Porous Si3N4 Research Articles

Fabrication and thermal shock behavior of Si3N4 whiskers toughened γ-Y2Si2O7 coating on porous Si3N4 ceramics

In order to prevent water from entering the porous Si3N4 ceramics, α-Si3N4 whiskers toughened γ-Y2Si2O7 coatings were prepared by a liquid infiltration and filling method. Effects of α-Si3N4 whiskers content on the microstructure, waterproof performance, mechanical properties and thermal shock behavior of the ceramic coatings were systematically studied in this paper. Results indicated that the addition of whiskers could effectively improve the performance of the coatings under thermal shock due to the toughening mechanism of whiskers including crack deflection, whisker bridging, whisker pulling out and other energy dissipation processes. The coating with a α-Si3N4 whisker/γ-Y2Si2O7 mass ratio of 1/9 possessed a flat, dense and defect-free surface microstructure. After five thermal cycles under a thermal shock temperature difference of 1200 °C, water absorption of the coating increased slightly from ~3.2% to ~6.0%. The whisker reinforced coating is promising to be used for high temperature wave-transmitting materials in long-time service.

Effects of N2 pressure and Si particle size on mechanical properties of porous Si3N4 ceramics prepared via SHS

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N2 pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N2 influences not only the thermodynamics but also the kinetics of the SHS as initial reactant. Flexural strength ranged between 67 MPa and 134 MPa with increasing N2 pressure. On the other hand, flexural strength ranged from 213 MPa to 102 MPa with different Si particle sizes. This plays an important role on the final diameter and length of β-Si3N4 grains and the formation mechanism of porous Si3N4 ceramics.

Enhanced mechanical properties of laminated ZrB2–SiC ceramics with porous Si3N4 interface

In this study, bionic laminated ZrB2–SiC ceramics (LZNw) were fabricated using Si3N4 whiskers as an interface layer. The room-temperature flexural strength and fracture toughness of LZNw were determined to be 528 MPa and 15.3 MPa m1/2, respectively. With increasing testing temperature up to 1500 °C, the flexural strength of LZNw remained higher than that of monolithic ZrB2–SiC ceramics (ZS). Specifically, at the peak temperature of 1500 °C, the flexural strength of LZNw was as high as 380 MPa, while that of ZS dropped to 219 MPa. This outstanding high-temperature strength of LZNw can be ascribed to the porous structure of the Si3N4 whisker interface layer, which absorbs the glassy phase accumulated at the crack tip, and the stepped fracturing pattern, which hinders the flow of the glassy phase, thus avoiding the grain boundary sliding caused by the enrichment of the glassy phase.

Impedance testing of porous Si3N4 scaffolds for skeletal implant applications

Si3N4 ceramics show excellent characteristics of mechanical and chemical resistance in combination with good biocompatibility, antibacterial property and radiolucency. Therefore, they are intensively studied as structural materials in skeletal implant applications. Despite their attractive properties, there are limited data in the field about in vitro studies of cellular growth on ceramic implant materials. In this study, the growth of bone cells was investigated on porous silicon nitride (Si3N4) ceramic implant by using electrochemical impedance spectroscopy (EIS). Partial sintering was performed at 1700 °C with limited amount of sintering additive for the production of porous Si3N4 scaffolds. All samples were then sterilized by using ethylene oxide followed by culturing MG-63 osteosarcoma cells on the substrates for in vitro assays. At 20 and 36 h, EIS was performed and results demonstrated that magnitude of the impedance as a result of the changes in the culture medium increased after incubation with osteosarcoma cells. The changes are attributed to the cellular uptake of charged molecules from the medium. Si3N4 samples appear to show large impedance magnitude changes, especially between 100 and 1 Hz. Impedance changes were also correlated with WST-1 measurements (36 h) and DAPI results.

Preparation and properties of porous silicon nitride ceramics with polymethyl methacrylate as pore-forming agent

Porous Si3N4 ceramics were prepared by adding polymethyl methacrylate (PMMA) as pore-forming agent, combined with cold isostatic pressing and pressureless sintering at nitrogen atmosphere. The composition of phases, microstructure, density and porosity, flexural strength, dielectric properties of samples with different content of PMMA were studied in this paper. With an increase of PMMA content, the density and flexural strength of samples were decreased, while the porosity and dielectric properties were improved. Two types of pores in the sample were found in the experiment, the micropores formed by β-Si3N4 bridging, while the macropores formed by the pore-forming agent, and the macropores were the main factor for reducing the flexural strength and improving the dielectric properties by increasing porosity. When the PMMA content was 15 wt%, the comprehensive performance of sample was optimal, the porosity of 33.49%, the flexural strength of 103.62 MPa, the dielectric constant of 5.82 and dielectric loss tangent of 6.0 × 10−3.

