SiC Fiber Surface Research Articles

Characterization of BN interface and its effect on the mechanical behavior of SiCf/SiC composites

The boron nitride (BN) interface of SiC fiber-reinforced SiC matrix (SiCf/SiC) composites was fabricated by chemical vapor infiltration (CVI) using BCl3 and NH3 as precursor. The microstructure and chemical composition of the BN interface were characterized. The influence of the coating thickness on the tensile behavior of SiCf/SiC composites was then evaluated. The BN interface synthesized at 800 °C has a low degree of crystallization and displays a quasi-isotropic microstructure. The ratio of element B to element N is near 1 and the oxygen content is about 10%. Meanwhile, the electron structure of B, N atoms near the SiC fiber surface have more sp2 hybridization state. Increasing the BN interface thickness exerts a critical role in decreasing the interfacial sliding stress which improves the tensile strength and toughness of SiCf/SiC composites. The optimal thickness of the BN interface is about 670 nm and further increasing the thickness exhibits little improvement on the mechanical properties of SiCf/SiC composites.

In Situ Construction of TiC-Ti3SiC2 Gradient Hybrid Interphase Coated SiC Fibers for Suppression of Specular Reflection and Non-Specular Scattering

TiC-Ti3SiC2 gradient hybrid interphase on the surface of SiC fibers was successfully obtained through the molten salt method. The electromagnetic parameters of the prepared samples can be accurately controlled by adjusting the reaction temperature. A significant bimodal effect is observed in electromagnetic parameters patterns, corresponding to the double interface layer. TiC-Ti3SiC2 gradient hybrid interphase plays a dominant role in impedance matching, as well as in the attenuation layer through multi-interfacial polarization and conduction loss. Through the co-evaluation of the suppression of specular reflection and non-specular scattering properties of the samples, the SiC fiber with the TiC-Ti3SiC2 gradient hybrid interphase is expected to be a high temperature resistant radar absorbing material for future stealth aircraft.

Synthesis and characterization of high-temperature resistant yttrium-doped lanthanum monazite coatings on SiC fibers

To improve the long-term oxidation resistance of SiC fibers, a novel yttrium-doped lanthanum monazite (Y-LaPO4) coating was deposited onto the surface of SiC fibers (Y-LaPO4/SiCf) by a simple hydrothermal method. The static oxidation behavior of Y-LaPO4/SiCf, SiC fibers coated with lanthanum monazite (LaPO4/SiCf), and SiC fibers without coatings (SiCf) was studied at 1350 °C for 50 h. The results indicated that yttrium diffused into the oxide scale during oxidation process. The thicknesses of the oxide scales for the oxidized fibers in Y-LaPO4/SiCf and LaPO4/SiCf were about 4.56 and 11.31 μm. SiC fibers without coatings were completely oxidized to silica. Furthermore, the oxidation kinetic curves of SiCf, LaPO4/SiCf, and Y-LaPO4/SiCf exhibited a parabolic trend during oxidation for 15, 20, and 50 h. Calculated the oxidation rate constants of SiCf, LaPO4/SiCf, and Y-LaPO4/SiCf were found to be 1.37, 1.08, and 0.40 μm2/h, respectively. The system of Y-LaPO4 coatings not only retains LaPO4 as a framework of the interface, but also incorporates yttrium to improve the oxidation resistance of fibers. Therefore, doping yttrium in lanthanum monazite can enhance the adherence between coatings and oxide scales and improve the oxidation resistance of SiC fibers.

Fabrication and performance of mini SiC/SiC composites with an electrophoresis-deposited BN fiber/matrix interphase

The feasibility of fabricating a BN matrix/fiber interphase of SiC/SiC composites via electrophoresis deposition (EPD) was investigated based on the simplicity and non-destructiveness of the process and the excellent interfacial modification effects of BN. The BN suspension and SiC fiber surface properties were both adjusted to generate suitable conditions for the EPD process of the BN interphase. Next, the deposition dynamics and mechanism were studied under different deposition voltages and time, and the relationship between the deposition morphology of the BN interphase and mechanical properties of the fabricated mini SiC/SiC composites were also discussed. After oxidation at high temperature (600–1000 ℃), the mechanical properties of the mini SiC/SiC composites were studied to verify the oxidation resistance effect of the EPD-deposited BN interphase, whose oxidation resistance mechanism was briefly analyzed as well.

