Pressureless-sintered SiC Research Articles

Corrosion behavior of pressureless-sintered SiC in molten NaCl–KCl–MgCl2 salt at 700 °C

The corrosion behavior of pressureless-sintered SiC in a molten NaCl–KCl–MgCl2 salt at 700 °C was investigated. Results indicate that it is prior for SiC and sintering aid B around grain boundaries to be corroded. MgCl2⋅xH2O(x=4,6) in the salt plays an important role in the corrosion of SiC. MgCl2⋅xH2O is hydrolyzed to HCl, H2O, and MgO, which react with SiC and B to form C, SiO2, SiCl4, B2O3, and Mg2SiO4. The corrosion depth of SiC and the concentrations of elements Si and B in the salt increase with the corrosion time, which are fitted well with Fick's law of diffusion.

Effect of stainless steel on the corrosion of pressureless-sintered SiC in the vapor of molten NaCl-KCl-MgCl2 salt

The effect of 316 stainless steel on the corrosion of pressureless-sintered SiC in the vapor of molten NaCl-KCl-MgCl2 salt at 700 °C was investigated. Results indicate that MgCl2·xH2O (x = 4, 6) was hydrolyzed to form HCl which corroded the stainless steel. Some corrosion products were dissolved to increase the concentration of elements Cr, Fe, and Ni in salt, and others reacted with SiC to form Cr7C3, Fe3Si, and SiCl4. Reactions among SiC, stainless steel, and their corrosion products (FeCl2, CrCl2 and SiCl4) drove the corrosion of SiC with a depth of ∼ 250 µm for 1000 h.

Effects of AlN on the densification and mechanical properties of pressureless-sintered SiC ceramics

In the present work, SiC ceramics was fabricated with AlN using B4C and C as sintering aids by a solid-state pressureless-sintered method. The effects of AlN contents on the densification, mechanical properties, phase compositions, and microstructure evolutions of as-obtained SiC ceramics were thoroughly investigated. AlN was found to promote further densification of the SiC ceramics due to its evaporation over 1800°C, transportation, and solidification in the pores resulted from SiC grain coarsening. The highest relative density of 99.65% was achieved for SiC sample with 15.0wt% AlN by the pressureless-sintered method at 2130°C for 1h in Ar atmosphere. Furthermore, the fracture mechanism for SiC ceramics containing AlN tended to transfer from single transgranular fracture mode to both transgranular fracture and intergranular fracture modes when the sample with 30.0wt% AlN sintered at 1900°C for 1h in Ar. Also, SiC ceramics with 30.0wt% AlN exhibited the highest fracture toughness of 5.23MPam1/2 when sintered at 1900°C.

Enhanced Fracture Toughness of Pressureless-sintered SiC Ceramics by Addition of Graphene

To enhance the fracture toughness of pressureless-sintered SiC ceramic, graphene was introduced as an additive. The effects of graphene contents on the fracture toughness, bending strength, micro-hardness, phase compositions, and microstructure evolutions of the SiC ceramics were investigated in detail by scanning electron microscopy, energy dispersive X-ray spectroscopy, and metallographic microscopy. The fracture toughness, bending strength and micro-hardness increased initially, and then decreased with the graphene content increasing from 0 to 5.0 wt%. The highest fracture toughness of 5.65 MPa m1/2 was obtained for sample with 1.0 wt% graphene sintered at 2130 °C for 1 h in Ar, which was about 22.6% higher than that of SiC sample without graphene. In addition, the highest bending strength and micro-hardness of 434.14 MPa and 29.21 GPa corresponded to the SiC samples with graphene content of 0.5 wt% and 2.0 wt%, respectively.

Thermal shock fracture of hot silicon carbide immersed in water

High purity CVD-SiC, considered as a nuclear grade cladding material, exhibits thermal shock tolerance ∼1260 °C in room temperature water and beyond it (>1260 °C) in saturated water. Being thinner than the tested specimen thickness (1.5 mm × 2.0 mm), the actual cladding (0.57 mm) is anticipated to exhibit enhanced thermal shock tolerance. This implies that thermal shock alone may not shatter the SiC cladding in reflood. Level of fuel rod internal pressure will be a decisive factor in predicting cladding fracture during reflood. Decreasing water subcooling significantly reduces thermal shock fracture danger of ceramic materials. Thermal shock experiments showed strength retention for both pressureless sintered-SiC and CVD SiC, as well as Al2O3 samples quenched from temperatures up to 1260 °C in saturated water. Solid–liquid contacts during nucleate and transition boiling, and boiling incipience upon water bath entering are a highly probable origin of thermal shock fracture in water quenching.

