Pyrolytic Carbon Interface Research Articles

Investigation on crack propagation of SiCf/SiC composite via PF-CZM method

SiCf/SiC composite materials had become a research hotspot in the fields of aerospace materials and nuclear materials in recent years. This materials have high strength, radiation resistance, high temperature resistance and oxidation resistance. However, SiC is a typical brittle material prone to being damaged due to stress concentration in some cases, leading to cracks and even component failure. Therefore, this study employed an accurate and efficient numerical simulation method, phase field cohesive zone method (PFCZM), to investigate the microscopic damage behavior of SiCf/SiC composite materials. This study aimed to illustrate the influence of the thickness of pyrolytic carbon (PyC) interface layer on the failure behavior of SiCf/SiC composite materials. Firstly, this study conducted relevant verification of PFCZM. Secondly, numerical simulations of crack initiation, crack propagation, and final damage were performed by establishing a representative volume element (RVE). Observing the simulation results, it was noted that at low strain, when PyC was thicker, the SiCf/SiC composite materials exhibited slight damage to both the SiC matrix and PyC interface. Conversely, when PyC was thinner, the SiCf/SiC composite materials displayed only slight damage to PyC interface. As the PyC thickness increases, the damage to PyC interface decreases, while the damage to SiC becomes more prominent. Simultaneously, a competitive relationship in the damage behavior of SiCf/SiC composite materials can be found after the SiC matrix has cracked: When PyC is thinner, the damage primarily manifests as interlayer delamination between the SiC matrix and the SiC fiber. Conversely, when PyC is thicker, it manifests in PyC failure. Finally, this study concluded that, in this investigation, a PyC interface layer with a thickness of 0.1 μm exhibited the most effective protective effect on SiCf/SiC composite material.

Changes in microstructure and mechanical properties of the carbon fiber and their effects on C/SiC composites

The changes in microstructure and mechanical properties of the carbon fiber and their effects on C/SiC composites were studied. The H, N and O atoms escaped and the carbon content in the carbon fiber increased when the heat treatment temperature exceeds 1200 °C. Meanwhile, the growth of graphite crystallite leaded to stress concentration and reduced the tensile strength of carbon fiber to 2.98 GPa by 1200 °C heat treatment. The active atoms on the surface of the carbon fiber were reduced, thus weakening the bonding force between the carbon fiber and the pyrolytic carbon interface layer and improving the pullout mechanism of the carbon fiber. This result was also observed by synchrotron radiation X-ray computed tomography. The flexural strength of the C/SiC composite increased from 300 MPa without treatment to 364 MPa after 1600 °C heat treatment.

Microstructure of irradiated AGR TRISO particle buffer layers as measured by X-ray computed tomography

Shrinkage of the initially low-density buffer layer in tristructural isotropic (TRISO) coated fuel particles during irradiation is a well-known phenomenon with potential implications for fission product and actinide transport, as well as potential pyrocarbon fracture that in some cases has been observed to impact particle performance. During post-irradiation examination, the buffer layer's structure is commonly determined using 2D microscopy of a particle cross section or a series of particle cross sections at staggered depths. Although these methods provide a general idea of the irradiated buffer microstructure, they do not provide a full picture of the TRISO coating microstructure. By contrast, x-ray computed tomography (XCT) provides full 3D imaging of the TRISO particle. Particles from various compacts irradiated for the Advanced Gas Reactor Fuel Development and Qualification Program were imaged using XCT. Some of these particles were selected because of abnormal fission product inventories (e.g., low 137Cs), whereas others were randomly selected from the center of the fission product inventory distribution. Qualitative and quantitative analysis techniques were applied to the randomly selected particles as representatives of typical TRISO behavior to study the post-irradiation structure of the buffer layer. These results showed that, while separation of the buffer and inner pyrolytic carbon layers was a common behavior during the radiation-induced shrinkage of the buffer, a portion of the original buffer/inner pyrolytic carbon interface remained intact throughout irradiation in nearly all cases. These results also indicated clear trends in the degree of buffer densification with irradiation temperature and fluence.

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

AbstractTo tailor the fiber–matrix interface of SiC nanowires‐reinforced SiC (SiCnw/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. SiCnw/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 m1/2 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 SiCnw from degradation as well as improving the more moderate interfacial bonding strengths during the RMI.

