Fabrication of carbon fiber reinforced ceramic matrix composites with improved oxidation resistance using boron as active filler

Boron was introduced into C_f/SiC composites as active filler to shorten the processing time of PIP process and improve the oxidation resistance of composites. When heat-treated at 1800 °C in N₂ for 1 h, the density of composites with boron (C_f/SiC-BN) increased from 1.71 to 1.78 g/cm³, while that of composites without boron (C_f/SiC) decreased from 1.92 to 1.77 g/cm³. So when boron was used, two cycles of polymer impregnation and pyrolysis (PIP) could be reduced. Meanwhile, the oxidation resistance of composites was greatly improved with the incorporation of boron-bearing species. Most carbon fiber reinforcements in C_f/SiC composite were burnt off when they were oxidized at 800 °C for 10 h. By contrast, only a small amount of carbon fibers in C_f/SiC-BN composite were burnt off. Weight losses for C_f/SiC composite and C_f/SiC-BN composite were about 36 and 16 wt%, respectively.     

Fabrication of two-layer SiC nanowire cladding tube with high thermal conductivity

Among the various concepts of SiC-based accident-tolerant fuel cladding, duplex SiC cladding, consisting of an inner composite layer and an outer monolithic SiC layer, is considered an optimal design due to its low load failure probability. In this study, SiC nanowires (SiCnw) were introduced on the substrate graphite rod to decrease the diameter of architectural valley-regions of SiC fiber (SiC_f) tubular preform. By avoiding the architectural valley-voids, a dense two-layer SiCnw tube consisting of an inner SiC fiber-reinforced SiC matrix (SiC_f/SiC) composite layer deposited by chemical vapor infiltration with a smooth inner surface was obtained. The microstructure and thermal properties of as-obtained two-layer SiCnw tubes were studied. Results showed that the thermal conductivity of the whole tube was highly sensitive to variations in thermal conductivity of the inner composite layer. By improving the thermal conductivity of the inner composite layer, the two-layer SiCnw tube exhibited a thermal conductivity of 23.8 W m⁻¹ K⁻¹ at room temperature, which had an improvement of 71 % compared to the two-layer SiC tube (13.9 W m⁻¹ K⁻¹). Moreover, the thermal transport properties of the two-layer SiCnw tube were significantly improved by a reduction in roughness of the inner surface.     

Flexible SiC-CNTs hybrid fiber mats for tunable and broadband microwave absorption

Flexible microwave absorbers with high stability are in increasing demand for the applications under harsh conditions. SiC as a functional ceramic material has the feature of high environmental tolerance and adjustable electromagnetic (EM) absorbing properties, making them suitable to be applied for harsh environments. However, the electrical property of SiC requires to be further enhanced to obtain qualified EM absorbing performance. In this work, multiwall carbon nanotubes (CNTs) were introduced to SiC to enhance the electrical properties. Flexible two-dimensional (2D) CNTs loaded SiC fiber mats were prepared as EM absorbers via electrospinning and polymer-derived-ceramic (PDC) methods. The CNTs inside the fibers can form conductive networks and act as reinforcement to ensure high flexibility and enhance the microwave absorption properties of SiC mats. Thus, a reflection loss of ?61 dB and an effective absorption band (EAB) of 2.9 GHz were obtained. More importantly, the EM absorption can be adjusted by tuning the content of CNTs and the EAB can cover the entire X-band by adjusting the material thickness. The work provided a facile strategy to fabricated flexible 2D ceramic mats with high environmental stability and tunable electrical properties, which may shed light on the production of reliable EM absorber for broadband EM absorption applications.     

