Damage development of sintered SiC ceramics with the depth variation in Ar ion-irradiation at 600 ℃

Irradiation damage of the materials depends on the irradiation dose and the intrinsic property of the material. In this paper, the high purity hot pressing sintered SiC ceramics with very few second phase and excellent crystallinity were prepared as the target materials, and the high irradiation dose up to 0.95 and 3.16?dpa respectively were chosen. The as-sintered SiC ceramics were irradiated with a 160?keV Ar ion beam at 600?°C. X-ray photoelectron spectroscopy, Raman spectrum, transmission electron microscopy and nanoindentation tests were utilized to analyze the microstructure variations on the surface of irradiated SiC, and it was found that the irradiated crystals kept crystallinity, although amorphization of SiC was generated with 10–25?nm depth, following with a mixture of point defect clusters and extended defects. Furthermore, it is also evident that there is a balance between irradiated-induced damages buildup and dynamic annealing of defects in high temperature.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Irradiation effects on Amosic-3 silicon carbide composites by Si ions implantation

SiC_f/SiC composites were irradiated to over 100 dpa with 300 keV Si ions at 300 ℃. Here, electron microscopy and Raman spectroscopy were utilized to study the microstructural evolution. The Raman spectra of fiber and matrix showed that the crystal structure was seriously damaged. TEM images revealed that the fiber underwent grain nucleation and growth in lower fluence region, accompanied by an amorphous layer near the damage peak area. Also, the matrix went through recrystallization, and the columnar grains turned into equiaxial ones. Moreover, stacking faults and massive amorphous islands were observed in high resolution TEM images. Following irradiation at 300 ℃, the matrix swelled, but the fiber and interphase shrunken along the axis. And, more remarkably, the hardness of fiber and matrix decreased to different extents, a result that was explained by the generation of amorphous islands and breakdown of covalent bonds, and recrystallization might be responsible for this.     

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.     

In-situ tensile damage and fracture behavior of PIP SiC/SiC minicomposites at room temperature

In-situ tensile damage and fracture behavior of original SiC fiber bundles, processed and uncoated SiC fiber bundles, SiC fiber bundle with PyC interphase, SiC/SiC minicomposites without/with PyC interphase are analyzed. Relationships between load-displacement curves, stress-strain curves, and micro damage mechanisms are established. A micromechanical approach is developed to predict the stress-strain curves of SiC/SiC minicomposites for different damage stages. Experimental tensile stress-strain curves of two different SiC fiber reinforced SiC matrix without/with interphase are predicted. Evolution of composite’s tangent modulus, interface debonding fraction, and broken fiber fraction with increasing applied stress is analyzed. For the BX™ and Cansas-3303™ SiC/SiC minicomposite with interphase, the composite’s tangent modulus decreased with applied stress especially approaching tensile fracture; the interface debonding fraction increased with applied stress, and the composite’s tensile fracture occurred with partial interface debonding; and the broken fiber fraction increased with applied stress, and most of fiber’s failure occurred approaching final tensile fracture.     

Crystallization of SiC and its effects on microstructure,hardness and toughness in TaC/SiC multilayer films

A series of TaC/SiC multilayer films with different SiC thicknesses (t_SiC) have been prepared by magnetron sputtering and their microstructure, hardness and toughness investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and nanoindentation. Results show that SiC crystallized and grew coherently with TaC layers at low t_SiC (≤ 0.8 nm), resulting from the template effect of TaC layers. Maximum hardness and toughness of 46.06 GPa and 4.21 MPa m^1/2 were achieved at t_SiC = 0.8 nm with good coherent interface. With further increasing of t_SiC, SiC layers partially transformed to an amorphous structure and gradually lost their coherent interface, leading to a rapid drop in hardness and toughness. The crystallization of SiC layers and the coherent growth are required to achieve superhardness and high toughness in the TaC/SiC multilayers.     

Additive-free low temperature sintering of amorphous SiBC powders derived from boron-modified polycarbosilanes: Toward the design of SiC with tunable mechanical,electrical and thermal properties

Additive-free SiC ceramics are prepared from polymer-derived powders with Si, C and B elements homogeneously distributed on the atomic level by rapid hot-pressing at 1750 °C and 1800 °C under argon and a load of 50 MPa. As-sintered samples display a Vickers hardness in the range of 9.6 ± 0.5 GPa–17.3 ± 1.9 GPa and an elastic modulus varying from 137 ± 3.4 GPa to 239 ± 6 GPa, both depending on the sample phase composition, crystallinity and porosity. Accordingly, the electrical conductivity changes from 340 to 3900 S/m whereas the thermal conductivity varies from 17.7 to 45.1 W/m⋅K as a function of these characteristics. Thus, we demonstrated that a polycarbosilane containing 0.7 wt.% of boron could produce boron-doped SiC powders that demonstrate tailored sinterability at temperatures as low as 1750 °C to form nearly dense SiC ceramics with adjusted hardness, Young’s modulus, electrical and thermal conductivities.     

