Polycrystalline 3C-SiC Research Articles

Molecular dynamics analysis of subsurface brittleness mechanism of nanocrystalline 3C-SiC rough friction surface

To study the effect of polycrystalline 3C-SiC rough friction surface on the mechanism of subsurface brittleness during nanocrystalline grinding. Initial grinding models of polycrystalline 3C-SiC and diamond abrasive grains on rough friction surfaces are developed using molecular dynamics methods and the Voronoi method for constructing polycrystalline abrasive grains. The processing mechanism of 3C-SiC is analyzed by post-processing methods such as dislocation defect analysis, atomic arrangement analysis and stress analysis. At 2.6 nm, "stress concentration" occurs between the abrasive particles and the workpiece, forming irregular force shapes. The larger the grain size, the smaller the crystal hardness, the greater the possibility of crystal fracture, and it is obvious in the crystal of larger grains. At 8 nm, the crystal breaks and creates vacancies. The roughness of the polycrystalline 3C-SiC friction surface and the cross-cutting mechanism between grains with grain boundaries are found to be effective in ameliorating the damage in the subsurface layer.

Grain boundary effects on chemical disorders and amorphization-induced swelling in 3C-SiC under high-temperature irradiation: From atomic simulation insight

High-temperature irradiation-induced amorphization in SiC is a crucial factor leading to swelling. This study employs atomistic simulations to clarify the effects of experimental irradiation damage (1.0 dpa) in polycrystalline 3C-SiC at 1000 K. The findings reveal amorphization and swelling thresholds at 0.45 dpa and 7.45 %, respectively. Interestingly, dose threshold for amorphization does not align with chemical bonding, volume swelling, or enthalpy thresholds. Instead, a competitive mechanism emerges between grain boundaries and the grain core in the evolution of chemical disorder and clusters. Moreover, as amorphization saturates and grain boundaries annihilate, the degradation of tensile strength and Young's modulus stabilizes at 21.1 GPa and 124.2 GPa, respectively. Notably, grain boundaries exert a significant influence on defect cluster formation during amorphous evolution and dominate the initial deterioration of mechanical properties at low damage doses. This study enhances our understanding of how grain boundaries and chemical disorder mechanisms impact amorphization SiC.

Impact of alternating precursor supply and gas flow on the LPCVD growth behavior of polycrystalline 3C-SiC thin films on Si

In this paper, we will provide information on the growth mechanism of 3C-SiC using alternating supply deposition (ASD). SiH4 and C3H8 were introduced successively in a low-pressure chemical vapor deposition (LPCVD) system at 1000 °C, forming a stoichiometric polycrystalline 3C-SiC thin film on Si substrates. We demonstrate the influence of different gas flow rates on thin film properties. In detail, altering the flow rates of the precursors and the carrier gas resulted in changes of the growth rate, the surface roughness, the crystal growth mechanisms as well as the crystal quality. Hydrogen inhibition and passivation are proposed to be the main effects influencing the growth behavior of ASD thin films. Applying ASD to grow 3C-SiC thin films with precursors from low to high flow rates resulted in a transition of different growth mechanisms influencing the surface roughness and grain size. Furthermore, we could prove that ASD is a cyclic step-by-step carbon-assisted redistribution of a prior deposited silicon film. This is shown by contact angle measurements of three differently terminated surfaces during the specific phases of one ASD cycle. High resolution transmission electron microscopy (HRTEM) analysis showed a continuous and homogenous thin film and confirmed the absence of silicon- or carbon-rich layers. ASD enables 3C-SiC thin films with customized properties for tailored micro electromechanical system (MEMS) applications.

Evidence of twin mediated growth in the CVD of polycrystalline silicon carbide

The origin of the 〈110〉 texture in polycrystalline 3C-SiC grown by chemical vapor deposition at different temperatures is investigated by thorough EBSD, SEM and XRD characterizations. Regarding the 〈110〉 orientation, although XRD suggests a fiber texture, closer inspection of isolated clusters of grains by EBSD reveals a high symmetrical order in-plane. The pole figures associated with these clusters are in good agreement with cross-twinned pentagonal structures in which each grain shares a common 〈110〉 axis. These structures form via twin-mediated growth involving the formation of twin plane reentrant edges that act as a preferential site for the incorporation of adatoms. A deviation from a perfect cross-twinned structure is observed and is attributed to the presence of additional parallel twins. In this study, the transition to the 〈110〉 orientation occurs when the temperature increases and the partial pressure of reactive species decreases. This can be explained by the enhancement of surface mobility that promotes surface diffusion to the twin plane reentrant edges, thus activating the twin mediated growth mechanism.

