α-SiC Ceramics Research Articles

αSiC - βSiC - graphene composites

In this study, the mechanical properties of the pressureless sintered samples of α-SiC based composite containing 0–3 wt.% graphene and 0–15 wt.% nano β-SiC were investigated. Simultaneous usage of nano β-SiC and graphene and transformation of β-SiC (3C) to α-SiC (6H/4H) resulted in elongation of secondary α-SiC grains, which significantly improved the mechanical properties (e.g. fracture toughness) of SiC ceramics. According to the results, the highest Relative density of 99.04%, Young’s modulus of 537.76 GPa and fracture toughness of 5.73 MPa × m1/2 were obtained in the sample containing 5 wt.% nano β-SiC and 1 wt.% graphene (5B1G). In addition, two methods of measuring bending strength including three-point bending tests and biaxial tests (piston-on-three-ball) were compared. Strip-shaped specimens were prepared for three-point bending test and disc-shaped specimens were prepared for biaxial bending test. Each bending test was evaluated using a universal testing machine. The results showed that the biaxial bending strength is less than the three-point bending strength. Also, the maximum three-point bending strength of 582.01 MPa and biaxial bending of 441.56 MPa were obtained in 5 wt.% Nano β-SiC and 1 wt.% Graphene samples (5B1G). Studies have shown that in addition to the many advantages of using the biaxial bending method, the results have a very similar trend to the three-point bending strength. Also, the most-increased hardnesses of 28.03 GPa and 29.97 GPa were seen in the sample containing 5 wt.% nano β-SiC (5B) with forces of 10 N and 1 N, respectively. One of the effective mechanisms in improving the fracture toughness of α-SiC ceramics is crack deflection/bridging. Also, the difference in thermal expansion of the α-SiC matrix and the reinforcements, leading to the creation of residual stresses between the matrix grains and the reinforcement, is effective in improving the mechanical properties (e.g. strength and fracture toughness).

Effects of Sintering Additives on Microstructure and Mechanical Properties of Hot-Pressed α-SiC Ceramics

Effects of Sintering Additives on Microstructure and Mechanical Properties of Hot-Pressed α-SiC Ceramics

The thermal, electrical and mechanical properties of porous α-SiC ceramics bonded with Ti3SiC2 and β-SiC via low temperature in-situ reaction sintering

Porous SiC ceramics have recently attracted wide attention for their applications in the electrically heatable filter. Further improvement of the thermal and electrical conductivity without sacrificing permeability is a critical parameter for such applications. In the present work, porous SiC/Ti3SiC2 ceramic composites with Ti3SiC2 and micro/nano SiC have been prepared from TiC/Si/α-SiC mixtures at a low sintering temperature (1400 °C). Nano-laminated Ti3SiC2 enhanced the electrical conductivity, while the good thermal conductivity was achieved through in-situ formed nano β-SiC and raw coarse α-SiC in the porous ceramics. Along with the increase of initial α-SiC particle size from 0.76 to 16.13 μm, the permeability, thermal and electrical conductivity improved due to the decreased porosity and increased pore size in porous SiC/Ti3SiC2 ceramics. The results suggested that the decoupling of the electrical conductivity from the thermal conductivity could be tuned by adjusting the initial α-SiC particle size.

Effect of sintering aids on microstructure and properties of textured SiC ceramics prepared in 6 T

ABSTRACT C-axis textured α-SiC ceramics were prepared by gel-casting in a strong magnetic field of 6 T and subsequent sintering at 1950 °C for 2 h. The effects of sintering aids on the microstructure and properties of textured SiC ceramics were discussed. Results showed that highly textured α-SiC green bodies were obtained by gel-casting in a strong magnetic field of 6 T. The sintering process slightly suppressed the texture development of α-SiC ceramic. The degree of texture in the α-SiC green bodies and sintered bodies both showed a decreasing trend with the increase of sintering aids content. And the increase of the sintering aids content promoted the densification and improved the bending strength of α-SiC ceramics. Besides, the bending strength of textured SiC ceramics showed an anisotropy based on the grain orientation.