Influence of power frequency on the performance of SiC thin films deposited by pulsed DC magnetron sputtering

Because of its high stability, good wear resistance, and high mechanical hardness, SiC is widely used in various mechanical parts as a protective film. However, there have been few reports published on the preparation of SiC films by pulsed DC magnetron sputtering. In this work, SiC films were deposited onto glass and ceramic substrates from a sintered SiC target through pulsed DC magnetron sputtering. The influence of the variation of the power pulse frequency (0?kHz, 50?kHz, 150?kHz, 250?kHz, and 300?kHz) on the film’s performance was studied. The surface morphology, structural characteristics, hardness, and adhesion strength of the deposited SiC films were investigated here. The results show that all the deposited films adhered well to the substrate. They were smooth, compact, and presented an amorphous structure. The film hardness was found to increase as the pulse frequency was increased. When the pulse frequency was 250?kHz, the resulting SiC film possessed optimal mechanical properties with a hardness of 25.74?GPa (measured using a nanometer indentation instrument) and an adhesion strength of about 36?N (measured by scratch tester).     

Microstructure and mechanical property of high growth rate SiC via continuous hot-wire CVD

An average growth rate of SiC at 3.4-28.5 μm/min can be achieved via continuous hot-wire CVD method with input powers of 300-380 W. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) analysis, and X-ray diffraction (XRD) method, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to investigate the microstructure of deposits. At 300 W, amorphous SiC and Si were main products in deposit, which were rapidly replaced by crystallite SiC containing high density stacking faults at 340 W and above. Moreover, with the grain size of SiC increasing from ~25 to ~420 nm, the stacking faults probability decreasing from 0.179 to 0.125, as well as the surface morphology changed from loose-packed granules to a well-defined faceted structure with strong (111) texture. Structural changes led to the increase of deposit's Young's modulus from 266.3 to 341.5 GPa, and the mean tensile strength of SiC filament from 1.57 to 3.03 GPa. The successive growth of W/SiC interfacial layer above 360 W resulted in the reduction in mean tensile strength and Weibull moduls of SiC monofilaments, which agrees with the prediction from critical interfacial layer thickness theory.     

Evaluation of Mechanical Properties Variations for Kr Ion‐Irradiated 6H‐SiC by Nanoindentation Methods

Nanoindentation experiments were performed to investigate the irradiation effects on the mechanical properties of 6H‐SiC irradiated by 4 MeV Kr ions at high fluences from room temperature (RT) to 550°C. The irradiation temperature is the primary factor that affects modifications of the comprehensive mechanical properties, while the effect of the fluence is less significant. Elastic modulus and hardness decrease drastically for RT‐irradiated samples, but they almost recover for elevated temperature samples, with hardness slightly higher than its original value. The hardness increases first and then decreases with increasing temperature, and the elastic modulus decreases linearly as the swelling increases. Meyer's index is related to the indentation size effect of hardness and the magnitude of the lattice damage. The ratio of irreversible work is associated with the degree of elastic recovery and the ratio of hardness to elastic modulus of SiC. Compared with the unirradiated value, fracture toughness changes slightly for RT irradiation, while increasing significantly for elevated temperature irradiation and has the same variation tendency of hardness. Results indicate that mechanical properties change with the variations of interatomic bond strength, dislocation mobility, and the behavior of crack propagation, which is strongly affected by the defects induced by heavy‐ion irradiation.     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

机械飞轮基体电泳沉积SiC涂层的研究

使用电泳技术在机械飞轮用30CrMo钢表面制备了SiC涂层,并研究了SiC的质量浓度对SiC涂层的厚度、表面形貌、硬度及耐蚀性的影响。结果表明:增加SiC的质量浓度有利于提高SiC涂层的厚度、硬度及耐蚀性。当SiC的质量浓度为35 g/L时,团聚作用和界面效应使得SiC涂层的厚度明显减小,表面裂纹增多,导致SiC涂层的硬度及耐蚀性大大降低。在SiC的质量浓度为30 g/L的条件下电泳沉积的SiC涂层具有最佳的硬度和耐蚀性。     

Kr ion irradiation study of polymer-derived SiFeOC–C–SiC nanocomposite

This study focuses on the pyrolysis and ion irradiation behaviors of polymer-derived SiFeOC–C–SiC ceramic. The pyrolyzed material is composed of SiO₂ and SiOC (amorphous), carbon (amorphous and turbostratic), and Fe₃Si and β-SiC (nanocrystalline). Irradiation was carried out at both room temperature and 600°C using 400 keV Kr ions with fluences of 4 × 10¹⁵ and 1 × 10¹⁶ ions cm⁻², respectively. The Fe₃Si and SiC nanocrystals are stable against irradiation up to 3 displacement per atom (dpa) at room temperature and up to 12 dpa at 600°C. The SiOC tetrahedrals show phase separation and minor carbothermal reduction. The high irradiation resistance and the dense, defect-free amorphous microstructure of SiFeOC–C–SiC after prolonged irradiation demonstrate its great potential for advanced nuclear reactor applications.     

