Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Fine study of hierarchical interphase constructed by boron nitride and carbon nanotubes in SiCf/SiC composites

A fine study of the interfacial part in the silicon carbide fiber (SiC_f) reinforced silicon carbide (SiC) composites was conducted by transmission electron microscopy. The boron nitride (BN) and carbon nanotubes (CNTs) were progressively coated on the SiC_f by chemical vapor deposition method to form a hierarchical structure. Three composites with different interfaces, SiC_f–CNTs/SiC, SiC_f@BN/SiC, and SiC_f@BN–CNTs/SiC, were fabricated by polymer infiltration and pyrolysis method. The interfaces and microstructures of the three composites were carefully characterized to investigate the improvement mechanism of strength and toughness. The results showed that BN could protect the surface of SiC_f from corrosion and oxidation so that improved the possibility of debonding and pullout. CNTs could avoid the propagation of cracks in the composites so that improved the damage resistance of the matrix. The synergistic reinforcement brought by BN and CNTs interfaces made the SiC_f@BN–CNTs/SiC composites with a tensile fracture strength as high as 359 MPa, with an improvement of 23% compared to that of SiC_f@BN/SiC.     

Microstructure and damage evolution of SiCf/PyC/SiC and SiCf/BN/SiC mini-composites: A synchrotron X-ray computed microtomography study

SiC_f/PyC/SiC and SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiC_f/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiC_f/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiC_f/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B₂O₃ molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B₂O₃, the SiC_f/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO₂ grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiC_f/PyC/SiC and SiC_f/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale.     

Fabrication and properties of SiCnw-reinforced SiC composites by reactive melt infiltration with BN and PyC interface

To tailor the fiber–matrix interface of SiC nanowires-reinforced SiC (SiC_nw/SiC) ceramic matrix composites (CMCs) for improved mechanical properties, SiC nanowires were coated with BN and pyrolytic carbon (PyC) compound coatings prepared by the dip-coating process in boric acid and urea solution and the pyrolysis of phenolic resin. SiC_nw/SiC CMC with PyC/BN interfaces were fabricated by reactive melt infiltration (RMI) at 1680°C for 1 h. The influences of phenolic resin content on the microstructure and mechanical properties of the CMC were investigated. The results showed that the flexural strength and fracture toughness reach the maximum values of 294 MPa and 4.74 MPa m^1/2 as the phenolic resin content was 16 and 12 wt%, respectively. The displacement–load curve of the sample exhibited a gradient drop with increasing phenolic resin content up to 12 wt%. The results demonstrated that the PyC/BN compound coatings could play the role of protecting the SiC_nw from degradation as well as improving the more moderate interfacial bonding strengths during the RMI.     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

Ring compression properties of SiCf/SiC composites prepared by chemical vapor infiltration

SiC fiber reinforced SiC matrix (SiC_f/SiC) composites prepared by chemical vapor infiltration are one of promising materials for nuclear fuel cladding tube due to pronounced low radioactivity and excellent corrosion resistance. As a structure component, mechanical properties of the composites tubes are extremely important. In this study, three kinds of SiC_f preform with 2D fiber wound structure, 2D plain weave structure and 2.5D shallow bend-joint structure were deposited with PyC interlayer of about 150–200?nm, and then densified with SiC matrix by chemical vapor infiltration at 1050?°C or 1100?°C. The influence of preform structure and deposition temperature of SiC matrix on microstructure and ring compression properties of SiC_f/SiC composites tubes were evaluated, and the results showed that these factors have a significant influence on ring compression strength. The compressive strength of SiC_f/SiC composites with 2D plain weave structure and 2.5D shallow bend-joint structure are 377.75?MPa and 482.96?MPa respectively, which are significantly higher than that of the composites with 2D fiber wound structure (92.84?MPa). SiC_f/SiC composites deposited at 1100?°C looks like a more porous structure with SiC whiskers appeared when compared with the composites deposited at 1050?°C. Correspondingly, the ring compression strength of the composites deposited at 1100?°C (566.44?MPa) is higher than that of the composites deposited at 1050?°C (482.96?MPa), with a better fracture behavior. Finally, the fracture mechanism of SiC_f/SiC composites with O-ring shape was discussed in detail.     