In-situ synthesis of ternary layered Y3Si2C2 ceramic on silicon carbide fiber for enhanced electromagnetic wave absorption

A novel ternary layered ceramic of Y 3 Si 2 C 2 was successfully in-situ synthesized on the surface of home-made third-generation KD-SA SiC fiber for the first time by molten salt method aimed at improving the electromagnetic wave (EMW) absorption. After in-situ synthesis of Y 3 Si 2 C 2 ceramic layer on SiC fiber (SiC f /Y 3 Si 2 C 2 ), significantly improved EMW absorption performance was obtained. The minimum reflection loss (RL min ) of −16.97 dB was reached in SiC f /Y 3 Si 2 C 2 composites at the thickness of only 2.19 mm, and the effective absorption bandwidth (EAB) was up to 5.44 GHz (12.56–18 GHz) at a thin thickness of 2.64 mm. The improvement in EMW absorption of SiC f /Y 3 Si 2 C 2 is mainly attributed to enhanced dielectric loss and conduction loss resulting from increased heterogeneous interfaces and multiple reflections and scattering originating from net structure. The SiC f /Y 3 Si 2 C 2 could be a promising EMW absorber for application in high-performance EMW absorbing materials.

Improvement of high‐temperature stability of PIP SiCf/SiC material through in situ grown BNNTs

AbstractIn this paper, the effect of in situ grown boron nitride nanotubes (BNNTs) and preparation temperature on mechanical behavior of PIP (Precursor Infiltration and Pyrolysis) SiCf/SiC minicomposites under monotonic and compliance tensile is investigated. In situ BNNTs are grown on the surface of SiC fibers using ball milling–annealing process. Composite elastic modulus, tensile strength, fracture strain, tangent modulus, and loading/unloading inverse tangent modulus (ITM) are obtained and adopted to characterize the mechanical properties of the composites. Microstructures of in situ grown BNNTs and tensile fracture surfaces are observed under scanning electronic microscopic (SEM). For SiCf/SiC minicomposites with BNNTs, the elastic modulus, tensile strength, and fracture strain are all lower than those of SiCf/SiC minicomposites without BNNTs, mainly due to high preparation temperature and the oxidation of the PyC interphase during the annealing process. Tensile stress–strain curves of SiCf/SiC minicomposites with and without BNNTs are predicted using the developed micromechanical constitutive model. The predicted results agreed with experimental data. This work will provide guidance for predicting the service life of SiCf/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.

Influence of the carbon interface on the mechanical behavior of SiC/SiC composites

SiC/SiC composites reinforced with 3rd generation SiC fibers (Hi-Nicalon S and Tyranno SA3 fibers) are promising candidates for thermomechanical applications in high technology industries. Both composites exhibited a pseudo-ductile mechanical behavior but the HNS/PyC/SiC composite reaches higher failure strains than TSA3/PyC/SiC ones. The mechanical behavior of SiC/SiC composites is linked to the way PyC is bonded to the fiber surface. Analyses have shown that these interactions and the Fiber/Matrix debonding behavior depend strongly on the nature of the carbon on the SiC fiber surface, which is different according to the SiC fiber. In order to understand the mechanism governing the chemical adhesion at the PyC/SiC fiber interface, the formation, the chemistry and the structure of the surface carbon layer were studied. Understanding the origin of this carbon has allowed elucidating the local interaction mechanisms of the studied SiC/SiC composites.

