Growth Of SiC Grains Research Articles

Role of boron addition on phase composition, microstructural evolution and mechanical properties of nanocrystalline SiBCN monoliths

Nanocrystalline SiBCN monoliths with the same Si/C/N mole ratio and various boron additions ranged from 0 to 3.0mol were prepared by mechanical alloying plus reactive hot pressing methods. Correction of boron content and microstructural/morphological evolution was investigated in detail by XRD, SEM, TEM and STEM-EDX structure characterization. Except for SiC and BN(C), boron addition contributes to BxC formation. Besides, boron addition promotes the crystallization of SiC, leading to the formation of poor crystallinity of spherical structures in inner SiC. Furthermore, boron addition significantly promotes the grain growth of SiC and BxC and therefore increases the relative volume ratios of BxC/BN(C) and BxC/SiC. Amorphous-like BN(C) changes to belt-like structures as boron addition increases. The new formed BxC effectively contributes to the improvement of Vicker’s hardness while pull-out of BN(C) belt-like structures benefits the fracture toughness.

The effects of B4C addition on the microstructure and mechanical properties of SiC prepared using powders recovered from kerf loss sludge

Abstract In this study, high purity SiC powders (~ 99.3%), which were recycled from the kerf loss sludge using hydro-cyclone, classifier and direct carbonization and which have purity similar to the commercial SiC powders used to fabricate structural components, were used to investigate the effect of B4C addition on the microstructure and mechanical properties of the sintered SiC. B4C phase and graphite were observed on the grain boundaries between SiC grains, and the amount of B4C phase at the grain boundaries was increased with increasing B4C contents. Both the phases on the grain boundaries impeded the growth of SiC grains and led to the high porosity, which resulted in the decrease in density and hardness. The addition of B4C was found to have negative effect on the density and hardness of SiC. The fracture toughness, however, was enhanced with the increase in the B4C content. The impediment of crack propagation by the B4C phase was found to be related to the improvement of fracture toughness as the amount of B4C content was increased

Conversion Process of Amorphous Si-Al-C-O Fiber into Nearly Stoichiometric SiC Polycrystalline Fiber

Tyranno SA (SiC-polycrystalline fiber, Ube Industries Ltd.) shows excellent heat-resistance up to 2000℃ with relatively high mechanical strength. This fiber is produced by the conversion process from a raw material (amorphous Si-Al-C-O fiber) into SiCpolycrystalline fiber at very high temperatures over 1500℃ in argon. In this conversion process, the degradation reaction of the amorphous Si-Al-C-O fiber accompanied by a release of CO gas for obtaining a stoichiometric composition and the subsequent sintering of the degraded fiber proceed. Furthermore, vaporization of gaseous SiO, phase transformation and active diffusion of the components of the Si-Al-C-O fiber competitively occur. Of these changes, vaporization of the gaseous SiO during the conversion process results in an abnormal SiC-grain growth and also leads to the non-stoichiometric composition. However, using a modified Si-Al-C-O fiber with an oxygen-rich surface, vaporization of the gaseous SiO was effectively prevented, and then consequently a nearly stoichiometric SiC composition could be obtained.

Reactive hot pressing of SiC-ZrC composites at low temperature

SiC-ZrC composites with relative density in excess of 99% were prepared by reactive hot pressing (RHP) of SiC and ZrH2 at 1800°C for 1h. The reaction between SiC and ZrH2 resulted in the formation of ZrC1-x. The formation process and densification behavior during RHP process were investigated. Low temperature densification of SiC-ZrC composites is attributed to the formed nonstoichiometric ZrC1-x and the removal of SiO2 impurity on the surface of SiC particles. As reinforced phase, ZrC1-x has inhibiting effect on the abnormal grain growth of SiC, resulting in homogeneous microstructure of fine SiC grains. Adding 10wt% ZrH2 to SiC, the formed SiC-4.62vol% ZrC composite exhibited better mechanical properties (Vickers hardness of 27.6±0.7GPa, flexure strength of 448±38MPa, fracture toughness of 6.0± 0.3MPa·m1/2, respectively) than monolithic SiC ceramic.