Wetting and joining of porous Si3N4 and dense Si3N4 ceramics with in-situ formed β-spodumene/spinel glass-ceramic interlayer

Efficient joining materials and techniques are of critical significance for the integration of ceramics in high performance structures. Porous Si3N4 has been successfully joined to dense Si3N4 by using a novel glass-ceramic based on the Li2O-MgO-Al2O3-SiO2 system. In terms of elucidating the microstructure of joints, systematic approaches were made, including scanning electron microscopy, transmission electron microscopy and X-ray diffraction, to characterize the glass/ceramic interface and to determine the influence of joining temperature on the microstructural evolution. In addition, the extent to which temperature the joint could withstand has been determined. It is suggested that the glass shows good wettability and thermomechanical compatibility with both porous and dense Si3N4 substrates. The micron-size β-spodumene, along with the submicronic spinel are identified as the main crystallization of the glass-ceramic interlayer forming an intergranular network structure. The infiltration layer, consisting of glass and rod-like Si3N4 grains, shows increased elastic modulus and microhardness, which helps improve the stress tolerance of the joint. Accordingly, this joint exhibits an extraordinary high room-temperature shear strength (112 ± 10 MPa) and a high-temperature resistance.

Synthesis of self-toughness porous Si3N4 ceramics with three-dimensional cage structures

Self-toughness porous Si3N4 ceramics are prepared by surface modification of Si3N4 particles. The as-fabricated porous Si3N4 ceramics have high number of three-dimensional cage structures that can cause the crack deflection, absorb the fracture stress to produce microcrack toughening effect and increase the structural reliability of porous Si3N4 ceramics. The flexural strength is 78.0 MPa. The dielectric constant and dielectric loss is 3.47 and 0.001 respectively. This kind of porous Si3N4 ceramics with special three-dimensional cage structures have high mechanical strength, good antioxidation properties and excellent dielectric properties.

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

AbstractA 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·m1/2.

Effect of β–Si3N4 seeds on microstructure and properties of porous Si3N4 ceramics prepared by gelcasting using DMAA system

Porous Si3N4 ceramics fabricated by gelcasting with DMAA demonstrated excellent mechanical and dielectric properties after introducing β–Si3N4 seeds into ceramic matrix. The microstructure and properties of slurries, green and sintered bodies were highly correlated with β–Si3N4 seed size, content and morphology. The slurry viscosity reached the maximum of 0.87 Pa s when introducing 10 wt% elongated seeds, but overall, exhibiting good fluidity. The green body strength dropped remarkably after adding small equiaxial seeds, but improved within 6 wt% elongated seeds and afterwards declined to the minimum of 25.99 MPa, which was quite ideal for processing. The elongated seeds impeded liquid phase flow and particle rearrangement more pronouncedly than equiaxial ones during sintering, thereby causing the highest porosity of 41.77% and the largest pore size of 1.04 μm when employing 10 wt% elongated seeds. The elongated seeds were more helpful in developing the strong self–reinforced bimodal microstructure due to the original hexagonal prism morphology and achieved the highest bending strength of 378.50 MPa and fracture toughness of 8.54 MPa m1/2 at 2 wt% and 6 wt% elongated seeds, respectively. The dielectric constant and loss tangent depended strongly on the porosity and separately varied in 4.70–4.82 and 0.041–0.047 ranges in 8.2–12.4 GHz when using 6 wt% elongated seeds, demonstrating superior microwave–penetrating properties.

Pore structure, porosity and compressive strength of highly porous reaction-bonded silicon nitride ceramics with various grain morphologies