Analysis of crystal structure transition of polycrystalline 3C-SiC in nanocrystalline grinding based on molecular dynamics simulation

In order to study the grain/grain boundary crossing mechanism of polycrystalline 3C-SiC materials under nano-grinding, molecular dynamics(MD) method was used to simulate the grinding process of polycrystalline 3C-SiC materials under diamond abrasive particles. Through the coupling method of Voronoi construction of polycrystalline 3C-SiC workpiece model and MD construction process, the nano-grinding process of polycrystalline 3C-SiC material was established, the potential function field parameters were optimized, and the residual stress in polycrystalline 3C-SiC was eliminated. The energy transition equation is modified to reduce the time for the system to enter dynamic equilibrium. The crystal structure and dislocation generation of polycrystalline 3C-SiC were analyzed by post-processing methods such as defect analysis and crystal structure recognition. The results found that: there are cubic lattice, simple cubic lattice, hexagonal lattice and graphene mechanism type atoms at the grain boundary. During the machining process, the cubic lattice type atoms are converted to other types atoms, and other types structure atoms are converted to each other. The internal structure atoms of grains are converted to the grain boundary structure atoms during the machining process, and the crystal atoms near the grain boundary have low structural stability. The grinding force mainly acts on the forward direction and just below the grinding grain, and also has a small amount of distribution at the grain boundary, but it is relatively weak. The dislocation caused by grinding mainly occurs at grain boundaries. The resulting debris was monolithic, and a few were granular. The interconversion between grains and different structural atoms at the grain boundary is mainly from the crystal lattice atoms in grains to other atoms at the grain boundary. The force is transferred to the grain through the grain boundary, which hinders the transboundary transfer of force and protects the grains. The shearing force plays a dominant role in the grinding process.

Thermal softening-suppressed inter-granular embrittlement of polycrystalline 3C-SiC under diamond cutting

While the deformation behavior of grain boundaries has a strong impact on the mechanical response of polycrystalline materials, investigating the coupled thermal–mechanical properties of grain boundaries in their real formats is crucial for enhancing the ductile machinability of hard brittle polycrystalline ceramic materials. In the present work, we report the thermal softening-suppressed inter-granular fracture, accompanied with enhanced stacking fault formation, increased healing ability of grain boundaries and promoted ductile material removal, in diamond cutting of polycrystalline 3C-SiC at elevated temperatures by multi-scale simulations. Molecular dynamics simulations and experiments of high temperature nanoindentations are performed to derive the temperature-dependent mechanical properties of bulk polycrystalline 3C-SiC. Furthermore, molecular dynamics simulations of Mode I (tension) and Mode II (shearing) loading of a Σ9<110>{122} symmetric tilt grain boundary at elevated temperatures are performed to extract the temperature-dependent mechanical properties of grain boundaries in polycrystalline 3C-SiC. A novel finite element model of heat treatment-assisted diamond cutting of polycrystalline 3C-SiC with the comprehensive consideration of thermal properties of both grains and grain boundaries at elevated temperatures is established to reveal the transition of material removal mode, grain boundary failure behavior, cutting force characteristics and chip profile with temperature.

Formation of high density stacking faults in polycrystalline 3C-SiC by vibration-assisted diamond cutting

Revealing the ductile deformation mechanisms of ultra-hard brittle cubic silicon carbide (3C-SiC), as well as their correlations with microstructure evolution, are crucial for facilitating the ductile machinability of the ceramic material. In the present work, we report the formation of highly oriented high density stacking faults accompanied with suppressed amorphization and cracking in polycrystalline 3C-SiC in ultrasonic elliptical vibration-assisted diamond cutting, which contributes to significantly enhanced ductile material removal of the ceramic material compared to ordinary cutting. Specifically, characterizations of Raman spectroscopy on machined surface and cross-sectional transmission electron microscopy on subsurface, as well as molecular dynamics simulations of the two kinds of cutting processes, are jointly performed to elucidate the mechanisms of phase transformation and microstructure evolution that govern the ductile material removal of polycrystalline 3C-SiC under the vibration assistance. In particular, the formation mechanisms of highly oriented high density stacking faults emitted from grain boundaries are revealed. Current findings provide insights into the ductile deformation behavior of hard brittle ceramics enhanced by field-assisted deformation process.