Simultaneously enhanced toughness and strain tolerance of SiC-based ceramic composite by in-situ formation of VB2 particles

SiC-30vol%VB2 ceramic composite was pressureless densified at 2150°C with excess B4C and C as sintering aids after in-situ formation of VB2 in SiC matrix. The sintered bulk gained a considerably high fracture toughness of 7.0±0.4MPam1/2, which was ∼2.4 times as high as that of the monolithic SiC ceramic, owing to the existences of weak heterophase boundaries, thermal residual stresses and microcracks. Meanwhile, since the VB2 particle has a lower elastic modulus than SiC and significantly suppressed the grain growth of SiC, the composite exhibited a high flexural strength of 458±36MPa and a relatively low Young’s modulus of 356±6GPa, resulting in an increase of ∼59.3% in mechanical strain tolerance (1.29×10−3) compared with that of single-phase SiC ceramic. Besides, the residual stresses and microcracks also induced a lower-than-expected Vickers hardness of 20.8±0.5GPa in the composite.

Effect of sintering techniques on the microstructure of liquid-phase-sintered SiC ceramics

Phase composition and microstructures of SiC ceramics with Al2O3 and Er2O3 as sintering additives were compared, which were prepared by pressureless liquid-phase sintering (PLPS), hot pressing sintering (HPS), and spark plasma sintering (SPS), respectively. Despite the different sintering temperatures and pressures, all samples were almost fully densified and exhibited fine-grained microstructure. Only SiC crystallized phase was detected in the SPS SiC because of the low sintering temperature and high cooling rate, while the ErAlO3 phase accompanied with a designed Er3Al5O12 phase was detected in the HPS SiC because of the absence of powder bed, which led to partial volatilization of Al2O3 powder at a higher sintering temperature. No obvious orientational growth of SiC grains was observed in all these ceramics, except a few 4H SiC grains along <100> axis in the SPS SiC. In addition, SiC–SiC grain boundary films differed from each other in these SiC ceramics.

Effect of grain growth on electrical properties of silicon carbide ceramics sintered with gadolinia and yttria

The effect of grain growth on electrical properties of SiC ceramics sintered with a new additive system (Gd2O3–Y2O3) was investigated. Hot-pressing of β-SiC with 3vol% Gd2O3–Y2O3 in a nitrogen atmosphere resulted in highly conductive SiC ceramics (∼10−2–10−3Ωcm), whereas hot-pressing of the same specimen in an argon atmosphere resulted in semi-insulating SiC ceramics (∼108Ωcm). This difference was due to the nitrogen (N) doping in SiC ceramics in the former case. The N-doping in SiC ceramics was achieved by grain growth of SiC grains via a solution-reprecipitation mechanism. The electrical resistivity of N-doped SiC ceramics decreased with an increase in grain size of SiC ceramics if there is no β→α phase transformation of SiC. The lowest electrical resistivity obtained was 8.9×10−3Ωcm after 6h sintering at 2000°C under an applied pressure of 40MPa in nitrogen.

Electrical, thermal and mechanical properties of silicon carbide–silicon nitride composites sintered with yttria and scandia

Abstract Composites of SiC–Si 3 N 4 , which consisted of uniformly distributed β-Si 3 N 4 grains, nitrogen-doped β-SiC grains, and a Y- and Sc-containing junction phase, were fabricated by conventional hot-pressing in nitrogen atmosphere, with a new additive system of 2 vol% equimolar Y 2 O 3 –Sc 2 O 3 additives. The effect of Si 3 N 4 content on the electrical, thermal, and mechanical properties of SiC–Si 3 N 4 composites were investigated. The electrical resistivity showed a minimum at 10 vol% Si 3 N 4 content, owing to the growth of nitrogen-doped SiC grains from a Y–Sc–Si oxycarbonitride glass via a solution-reprecipitation mechanism. The thermal conductivity decreased with increasing Si 3 N 4 content, whereas the fracture toughness increased with increasing Si 3 N 4 content. Typical electrical resistivity and thermal conductivity of the SiC–10 vol% Si 3 N 4 composites were 0.09 Ω cm and 83 W/m K at room temperature, respectively. The flexural strength, fracture toughness, and Vickers hardness values of the SiC–35 vol% Si 3 N 4 composite were ∼720 MPa, ∼7 MPa m 1/2 , and ∼19 GPa, respectively.