Wet chemical deposition of BN,SiC and Si3N4 interphases on SiC fibers

Fiber coatings based on BN, BN/SiC and BN/Si₃N₄ were deposited on Hi Nicalon type S SiC fibers. The coating parameters were optimized using a design of experiments study. With optimized parameter sets, the coatings exhibited a high degree of coverage on the fibers and almost no fiber bridging could be observed. The coated fiber bundles are flexible and can be processed further by techniques such as filament winding. In comparison to a non-processed reference sample, the maximum tensile load of the fiber bundles with BN, BN/SiC and BN/Si₃N₄ coatings was reduced by only 5 %, 13 % and 10 %, respectively. The coated fiber bundles retained their tensile strength after thermal annealing up to 1650 °C in a nitrogen atmosphere for 0.5 h. SiC_f/SiC samples with BN/SiC fiber coatings exhibited higher values of bending strength and strain-to-failure as a reference sample without fiber coating indicating the functionality of the fiber coatings.     

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.     

Preparation of Gadolina Stabilized Bismuth Oxide Doped with Boron via Electrospinning Technique

In this study, boron doped and undoped poly (vinyl) alcohol/bismuth–gadolina acetate (PVA/Bi–Gd) nanofibers were prepared using electrospinning technique then calcinated at 800 °C for 2 h. The originality of this study is the addition of boron to metal acetates. The effects of boron doping were investigated in terms of solution properties, morphological changes and thermal characteristics. The characteristics of the fibers were investigated with FT-IR, XRD, SEM and BET. The addition of boron did not only increase the thermal stability of the fibers, but also their diameters, which yielded stronger fibers. XRD analyses showed that boron doping increased the peak intensities and indicated that the boron doping enhanced the crystallite size. Moreover, no shifts were noticed in diffraction angles for boron doped and undoped samples. Therefore, boron doping did not significantly alter the lattice spacing. The SEM micrograph of the fibers showed that the addition of boron resulted in the formation of cross-linked bright-surfaced fibers. The average fiber diameter for boron doped and undoped fiber mats were 204 and 123 nm, respectively. Also, grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 140 and 118 nm, respectively. The BET results showed that boron undoped and doped Bi₂O₃–La₂O₃ nanocrystalline powder ceramic structures sintered at 800 °C have surface areas of 59.72 and 39.80 m²/g, respectively.     

Synthesis and structural characterization of SiBOC ceramic fibers derived from single-source polyborosiloxane

SiBOC ceramic fibers have been successfully prepared from single-source polyborosiloxanes which are synthesized from polymethylethoxysiloxane and B(OH)₃ via a sol-gel route. The morphological change, structural evolution and crystallization behavior of fibers as a function of thermal treatment are studied by several techniques. Polyborosiloxane sols exhibit remarkable spinnability, in which B atoms are homogenously incorporated into the linear Si-O-Si skeleton via Si-O-B bridges. SiBOC ceramic fibers with diameters of about 10 μm are prepared with high ceramic yield ranging from 80.5 to 86%. Although a continuous structural evolution occurs with increasing pyrolysis temperature, the SiBOC ceramic fiber with B/Si atom ratio of 0.14 is thermal stable at 1500 °C. The thermolysis and crystallization behaviors are closely related to the boron content. The tendency toward crystallization of SiC and graphitization of free carbon is strengthened with the increase of boron content and pyrolysis temperature.     

Deformation Control During Thermal Treatment of Electrospun PbZr0.52Ti0.48O3 Nanofiber Mats

This paper focuses on the deformation origin of PbZr_0.52Ti_0.48O₃ (PZT) fiber mats obtained by electrospinning. The main cause of deformation of the green mats during heating was found to be a nonuniform relaxation of the stretched PVP polymer, due to nonuniform thermal decomposition of the Pb‐hexanoate in the fibers. This relaxation starts under 100°C, well below the polymer decomposition temperature. The shrinkage was found to accelerate above the polymer glass transition point, giving rise to an overall linear change of almost 50%. The “green” PZT mats were easily separated from the collector by first depositing a pure PVP sublayer on the collector. An optimal fabrication and slow multistep thermal treatment process that provides fiber mats with desired PZT phase and overcomes the nonuniform deformation is described.     

Silicon carbide fiber oxidation behavior in the presence of boron nitride

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O₂ at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO₂ due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.     