How does pore-induced crack change as temperatures decrease from 293 K to 77 K?

The pore-induced cracks in pressureless-sintered SiC ceramics have been investigated at ambient and cryogenic temperatures. The Weibull plots of flexural strength were determined to compare the effects of pore-induced cracks on fracture behavior at 77K and 293K. The stress field near crack tip was investigated based on the microstructure of sintered SiC. The measurements of lattice constants of single crystal SiC were carried out by in situ X-ray diffraction. It was found that the effect of defect-induced crack on fracture was diminished at cryogenic temperatures, owing to the decreases of cracks׳ effective size and increases of propagate resistance. The improved fracture reliability and enhanced flexural strength of SiC ceramics facilitate their applications in space-based reflectors.

Effects of graphene on the thermal conductivity of pressureless-sintered SiC ceramics

SiC ceramics with graphene additives have been fabricated by a pressureless-sintered method. The effects of graphene contents on the densification, thermal conductivity, electrical conductivity, phase compositions, and microstructure evolutions of the SiC ceramics were investigated in detail. As the graphene content increased from 0 to 5.0wt%, the relative density of the SiC samples monotonically decreased, the electrical conductivity monotonically increased, whereas the thermal conductivity initially increased and then decreased. The highest thermal conductivity of 145.14W/(mK) was obtained for SiC sample with 2.0wt% graphene sintered at 2130°C for 1h in Ar.

Wetting and evaporation behaviors of molten Mg on partially oxidized SiC substrates

The wetting and evaporation behaviors of molten Mg drops on pressureless-sintered SiC surfaces were studied in a flowing Ar atmosphere at 973–1173 K by an improved sessile drop method. The initial contact angles are between 83° and 76°, only mildly depending on temperature. The formation of a ridge at the triple junction as a result of reaction between molten Mg and the SiO 2 film on the SiC surface pins the triple line and leads to a constant contact diameter mode during the entire evaporation process. Moreover, the diffusion coefficients of the Mg vapor at different temperatures were evaluated based on a simple model.

Effects of oxidation on surface stresses and mechanical properties of liquid phase pressureless-sintered SiC–AlN–Y 2O 3 ceramics

The understanding of the oxidation mechanism of 50 wt% SiC–50 wt% AlN composites obtained by means of pressureless sintering without the protective powder bed and with Y 2O 3 as sintering-aid were significantly improved by means of Raman spectroscopy. These analyses put in evidence that “amorphous carbon” started to be formed at 1300 °C as main effect of active oxidation of SiC. At higher temperature the crystallization process began and it was completed at 1500 °C when only graphite could be recognized. On the basis of these new evidences, oxidation effects on the mechanical properties of SiC–AlN–Y 2O 3 composites were defined. First of all, heat treatment in air was able to induce a compressive surface stress due to the volume gain associated to the oxidation of the intergranular phase. As a consequence apparent fracture toughness showed a value of 6.6 MPa m 1/2 after a heat treatment at 1300 °C, while at higher temperature effects of active oxidation caused a decreasing up to 4.7 MPa m 1/2. This toughening mechanism was also used to improve the resistance to thermal shock, which was evaluated by performing quenching tests. Furthermore, passive oxidation induced the healing of superficial flaws by means of the formation of α-cristobalite. This phenomenon was assumed to be responsible for the increasing of the flexural strength.

Relationship between dimensional changes and the thermal conductivity of neutron-irradiated SiC

Three kinds of commercially available SiC ceramics, hot-pressed, pressureless-sintered and reaction-bonded SiC were neutron irradiated at temperatures of 300 and 650 °C and fluences 8.0 × 10 23 and 4.3 × 10 24 n/m 2 ( E>0.18 MeV), respectively. The thermal conductivity and dimensional change were measured by isochronal annealing experiment up to 1700 °C. The change in thermal conductivity was analyzed in terms of phonon scattering by point defects. The defect concentration calculated from the thermal conductivity agreed well with that calculated from the dimensional change of the same specimen. We obtained quite good agreement between changes of thermal conductivity and swelling. Therefore, it is confirmed that the same kind of irradiation induced defects are involved in the degradation of thermal conductivity and swelling of SiC.