Processing of solid state sintered α-SiC ceramics with carbon-black addition

The efficient removal of surface silica layer from SiC particles by the addition of carbon is required for solid state sintering (SSS) of SiC ceramics. While wax-based organics are generally utilized as carbon source in many commercial SSS-SiC ceramics, carbon black powders with high specific surface area (SSA) are potential alternatives as carbon source, which can be dispersed in ceramic slurries directly without needing any additional pyrolysis reactions. In this study, carbon black powders were utilized as carbon source for sintering of SiC. The effect of particle size of carbon black powders on densification and microstructural development during pressureless and spark plasma sintering (SPS) were investigated. It was observed that while the density of the SiC samples increased with increasing SSA of carbon black powders, complete densification was not achieved for all samples in case of pressureless sintering due to the flaws originated by carbon black segregation on granule surfaces. SPS was found an effective sintering method for the elimination of such flaws and SiC ceramics were sintered to 100% of theoretical density independent from the particle size of the carbon black powders in this sintering technique.

Localized plasticity in silicon carbide ceramics induced by laser shock processing

This paper investigates how laser shock processing (LSP) changes the microstructures and mechanical properties of polycrystalline α-SiC ceramics. Three-dimensional (3-D) topography suggests that the grain boundaries experience larger height changes and deformation than the grains after LSP. Transmission electron microscopy characterizations revealed dislocation activities near the surface and grain boundaries in LSP-treated SiC ceramics, suggesting that localized plasticity could be generated during the LSP process at room temperature. X-ray diffraction analysis shows that significant compressive residual stress was introduced in SiC surfaces, which could be extended to a depth of 750 µm below the surface. The LSP-induced localized plasticity can improve the mechanical properties of ceramics, such as the apparent fracture toughness and bending strength.

Thermal transformation of nanohafniumcarbosilanes

The process of thermochemical transformation of nanohafniumcarbosilane (nano-HfCS) at 1500 °C in various media was investigated. The regularities of the formation of SiC ceramic structures modified with hafnium compounds were analyzed by physicochemical methods.The two-stage thermochemical transformation of nano-HfCS first at 1100 °C in argon and then at 1500 °C in various media: argon, nitrogen, vacuum, air results in formation of a nanocrystalline α-SiC ceramics with a hexagonal crystal lattice, modified with HfC, as the main phase. It is demonstrated that the medium in which pyrolysis of nano-HfCS is carried out at 1500 °C affects only the surface layer of the resulting ceramics: at pyrolysis in argon, carbon oxides and hafnium are produced as additional phases on the surface; in nitrogen, silicon nitride and carbon are formed as additional phases on the surface, and in air SiO2 and carbon are formed on the surface as additional phases.

Electrical resistivity of α-SiC ceramics sintered with Al2O3 or AlN additives

Abstract Highly resistive SiC ceramics were prepared by hot pressing α-SiC powders with Al 2 O 3 -Y 2 O 3 additives with a 4:1 molar ratio. X-ray diffraction patterns, Raman spectra, electron probe microanalysis (EMPA), and scanning electron microscopy (SEM) images revealed that the bulk SiC ceramics consisted mostly of micron-sized 6H-SiC grains along with Y 2 O 3 and Si clusters. As the additive content increased from 1 to 10 vol%, the electrical resistivity of the ceramics increased from 3.0 × 10 6 to 1.3 × 10 8 Ω cm at room temperature. Such high resistivity is ascribed to Al 2 O 3 in which Al impurities substituting Si site act as deep acceptors for trapping carriers. More resistive α-SiC ceramics were produced by adding AlN instead of Al 2 O 3 . The highest resistivity (1.3 × 10 10 Ω cm) was achieved by employing 3 vol% AlN-Y 3 Al 5 O 12 (yttrium aluminum garnet, YAG) as an additive.

Thermoelectric properties of aluminum compound-doped α-SiC ceramics

Thermoelectric properties of aluminum compound-doped α-SiC ceramics

Effect of removal of silicon on preparation of porous SiC ceramics following reaction bonding and recrystallization

Porous α-SiC ceramics with distributed pore structure were prepared by recrystallization method using silicon as the template. The microstructure evolution of this material (sintered at 2100°C) was investigated by varying particle size of the silicon powders (3.5μm and 37μm). The results demonstrated that finer silicon powder could strengthen grain growth along close-packed directions and made the material easily to form plate-like SiC crystals. Pore diameter was mainly controlled by silicon raw powder when the silicon had a greater median diameter value than the SiC powder. In this study, grain growth was determined by a mixed mechanism during silicon removing. The porosity increased with the increasing of silicon contents while the strength was in a reverse proportional relationship. High porosity ceramics with relative high strength, which were sintered at 2100°C for 15min, were obtained using 3.5μm silicon powder as the template.

