sintering process, the oxidation weight gain of Si₃N₄/O′–SiAlON composite ceramics was 0.432 mg/cm² after two-time sintering when oxidized at 1500 C for 30 h, which was reduced by 43.3%. The mechanism of the improved oxidation resistance properties was ascribed to the formation of more O′–SiAlON and the enhancement of the densification.     

Production and EDM of Si3 N4–TiB2 ceramic composites

Abstract

Ceramic matrix composites containing TiB₂ as a particulate phase have been produced by hot pressing. The problems of the reactivity of TiB₂ with the Si₃ N₄ matrix and with the sintering environment have been successfully addressed. A novel dual atmosphere sintering profile combined with low temperature hot pressing has been used to successfully produce fully dense materials. The effect of TiB₂ addition on mechanical properties has been investigated. Composites containing TiB₂ show significant improvements in hardness and fracture toughness. The addition of TiB₂ at a level of 40 vol.-% raises the conductivity of the composite to a level where electro-discharge machining (EDM) is possible. A comprehensive study of the application of EDM has been carried out and the optimum machining conditions have been identified. Under these conditions Si₃ N₄–TiB₂ composites have been shaped with relative ease and proved to be ‘robust’ materials under EDM.     

SiAlON–Al2O3 ceramics as potential biomaterials

When used as implants, Al₂O₃ is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al₂O₃ ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al₂O₃ ceramics was identified as Si₄Al₂O₂N₆, formed by reaction of Si, Al and Al₂O₃ under nitrogen atmosphere at high temperature. As Al₂O₃ content increased, porosity and compressive strength decreased. Therefore, Si₄Al₂O₂N₆ phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si₄Al₂O₂N₆ phase in SiAlON–Al₂O₃ ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al₂O₃ bioceramics.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Interfacial microstructure characterization and strength evaluation of Si3N4/Si3N4 joints with Si–Mg composite filler for high-temperature applications

Silicon-nitride (Si₃N₄) components were joined under vacuum at 1100 °C for 10 min using Si–Mg composite fillers with Mg contents (X_Mg) that ranged from 0 at.% to 59 at.%. The Si₃N₄/Si₃N₄ joints were fabricated via Si layer formation at the joint interface; the molten Si–Mg liquid was transformed into a solid Si layer after Mg-evaporation-induced isothermal solidification. The joint tensile strength at room temperature increased considerably when X_Mg exceeded the liquidus composition of 37 at.% because of the enhanced densification/thinning of the Si layer. In these cases, some Mg atoms reacted with Si₃N₄ to form a fine-grained MgSiN₂-based layer, whereas relatively large (>0.1 μm) and smaller MgO precipitates (<10 nm) were observed in the Si layer. At a high X_Mg, the MgO precipitates were arranged in a network-like structure, which improved the fracture strength of the Si layer. The joints with a high strength at room temperature were examined using a three-point bending test at 1200 °C in air and endured a maximum fracture stress of ~200 MPa, which confirmed their potential for use in oxidizing atmospheres at least 100 °C above the bonding temperature.     

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.     

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.     

Tribological properties of Si3N4–graphene nanocomposites

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt% of various types of graphene platelets were studied. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at room temperature in dry sliding. Coefficient of friction and specific wear rates were calculated and related to the damage mechanisms observed in the wear tracks. The measured properties were then assessed with respect to the type and volume fraction of the graphene additives. It is shown that addition of such amounts of carbon phases does not lower the coefficient of friction. Graphene platelets seem to be integrated into the matrix very strongly and they do not participate in lubricating processes. The best performance offers materials with 3 wt% of larger sized graphene, which have the highest wear resistance.     

Properties of Si3N4–TiN composites fabricated by spark plasma sintering by using a mixture of Si3N4 and Ti powders

Si₃N₄–TiN composites were successfully fabricated via planetary ball milling of 70 mass% Si₃N₄ and 30 mass% Ti powders, followed by spark plasma sintering (SPS) at 1250–1350 °C. The sintering mechanism for SPS was a hybrid of dissolution–reprecipitation and viscous flow. The electrical resistivity decreased with increasing sintering temperature up to a minimum at 1250 °C and then increased with the increasing sintering temperature. The composites prepared by SPS at 1250–1350 °C could be easily machined by electrical discharge machining. Composite prepared by SPS at 1300 °C showed a high hardness (17.78 GPa) and a good machinability.     

Mechanical properties of Si3N4 ceramics from an in-situ synthesized a-Si3N4/β-Si3N4 composite powder

Sintered Si₃N₄ ceramics were prepared from an ɑ-Si₃N₄/β-Si₃N₄ whiskers composite powder in-situ synthesized via carbothermal reduction at 1400–1550 °C in a nitrogen atmosphere from SiO₂, C, Ni, and NaCl mixture. Reaction temperatures and holding time for the composite powder, and mechanical properties of sintered Si₃N₄ were investigated. In the synthesized composite powder, the in-situ β-Si₃N₄ whiskers displayed an aspect ratio of 20–40 and a diameter of 60–150 nm, which was mainly dependent on the synthesis temperature and holding time. The flexural strength, fracture toughness and hardness of the sintered Si₃N₄ material reached 794±136 MPa, 8.60±1.33 MPa m^1/2 and 19.00±0.87 GPa, respectively. The in-situ synthesized β-Si₃N₄ whiskers played a role in toughening and strengthening by whiskers pulling out and crack deflection.     

