Pressureless sintered silicon carbide tailored with aluminium nitride sintering agent

This study reports the influence of aluminium nitride on the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Pressureless sintering was achieved at 2000 °C for 5 min with the additions of boron carbide together with carbon of 1 wt% and 6 wt%, respectively, and a content of aluminium nitride between 0 and 10 wt%. Sintered samples present relative densities higher than 92%. The sintered microstructure was found to be greatly modified by the introduction of aluminium nitride, which reflects the influence of nitrogen on the β-SiC to α-SiC transformation. The toughness of sintered sample was not modified by AlN incorporation and is relatively low (around 2.5 MPa m^1/2). Materials exhibited transgranular fracture mode, indicating a strong bonding between SiC grains.     

Effect of pressure and temperature on densification,microstructure and mechanical properties of spark plasma sintered silicon carbide processed with β-silicon carbide nanopowder and sintering additives

The effects of applied pressure and temperature during spark plasma sintering (SPS) of additive-containing nanocrystalline silicon carbide on its densification, microstructure, and mechanical properties have been investigated. Both relative density and grain size are found to increase with temperature. Furthermore, with increase in pressure at constant temperature, the relative density improves significantly, whereas the grain size decreases. Reasonably high relative density (~96%) is achieved on carrying out SPS at 1300 °C under applied pressure of 75 MPa for 5 min, with a maximum of ~97.7% at 1500 °C under 50 MPa for 5 min. TEM studies have shown the presence of an amorphous phase at grain boundaries and triple points, which confirms the formation of liquid phase during sintering and its significant contribution to densification of SiC at relatively lower temperatures (≤1400 °C). The relative density decreases on raising the SPS temperature beyond 1500 °C, probably due to pores caused by vaporization of the liquid phase. Whereas β-SiC is observed in the microstructures for SPS carried out at temperatures ≤1500 °C, α-SiC evolves and its volume fraction increases with further increase in SPS temperatures. Both hardness and Young׳s modulus increase with increase in relative density, whereas indentation fracture toughness appears to be higher in case of two-phase microstructure containing α and β-SiC.     

Pressureless densification and mechanical properties of hafnium diboride doped with B4C: From solid state sintering to liquid phase sintering

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.     

Formation of porous SiC ceramics via recrystallization

In this study, porous SiC ceramics with interconnected huge plate-like grains were fabricated from oxidized β-SiC powder with 1 wt% B₄C. When the β–α SiC phase transformation occurred at 2100 °C, rapid grain growth of α-SiC consumed the unstable β-SiC matrix resulting in an interconnected network structure with huge plate-like grains. The oxidation of β-SiC powder and the addition of B₄C are necessary conditions for rapid grain growth. The observed results are discussed based on thermodynamic considerations. The measured porosity of the specimens sintered at 2100 °C for 30 min was 47% and the mean pore size was 6–7 μm. The strength of the sintered specimen was 45 ± 5 MPa.     

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.     

Wear resistant boron carbide compacts produced by pressureless sintering

Boron carbide compacts were produced by pressureless sintering at 2200 °C/2 h and 2250 °C/2 h in Ar atmosphere, using a starting powder with a particle size smaller than 3 µm. Effects of carbon addition (3.5 wt%) and methanol washing of the starting powder were investigated on the densification, Vickers hardness, and micro-abrasive wear resistance of the samples. The removal of oxide phases by methanol washing allowed the production, with no sintering additive, of highly densified (93.6% of theoretical density), hard (25.4 GPa), and highly wear resistant (wear coefficient =2.9×10^–14 m³/N.m) boron carbide compacts sintered at 2250 °C. This optimized combination of properties was a consequence of a reduced grain growth without the deleterious effects associated to the carbon addition. Methanol washing of the starting powder is a simple and general approach to produce, without additives, high quality, wear resistant boron carbide compacts by pressureless sintering.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Properties of liquid phase pressureless sintered silicon carbide obtained without sintering bed

High density pressureless sintered silicon carbide bodies with yttria and alumina as sintering aids were obtained without sintering bed (LPSSC-NB). Sintering behavior of this material was studied between 1850 °C and 1950 °C and it was compared to the liquid phase sintered SiC material obtained using sintering bed (LPSSC-B). Sintered density was 97% of the theoretical density (T.D.) at 1875 °C. Mechanical properties like fracture toughness, hardness, flexural strength were determined and compared to other SiC-based materials. In this manner we were able to demonstrate that silicon carbide could successfully be sintered by means of liquid phase mechanism also without sintering bed. This fact opens liquid phase sintered silicon carbide to a wide range of industrial application.     

