Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

Effects of whisker-like β-Si3N4 seeds on phase transformation and mechanical properties of α/β Si3N4 composites using MgSiN2 as additives

α/β Si₃N₄ composites with β-Si₃N₄ content ranging from 26% to 100% were hot-pressed with or without β-Si₃N₄ seeds, using MgSiN₂ as additives, and their mechanical properties were investigated. When the α-Si₃N₄ content was over 58%, the microhardness of α/β Si₃N₄ composites was in the range of 23–24 GPa, and then the indentation hardness decreases with decreasing the content of α-Si₃N₄, whether with β-Si₃N₄ seeds or not. The toughness increased with increasing elongated β-Si₃N₄ grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0 MPa m^1/2 with 1 wt% β-seeds. In comparison with α/β Si₃N₄ composite with a similar phase composition, the fracture strength was improved by adding β-Si₃N₄ seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si₃N₄ composite with 5 wt% β-seeds showed a high strength of 1253 MPa, a high hardness of 20.9 GPa and a toughness of 6.9 MPa m^1/2.     

MoSi2–Si3N4 composites: Influence of starting materials and fabrication route on electrical and mechanical properties

The fabrication method and the mechanical and electrical properties of different MoSi₂–Si₃N₄ composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi₂–Si₃N₄ and submicron MoSi₂ powders were used as starting materials, followed by hot pressing. It was found that the sintering atmosphere used, nitrogen or argon, had a significant effect on the phase composition, mechanical and electrical properties of the final materials. It was shown that in some cases partial nitridation of MoSi₂ occurred with the formation of MoSi₂–Mo₅Si₃–Si₃N₄ ternary composites. The electrical conductivity of the composites depends also on the microstructure of materials. It was shown that the composites fabricated using combustion synthesized MoSi₂ powders (500 nm) are characterized by higher flexural strength at room temperature compared to those from commercial powders. On the other hand, the composites fabricated from the commercial powders had higher strength and fracture toughness at elevated temperatures (up to 1200 °C). For all composites, the strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

Preparation,microstructure, and properties of GPS silicon nitride ceramics with β-Si3N4 seeds and nanophase additives

Si₃N₄ ceramics with high thermal conductivity and outstanding mechanical properties were prepared by adding β-Si₃N₄ seeds and nanophase α-Si₃N₄ powders as modifiers. The introduction of β-Si₃N₄ seeds enhanced the growth of β-Si₃N₄ grains. Owing to the interlocked structure induced by the β-Si₃N₄ grains, the fracture toughness of Si₃N₄ ceramics reached a high value of 7.6 MPa·m^1/2; also, the large-sized grains increased the contact possibility of Si₃N₄ grains, improving the thermal conductivity of Si₃N₄ ceramics (64 W/(m·K)). Because of the introduction of nanophase α-Si₃N₄, the flexural strength, fracture toughness, and thermal conductivity of the Si₃N₄ ceramics increased to 754 MPa, 7.2 MPa·m^1/2, and 54 W/(m·K), respectively. According to the analysis of the growth kinetics of Si₃N₄ grains, the rapid growth of Si₃N₄ grains was ascribed to the reduction in the activation energy resulting from the introduction of β-Si₃N₄ seeds and nanophase α-Si₃N₄.     

Feasibility of SiAlON–Si3N4 composite ceramic as a potential bone repairing material

In this study, SiAlON–Si₃N₄ composite ceramic are prepared by direct nitridation and investigated to overcome the limitations associated with ceramic Si₃N₄, which includes the difficulty in fabricating ceramic Si₃N₄ into shaped parts for use in the human body. Phase composition and microstructure of the SiAlON–Si₃N₄ composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the porosity, bulk density, compressive strength, and ion release were also measured. The biological properties were evaluated by bone cell cultures on the ceramic surfaces. Results show that Si₄Al₂O₂N₆ is formed by the reaction of Al, Si, and Al₂O₃ with nitrogen at high temperature that forms Si₃N₄, thereby fabricating SiAlON–Si₃N₄ composite ceramics. Some α-Si₃N₄ grains underwent a phase transition from α-to β-Si₃N₄ fiber at high temperature. Porosity of the samples increases with increasing Si₃N₄ content, while the bulk density of the samples decreases. The compressive strength increases and then slightly decreases with increasing Si₃N₄ content. Water leaching experiments of the SiAlON–Si₃N₄ composite ceramics reveal that the composites exhibit outstanding chemical stability. Studies using bone cell culture indicate that the cells present a fusiform and extend two or three thin pseudopodia. The phenomena demonstrate that MC3T3-E1 cells have excellent growth activity and have the potential ability to proliferate to osteocytes on the surfaces of the samples, thus suggesting that SiAlON–Si₃N₄ based ceramics are biocompatible and could be implemented as a potential bone-repairing material.     