Study of the Ion‐Irradiation Behavior of Advanced SiC Fibers by Raman Spectroscopy and Transmission Electron Microscopy

6H–SiC single crystals and two types of SiC fibers, Hi‐Nicalon type S and Tyranno SA3, have been irradiated with 4‐MeV Au³⁺ up to 2 × 10¹⁵ cm^?2 (4 dpa) at room temperature, 100°C and 200°C. These fibers are composed of highly faulted 3C–SiC grains and free intergranular C. Stacking fault linear density and grain size estimations yield, respectively, 0.29 nm^?1 and 26–36 nm for the Hi‐Nicalon type S fibers and 0.18 nm^?1 and 141–210 nm for the Tyranno SA3 fibers. Both transmission electron microscopy and surface micro‐Raman spectroscopy reveal the complete amorphization of all the samples when irradiated at room temperature and 100°C and a remaining crystallinity when irradiated at 200°C. The latter observations reveal a multi‐band irradiated layer consisting in a partially amorphized band near the surface and an in‐depth amorphous band. Also, nanocrystalline SiC grains with high stacking fault densities can be found embedded in amorphous SiC at the maximum damage zone of the Hi‐Nicalon type S fibers irradiated at 200°C.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

In-situ TEM investigations on the microstructural evolution of SiC fibers under ion irradiation: Amorphization and grain growth

SiC/SiC composites are attractive candidates for many nuclear systems. As reinforcements, SiC fibers are critical to the in-service performance of composites. In this work, the temperature effects on the irradiation-induced microstructural evolution of Cansas-III SiC fibers were investigated using in-situ transmission electron microscopy (TEM). With in-situ 800 keV Kr ion irradiation, at room temperature (RT) the SiC fiber experienced heterogeneous amorphization and became completely amorphous at ～2.6 dpa. Above the critical temperature of crystalline-to-amorphous (T_c), SiC fibers underwent a simultaneous process of carbon packet disappearance and nano-grain growth at 300 °C and 800 °C. Possible mechanisms were discussed.     

Flexural modulus of SiC/SiC composites sintered by microwave and conventional heating

To deeply study the variation mechanisms of mechanical properties, flexural modulus of SiC fibers reinforced SiC matrix (SiC/SiC) composites prepared by conventional and microwave heating at 800?°C–1100?°C was discussed in detail. The elastic modulus of fibers and matrix, interface bonding strength and porosity of SiC/SiC composites were considered together to analyze the changing tendencies and differences in their flexural modulus. The elastic modulus of fiber and matrix was determined by nanoindentation technique and interface characteristics applying fiber push-out test. The porosity and microstructure examinations were characterized by mercury intrusion method, X-ray Diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, two conflicts between the changing trends of elastic modulus and chemical compositions of composite components were focused and explained. Results indicate that three factors played different roles in the flexural modulus of SiC/SiC composites and residual tensile stress in composite components led to the conflicts between their elastic modulus and chemical compositions.     

Advanced technology for fabrication of reaction-bonded SiC with controlled composition and properties

An advanced fabrication technology of reaction-bonded SiC is developed, which includes the preparation of a C/SiC preform by repeated cycles of phenolic resin impregnation and pyrolysis, followed by infiltration with silicon melt. The use of different number of impregnation stages provides control of carbon content in the preform and the corresponding SiC content in final ceramics. The effect of the impregnation number on the preform characteristics and ceramics composition, thermal and mechanical properties are investigated comprehensively. With an increase of impregnation number up to four, SiC fraction in the ceramics enlarges to 93 vol%, thermal conductivity and Young’s modulus increase to 186 W/(m?K) and 427 GPa respectively, which are superior to most reaction-bonded SiC. Flexural strength (225 MPa) and thermal expansion coefficient (2?10^?6 K^-1) are not dependent on the impregnation number. The obtained results provide an opportunity to design and fabricate reaction-bonded SiC ceramics with a given set of properties.     

Thermal stability and heat flux investigation of neutron-irradiated nanocrystalline silicon carbide (3C–SiC) using DSC spectroscopy

Nanocrystalline silicon carbide (3C–SiC) particles have been irradiated by neutron flux (2 × 10¹³ n∙cm⁻²s⁻¹) up to 5 h at the TRIGA Mark II type research reactor. At the present work, thermal properties of nanocrystalline 3C–SiC are comparatively investigated before and after neutron irradiation at the 300 K < T < 1300 K ranges. Simultaneously, the DSC (Scanning Calorimetry), TGA (Thermogravimetric Analysis) and DTG (Differential Thermogravimetric Analysis) experiments were conducted from 300 K up to 1300 K. Oxidation mechanism of nanocrystalline 3C–SiC particles have been theoretically and experimentally studied before and after neutron irradiation. The kinetics of mass and heat flux were analysed at the heating and cooling processes using DSC spectroscopy.