Electrical properties of SiC ceramics sintered with 0.5 wt% AlN–RE2O3 (RE=Y, Nd, Lu)

Silicon carbide ceramics with 0.5wt% AlN–RE2O3 (RE=Y, Nd, Lu) additives were prepared by hot-pressing the powder mixtures at 2050°C for 6h under an applied pressure of 40MPa in a nitrogen atmosphere. The effects of additive composition on the sinterability, microstructure, and electrical properties of hot-pressed liquid-phase sintered (LPS)-SiC ceramics were investigated. It was found that all specimens could be sintered to >95% of the theoretical density by adding 5wt% in situ-synthesized nano-sized SiC and 0.5wt% AlN–RE2O3 additives. SiC ceramics sintered with AlN–Y2O3 and AlN–Nd2O3 showed low electrical resistivity on the order of 10−2Ωcm. This low electrical resistivity was attributed to the growth of nitrogen-doped SiC grains and the confinement of non-conducting RE-containing phases in the junction areas. The electrical resistivity of LPS-SiC ceramics sintered with AlN–RE2O3 was dependent on the chemistry of the additive composition.

Effect of Carbon Source and Adding Ratio on the Microstructure and Properties of Solid-state Sintering Silicon Carbide

With 6wt% total carbon additive amount, the microstructure and properties of pressureless solid-state sintered SiC(SSiC) were studied with different carbon sources and carbon source adding ratio. It is found that SSiC ceramics with inorganic carbon(carbon black) as carbon source have finer SiC grains and lower density. As the carbon additive amount derived from organic carbon source(phenolic resin) increases, the grain size of SiC grains in SSiC ceramics increases, the distribution of carbon phase in the ceramics is more homogeneous, the density and the mechanical properties of SSiC ceramics increase. When the carbon additive amount derived from organic carbon source is3.0wt%, the ceramics with highest density have the largest elastic modulus, which is up to 486 GPa, the fracture toughness of it is 4.65 MPa·m1/2. When the carbon additive amount derived from organic carbon source is over 3.0wt%,the abnormal grain growth of SiC grains appears in the ceramics, the fracture strength and toughness further increase.At the same time, the thermal diffusion coefficient of SSiC ceramics with different carbon sources and adding ratio is also discussed.

Synthesis of Ceramic Bonded Carbon Using SiC‐Coated Carbon Particles and Spark Plasma Sintering

A dense SiC coating on carbon particles has been developed using SiO powders as the Si source and a chemical vapor reaction method. The coating process was controlled by atmosphere, coating temperature, time, and weight ratio of SiO/C. The SiC‐coated carbon particles, with a mean particle size of 20 μm, were sintered at 2000°C under 40 MPa for 20 min by spark plasma sintering (SPS). The product, called ceramic bonded carbon (CBC), had a unique microstructure, with a 3D micro‐network structure consisting of carbon particles covered with 1~5 μm thick SiC boundary layers. The sintering behavior of SiC/CBCs was analyzed in terms of the network formation of SiC boundary layers at over 1800°C and the grain growth of SiC at 2000°C. SiC/CBCs showed a bending strength of 150 MPa and a low shore hardness of 40. These materials could also be easily joined to SiC ceramics using SPS.

Electrical resistivity of silicon carbide ceramics sintered with 1 wt% aluminum nitride and rare earth oxide

Abstract The influence of additive composition on the electrical resistivity of hot-pressed liquid-phase sintered (LPS)-SiC was investigated using AlN–RE2O3 (RE = Sc, Nd, Eu, Gd, Ho, Er, Lu) mixtures at a molar ratio of 60:40. It was found that all specimens could be sintered to densities >95% of the theoretical density by adding 5 wt% in situ-synthesized nano-sized SiC and 1 wt% AlN–RE2O3 additives. Six out of seven SiC ceramics showed very low electrical resistivity on the order of 10−4 Ω m. This low electrical resistivity was attributed to the growth of nitrogen-doped SiC grains and the confinement of non-conducting RE-containing phases in the junction areas. The SiC ceramics sintered with AlN–Lu2O3 showed a relatively high electrical resistivity (∼10−2 Ω m) due to its lower carrier density (∼1017 cm−3), which was caused by the growth of faceted grains and the resulting weak interface between SiC grains.