Failure evaluation of neutron-irradiated SiC/SiC composites by underwater acoustic emission

Understanding the statistical properties of mechanical properties of non-irradiated and neutron-irradiated SiC/SiC composites is essential for component design. This study aims to evaluate the detailed damage accumulation behavior of composites focusing on two fracture parameters: proportional limit stress (PLS) and acoustic emission (AE) onset stress. The developmental underwater AE technique, which is benefit in non-contact in-situ failure monitoring method during mechanical testing and in handle of the irradiated material, was first applied to evaluate damage accumulation behavior. Two types of chemical vapor infiltration SiC/SiC composites were used: one reinforced with Hi-Nicalon Type-S SiC fiber and one reinforced with Tyranno-SA3 SiC fiber in the form of satin-woven 2D fabrics with pyrolytic carbon interface. Neutron irradiation in the High Flux Isotope Reactor at Oak Ridge National Laboratory reached a fluence of 30 dpa at a temperature of 620–670°C. Four-point flexural tests were conducted to evaluate post-irradiation strength. Weibull statistics did not suggest marked degradation of composite strength. Detailed failure behavior evaluated by AE demonstrated no irradiation-induced change of the AE onset stress (i.e., crack initiation equivalent stress). Failure probability analysis suggests that increasing the reliability of composites (i.e., the Weibull modulus rather than strength itself) is essential to expanding the design margin and benefiting from a probabilistic design approach.

Effect of slurry and sol‐gel introduce SiCnws on ablation and bending behaviors of modified SiCf/HfC‐SiC composites

AbstractThe slurry and sol‐gel methods were used to introduce SiC nanowires (SiCnws) into the SiCf/HfC‐SiC composites. The microstructures, ablation, and bending behaviors of the SiCnws modified composites prepared by the two methods were compared. The bending strengths of the modified composites obtained by introducing SiCnws by the slurry and sol‐gel methods were 224 ± 19 and 154 ± 14 MPa, respectively. The results showed that SiC fibers with chemical corrosion and thermal damage during the sol‐gel process decreased the bending strength of the SiCnws‐modified SiCf/HfC‐SiC composites. Meanwhile, the pyrolytic carbon interface accompanying corrosion damage in the sol‐gel process led to the degradation of interface function, which hindered the interface debonding and fiber sliding of the composites during the bending test. After ablation, the bending strengths of the two composites were 188 ± 19 and 50 ± 7 MPa, respectively. The bending strength retention of the modified composites fabricated by the slurry method (83.9%) was higher than that (32.5%) of the composites fabricated by the sol‐gel method after ablation. As the composites fabricated by the slurry method exhibited a good ablation resistance under the oxyacetylene flame (∼2350°C).

Fiber/matrix debonding evaluation of SiCf/SiC composites using micropillar compression technique

Third-generation silicon carbide (SiC) composites reinforced by SiC fibers (Hi-Nicalon S [HNS] and Tyranno SA3 [SA3]) are attractive for use in next generation reactors owing to their high strength and chemical inertness at high temperatures, as well as enhanced radiation tolerance under neutron irradiation environments. To optimize composite performance, the interfacial mechanical properties of chemical vapor–infiltrated (CVI) SiCf/SiC composites are investigated in this effort by using a slant interface micropillar compression testing procedure. The micropillar test specimens, containing an inclined pyrolytic carbon (PyC) interphase, are prepared using a focused ion beam. The novel microcompression testing successfully quantifies the debond shear strength and internal friction coefficient of micropillar test samples by using the Mohr-Coulomb formulation. According to four types of SiCf/SiC composite microcompression test results, interfacial properties and debond mechanisms are significantly affected by the PyC layer thickness, the local bonding mechanism of the PyC interphase on the SiC fiber surface, and the surface roughness of fibers. Regardless of PyC thicknesses, SA3-reinforced CVI SiCf/SiC composites are found to have much higher debond shear strengths than HNS-reinforced SiCf/SiC CVI composites. By using this micropillar compression technique alongside analytical methods, we uncover new understandings of PyC interface properties. Additionally, the micropillar test results obtained are correlated with macroscopic mechanical properties of neutron-irradiated CVI SiCf/SiC composites.

Effects of heat treatment on the mechanical properties at elevated temperatures of plain-woven SiC/SiC composites

The effects of heat treatment on the mechanical properties of plain-woven SiC/SiC composites at 927 °C and 1200 °C in argon were evaluated through tensile tests at room temperature and at elevated temperature on the as-received and heat-treated plain-woven SiC/SiC composites, respectively. Heat treatment can improve the mechanical properties of composites at room temperature due to the release of thermal residual stress. Although heat treatment can damage the fiber, the effect of this damage on the mechanical properties of composites is generally less than the effect of thermal residual stress. Heat treatment will graphitize the pyrolytic carbon interface and reduce its shear strength. Testing temperature will affect the expansion or contraction of the components in the composites, thereby changing the stress state of the components. This study can provide guidance for the optimization of processing of ceramic matrix composites and the structural design in high-temperature environments.