Fabrication and contact resistivity of W–Si3N4/TiB2–Si3N4/p–SiGe thermoelectric joints

The design and fabrication of silicon germanium (SiGe) thermoelectric elements, typically including the selection of electrode and intermediate materials, the process of joining electrode and intermediate layer onto thermoelectric materials, are the major challenge for SiGe thermoelectric device technology. In this study, W–Si₃N₄ and TiB₂–Si₃N₄ composites are designed as the electrode and intermediate layer, respectively, and the W–i₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 joints are fabricated by a one-step spark plasma sintering process. The influences of the composition of TiB₂–Si₃N₄ intermediate layer on the interfacial structure, contact resistivity and interfacial thermal stability are investigated. The interfacial thermal stability is improved with increasing Si₃N₄ content in TiB₂–Si₃N₄ intermediate layer due to the reduced mismatch of coefficients of thermal expansion between the intermediate layer and SiGe. On the other hand, the contact resistivity increases with the rising of Si₃N₄ content due to the weakened TiB₂/SiGe ohmic contact, which degrades the device efficiency. As the balanced point, the intermediate layer with the composition of 80 vol% TiB₂+20 vol% Si₃N₄ provides good interfacial thermal stability and moderately small contact resistivity (~75 μΩ cm² after aging at 1000 °C for 120 h) simultaneously, which is an optimized intermediate layer composition for W–Si₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 thermoelectric element. The TiB₂–Si₃N₄ intermediate layer has excellent chemical stability to both W–Si₃N₄ electrode and SiGe thermoelectric material at high temperatures, which contributes to the sharp interface of the joint and effectively prevents the inter-diffusion between the electrode and the SiGe.     

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si₃N₄ based materials was investigated. The location of SiC particles inside the Si₃N₄ grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE³⁺ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La₂O₃ was significantly higher compared to the composite and the monolith doped with Lu₂O₃ (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.     

Preparation of Si–C–N–Fe magnetic ceramic derived from iron-modified polysilazane

In this paper, Si–C–N–Fe magnetoceramics were obtained by pyrolysis of iron-modified polysilazane (PFSZ) precursors which were synthesized by using polysilazane (PSZ) and iron (III) acetylacetonate (Fe(acac)₃) as starting materials. The as-synthesized PFSZ precursors were characterized by Fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography. The polymer-to-ceramic conversion of the PFSZ was studied by FT-IR and thermal gravimetric analysis. It is found that the ceramic yield of the PFSZ precursor is ca. 25% higher than that of the original PSZ. The crystallization behavior, microstructures and magnetic properties of the PFSZ-derived Si–C–N–Fe magnetoceramics were studied by techniques such as X-ray diffraction, transmission electron microscopy and vibrating sample magnetometer. The results indicate that the formed α-Fe nanoparticles are uniformly dispersed in amorphous Si–C–N(O) matrix, leading to the soft magnetization of the resultant Si–C–N–Fe ceramics. Moreover, the iron content and the magnetic properties of the Si–C–N–Fe ceramic could be easily controlled by the amount of Fe(acac)₃ in the precursor.     

Generation of ceramic–ceramic layered composite microstructures using electrohydrodynamic co-axial flow

In this work different loading volumes (10 and 20 vol.%) of alumina and zirconia were suspended in selected liquid carriers to prepare suspensions. These suspensions were infused through inner and outer capillaries of a co-axial nozzle arrangement, which was then subjected to an electric field. The rate at which these suspensions were perfused (flow rate) and the applied voltage was varied during the experiments. Under optimal conditions, stable cone and jet formation was achieved for selected co-flowing suspensions which subsequently breaks-up into ceramic–ceramic composite droplets. Thus, droplets containing alumina and zirconia were produced and the resulting relics were collected on quartz substrates. These were sintered at 1200 °C and analysed by optical and scanning electron microscopy. The relic size increased as a function of the volume loading and layered encapsulation (shown using SEM and EDX) of alumina by zirconia and vice-versa, using this method, is achievable.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Microstructural and mechanical characterization of the Si3N4/Si3N4 joint brazed using Au–Ni–V filler alloys

In this study, two Au–Ni–V filler metals were used to braze Si₃N₄ ceramic in the form of foils. The effects of brazing temperature and V content in the filler alloy on microstructure and bonding strength of the joint were studied. The results reveal that a VN reaction layer with a thickness about 4 μm was formed at the interface between Si₃N₄ substrate and filler alloy. With increasing brazing temperature or V content the thickness of VN reaction layer increased. A maximum joint bending strength of 242 MPa was achieved when the joint was brazed at the temperature of 1423 K for 30 min using Au58.7Ni36.5V4.8 filler alloy. The bonding mechanism was discussed with reference to the discovered phases and brazing parameters.     

Si3N4／Si3N4陶瓷连接的研究进展

本文综述了Ｓｉ３Ｎ４／Ｓｉ３Ｎ４陶瓷连接的研究现状，论述不同连接工艺对接头强度的影响。     

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.     

Si3N4／Si3N4陶瓷连接的研究进展

连接技术是Ｓｉ３Ｎ４陶瓷实用过程中必须解决的难题之一。本文综述了Ｓｉ３Ｎ４／Ｓｉ３Ｎ４陶瓷连接的研究现状以及不同连接工艺对连接强度的影响。