Mechanical properties and ablation resistance of HfC nanowire modified carbon/carbon composites

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.     

Microstructures and enhanced flexural properties of single-crystalline HfC nanowires in-situ modified carbon/carbon composites by electrophoresis-thermal evaporation using CNTs as the template

To improve the mechanical properties of carbon/carbon (C/C) composites, in-situ synthetized single-crystalline hafnium carbide nanowires (HfCnws) were introduced into the carbon fiber preforms by electrophoresis-thermal evaporation method. The Multi-walled carbon nanotubes (MWCNTs) were utilized as the carbon source and templates for forming HfCnws. The microstructure, chemical composition and mechanical properties of the HfCnws modified carbon/carbon (HfCnws-C/C) composites were characterized. Results reveal that HfC is produced preferentially in the inner nodular parts and end parts of MWCNTs. The raising heat-treatment temperature would influence the diffusion rate of Hf atoms and then the number of nucleation sites, which further changed the aspect ratio and morphology of HfCnws. The HfCnws have refined the grain size of pyrolytic carbon (PyC), and significantly improve the flexural strength of C/C composites by 79.3%.     

Thermophysical properties of carbon/carbon composites and physical mechanism of thermal expansion and thermal conductivity

Five different carbon/carbon composites (C/C) have been prepared and their thermophysical properties studied. These were three needled carbon felts impregnated with pyrocarbons (PyC) of different microstructures, chopped fibers/resin carbon + PyC, and carbon cloth/PyC. The results show that the X-Y direction thermal expansion coefficient (CTE) is negative in the range 0-100 °C with values ranging from −0.29 to −0.85 × 10⁻⁶/K. In the range 0-900 °C, their CTE is also very low, and the CTE vs. T curves have almost the same slope. In the same temperature range composites prepared using chopped fibers show the smallest CTE values and those using the felts show the highest. The microstructure of the PyC has no obvious effect on the CTE for composites with the same preform architecture. Their expansion is mainly caused by atomic vibration, pore shrinkage and volatilization of water. However, the PyC structure has a large effect on thermal conductivity (TC) with rough laminar PyC giving the highest value and isotropic PyC giving the lowest. All five composites have a high TC, and values in the X-Y direction (25.6-174 W/m K) are much larger than in the Z direction (3.5-50 W/m K). Heat transmission in these composites is by phonon interaction and is related to the preform and PyC structures.     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Effects of silanization of C/C composites and grafting of h-BN fillers on the microstructure and interfacial properties of CVI-based C/C-BN composites

A low-density carbon/carbon (C/C) composite/silane coupling agent/hexagonal boron nitride (h-BN) hybrid reinforcement was prepared by grafting polyethyleneimine (PEI)-encapsulated modified h-BN fillers onto a carbon fiber surface using 3-aminopropyltriethoxysilane (APS) as the connection to improve the distribution uniformity of h-BN fillers in quasi-three-dimensional reinforcements and the interfacial properties between the fibers/pyrocarbon (PyC) in the C/C-BN composites obtained after densification by chemical vapor infiltration (CVI). The microstructure and chemical components of the hybrid reinforcement were investigated. The transmission electron microscopy (TEM) sample was prepared using a focused-ion beam (FIB) for the h-BN/PyC interfacial zone. The interlaminar shear strength (ILSS) and impact toughness were analyzed to inspect the composites’ interfacial properties. The results show that APS and h-BN are uniformly grafted on the fiber surface in the chopped fiber web inside the C/C composite without a density gradient, and agglomeration occurred and significantly increasing the fiber surface roughness. The highly ordered h-BN basal plane may affect the order degree of PyC near the h-BN/PyC interface. The addition of h-BN reduces the PyC texture near it, causing the annular cracks to disappear gradually. The lower PyC texture and the rougher fiber surface strengthen the interfacial bond of the fiber/matrix. Consequently, the ILSS strength of the C/C-BN composites first increases and then decreases as the h-BN filler content increases and is always higher than that of the C/C composite, while the addition of h-BN fillers weakens its impact toughness. When the h-BN content in the C/C-BN composite is 10 vol%, the ILSS of the C/C-BN composites was 15.6% higher than that of the C/C composites. However, when the h-BN content is excessive (15 vol%), the densely grafted h-BN will bridge each other, reducing the subsequent CVI densification efficiency to form a loose interface, causing a decrease in the shear strength.     