In situ preparation of Si3N4–TiN composite by pyrolysis of polytitanosilazane (PTSZ)

Si₃N₄–TiN composite powders were obtained by in situ pyrolysis of polytitanosilazane. Dense Si₃N₄–TiN composites were prepared by hot-pressing at 1800 °C under 20 MPa for 2 h without sintering additive. Crystallization of amorphous PTSZ powders occurred between 1400 and 1500 °C with major phases, α-Si₃N₄, β-Si₃N₄, and small amount of phase TiN. Mechanical properties and microstructure of Si₃N₄–TiN composites were characterized. The results showed that the mechanical strength was 620 MPa, the fracture toughness was 7.8 MPa m^1/2 and the Vickers hardness was 8.5 GPa. SEM analysis indicated that Si₃N₄–TiN composite possessed excellent fracture toughness because TiN grains produced by in situ pyrolysis were well dispersed in Si₃N₄ matrix.     

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si₃N₄ and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si₂N₂O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si₃N₄ matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si₂N₂O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si₂N₂O grains, the texture formation, and the coarsening of β-Si₃N₄. Fracture toughness, however, was still low (∼5.2 MPa m^1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si₂N₂O-matrix interfacial bond and nearly equiaxed β-Si₃N₄ with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Formation mechanism of Si3N4 in reaction-bonded Si3N4-SiC composites

The formation mechanism and thermodynamics of Si₃N₄ in reaction-bonded Si₃N₄-SiC materials were analyzed. There are two kinds of Si₃N₄, fibroid α-Si₃N₄ and columnar β-Si₃N₄, which are formed by different processes in Si₃N₄-SiC materials. Silicon reacts with oxygen, forming gaseous SiO and reducing oxygen partial pressure. SiO(g) diffuses from central to peripheral sections of blocks and reacts with nitrogen, thus forming Si₃N₄, mainly in peripheral sections. The reaction between silicon and oxygen causes the consumption of oxygen and leads to low oxygen partial pressure in the sintering system, which allows silicon to react with nitrogen directly generating Si₃N₄in situ. SiO(g) reacts with nitrogen forming Si₃N₄ at both central and peripheral sections of block. The non-uniform distribution of Si₃N₄ and uneven microstructure is caused by the generation process, indicating that it is unavoidable in Si₃N₄-SiC composites.     

Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers

Porous Si₃N₄-bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si₃N₄ was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si₃N₄-bonded SiC ceramics were SiC, α-Si₃N₄, and β-Si₃N₄, respectively. The contents of β-Si₃N₄ were increased following the sintering temperature. The morphology of Si₃N₄ whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sintering-temperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si₃N₄ bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6?MPa. The α-Si₃N₄ whiskers were derived from the reaction between N₂ and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si₃N₄ was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si₃N₄ was dissolved into the liquid then the β-Si₃N₄ was precipitated successfully.     

Effect of the diluent on combustion synthesis of silicon nitride

Si₃N₄ powders were prepared by combustion synthesis with 1- and 3-μm α-Si₃N₄, β-Si₃N₄ diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α-Si₃N₄ and β-Si₃N₄ as diluents are about 1600–1800°C. Moreover, the newly formed α-Si₃N₄ contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α-Si₃N₄ and β-Si₃N₄ as diluent about 20–40 wt%. The strip-like α-Si₃N₄, rod-like β-Si₃N₄ grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α-phase content of combustion synthesized Si₃N₄ is discussed, which provides key guidance for preparing Si₃N₄ powders with high α-phase content.     

Effect of large β-Si3N4 particles on the thermal conductivity of β-Si3N4 ceramics

Mean-field micromechanics model, the rule of mixture is applied to the prediction of the thermal conductivity of sintered β-Si₃N₄, considering that the microstructure of β-Si₃N₄ is composed of a uniform matrix phase (which contains grain boundaries and small grains of Si₃N₄) and the purified large grains (⩾2 μm in diameter) of Si₃N₄. Experimental results and theoretical calculations showed that the thermal conductivity of Si₃N₄ is controlled by the amount of the purified large grains of Si₃N₄. The present study demonstrates that the high thermal conductivity of β-Si₃N₄ can be explained by the precipitation of high purity grains of β-Si₃N₄ from liquid phase.     

