Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

BN/Si3N4 nanocomposite with high strength and good machinability

BN/Si₃N₄ nanocomposite was prepared using BN/Si₃N₄ powder obtained by nitriding Si₃N₄/NH₄HB₄O₇ mixture in ammonia gas as the starting powder. Microstructural investigations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that BN particles were homogeneously distributed within the matrix grains as well as at the matrix grain boundaries, and the growth of Si₃N₄ matrix grain was significantly retarded by BN particles. The BN/Si₃N₄ nanocomposite showed a higher strength than the conventional BN/Si₃N₄ microcomposite due to the formation of fine and homogeneous microstructure in it. BN/Si₃N₄ nanocomposite with a BN content of 20 vol% and above showed excellent machinability, because of the formation of weak BN/Si₃N₄ interfaces and the cleavage behavior of BN particles.     

Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si₃N₄) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si₃N₄ powders was studied, and the results were compared with a commercial powder. The α-Si₃N₄ powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm^?3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si₃N₄ powders produced by CRN from silica initially containing 5 wt.% Y₂O₃. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.     

Microstructure and properties of porous silicon nitride ceramics prepared by gel-casting and gas pressure sintering

Wave-transparent porous Si₃N₄ ceramics were prepared by gel-casting and gas pressure sintering, and the effects of solid loading on microstructure, mechanical and dielectric properties were investigated. Microstructures with interlocked elongated β-Si₃N₄ grains and uniformly distributed pores were observed, while both the β-Si₃N₄ phase content and grain aspect ratio reduced as the solid loading increased due to the restrained anisotropic growth of β-Si₃N₄ grains. As the solid loading increased from 30 to 45 vol.%, the porosity of ceramics declined from 57.6% to 36.4%. The flexural strength increased linearly from 108.3 to 235.1 MPa, and the dielectric constant and loss tangent of ceramics increased from 2.63 and 2.85 × 10⁻³ to 3.68 and 3.56 × 10⁻³ (10 GHz), respectively.     

A novel fiber-reinforced polyethylene composite with added silicon nitride particles for enhanced thermal conductivity

A novel thermally conductive plastic composite was prepared from a mixture of silicon nitride (Si₃N₄) filler particles and an ultrahigh molecular weight polyethylene–linear low density polyethylene blend. The effects of Si₃N₄ particle sizes, concentration, and dispersion on the thermal conductivity and relevant dielectric properties were investigated. With proper fabrication the Si₃N₄ particles could form a continuously connected dispersion that acted as the dominant thermally conductive pathway through the plastic matrix. By adding 0–20% Si₃N₄ filler particles, the composite thermal conductivity was increased from 0.2 to ～1.0 W m^?1 K^?1. Also, the composite thermal conductivity was further enhanced to 1.8 W m^?1 K^?1 by decreasing the Si₃N₄ particle sizes from 35, 3 and 0.2 μm, and using coupling agent, for the composites with higher filler content. Alumina short fibers were then added to improve the overall composite toughness and strength. Optimum thermal, dielectric and mechanical properties were obtained for a fiber-reinforced polyethylene composite with 20% total alumina–Si₃N₄ (0.2 μm size) filler particles.     

Fracture behaviour of α-Al2O3 ceramics reinforced with a mixture of single-wall and multi-wall carbon nanotubes

The extraordinary mechanical properties of single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) have generated interest in incorporating them as toughening agents in ceramics. This work describes the fracture behaviour of an alumina (Al₂O₃) ceramic reinforced with a mixture of 0.05 wt% MWCNTs + 0.05 wt% SWCNTs. The CNT/Al₂O₃ nanocomposite was pressureless sintered in air using graphite powder as bed powder at 1520 °C for 1 h. The hardnesses and fracture toughnesses were lower than for pure Al₂O₃ and Al₂O₃ + 0.1 wt% SWCNTs and Al₂O₃ + 0.1 wt% MWCNTs. A predominantly transgranular fracture mode with a decrease in crack deflection and no pull-out was observed in the SWCNT + MWCNT–Al₂O₃ nanocomposite. MWCNTs had to the best reinforcing effect in Al₂O₃ nanocomposite.     