Tiインサート金属を用いた炭化ケイ素と無酸素銅との摩擦圧接継手の界面構造

In order to discuss the effect of a Ti intermediate layer on the strength of the friction-bonded joint of silicon carbide to copper, the microstructure of the joint interface was observed with a TEM. Specimens to be bonded were rods of pressureless-sintered SiC and oxygen-free copper. TEM observations revealed that reaction layers less than a few 10 nm thick were formed, which were identified as Cu, TiC, and Ti5Si3 on the basis of SAD pattern and EDX analyses. The Ti5Si3 layer was partly formed as discrete islands on the Cu side of the TiC layer. The Cu layer was located between SiC matrix and TiC layer, forming TiC/Cu double layers. The TiC/Cu double layers presented preferred orientation relationships with SiC, which can be expressed by the following equations.(This article is not displayable. Please see full text pdf.) \ oindentOn the other hand, the Ti5Si3 layer showed no preferred orientation relationship with SiC or Cu. In addition to these reaction layers, amorphous layers of Si oxide ∼100 nm thick were occasionally observed at the SiC/Cu interface. In the Cu-Ti mixing region adjacent to the joint interface, Cu4Ti particles ∼100 nm size in diameter were observed.

Phase Reactions and Diffusion Path of the SiC/Cr System

Solid-state bonding at pressureless-sintered SiC has been carried out using 25-μm Cr foil at temperatures from 1373 to 1773 K for 1.8 ks in vacuum. The formation of reaction phases and microstructures at the interface between SiC and Cr was investigated by X-ray diffraction and microprobe analysis. At the bonding temperature of 1373 K the cubic Cr23C6phase formed next to Cr, and the hexagonal Cr7C3 phase formed next to SiC. At 1473 K the cubic phase Cr3SiCx appeared additionally on the SiC side. At 1573 K the complete diffusion path was established. Upon increasing the joining temperature beyond 1573 K all the chromium was consumed, and Cr23C6 and Cr3SiC x dissolved. A layered structure consisting of SiC/Cr5Si3 C x /Cr7C3/Cr5Si3 C x /SiC occurred.

SiC/V接合層の構造と破断強度

Pressureless-sintered (PLS) SiC was bonded to SiC using V foils by solid state bonding at 1373∼1673 K for 1.8∼64.8 ks and 30 MPa in vacuum. The interfacial reaction layers and microstructure were investigated by electron probe microanalysis and X-ray diffractometry. At 1573 K for 7.2 ks, granular V2C and V3Si layer zone were formed on the V and SiC side, respectively. A hexagonal V5Si3Cx phase was formed at the interface between V3Si and SiC at 1573 K for 7.2-14.4 ks, and the interface structure of the joint became SiC/V5Si3Cx/V3Si/V2C+V/V. With the further bonding at 1573 K, V was completely consumed, and the interface structure of the joint became SiC/V5Si3Cx/V3Si/V2C.The fracture shear strength of the SiC/V/SiC couples bonded at 1573 K for 14.4 ks, comprized a thin layer of V5Si3Cx adjacent to SiC and showed a maximum of 130 MPa at room temperature and exhibited stable strength at the temperatures up to 1073 K.

Effect of neutron irradiation on passive oxidation of silicon carbide

Hot-pressed SiC and pressureless-sintered SiC polycrystalline specimens were neutron-irradiated in the Japan Materials Testing Reactor up to 4.5 × 10 24 n/m 2 ( E ≫ 1.0 MeV). The bulk specimens were crushed into powders to increase surface area. Weight change of the specimen and released CO 2 gas content were measured after isothermal annealing between 500 and 1300°C in oxidizing atmosphere using an electric balance and a gas chromatograph, respectively. There was no significant difference between irradiated and unirradiated specimens, for both hot-pressed and pressureless-sintered specimens. All of the weight gain data and amount of evolved CO 2 satisfied a parabolic rate law against oxidation time. Apparent activation energy of the reaction confirmed the diffusion of O 2 molecules through amorphous SiO 2 layer. However, apparent activation energy decreased by the irradiation only in pressureless-sintered SiC containing B, suggesting the effect of transmuted atoms, or Li.

ＳｉＣ／Ｔｉ接合界面の構造と強度

The solid state bonding of pressureless-sintered SiC to SiC using 20 μm Ti foil was conducted at bonding temperatures ranging form 1373 to 1673 K for 0.3 to 72 ks in vacuum. For a constant bonding time of 3.6 ks, the granular TiC next to Ti and a mixture of Ti5Si3Cx+TiC phases next to SiC were formed. Increasing the bonding temperature in 100 K intervals from 1473 K to 1773 K, the Ti5Si3Cx single phase, Ti3SiC2 phase and TiSi2 phase sequentially appeared. Fracture shear testing was used to measure bonding strength. The bonding strength was found to increase up to a maximum of 153 MPa at 1473 K. At higher temperatures it decreases to 54 MPa at 1573 K. At still higher temperatures bonding strengh again increases. The highest strengh of 250 MPa was measured at 1773 K. This variation of bonding strength with bonding temperature will be correlated with the microstructure observed at the interfase of the joints. The SiC/Ti joint with a duplex phase of Ti3SiC2+TiSiz shows the stable strength up to testing temperature 973 K.

