Low-temperature fabrication and characterization of porous SiC ceramics using silicone resin as binder

Commercially available silicone resin and silicon carbide (SiC) powders were adopted as the starting materials for the fabrication of porous SiC ceramics. During the heat treatment process, silicone resin experienced an organic–inorganic transformation and acted as the bonding material between SiC particles at a low temperature of 1000 °C. The mean particle size of starting SiC powders and silicone resin content can control the pore size, open porosity and fracture strength. The flexural strength of porous SiC ceramics increases with increasing silicone resin content and decreasing mean particle size of SiC powders. Larger pores can be obtained with coarser starting SiC powders and higher silicone resin content. The fracture surface of porous SiC ceramics was observed.     

Microstructural characterization of C-SiC-carbon nanotube composite flakes

Composite flakes of carbon, silicon carbide (SiC) and carbon nanotubes were synthesized by spray pyrolysis employing a slurry of ferrocene (Fe(C₅H₅)₂) and SiC powders in xylene at 1000 ± 50 °C. These flakes were characterized for their microstructure and composition by transmission electron microscopy, scanning electron microscopy and high-resolution electron energy loss spectroscopy. These studies suggest the in situ formation of nanotubes and sintering of the SiC nanoparticles to form the freestanding composite flakes.     

Mechanical and tribological properties of silicon carbide coating on Inconel alloy from liquid pre-ceramic precursor

Liquid polycarbosilane (LPCS) derived hard coatings of silicon carbide (SiC) were deposited on Inconel alloy at three different moderately high temperatures by chemical vapour deposition. The deposited films were characterized by X-ray diffractometry and Field emission scanning electron microscopy. Liquid PCS yielded a mixture of α-SiC and β-SiC during decomposition having uniform round-shaped particles of dimension around 200–300 nm without extensive cracking and few discrete shaped particles were also found to form at higher temperature (i.e. 1100 °C and 1200 °C) deposited films. The coated samples showed substantial increment in hardness and fracture toughness as compared to the uncoated sample. The fracture toughness (K_IC) values of the deposited films were in the range of 6.7–10.7 MPa(m)^1/2. The tribological properties and hardness of the films were also found to vary with deposition temperature. The scratch tracks of the films revealed that brittle failures occurred in all SiC coated substrates.     

Preparation of SiC nanowires-filled cellular SiCO ceramics from polymeric precursor

SiC nanowires-filled cellular SiCO ceramics were prepared using polyurethane sponge as a porous template infiltrated with silicone resin by pyrolysis at 1400 °C under Ar atmosphere. The pyrolysis temperature was an important parameter affecting the formation of SiC nanowires. The as-prepared sample obtained at 1000 °C was composed of SiCO glasses and turbostratic carbon. The SiCO ceramic was further converted into SiO₂ crystals and amorphous carbon by pyrolysis at 1200 °C. With the increasing pyrolysis temperature, SiC nanocrystals embedded in the non-crystalline SiCO matrix were observed. Furthermore, the SiC nanowires were formed in the pores of the SiCO ceramic. The diameters of the SiC nanowires are in the range 80–150 nm and the lengths are up to several tens of micrometers. The growth mechanism of the nanowires was supported by the vapor-solid mechanism.     

Facile synthesis of boron carbide elongated nanostructures via a simple in situ thermal evaporation process

Boron carbide elongated nanostructures such as nanowires, nanobelts and nanosheets have been synthesized via a low-cost and simple in situ thermal evaporation process using commercially available B₄C powders as the main precursor. Heat treatments were done in the temperature range of 1400-1600 °C in the presence of Co nanoparticles (and NiCl₂ in some experiments) as the catalyst material. The growth mechanism of the nanostructures was proposed to be a cooperative growth procedure including surface diffusion, vapor-liquid-solid (VLS) and solid-liquid-solid (SLS) growth mechanisms. The final product, containing some of the initial B₄C particles and as-synthesized elongated nanostructures may be potentially applicable as an excellent reinforcing phase in composite materials. Moreover, nanostructures with right angle junctions were obtained from the sidewalls of the graphite boats, which may be operative in MEMS and NEMS devices. The samples have been characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and photoluminescence spectroscopy.     

Effect of SiC particles on rehological and sintering behaviour of alumina–zircon composites

The effect of additions of SiC particulates on rheological and sintering behaviour of slip-cast alumina–zircon composites has been investigated. Finely divided alumina, zircon and silicon carbide powders were first processed into slips, using polyacrylite dispersant (0.5 wt.%) to create highly concentrated, stable aqueous suspensions at 40 vol.% loadings, from which test specimens which were then slip cast and dried. They were subsequently sintered in air for 2 h at 1650 °C. Rheological properties of the prepared slips were evaluated and related to the amount of added SiC. After sintering, the resultant porosities, fractional densities, crystallographic phases present, and microstructures were determined.     

