Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si₃N₄/BN composites. In addition, the fracture toughness of Si₃N₄/micro BN composites was enhanced comparing to monolithic Si₃N₄ because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si₃N₄/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si₃N₄ matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.     

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 characterization of Si3N4 whisker-reinforced SiO2 ceramic for radome materials

Fused silica ceramic has become one of the most widely used radome materials in the world since the 1970s. But its poor mechanical properties restricted its application to some extent. To improve the mechanical properties of the fused silica ceramic and keep its characteristic for radome materials, silicon nitride (Si₃N₄) whisker-reinforced fused silica ceramics were prepared by a slip-casting method in the work. The influence of Si₃N₄ whisker contents on the properties of the slurry was studied, indicating that the preferable pH values of the slurry were 4–6 and whisker contents were 10 wt.%. The flexural strength of as-prepared Si₃N_4w/SiO₂ ceramic was about 74.35 MPa, exhibiting an increase of 7.75% over that of the pure silica sample. Its dielectric constant in the range from 8 to 12 GHz and tanδ under 10 GHz were, respectively, 3.37 and .0011. It is of great interest to find that Si₃N_4w/SiO₂ has excellent oxidization resistance and its mass maintains even at 1270°C.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

In-situ formation of BN layers by nitriding boron powders in BN/Si3N4-based composite ceramics

Boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were fabricated via the in-situ nitridation of boron (B) and silicon (Si) powders in forming gas (95%N₂/5%H₂) at 1390?°C. The effect of the B content on the phase composition, microstructure, density/porosity, machinability as well as mechanical properties of nitridized BN/Si₃N₄ composite ceramics was investigated. The addition of B slightly increased the nitridation degree of the Si and B powders mixture, and improved the ratio of the β-Si₃N₄ phase significantly at low B contents. B powders may have acted as a nucleating agent to promote the formation of β-Si₃N₄ crystals. A core-shell Si₃N₄/BN structure was revealed by the TEM technique, and the number of BN layers increased with the increase of the B content. The in-situ BN formed by the nitridation of B played a similar role with the BN directly added in enhancing the machinability of the BN/Si₃N₄ composite ceramics. The method of the in-situ nitridation of B is also effective to prepare SiC fiber-reforced BN/Si₃N₄ ceramic matrix composites.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.     

Influence of α-Si3N4 coarse powder on densification,microstructure, mechanical properties,and thermal behavior of silicon nitride ceramics

Silicon nitride (Si₃N₄) ceramics, with different ratios of fine and coarse α-Si₃N₄ powders, were prepared by spark plasma sintering (SPS) and heat treatment. Further, the influence of coarse α-Si₃N₄ powder on densification, microstructure, mechanical properties, and thermal behavior of Si₃N₄ ceramics was systematically investigated. Compared with fine particles, coarse particles exhibit a slower phase transition rate and remain intact until the end of SPS. The remaining large-sized grains of coarse α-Si₃N₄ induce extensive growth of neighboring β-Si₃N₄ grains and promote the development of large elongated grains. Noteworthy, an appropriate number of large elongated grains distributed among fine-grained matrix forms bimodal microstructural distribution, which is conducive to superior flexural strength. Herein, Si₃N₄ ceramics with flexural strength of 861.34 MPa and thermal conductivity of 65.76 W m⁻¹ K⁻¹ were obtained after the addition of 40 wt% coarse α-Si₃N₄ powder.     

The preparation of silicon nitride ceramics by gelcasting and pressureless sintering

A new gelling system based on the polymerization of hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was successfully developed for fabricating silicon nitride (Si₃N₄) ceramics. The effects of pH value, the dispersant content, solid volume fraction and hydantion epoxy resin amount on the rheological properties of the Si₃N₄ slurries were investigated. The relative density of green body obtained from the solid loading of 52 vol% Si₃N₄ slurry reached up to 62.7%. As the concentration of hydantion epoxy resin increased from 5 wt% to 20 wt%, the flexural strength of Si₃N₄ green body enhanced from 5.3 MPa to 31.6 MPa. After pressureless sintering at 1780 °C for 80 min, the sintered samples exhibited the unique interlocking microstructure of elongated β-Si₃N₄ grains, which was beneficial to improve the mechanical properties of Si₃N₄ ceramics. The relative density, flexural strength and fracture toughness of Si₃N₄ ceramics reached 97.8%, 687 MPa and 6.5 MPa m^1/2, respectively.     