Effect of microstructure on the mechanical properties of liquid-phase-sintered silicon carbide at pre-creep temperatures

The effect of the microstructure on the mechanical properties of pressureless, liquid-phase-sintered (LPS) α-SiC ceramics above room-temperature was studied. LPS-SiC ceramics were fabricated with different microstructural features (grain size and morphology, and content of the intergranular phase), and their mechanical behaviour under contact stresses was evaluated by high temperature Hertzian testing (HTHT) from room temperature up to the creep temperature (1000 °C). The amount of intergranular phase was found to control the elasto-plastic properties of LPS-SiC at intermediate temperatures. Grain size and morphology had a significant influence only on toughness, since the crack bridging mechanism was enhanced by elongated grains, the more so the larger their size. Implications of these results for the design and fabrication of LPS-SiC ceramics with tailored contact-mechanical properties are discussed.

Recovery behavior of point defects after low-dose neutron irradiation of sintered SiC by thermal diffusivity and swelling measurements

Effects of neutron irradiation on physical property change of two kinds of sintered α-SiC ceramics were examined through the post-irradiation recovery experiments. Changes of macroscopic length and thermal diffusivity due to post-irradiation isothermal annealing up to 6h between room temperature and 1400°C were measured. Results of both measurements indicated almost same recovery tendency for each specimen. The SiC specimen containing Al2O3 showed large change in the activation energy of both recoveries at ∼950°C and 1200°C, and these of the SiC specimen containing B4C showed large change at ∼800°C, 950°C and 1200°C. Activation energies of macroscopic length recovery for both specimens from irradiation temperature to the lower temperature boundary were 0.1–0.2eV and those of the second temperature range were 0.8–0.9eV, respectively. These values indicated that the recovery of these properties is proceeded by the recombination of Frenkel pairs of both C and Si at various configurations.

A Feasibility Study on Laser-Mechanical Drilling of SiC Ceramics

Structural SiC (α-type) is believed to be widely applied in hostile environments such as high-temperature, high-corrosive applications in the semiconductor industries due to its superior thermo-physical and mechanical properties. However, the extremely high hardness and brittleness of SiC makes hole drilling difficult by the conventional mechanical drilling (CMD) technique. Laser can be used to drill SiC; but the resultant holes are often tapered and uneven, with tendency for microcracks and thermal damage to occur at the hole entry due to the high thermal shock from the laser. This paper reports on the experimental results of a sequential laser-mechanical drilling (LMD) technique for drilling α-SiC. At first, an Nd:YAG laser was used to drill a series of pilot holes on a 3 mm thick SiC plate. Then a diamond-coated carbide drill was sequentially applied to these holes to obtain desired hole diameter of 0.5 mm. A number of through holes on SiC (aspect ratio: 6) were successfully obtained using this approach. The quality of the drilled holes were assessed in terms of the entrance and exit sizes and conditions, hole taper angle, hole edge shapes, and microcracks. Finally, comparisons of the LMD performances were also made against the holes predrilled by the laser itself and holes of the similar size drilled separately with the CMD technique. The experiment results show that the proposed drilling approach can effectively drill α-SiC ceramics.

Laser irradiation of α-SiC ceramics

The process of laser irradiation of a surface of SiC ceramics in air was investigated. As a result of SiC oxidation and evolution of CO 2, porous SiO 2 forms on the target surface. During deposition of ablation products on the substrate, a loose SiO 2 film forms.

Effect of the nature of the intergranular phase on sliding-wear resistance of liquid-phase-sintered α-SiC

The effect of the nature of the intergranular phase on sliding-wear resistance of equiaxed-grained, liquid-phase-sintered (LPS) α-SiC ceramics was studied. The nature of the intergranular phase in LPS α-SiC ceramics was modified using Ar or N2 sintering atmospheres, while maintaining the same grain size and morphology. Sintering under N2 leads to the incorporation of nitrogen in the intergranular phase, and attendant solid-solution hardening of that phase. The solid-solution hardened intergranular phase results in LPS SiC ceramics with improved sliding-wear resistance.

Helium behavior in α-SiC ceramics investigated by NRA technique

The mechanisms involved in helium migration in α-SiC are investigated through the evolution of its microstructure and of the concentration profiles following annealing at 1300°C/30min for fluences of 1 and 5×10153Hecm−2 and a He implantation energy of 500keV. Helium profiling is performed using the 3He(d,α)1H NRA technique with an improved detection limit of 5atppm. The NRA and TEM techniques clearly show that depending on the initial fluence, a proportion of the helium is trapped within the grain and a part of the helium is released. Analysis of the helium profile changes after annealing enabled to determine a value of the volume diffusion coefficient close to (8±1)×10−17m2s−1 for both fluences studied.