Influence of Supersaturation on Growth Behavior and Mechanical Properties of Polycrystalline 3C-SiC on W Wire Substrate

As an important reinforcement for metal matrix composites, the microstructure and mechanical properties of W-core SiC filament have drawn increasing attentions among researchers. In this work, the growth behavior of polycrystalline 3C-SiC on W-wire substrate in the chemical vapor deposition (CVD) process and the evolution of mechanical properties in preparation of W-core SiC filament, were investigated as a function of gas-phase supersaturation. Kinetic studies revealed that the growth of 3C-SiC grains was limited by surface reactions at both 850 °C and 1050 °C, and the deposit experienced similar morphological changes from a porous structure to large clusters, with the increase in supersaturation. Structural analyses and mechanical tests show that the production of pores and the amorphous phase with a low supersaturation, of 9.6 × 107 at 850 °C, resulted in a reduction in the modulus and hardness of the polycrystalline deposits, to 270.3 GPa and 33.9 GPa, while the reduced structural defects (e.g., stacking faults and twins) in highly (111) orientated 3C-SiC grains, as well as the improved surface quality obtained with the medium supersaturation of 1.6 × 107 at 1050 °C, enhanced the tensile strength and the Weibull modulus of W-core SiC filament to 2.88 GPa and 11.2, respectively. During the growth of 3C-SiC grains, the variation in structural defects density is controlled by the critical nucleation energy of the two-dimensional (2D) nucleus.

Observations of He platelets during He ion irradiation in 3C SiC

Polycrystalline 3C SiC was irradiated and observed in-situ via Transmission Electron Microscopy with a 20 keV He ion beam at 400, 800, 1000 and 1200 °C at the Microscopes and Ion Accelerators for Materials Investigations facility. During the 400, 800, 1000 and 1200 °C irradiations, black-spot damage was observed at 3.1, 1.1, 2.1 and 2.1 dpa respectively. Helium bubbles were observed after 6.3 dpa at 400 °C and 2.1 dpa at 800 °C, and He platelets were seen after 1.1 dpa at 800, 1000 and 1200 °C but not observed during the 400 °C irradiation. This work shows for the first time, the preferential nucleation of platelets within stacking faults in 3C SiC. The dependence of He platelet diameter with temperature and dose has also been observed.

Atomistic origin of brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting

The machinability of hard brittle polycrystalline ceramic has a strong correlation with internal microstructures and their accommodated deformation behavior. In the present work, we investigate the mechanisms governing the brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting by means of molecular dynamics simulations. Simulation results reveal the co-existence of dislocation slip and amorphization-dominated ductile deformation and cracking along grain boundaries-mediated brittle fracture, as well as the correlation of individual deformation modes with machining force variation and machined surface morphology. In addition, inter-granular fracture, grain boundary sliding and grain pull-up are also operating brittle deformation modes of polycrystalline 3C–SiC. The strong competition between above heterogeneous deformation modes determines the brittle-to-ductile transition behavior in grooving of polycrystalline 3C–SiC. Simulation results also demonstrate that grain size has a strong impact on the brittle-to-ductile transition and material deformation behavior of polycrystalline 3C–SiC under diamond cutting.

Crystal Growth of Epitaxial 3C-SiC Thin Film on Si Substrate by Chemical Vapor Deposition using Single Precursor of Vinylsilane

