SiC Specimens Research Articles

Modeling and testing miniature torsion specimens for SiC joining development studies for fusion

The international fusion community has designed a miniature torsion specimen for neutron irradiation studies of joined SiC and SiC/SiC composite materials. Miniature torsion joints based on this specimen design were fabricated using displacement reactions between Si and TiC to produce Ti3SiC2 + SiC joints with SiC and tested in torsion-shear prior to and after neutron irradiation. However, many miniature torsion specimens fail out-of-plane within the SiC specimen body, which makes it problematic to assign a shear strength value to the joints and makes it difficult to compare unirradiated and irradiated strengths to determine irradiation effects. Finite element elastic damage and elastic–plastic damage models of miniature torsion joints are developed that indicate shear fracture is more likely to occur within the body of the joined sample and cause out-of-plane failures for miniature torsion specimens when a certain modulus and strength ratio between the joint material and the joined material exists. The model results are compared and discussed with regard to unirradiated and irradiated test data for a variety of joint materials. The unirradiated data includes Ti3SiC2 + SiC/CVD-SiC joints with tailored joint moduli, and includes steel/epoxy and CVD-SiC/epoxy joints. The implications for joint data based on this sample design are discussed.

High temperature image correction in DIC measurement due to thermal radiation

In this article, the basic principle of high temperature image correction due to thermal radiation and its application in DIC high temperature measurement are investigated. First, the basic principle of high temperature image and the image gray level model due to thermal radiation are analyzed. Second, the high temperature image correction method in DIC measurement is proposed and the relationship between the gray level of high temperature image and the thermal radiation is established. Finally, the simulated high temperature image is corrected to validate the effectiveness of eliminating the thermal radiation effect in high temperature DIC, and a high temperature DIC experiment using a SiC specimen is performed to extract the high temperature displacement with high accuracy.

Fabrication and plasma resistance of Y2O3 ceramics

Y2O3 granular powder was fabricated using a spray drying process from slurry prepared by mixing PVA binder, NaOH for pH control, PEG and milled Y2O3 powder. The Y2O3 specimen was sized to ∅14mm and then sintered at 1650°C. The decrease in particle size due to the milling process with attrition mill and granulation of milled powder promoted the densification of the Y2O3 specimen without cracks. The characteristics, microstructure, density and plasma resistance of the Y2O3 specimen were investigated as functions of the forming pressure and sintering time. The plasma resistance and the edge depth of the Y2O3 specimens were compared with those of Al2O3 and SiC specimens using a laser microscope. After a plasma etching test, the Al2O3 specimen showed a very high edge depth value and a severely etched morphology in laser microscope images. Upon increasing the sintering time, the Y2O3 specimens showed relatively high density and strong resistance compared with those of the Al2O3 and SiC specimens in the plasma etching test.

Liquid polycarbosilane-derived C/C–SiC composites with improved mechanical strength for high temperature applications

In the present study, liquid polycarbosilane (LPCS) has been synthesized in the form of low viscous resin, by the heat-treatment of polydimethylsilane (PDMS) in an autoclave at 400°C for 5h. The spectral and thermal characterizations carried out at different processing stages indicate the formation of polycarbosilane with high Si–H content. LPCS gave ceramic residue of 34% at 900°C in argon atmosphere. XRD pattern of 1500°C heat-treated LPCS, indicates the formation of silicon carbide. LPCS was used as infiltrating resin for the preparation of C/C–SiC composite without any interfacial coating over the T-300 carbon fabric reinforcement. Flexural strength value of 64MPa for C/C–SiC specimen with density of 1.36g/cm3 was obtained after six infiltration and pyrolysis cycles. Fractured surface was characterized by SEM which indicates the formation of nano- to submicron wires in the pores of the hybrid composite.

Effects of oxidation of SiC aggregates on the microstructure and properties of bauxite–SiC composite refractories

Bauxite–SiC composite refractories are widely used in the transition zone of cement rotary kilns, whose properties (e.g. strength, abrasion and thermal shock resistance) are mainly associated with the nature of the interfacial bonding between SiC and the matrix of refractories. In the present paper, the effects of oxidation of SiC aggregates on the microstructure, and properties of bauxite–SiC composite refractories during the firing process were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) as well as the thermodynamics analysis. The results show that the oxidation of SiC aggregates is closely associated with oxygen partial pressure in preparation of bauxite–SiC specimens, and can markedly affect the microstructure and properties of bauxite–SiC refractories. The active oxidation of SiC aggregates takes place when the oxygen partial pressure is lower than 10−15atm at 1500°C. Consequently, large interstices between SiC aggregates and matrix of bauxite–SiC specimens occur. However, strong interfacial bonding between SiC aggregates and matrix can be produced as Si, SiC powder and their combination are added. As a result, the strength of bauxite–SiC specimens could be greatly improved.