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.

Effect of Pyrolytic Carbon Interface Thickness on Conductivity and Tribological Properties of Copper Foam/Carbon Composite

To improve electrical conductivity of the carbon-based pantograph strip, a sliding contact material of pyrolytic carbon (PyC) coated-copper foam/carbon composite was fabricated by chemical vapor deposition technology, followed by densifying processes of furan resin impregnation and carbonization. Morphology, electrical conductivity, wear and friction behavior of the composites are investigated to clarify the effect of PyC thickness (from 0 μm to 280 μm) on tribological behaviors and current transfer characteristic. The results show that there is a good interface combination between copper foam and resin carbon matrix in the composites after the improvement of interface wettability by inserting PyC layer. The composite has great advantages in electrical conductivity and density due to the copper foam and PyC layer with three-dimensional structure. With increasing the PyC thickness, both electrical conductivity and wear resistance of the composite have steadily increasing tendency, but friction coefficient has no obvious change. In addition, an analytical model is developed to explore current transfer mechanisms of the PyC-copper foam/carbon composite, and the theoretical predictions are in agreement with the experimental observations, in term of electrical conductivity and porosity of composite preform.

High-temperature cyclic fatigue in air of SiCf/SiC ceramic matrix composite with a pyrolytic carbon interface

A cyclic fatigue test of SiCf/SiC ceramic matrix composites was conducted at 1400°C and compared to the monotonic tensile test. The specimens were prepared with an interface layer of pyrolytic carbon and densified through chemical vapor infiltration. In the monotonic tensile test, at 1400°C, the specimen fractured at a strain of 0.35% with a proportional limit stress of 175 MPa, showing a typical fiber pull-out. However, after a prolonged cyclic test with increasing stresses from 65 to 95 MPa, the specimen fractured brittlely with almost no fiber pull-out. The microstructure analysis of the fracture surface showed different oxidation levels with respect to fracture locations, indicating that the crack propagated during the cyclic fatigue test. Transmission electron microscopy analysis revealed that the interface layer of pyrolytic carbon was removed by oxidation and oxide layers were formed on both sides of the fiber and matrix at the later stage of the cyclic test, resulting in a strong interface between the fibers and matrix and brittle fracture during the cyclic test at 1400°C.

Depositing CNTs on the interface and surface of C/PyC/SiCs for tunable mechanical and electromagnetic properties

The incorporation of carbon nanotubes (CNTs) in carbon fiber reinforced silicon carbide composites with pyrolytic carbon interface (C/PyC/SiCs) can be very beneficial for aerospace and military applications. In the present study, CNTs were introduced onto internal PyC interface (I-CNTs) and onto external SiC surface (C-CNTs) through electrophoretic deposition (EPD) method to tune the mechanical and electromagnetic properties of C/PyC/SiCs. Results indicated that with the increase of EPD duration, flexural strength of I-CNTs and C-CNTs increased by 25.3% and 11.4%, respectively, owing to energy consumption of CNTs between PyC interface and SiC matrix. The electrical conductivity of I-CNTs and C-CNTs were found to be in a quadratic relation with EPD duration, while the conductivity of I-CNTs increases from 4.21 to 5.96 S/cm and that of C-CNTs increases from 4.47 to 5.77 S/cm. The absorption and total electromagnetic shielding effectiveness of I-CNTs was found to be higher than that of C-CNTs due to the increase of CNTs content on PyC interface and electrical conductivity. Therefore, the I-CNTs with excellent mechanical and electromagnetic shielding performance can be used in various applications.

Effect of pyrolytic carbon interface thickness on conductivity and mechanical and wear properties of copper mesh modified carbon/carbon composite

Carbon fiber/copper fiber-reinforced carbon matrix composite, a kind of novel sliding contact strip material, was fabricated using a chemical vapor infiltration (CVI) technology, following by furan resin impregnation and carbonization processes. To improve the interface combination, pyrolytic carbon (PyC) with different layer thicknesses (0–18.86 μm) were formed on the copper and carbon fiber surfaces via controlling deposition time of the CVI. Microstructure, electrical conductivity, flexural strength and current-carrying friction performances were investigated. The results showed the dominant affecting factor on the material performances was the microstructure of matrix carbon. The electrical conductivity increased with increasing of the PyC layer thickness, which was an approximate linear collection. As compared to the one without the PyC interface, Cf/Cuf-C composites with PyC interface increased flexural strength and wear resistance by more than two times owing to the effective protection for carbon fibers and copper fibers. In addition, three physical models were used to describe wear mechanisms of Cf/Cuf-C composites with different PyC layer thicknesses.