Effect of BN content on microstructures,mechanical and dielectric properties of porous BN/Si3N4 composite ceramics prepared by gel casting

Porous BN/Si₃N₄ composite ceramics with different BN contents have been fabricated by gel casting. The rheological behaviors of the suspensions, microstructure, mechanical properties, dielectric properties and critical temperature difference of thermal shock (ΔT_C) of porous BN/Si₃N₄ composite ceramics with different BN contents were investigated. With BN contents increasing, the mechanical properties of the porous BN/Si₃N₄ composite ceramics were partially declined, but the dielectric properties and thermal shock resistances were enhanced at the same time. For the porous Si₃N₄ ceramic without BN addition, the porosity, flexural strength, dielectric constant and critical temperature difference were 48.1%, 128 MPa, 4.1 and 395 °C, while for the 10 vol% BN/Si₃N₄ porous composite ceramics, they were 49.4%, 106.6 MPa, 3.8, and 445 °C, respectively. The overall performance of the obtained porous BN/Si₃N₄ composite ceramics indicated that it could be one of the ideal candidates for high-temperature wave-transparent applications.     

Sintering behavior of ZrB2–SiC composites doped with Si3N4: A fractographical approach

ZrB₂–SiC composite ceramics were doped with 0, 1, 3 and 5 wt% Si₃N₄ plus 1.6 wt% carbon (pyrolized phenolic resin) as sintering aids and fabricated by hot pressing process under a relatively low pressure of 10 MPa at 1900 °C for 2 h. For a comparative study, similar ceramic compositions were also prepared by pressureless sintering route in the same processing conditions, with no applied external pressure. The effect of silicon nitride dopant on the microstructural evolution and sintering process of such ceramic composites was investigated by a fractographical approach as well as a thermodynamical analysis. The relative density increased by the addition of Si₃N₄ in hot pressed samples as a fully dense composite was achieved by adding 5 wt% silicon nitride. A reverse trend was observed in pressureless sintered composites and the relative density values decreased by further addition of Si₃N₄, due to the formation of gaseous products which resulted in the entrapment of more porosities in the final structure. The formation of ZrC phases in pressureless sintered samples and layered BN structures in hot pressed ceramics was detected by HRXRD method and discussed by fractographical SEM-EDS as well as thermodynamical analyses.     

A novel Si3N4/BAS/BN composite synthesized by spark plasma sintering

Based on good thermomechanical and electromagnetic properties of silicon nitride (Si₃N₄), barium aluminosilicate (BaO–BaTiO₃–SiO₂ or BAS), and boron nitride (BN), a novel combination of Si₃N₄/BAS/BN composites was fabricated by spark plasma sintering (SPS) after traditional powder mixing process. The effect of different amounts of BN (3–9 wt%) on the mechanical properties of composite was studied. The phases were observed by X-ray diffraction, and the microstructures were identified by scanning electron microscopy (SEM). The optimal sample is the one containing 3 wt% of BN and is sintered under a final pressure of 50 MPa. This sample has a hardness of 9.03 GPa, a flexural strength of 418.75 MPa, an elastic modulus of 934.46 MPa, and a loss tangent of less than 0.002 in 38% of the X-band frequencies. The optimal sample thickness was determined via the Nicolson-Ross-Weir (NRW) technique considering the mechanical strength limits.     

Effect of polystyrene addition on properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics with high strength and high transmittance have been widely used in the field of defense and military. Additive manufacturing (AM) technology is one of the effective means to fabricate porous Si₃N₄ ceramics. Nevertheless, it is difficult to prepare porous Si₃N₄ ceramics by using digital light processing (DLP) because of the large refractive index difference between Si₃N₄ powders and photosensitive resin. In this study, the effects of the amount of polystyrene (PS) powders on the properties of Si₃N₄ ceramic slurries and sintered ceramics were systematically discussed. The addition of PS reduced the overall refractive index of powders and increased the average particle size of powders, thus improving the cure depth of Si₃N₄ ceramic slurries from 11.0 ± 2.0 μm to 55.7 ± 1.8 μm. With the increase of PS content, the shrinkage and porosity of Si₃N₄ ceramics gradually increased, and the bulk density and flexural strength showed the opposite trend. The slurry with low viscosity (2.38 Pa٠s at a shear rate of 30 s⁻¹) and high cure depth (51.2 ± 4.6 μm) was obtained when the content of PS was 15 wt%, which met the thickness requirements for printing. The total porosity of Si₃N₄ ceramics reached the maximum values at 28.21 ± 2.58%. The addition of PS solved the problem of low cure depth of slurries, and PS as a pore-forming agent could help Si₃N₄ ceramics form porous structure. This research provides valuable insights into the fabrication of non-oxide ceramics with high refractive index using DLP technology.     