In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

Alumina/silicon carbide composites fabricated via in situ synthesis of nano-sized SiC particles

Alumina/silicon carbide composites have been fabricated by a new technique involving the in situ synthesis of nano-sized SiC particles. A mixture of alumina powder and silicon carbide precursors was prepared in an aqueous suspension. Green bodies were formed by cold isostatic pressing of granules obtained by freeze granulation, and pressureless sintered at 1750 °C for 4 h in an argon atmosphere. Mullite (10–20 vol%) formed in addition to SiC during sintering. The SiC particles were located predominantly to the interior of the mullite and alumina matrix grains.     

SiC coating toughened by SiC nanowires to protect C/C composites against oxidation

A dense SiC coating toughened by SiC nanowires was prepared on carbon/carbon (C/C) composites using a two-step technique of chemical vapor deposition (CVD) to protect them against oxidation. The morphologies and crystalline structures of the coatings were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. SiC nanowires played a role in decreasing the size of the cracks and improving the thermal shock resistance of the coating. The result of thermal shock between 1773 K and room temperature for 21 times indicates that, compared with the SiC coating without SiC nanowires, the average size of the cracks in the SiC coating toughened with SiC nanowires reduced from 5 ± 0.5 to 3 ± 0.5 μm. The weight loss of the SiC coated C/C composites decreased from 9.32 to 4.45% by the introduction of SiC nanowires.     

Microstructure characteristics of silicon carbide coatings fabricated on C/C composites by plasma spraying technology

A functional gradient SiC coating on C/C composites has been developed using a novel process which is the combination of plasma spraying technology with reaction-formed heat-treatment. Microstructure observation and phase identification of the SiC coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Experimental results showed that a uniform silicon coating was deposited on C/C composite by plasma spraying technology. The reaction between the silicon coating and C/C substrate occurred during the heat-treatment at temperature of 1450 °C and 1600 °C in argon environment, respectively. A continuous SiC coating was formed on the surface of the C/C substrate. And a layer of SiC/C convention layer was formed on the near-surface area of the substrate, which was resulted from the molten silicon penetrating into the open pores and consequently reacting with the C/C composites. The thickness of the formed SiC coatings was closely related to the original silicon coatings.     

The wetting behaviour and reaction kinetics in diamond–silicon carbide systems

The wetting behaviour of silicon on diamond and the interaction of diamond with molten silicon were investigated. It was found that diamond is well wetted by molten silicon reaching a contact angle of about 20° after melting. The wetting is caused by the rapid formation of a SiC interlayer by nucleation of silicon carbide grains on the surface of the diamond. Investigations of the interaction of silicon with CVD diamond, using SEM, showed that the initial rate of SiC formation is very fast and is significantly reduced after the formation of a 4–6 μm thick dense SiC interlayer. At that stage further growth is likely to be controlled by the diffusion of Si and C through the grain boundaries of the silicon carbide interlayer. The results were compared with the interaction of silicon with glassy carbon.     

Graphite blocks with high thermal conductivity derived from natural graphite flake

In this work, natural graphite flake (NG) and mesophase pitch were used as precursor carbons to prepare the graphite blocks, which were doped with Si and Ti powders. After hot-pressed at 2700 °C, we investigated the effect of mean size of NG on properties and microstructure of the graphite blocks. Results showed that both thermal conductivity and flexural strength of the graphite blocks were improved as mean size of NG in raw material increased from 50 to 246 μm. However, a decrease of thermal conductivity was observed when mean size of NG was higher than 246 μm. The density and open porosity were respectively 2.26 g/cm³ and 5.82% when mean size of NG in raw material was 246 μm. The thermal conductivity was enhanced, however, the flexural strength was reduced as hot-pressing temperature increased from 2300 to 3000 °C. The thermal conductivity and flexural strength of the graphite block were respectively 704 W/m K and 21.1 MPa when hot-pressing temperature was 3000 °C. Phase analysis demonstrated there were diffraction peaks of graphite, TiC and α-SiC crystals in the graphite block as the hot-pressing temperature was less than 2500 °C. No SiC crystals were evident when the hot-pressing temperature was 2700 °C or above.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

Erosion behavior of SiC–WC composites

Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 °C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m^1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 30° to 90°, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm³/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated.     