Study on tribological behavior of electrodeposited Cu–Si3N4 composite coatings

Cu–Si₃N₄ composite coatings were prepared by electrolysis from a copper sulphate solution containing dispersed Si₃N₄ particles of 0.4 or 1 μm mean size. Wear behavior of Cu–Si₃N₄ composite and pure copper coatings were evaluated using a pin-on-disc test machine under dry condition sliding. Effects of current density and particle concentration on the incorporation percentage of Si₃N₄, the preferred orientation of copper crystallites, the microstructure, the microhardness and the wear resistance of the coatings were determined. Si₃N₄ particles in the copper matrix resulted in the production of composite deposits with smaller grain sizes and led to change the preferred orientation growth from [1 0 0] to [1 1 0]. It was proved that the presence of Si₃N₄ particles decreases the wear loss and the friction coefficient of the coating. According to the results, the friction coefficient decreased dramatically from 0.52 to 0.26 for pure copper coatings to 0.16–0.24 for Cu–Si₃N₄ composite coatings. In addition, fluctuation of friction coefficient values for Cu–Si₃N₄ composite coating was lower compared with the pure copper coating. The wear properties of Cu–Si₃N₄ composite coatings were shown to depend on the weight fraction, the size and the distribution of co-deposited particles.     

Microstructure and fracture-mechanical properties of carbon derived Si3N4 + SiC nanomaterials

The microstructure and basic mechanical properties, as hardness, fracture toughness, fracture strength and subcritical crack growth at room temperature were investigated and creep behavior at high temperatures was established. The presence of SiC particles refined the microstructure of Si₃N₄ grains in the Si₃N₄ + SiC nanocomposite. Higher hardness values resulted from introducing SiC nanoparticles into the material. A lower fracture toughness of the nanocomposite is associated with its finer microstructure; crack bridging mechanisms are not so effective as in the case of monolithic Si₃N₄. The strength value of the monolithic Si₃N₄ is higher than the characteristic strength of nanocomposites. Fractographic analysis of the fracture surface revealed that a failure started principally from an internal flaw in the form of cluster of free carbon, and on large SiC grains which degraded strength of the nanocomposite. The creep resistance of nanocomposite is significantly higher when compared to the creep resistance of the monolithic material. Nanocomposite exhibited no creep deformation, creep cracks have not been detected even at a test at 1400 °C and a long loading time, therefore the creep is probably controlled mainly by diffusion. The intergranular SiC nanoparticles hinder the Si₃N₄ grain growth, interlock the neighboring Si₃N₄ grains and change the volume fraction, geometry and chemical composition of the grain boundary phase.     

Densification and alloying of ball milled Silicon-Germanium powder mixture during spark plasma sintering

In this research, the influence of process parameters such as sintering temperature and current during alloying and densification of silicon-germanium (Si₈₀-Ge₂₀) powder mixture using spark plasma sintering (SPS) was reported. Si₈₀-Ge₂₀ powder mixture was consolidated at the temperature range 900–1200 °C with 40 MPa pressure for 5 min. soaking. X-ray diffraction (XRD) study was made on sintered compacts to confirm the Si(Ge) alloy formation. Scanning electron microscope (SEM) was used to understand the morphology, particle size and distribution of un-milled and milled Si₈₀-Ge₂₀ powder mixture. Transmission electron microscope (TEM) study was made on milled Si₈₀-Ge₂₀ powder mixture and bulk SiGe alloy to confirm the nano-crystallinity and alloy formation. Fracture toughness of sintered bulk SiGe alloy was determined from Palmqvist cracks geometry model using Vickers hardness testing. It is understood that, during spark plasma sintering nano-structured Si₈₀-Ge₂₀ powder simultaneously increases the densification and reaction kinetics. It helps to achieve homogenous nanostructured SiGe alloy of near theoretical density. The superior hardness and benchmarked fracture toughness (K_IC) values of 630 VHN and 2.19 MPa√m was achieved for SiGe alloy sintered at 1200 °C, respectively.     

Structure and strength of aluminum with sub-micrometer/micrometer grain size prepared by spark plasma sintering

A spark plasma sintering (SPS) technique has been applied to prepare fully dense Al samples from Al powder. By applying a sintering temperature of 600 °C and a loading pressure of 50 MPa, fully recrystallized samples of nearly 100% density with average grain sizes of 5.2 μm, 1.3 μm and 0.8 μm have been successfully prepared using a sintering time of less than 30 min and without the need for a nitrogen atmosphere. A similarity between the grain size and powder particle size is found, which suggests a potential application of the SPS technique to prepare samples with a variety of grain sizes by tailoring the initial powder particle size. The SPS samples show higher strength than Al samples with an identical grain size prepared using thermo-mechanical processing, and a better strength–ductility combination, with the 1.3 μm grain size sample showing a yield strength (σ_0.2%) of 140 MPa and a uniform elongation of more than 10%. This higher strength is related to the presence of oxide particles in the grain boundaries of the samples. It is concluded that SPS is an excellent technique for the production of very fine grained Al materials with high strength, by combining both grain boundary and oxide dispersion strengthening.     