Structure–property relations of ferroelectric BaTiO3 ceramics containing nano-sized Si3N4 particulates

Barium titanate/silicon nitride (BaTiO₃/xSi₃N₄) powder (when x = 0, 0.1, 0.5, 1 and 3 wt%) were prepared by solid-state mixed-oxide method and sintered at 1400 °C for 2 h. X-ray diffraction result suggested that tetragonality (c/a) of the BaTiO₃/xSi₃N₄ ceramics increased with increasing content of Si₃N₄. Density and grain size of BaTiO₃/xSi₃N₄ ceramic were found to increase for small addition (i.e. 0.1 and 0.5 wt%) of Si₃N₄ mainly due to the presence of liquid phase during sintering. BaTiO₃ ceramics containing such amount of Si₃N₄ also showed improved dielectric and ferroelectric properties.     

Effect of BN content on the structural,mechanical, and dielectric properties of PDCs-SiCN(BN) composite ceramics

Hexagonal boron nitride (h-BN) with low dielectric loss and high temperature resistance opens up new opportunities for the preparation of polymer-derived SiCN ceramics (PDCs-SiCN ceramics) with excellent mechanical and dielectric properties. BN-containing polymer-derived SiCN composite ceramics (PDCs-SiCN(BN) composite ceramics) with different BN content were prepared via a pyrolysis process of ball-milling-blended Polyvinylsilazane/boron nitride (PVSZ/BN) precursors. BN is stably embedded in the SiCN tissue and tightly bound with it. The appropriate content of BN greatly improves the mechanical properties of PDCs-SiCN ceramics, as BN reduces the number of pores and prevents crack expansion. Additionally, BN is also beneficial in lowering the dielectric loss of PDCs-SiCN ceramics because of the weakened polarization relaxation behavior. PDCs-SiCN (BN) composite ceramics have optimal mechanical and dielectric properties when the BN content is 1 wt%. The flexural strength, flexural modulus and compression strength of PDCs-SiCN(BN) composite ceramics with 1 wt% BN doping content were 189.37 MPa, 46.38 GPa, and 399.02 MPa, respectively. Its average dielectric loss (tanδ_ε) at 12.4-18 GHz is 0.0049.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Preparation,oxidation property and mechanism of Si3N4/O′-SiAlON composite ceramics

The applications of Si₃N₄ ceramics were significantly restricted because of the disastrous failure resulted from the oxidation weight gain. The generation of O′-SiAlON could effectively address this issue. The effect of N₂ gas pressure on the phase evolution of the Si₃N₄/O′-SiAlON was studied. It was found that high N₂ gas pressure (3 MPa) was more favorable for the formation of the O′-SiAlON than low N₂ gas pressure (0.6 MPa). Furthermore, the effects of SiO₂ content on the phase evolution, microstructure, oxidation properties and mechanism of the Si₃N₄/O′-SiAlON ceramics were investigated. The results revealed that the relative content of the O′-SiAlON phase evidently enhanced from 0 wt% to 18.15 wt%, and the bulk density decreased from 3.01 g/cm^?3 to 2.62 g/cm^?3 with an increase in SiO₂ from 0 wt% to 12.5 wt%. Additionally, the weight gain, oxide layer thickness and roughness similarly reduced from 2.02 mg/cm² to 0.85 mg/cm², 133.87 μm to 2.31 μm and 21.91 μm to 6.34 μm, respectively. The addition of SiO₂ could also reduce bubbles and cracks formation and hinder the diffusion of Al and Y elements from the interior to the surface. Finally, the oxidation resistance mechanism was mainly credited to the pinning effect of O′-SiAlON phases at the grain boundaries.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si₃N₄/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si₃N₄/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si₃N₄/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.