Integrative improvement on thermophysical properties and ablation resistance of laminated carbon/carbon composites modified by in situ grown HfC nanowires onto carbon fiber cloths

HfC nanowires modified carbon fiber cloth laminated carbon/carbon (HfC_nw-C/C) composites were fabricated by in situ growth of HfC nanowires on carbon cloths via catalytic CVD, followed with lamination of the cloths and densification by pyrolytic carbon (PyC). Morphologies, thermal conductivity, coefficient of thermal expansion (CTE), and ablation resistance of the composites were investigated. Due to the loading of HfC nanowires, the matrix PyC with low texture was obtained; the thermal conductivity of the composites in the Z direction was enhanced from 100℃ to 2500℃; CTE along the X–Y direction also decreased in the range of 2060 ℃ – 2500 ℃, which reaches the maximum of 24 % at 2500℃. Moreover, the 20s-ablation-resistance of HfC_nw-C/C composites exhibits mass and linear ablation rates of 5.3 mg/s and 21.0 μm/s, which are 40 % and 37 % lower than those of pure C/C composites, respectively. Our work shows laminated HfC_nw-C/C composites are a promising candidate for high-temperature applications.     

Improved thermal shock resistance of SiCnw/PyC core-shell structure-toughened CVD-SiC coating

In-situ SiC nanowire (SiCnw)/pyrolytic carbon (PyC) core-shell structures were introduced to mainly improve the thermal shock performance of chemical vapor deposition (CVD)-SiC coating on carbon/carbon (C/C) composites. The microstructure, phase composition, and mechanical properties of the CVD-SiC coating toughened by SiCnw/PyC core-shell structures were studied as well. The results show that the introduction of SiCnw/PyC core-shell structures can effectively alleviate the mismatch of coefficient of thermal expansion (CTE) between SiC coating and C/C substrate, thus enhancing the thermal shock resistance of the coating. Furthermore, the increased numbers of interfaces in the SiC coating owing to the addition of core-shell structures are beneficial to the mechanical properties of the coating after thermal shock test.     

Effect of PyC interphase thickness on mechanical behaviors of SiBC matrix modified C/SiC composites fabricated by reactive melt infiltration

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.     

Effect of preparation method on the mechanism for oxidation of C/C-BN composites

C/C-BN composites and C_f/BN/PyC composites exhibiting different structures for pyrolytic carbon (PyC) and boron nitride (BN) were studied comparatively to determine their oxidation behavior. This study used five types of samples. Porous C/C composites were modified with silane coupling agents (APS) and then fully impregnated in water-based slurry of hexagonal boron nitride (h-BN); the resulting C/C-BN preforms were densified by depositing PyC by chemical vapor infiltration (CVI), resulting in three types of C/C-BN composites. The other two C_f/BN/PyC composites were obtained by depositing a BN interphase and PyC in carbon fiber preforms by CVI; one was treated with heat, and the other was not. This study was focused on determining how the PyC deposition mechanism, morphology and pore structure were affected by the method of BN introduction. In the 600–900 °C temperature range, the C_f/BN/PyC composites and C/C composites underwent oxidation via a mixed diffusion/reaction mode. The C/C-BN composites had a different pore structure due to the formation of nodules comprising h-BN particles; both interfacial debonding and cracking were reduced, resulting in higher resistance to gas diffusion, lower oxidation rate and larger activation energy (Ea) in the temperature range 600–800 °C. In addition, the mechanism for oxidation of C/C-BN composites gradually exhibited diffusion control at 800–900 °C because the formation of h-BN oxidation products healed the defects. The oxidation mechanism was more dependent on pore structure than on BN structure or content.     

