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.     

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 behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Comparison among different sintering routes for preparing alumina-YAG nanocomposites

Al₂O₃-YAG (50 vol.%) nanocomposite powders were prepared by wet-chemical synthesis and characterized by DTA-TG, XRD and TEM analyses. Amorphous powders were pre-heated at different temperatures (namely 600 °C, 800 °C, 900 °C and 1215 °C) and the influence of this thermal treatment on sintering behavior, final microstructure and density was investigated. The best performing sample was that pre-calcined at 900 °C, which yields dense bodies with a micronic/slightly sub-micronic microstructure after sintering at 1600 °C. A pre-treatment step to induce controlled crystallisation of the amorphous powder as well as a fast sintering procedure for green compacts, were also performed as a comparison.Finally, the previously stated thermal pre-treatment of the amorphous product was coupled to an extensive mechanical activation performed by wet planetary/ball milling. This procedure was highly effective in lowering the densification temperature, so that fully dense Al₂O₃-YAG composites, with a mean grain size smaller than 200 nm, were obtained by sintering in the temperature range 1370–1420 °C.     

Liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing

The effects were investigated of the starting particle size (i.e., nanometer or submicrometer powders), content of Y₃Al₅O₁₂ additives (YAG; in the range 5–20 wt.%), and difference of size scales between the two particle types on the liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing. It was found that flash sintering benefits from the refinement of the particles size, the increase in additive content, and the smaller size scale of the particulate additive. It was also found that under the present flash sintering conditions (i.e., 900 °C furnace temperature, 13 A current, and 50 s in flash state) the resulting ceramics are, despite the formation of liquid phase, porous to a greater or lesser extent, and exhibit decreasing porosity gradients from their surface to the centre. These observations are rationalized to extract guidelines for powder batch design contributing to the pressureless ultrafast sintering of non-oxide advanced ceramics.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

High pressure sintering of cubic boron nitride compacts with Al and AlN

Cubic boron nitride (cBN) compacts, using 15 wt.% Al and 20 wt.% AlN respectively as additives, were sintered in the temperature range of 1300–1700 °C for 20 min under high pressure of 5.0 GPa. The hardness, microstructure, phase composition and cutting performance of the high pressure sintered samples were investigated. A liquid phase sintering and reaction process was observed in the cBN–Al system, which leads to the formation of AlN and AlB₂ as confirmed by X-ray diffraction (XRD) in the sintered compacts. Scanning electron microscopy (SEM) analysis shows that the samples have a homogeneous microstructure. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. While in the cBN–AlN system, AlN grains agglomerate heavily at temperature below ~ 1500 °C. As the sintering temperature increasing, Al₂O₃ appeared and the AlN agglomeration disappeared gradually. A highest cBN–AlN composite hardness of 29 GPa was achieved when sintered at 1600 °C. Turning tests showed that cBN compacts with 15 wt.% Al as the additive has a longer tool life as compared to that with 20 wt.% AlN. Our results indicated that cBN–Al system is more favourable to obtain well-sintered cBN compacts comparing with the cBN–AlN system.     

Pressureless sintering of HfB2–SiC ceramics doped with WC

Using WC as sintering aid, nearly full dense (～99%) HfB₂–20 vol% SiC ceramics were sintered at 2200 °C for 2 h without external pressure. The densification mechanism, microstructure evolution, mechanical properties and oxidation resistance were investigated. The results indicated that complex chemical reactions of WC in HfB₂–SiC system strongly related to the densification, microstructure and properties. The Young's modulus, fracture toughness and 3-pt bending strength of HfB₂–20 vol% SiC with 10 wt% WC were 511 GPa, 4.85 Mpa m^1/2 and 563 MPa, respectively, which were comparable to some hot pressed HfB₂–SiC ceramics in literature. The oxidation of HfB₂–20 vol% SiC with 10 wt% WC at 1500 °C in air exhibited parabolic kinetics. After oxidation at 1500 °C for 10 h, its weight gain and SiC-depleted layer thickness were 3.7 mg/cm² and 43 μm, respectively, and its residual flexural strength was comparable to or even a little higher than the value before oxidation.     

Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering

The development of reliable joining technology is of great importance for the full use of SiC. Ti₃SiC₂, which is used as a filler material for SiC joining, can meet the demands of neutron environment applications and can alleviate residual stress during the joining process. In this work, SiC was joined using different powders (Ti₃SiC₂ and 3Ti/1.2Si/2C/0.2Al) as filler materials and spark plasma sintering (SPS). The influence of the joining temperature on the flexural strength of the SiC joints at room temperature and at high temperatures was investigated. Based on X-ray diffraction and scanning electron microscopy analyses, SiC joints with 3Ti/1.2Si/2C/0.2Al powder as the filler material possess high flexural strengths of 133 MPa and 119 MPa at room temperature and at 1200 °C, respectively. The superior flexural strength of the SiC joint at 1200 °C is attributed to the phase transformation of TiO₂ from anatase to rutile.     

Alon-based materials prepared by SHS technique

The aim of the presented paper was preparation of the highly reactive in the sintering powders in the Al–O–N system by SHS method. Combustion reactions of metallic aluminium and corundum powder mixtures (from 15% Al–85% Al₂O₃ to 50% Al–50% Al₂O₃) were performed in nitrogen atmosphere. The obtained powders were ground and hot-pressed at 1750, 1850 and 1950 °C for 1 h under 25 MPa in nitrogen flow. In contrast to conventional methods, which require 24 h of the precursor heat treatment at 1200 °C our studies, showed that it is possible to prepare almost pure γ-alon materials using SHS reaction. Sintering of the powders led to obtained dense materials composed of pure γ-alon or γ-alon–AlN composites. The phase composition of the sintered bodies was controlled by the chemical composition of the starting mixture and the sintering temperature.     

Fabrication of tough SiCf/SiC composites by electrophoretic deposition using a fabric coated with FeO-catalyzed phenolic resin

A hybrid processing route based on vacuum infiltration, electrophoretic deposition, and hot-pressing was adopted to fabricate dense and tough SiC_f/SiC composites. The as-received Tyranno SiC fabric preform was infiltrated with phenolic resin containing 5 wt.% FeO and SiC powders followed by pyrolysis at 1700 °C for 4 h to form an interphase. Electrophoretic deposition was performed to infiltrate the SiC-based matrix into the SiC preforms. Finally, SiC green tapes were sandwiched between the SiC fabrics to control the volume fraction of the matrix. Densification close to 95% ρ_theo was achieved by incorporating 10 wt.% Al₂O₃-Sc₂O₃ sintering additive to facilitate liquid phase sintering at 1750 °C and 20 MPa for 2 h. X-ray diffraction and Raman analyses confirmed the catalytic utility of FeO by the formation of a pyrolytic carbon phase. The flexural response was explained in terms of the extensive fractography results and observed energy dissipating modes.     

Al2O3–SiC composites prepared by warm pressing and sintering of an organosilicon polymer-coated alumina powder

Al₂O₃/SiC micro/nano composites were prepared by axial pressing of poly(allyl)carbosilane-coated submicrometre alumina powder at elevated temperature (called also warm pressing, or plastic forming) with subsequent pressureless sintering in the temperature interval between 1700 and 1850 °C. Warm pressing at 350 °C and 50 MPa resulted in green bodies with high mechanical strength and with markedly higher density than in green bodies prepared by cold isostatic pressing of the same powder at 1000 MPa. The sintering of warm pressed specimens moreover yielded the composites with higher final density (less than 4% of residual porosity) with the microstructure composed of micrometer-sized alumina grains (D₅₀ < 2 μm) with inter- and intragranular SiC precipitates. High sintering temperatures (>1800 °C) promoted the formation of intergranular platelets identified by TEM as 6H polytype of α-SiC. The maximum hardness (19.4 ± 0.5 GPa) and fracture toughness (4.8 ± 0.1 MPa m^1/2) were achieved in the composites containing 8 vol.% of SiC, and sintered for 3 h at 1850 °C. These values are within the limits reported for nanocomposites Al₂O₃/SiC by other authors and do not represent any significant improvement in comparison to monolithic alumina.     

