Si3N4-SiC Composites Research Articles

Mechanical properties of porous Si3N4–SiC composites fabricated at 1200 °C

Porous Si3N4–SiC composites are potentially very attractive for various advanced applications such as aerospace and solar thermal power plants. However, processing of these porous composites is difficult and expensive due to the requirement of very high temperatures. In the present work, porous Si3N4–SiC composites were fabricated at only 1200 °C using phosphoric acid both as a pore former and binder. Porous composites with a constant SiC content were fabricated using various H3PO4 content and the morphology and mechanical properties of the porous composites were systematically investigated. The addition of SiC particles resulted in significant improvement of stiffness, compressive strength, and flexural strength over monolithic porous Si3N4, and porous composites fabricated using 40 vol% H3PO4 yielded the best properties of longitudinal elastic constant >50 GPa, flexural strength >80 MPa, and compressive strength >180 MPa. The achieved mechanical properties have been discussed in light of the analytical micromechanical models for Young's modulus and minimum solid area theory. Finally, the influence of processing temperature on the evolution of porosity and crystalline phases has been systematically studied and the dependence of porous composites' stiffness on these parameters has been investigated in detail.

A TEM study of nanostructures and interfaces in the hot-press sintered ZrB2–SiC–Si3N4 composites

A fully dense ZrB2–30 vol% SiC composite containing 5 wt% Si3N4 and 4 wt% phenolic resin (1.6 wt% carbon) was sintered using the hot-pressing route under the external pressure of 10 MPa at 1900 ºC for 2 h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB2 and SiC grains without any evidence of Si3N4. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of 20 nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB2 grain growth. Two types of ZrB2/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si3N4 addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB2 grain (0.335 nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB2 grains confirmed dislocation creep as the main densification mechanism.

Silicon Carbide–Silicon Nitride Refractory Materials: Part 1 Materials Science and Processing

Silicon carbide and silicon nitride materials were intensively studied in the end of the past century, yet some aspects of its physical chemistry require investigation. The strength characteristics of Si3N4-SiC refractories are moderate; however, these materials sometimes demonstrate “stress–strain” behavior, more typical for composite materials than for the brittle ceramics. These materials may be considered to be ceramic composites because they consist of big grains of silicon carbide surrounded by small grains of silicon nitride, with strict interfaces between them. There is no direct certainty whether Si3N4-SiC compositions may be called composite materials or brittle ceramic materials from the viewpoint of mechanics and strength. The balance of α/β modifications of silicon nitride in Si3N4-SiC composite material and, the occurrence and the role of silicon oxynitride Si2ON2 are also a matter of scientific interest in processing of Si3N4-SiC composite material. The same may be said about the particles of silicon nitride between the grains of silicon carbide—there is no direct understanding whether silicon nitride grains will be isometric grains or needle-like crystals.

Influence of fluoride phase on the tribological behaviour of Si3N4–SiC composites under extreme operating conditions

For extreme operating conditions such as high temperature and load, Si3N4–SiC composite is often used as wear resistant material. However, it suffers from a moderately high friction coefficient and wear rate at such working conditions; therefore, it is still an ongoing area of research for further development. This work strives to develop a state-of-the-art wear-resistant Si3N4 composite with SiC nanowire as reinforcement phase and MgF2 as an internal lubricating additive, particularly suitable for harsh operating conditions at both high temperatures and high loads. A significant improvement in tribological characteristics is observed when MgF2 is added into the composite matrix tested at 500 °C as compared to its parent Si3N4–SiC composite. When the load is increased from 6 to 14 N, the composite shows better wear resistance by forming a lubricious tribolayer at the contacting surface. Delamination is found to be the dominating wear mode for composites containing fluoride phase as identified by the morphological characterization of the worn surfaces. A comprehensive analysis of the evolution of different wear modes with respect to the variation in extreme operating conditions is articulated here.