Pressureless sintering of single-phase tantalum carbide and niobium carbide

This work presents the results of studies on the preparation of single-phase polycrystalline tantalum carbide and niobium carbide. It has been found that it is possible to obtain polycrystals with high density in the pressureless sintering process at temperatures up to 2000 °C and therefore relatively low temperatures such as for the compounds with one of the highest melting points; TaC – 3985 °C and NbC – 3600 °C. Only carbon as a sintering additive was used. The main role of carbon is to reduce of oxide contamination. It has been shown that the determination of the amount of carbon required to reduce oxide contamination is only possible through the experimental method.     

Spark plasma sintered ZrC-Mo cermets: Influence of temperature and compaction pressure

The microstructure analysis and mechanical characterisation were performed on a ZrC-20 wt%Mo cermet that was spark plasma sintered at various temperatures ranging between 1600 and 2100 °C under either 50 or 100 MPa of compaction pressure. The composite reached ~98% relative density for all experiments with an average grain size between 1 and 3.5 µm after densification. The nature of SPS technology caused a faster densification rate when higher compaction pressures were applied. The difference in compaction pressures produced different behaviors in densification and grain structure: 1900 °C, 100 MPa produced excessive grain growth in ZrC; 1600 °C, 50 MPa revealed a very clear ZrC grain structure and Mo diffusion between carbide grains; and 2100 °C, 50 MPa exhibited the highest overall mechanical properties due to small clusters of Mo phases across the microstructure. In fact, this particular sintering regime gave the most optimal mechanical values: 2231 HV10 and 5.4 MPa*m^1/2, and 396 GPa Young's modulus. The compaction pressure of SPS played a pivotal role in the composites’ properties. A moderate 50 MPa pressure caused all three mechanical properties to increase with increasing sintering temperature. Conversely, a higher 100 MPa pressure caused fracture toughness and Young modulus to decrease with increasing sintering temperature.     

Pressureless sintering of titanium carbide doped with boron or boron carbide

Titanium carbide ceramics with different contents of boron or B₄C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB₂ platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B₄C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m^1/2) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B₄C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB₂ platelets.     

High temperature oxidation of SiC under helium with low-pressure oxygen. Part 2: CVD β-SiC

In the frame of the Generation IV International Forum, Gas-cooled Fast Reactor (GFR) is one system studied by CEA (France). Helium pressurized at 7 MPa is the coolant and the nominal temperature of use is about 1300 K. The cladding materials currently considered is a SiC/SiC composite with a β-SiC coating. In case of accident, reactor temperatures can reach 1900–2300 K. A previous study was carried out to determine the physico-chemical behavior of another polytype, α-SiC, for comparison on the position of the active to passive transition and of the mass loss rates under active conditions to simulate a typical accident. Experimental oxidation tests at high temperature (1400–2300 K) on massive β-SiC samples processed by Chemical Vapor Deposition (CVD) coupled to mass variation, SEM, XPS, AFM and roughness analyses enabled to determine the transition between passive and active oxidation regimes, and to study the resistance to oxidation of such material in some conditions that might be encountered in case of accident (high temperature increase up to 2300 K). Finally, the experimental results have shown that the transition from passive to active regime occurs at higher temperature for β-SiC than for α-SiC and that the mass loss rate of β-SiC is lower than the one measured for α-SiC on the common temperature range investigated (up to 2100 K).     

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.     

Pressureless sintering of binderless tungsten carbide

The aim of this study was to produce dense, single phase polycrystals. The research was carried out on the submicron tungsten carbide powder without additives, with either a carbon or tungsten additive and on the powder with both additives. The primary task of carbon was to reduce surface oxide impurities which passivate WC grains; tungsten in turn bounds free carbon in the WC. The authors manufactured fine-grained, dense (96–98% T.D.) and single-phase WC polycrystals, using the technique of pressureless sintering at the temperature not exceeding 2000 °C. A positive effect of carbon addition on tungsten carbide sinterability was observed, whereby a dense, fine-grained polycrystals can be obtained at 1900 °C. It was also observed that a significant excess of temperature of sintering process resulted in a strong abnormal grain growth of WC grains.     