Complex characteristics of the pores and properties of porous reaction-bonded Si3N4 have been investigated and correlated with the microstructure of Si3N4 grains. Porous ceramics with porosities of ≤ 75 vol% and α-Si3N4 matte grains (α/β phase ratio of 1.5) or α-Si3N4 whiskers (α/β phase ratio of 0.36) were prepared by in situ nitridation of silicon powder. To obtain various microstructures by α → β-phase transformation and grain morphology modification, samples were heat-treated at 1700 °C while embedded in a Si3N4 powder bed containing MgO. By the growth of α-matte or β-Si3N4 grains on the pore walls, highly interconnected structures with spherical cavities and unimodal pore size distributions resulted with d50 ≈ 8.8 µm and ≈ 6.5 µm, respectively. In contrast, α-whiskers grew inside the pore cavities; thus, complex and irregular inter-particle pores appeared which generated an extra peak near d50 ≈ 1 µm forming a bimodal pore size distribution. Compared to the α-matte grains, α-whiskers densified upon heat treatment and produced a large drop in porosity, which resulted in a structure with less interconnectivity. As a consequence of growth of fine β-rods, pore walls became relatively smooth and whisker free; thus, inter-cluster channels were modified to spherical cavities with d50 ≈ 3.7 µm. Samples exhibiting networked whiskers and fine pores or low porosity demonstrated higher compressive strength than the interconnected structures with spherical cavities.

Preparation and Performance of Si3N4 Hollow Microspheres by the Template Method and Carbothermal Reduction Nitridation.

Silicon nitride (Si3N4) hollow microspheres with smaller particle size and narrower distribution can be used to prepare closed-cell ceramics as the pore-forming agent to improve heat insulation performance and wave-transparent performance of porous Si3N4 ceramics. In this work, Si3N4 hollow microspheres with a diameter of about 1 μm and a wall thickness of approximately 150 nm were prepared by using the template method combined with the carbothermal reduction nitridation method. The optimum preparation temperature of the Si3N4 hollow microspheres is 1450 °C. The morphology, microstructure, and phase composition of the prepared Si3N4 hollow microspheres were characterized. The formation mechanism of the Si3N4 hollow microspheres was discussed. The dielectric properties of Si3N4 hollow microspheres were measured using the waveguide method at 8.2-12.4 GHz. The results show that the wave-transparent performance of the Si3N4 hollow microspheres is similar to those of α-Si3N4 particles. It can be used as the pore-forming agent or matrix for preparing lightweight, heat-insulating, wave-transparent, and high-strength porous Si3N4 ceramics.

Microstructure, mechanical properties and sintering mechanism of pressureless-sintered porous Si3N4 ceramics with YbF3-MgF2 composite sintering aids

Porous Si3N4 ceramics were fabricated by pressureless sintering at 1550 °C for 2 h with YbF3-MgF2 composite sintering aids, systematically investigated for the effect of sintering aids addition on the microstructure and mechanical properties, further explored the formation process and sintering mechanism. The α-β phase transition rate of all samples approached 100% and MgF2 showed a positive effect on the elongating of β-Si3N4 crystal grains in morphology. The apparent porosity of porous Si3N4 ceramics ranged from 34% to 39%. In addition, when 2.5 wt% YbF3 was replaced by the same amount of MgF2, the flexural strength sharply increased to 298 MPa, an increase of ~49%, while the porosity increased to 37%, an increase of ~15%. With MgF2 content increasing from 0 to 7.5 wt%, the structure factor reached a minimum of 3.23 when MgF2 addition was 2.5 wt%, which demonstrated unparalleled superiority in mechanical properties compared with previous studies due to a more preferred form of morphology of connected rod-like β-Si3N4 crystals network. Besides, our research indicated that MgF2 addition could develop higher aspect ratio of β-Si3N4 grains and had obviously positive effect on the elongating characteristic of β-Si3N4 grains, which was related to the activation energy variation for nucleation on the different facets.

Synthesis and textural evolution of mesoporous Si3N4 aerogel with high specific surface area and excellent thermal insulation property via the urea assisted sol-gel technique

Si3N4 aerogel, which is one of the most promising materials for application as high-temperature thermal insulation, is synthesized by the urea-assisted sol-gel technique, followed by the supercritical drying, carbonization, carbothermal reduction and carbon combustion processes. The effects of heat treatment temperatures on the physicochemical and textural evolution of mesoporous Si3N4 aerogel are investigated. Si3N4 aerogel is formed when the heat treatment temperature increases to 1500 °C, whereas it transforms into SiC phase with a further higher temperature of 1600 °C. The SEM analysis confirms the existence of Si3N4 nanoparticles, and the particle size of Si3N4 nanoparticles is around 20–40 nm, with pore size around 20–40 nm. The XPS measurement reveals that Si3N4 aerogel is composed of 74.4% Si3N4 phase, with lower content of 25.6% SiO2. Si3N4 aerogel possesses a large BET specific surface area of 519.6 m2/g, which results from the mesoporous pores left after carbon combustion. This value is much larger than that of the conventional Si3N4 porous ceramics and Si3N4 nano materials ever reported. The BJH adsorption average pore diameter of Si3N4 aerogel is around 11.8 nm, with a large pore volume of 3.5 cm3/g. Si3N4 oxidation with forming amorphous SiO2 layer on the surface is observed by the TG measurement. Moreover, the resulting Si3N4 aerogel shows low bulk density of 0.075 g/cm3, as well as low thermal conductivity of 0.045 W/(m·K) at room temperature. The mechanism of Si3N4 formation is based on the VS growth between C, SiO2 and N2. The structures evolution and formation mechanism of Si3N4 aerogel are also investigated in this study.