Thermophysical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Three kinds of carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites (n = 1, 2 and 4) were prepared by means of layer-by-layer deposition of PyC and SiC via chemical vapor infiltration. Thermal expansion behaviors in the temperature range of 800–2500 °C and thermal conductivity from room temperature to 1900 °C of C/(PyC–SiC)_n composites with various microstructures were investigated. The results show that with increasing PyC–SiC sequences number (n), the coefficients of thermal expansion of the composites decrease due to the increase of interfacial delamination, providing room for thermal expansion. The thermal diffusivity and thermal conductivity also decrease with the increase of sequences number, which are attributed to the enhancement of phonon-interface scattering resulted from the increasing number of interfaces. Modified parallel and series models considering the interfacial thermal resistance are proposed to elaborate thermal conductivity of the composites, which is in accordance with the experimental results.     

Synthesis of high conductivity Polyaniline/Ag/graphite nanosheet composites via ultrasonic technique

A new process for the synthesis of high conductivity polyaniline/Ag/graphite nanosheet (PANI/Ag/NanoG) composites was developed. Graphite nanosheet was prepared by treating the expanded graphite in aqueous alcohol solution using sonication, and a uniform silver film about 470 nm thick was obtained on graphite nanosheet surface via an improved electroless plating method. Then PANI/Ag/NanoG composites were fabricated via in situ polymerization of aniline monomer in the presence of silver coated graphite nanosheet through using ultrasonic technique. The sliver particles and composites were evidenced by scanning and transmission electron microscopy examinations, the results showed that the silver coated graphite nanosheet particles played an important role in forming conducting bridge in polyaniline matrix. According to the electrically test, the conductivity of the PANI/Ag/NanoG composites was dramatically increased compared with pure PANI. From the thermogravimetric analysis, the PANI/Ag/NanoG composites exhibited a beneficial effect on the thermal stability of pure PANI.     

Large-scale synthesis of SiC/PyC core-shell structure nanowires via chemical liquid-vapor deposition

In carbon/carbon (C/C) composites, SiC/PyC core-shell structure nanowires were successfully fabricated via chemical liquid-vapor deposition (CLVD). The influences of heat-treatment temperature on the microstructure and composition of SiC nanowires were studied, and meanwhile the growth mechanism of SiC nanowires was discussed. Additionally, the microstructure and morphology of SiC/PyC core-shell structure nanowires were also investigated. The results displayed that the low heat-treatment temperature could not meet the requirements of SiC nanowires growth, but the too high temperature made the nanowires appear agglomerate easily. Only when the heat-treatment temperature was 1800 °C, SiC nanowires possessed a uniform distribution. The diameter of SiC nanowire was about 300 nm, and there was a SiO₂ layer with the thickness of about 1 nm existing on the surface of SiC nanowire. The growth behavior of SiC nanowire was governed by vapor-solid (V–S) mechanism. After the PyC deposition, SiC/PyC core-shell structure nanowires were constructed, and the nanowires were about 450 nm in diameter. These nanowires displayed a core-shell structure with three layers, which were SiC nanowire core, SiO₂ interlayer and PyC shell, respectively. Meanwhile, SiC/PyC core-shell structure nanowires connected the matrices with each other, and the core-shell structure nanowires generated a stable network.     

PyC interphase growth mechanism and control models of C/SiC composites

To better understand the pyrocarbon (PyC) interphase growth mechanism, a series of experiments was conducted on the PyC deposited on T-300™ and T-700™ carbon fibers by the chemical vapor infiltration (CVI) method. Nine groups of fabrication parameters were used to analyze the effects of deposition temperature, pressure, and residence time on the PyC interphase growth mechanism. Atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), Raman spectroscopy, and nanoindentation tests were performed to characterize the microstructures of carbon fibers and PyC interphase. The PyC interphase growth mechanism was discussed, and the relationships between the fabrication parameters, R (C₂/C₆) value, texture type, and interphase thickness were established through numerical simulations. The hardness and modulus of PyC for T-300™ and T-700™ carbon fibers were measured. The tensile behaviors of C/SiC minicomposites with medium and high textures PyC interphases were analyzed. The C/SiC composite with the medium texture PyC interphase possessed the higher fracture strength and failure strain with a longer fiber pullout length at the fracture surface.     

