Densification of liquid phase sintered silicon carbide

The densification and phase formation of liquid phase sintered silicon carbide (LPSSiC) with 10 wt.% additives were investigated. The ratio of the Al₂O₃/Y₂O₃-additives was changed between 4:1 and 1:2. Densification was carried out by hot pressing and gas pressure sintering. The different densification routes result in different major grain boundary phases—aluminates in gas pressure sintered materials and silicates in hot pressed samples. Thermodynamic calculations were carried out to determine the amount of liquid phase during densification and for the interpretation of the results.     

Vibration milling of silicon carbide and sintered corundum

The paper reports a study concerned with size reduction of corundum grog, sintered corundum (VK-94-1 ceramic waste), and silicon carbide. Corundum and silicon-carbide powders with a specific surface area of 17 and 12 m²/g were obtained. They can serve as fillers for fire-resistant polymer-ceramic composites. Also, the silicon carbide powder can be used as a finely divided additive to heat-stable corundum ceramic products. Translated from Steklo i Keramika, No. 6, pp. 17–20, June, 1997.     

Thermal conductivity of liquid phase sintered silicon carbide

This study investigates the thermal diffusivity and conductivity of YAG/AlN-alloyed LPS-SiC as a function of composition and temperature using the laser-flash technique. Maxwell's model for the thermal conductivity of composites with spherical inclusions is adapted to typical features of the LPS-microstructure and its predictions are compared with experimental data. The results indicate that the thermal conductivity of LPS-SiC is controlled by (i) concentration of impurity atoms in the SiC-phase, (ii) fraction of bulk oxide phase and (iii) amorphous interphases which act as thermal resistance barriers.     

Micro-segregations in liquid phase sintered silicon carbide ceramics

The densification behaviour of LPSSiC ceramics with different amount of secondary phases was investigated during Field Assisted Sintering (FAST). In the densified materials micro-segregations were found with dimensions of several 100 μm. Sometimes such segregations were found in gas pressure sintered materials. The investigation of the state of crystallisation by EBSD and XRD revealed that these micro-segregations are connected with the formation of large YAG (Yttrium aluminium garnet) crystals. The mobility of yttrium in the grain boundaries was investigated by measuring concentration profiles in diffusion couples. The high diffusion coefficient determined at 1850 °C (10^?6 cm/s) indicates that the observed segregations are caused by the crystallisation kinetics of the secondary phases during cooling.     

热处理温度对反应烧结碳化硅材料组织与性能的影响

研究了真空热处理温度对反应烧结碳化硅材料显微组织和断裂强度的影响.结果表明反应烧结碳化硅中的游离硅在1600℃、1800℃真空热处理过程中已全部去除;经过1800℃真空热处理材料的强度均高于1600℃真空热处理材料的强度.在1800℃真空热处理过程中发生的碳化硅再结晶以及气孔形状的变化,是其强度较高的主要原因.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Damage formation mechanisms of sintered silicon carbide during single-diamond grinding

In this research, a single-diamond grinding test was performed on sintered silicon carbide (SSiC) to explore the damage formation mechanism. A scanning electron microscope and a transmission electron microscope (TEM) were used to examine the surface and subsurface morphologies of the grinding groove, respectively. The characteristics of the ground surface morphologies reveal that the single-diamond grinding process of SSiC can be classified into purely ductile, primarily ductile, primarily brittle, and purely brittle stages. Based on the high-resolution TEM (HRTEM) images and the corresponding Fast Fourier transform images of the near-surface region, results reveal that the high density of dislocations and amorphization of SiC grains are responsible for the plastic deformation of SSiC. Most of the cracks congregate on the top grains of the ground surface due to the distinct obstruction of the grain boundary on the cracks propagation, and the cracks generated at the grain boundaries emit into the top grain interiors and go up toward the exposed surface for the distortedly deformed region with higher strain energy; Furthermore, stress concentration caused by the dislocation pileups at grain boundaries represents the crack initiation mechanisms for SSiC. Finally, based on the dislocations pile-up theory, a critical undeformed chip thickness model for boundary crack system nucleation is established, which considers the cutting-edge radius, grinding wheel speed, material properties, and grain size of ceramics.     

反应烧结碳化硅高温性能的研究进展

反应烧结碳化硅具有优良的力学性能、抗侵蚀性能和抗氧化性能等优点,是一种高致密度、低成本和净尺寸成型的材料.但由于反应烧结法的特殊工艺,反应烧结碳化硅中常含有较多游离硅,严重损害了材料的高温性能.主要阐述了反应烧结碳化硅高温力学性能、抗氧化性能、导热性能和抗热震性能的研究现状,并总结了近年来降低游离硅含量、提高反应烧结碳...     