Synthesis and characterization of aryl boron-containing thermoplastic phenolic resin with high thermal decomposition temperature and char yield

Phenol, zinc acetate dihydrate and paraformaldehyde are firstly performed to synthesize thermoplastic phenolic resin (PR), then phenylboronic acid (PBA) and other two boron compounds (4-hydroxymethyl phenylboronic acid & boronic acid) are introduced to fabricate the boron-containing thermoplastic phenolic resins (BPRs). The corresponding molecular structure, softening points, thermal decomposition temperature and char yield ratio of the BPRs are characterized and investigated by FTIR, NMR, XPS and TGA. Compared to pure thermoplastic PR, all the BPRs present relatively higher softening points, more excellent thermal decomposition temperature and higher char yield values. BPR-a exhibits the optimal thermal decomposition temperature (T ₅ of 317.4 °C) and char yield ratio at 800 °C (69.6 %).     

In-plane thermal expansion behavior of dense ceramic matrix composites containing SiBC matrix

In order to reveal the effect of matrix cracks resulted from thermal residual stresses (TRS) on the thermal expansion behavior of ceramic matrix composites, SiBC matrix was introduced into C_f/SiC and SiC_f/SiC by liquid silicon infiltration. The TRS in both two composites were enlarged with incorporating SiBC matrix which has higher coefficients of thermal expansion (CTEs) than SiC matrix. Due to the relatively high TRS, matrix cracks and fiber/matrix (f/m) debonding exist in C_f/SiC-SiBC, which would provide the space for the expansion of matrix with higher CTEs. For SiC_f/SiC, no matrix cracking and f/m debonding took place due to the close CTEs between fiber and matrix. Accordingly, with the incorporation of SiBC matrix, the in-plane CTE of C_f/SiC between room temperature to 1100 °C decreases from 3.65 × 10⁻⁶ to 3.19 × 10⁻⁶ K^-1, while the in-plane CTE of SiC_f/SiC between room temperature to 1100 °C increases slightly from 4.97 × 10⁻⁶ to 5.03 × 10⁻⁶ K^-1.     

Influence of SiC fiber on thermal cycling lifetime of SiC fibers /YSZ thermal barrier coatings by atmospheric plasma spraying

The initiation and propagation of cracks under thermal stresses easily is one of the problems limiting the thermal cycling lifetime of thermal barrier coatings (TBCs). In order to improve the thermal cycling lifetime, SiC fibers were introduced to yttria stabilized zirconia (YSZ) coating deposited on In738LC substrate by atmospheric plasma spray (APS). Phase composition, thermal cycling behaviors and fiber toughening mechanisms of coatings were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal cycling test. Results showed that the thermal cycling lifetime and fracture toughness of SiC fibers/YSZ coatings could reach 442?±?13 and 1.54?±?0.19?MPa $\sqrt{m}$ respectively, which were 1.6 times and 1.3 times higher than that of conventional TBCs. There are two stages of fiber reinforced during thermal cycling, and the first is crack deflection and termination, the second is fiber debonding, pull-out, breakage and bridging. Meanwhile, SiC fibers could prevent the stress-activated ZrO₂ martensitic transformation by reducing the stress in the lattice.     

Flexible ultrafine nearly stoichiometric polycrystalline SiC fibers with excellent oxidation resistance and superior thermal stability up to 1900 °C

Flexible ultrafine SiC fibers with superior high-temperature stability and excellent oxidation resistance are regarded as one of the most promising materials for high-temperature applications. However, excess oxygen and carbon in the ultrafine SiC fibers limit their thermal stability due to decomposition of the SiC_xO_y phase. In the present work, flexible ultrafine nearly stoichiometric polycrystalline SiC fibers were fabricated by combining the electrospinning technique and polymer-derived ceramic method. The ultrafine SiC fibers exhibited superior high-temperature stability and oxidation resistance. The retention rates of tensile strength were 90.0 %, 94.2 % and 86.4 % after heat treatment in argon at 1800 °C, 1900 °C and 2000 °C, respectively. TG results of the fibers showed little weight loss of only 1.52 % at 1900 °C in Ar and the weight gain of only 4.1 % up to 1500 °C in air. Such improved thermal stability was achieved through sintering at high temperature for elimination of excess oxygen and carbon with Al doped as the sintering aid to restrain the grain coarsening. The ultrafine SiC fibers still exhibited excellent flexibility without obvious damage when they were heated by the butane blowtorch flame of about 1100 °C in air. Furthermore, the infrared thermography illustrated that the ultrafine SiC fiber membrane also had good thermal insulation performance. The outstanding mechanical properties and thermal stability of ultrafine SiC fibers suggest their potential applications at the high temperature and harsh environment.     