Microstructure and ablation performance of HfC/PyC core-shell structure nanowire-reinforced Hf1-xZrxC coating

To improve the ablation performance of C/C composites, HfC/PyC core-shell structure nanowire (HfCnw/PyC)-reinforced Hf_1-xZr_xC coating was prepared via three-step chemical vapor deposition (CVD) method. Effects of HfCnw/PyC and PyC layer thickness on the microstructure, residual stress and ablation performance of Hf_1-xZr_xC coating were studied. HfCnw/PyC-reinforced coatings exhibited equiaxial crystal structure. After incorporating HfCnw/PyC, ablation property of Hf_1-xZr_xC coating was enhanced because of the skeleton role of HfO₂ nanowires. PyC possessed low coefficient of thermal expansion (CTE) and high heat conductivity, but poor ablation performance. Hence, with the increase in thickness of PyC layer, ablation property of the coating first increased and then decreased. HfCnw/PyC-reinforced Hf_1-xZr_xC coating with PyC layer thickness of about 50 nm exhibited the best ablation property.     

Preparation and mechanical properties of C/C composites reinforced with arrayed SiC columnar pins

C/C composites with SiC columnar pins were fabricated by a recently developed space-holder method. Effects of SiC columnar pins with pins-row spacing of 5 mm and 4 mm on mechanical properties and toughening of C/C composites were characterized and discussed. Corresponding porous C/C composite matrices were also characterized. The results show that introduction of SiC columnar pins not only improves the compressive and shear properties of C/C composites, but significantly affects the PyC texture of the C/C composite matrix. Under identical TG-CVI deposition conditions, the pristine C/C composites (S₀), the unidirectional porous C/C composites (S₁ and S₂), and the C/C composites with SiC columnar pins (S₃ and S₄) show typical low-textured PyC, high-textured PyC, and medium-textured PyC, respectively. The mechanical properties of unidirectional porous C/C composites with channels-row spacing of 5 mm (S₁) are higher than those of unidirectional porous C/C composites with channels-row spacing of 4 mm (S₂). Conversely, for the C/C composites with SiC columnar pins, the mechanical properties of samples with columns-row spacing of 5 mm (S₃) are lower than those with columns-row spacing of 4 mm (S₄). Moreover, the compressive strength P_//(load direction parallel to the channel), P_⊥ (load direction vertical to the channel), and shear strength of S₃ and S₄ is respectively higher than that of S₁. Therefore, introduction of SiC columnar pins can effectively improve the mechanical properties of composites without significantly changing the density.     

Siliconization elimination for SiC coated C/C composites by a pyrolytic carbon coating and the consequent improvement of the mechanical property and oxidation resistances

Carbon/carbon (C/C) composites have a wide application as the thermal structure materials because of their excellent properties at high temperatures. However, C/C composites are easily oxidized in oxygen-containing environment, which limits their potential applications to a great degree. Silicon carbide (SiC) ceramic coating fabricated via pack cementation (PC) was considered as an effective way to protect C/C composites against oxidation. But the mechanical properties of C/C composites were severely damaged due to chemical reaction between the molten silicon and C/C substrate during the preparation of SiC coating by PC. In order to eliminate the siliconization erosion, a pyrolytic carbon (PyC) coating was pre-prepared on C/C composites by the chemical vapor infiltration (CVI) prior to the fabrication of SiC coating. Due to the retardation effect of PyC coating on siliconization erosion, the flexural strength retention of the SiC coated C/C composites with PyC coating increased from 46.27 % to 107.95 % compared with the specimen without PyC coating. Furthermore, the presence of homogeneous and defect-free PyC coating was beneficial to fabricate a compact SiC coating without silicon phase by sufficiently reacting with molten silicon during PC. Therefore, the SiC coated C/C composites with PyC coating had better oxidation resistances under dynamic (between room temperature and 1773 K) and static conditions in air at different temperatures (1773?1973 K).     