In-situ formation of Ti3SiC2 interphase in SiCf/SiC composites by molten salt synthesis

Titanium silicon carbide (Ti₃SiC₂) film was synthesized by molten salt synthesis route of titanium and silicon powder based on polymer-derived SiC fibre substrate. The pre-deposited pyrolytic carbon (PyC) coating on the fibre was utilized as the template and a reactant for Ti₃SiC₂ film. The morphology, microstructure and composition of the film product were characterized. Two Ti₃SiC₂ layers form the whole film, where the Ti₃SiC₂ grains have different features. The synthesis mechanism has been discussed from the thickness of PyC and the batching ratio of mixed powder respectively. Finally, the obtained Ti₃SiC₂ film was utilized as interphase to prepare the SiC fibre reinforced SiC matrix composites (SiC_f/Ti₃SiC₂/SiC composites). The flexural strength (σ_F) and fracture toughness (K_IC) of the SiC_f/Ti₃SiC₂/SiC composite is 460 ± 20 MPa and 16.8 ± 2.4 MPa?m^1/2 respectively.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases

The effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)_n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (C_f/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated C_f/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of C_f/SiC composites having multilayered interphase.     

Preparation and tensile behavior of SiCf/SiC mini composites containing different types and thicknesses of interphase

SiC fiber-reinforced SiC matrix (SiC_f/SiC) composites are promising materials for high-temperature structural applications. In this study, KD-II SiC fiber bundles with a C/Si ratio of approximately 1.25 and an oxygen amount of 2.53%, were used as reinforcement. PyC interphase, PyC-SiC co-deposition interphase I and II, with different thicknesses, and SiC matrix were deposited into the SiC fiber bundles by using chemical vapor infiltration (CVI) to form SiC_f/SiC mini composites. When the thickness of the interphase is approximately 1000 nm, the ultimate tensile stress and strain of SiC_f/SiC mini composites with PyC-SiC co-deposition interphase I can reach 1120.0 MPa and 0.72%, respectively, which are significantly higher than those of SiC_f/SiC mini composites with a PyC interphase (740.0 MPa, 0.87%) and PyC-SiC co-deposition interphase II (645.0 MPa, 0.54%). The effect of thicknesses and types of interphase on tensile fracture behavior of mini composites and then the fracture mechanism are discussed in detail.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Mechanical and dielectric properties of SiCf/BN/SiBCN composites via different synthesis technologies

SiC fibers reinforced SiBCN ceramic matrix composites (SiC_f/BN/SiBCN composites) were synthesized by direct chemical vapor infiltration (CVI), polymer infiltration pyrolysis (PIP) or chemical vapor infiltration combined with polymer infiltration pyrolysis (CVI + PIP). It is shown that the insertion of a continuous and dense SiBCN matrix via the CVI process improves the flexural strength and modulus. Interface debonding and fiber pullout happened with 50–100 nm BN interface in CVI and CVI + PIP SiC_f/BN/SiBCN composites. The relative complex permittivity was measured in X-band. Higher ε′′ values in CVI-containing composites can be observed, which can be attributed to the accumulation of C and SiC phases and a multilayer matrix. Strong electromagnetic wave attenuation ability was obtained with high dielectric loss.     

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiC_f)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiC_f using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiC_f/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiC_f/PyC/SiCNWs. Three mini-composites, SiC_f/PyC/SiC, SiC_f/PyC/SiCNWs/SiC, and SiC_f/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiC_f/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiC_f/SiC mini-composites were investigated.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Oxidation and microstructure of SiCf/SiC composites in moist air up to 1600°C by X-ray tomographic characterization

SiC_f/SiC composites that possess PyC or BN interface layers were fabricated and then oxidized in moist air at 1000, 1200, 1400, and 1600°C. High-resolution CT was used for capturing 3D images and quantifying the SiC phase, mesophase, and voids. The oxidation behavior and microstructural evolution of SiC_f/SiC with PyC or BN interface are discussed in this study. The microstructure of the SiC_f/SiC with a PyC layer was seriously damaged in moist air at high temperature, whereas the BN interface layer enhanced the oxidation resistance of the SiC_f/SiC. These results are also confirmed by using XRD, oxidation mass gain, tensile testing, and SEM measurements. The results of the oxidation behavior and microstructural evolution for SiC_f/SiC oxidized in dry air are also compared with the results of this study. Comparing the SiC_f/SiC with a PyC interface layer, the composite with a BN interface layer oxidized in moist air exhibits a high void growth rate and a low SiO₂ grain growth rate from 1000 to 1600°C. This work will provide guidance for predicting the service life of SiC_f/SiC for multiscale damage rate models of materials at a local scale and will also provide guidance on the life service design of SiC_f/SiC materials.     