Densification behavior of high purity SiC by hot pressing

Densification behavior of 99.998% purity SiC by hot pressing without metallic additives was investigated. A theoretical densification of 92% was achieved at 2350°C with 50MPa in 3h. The densification was more affected by the sintering pressure rather than by the sintering temperature and was independent of initial particle size. Data were interpreted according to the power-law creep model. The β-SiC phase was preserved in the high purity, hot-pressed SiC specimen without β→α phase transformation even at 2350°C.

Swelling of ion-irradiated 3C–SiC characterized by synchrotron radiation based XRD and TEM

An experimental technique was established to characterize irradiation-induced volume swelling through a combined utilization of synchrotron radiation-based X-ray diffraction (XRD) and transmission electron microscopy (TEM). 3C–SiC specimens were irradiated by Si2+ ions (5MeV) with fluences up to 5×1017ion/cm2 at 1000°C. In order to avoid the accumulation of implanted Si ions in the SiC layer, specific thicknesses of the epitaxy layer and implanted ion energy were chosen. Unresolvable black spot defects were studied by TEM, and the average size and density were calculated. XRD radial scan results of surface (002), (111), (022), (113), and (200) including peak shift and asymmetry peak broadening were observed. Different interplanar spacing information of single crystal SiC can be obtained from this XRD measurement method, making it possible to investigate the lattice expansion and volume swelling more precisely. While TEM provided a direct visualization of the microstructures and the interplanar spacing was measured from HRTEM images. It is suggested that irradiation induced point defects and compressive stress from the Si substrate were the cause of anisotropic (a=b<c) volume swelling of irradiated 3C–SiC in this study.

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y2O3 addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 101 Ω·cm) that is larger by a factor of ~102 compared to that (1.3 × 10−1 Ω·cm) of a baseline specimen sintered with Y2O3 only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10−1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10−2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.

A Finite Fracture Approach for Determining the Fracture Onset of a Brazed SiC Specimen

The failure initiation of a brazed sample made of silicon carbide substrates and submitted to bending is analyzed with the help of a criterion which combines a maximum incremental energy release rate and a maximum tensile stress conditions. Two different modes of cracking are considered to develop in the vicinity of the free edge between the brazed layer and the ceramic substrate: edge debonding and substrate cracking. The comparison of the predictions with the experimental results allows estimating the fracture properties of the bonding.

Reactive sintering of tungsten-doped high strength ZrB2–SiC porous ceramics using metastable precursors

Tungsten-doped porous ZrB2-20vol.% SiC specimens with a high strength skeleton were fabricated by reactive sintering using metastable reactants, i.e., as-synthesized amorphous hydrous ZrO2–WO3/ZrO2, amorphous boron, nanocarbon, and β-SiC. Doping with tungsten clearly promoted sintering and improved mechanical properties; the optimum tungsten concentration to obtain high mechanical properties was investigated. A single-phase solid solution (Zr,W)B2 was formed in the specimens with tungsten. We suggest that sintering was promoted by self-accelerated diffusion owing to the formation of point defects caused by doping with tungsten. The improvement of mechanical properties could be attributed to solid solution strengthening.

Long-term corrosion behavior of CVD SiC in 360 °C water and 400 °C steam

Long-term corrosion tests were performed using chemical vapor deposition (CVD) SiC specimens in 360°C water and 400°C steam to evaluate the corrosion behavior of SiC cladding under normal operation conditions. The CVD SiC specimens exhibited weight loss caused by dissolution in both 360°C water and 400°C steam. For the corrosion test in 360°C water the dissolution rate was much higher in the static autoclave, where the dissolved oxygen content was higher, than in the loops, where the dissolved oxygen content was maintained at approximately 5ppb. The grain boundaries of the SiC dissolved preferentially during the early stage of corrosion, and the grains became thinner and detached from the surface, thereby leading to an acceleration of the weight loss. The pre-oxide was not able to protect the SiC from dissolving in water, even though it was effective, to a certain extent, at mitigating the dissolution of the SiC. In the corrosion test in 400°C steam, the weight loss was significant from the early stage of corrosion. No acceleration in the corrosion rate was observed in 400°C steam for up to 135days, and the weight loss at 90days was similar to that obtained from the 360°C water autoclave test.

