High Hardness and Ductile Mosaic SiC/SiO2 Composite by Spark Plasma Sintering

SiC powder was coated with SiO₂ layer by chemical vapor deposition, and the SiC(core)/SiO₂(shell) composite powder was consolidated to a SiC/SiO₂ composite with a mosaic microstructure by spark plasma sintering (SPS) at 1923 K for 1.8 ks. The SiC(core)/SiO₂(shell) powder with a 80–100 nm thick SiO₂ layer resulted in a SiC/SiO₂ composite with a relative density of 97% and hardness and fracture toughness of 17.1 GPa and 8.4 MPa m^1/2, respectively.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

Fabrication of transparent SiO2 glass by pressureless sintering and spark plasma sintering

Transparent SiO₂ bodies were prepared by pressureless sintering (PLS) and spark plasma sintering (SPS). The effects of sintering and annealing temperature on the transmittance of the SiO₂ bodies were investigated. The SiO₂ bodies sintered by SPS and PLS at 1073–1573 K were amorphous. With increasing the sintering temperature to 1673 K, the SiO₂ bodies sintered by PLS were crystallized while those sintered by SPS were still amorphous. The relative density of the SiO₂ bodies sintered by SPS was 98.5% at 1373 K and 100% at 1573 K, whereas that sintered by PLS was 92.6% at 1373 K and 98.9% at 1573 K. The transmittance was 91.0% and 81.5% at a wavelength (λ) of 2 μm for the SiO₂ sintered bodies by SPS and PLS, respectively. In the ultraviolet range, the transmittance of the SiO₂ bodies sintered by SPS at 1573 K was about 40% at λ = 200 nm and increased to 75% after annealing at 1073 K for 1 h, which was about three times of the transmittance of the SiO₂ bodies sintered by PLS (24.8%).     

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

Spark plasma sintering of ZrC–SiC ceramics with LiYO2 additive

Spark plasma sintering (SPS) of ZrC–SiC composite powders in the presence of LiYO₂ sintering additive was studied. The starting powders were obtained by a carbothermal reduction (CTR) of natural mineral zircon (ZrSiO₄), which provided an intimate mixing of in-situ created ZrC and SiC powders. This composite powder and LiYO₂ as additive were densified by spark plasma sintering. Microstructural features of the composite were investigated by XRD, SEM/EDS and AFM analysis. The sintered composite material possesses promising mechanical properties and excellent cavitation resistance which was observed with a cavitation erosion test. The values of Vickers microhardness and fracture toughness of the composite material are 20.7 GPa and 5.07 MPam^1/2, respectively.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Silicon carbide ceramics: Mechanical activation,combustion and spark plasma sintering

Single-stage fabrication of SiC ceramics by a combination of self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS) is reported. SHS+SPS is demonstrated to be an efficient method for production of SiC ceramics with density 3.1 g/cm³, hardness of 24 GPa and toughness of 5 MPa m^1/2. The starting material for the process is fine (50–300 nm in size) highly reactive powder, which involves composite particles of elemental carbon and silicon. This powder was prepared using a high-energy ball milling (HEBM). To optimize precursor preparation conditions, the structure transformation in nano-composite Si/C particles at different HEBM stages is also investigated.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Reclamation of used SiC grinding powder and its sintering characteristics

A process of recycling used abrasive SiC powder after grinding Si wafer was proposed to raw powder for sintering. The used SiC powder could be successfully converted to composite powders consisting of SiC particle and Si₃N₄ whisker via a heat treatment in N₂ atmosphere, in which iron oxide acted as a catalyst in the vapor–liquid–solid (VLS) formation of Si₃N₄. With the addition of 3 mass% Al₂O₃ and 1 mass% Y₂O₃, the composite powders sintered at 1900 °C for 2 h exhibited a 3-point bending strength of 626 ± 48 MPa and a fracture toughness of 3.9 ± 0.1 MPa m^1/2, which were significantly enhanced as compared with those of using recovered powder merely composed of SiC particle. The strength and fracture toughness of the sintered material could be improved by optimization of chemical and heat treatment parameters and controlling the amount of sintering additives and hot pressing conditions.     

Densification of SiO2–cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder

Cubic boron nitride (cBN) powder was coated with Ni nanoparticle and SiO₂ nanolayer (abbreviated as cBN/Ni and cBN/SiO₂, respectively) by rotary chemical vapor deposition (RCVD), and compacted with SiO₂ powder by spark plasma sintering at 1473–1973 K for 0.6 ks. The effects of Ni and SiO₂ coatings on the densification, phase transformation of cBN and hardness of SiO₂–cBN composites were compared. The phase transformation of cBN to hBN was identified at 1973 K in SiO₂–cBN/SiO₂ composites, 300 K higher than that in SiO₂–cBN/Ni composites, indicating that SiO₂ retarded the transformation of cBN. The relative density of SiO₂–cBN/SiO₂ with 50 vol% cBN sintered at 1873 K was 99% with a hardness of 14.5 GPa.     