Effects of post-sintering annealing on the microstructure and toughness of hot-pressed SiCf/SiC composites with Al2O3-Y2O3 additions

This study examined the effects of post-sintering heat treatment on enhancing the toughness of SiC_f/SiC composites. Commercially available Tyranno^® SiC fabrics with contiguous dual ‘PyC (inner)-SiC (outer)’ coatings deposited on the SiC fibers were infiltrated with a SiC + 10 wt% Al₂O₃-Y₂O₃ slurry by electrophoretic deposition. SiC green tapes were stacked between the slurry-infiltrated fabrics to control the matrix volume fraction. Densification of approximately 94% ρ_theo was achieved by hot pressing at 1750 °C, 20 MPa for 2 h in an Ar atmosphere. Sintered composites were then subjected to isothermal annealing treatment at 1100, 1250, 1350, and 1750 °C for 5 h in Ar. The correlation between the flexural behavior and microstructure was explained in terms of the in situ-toughened matrix, phase evolution in the sintering additive, role of dual interphases and observed fracture mechanisms. Extensive fractography analysis revealed interfacial debonding at the hybrid interfaces and matrix cracking as the key fracture modes, which were responsible for the toughening behavior in the annealed SiC_f/SiC composites.     

Microstructure and mechanical properties of in-situ hot pressed (TiB2 + SiC)/Ti3SiC2 composites with tunable TiB2 content

Without impurity phases detected, fully dense (TiB₂?+?SiC)/Ti₃SiC₂ composites have been successfully synthesised by in-situ reaction hot pressing. The effect of TiB₂ content on phase composite, sintering properties, microstructure, and mechanical properties of the composites were thoroughly investigated. With TiB₂ content increasing from 0 to 50?vol.-%, the flexural strength increases first and then decreases, whereas fracture toughness, hardness and modulus show a linear increase. The maximum strength of 826?MPa was obtained at 20?vol.-% TiB₂. On the whole, the (TiB₂?+?SiC)/Ti₃SiC₂ composites exhibit a superior comprehensive mechanical properties superior to other reported Ti₃SiC₂-based composites reinforced by singular reinforcement. The significant strengthening and toughening effect induced by the in-situ incorporated TiB₂ can be ascribed to the unique properties of TiB₂ and the synergistic action of many mechanisms including particle reinforcement, pulling out of grains, crack deflection and grain refinement strengthening.     

Effects of in-situ carbon layer and volume fraction of SiC fiber on the mechanical properties and fracture behaviors of SiCf/SiC composites fabricated by a modified PIP method

Unidirectional SiC_f/SiC composites (UD SiC_f/SiC composites) with excellent mechanical properties were successfully fabricated by a modified PIP method which involved the preparation of film-like matrix containing carbon layer with a low concentration PCS solution followed by the rapid densification of composites with a high concentration PCS solution. Carbon layers were in-situ formed and alternating with SiC layers in the as-received matrix. The unique microstructure endows the composites with appropriate interfacial bonding state, good load transfer ability of interphase and matrix and load bearing ability of fiber, and great crack deflection capacity, which ensures the synergy of high strength and toughness of composites. It is also found that the fiber volume fraction in the preform makes a non-negligible effect on the distribution of interphase and matrix, of which the reasonable adjustment can be utilized to optimize the mechanical properties of composites. Compared with the composites only using high concentration PCS solution, the UD SiC_f/SiC composites prepared by the modified PIP method exhibit superior mechanical properties. Ultrahigh flexural strength of 1318.5 ± 158.3 MPa and fracture toughness of 47.6 ± 5.6 MPa·m^1/2 were achieved at the fiber volume fraction of 30%.     

Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m^1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si₃N₄ matrix composites (SiC_f/Si₃N₄) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiC_f/Si₃N₄ composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than ?7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa?m^1/2).