Microstructure,mechanical and anti-ablation properties of SiCnw/PyC core-shell networks reinforced C/C–ZrC–SiC composites fabricated by a multistep method of chemical liquid-vapor deposition

C/C–ZrC–SiC composites reinforced by SiC nanowire (SiCnw)/pyrocarbon (PyC) core-shell networks were prepared by a multistep method of chemical liquid-vapor deposition (CLVD). The microstructure, mechanical property and ablation resistance were researched. The investigations presented that the PyC was deposited on the SiC nanowires, and the micro-scale core-shell structures were produced. Moreover, these micro-scale structures not only connected with the fibers and matrices, but also filled the pores in the composites. In contrast with C/C–ZrC–SiC composites, the flexural modulus and strength of SiCnw/PyC-C/C–ZrC–SiC composites increased by 36.91% and 44.53%, and the fracture mode was changed from the brittle to pseudo-plastic fracture. After the oxyacetylene torch ablation at two temperatures for 90s, the composites strengthened by SiCnw/PyC core-shell possessed a better resistant ablation. At ablation temperature of 2300 °C, the mass loss rate and linear reduction rate of the composites with core-shell networks decreased by 66.18% and 57.55% in contrast with the non-reinforced composites, and declined by 56.46% and 57.48% at ablation temperature of 3000 °C. The obvious decrease of ablation rates was ascribed to the dense microstructure, the small coefficient of thermal expansion (CTE), the good thermal conductivity, and the resistant ablation roles of SiCnw/PyC core-shell systems.     

Carbon nanotube reinforced pyrocarbon matrix composites with high coefficient of thermal expansion for self-adapting ultra-high-temperature ceramic coatings

The mismatch in the coefficients of thermal expansion (CTE) of the carbon fiber reinforced pyrocarbon (Cf/C) composites and their thermal barrier coatings (TBCs) has significantly restricted the service life of Cf/C composites in high-temperature environments. Owing to the high CTE of TBCs, it is vital to find a material with similar mechanical properties and higher CTE than Cf/C composites. In this work, carbon nanotube reinforced pyrocarbon (Ct/C) nanocomposites with high CTEs were prepared to self-adapt to the TBCs. Different CTEs (～4.0–6.5 × 10^?6/°C) were obtained by varying the carbon nanotube (CNT) content of the Ct/C composites. Owing to the decreased mismatch in the CTEs, no cracks were formed in the TBCs (SiC and HfB₂-SiC-HfC coatings) deposited on the Ct/C composites. After heat treatment at 2100 °C, several wide cracks were found in the TBCs on the Cf/C composite, whereas the TBCs on the Ct/C composites were intact without cracks. We found that the CTE-tunable Ct/C composites can self-adapt to different TBCs, protecting the composites from oxidation at high temperatures.     

Floating catalyst chemical vapor infiltration of nanofilamentous carbon reinforced carbon/carbon composites – Integrative improvement on the mechanical and thermal properties

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were produced by floating catalyst chemical vapor infiltration with ferrocene content ranging 0–2.0?wt%. The NFCs and increased graphitization degree led to an improvement on the mechanical and thermal properties. An excellent combination of high strength and thermal conductivity (TC), and low coefficient of thermal expansion (CTE) was reached by adding 0.5–0.8?wt% catalyst. When the content exceeded 0.8wt%, the strength and TC were decreased by the limited NFC growth and matrix transited from rough laminar to isotropic pyrocarbon. After the treatment of 2500?°C, the strength and CTE decreased whereas the TC was increased. With the catalyst contents at 0.5–0.8?wt%, the flexural and shear strength retention ratios achieved a high value of 73.1–74.5 and 79.1–79.4%, respectively, and the in-plane and out-of-plane TCs exhibited maxima of 339.1 and 72.5?W/(m?K). Relatively low CTE was obtained at 2.0?wt% catalyst owing to the increased amount of cracks and pores.