SiC/Nb接合界面の構造と強度

Pressureless-sintered (PLS) SiC was joined to Nb by solid state bonding at 1790 K for 0.6∼144 ks and 7.26 MPa in vacuum. Interfacial reaction layers and microstructures were investigated by an electron probe microanalyser and X-ray diffractometry. The hexagonal Nb2C phase in the Nb side and the mixture of hexagonal Nb5Si3C and tetragonal Nb5Si3 phases in the interface of Nb2C and SiC were formed at 1790 K for 0.6∼7.2 ks. With increasing joining time, the Nb2C containing small amounts of NbC changed to NbC. At the long joining time of 72 ks, the hexagonal NbSi2 phase appeared at the interface between NbC adjacent to SiC and Nb5Si3C. The fracture shear strength of SiC/Nb/SiC couples showed the maximum of 187 MPa at room temperature, where a uniform thin layer of NbC at the interface adjacent SiC at 1790 K for 36 ks was formed. The couples exhibited the stable strength at temperatures up to 973 K.

Effect of BeO on the Sintering and Properties of Boron-Doped High-Purity SiC Ceramics

In order to obtain pressureless-sintered SiC ceramics with high thermal conductivity, effect of BeO addition on sintering properties boron-doped β-SiC powder was investigated. The β-SiC powder was produced by the plasma arc method from SiH4, B2H6 and CH4 gases. Pressureless-sintered boron-doped β-SiC ceramics showed high thermal conductivity of 180 W·m-1·K-1 and electrical resistivity of 1100Ω·m (10mV). On the other hand, pressureless-sintered boron-doped β-SiC ceramics containing BeO showed varistor characteristics with electrical resistivity of 1011Ω·m at 10V, and thermal conductivity of 230 W·m-1·K-1. These ceramics consisted of net-like elongated β-SiC grains. Thus, addition of BeO increased the thermal conductivity and electrical resistivity.

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using Β-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the Β-SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the Β→α phase transformation. With increasing holding time, the fraction of Β→α phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m1/2 to 8.3 MPa m1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.

Hardening by Point Defects in Neutron Irradiated AIN and SiC.

Abstract Pressureless-sintered A1N and hot-pressed, pressureless-sintered and reaction-bonded SiC were neutron irradiated at temperatures between 100 and 785°C up to a fluence of 5.2 × 1024 n/m2. The hardness was increased by up to 51% in A1N and 84% in SiC. The hardness decreased after annealing at temperatures around the irradiation temperature. At the same temperatures, the macroscopic length, which was increased by irradiation, also began to decrease. The hardness and length were almost recovered after 1,200–1,400°C annealing. Thus, hardening in irradiated A1N and SiC is controlled by the number of point defects, or, more precisely, by the strain caused by small point defect clusters which pin down dislocation movement. Dislocation loops were still observed in some samples after 1,400°C annealing while the hardness was almost recovered to that in the unirradiated state. Thus, the existence of dislocation loops is not grounds for hardening in irradiated AIN and SiC.

Effect of a silicon sintering additive on solid state bonding of SiC to Nb

Pressureless-sintered (PLS) SiC was joined to Nb by solid state bonding in a vacuum. The joining strength of the PLS SiC/Nb joint increases to the saturated value of 108 MPa with increasing joining pressure at a joining condition of 1673 K and 7.2 ks. This saturated value of PLS SiC/Nb joint is higher than that of the reaction sintered (RS) SiC/Nb joint. The strength of SiC itself affects the strength of the SiC/Nb joint. The high stability of the intermediate phase Nb5Si3 in the interface at elevated temperature leads to the high heat resistance of the joint. The thickness of the intermediate phase Nb5Si3 in the PLS SiC/Nb system is lower than that of the RS SiC/Nb system at a constant joining time, although the activation energy, 452 kJ mol−1, of growth for the phase in the PLS SiC/Nb system is almost the same as the RS SiC/Nb system, 456 kJ mol−1. The rate constant of the growth for the phase in the PLS SiC/Nb system is lower than that in the RS SiC/Nb system. The excess silicon in RS SiC promotes the formation of the Nb5Si3 phase at the interface between SiC and Nb.