Preliminary investigation of flash sintering of SiC

The feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time. Flash sintering involves the application of an electrical potential difference across a powder compact during heating, which leads to sintering at low furnace temperatures in a few seconds and has only been demonstrated with ionic ceramics previously. Near-theoretical density was achieved using Al₂O₃ + Y₂O₃ sintering aids at a furnace temperature of only 1170 °C and in a time of 150 s. Specimen temperatures were significantly higher than the furnace temperature owing to Joule heating and consequently heat loss limited densification in the near surface region. It was not possible to reach high densities using “ABC” sintering aids (aluminium–boron–carbon) or pure SiC. The mechanisms involved and potential commercial advantages are briefly discussed.     

Optimization of spark plasma sintering parameters of Si3N4-SiC composite using response surface methodology (RSM)

The Si₃N₄-SiC micro-nano composites were fabricated via the spark plasma sintering method using MgSiN₂ as an additive. Response surface methodology and central composite design were applied to optimize the spark plasma sintering process for the fabrication of Si₃N₄-SiC/MgSiN₂ with improved density. The relation between the three parameters of sintering including temperature, pressure, and holding time was modeled and the optimized parameters were obtained. The best sintering results obtained for the sintering temperature, holding time, and pressure are 1700 °C, 487 s, and 49 MPa, respectively. The addition of MgSiN₂ as an additive and SiC as a secondary phase were also investigated in the present work. The Si₃N₄−5 vol% SiC composite exhibited high hardness (19 GPa) and fracture toughness values (6.5 MPa m^1/2) at room temperature.     

Microstructures and mechanical properties of B4C–SiC intergranular/intragranular nanocomposite ceramics fabricated from B4C,Si, and graphite powders

B₄C–SiC intergranular/intragranular nanocomposites with high hardness and high toughness were fabricated through mechanochemical processing with B₄C, Si, and graphite powders and subsequent hot pressing without any sintering aid. The milled powders are composed of stacking-disordered SiC and nanocrystalline B₄C. Most nano/micron-sized SiC particles are homogeneously dispersed in B₄C matrix, and some nano-sized SiC and B₄C particles are embedded into B₄C grains to form an intergranular/intragranular structure. The disordered characteristic of the milled powders is the essential factor for the formation of the intragranular structure, sudden densification within the narrow temperature range (1700–1900 °C), and the preparation of dense samples under a relatively low temperature (1900 °C). The relative density, Vickers hardness, and fracture toughness of the samples sintered at 1950 °C are 98.6%, 34.3 GPa, and 6.0 MPa m^1/2, respectively. The intergranular/intragranular structure plays an important role in improving fracture toughness and hardness of the composites.     

ZrB2–SiC composites prepared by reactive pulsed electric current sintering

Fully densified ZrB₂–20 vol% SiC composites were produced by reactive pulsed electric current sintering (PECS) of a powder mixture containing ZrH₂, B, SiC and B₄C within a total thermal cycle time of only 50 min. During the combined synthesis and sintering process, the ZrH₂ powder decomposed gradually from ZrH₂ into ZrH_m and finally metal Zr that reacted with elemental B to form the ZrB₂ matrix. Reducing the ZrH₂ particle size by attritor milling significantly enhanced densification and allowed initiation of self-propagating high temperature synthesis (SHS) during PECS. The PECS grades exhibited a slightly textured structure, with ≤17% of the ZrB₂ grains oriented with their (0 0 1) planes perpendicular to the direction of pressure and DC current. Because of the ZrB₂ grain orientation, anisotropic mechanical properties were observed. Ceramics prepared from attritor milled powders and PECS with a pressure applied after 5 min upon reaching 1900 °C achieved excellent flexural strengths of 901–937 MPa. The hardness and fracture toughness were respectively 19.7–19.8 GPa and 4.0–4.7 MPa m^1/2 in the direction parallel and 20.2–21.3 GPa and 3.8–3.9 MPa m^1/2 in the direction perpendicular to the applied pressure.     