Effect of SiC particle size on the strength and stiffness of porous Si3N4–SiC composites fabricated via a low-temperature sintering process

Porous Si3N4–SiC composites are potentially very attractive in various applications such as chemical-gas industries, nuclear power plants, automobile industries, and as heat-absorbing materials for solar thermal power plants and electromagnetic radiation shielding materials for commercial and military applications. The fabrication of these advanced composites generally requires expensive raw materials and very high temperatures necessitating advanced sintering techniques, thereby making these materials very expensive. Within the scope of this work, a novel technique for fabricating these porous composites at a low temperature of 1200 °C using phosphoric acid both as a binder and pore former has been proposed. Two different Si3N4–SiC composites with different SiC particle sizes and varying SiC content, with overall porosities in the range of 33–43 vol%, have been fabricated. A detailed study of the influence of the composites’ composition on their microstructure and mechanical properties has been carried out. The results show that both the SiC particle size and its content strongly influence the strength, stiffness, and elastic anisotropy of the fabricated composites. The most optimum composition resulting in the best combination of stiffness and strength in the porous composites has been identified. This yields a longitudinal elastic constant of 50 GPa, a flexural strength of ∼80 MPa, and compressive strength of ∼200 MPa. These values fare well in comparison to porous Si3N4–SiC composites reported in the literature, albeit fabricated at much higher temperatures in comparison to this work. The strength and stiffness of the porous composite with composition Si3N4-20 vol% fine SiC are considerably higher than monolithic porous Si3N4.

Synthesis mechanism of α-Si3N4 whiskers via SiO vapor in reaction bonded Si3N4-SiC composite

An indirect nitridation mechanism was designed to realize a high conversion rate to α-Si3N4 whiskers in reaction bonded Si3N4-SiC composite via VS mechanism, in which the dominant vapor-phase source of reactants is SiO vapor produced by the active oxidation of Si. The role played by the temperature-time program and addition of ultra-fine Si powder on the formation of SiO vapor and the resulting Si3N4 whiskers was clarified based on the experimental results and the thermodynamic calculations. The result shows that during sintering, in the presence of trace O2, ultra-fine Si particles preferentially undergo active oxidation and volatilized completely in the form of SiO(g). While an equilibrium of SiO(g) and Si(s) is formed on the surface of the coarse Si particle meanwhile. By stepped holding temperature further, this equilibrium is constantly upgraded, so the coarse Si particles is evaporated as SiO(g) shell layer by layer. Massive α-Si3N4 whiskers is formed by SiO(g) nitridation while the stepped holding temperature sintering. A reasonable model of the controllable indirect nitridation mechanism was established.

Dielectric properties and thermal conductivity of Si3N4–SiC composite ceramics

In this work, Si3N4–SiC composite ceramics were prepared by hot pressing sintering with Y2O3–MgO–Al2O3 as sintering additives in a flowing nitrogen atmosphere. The crystal structure and microstructure of Si3N4–SiC specimens with different SiC content were studied. The XRD results show that the peaks of β-Si3N4 and SiC were displayed and matched well on XRD patterns. Si3N4 particles were basically long rod-shaped. Besides, the effects of SiC content on thermal conductivity and dielectric properties in W band (75–110 GHz) of sintered Si3N4–SiC ceramics were researched. The dielectric constant first decreased then increased with the increase of SiC content, while the trend of dielectric loss (tgδ) was opposite. The dielectric constant of Si3N4–SiC composites for 35 wt% SiC reached a minimum range of 13–20 and tgδ values reached a maximum range of 0.72–0.96 in the frequency of 75–110 GHz. Meanwhile, the thermal conductivity of Si3N4–SiC composites for 35 wt% SiC was 63.008 W/(m K) at room temperature. The variation of thermal conductivity as test temperature was also studied. The thermal conductivity decreased with increasing test temperature. When the test temperature was 300 °C, the thermal conductivity of the Si3N4–SiC ceramics for 35 wt% SiC content was 36.022 W/(m K). The Si3N4–SiC composite ceramics were expected to be applied as microwave absorbing materials.