SiC Ceramic‐Bonded Carbon Fabricated With Si3N4 and Carbon Powders

New composites called ceramic‐bonded carbon (CBCs), consisting of a three‐dimensional structure of carbon particles bonded with thin ceramic boundaries, were developed. To fabricate light and tough CBCs and to understand their reinforcing mechanism, Si3N4 and carbon powders (25:75 in volume ratio) were gelcasted and then sintered by spark plasma sintering at temperatures of 1700–1900°C. The ceramic boundary of SiC was formed in situ at above 1700°C by the reaction of Si3N4 and C. The sintered CBCs showed a unique microstructure consisting of carbon particles and ceramic boundaries of 15 μm in size and 0.5–3 μm in thickness, respectively. With an increase in sintering temperature, physical bonding of ceramic grains to the carbon particles was enhanced as the grain growth of SiC increased. The SiC/CBCs sintered at 1900°C were highly dense (97% theoretical density), lightweight (2.36 mg/m3), and had both relatively high bending strength and thermal conductivity (135 MPa and 140 W/mK, respectively).

Taguchi Method Optimization of Parameters for Growth of Nano Dimensional SiC Wires by Chemical Vapor Deposition Technique

SiC wires of different morphology were grown using methyltrichlorosilane (MTS) and hydrogen by chemical vapor deposition under ambient pressure. Taguchi method has been used to design experiments to get the optimum parameters for growing SiC wires of diameter in nanometer range. Results from XRD and SEM analyses showed the growth of β-SiC wires having different morphology. At higher temperature (1500 °C), the growth of SiC grains was observed rather than wires. The optimum deposition conditions for uniform diameter growth of SiC nano wires, smoothness of the surface and homogeneous growth of SiC on the surface have been obtained. The hydrogen to MTS flow rate ratio should be above 20 for the growth of SiC wires of nanometer diameter. The deposition temperature for the growth of crystalline SiC wires should be 1100-1300 °C. The total flow rate of carrier gas comprising of argon and hydrogen for a particular H2/MTS flow rate ratio is critical for morphological outcome of SiC. In the present study it was 2 lpm for H2/MTS flow rate ratio 14 to obtain wire morphology. When the total gas flow rate was increased to 6 lpm for the same H2/MTS flow rate ratio 14, the wire morphology of SiC disappeared and the formation of grains occurred. The optimum deposition temperature i.e. 1300 °C was kept constant and further experiments were conducted by changing H2/MTS mole ratio to verify morphological outcome of SiC. A plausible mechanism has been suggested for the above observations using vapor-solid mechanism.

Microstructure and mechanical properties of short carbon fibre/SiC multilayer composites prepared by tape casting

Silicon carbide multilayer composites containing short carbon fibres (Csf/SiC) were prepared by tape casting and pressureless sintering. The C fibres were dispersed in solvents with dispersant (Triton X-100) firstly and then mixed with the SiC slurry to make green Csf/SiC tapes. Fibres were homogeneously distributed in the tape and tended to align fairly well along the tape casting direction. The addition of short C fibre hindered the shrinkage in the plane containing the fibres as well as the grain growth of SiC during sintering. The weight loss occurring during oxidation tests of Csf/SiC multilayer composites increased with fibre amount and material porosity. Elastic modulus of Csf/SiC multilayer composites decreased linearly with fibre amount. Bending strength presented clear relationship with the relative density, that is with the total porosity.

Effect of Al4SiC4 additive on the densification of β-silicon carbide under vacuum

Abstract The potential of ternary compound (Al 4 SiC 4 ) powders as an effective sintering additive to concurrently achieve SiC densification and grain refinement was evaluated under vacuum. Nearly fully densified SiC ceramic was successfully obtained in the absence of a residual liquid phase at the grain boundaries using low temperature hot pressing at 1700 °C by adding 10 wt% Al 4 SiC 4 as an additive. The main mechanism for obtaining SiC densification was analyzed with changing additive contents. A larger amount of additive content was effective in suppressing the grain growth of SiC due to the formation of newly generated carbon by the thermal decomposition of Al 4 SiC 4 . Regardless of the additive content, sintering temperature and grain size, the fracture mode of the Al 4 SiC 4 -doped SiC mainly consisted of intragranular fractures due to the high interfacial bonding strength.