Design and manufacture of an oven for high temperature experiments of erosion–corrosion of SiCf/SiC into LiPb

SiC composites are studied as structural and functional materials in the designs of DEMO liquid blankets and their applications are foreseen also for the test blanket modules of ITER. Among the chemical–physical characteristics to be studied, the assessment of the erosion and corrosion of the SiC composites in liquid blanket (lithium–lead) is of main importance in order to verify the stability of these materials under the operating conditions typical of fusion power reactors.In the framework of the Broader Approach research activities, an experiment with a relative velocity between the SiC composite specimen and the lithium–lead of 0.1–0.5ms−1 at temperature of 1000°C has been planned. The experimental setup mainly consists of a high temperature oven wherein the SiC specimen is rotating into a lithium–lead bath. The experimental plan foresees the chemical analysis of the lithium–lead before and after exposure and the post exposure examination of the SiC composite samples in order to study the behavior of both fibers and interphase.The design of the experimental device has considered the compatibility of the materials used at high temperature: in this view, a preliminary test campaign has been carried out at ENEA laboratories on SiCf–SiC samples obtained via chemical vapor infiltration (CVI) of Tyranno SA fibers.In this paper, the results of the preliminary tests and the design of the experimental apparatus are described in details as well as the status of its construction.

Temperature Dependence of Electrical Resistivity (4–300 K) in Aluminum‐ and Boron‐Doped SiC Ceramics

Al‐ and B‐doped 3C–SiC ceramics were prepared by hot‐pressing powder compacts containing submicrometer‐sized β‐SiC, precursors of 5 wt% nanosized β‐SiC, and an optional additive (Al or B) in an Ar atmosphere. Electron probe microanalysis (EPMA) investigation on the obtained specimens revealed that a portion of the doped Al and B atoms substituted the zinc blende lattice sites. The temperature‐dependent electrical resistivity data of the Al‐ and B‐doped SiC specimens were measured in the 4–300 K range and compared with those of an undoped specimen. The Al‐ and B‐doped SiC specimens exhibited resistivities that were as high as ~103 Ω cm at room temperature and ~105 and ~104 Ω cm, respectively, below 100 K. These values are larger than those of the undoped SiC specimen by a factor of ~104. Such high resistivities of the impurity‐doped specimens are attributable to the carrier compensation by the Al‐ and B‐derived acceptors located well above the valence‐band edge of 3C–SiC. Photoluminescence investigation revealed that the Al‐ and B‐doped specimens exhibited emission profile below 2 eV, implying the existence of the acceptors.

Generalized Wien’s Displacement Law in Determining the True Temperature of $$\text{ ZrB }_{2}$$ ZrB 2 –SiC-Based Ultrahigh-Temperature Ceramic: Thermodynamics of Thermal Radiation

The temperature dependence of the generalized Wien displacement law is investigated. For determining the true temperature of a \(\text{ ZrB }_{2}\)–SiC-based ultrahigh-temperature ceramic, the experimental values of the position of the maximum of the spectral density power are needed. Thermodynamics of the thermal radiation of \(\text{ ZrB }_{2}\)–SiC is constructed by using the temperature dependence of the generalized Stefan–Boltzmann law. The calculated values of the normal total emissivity for \(\text{ ZrB }_{2}\)–SiC at different temperatures are in good agreement with experimental data. The total radiation power emitted from a surface of \(\text{ ZrB }_{2}\)–SiC specimens at different temperatures is calculated. The temperature dependences of the Helmholtz free energy, entropy, heat capacity at constant volume, pressure, enthalpy, and internal energy of the thermal radiation of \(\text{ ZrB }_{2}\)–SiC are obtained. For determining the true temperature, experimental values of either the normal total emissivity or the normal total energy density are needed. The uncertainty in the determination of the true temperature is no greater than 1 %. A new universality class of bodies with a new relationship between the temperature \(T\) and the position of the spectral energy density maximum is established.