Effect of SiC Particle Size on the Strength and Stiffness of Porous Si3N4-SiC Composites Fabricated Via a Low-Temperature Sintering Process

Effect of SiC Particle Size on the Strength and Stiffness of Porous Si3N4-SiC Composites Fabricated Via a Low-Temperature Sintering Process

Microstructure, phase compositions and mechanical properties of slip cast sintered SiC/Si3N4 composites

In this study, a slurry containing a mixture of Si and SiC powders was used to fabricate the SiC/Si3N4 composites by slip casting method and sintering in a nitrogen-controlled furnace. The effects of mixing ratios of Si and SiC powders, different particle sizes of SiC powders as well as three sintering temperatures (1400, 1450, 1490 °C) were evaluated for SiC–Si3N4 composites. The results demonstrated that the best slurry was prepared using the weight ratio of Si/SiC of 30/70 and the fine-particle size of SiC powders with an average particle size of d50 ≤ 0.45 μm. The results of XRD and FESEM determined that the sintered composites at 1450 °C have the minimum porosity without the SiO2 phase. Besides, with an increase in the sintering temperature, the α/β-Si3N4 phase ratio increased while the hardness and flexural strength were strongly dependent on the α/β-Si3N4 phase ratio in the SiC–Si3N4 composites.

SiC–Si3N4 composite with multi-laminate nanowires originated from oriented porous structure by freeze-casting

Novel form of SiC–Si3N4 composite has been prepared via a simple and versatile route, namely carbon-doped sol with aligned pores from freeze casting method in combination with the subsequent carbothermal reduction process. The oriented pores of green bodies supply both nitrogen and space for the nucleation and growth of Si3N4 nanowires following the VS mechanism. Meanwhile, lack of nitrogen at partial region leads to the generation of aggregated β-SiC particulates. As a result, multiply laminate nanofibrous ceramic foams embedded with fine particles have been prepared. Owing to the low bulk density of 0.15 g/cm3 and fine building blocks, this novel material possesses relatively high specific surface area of 29.89 m2/g and ultra-low thermal insulation of 0.058 W/(m·K), which would have potential applications in the catalysts, thermal insulators or gas/liquid filters fields.

Basınçsız ve Spark Plazma Sinterlenmiş Si3N4 Kompozitlerin Özelliklerine SiC’ün Etkisi

Si3N4-SiC composites were prepared by adding SiC particles to the matrix in different weight ratios.The samples were sintered with two different methods. The samples were produced by pressureless sintering and spark plasma sintering method. In the first method, cold isostatic pressing method was used to press the composites. The samples were pressureless sintered at 1850 0C for 1 hour under the nitrogen atmosphere. In the second method, the samples were spark plasma sintered at 1650 0C under nitrogen atmosphere under 40 MPa for 5 minutes. AlN and Y2O3 were also added to Si3N4-SiC composite mixture to aid sintering. Addition of SiC particles to the Si3N4 matrix play critical role in controlling of densificication and microstructure properties of the sintered samples. The effect of composition and microstructure on properties of sintered samples was analyzed.

Synthesis and characterization of SiC/Si3N4 composites from rice husks

SiC-Si3N4 composites have been obtained by carbothermal reduction of rice husk under a nitrogen-argon atmosphere at 1450 °C, which is a lower temperature than those used by other authors. On the other hand, tailoring the argon/nitrogen ratio led to the obtained of SiC-Si3N4 composites across the whole range of compositions. Phosphoric acid treatment permited the synthesis of the composite without a pyrolysis step. The final products were characterized by X-ray diffractometry, IR spectroscopy and scanning electron microscopy.

Sintering and kinetic mechanism of Si3N4-SiC micro-nano composite by spark plasma sintering

Si3N4–SiC composites were fabricated by spark plasma sintering at 1700[Formula: see text]C for 480 S with MgSiN2 and Y2O3 as additives. The morphology and phase characterization of the composites were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The values of [Formula: see text] parameter indicate that the grain boundary reaction is the rate controller at 1500[Formula: see text]C and diffusion becomes the controlling step at 1550[Formula: see text]C. The nanohardness and Young’s modulus attained the maximum value of 18.5 and 316[Formula: see text]GPa, respectively.

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.