Fabrication of a polymer composite with high thermal conductivity based on sintered silicon nitride foam

A novel route was developed to fabricate Si₃N₄/epoxy composite. In this route, the Si₃N₄ particles were constructed into the foamed shape by using protein foaming method, firstly. Then the Si₃N₄ foams were sintered to bond these Si₃N₄ particles together. Finally, the Si₃N₄/epoxy composite was fabricated by infiltrating the epoxy resin solution into the sintered Si₃N₄ foams. This route was proved to be an efficient way in enhancing the thermal conductivity of epoxy matrix at a low loading fraction. For example, the thermal conductivity of the as-prepared Si₃N₄/epoxy composite with a loading fraction of 22.2 vol% was up to 3.89 W m⁻¹ K⁻¹, which was about 17 times higher than that of neat epoxy.     

Microstructure characterization,mechanical properties,compressibility and sintering behavior of Al-B4C nanocomposite powders

In the present work, Al-xB₄C nanocomposite (x = 0, 1, 2, 3, 4 and 5 in wt%, having the average B₄C size of 50 nm) were prepared using a high-energy ball mill. The milling times up to 16 h were applied. Then, the microstructural evolutions, mechanical properties, compressibility and sintering behavior of nanocomposites were investigated. The changes in powders morphology and microstructure during the milling process were characterized by laser diffraction particle size analyzer (LDA), SEM, XRD, EDS and TEM techniques. Compressibility and sintering behavior of milled powders compacted under different pressures (100–900 MPa) and at different sintering temperatures (500, 550 and 600 °C) were also studied. The pressing behavior of the nanocomposites was analyzed using linear compaction equations developed by Heckel, Panelli-Filho and Ge. The results showed the significant effects of B₄C amounts and sintering temperatures on the compressibility and sintering behavior of nanocomposites. The increase in the B₄C amount led to a decrease in both the compressibility rate and the sinterability of specimens. The maximum compression strength of 265 MPa and Vickers hardness of 165 VHN were obtained for Al-5 wt.% B₄C nanocomposite milled for 16 h followed by sintering at 600 °C.     

Intergranular alumina–SiC micro-nanocomposites sintered by spark plasma sintering

Alumina–5 vol% SiC micro-nanocomposites with almost completely intergranular microstructure were produced by spark plasma sintering (SPS). Before sintering, well dispersed slurries were obtained by ball-milling of powders using an electrosteric dispersant in aqueous media. High density composites could be achieved at lower sintering temperatures by SPS, as compared with hot pressing (HP) sintering process. Microstructure studies show that the nano-SiC particles were mainly located at the alumina grain boundaries.     

Improvement of the bonding interface of a sintered Al 2014–(Ti5Si3)p composite by the copper coating of the reinforcement

AA 2014 aluminium-based composites reinforced with (5–20 wt.%) Ti₅Si₃ intermetallic particles, with and without Cu coating, were obtained in a Turbula powder mixer from commercially-available prealloyed powders. Mechanical alloying was used for the deposition of Cu on the surface of the Ti₅Si₃ particles. Compaction of the specimens was performed using a hydraulic press and a floating die. The results show that the liquid formation and phase distribution are modified by the copper coating of the ceramic reinforcement, resulting in changes in the materials microstructure and the mechanical properties. The presence of the reinforcement particles improves densification of the composites. Improved densification was found for the 2014 + Ti₅Si₃ composites. 2014 + Ti₅Si₃–Cu composites exhibit superior mechanical properties compared to the 2014 + Ti₅Si₃ composites.     