Cubic silicon carbide (3C-SiC) has attracted much attention since it has a wide bandgap, high dielectric breakdown strength, and high thermal stability. 3C-SiC promises various electronic applications such as power devices and MEMS. Also, 3C-SiC has an advantage for a lower cost among many SiC polytypes, as it can grow epitaxially on Si substrate. Generally, two types of precursor source materials, including Si and C, are necessary to grow 3C-SiC thin films for chemical vapor deposition (CVD) method. In addition, complex processes including carbonization with a temperature as high as about 1200 °C are required for conventional epitaxial growth of 3C-SiC [1]. We focus on vinylsilane as a single precursor material for SiC growth. The Si-C bond in vinylsilane is relatively stable due to the relationship of intramolecular bond energy, that is appreciable for SiC growth. The growth of SiC crystals using vinylsilane can be achieved at a low temperature of 1000 ºC or less and the deposition process will be simplified. Recently, we have achieved the formation of polycrystalline SiC thin films on metal substrate using CVD method only with vinylsilane [2]. In this study, we examine the epitaxial growth of 3C-SiC and also report phosphorous (P) doping for SiC thin films grown on Si substrate.SiC thin films were deposited on Si(001) and Si(111) substrates using CVD method with a vinylsilane precursor (manufactured by Japan Advanced Chemicals Co., Ltd.). The substrate temperature of the growth was ranging from 900 to 1000 °C, the growth pressure was 17 kPa, and the growth time was 60 min.The surface morphology of deposited film was observed using scanning electron microscopy (SEM). We can confirm that the uniform morphology of SiC film was formed on the Si substrate for a sample prepared with Si(111) substrate at 1000 ºC and also that there are no voids or defects at the interface.We also examined X-ray diffraction measurements to characterize the crystalline structure. Out-of-plane XRD measurement (2θ/ω method) shows a sufficiently large diffraction peak related to 3C-SiC(111) for the Si(111) sample prepared at 1000 ºC, which means the growth of a well (111)-oriented 3C-SiC thin film on Si(111) substrate. On the other hand, in the case of the grazing angle XRD measurement (2θ method) for the SiC/Si(001) samples prepared at both 900 and 1000 ºC, we observed many diffraction peaks meaning the formation of polycrystalline 3C-SiC. The analysis of peak intensity and full width at half maximum value (FWHM) revealed that the crystallinity of SiC film improved with increasing the growth temperature. Raman scattering spectroscopy measurement also revealed that the dominant bonding structure in the film is Si-C and not C-Hn or C-C for samples prepared at higher than 900 ºC, that means the formation of a high crystalline quality SiC thin film.In our presentation, we will discuss pros and cons of epitaxial growth of SiC films on Si(111) substrates and also reports P-doping into the SiC films will be discussed.

Crystal Growth of Epitaxial 3C-SiC Thin Film on Si Substrate by Chemical Vapor Deposition using Single Precursor of Vinylsilane

We have examined the formation of 3C SiC thin films on Si substrates by chemical vapor deposition (CVD) method using vinysilane as a single precursor. The epitaxial 3C-SiC film was successfully grown on Si(111) substrate at a lower temperature of 1000 °C. In addition, in-situ phosphorus doping for polycrystalline 3C-SiC was achieved using tri-ethyl-phosphorus as a dopant source. The active dopant concentration as high as 8.2×1019 cm−3 and the resistivity as low as 1.9×10−2 Ωcm was obtained. This resistivity value is comparable to the epitaxial 3C-SiC film formed on Si substrate ever reported at the same dopant concentration region. The fact that resistivity as low as single crystalline films can be obtained at a low temperature of 900°C promises that CVD-grown 3C-SiC films using a single precursor of vinylsilane is a suitable material for a wide range of applications.

Microstructure and texture of polycrystalline 3C–SiC thick films characterized via EBSD

Cubic silicon carbide (3C–SiC) is an excellent protective film on graphite and has been fabricated via laser chemical vapor deposition (LCVD) with an extremely high deposition rate by our research group. To understand comprehensively its growth behavior, the polycrystalline 3C–SiC thick films with the preferred orientation of <111> and <110> were characterized by diverse measurements, especially electron back-scatter diffraction (EBSD). Along the growth direction of the <110>-oriented 3C–SiC, the microstructure changed from equiaxed grains to elongated grains with <111> orientation, and eventually the <110>-oriented columnar grains. The stacking faults in the <110>-oriented 3C–SiC could be marked as <11–20>-oriented 6H–SiC. On the other hand, in the <111>-oriented 3C–SiC films, the microstructure changed from mainly equiaxed grains to large columnar grains. The high-density stacking faults in <111>-oriented 3C–SiC films may lead to the identification of the nominal 2H, 4H and 6H polytypes by Raman spectra and EBSD. The (0001) planes of 2H-, 4H–SiC are perpendicular to the growth direction, while that of 6H–SiC are parallel.

