Microstructure and interlaminar shear property of carbon fiber-SiC nanowire/pyrolytic carbon composites with SiC nanowires growing at different positions

In order to improve the interlaminar shearing strength of carbon fiber/pyrolytic carbon (Cf/PyC) composites, SiC nanowires (SiCNWs) growing at different positions were introduced into carbon fiber/pyrolytic carbon composites to generate carbon fiber-SiC nanowire/pyrolytic carbon (Cf-SiCNWs/PyC) composites. Cf-SiCNWs/PyC composites were prepared by sol-gel and isothermal chemical vapor infiltration (ICVI) method. The morphology, microstructure and compositions of composites were investigated by SEM, TEM, XRD and XPS. The interlaminar shearing strength was tested and the effect of SiCNWs growth positions on the interlaminar shearing strength was investigated. The results showed that SiCNWs were consisted of perfect single crystalline structure of β-SiC with diameter of 160–200 nm. The SiCNWs could grow at four kinds of positions to combine with carbon fibers to form multi-scaled reinforcements (micro-scaled carbon fibers and nanoscaled SiCNWs). The interlaminar shear strength of Cf-SiCNWs/PyC composites were increased by 78% compared with Cf/PyC composites without SiCNWs. The improvement of interlaminar shear strength was attributed to bridging and pull-out of multi-scaled reinforcements composed of carbon fibers and SiCNWs as well as the enhancement of fiber/matrix interface bonding generated by SiCNWs growing at different positions.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.     

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     

Oxidation behavior of SiC/glaze-precursor coating on carbon/carbon composites

In order to improve the oxidation behavior of carbon/carbon composites in a wide range of temperature, a new SiC/glaze-precursor coating was developed.The SiC layer was produced by slurry and sintering, while the glaze precursor layer was prepared by slurry and drying. The microstructures and phase compositions of the coating were analyzed by SEM and XRD, respectively. The oxidation resistance of the coated composites was investigated using both isothermal and temperature-programmed thermogravimetric analysis in the temperature range from room temperature to 1600 °C. The results showed that the oxidation behavior of the coating was mainly controlled by the diffusion of oxygen during the test.The coating showed excellent oxidation resistance and self-healing ability in a wide range of temperature.     

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.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Effect of the pyrolytic carbon (PyC) content on the dielectric and electromagnetic interference shielding properties of layered SiC/PyC porous ceramics

A novel layered SiC/pyrolytic carbon (PyC) porous ceramic was synthesized from a nickel foam substrate via low-pressure chemical vapor infiltration (LPCVI) with SiCl₃CH₃-NH₃-BCl₃-H₂-Ar. The microstructure and phase composition of the PyC deposited via Ni catalysis were investigated. In addition, the effect of the PyC content on the microstructure, conductivity, and electromagnetic shielding effectiveness of a two-layered SiC/PyC porous ceramic were discussed. Both the electrical conductivity (from 0.090 to 0.319?S/cm) and the total shielding effectiveness (from 19.2 to 29.0?dB) of the two-layer SiC/PyC porous ceramic (pore size: 200–400?µm) increased with the PyC content. The high-temperature shielding effectiveness of the sample showed an outstanding stability with temperature and remained nearly unchanged (only 2?dB variation) over the 25–600?°C temperature range.     

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.     

Microstructure and anti-oxidation properties of multi-composition ceramic coatings for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at elevated temperature, an effective WSi₂-CrSi₂-Si ceramic coating was deposited on the surface of SiC coated C/C composites by a simple and low-cost slurry method. The microstructures of the double-layer coatings were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy analyses. The coating exhibited excellent oxidation resistance and thermal shock resistance. It could protect C/C composites from oxidation in air at 1773 K for 300 h with only 0.1 wt.% mass gain and endure the thermal shock for 30 cycles between 1773 K and room temperature. The excellent anti-oxidation ability of the double-layer WSi₂-CrSi₂-Si/SiC coating is mainly attributed to the dense structure of the coating and the formation of stable vitreous composition including SiO₂ and Cr₂O₃ produced during oxidation.     

SiC/SiC复合材料及其应用

日本开发的Nicalon和Tyranno两种品牌的SiC纤维占有世界上绝对性的市场份额。SiC/SiC复合材料典型的界面层是500 nm厚的单层热解碳(PyC)涂层或多层(PyC-SiC)n涂层,在湿度燃烧环境及中高温条件下界面层的稳定性是应用研究的重点。SiC/SiC复合材料,包括CVI-SiC基体和日本开发的Tyranno hex和NITE-SiC基体等,具有耐高温、耐氧化性和耐辐射性的特点,在航空涡轮发动机部件、航天热结构部件及核聚变反应堆炉第一壁材料等方面正开展工程研制应用。     

Effects of channel modification on microstructure and mechanical properties of C/SiC composites prepared by LA-CVI process

Carbon fiber reinforced SiC matrix composites (C/SiC) with four different deposition channel sizes were fabricated via a novel laser-assisted chemical vapor infiltration (LA-CVI) method. Effects of infiltration channel sizes on microstructure and mechanical properties of C/SiC composites were investigated. The results showed that increasing the size of channels could expand infiltration passages and densification bands, which was consistent with theoretical calculations. Due to the presence of channels, the flexural strength of C/SiC composite increased by 14.47% when the channel diameter was 0.3?mm, compared to C/SiC composites prepared via conventional CVI process. Characteristics of matrix cracking and crack propagation on fracture surface were analyzed by using scanning electron microscopy. LA-CVI C/SiC composites displayed significantly improved damage-tolerant fracture behavior. Thus, findings of this work demonstrate that LA-CVI fabricated C/SiC composites are promising for a wide range of applications, particularly for enclosed-structure and thick-section C/SiC composites.     