Synergistic effect of organic and inorganic nano fillers on the dielectric and mechanical properties of epoxy composites

Titanium oxide TiO₂/epoxy and TiO₂ with detonation nano-diamond (DND)/epoxy nanocomposites were prepared by using ultrasonication method. TiO₂ and DND particles as reinforcement species and epoxy as matrix were used to produce nanocomposites. The addition of DND particles into TiO₂/epoxy composite improved the dielectric and mechanical properties of nanocomposites in significant amount. The dielectric properties of TiO₂-DND/epoxy nanocomposite demonstrated increase in permittivity and conductivity after addition of the DND particles. The maximum and minimum reflection losses of TiO₂-DND/epoxy nanocomposite for 0.6 and 0.2 wt% DND loading were detected at ?14.5 and ?1.3 dB, respectively. The flexural and tensile strength of TiO₂-DND/epoxy nanocomposites with the addition of 0.4 wt% DNDs were enhanced to 220% and 223%, respectively. Additionally, the energy to break and percent break strain were 3.9 J and 3.86, respectively for 0.4 wt% DND loading in TiO₂-DND/epoxy nanocomposite. Therefore, the present work findings claim that DND particles are well suitable to enrich the dispersion of TiO₂ nanoparticles in epoxy matrix, which develops a strong load transfer interface between the nanoparticles and epoxy matrix and consequently leads to superior properties.     

Microstructural control of green bodies prepared from Si-based multi-component non-aqueous slurries and their effects on fabrication of Si3N4 ceramics through post-reaction sintering

A Si-based slurry containing Si particles covered with Y₂O₃ and MgO nanoparticles (NPs) has been successfully prepared and then applied to shape Si-based green compacts for the fabrication of silicon nitride (Si₃N₄) ceramics via post-reaction sintering. It was found that Y₂O₃ and MgO NPs modified with polyethyleneimine-oleic acid complex (PEI-OA) could be effectively attached to Si particles by simple mixing in dense toluene slurry. Field emission scanning electron microscopy observations confirmed the attachment of PEI-OA-modified sintering aids to Si particles without forming large NP agglomerates. The adsorption of the PEI-OA-modified sintering aids and PEI-OA on the surface of Si particles drastically improved the stability of the Si-based toluene slurry, which was subsequently molded through wet vacuum casting and dewaxed to fabricate a Si-based green body. The obtained green body was nitrided at 1375?°C for 4?h at a N₂ pressure of 0.15?MPa and further sintered at 1850?°C for 2?h at a N₂ pressure of 0.9?MPa. The adsorption of sintering aid particles on the Si surface reduced the number of contact points between Si particles in the green body, which effectively suppressed the Si melting process during nitriding and improved the characteristics of the produced nitride body such as the degree of nitriding and α/(α+β) ratio of Si₃N₄, leading to the successful fabrication of high-density Si₃N₄ ceramics during the subsequent densification step.     

Preparation and characterization of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics prepared by compression molding and slip casting methods

Porous silicon nitride (Si₃N₄) ceramics were fabricated by compression molding and slip casting methods using petroleum coke as pore forming agent, and Y₂O₃-Al₂O₃ as sintering additives. Microstructure, mechanical properties and gas permeability of porous Si₃N₄ ceramics were investigated. The mechanical properties and microstructure of porous Si₃N₄ ceramics prepared by compression molding were better than those which were prepared by slip casting method, whereas slip casting method is suitable for the preparation of porous Si₃N₄ ceramics with higher porosity and excellent gas permeability.     

Mechanical properties of carbon nanotube–alumina nanocomposites synthesized by chemical vapor deposition and spark plasma sintering

Carbon nanotubes–alumina (CNT–Al₂O₃) nanocomposites with variable CNT content were directly synthesized by chemical vapor deposition (CVD). The as-grown CNT–Al₂O₃ mixture was densified by spark plasma sintering (SPS) at 1150 and 1450 °C. Vickers hardness of 9.98 GPa and fracture toughness of 4.7 MPam^1/2 were obtained for 7.39 wt.% CNT–Al₂O₃ nanocomposite. The addition of CNTs gives rise to 8.4% increase in hardness and 21.1% increase in toughness over that of the pure Al₂O₃. The optimum amount of CNTs is considered to be able to significantly enhance the mechanical property of ceramics in composites.     

Evaluation of processing parameters effects on the formation of Si3N4 wires synthesized by means of ball milling and nitridation route

Crystalline silicon nitride (Si₃N₄) wires have been synthesized by means of ball milling and nitridation route. The influence of temperature of reaction and starting condition of the powder (milled or unmilled) on the synthesis of Si₃N₄ wires were studied. The reduced size of silicon particle during the milling process led to an increased degree of nitridation.Silicon powders with higher surface energy can react incessantly with nitrogen to form silicon nitride wires. The results show that the Si₃N₄ was fully formed with two kinds morphologies including globular and wire with a width of 100–300 nm and a length of several microns at temperature of 1300 °C for 1 h by employing the milled silicon powder. The infrared adsorption of wires exhibit absorption bands related to the absorption peaks of Si–N band of Si₃N₄.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.