Investigation of the structure and chemical nature of Pd fission product agglomerations in irradiated TRISO particle SiC

Tristructural-isotropic (TRISO)-coated fuel particles are used in high-temperature gas-cooled nuclear reactors. Although the polycrystalline 3C–SiC layer acts as the main barrier to fission product release, post-irradiation examinations have shown that certain metallic fission products are found on the outside of the TRISO coated particle, with no observable evidence of micro-cracks or mechanical failure. In this study, an atomic resolution transmission electron microscopy investigation of a SiC layer from a neutron irradiated (19.38% fissions per initial metal atom average burnup; 1072 °C time-averaged temperature) TRISO-coated particle from the first fuel irradiation experiment as part of the US DOE Advanced Gas Reactor Fuel Qualification and Development (AGR) Program, was conducted. The fission product Pd was found to be present at dislocation cores associated with twins, stacking faults, and their intersections as atomic scale silicide inclusions. EELS elemental quantification of the silicide region showed a reduction of C and increase of Si in the Pd containing inclusion. Pd was also observed as distributed along grain boundaries down to the atomic scale. The local corrosion and dissociation of C from Pd containing grain boundary agglomerations was observed at the atomic level. Elemental quantification of atomically dispersed Pd grain boundary regions using EELS spectrum imaging similarly showed a measurable decrease in C concentration at Pd regions and an increase in Si concentration.

Migration behaviour of selenium implanted into polycrystalline 3C–SiC

The migration behaviour of selenium (Se) implanted into polycrystalline SiC was investigated using Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and Raman spectroscopy. Se ions of 200 keV were implanted into polycrystalline SiC samples to a fluence of 1 × 1016 cm−2 at room temperature. The implanted samples were isochronal annealed in vacuum at two different regimes. Initially, the samples were annealed from 1000 to 1500 °C in steps of 100 °C for 10 h. Another set, at 1300 °C, 1350 °C and 1450 °C for 20 h. Implantation of Se at room temperature amorphized the near-surface region of the SiC substrate. Annealing at 1000 °C already resulted in the recrystallization of the amorphous SiC. The SEM images showed that the polishing marks became less visible after implantation due to the sputtering and swelling effects. These crystallites became larger and more visible with increase in annealing temperature. No diffusion was observed for annealing up to 1200 °C. Slight peak broadening indicating diffusion was observed after annealing at 1300 °C. This broadening increased with increase in annealing temperature. In the second set of annealing, the diffusion coefficients at 1300 and 1350 °C were estimated to be 6.3 × 10−21 and 2.0 × 10−20 m2 s−1, respectively.

Impact of the dangling bond defects and grain boundaries on trapping recombination process in polycrystalline 3C SiC

The bulk polycrystalline (pc) 3C SiC of n- and p-type obtained by thermal decomposition of methyltrichlorosilane vapor have been studied by applying the electron paramagnetic resonance (EPR), direct current (DC) conductivity, photoconductivity (PC) and PC time-resolved decay methods. The energy level of the two donor-like minority carrier traps at ε1 = 77 meV and ε2 = 92.6 meV located at the grain boundaries (GB) of pc-3C SiC has been obtained using the measurements of the temperature dependence of DC conductivity and PC in the range of 80–600 K. The minority carrier traps assigned to carbon and silicon dangling bonds with the carbon back bonds were observed in the EPR spectra of pc-3C SiC of n- and p-type at g = 2.0029(3), g = 20042(3), respectively. The PC time decay after the termination of the photo-excitation was studied in monocrystalline and pc-3C SiC of n- and p-type at 80 K. The persistent relaxation of PC has been described by kinetic equations accounting the trapping, ionization, and recombination processes of non-equilibrium charge carriers bound dynamically to shallow donors and acceptors. We have concluded that the main process responsible for the long-lived relaxation of the PC is trap-assisted electron-hole recombination in n-type pc-3C SiC and ionization of boron acceptors, as well as the hole escape/capture at the boron level in p-type pc-3C SiC. The differences in the relaxation process of the PC in n- and p-type pc-3C SiC were explained by the presence of the potential barrier height of about 8.6 meV at GB for the capture of the majority carriers in p-type pc-3C SiC.