Fabrication of a 3D4d braided SiCf/SiC composite via PIP process assisted with an EPD method

In order to improve the thermal conductivity and full-fill the gaps between the fiber bundles for three-dimensional four-directional (3D4d) braided SiC_f/SiC composites, 500?nm submicron-sized β-SiC particles were introduced into the 3D4d preform by an electrophoretic deposition (EPD) method. ζ-potential of the KD-Ⅱ SiC fibers and the aqueous suspension of the β-SiC particles were analyzed, as well as the efficiency of the deposition. After densified via PIP process, microstructure, three-point bending strength and thermal conductivity of the composite were investigated. The results showed that, SiC particles filled the gaps between the SiC fiber bundles efficiently, and thermal conductivity of the composites fabricated through PIP process assisted by EPD was 2.3 times that of the composites fabricated via PIP only. The bending strength of the EPD-composites was 647.08?±?69.53?MPa, which decreased to 2/3 of that of the composites manufactured only by PIP, owing to the reduction of fiber volume fraction and the damages to the interface coatings and fibers under the action of the electric field.     

Boron‐dependent microstructural evolution,thermal stability,and crystallization of mechanical alloying derived SiBCN

Amorphous boron‐rich SiBCN were prepared by high‐energy ball‐milling of the mixtures of Si, graphite, h‐BN, and inorganic boron, which acted as extra boron source. The solid‐state amorphization, thermal stability, and crystallization of the boron‐rich SiBCN were studied in detail. It was suggested that mechanical alloying can drive solid‐state amorphization but also can be an initiation step for the nucleation of nanocrystals. The amorphous networks of Si‐C, C‐B, C‐C, C‐N, B‐N, and C‐B‐N bonds are detected by XPS; however, solid‐state NMR further confirms the formation of a new chemical environment around B atoms, BC₃. The increases in boron content improve the thermal stability of SiBCN ceramics but weaken their oxidation resistance. Nano‐SiC crystallizes first while BN(C) forms subsequently. Boron promoting SiC crystallization may result from the reduced hindering effects of B‐N‐C nanodomains that retard SiC crystallization.     

Tensile creep behavior of SiCf/SiC ceramic matrix minicomposites

Single fiber-tow minicomposites represent the major load-bearing element of woven and laminate ceramic matrix composites (CMCs). To understand the effects of fiber type, fiber content, and matrix cracking on tensile creep in SiC_f/SiC CMCs, single-tow SiC_f/SiC minicomposites with different fiber types and contents were investigated. The minicomposites studied contained either Hi-Nicalon™ or Hi-Nicalon™ Type S SiC fibers with a boron nitride (BN) interphase and a chemical-vapor-infiltrated-silicon-carbide (CVI-SiC) matrix. Tensile creep was performed at 1200 °C in air. A bottom-up creep modeling approach was applied where creep parameters of the fibers and matrix were obtained separately at 1200 °C. Next, a theoretical model based on the rule of mixtures was derived to model the fiber and matrix creep-time-dependent stress redistribution. Fiber and matrix creep parameters, load transfer model results, and numerical modeling were used to construct a creep strain model to predict creep damage evolution of minicomposites with different fiber types and contents.     