A development mechanism of graded microstructures in iron-containing SiC fibers revealed by electron microscopy

Transmission electron microscopy is utilized to investigate the microstructure development of iron-containing SiC fibers as a function of their annealing temperatures. Compositional and structural gradients along the radial directions start to form within SiC fibers fabricated at temperatures above 1200 °C. The graded microstructures are associated with the outward diffusion of iron-rich particles along the radial direction. In-situ heating TEM experiments suggest that the iron-rich particles within SiC fibers are highly mobile at elevated temperatures. Therefore, we propose that the graded microstructures along the radial directions are the synergetic effects of iron catalyzation and outgassing, namely, iron-rich particles are progressively driven to the surface of SiC fibers and in the trajectory of iron-rich particles, the decomposition of the amorphous SiCxOy phase is accelerated, promoting the growth of SiC grains. The current study emphasizes the important role of the outgassing process to the microstructure evolution of SiC fibers. Our proposed mechanism can be extended to understand the microstructural evolution of SiC fibers that use metallic additives as sintering aids.

Synthesis and electromagnetic wave absorption properties of peanut shell-like SiC fibers

The peanut shell-like SiC fibers were prepared in order to improve the electromagnetic wave absorption properties. The phase of fibers was mainly β-SiC and less amorphous SiO2. The peanut shell-like holes with the average size of 89 nm appeared in the surface of SiC fibers. Their optimal reflection loss was high as −48 dB at 11.1 GHz as absorber thickness was 2.53 mm. The SiC fibers exhibit excellent electromagnetic wave absorption ability in X-band. It was attributed to the fibers’ biomimetic structure, which caused the microwave multiple reflection, scattering and Debye relaxation.

Effect of BNNTs/matrix interface tailoring on toughness and fracture morphology of hierarchical SiCf/SiC composites

A thin BN interphase is applied on BNNTs surface to tailor the interfacial bonding between BNNTs and SiC matrix in hierarchical SiCf/SiC composites. The thickness of BN interphase ranging from 10 to 70 nm can be optimized by chemical vapor deposition after BNNTs are in situ grown on SiC fiber surface. Without BN interphase, the fracture toughness of hierarchical SiCf/SiC composites can be impaired by 13.6% due to strong interfacial bonding. As long as BN interphase with 30–45 nm thickness is applied, the interfacial bonding can be optimized and fracture toughness of hierarchical composites can be improved by 27.3%. It implies that tailoring BNNTs/matrix interface by depositing a layer of BN interphase is in favor of activating energy dissipation mechanisms at nanoscale induced by BNNTs.

Microstructure and Failure Behavior of Double Anti-Oxidation Coating on SiCf/SiC-CMC

In this paper, BN interfacial phase was prepared on SiC fiber surface and CVD SiC gradient coating was prepared on the surface of SiCf/SiC-CMCs composite. CVD SiC coatings were fabricated by the pyrolysis of methyltrichlorosilane (MTS) in hydrogen at a low pressure of 5-10kPa. The ratio of MTS to hydrogen is 1 to 12.Oxidation behaviour is examined by placing the material with coats in air. The oxidizing temperatures were varied from 100°C to 1200°C. Optical microscope and SEM were used to observe the surface morphology and microstructure of coatings. XRD was used to characterize the phase composition. The results indicated that the Oxidation of SiC/SiC-CMC is the most serious at 800°C. Tensile strength test shows that the fracture strength of the material is the lowest and with the increase of temperature to 1200°C, the tensile strength of the material increases. Tensile strength test shows that the fracture strength of the material is the lowest and with the increase of temperature to 1200°C, the tensile strength of the material increases.

Growing SiC nanowires on modified SiC fibers surface via a chemical vapor deposition route

SiC nanowires were grown successfully on the surface of SiC fibers via an in situ chemical vapor deposition (CVD) route with Fe(NO3)3 as catalyst. Ethylenediaminetetraacetic acid (EDTA) and dimethylformamide (DMF) were mixed to prepare surface modification agent which was used for surface grafting treatment of SiC fibers. The changes of surface chemical groups between modified and non-modified SiC fibers were analyzed by fourier transform infrared spectroscopy. The as grown products were investigated by scanning electron microscopy (SEM) and transmission electron microscopy(TEM). The results show that the surface modification could bring COOH groups onto the surface of SiC fibers, which could strongly improve the interfacial adhesion strength between fibers surface and catalyst particles. In addition, the surface modification can also make the catalytic particles distribute uniformly throughout the fiber surface which provided a good surface condition for the growth of SiC nanowires.