Additively manufactured mesostructured MoSi2-Si3N4 ceramic lattice

Lattice structures, their shape, orientation, and density make the critical building blocks for macro-scale geometries during the AM process and, therefore, manipulation of the lattice structure extends to the overall quality of the final product. This work reports on manufacturing of MoSi₂-Si₃N₄ ceramic lattices through a selective laser melting (SLM) approach. The strategy first employs the production of core-shell structured MoSi₂/(10-13?wt%)Si composite powders of 3–10?μm particle size by combustion synthesis followed by SLM assembly of MoSi₂/Si lattices and their further nitridation to generate MoSi₂-Si₃N₄ mesostructures of designed geometry. Experimental results revealed that the volumetric energy density of SLM laser has remarkable influence on the cell parameters, strength, porosity and density of lattices. Under compressive test, samples sintered at a higher laser current demonstrated a higher strength value. Selective laser melting has shown its potential for production of cellular lattice mesostructures of ceramic-based composites with a low content of a binder metal, which can be subsequently converted into a ceramic phase to produce ceramic-ceramic structure.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

Fabrication of Si3N4-based composite containing needle-like TiN synthesized using NH3 nitridation of TiO2 nanofiber

A Si₃N₄ composite containing needle-like TiN particles (7 vol%) was fabricated. Needle-like TiN particles several micrometers long were synthesized using NH₃ nitridation of TiO₂ nanofiber, which was obtained using hydrothermal treatment. A mixed powder of α-Si₃N₄ and the needle-like TiN particles with additives was hot pressed at 24 MPa and 1850 °C for 1 h in N₂ atmosphere. Mechanical properties of the composite were compared with those of a composite containing rounded TiN particles and a monolithic β-Si₃N₄ ceramic. The Si₃N₄ matrix of the composites containing TiN was mainly a-phase, suggesting that the α–β phase transformation of Si₃N₄ was inhibited by the presence of TiN. Although fracture strength of the composites was lower, fracture toughness was comparable to that of monolithic β-Si₃N₄ ceramics. Hardness of the composites was about 19 GPa and was greater than that of the monolithic β-Si₃N₄ ceramic.     

The effect of different sintering additives on the electrical and oxidation properties of Si3N4–MoSi2 composites

The fabrication and properties of electrically conductive Si₃N₄–MoSi₂ composites using two different sintering additive systems were investigated (i) Y₂O₃–Al₂O₃ and (ii) Lu₂O₃. It was found that the sintering atmosphere used (N₂ or Ar) had a critical influence on the final phase composition because MoSi₂ reacted with N₂ atmosphere during sintering resulting in the formation of Mo₅Si₃. The electrical conductivity of the composites exhibited typical percolation type behaviour and the percolation concentrations depended on the type of sintering additive and atmosphere used. Metallic-like conduction was the dominant conduction mechanism in the composites with MoSi₂ content over the percolation concentrations due to the formation of a three-dimensional percolation network of the conductive MoSi₂ phase. The effect of the sintering additives on the electrical and oxidation properties of the composites at elevated temperatures was investigated. Parabolic oxidation kinetics was observed in the composites with both types of additives. However, the Lu₂O₃-doped composites had superior oxidation resistance compared to the composites containing Y₂O₃–Al₂O₃. It is attributed to the higher eutectic temperature and crystallisation of the grain boundary phase and the oxidation layer in the Lu₂O₃-doped composites.     

A MoSi2-SiOC-Si3N4/SiC anti-oxidation coating for C/C composites prepared at relatively low temperature

In this study, SiC coating for C/C composites was prepared by pack cementation method at 1773 K, and MoSi₂-SiOC-Si₃N₄ as an outer coating was successfully fabricated on the SiC coated samples by slurry method at 1273 K. The microstructure and phase composition of the coatings were analyzed. Results showed that a porous β-SiC inner coating and a crack-free MoSi₂-SiOC-Si₃N₄ coating are formed. Effect of Si₃N₄ content on the oxidation resistance of the coated C/C composites at 1773 K in air was also investigated. The weight loss curves revealed that introducing the appropriate proportion of Si₃N₄ could improve the oxidation resistance of coating. The MoSi₂-SiOC/SiC coated C/C sample had an accelerated weight loss after oxidation in air for 20 h. However, the coating containing 45% Si₃N₄ could protect C/C composition from oxidation for 100 h with a minute weight loss of 0.63%.     