Ablative and mechanical properties of C/C-SiC-ZrC composites modified with PyC/SiC and PyC/BN interface layers

Two kinds of novel modified C/C-SiC-ZrC composites were prepared via precursor infiltration and pyrolysis, as pyrocarbon (PyC)/silicon carbide (SiC) and PyC/boron nitride (BN) dual-layer interphases were separately structured on the fibers by means of chemical vapor infiltration. Data analysis and conclusions are served for investigating the effects of these two interface layers on mechanical and anti-ablative properties. On the mechanical property hand, both PyC/BN and PyC/SiC interphase layers play positive roles, resting with the reduction of fiber damage during the fabrication process. Compared with BN, SiC shows better enhancement as the flexural strength of PyC/BN and PyC/SiC interphase-modified composites are 214.9 and 229.2 MPa, respectively. On the ablative property hand, after oxyacetylene flame ablation for 60 s, the mass and linear ablation rates of the composites modified by PyC/SiC interface were 2.2 mg/s and 9.7 μm/s, which is much lower than that modified by PyC/BN. The inferior ablation properties of PyC/BN-CSZ were attributed to the vaporization of the B₂O₃ gas that destroys the integrity of the oxide film and oxygen erosion on the substrate through the damaged BN interface.     

Wear behavior of SiC nanowire-reinforced SiC coating for C/C composites at elevated temperatures

To improve the wear resistance of SiC coating on carbon/carbon (C/C) composites, SiC nanowires (SiCNWs) were introduced into the SiC wear resistant coating. The dense SiC nanowire-reinforced SiC coating (SiCNW-SiC coating) was prepared on C/C composites using a two-step method consisting of chemical vapor deposition and pack cementation. The incorporation of SiCNWs improved the fracture toughness of SiC coating, which is an advantage in wear resistance. Wear behavior of the as-prepared coatings was investigated at elevated temperatures. The results show that the wear resistance of SiCNW-SiC coating was improved significantly by introducing SiC nanowires. It is worth noting that the wear rate of SiCNW-SiC coating was an order of magnitude lower than that of the SiC coating without SiCNWs at 800 °C. The wear mechanisms of SiCNW-SiC coating at 800 °C were abrasive wear and delamination. Pullout and breakage of SiC grains resulted in failure of SiC coating without SiCNWs at 800 °C.     

Microstructures and oxidation behaviors of Al-modified and Al2O3-modified SiC coatings on carbon/carbon composites via pack cementation

Al₂O₃-modified SiC (AOSC) and Al-modified SiC (ASC) coatings were prepared on carbon/carbon (C/C) composites by one-time pack cementation (PC). Their microstructures and anti-oxidation performances were studied. Compared with ASC coating, AOSC coating shows more conspicuous defects (micro-cracks and holes) and lower densification. ASC coating can offer better oxidation resistance and thermal shock resistance to C/C composites than AOSC coating. Al additive can more efficiently improve the sinterability of SiC, which causes the above results. Besides, Al₂O₃ oxidation product is more stable than SiO₂ (l) of oxidized SiC at 1500 °C based on the thermodynamic analysis.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

The mechanical and repair behavior of damaged carbon/carbon composites

With the aim of understanding the effect of defect types on the mechanical performance of carbon/carbon (C/C) composites, three kinds of defects such as circle arc, square, and triangle shapes were prefabricated on their surfaces. The results show that the prefabricated defects damage the flexural strength of C/C composites compared to the pristine sample (101 ± 6 MPa). The flexural strength of C/C decreased by 30.84%, 45.84%, and 42.58% corresponding to the circle arc, square, and triangle type defects respectively. The defect-repair method with Ni-based solder as the additive was employed to repair the damaged C/C composites. After repair, the stress concentration of C/C composites decreases, and there is a good connection between carbon fiber and the repaired solder so that the load can be transferred continuously, therefore the flexural strength of C/C composites can be improved by 20–28%.     

Effects of multilayer hydrothermal carbon interphases on mechanical properties and thermal shock resistance of CF/ZrB2-SiCBN

Multilayer hydrothermal carbon coatings (HTCCs) with various thicknesses were constructed on CFs using alternating hydrothermal carbonization and heat treatment approach. Well-established CF/ZrB₂-SiCBN composites with distinct multilayer interphases were processed via slurry injection and precursor infiltration pyrolysis method. The multilayer HTCCs with suitable thickness would be pulled-out stepwise from the matrix, which could absorb the impact energy and toughen the composites effectively. Although the outer HTCC would still react with the matrix to form a strong interface, the moderate interfacial binding force between CF and inner HTCC could effectively transfer the load and assure the fiber debonding when the composites deformed drastically. Moreover, the increasing of coating thickness could alleviate the damage of fibers in oxidation environment and improve the critical thermal shock temperature difference of composites, while the excessive thick coating would weaken the fiber strength and limit toughening abilities of the CFs.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.