Thermophysical Properties of Short Carbon Fiber/SiC Multilayer Composites Prepared by Tape Casting and Pressureless Sintering

C_sf/SiC multilayer composites for thermal conductivity (TC) test in three directions were successively prepared by tape casting and pressureless sintering. After 1500°C/5 h oxidation treatment, short carbon fibers were oxidized which produced many pores. However, a core area, which was composed by short carbon fiber, SiC, and few SiO₂, was still observed. TC properties of C_sf/SiC multilayer composites were highly anisotropic. The TC was decreased with the increase in fiber amount. C_sf/SiC multilayer composites demonstrated the highest TC along the tape casting direction and the lowest TC through the thickness direction, which is favorable for thermal protection purpose.     

Excellent ablation resistance and reusability of silicon carbide fiber reinforced silicon carbide composite in dissociated air plasmas

This work explores the potentials of SiC fiber reinforced SiC matrix composites (SiC_f/SiC) with SiC coating to resist aerodynamic ablations for thermal protection purpose. A plasma wind tunnel is employed to evaluate their anti-ablation property in dissociated air plasmas. The results suggest a critical ablation temperature of SiC coated SiC_f/SiC, ≈ 1910 °C, which is the highest ever reported in literatures. Benefited by ‘all-SiC’ microstructures and relative flat ablated surfaces, the SiC_f/SiC is still ablation-resistant up to ≈ 1820 °C after the occurrence of ablation. This implies an excellent ablation resistance and reusability property of SiC_f/SiC, which surpasses that of traditional carbon fiber reinforced composites. Finally, an ablation mechanism dominated by surface characteristic is proposed. For the SiC coated SiC_f/SiC, ablation is prone to take place at surface cracks formed by thermal mismatch; while for the ablated SiC_f/SiC, ablation is triggered at the exposed fiber bundles which is over-heated in the plasmas.     

Fabrication of CNT-SiC/SiC composites by electrophoretic deposition

Due to the outstanding mechanical and thermal properties of carbon nanotubes (CNTs), they are considered suitable reinforcement for structural materials. In this study, for the first time, electrophoretic deposition (EPD) was used to deposit (multi-walled) CNTs onto SiC fibres (SiC_f) to form an effective CNT interphase layer for SiC_f/SiC composites. This deposition was followed by electrophoretic infiltration of the CNT-coated SiC fibre mats with SiC powder to fabricate a new CNT-SiC-fibre-reinforced SiC-matrix (SiC_f/SiC) composite for fusion applications. In these EPD experiments, a commercial aqueous suspension of negatively charged CNTs and an optimized aqueous suspension of negatively charged SiC particles were used. The CNT-coatings on the SiC fibres were firm and homogenous, and uniformly distributed nanotubes were observed on the fibre surfaces. In a following step of EPD, a thick SiC layer was formed on the fibre mat when the CNT-coated SiC fibres were in contact with the positive electrode of the EPD cell; however, spaces between the fibres were not fully filled with SiC. Conversely, when CNT-coated SiC fibres were isolated from the electrode, the SiC particles were able to gradually fill the fibre mat resulting in relatively high infiltration, which leads to dense composites.     

A novel B–Si–Zr hybridized ceramizable phenolic resin and the thermal insulation properties of its fiber-reinforced composites

A novel B–Si–Zr hybridized ceramicizable resin(BSZ-PR) was fabricated by chemical reaction of boric acid, zirconium hydroxyl-containing polyhedral oligomeric silsesquioxane(Zr-POSS) and phenolic. The incorporation of boric acid and Zr-POSS improved the thermal stability of the resin effectively, and the residual carbon rate increased to 72.63% at 800 °C under nitrogen atmosphere. The flexural strength of carbon fiber/BSZ-PR and high silica fiber/BSZ-PR composites were increased by 25.7% and 175.5%, and linear ablation rates were reduced by 37% and 44.75%, respectively. It was discovered that the ceramic structures such as SiO₂, ZrO₂ and SiC can be formed at high temperatures as well as under extreme ablative conditions from both BSZ-PR and its fiber-reinforced composites, which may be the key to the improved thermal, ablative properties.