Mechanical and dielectric properties of porous and wave-transparent Si3N4-Si3N4 composite ceramics fabricated by 3D printing combined with chemical vapor infiltration

Porous Si3N4-Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration (CVI). This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties. The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4-Si3N4 composite ceramics were studied. The study highlights following: When CVI processing time was increased from 0 to 12 h, the porosity decreased from 68.65% to 26.07% and the density increased from 0.99 to 2.02 g/cm3. At the same time, the dielectric constant gradually increased from 1.72 to 3.60; however, the dielectric loss always remained less than 0.01, indicating the excellent electromagnetic (EM) wave-transparent performance of porous Si3N4-Si3N4 composite ceramics. The maximum flexural strength of 47±2 MPa was achieved when the deposition time attained 6 h. After heat treatment, the porosity increased from 26.07% to 36.02% and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01. It has been demonstrated that the porous Si3N4-Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.

Fabrication, mechanical properties and thermal shock resistance of a dense SiC NWs/α-Si3N4 composite coating for protecting porous Si3N4 ceramics

To prevent porous Si3N4 ceramics from absorbing moisture in practical application, a dense SiC NWs/α-Si3N4 composite coating was fabricated using the two-step technology involving slurry and liquid infiltration method. The results revealed that the SiC NWs not only can promote densification sintering of the coating but also can reduce thickness of the coating effectively due to the block effect of the SiC NWs. Furthermore, the SiC NWs in the coating had a good interfacial bonding with α-Si3N4 matrix, meanwhile the toughening effects of the SiC NWs involving crack deflection, crack branching, SiC NWs bridging and pullout, also presented during mechanical damage. The strengthening and toughening mechanisms of the SiC NWs not only enhanced mechanical properties of the coating but also raised significantly critical crack size (L), thereby which made the coating have a good ability for thermal shock resistance. After thermal shock at ΔT = 1000 °C for 15 times, water absorption of the coated porous Si3N4 ceramics increased only to 7.21% from 4.54%, and its residual strength ratio was up to 83.4%. More importantly, the coating exhibited an ability for crack healing during thermal shock testing. All those above results indicated that as-prepared SiC NWs/α-Si3N4 composite coating can meet the applied requirements in long-term extreme conditions.

Gas permeation performance of porous silicon nitride ceramics with controllable pore structures

Porous Si3N4 ceramics with controllable pore structures were prepared by polymethyl methacrylate (PMMA) as the pore agent combined with cold isostatic pressing (CIP) method. The pore structures of specimens were tailored via the changes in the content and particle size of PMMA. The higher the addition and particle size of PMMA, the larger the open porosity and pore size respectively. The increased Darcian and non-Darcian permeability coefficients are obtained as the open porosity and pore size of pore structures increase. The ratio of Darcian and non-Darcian permeability coefficient representing the individual contribution of viscous and inertial resistance during permeation process, that is strongly dependent on the pore size and open porosity, but there is distinct relationship for small and larger pores. Moreover, the comparison of the theoretical permeability coefficients with the experimental permeability constants of porous materials is presented in this work.

Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds

Porous Si3N4 with various grain morphologies was prepared by direct nitriding of silicon powder and subjected to heat-treatment while embedded in a Si3N4 powder bed. The influence of MgO, added either to the starting silicon or to the powder bed, on the microstructural transformation and morphology of pores at temperatures between 1425 °C and 1700 °C are discussed. In the presence of MgO, α-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an α-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the α-whiskers gradually vanished, and grains recrystallized as very fine β-Si3N4 rods. Consequently, pores became spherical, large and whisker-free. In agreement with the results of SEM-EDX along with XRD analysis, the observed morphology transition and full phase transformation occurred by vapor phase transport of MgO from the powder bed and a subsequent solution-precipitation mechanism. The presence of volatile MgO in the powder bed caused a substantial decrease in weight losses, while enhancing β phase formation, grain coarsening and linear shrinkage. The development of coarse β-rods from α-matte grains in a Si–Mg–O–N glassy phase was related to the presence of substantial liquid phase during the growth mechanism. Compared to the granular morphology, whiskers with high aspect ratio gave rise to a high sintering driving force and led to a maximum value of ≈2% linear shrinkage and porosity of ≈30 vol%. Consequently, this ceramic exhibited the highest compressive strength of ≈10 MPa.