Densification, Phases, Microstructures and Mechanical Properties of Liquid Phase-Sintered SiC

This paper reports the influence of sintering additives (RE2O3, Al2O3RE2O3, RE = Yb, Y and Gd, 13 vol%) and mixing effect of 30 nm SiC powder with 800 nm SiC powder on phases of grain boundaries, grain size of SiC, fracture toughness and strength of SiC hot-pressed at 1950°C under 39 MPa of applied pressure. Rare earth ions were uniformly adsorbed on negatively charged SiC particles with 150 nm Al2O3 particles in aqueous suspensions at pH 5. A rapid densification of SiC with one component RE2O3 occurred above 1700°C when a liquid of SiO2 (formed on SiC particles)RE2O3 system was formed. The Al2O3RE2O3 additives lowered a liquid formation temperature to 14001500°C and enhanced the densification rate of SiC. An increased solubility of 30 nm SiC in a liquid during dissolution-precipitation process provided an amorphous phase of SiCSiO2Al2O3RE2O3 system at grain boundaries and suppressed the grain growth of SiC. The fracture toughness of dense SiC was dominated by the grain boundary thickness controlled by grain size of SiC and amount of oxide additives. Mixing of 30 nm SiC with 800 nm SiC improved greatly the strength of SiC with two component oxides and the mean flexural strengths reached 740810 MPa.

Pressureless sintering of internally synthesized SiC-TiB 2 composites with improved fracture strength

SiC-TiB 2 particulate composites were fabricated by converting TiO 2 to TiB 2 through the reaction between TiO 2, B 4C and C. The presence of initially very fine, in-situ created, TiB 2 particles increased driving force for sintering and enabled fabrication of a dense composite utilizing pressureless sintering and the liquid phase created between Al 2O 3 and Y 2O 3 additives. The effect of volume fraction of the in-situ formed TiB 2 on density, microstructure and flexural strength was discussed. It was found that the presence of TiB 2 particles suppressed the growth of SiC grains and enhanced fracture strength. The fracture strength of samples containing 12 vol% TiB 2 was more than 30% higher than that of the monolithic SiC. The effect of SiC grain size on fracture strength was also analyzed.

Toughening of SiC matrix with in-situ created TiB 2 particles

SiC–TiB 2 composites with up to 50 vol% TiB 2 were fabricated by in-situ reaction between TiO 2, B 4C and C. The densification of the uniaxially pressed samples was done using pressureless sintering in the presence of sintering aids consisting of Al 2O 3 and Y 2O 3. The influence of the volume fraction of TiB 2 and sintering temperature on density and fracture toughness was examined. It was found that fracture toughness is strongly affected by the volume fraction of TiB 2. The presence of TiB 2 particles suppresses the grain growth of SiC and facilitates different toughening mechanisms to operate which, in turn, increases fracture toughness of the composite. The highest value for fracture toughness of 5.7 MPa m 1/2 was measured in samples with 30 vol% TiB 2 sintered at 1940 °C.

Al-B-C 조제 β-SiC의 스파크 플라즈마 소결에 미치는 α-SiC seed 첨가 영향: 미세 구조 변화

The unique features of spark plasma sintering process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. <TEX>$\beta$</TEX>-SiC powder with 0, 2, 6, 10 wt% of <TEX>$\alpha$</TEX>-SiC particles (seeds) and 4 wt% of Al-B-C (sintering aids) were spark plasma sintered at <TEX>$1700-1850^{\circ}C$</TEX> for 10 min. The heating rate, applied pressure and sintering atmosphere were kept at <TEX>$100^{\circ}C/min$</TEX>, 40 MPa and a flowing Ar gas (500 CC/min). Microstructural development of SiC as function of seed content and temperature during spark plasma sintering was investigated quantitatively and statistically using image analysis. Quantitative image analyses on the sintered SiC ceramics were conducted on the grain size, aspect ratio and grain size distribution of SiC. The microstructure of SiC sintered up to <TEX>$1700^{\circ}C$</TEX> consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at <TEX>$1750^{\circ}C$</TEX> and the plate-like grains interlocking microstructure had been developed by increasing sintering temperature. The introduction of <TEX>$\alpha$</TEX>-SiC seeds into <TEX>$\beta$</TEX>-SiC accelerated the grain growth of elongated grains during sintering, resulting in the plate-like grains interlocking microstructure. In the <TEX>$\alpha$</TEX>-SiC seeds added in <TEX>$\beta$</TEX>-SiC, the rate of grain growth decreased with <TEX>$\alpha$</TEX>-SiC seed content, however, bulk density and aspect ratio of grains in sintered body increased.