The effect of grain-size on fracture of polycrystalline silicon carbide: A multiscale analysis using a molecular dynamics-peridynamics framework

A robust atomistic to mesoscale computational multiscale/multiphysics modeling framework that explicitly takes into account atomic-scale descriptions of grain-boundaries, is implemented to examine the interplay between grain-size and fracture of polycrystalline cubic silicon carbide (3C-SiC). A salient feature of the developed framework is the establishment of scale-parity between the chosen atomistic and the mesoscale methods namely molecular dynamics (MD) and peridynamics (PD) respectively, which enables the ability to model the effect of the underlying microstructure as well as obtain relevant new insights into the role of grain-size on the ensuing mechanical response of 3C-SiC. Material properties such as elastic modulus, and fracture toughness of single crystals and bicrystals of various orientations are obtained from MD simulations, and using appropriate statistical analysis, MD derived properties are interfaced with PD simulations, resulting in mesoscale simulations that accurately predict the role of grain-size on failure strength, fracture energy, elastic modulus, fracture toughness, and tensile toughness of polycrystalline 3C-SiC. In particular, it is seen that the fracture strength follows a Hall-Petch law with respect to grain-size variations, while mode-I fracture toughness increases with increasing grain-size, consistent with available literature on brittle fracture of polycrystalline materials. Equally importantly, the developed MD-PD multiscale/multiphysics framework represents an important step towards developing materials modeling paradigms that can provide a comprehensive and predictive description of the microstructure-property-performance interplay in solid-state materials.

Mesoporous SiC with Potential Catalytic Application by Electrochemical Dissolution of Polycrystalline 3C-SiC

Electrochemical dissolution of highly doped (ρ ∼ 1 mΩ·cm, n-type) polycrystalline 3C-SiC in HF/H2O and HF/H2O/ethanol solutions allowed production of porous silicon carbide (por-SiC) and soluble carbon fluorooxide nanoparticles as a byproduct. The por-SiC is a crystalline material with large pore volume, surface area close to 100 m2 g–1, and open mesoporous structure. The surface of por-SiC is covered with a thin carbon-enriched layer, bearing carboxylic acid groups. Depending on the SiC resistivity, etchant composition, and current density, three different types of por-SiC morphology, namely, a macroporous tubular, mesoporous hierarchical, and mesoporous filamentary were revealed. A qualitative physical model of SiC electrochemical dissolution, based on the phenomena of quantum confinement, charge carriers trapping onto the surface defects, and the surface passivation, was proposed, and the model successfully interpreted the dependencies of por-SiC morphology and material balance on the etching conditions. The por-SiC is anticipated to be a prospective material for catalytic, nanofiltration, and sensing applications.

Properties of SiC Films Obtained by the Method of Substitution of Atoms on Porous Silicon

The paper is devoted to the growth of thin SiC films by the method of substitution of atoms on macro- and mesoporous substrates of p- and n-type silicon of (100) orientation. On the mesoporous Si (100) substrates polycrystalline 3C-SiC films were formed, the crystallite size determined from XRD patterns was 27.5 nm. The obtained structures are studied by the methods of scanning electron and atomic-force microscopy, micro-Raman spectroscopy and X-ray diffraction analysis.

Vacancy effects on the formation of He and Kr cavities in 3C-SiC irradiated and annealed at elevated temperatures

Polycrystalline 3C-SiC was sequentially irradiated at 400 and 750°C with 120keV He2+ and 4MeV Kr15+ ions to 1017 and 4×1016cm−2, respectively. The Kr15+ ions penetrated the entire depth region of the He2+ ion implantation. Three areas of He2+, Kr15+ and He2++Kr15+ ion implanted SiC were created through masked overlapping irradiation. The sample was subsequently annealed at 1600°C in vacuum and characterized using cross-sectional transmission electron microscopy and energy-dispersive X-ray spectroscopy. Compared to the He2+ ion only implanted SiC, He cavities show a smaller size and higher density in the co-implanted SiC. At 25dpa, presence of He in the co-implanted 3C-SiC significantly promotes cavity growth; much smaller voids are formed in the Kr15+ ion only irradiated SiC at the same dose. In addition, local Kr migration and trapping at cavities occurs, but long-range Kr diffusion in SiC is not observed up to 1600°C.