Cost-effective manufacture and synthesis mechanism of ferrosilicon nitride porous ceramic with interlocking structure

To realize cost-effectively manufacture of high-performance Si₃N₄ porous ceramic, a ferrosilicon nitride porous ceramic with an optimized interlocking structure was synthesized by flash combustion synthesis using FeSi75 powder as raw material. And the technology has been improved in many ways to ensure stable industrial production. The theoretical combustion temperature of FeSi75 in N₂(g) is up to 4608K, while Si₃N₄ is unstable. Both adding diluent and designing the preheat temperature of nitrogen are taken to control synthesis temperature below 1600 °C. During synthesis, the Fe–Si liquid phase and SiO(g), which are essential for the selective growth of elongated columnar β-Si₃N₄ and whisker α-Si₃N₄ respectively, are formed firstly. Then, nitriding proceed in multiple ways. N diffuses through Fe–Si(l) and reacts with Si to form β-Si₃N₄, and the growth of elongated β-Si₃N₄ in Fe–Si liquid follows the dynamic ripening model, which is very fast and effective. Thus, an interlocking structure composed of elongated β-Si₃N₄ with an aspect ratio above 20 is reached. There is also an indirect nitridation reaction, that is, FeSi75 preferentially reacts with trace O₂ in atmosphere to form SiO(g), which is further nitrided to form needle-like α-Si₃N₄. Needle-like α-Si₃N₄ is interspersed in the well-developed columnar β-Si₃N₄, making the structure stronger. Fe finally exists in the form of Fe₃Si, which binds the surrounding elongated Si₃N₄ to form a sea-urchin like unit, making the structure more stable and strengthened. Through control of these reactions, optimizations in microstructure are reached, and the annual output of has reached 25,000 tons. The reaction model is established.     

Simultaneous improvement in porosity and strength of porous β-Si3N4 ceramics by formation of ultrafine fibrous grains

Si₃N₄ ceramic with ultrafine fibrous grains are expected to exhibit remarkable mechanical properties. In this work, highly porous Si₃N₄ ceramic monoliths composed of ultrafine fibrous grains were developed via a novel vapor-solid carbothermal reduction nitridation (V-S CRN) reaction between SiO vapor and green bodies comprised of carbon nanotubes (CNTs), α-Si₃N₄ diluents and Y₂O₃ in a N₂ atmosphere. The unique fibrous grains-interconnected structure was developed through in-situ formation of Si₃N₄ and following liquid phase sintering. The porous Si₃N₄ monoliths with porosity of 61–78% was developed by controlling the contents of α-Si₃N₄ diluents and densities of the CNT green bodies. With increasing of the α-Si₃N₄ contents, Si₃N₄ fibrous grains with an aspect ratio of approximate or higher than 20 could be achieved, and the grains were gradually refined. For the samples with 40 wt% α-Si₃N₄, the minimum mean grain diameter and pore size of 164 nm and 0.79 μm were achieved, respectively, and the resultant porous Si₃N₄ monolith exhibited a flexural strength of as high as 73–102 MPa with the porosity of 61–73%, which is much higher than that of the reported in literature. The improvement of mechanical strength could be attributed to the densely interconnected bird's nests structure formed by the ultrafine fibrous grains. The effects of the α-Si₃N₄ diluents on the resulting porous Si₃N₄ monolith via this method were analyzed.     

Mechanical and dielectric properties of direct ink writing Si2N2O composites

Si₂N₂O composites were achieved via direct ink writing technique and pressureless sintering. The design and optimization of inks and the effect of mass ratio of Si₃N₄ and SiO₂ on the phase composition, microstructure, mechanical properties and dielectric properties of composites were systematically investigated. The inks exhibit superior stability and printability. Increasing SiO₂ content facilitated densification and enhancement of mechanical property. The generation of β-Si₃N₄ and Si₂N₂O and the nucleation of cristobalite were restrained by high content SiO₂. The ceramic composites with the flexural strengths from 74.1MPa to 104.9MPa and low dielectric constant (≤4.68) were fabricated. This strategy provides a systematic reference for synthesis of high-performance porous Si₂N₂O composites based on the DIW.