Effect of carbon nanotubes on the electromagnetic shielding properties of SiCf/SiC composites

Carbon nanotube (CNT)-reinforced SiCf/SiC composites were prepared through in-situ growth of CNTs on SiC fiber via catalytic chemical vapor deposition (CCVD) and subsequent densification by precursor infiltration and pyrolysis (PIP) to optimize electromagnetic interference (EMI) shielding performance in the frequency range of 8.2–12.4 GHz. The influence of CNTs on the dielectric properties and EMI shielding properties of the SiCf/SiC composite were studied. The SEM/TEM morphology showed that the CNT network was formed on the surface of SiC fibers. The electrical conductivity of composites increased owing to the introduction of CNTs, leading to markedly increased conductive loss. Elevated CNT concentration was found to enhance complex permittivity and tan δ. Experimental results exhibited that absorption shielding effectiveness was the dominant EMI shielding mechanism. With the increase in CNT amount, the EMI total shielding effectiveness of the composite increased and reached a maximum of 39 dB with Co loading of 8%, an increase of 60% over that of plain SiCf/SiC composites. The achieved EMI shielding properties suggested the potential of CNT-reinforced SiCf/SiC composites as new-generation EMI shielding materials.

Formation of carbon-rich layer on the surface of SiC fiber by sintering under vacuum for superior mechanical and thermal properties

SiC fibers have been intensively developed for application in advanced aerojet engines, stationary gas turbines and nuclear reactors of the future. In this work, SiC fibers with controllable carbon-rich layer were prepared by sintering under vacuum, which could be attributed to the release of gaseous silicon under vacuum at high temperature. The thickness of the carbon-rich layer on the fiber surface could be adjusted by changing the sintering temperature. Moreover, the carbon-rich layer was well distributed on the fiber surface, combining closely with the interior of the fiber. The as-prepared SiC fibers had high tensile strength and relatively low elastic modulus, which was favorable for weaving into different fabrics. Furthermore, the fibers also exhibited excellent ultra-high temperature resistance due to the presence of carbon-rich layer on the surface, which was better than that of the Hi-Nicalon and Hi-Nicalon S fibers.

Microstructural Difference between Unreinforced Canning of TC17 Alloy and the Matrix in SiC<sub>f</sub>/TC17 Composite Fabricated by HIP Process

Continuous unidirectional SiCf/TC17 composite has been fabricated by hot isostatic pressing (HIP). After consolidation, the TC17 canning (the unreinforced ambient portion of the specimen) showed an equiaxed microstructure, whereas the matrix of SiCf/TC17 composite (deposited on the continuous SiC fibers by magnetron sputtering) exhibited a typical lamellar structure. In this work, the heat treatments under different condition, XRD, SEM and WDS have been employed to characterize and analyze the microstructural difference. The results indicated that the difference in β transus temperature (Tβ) between the canning and matrix of TC17 alloy induced the microstructural diversity. The introduction of C element (an intensive α stabilizing element) into the matrix alloy may be ascribed to the diffusion of carbon layer at the surface of SiC fiber. As a result, Tβ of matrix TC17 alloy increased to above 1000 °C, much higher than that of the canning TC17 alloy (890 °C). The investigation of microstructure difference reveals the microstructure evolution in SiCf/TC17 composite, which can provide an effective reference for following processing design.