Synthesis and mechanical properties of highly porous ultrafine-grain Si3N4 ceramics via carbothermal reduction-nitridation combined with liquid phase sintering

Highly porous Si3N4 ceramics have been prepared via a novel carbothermal reduction-nitridation (CRN) reaction between carbon nanotubes (CNTs) and SiO vapor in a N2 atmosphere. The green bodies consisted of 49.0 wt% CNT, 40.4 wt% α-Si3N4 as seed and 10.6 wt% Y2O3 as sintering additive, and SiO powders were located under the green bodies during sintering. By heating at temperatures ranging from 1550 to 1650 °C, fibrous β-Si3N4 nanograins were obtained. Complete reaction was achieved after soaking at 1650 °C, and a mean grain diameter of approximately 290 nm and aspect ratio of as high as 20 were obtained, and a porosity of 73% was reached due to the low density of the green bodies. The bending and compressive strengths were up to 62 MPa and 82 MPa, respectively, which were higher than the values in the literature, as a result of the fine elongated fibrous β-Si3N4 grains. It should be noted that the Si3N4 ceramics exhibited a large strain to failure and ‘pseudoplastic’ deformation behavior during the compressive test, which was attributed to the high porosity.

Induced crystallization behavior and EMW absorption properties of CVI SiCN ceramics modified with carbon nanowires

Herein we reported on the crystallization mechanism and electromagnetic wave (EMW) absorption properties of in-situ carbon nanowires (CNW) modified amorphous SiCN ceramics. The composite ceramics were prepared by catylitic chemical vapor deposition (CCVD) CNW and chemical vapor infiltration (CVI) SiCN on porous Si3N4 matrix, forming Si3N4-CNW-SiCN composite ceramics with sandwich structure. The high specific area and unique microstructure of CNW have been shown to significantly affect the crystallization behavior and composition of the composite ceramics during heat-treatment. Furthermore, both complex permittivity and electrical conductivity of Si3N4-CNW-SiCN composite ceramics firstly increased and then decreased with the increase of annealing temperature, because carbon nanotubes (CNTs) with higher electrical conductivity were formed with the help of catalytic Ni at the temperature of 1200 °C, and then SiCN ceramics was gradually crystallized into graphite, β-SiC and α-Si3N4 ceramics with lower electrical conductivity at the higher temperatures. The completely crystallization temperature of Si3N4-CNW-SiCN composite ceramics was about 1400 °C compared with 1600 °C of Si3N4-SiCN.

Porous Si3N4 ceramics with hierarchical pore structures prepared by gelcasting using DMAA as gelling agent and PS as pore–forming agent

Porous Si3N4 ceramics with bimodal hierarchical pore structures were fabricated through combining aqueous gelcasting and adding pore–forming agent method wherein a low–toxic monomer N, N–dimethylacrylamide (DMAA) and spherical pore–forming agent polystyrene sphere (PS) were respectively applied. The results demonstrated that PS size and content played an important function in regulating microstructure and properties of slurries, green and sintered bodies. The slurry viscosity increased sharply with the introduction of fine PS and reached a maximum of 1.78 Pa⋅s when adopting 7 wt% PS with d50 = 2.51 μm. Fine PS addition caused higher porosity in green bodies than coarse one, resulting in a significant reduction in bending strength from 28.50 MPa to 11.50 MPa, however, overall, demonstrating high enough strength for mechanical processing. The formed spherical pores from PS in ceramic matrix facilitated high porosity and highly crosslinked microstructure, thus contributing to lowering dielectric constant and loss. With PS size increasing, the dominant role in mechanical properties was transferred from porosity to microstructure, whilst dielectric properties were merely controlled by porosity and spherical pores. The porosity, bending strength, fracture toughness, dielectric constant and loss at 15 GHz varied in ranges of 36.34–52.91%, 258.42–101.21 MPa, 6.02–3.38 MPa⋅m1/2, 4.29–2.98 and 1.48–1.18 × 10−3, respectively, displaying an excellent comprehensive properties.