De-sintering of Ba5Nb4O15 ceramic and its influence on microwave characteristics

The sintering behaviour of a cation-deficient perovskite, Ba₅Nb₄O₁₅, is investigated in the present study. The highest density can be achieved through pressureless sintering at 1250 °C is only 93%. A further increase in sintering temperature results in a decrease in density; for example, the density is only 82% after sintering at 1435 °C. The density decrease, de-sintering, can be related to the formation of abnormal grains ( > 1250 °C) and the reduction of niobium ions ( > 1400 °C). The permittivity of sintered Ba₅Nb₄O₁₅ specimen shows strong dependence on density; however, the quality factor (Qxf) increases to 45,000 with the increase of sintering temperature. The increase in quality factor is attributed to the ordering of cations in the perovskite structure.     

Liquid poly(silylacetylene)siloxane resin as a novel precursor of silicon carbide and silicon oxycarbide ceramics

Liquid preceramic poly(silylacetylene)siloxane resin was synthesized via a two-step protocol including organometallic condensation and hydrolysis reactions. The preceramic resin was well soluble in acetone, toluene, and tetrahydrofuran (THF), etc. By thermal cure at 180–250 °C a hard monolithic solid was formed through radical polymerization of secondary ethynyl groups. The poly(silylacetylene)siloxane resin was processed easily to various nonporous shapes to silicon carbide (SiC) and silicon oxycarbide (SiCO). SiCO ceramic was obtained at a yield of >75% by pressureless pyrolysis at 900–1200 °C; while SiC ceramic was obtained at 1500 °C at a yield of ≈67%. The molar ratio of Si/C in the SiC was found at 1:1.1–1:3, based on ICP-MS elemental analysis. X-ray diffraction (XRD) results revealed the typical β-SiC structure in the poly(silylacetylene)siloxane derived SiC ceramics. The SiC ceramics exhibited high thermo-oxidation resistance at elevated temperatures in air atmosphere.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Pressureless sintered self-reinforced Y-α-SiAlON ceramics

Y-doped α-SiAlONs, where the level of aluminum and oxygen was varied, have been successfully densified by pressureless sintering. The effect of parameters such as starting composition, morphology of grains and sintering temperature on densification and mechanical properties of Y-SiAlON ceramics were study. It was found that Y-α-SiAlONs with different compositions could be pressureless sintered in excess of 98% theoretical density at temperatures ranging from 1750 to 1920 °C. The typical microstructure of the SiAlON investigated consists of elongated α-SiAlON grains uniformly distributed in a matrix of equiaxed grains. Fracture toughness of 6.2 MPa m^1/2 and hardness of 17.6 GPa were found for Y_0.667Si_8.5Al_3.5O_1.5N_14.5 (m = 2.0, n = 1.5) sintered at 1850 °C. The results indicate that both crack deflection and crack bridging mechanisms of toughening operate in the materials, with the latter being the dominant one.     

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB₂–SiC ultra high temperature ceramics containing B₄C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB₂–SiC suspension were investigated in detail. A well-dispersed suspension with 50 vol.% solid loading was prepared at pH 10 with 0.4 wt.% PAA after ball milling at 240 rpm for 24 h. Then crack-free green ZrB₂–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100 °C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405 ± 27 MPa and 4.3 ± 0.3 MPa m^1/2, respectively.     

Microstructure and mechanical properties of the spark plasma sintered Ta2C ceramics

Single phase hexagonal α-Ta₂C ceramics were synthesized by spark plasma sintering and using TaC and Ta as the starting powders. Effects of sintering temperatures and holding times on the densification process, phase formation, microstructure development, and mechanical properties of the α-Ta₂C ceramics were investigated. Densification occurred in the temperature range of 1520–1675 °C in less than 2.5 min. But completion of the Ta₂C formation took about 40 min at 1500 °C, and 5 min at 1900 °C. The materials sintered at 1500 °C consisted of fine equiaxed grains. The Ta₂C grains grew anisotropic to form an elongated self-toughening microstructure at 1700 °C. At 1900 °C, the neighboring Ta₂C individual crystals coalesced to form large Ta₂C blocks to entrap the residual pores. Although higher flexural strength and fracture toughness were reached at 1700 °C, the unstable microstructures of the Ta₂C materials indicated limited applications at high temperatures.