Preparation of titanium nitride ultrafine powders by sol–gel and microwave carbothermal reduction nitridation methods

Titanium nitride ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction methods. The influences of reaction temperature, molar ratio of Ti to C, addition of crystal seeds and amount of NH₄F on the synthesis of titanium nitride were studied. The results show that excess amount of carbon, addition of crystal seeds and NH₄F plays a positive effect on the preparation of TiN at low temperature. The inceptive formation temperature of TiN ultrafine powders is about 800 °C, and pure TiN can be prepared at 1000 °C. Field emission-scanning electron microscopy (FE-SEM) was used to get the micrograph of the TiN powder, it shows that the size of the powders synthesized at 1000 °C is about 0.1–0.5 μm.     

Effect of the starting materials on the reaction synthesis of Ti3SiC2

Ternary carbide of titanium and silicon was produced via mechanical milling and following heat treatment. Effects of the starting materials, milling time and heat treatment temperature were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to evaluate the structural and morphological evolutions of the ball-milled and annealed powders. Results showed that the ball milling of TiO–Si–C as the starting materials failed to synthesize Ti₃SiC₂. Additionally, ball milling the elemental powders for shorter milling times resulted in the activation of the powders. However, after longer milling times, Ti–TiC nanocomposite was obtained. Furthermore, during annealing the milled powders, Ti₃SiC₂–TiC nanocomposite with the mean grain size of 16 nm was synthesized. After 20 h of milling, a very fine microstructure with narrow size of distribution and spheroid particles was achieved.     

Influence of precursor morphology on the microstructure of silicon carbide nanopowder produced by combustion syntheses

Silicon carbide nanopowder was synthesized using the combustion-based approach. Combustion synthesis was performed in reduction type SiO₂–Mg–C system. Silicon oxide powders with different morphologies and average particle size were used as starting powders. It was shown that even micro-size silica allows formation of nano-size silicon carbide powder. However, the specific surface area of synthesized SiC particles increases with the decrease the size of the silicon oxide precursor. The mechanism of silicon carbide formation in the combustion wave is also discussed.     

Synthesis of 6H-SiC nanowires on bamboo leaves by carbothermal method

6H silicon carbide (SiC) nanowires were fabricated on bamboo leaves infiltrated with tetraethyl orthosilicate (TEOS) by carbothermal method at 1300–1400 °C. The bamboo leaves were the carbon source and template for the growth of SiC. The TEOS was the silicon source. The crystalline structure, morphology, and the distribution of the prepared SiC were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. The SiC had a hexagonal 6H- structure with diameter of 60 to 160 nm and length up to tens of microns. The yield of SiC grown on the top surface of the bamboo leaf was higher and had a branched structure. The SiC on the bottom surface showed a bamboo-like structure. The nanowires were mainly 6H phase, however cubic 3C-SiC phase was found on the divisions in the branch structure and the nodes in the bamboo-like structure. The difference in density of SiC nanowires between the two surfaces is proposed to be related to the structural and compositional differences of the two surfaces. TEOS was preferred to attach the hydrophilic top surface, which led to larger amount of SiC. Meanwhile, the Al content inside the bottom surface prohibited the growth of the SiC nanowires. The growth mechanism of the SiC nanowires is also discussed.     

Effect of silicon carbide additive on microstructure and properties of porcelain ceramics

Porcelain green bodies with various silicon carbide contents (0-3 wt.%) were prepared from a porcelain tile powder as a major raw material and SiC particle as an additive, and were sintered at 1000-1240 °C. The samples were systematically characterized by the X-ray diffraction (XRD), scanning electron microscope (SEM) and metallurgical microscope. Effects of the SiC content and sintering temperature on the pore size, SiC particle size and sintered density were investigated in detail, and the correlative mechanism was also discussed. The SiC particle size decreased and the pore size augmented with increasing the sintering temperature. The sintered density decreased and the pore size enlarged with increasing the SiC content. The experimental results indicate that a small amount of SiC can cause porcelain ceramics to foam during sintering, and a foaming origin of the polishing porcelain waste during sintering could be attributed to the oxidation reaction of SiC particles under high temperature and alkaline molten salt conditions.     

Periodically twinned 6H-SiC nanowires with fluctuating stems

Periodically twinned 6H-SiC nanowires with fluctuating stems were successfully synthesized on SiC substrate by chemical vapor deposition with ferrocene as the catalyst. The morphology and structures of the products were systematically characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the nanowires consisted of periodically twinned segments and presented stem-fluctuating morphology, whose diameters fluctuated in the range of 10–150 nm along the axial direction. A model based on vapor–liquid–solid mechanism was proposed to explain the growth of periodically twinned SiC nanowires with fluctuating stems, which revealed that the alternating high-density stacking faults facilitated the formation of stem-fluctuating morphology.