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.     

Thermal conductivity of liquid-phase sintered silicon carbide ceramics: A review

Silicon carbide (SiC) exhibits excellent thermal conductivity. Recently, thermal conductivity that amounts to 261.5 W/m-K has been obtained in polycrystalline SiC ceramic liquid-phase sintered (LPS) with Y₂O₃-Sc₂O₃ additives at 2050 °C under a nitrogen atmosphere. From the additive used to the sintering atmosphere selected, many factors affect the thermal conductivity of the SiC. In this review, important factors that are known to determine the thermal conductivity of LPS-SiC (lattice oxygen/nitrogen content, porosity, grain size, grain boundary structure, phase transformation, and additive composition) have been evaluated. While reviewing the impact of each factor on thermal conductivity, hidden correlations among different factors are also discussed. Among the factors that are claimed to be important, we suggest a few factors that are more critical to thermal conductivity than others. Based on the most critical factors on the thermal conductivity of LPS-SiC, a complete engineers’ guide for high thermal conductivity LPS-SiC is proposed.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Microstructure and mechanical properties of silicon carbide pressureless sintered with oxide additives

It is known that SiC powders can be densified at relatively low temperatures (1850–2000 °C) with some oxide additions. In this work the densification behavior, microstructure and mechanical properties (bending strength, fracture toughness, hardness) of SiC ceramics pressureless sintered with different additions chosen from oxide groups: Al₂O₃ + Y₂O₃, Al₂O₃ + Y₂O₃ + MgO, were investigated. It was found that oxide additives facilitate densification of sinters and significantly improve mechanical properties of SiC ceramics. The best activating oxide additions have been identified.     

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.     

High temperature strength of silicon carbide sintered with 1 wt.% aluminum nitride and lutetium oxide

A heat-resistant SiC ceramic was developed from submicron β-SiC powders using a small amount (1 wt.%) of AlN–Lu₂O₃ additives at a molar ratio of 60:40. Observation of the ceramic using high-resolution transmission electron microscopy (HRTEM) showed a lack of amorphous films in both homophase (SiC–SiC) boundaries and junction areas. The junction phase consisted of Lu–Si–O elements, and the homophase boundaries contained Lu, Al, O, and N atoms as segregates. The ceramic maintained its room temperature (RT) strength up to 1600 °C. The flexural strength of the ceramic was 630 MPa and 633 MPa at RT and 1600 °C, respectively.     

反应烧结SiC材料的高温氧化行为研究

研究了反应烧结SiC材料在 110 0℃空气中的高温氧化行为。结果表明 :反应烧结SiC在110 0℃的氧化动力学曲线符合抛物线规律 ;材料的氧化受O2 和CO在玻璃态硅酸盐中的扩散所控制 ;材料中的杂质元素降低了SiO2 氧化膜的粘度 ,促进了O2 和CO在氧化膜中的扩散     

High temperature strength and fractography of sintered silicon nitride

Influences of the sintering liquid system, temperature, microstructure and post sintering heat treatment of high temperature (30–1250°C) strength, Young's modulus and fracture toughness of sintered silicon nitride (SSN) have been studied. Based on quantitative fractography, typical fracture origin statistics has been presented for SSN. The measured strength of the SSN is in good agreement with the fractographically predicted strength.     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

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.     

High temperature oxidation of two- and three-dimensional hafnium carbide and silicon carbide coatings

The difficulty in using C/C composites as structural components above 2000 °C in an oxidizing atmosphere is their poor lifetime. The solution proposed here consisted in combining two refractory carbides, hafnium and silicon carbides, in coating with a complex architecture, named a three dimensional coating, over a C/C substrate. Such a coating protects the C/C composite at 2000 °C under air. The oxidation of the coating leads to the formation of a Si_xO_yHf_z hafnium-containing silicate liquid, combined with HfO_2(s). This liquid limits oxygen diffusion more than pure SiO₂ does, so it is a better protection against oxidation. Furthermore, HfO_2(s) acts as a frame holding Si_xO_yHf_z in place. From these results, an oxidation mechanism is proposed and discussed.     

Growth and high temperature performance of semi-insulating silicon carbide

The problems of obtaining insulating properties in bulk single-crystal silicon carbide by vanadium doping under the LETI method growth process are considered. The prime novelty of this work is the growth of a semi-insulating bulk single-crystal n-4H-SiC:V by the LETI method in vacuum from a vanadium and aluminium-containing source. The obtained 4H-SiC:V material possesses resistivity >10⁷ Ω cm at 20°C and activation energy ∼1.6 eV at 20–800°C and can be applied as a semi-insulating substrate material for extreme electronics based on silicon carbide or nitrides.