TiN modified SiC with enhanced strength and electrical properties

SiC based composites were manufactured with varying TiN content (0–50 V%) using Al₂O₃ and Y₂O₃ sintering aids. Basic dilatometry measurements were performed to determine when densification begins within the composite system. Samples were consolidated via uni-axial hot pressing at 1900 °C to produce ceramic composites with >98% theoretical density. Electrical measurements show increasing TiN additions reduce resistivity and begin to plateau at 40–50V%. Resistivity decreased from 2.0 × 10⁵ Ω − cm (0% TiN) to 2.0 × 10⁻⁴ Ω − cm (50V% TiN). Flexural strengths were characterized and compared against a baseline (0% TiN) SiC. Strengths increased gradually with TiN content. A maximum strength 921 MPa was observed at 40V% TiN content vs. 616 MPa for the baseline SiC. This was a gain of 50% over baseline. Additions beyond that range did not produce further gains in strength.     

Mechanical Properties and Microstructure of Nano-SiC–Al2O3 Composites Densified by Spark Plasma Sintering

Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al₂O₃ powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al₂O₃ composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al₂O₃ densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al₂O₃ grains and the fracture mode of the nanocomposites was mainly transgranular fracture.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Large scale synthesis and photoluminescence properties of necklace-like SiC/SiOx heterojunctions via a molten salt mediated vapor reaction technique

Necklace-like SiC/SiO_x heterojunctions were synthesized by a simplified vapor reaction technique using silicon powders as raw material without metallic catalyst or flowing gas at 1200 °C. Phase composition, morphology and microstructural characteristics of as-achieved products were characterized. The as-synthesized SiC/SiO_x heterojunctions, up to millimeters in length, are composed of SiC/SiO₂ core–shell chains with diameters of 0.3–1 µm and amorphous SiO₂ beans with diameters of 2–5 µm. The growth mechanism of the as-synthesized SiC/SiO₂ core–shell chains is most likely controlled by vapor–solid mechanism. The formation of SiO₂ beans can be mainly attributed to the Rayleigh instability and surface tension. The photoluminescence spectra of the as-received products exhibit a significant violet-blue emission, making a potential application in violet-blue emitting devices.     

Thermoelectric Ca0.9Yb0.1MnO3−δ grain growth controlled by spark plasma sintering

n-Type Ca_0.9Yb_0.1MnO_3?δ thermoelectric (TE) powders were prepared by solid state synthesis (SSS) and co-precipitation method (Cop). The bulk TE materials were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) respectively. The shrinkage behavior, as well as the sample densification strongly depends on the starting particle size. Consequently, the bulk samples from normal powder (SSS) and nano-powder (Cop) were prepared with similar density by using different sintering temperatures, of 1400 °C and 1200 °C, then 1200 and 950 °C for CS and SPS respectively. Such a decrease (up to 200 °C) of the sintering temperature is a consequent progress in terms of engineering for applications. Another advantage of the co-precipitation process compared to the conventional solid state synthesis is that, due to the small particle sizes and the decreased sintering temperature, grain growth was limited and TE properties were enhanced. The interest of the SPS process was also evidenced and we are presenting here the structural and microstructural investigations. In addition, the thermoelectric properties of samples prepared with two different processes were studied with the figure of merit of 0.18 at 750 °C.     

Microstructure characterization of ZrB2–SiC composite fabricated by spark plasma sintering with TaSi2 additive

Dense ZrB₂–SiC composite was synthesized by spark plasma sintering with 10 vol.% TaSi₂ additive. When sintered at 1600 °C, core–shell structure was found existing in the sample. The core was ZrB₂ and the shell was (Zr,Ta)B₂ solid solution. This result was ascribed to the decomposition of TaSi₂ and the solid solution of Ta atoms into ZrB₂ grains. The solid solution process probably decreased the boride grain boundary active energy, contributing to the formation of coherent structure of grain boundaries. Additionally, the existence of dislocations in the boride grains indicated that the applied pressure also imposed an important effect on the densification of composite. When sintered at 1800 °C, owing to the atom diffusion, Ta atoms homogeneously distributed in the boride grains, leading to the disappearance of core–shell structure. The boundaries between (Zr,Ta)B₂ grains, as well as between boride grains and SiC particles, were still clear without amorphous phase existing.     

Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Processing and properties of cordierite-silica bonded porous SiC ceramics

Cordierite-silica bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC with cordierite sol followed by sintering at 1300–1400 °C in air. The porosity, average pore diameter and flexural strength of the ceramics varied 30–36 vol%, ~ 4–22 µm and ~ 13–38 MPa respectively with variation of sintering temperature and SiC particle sizes. In the final ceramics SiC particles were bonded by the oxidation-derived SiO₂ and sol-gel derived cordierite. The corrosion behaviour of sintered SiC ceramics was studied in acidic and alkaline medium. The porous SiC ceramics were observed to exhibit better corrosion resistance in acid solution.