PENETRATION ABILITY OF ABRASIVE WATERJETS IN CUTTING OF ALUMINUM-SILICON CARBIDE PARTICULATE METAL MATRIX COMPOSITES

This article presents a set of studies performed on aluminum-silicon carbide particulate metal matrix composites prepared by adding 5, 10, 15 and 20% of SiC in aluminum alloy and processed with abrasive water jets that are formed with garnet and silicon carbide abrasives of 80 mesh size. These studies are essentially meant to assess the penetration ability of abrasive water jets on different compositions of Al-SiC p MMCs produced by stir casting method. Abrasive waterjet cutting experiments were conducted on trapezoidal shaped specimens of different composites as well on the constituent materials i.e., 100% aluminum alloy and 100% SiC specimens by varying water pressure, jet traverse speed and abrasive mass flow rate, each at three different levels. The percentage contribution of individual and combined effects of process parameters on penetration ability was analyzed by means of analysis of variance. Contribution of waterjet pressure and traverse speed on jet penetration in these meaterials are found to be more than abrasive flow rate. Among the interaction effects, waterjet pressure and jet traverse speed combinations contribute more to jet penetration. The results presented in this study can be used to build statistical models that can predict the depth of penetration of jet in different MMCs. This study also highlights the need to choose suitable abrasive mass flow rates and jet traverse speeds for effective processing of MMCs with abrasive water jets.

The Pyrolysis Processing of Polycarbosilane Studied by TG, XRF, IR, XRD, and XPS

Polycarbosilane (PCS) is one of precursor systems applied in industry for the Polymer Infiltration Pyrolysis (PIP) processing of C or SiC fiber reinforced SiC matrix composites (Cf/SiC or SiCf/SiC CMC) materials. In this work, a series of PCS-derived SiC specimens treated at 300, 550, 800, and 1100 °C was studied by XRF, IR, XRD and XPS methods. The results show that a slight amount of PCS molecules with low softening point evaporates below 300 °C; the cross-link between PCS chains takes place to form a 3D -Si-C-Si- network via the reaction between -Si-CH3, -Si-CH2-Si- and -Si-H groups in the temperature range of 300-550 °C; an amorphous phase of SiC was observed when PCS was heated up to 800 °C, which indicates the completion of transformation from organic to inorganic; above 800 °C, β-SiC domain grows. Furthermore, a surface structure evolution of PCS-derived SiC specimens with temperature is proposed after the analyzing the XPS spectra.

Study the Formation Mechanism of Silicon Carbide Polytype by Silicon Carbide Nanobelts Sintered Under High Pressure

In this paper, in order to reveal the formation mechanism of SiC polytype, four SiC specimens sintered under high pressure has been investigated, after being prepared from SiC nanobelts as initial powders. The structure and morphology variation dependence of SiC specimens with temperature and pressure was studied based on experimental data obtained by XRD, SEM, and Raman. The results show that SiC lattice structure and the crystallite size are greatly affected by pressure between 2 and 4 GPa under different sintering temperatures of 800 and 1200 degrees C. At the largest applied pressure and temperature, 4 GPa and 1200 degrees C, 3C-SiC crystal structure can be changed into to R-SiC due to the stress resulted in dislocations instead of planar defects. Based on our results, the multiquantum-well structure based a single one-dimensional nanostructure can be achieved by applying high pressure at certain sintered temperature.

Dislocation microstructure of 4H–SiC single crystals plastically deformed around the transition temperature

4H–SiC specimens were plastically deformed by basal slip at temperatures between 800 °C and 1300 °C. Samples were investigated by means of transmission electron microscopy and high-resolution techniques. The accepted transition temperature (1030 °C) was found not to be actually so well defined since the two mechanisms were operating together between 1000 °C and 1100 °C. Dissociation of basal dislocations takes place over the entire temperature range investigated, having a different influence on each regime. In the high temperature regime, after dissociation the two partials slip together in the basal plane fringing a stacking fault. We have determined the dissociation width, obtaining a stacking fault energy of 20 ± 5 mJ/m 2 . However, below the transition temperature the difference in mobility of the partials and the low stacking fault energy allow the leading partial to glide alone. We discuss the consequences of this finding for the crystal structure (cubic bands nucleation) and mechanical behaviour (high work-hardening rate).

Concentric ring on ring test for unirradiated and irradiated miniature SiC specimens

The flexure strength of miniature disk specimens was evaluated for both the unirradiated and irradiated CVD SiC by equibiaxial flexural test developed in this work. The results for the unirradiated specimens indicated no stress magnification at the loading point, which is often concerned in the biaxial tests for the disk specimens. Although the strength was retained after the irradiation at 1100 and 1300°C, ∼20% of reduction was observed for the samples irradiated at 1500°C. It is clearly seen that the smooth cleavage of large grains were frequently observed in the sample irradiated at 1500°C comparing to specimens irradiated at lower temperatures. A substantially lower population of finer defect clusters such as loops, vacancy, and vacancy clusters may be attributed to the inhibition of the strengthening at the higher irradiation temperatures.

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.