Effect of liquid-phase content on the contact-mechanical properties of liquid-phase-sintered α-SiC

Effect of the content of the intergranular phase (Y 3Al 5O 12 or yttrium aluminum garnet or YAG) on the room-temperature contact-mechanical properties of pressureless, liquid-phase-sintered (LPS) α-SiC ceramics has been studied. An increase in YAG vol.% is found to result in the expected degradation of the elastic modulus and the indentation yield strength in LPS SiC. However, with increasing YAG vol.% the degradation in the hardness and the sliding-wear resistance is found to be severe, while the indentation toughness first increases and then decreases. These results are analyzed, and discussed in the context of providing guidelines for the design and fabrication of low-cost LPS SiC ceramics with tailored contact-mechanical properties.

Thermoelectric Properties of Boron Compound-Doped .ALPHA.-SiC Ceramics

The effects of boron compound additive on the thermoelectric properties of α-SiC ceramics were studied. Porous SiC ceramics with 57-62% relative density were fabricated by sintering the pressed α-SiC powder compacts with B, B 4 C, and BN at 2000-2100°C for 3h in Ar and/or N 2 atmosphere. The sintered bodies were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameter measurements revealed incorporation of a certain amount of added B into the SiC lattice and negligible phase transformation during sintering. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured at 600-900°C in Ar and/or vacuum atmosphere. The kind of additives, the amount of addition and sintering atmosphere had significant effects on the thermoelectric properties. On the whole, the excess addition had a harmful influence upon electrical conductivity. The thermoelectric figure of merit of B-doped SiC was lower than that of n-type SiC.

Helium Release and Physical Property Change of Neutron-Irradiated α-SiC Containing B4C of Different 10B Concentrations

Sintered α-SiC ceramics containing B4C with various 10B concentrations were neutron-irradiated, and the helium release rate during increasing annealing temperature was observed. The lattice parameter and the macroscopic length changes of the same specimens were also observed. The helium release rate from powdered specimens increased markedly above 650°C, and showed a sharp peak at ~1,160°C independent of the 10B concentration, but the rate was higher for the specimen containing higher concentration of 10B. The macroscopic volume of the SiC specimen with higher 10B concentration was expanded larger than that of the lower one by the irradiation. The unit cell volume change was mostly independent of 10B concentration, whereas the each axis changed different manner. A-axis started to shrink around the irradiation temperature, and contracted mostly linearly with increasing annealing temperature. On the other hand, contraction of the c-axis was not parallel to that of a-axis, and was retarded between 500–1,100°C. It is supposed that helium will induce some lattice defects, which expands c-axis length of SiC. These defects retained up to 1,100°C with support of helium migration. During the annealing, the expansion of macroscopic length occurred at higher temperature than 1,300°C, where the higher 10B concentration specimen gave the larger expansion, indicating the formation of grain boundary helium bubbles.

Helium Release and Physical Property Change of Neutron-Irradiated .ALPHA.-SiC Containing B4C of Different 10B Concentrations

Sintered α-SiC ceramics containing B4C with various 10B concentrations were neutron-irradiated, and the helium release rate during increasing annealing temperature was observed. The lattice parameter and the macroscopic length changes of the same specimens were also observed. The helium release rate from powdered specimens increased markedly above 650°C, and showed a sharp peak at ~1,160°C independent of the 10B concentration, but the rate was higher for the specimen containing higher concentration of 10B. The macroscopic volume of the SiC specimen with higher 10B concentration was expanded larger than that of the lower one by the irradiation. The unit cell volume change was mostly independent of 10B concentration, whereas the each axis changed different manner. A-axis started to shrink around the irradiation temperature, and contracted mostly linearly with increasing annealing temperature. On the other hand, contraction of the c-axis was not parallel to that of a-axis, and was retarded between 500–1,100°C. It is supposed that helium will induce some lattice defects, which expands c-axis length of SiC. These defects retained up to 1,100°C with support of helium migration. During the annealing, the expansion of macroscopic length occurred at higher temperature than 1,300°C, where the higher 10B concentration specimen gave the larger expansion, indicating the formation of grain boundary helium bubbles.