Carbon nanotube/boehmite-derived alumina ceramics obtained by hydrothermal synthesis and spark plasma sintering (SPS)

Preparation, structure and properties of hydrothermally treated carbon nanotube/boehmite (CNT/γ-AlOOH) and densification with spark plasma sintering of Al₂O₃ and CNT/Al₂O₃ nanocomposites were investigated. Hydrothermal synthesis was employed to produce CNT/boehmite from an aluminum acetate (Al(OH)(C₂H₃O₂)₂) and multiwall-CNTs mixture (200 °C/2 h.). TEM observations revealed that the size of the cubic shape boehmite particles lies around 40 nm and the presence of the interaction between surface functionalized CNTs and boehmite particles acts to form ‘nanocomposite particles’. Al₂O₃ and CNT/Al₂O₃ compact bodies were formed by means of spark plasma sintering (SPS) at 1600 °C for 5 min using an applied pressure of 50MPa resulting in the formation of stable α-Al₂O₃ phase and CNT–alumina compacts with nearly full density. It was also found that CNTs tend to locate along the alumina grain boundaries and therefore inhibit the grain coarsening and cause inter-granular fracture mode. The DC conductivity measurements reveal that the DC conductivity of CNT/Al₂O₃ is 10^?4 S/m which indicate that there is a 4 orders of magnitude increase in conductivity compared to monolithic Al₂O₃. The results of the microhardness tests indicate a slight increase in hardness for CNT/Al₂O₃ (28.35 GPa for Al₂O₃ and 28.57 GPa for CNT/Al₂O₃).     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Synthesis and sintering of pre-mullite powders obtained via carbonate precipitation

Ultrafine pre-mullite powders, which yield mullite at high temperatures, have been prepared from colloidal silica and aluminium nitrate via carbonate coprecipitation and followed by calcination. The chemical and structural evolutions of the as-prepared precipitation powder during thermal treatment were studied and the sinterability of pre-mullite powders were investigated. The as-prepared powders are comprised of ammonium aluminum carbonate hydroxide and amorphous silica, which convert to mullite via the Al–Si spinel phase at 1250 °C. Calcination of the as-prepared powders at 1000 °C gives a very active powder which can be reactively sintered to 98.2% theoretical density at 1550 °C. The sintered body possesses a relatively uniform chemical composition with Al₂O₃/SiO₂ mole ratio of 1.48 and exhibits a very fine interlocking equiaxed and polygonal grain morphology with grain size of 100–200 nm.     

Effects of oxidation curing and Al atoms on the formation of near-stoichiometric freestanding SiC(Al) films derived from polyaluminocarbosilane (PACS)

The effects of oxygen pick-up and Al atoms on the formation and microstructure of freestanding SiC(Al) films by melt spinning of polyaluminocarbosilane (PACS) precursor were studied. PACS green films were cross-linked for 1 h, 2 h, 3 h and 4 h, pre-pyrolyzed at 900 °C, respectively. They were continuously pyrolyzed at 1800 °C to convert initial PACS into SiC(Al) ceramic films. Results reveal that the strict control of oxygen content during the oxidation curing is essential to produce near-stoichiometric SiC(Al) films. The microstructure of the dense films is a mixture of β-SiC crystals, α-SiC nano-crystals, C clusters and a small amount of Al₄O₄C and Al₄SiC₄. Al atoms which play important roles as both sintering aids and grain growth inhibitor are well distributed in the films due to the presence of stable composition and structure. SiC(Al) films with excellent mechanical properties would be attractive candidate materials for MEMS in harsh environments.