Formation of Ti3SiC2 Interphase of SiC Fiber by Electrophoretic Deposition Method

Due to its stability at high temperature and its layered structure, Ti3SiC2 MAX phase was considered to the interphase of SiC f /SiC composite. In this study, Ti₃SiC₂ MAX phase powder was deposited on SiC fiber via the electrophoretic deposition (EPD) method. The Zeta potential of the Ti₃SiC₂ suspension with and without polyethyleneimine as a dispersant was measured to determine the conditions of the EPD experiments. Using a suspension with 0.03 wt.% ball milled Ti₃SiC₂ powder and 0.3 wt.% PEI, Ti₃SiC₂ MAX phase was successfully coated on SiC fiber with an EPD voltage of 10 V for 2 h. Most of the coated Ti₃SiC₂ powders are composed of spherical particles. Part of the Ti₃SiC₂ powders that are platelet shaped are oriented parallel to the SiC fiber surface. From these results we expect that Ti₃SiC₂ can be applied to the interphase of SiC f /SiC composites.

Formation mechanism of large SiC grains on SiC fiber surfaces during heat treatment

Three low oxygen-containing SiC fibers with different surface compositions were heat-treated at high temperature in argon, and large SiC particles were observed on the surfaces of SiC fibers with oxygen-containing surface layers. The SiC particles were identified as single crystal β-SiC. The growth process, formation mechanism of these large SiC grains and their influence on fiber strength were investigated. Nano-sized SiC particles were precipitated from the decomposition of SiCxOy in the fiber surface that acted as nuclei, and these nuclei grew gradually to form large grains because of the gaseous reaction of SiO and CO. The formation of large SiC grains was related closely to the SiC fiber surface composition, which affected SiC nucleation and SiO diffusion from the core to the fiber surface. The large grains influenced the fiber strength degradation significantly and should be avoided to retain high fiber strength.

Electrochemical Deposition of Magnesium on SiC Fibers from the LiCl-KCl-MgCl2 Molten Salt

The preparation of Mg/SiC fiber precursor is the key process for the application of SiC fiber in the Mg alloys. Electrochemical deposition method was firstly adopted for the preparation of Mg/SiC fiber precursor in the LiCl-KCl molten salt. However, metal Li will be deposited prior to the deposition of Mg to form Li-C alloys with the carbon layer on the surface of SiC fiber due to the under potential deposition of Li+ ion. A Cu intermediate layer was introduced to avoid the formation of Li-C alloys, so that SiC fiber could be stable in a wide electrochemical window in the LiCl-KCl molten salt. A thin Cu layer was coated on SiC fiber using chemical plating method. The effect of NaOH concentration on the Cu coating was investigated and bright Cu coating was obtained. The electrochemical deposition process of Mg2+ ion was investigated through cyclic voltammetry and the detailed deposition parameters were obtained. Mg/SiC fiber precursor was prepared using pulse current electrodeposition method following the deposition parameters obtained by cyclic voltammetry. The morphology and microstructure of SiC/Mg fiber were explored through SEM. After the optimization of electrodeposition condition, compact and even Mg layer were prepared on SiC fiber.

Zirconium and hafnium germanate-based thin films on SiC fibers

A technique has been developed for producing coatings from sols of hydrous zirconium and germanium dioxides or hafnium and germanium dioxides on SiC microfibers. The technique allows one to produce homogeneous, dense nanocoatings of controlled phase composition. SiC fiber surface modification under optimal conditions has been shown to have no significant effect on the mechanical tensile strength of the fibers. Zirconium germanate and hafnium germanate coatings on SiC fibers favor fiber debonding and pullout from the SiC matrix.

A Study Designed to Improve High-Temperature Thermal Stability of SiC Fiber

In a high-temperature environment, the pyrolytic reaction of the SiC fiber destroyed the chain structure of the fiber and cause fiber apparent weightlessness, the surface and internal of fiber produce porous structure defects. In order to improve high-temperature thermal stability of SiC fiber that the reunion composite powder (CP) of perlite and nanozirconia was prepared by atomized granulation technology, and the CP is sprayed the surface of SiC fiber by plasma spraying technology. The SiC fiber was completely ablation at 1200°C for 3h. On the contrary, the spraying CP of fiber is not affected, showing favorable high-temperature stability. The surface appearance of the fiber was analyzed by means of SEM, and the flame temperature Spray Watch.