Research on Rheological Behavior of Si<sub>3</sub>N<sub>4</sub>-SiC Composites by Gelcasting

Using SiC particles and powder, the modified Si and α-Si3N4 powder as the main raw material, Si3N4-SiC composite materials have been fabricated by gelcasting. The Si3N4-SiC suspension with 75vol% (volume fraction) solid loading exhibits shear thinning rheological behavior under a shear rate from 10 to 160s-1, which meets the requirement of the gelcasting process. The bulk density of the green bodies after solidification is 2.57 g·cm-3 and the flexural strength 16 MPa, the green bodies have a dense and uniform structure. The bulk density and flexural strength of the Si3N4-SiC composites after nitriding are 2.67g·cm-3 and 48 MPa respectively; the strength is higher than that of other industrial products without gelcasting. Well-developed intertwined fibrous Si3N4 crystals grow on the surface of the SiC particles to form a tight microstructure in the Si3N4-SiC composites after firing.

Si3N4–TiN–SiC three particle phase composites for wear applications

A three phase ceramic composite consisting of a Si3N4 matrix reinforced with TiN and SiC particles was prepared by hot pressing. The Si3N4–TiN–SiC composite material was investigated for microstructure and mechanical properties. Dry wear tests were carried out and the results compared with two phase Si3N4–TiN and Si3N4–SiC composites. The Si3N4–TiN–SiC was found to have an interesting combination of high abrasive wear resistance, low coefficient of friction and high hardness, which could lead to its use in very interesting wear applications.

Influence of the heat treatment on mechanical properties and oxidation resistance of SiC–Si3N4 composites

The basic mechanical properties and oxidation behaviour of liquid-phase-sintered SiC–Si3N4 composites were investigated as a function of the heat treatment at oxidising condition at 1350 °C/0–204 h. The results were compared to those obtained for a reference silicon carbide material, prepared by the same fabrication route. The heat treatment at higher temperature had a positive effect on fracture toughness values but no changes of hardness were observed. It was shown that the oxidation resistance increases with increasing temperature of heat treatment from 1650 °C to 1850 °C. Oxidation always followed parabolic rate law indicating diffusion as the rate limiting mechanisms. The addition of silicon nitride content had no significant influence on oxidation resistance of SiC–Si3N4 composites.

Fabrication of Porous Reaction-Bonded Si3N4-SiC Composites

Porous reaction-bonded Si3N4-SiC composites were fabricated by using potassium chloride(KCl) as pore-forming agent. Green bodies with 20% SiC and 30% KCl in mass were subjected to presinter in Ar atmosphere at 1000~1200 and then reaction-sintered. The results indicated that Si3N4-SiC composites which with about 58.5% porosity could be prepared, and the main phase compositions were α-Si3N4 and SiC. After presintered, the porosity of composites had small decrease, but bending strength increased obviously and little amounts of Si2N2O had been found. The distribution of pores of porous ceramics was homogeneous and SiC grains were surrounded by Si3N4 grains and fibres. Lots of needle-like α-Si3N4, especially in the high porosity ceramics, could be observed.

Consolidation and mechanical properties of nanostructured MoSi2–SiC–Si3N4 from mechanically activated powder by high frequency induction heated sintering

A dense nanostructured MoSi2–SiC–Si3N4 composite was sintered by the high frequency induction heating method within 2min from mechanically activated powder of Mo2N, Mo2C and Si. Highly dense MoSi2–SiC–Si3N4 composite was produced under simultaneous application of a 80MPa pressure and the induced current. Mechanical properties and grain size of the composite were investigated. The average hardness and fracture toughness values obtained were 1420kg/mm2 and 4.5MPam1/2, respectively.

Fabrication and microstructure characterization of dense and porous SiC-Si3N4/AlN composites using multi-pass extrusion process

Novel dense and porous SiC-Si3N4/AlN composites were fabricated using a multi-pass extrusion process. In the 2nd and 3rd pass dense composites, the diameter of AlN fibers in the SiC-Si3N4 matrix was about 170 μm and 30 μm, respectively. The AlN fibers were refined in the SiC-Si3N4 composites with the increase of extrusion pass number. However, in the porous SiC-Si3N4/AlN composites, the thin layer of AlN was well coated on the continuous pore surface of the porous SiC-Si3N4 matrix. The thickness of the AlN layers in the porous composite was about 40 μm. The detailed microstructures and material properties of the dense and porous SiC-Si3N4/AlN composites were investigated using OM, SEM, XRD, TEM and HRTEM techniques.