Effect of pyrolytic carbon interface thickness on microstructure and mechanical properties of lightweight zirconium boride modified carbon-bonded carbon fiber composites

Abstract To improve the mechanical properties of carbon-bonded carbon fiber (CBCF) composites, they are firstly fabricated by chemical vapor deposited (CVD) pyrolytic carbon (PyC) coating layer on the carbon fiber surface, and then modified by zirconium boride (ZrB 2 ) using three cycles of precursor infiltration and pyrolysis (PIP) process. The effects of different PyC interface thickness on the microstructure and mechanical properties of ZrB 2 modified PyC coated CBCF (PyC–CBCF/ZrB 2 ) composites were studied and characterized. As the PyC thickness increased from 0.5 to 3.6 μm, the flexural properties of PyC–CBCF/ZrB 2 composites are noticeably enhanced in x / y and z direction, respectively. Mechanical enhancements for PyC–CBCF/ZrB 2 composites are mainly attributed to the effective interface bonding between carbon fiber and PyC, crack deflection and branching within the laminar PyC layer and carbon fiber pullout from PyC interface coating.

Effect of pyrolytic carbon interface on the properties of 3D C/ZrC–SiC composites fabricated by reactive melt infiltration

3D C/ZrC–SiC composites have been fabricated by slurry infiltration combined with reaction melt infiltration method. In order to clarify the effect of fiber/matrix (F/M) interface on properties on 3D C/ZrC–SiC, pyrocarbon (PyC) interface with various thickness (160–1100nm) was introduced by chemical vapor infiltration. The microstructure, mechanical properties and ablation properties were investigated systematically. Results showed that thicker PyC could protect fiber bundles from the erosion caused by the melt, leading to the as-prepared 3D C/ZrC–SiC with more excellent mechanical properties. The flexural strength and fracture toughness of C/ZrC–SiC composites with 850nm PyC was 125.67±2.49MPa and 4.82±0.48MPam1/2, respectively, which obtained 181% and 81% improvement compared with that of C/ZrC–SiC with 160nm PyC. However, as the thickness of PyC rised from 850nm to 1100nm, mechanical properties of C/ZrC–SiC decreased obviously. Also, after the ablation test, it was found that C/ZrC–SiC with 470nm PyC showed the best ablation resistance as SiC content was improved and porosity was lower in contrast with that of other composites.

Microstructure-flexural strength correlation and efficacy of rare earth nitrates on the sintering of SiCf/SiC composites

A vacuum infiltration technique was used to infiltrate a SiC-based matrix phase, containing rare earth (Sc, Er and Yb) nitrate (10wt.%) as a sintering additive, into a 2D-woven Tyranno SiC fabric preform coated with pyrolytic carbon (PyC) for the fabrication of a dense SiCf/SiC composite. By inserting a SiC green tape, a high composite density >95% was achieved after hot pressing at 1750°C for 2h under a pressure of 20MPa. The effects of the addition of each rare earth nitrate on the density, microstructure and mechanical properties of the SiCf/SiC composites were investigated. In particular, Sc-nitrate was found to be the most effective sintering additive in terms of retaining an ultrafine grain size distribution and the retention of structural integrity by preserving the weak PyC interface. Finally on the basis of identified key failure mechanisms, a correlation between microstructure and variation of flexural strength was proposed.

Interface engineering of fiber-reinforced all-oxide composites fabricated by the sol–gel method with fugitive pyrolytic carbon coatings

This paper describes the interface engineering of three–dimensional (3D) Nextel™440 fiber-reinforced aluminosilicate composites fabricated by the sol–gel method with fugitive pyrolytic carbon (PyC) coatings. The coating thickness on the fiber strength, interfacial characteristics and there corresponding effects on mechanical properties of the composites were investigated. The study shows that the fiber strength was influenced by the coating thickness and optimized with the thickness of 0.15 μm. The composites with uncoated fibers showed brittle fracture behavior without fiber pullout because of strong interactions between the fiber and the matrix. However, higher strengths and pseudo-ductile fracture behaviors were obtained in the composites with PyC interphases, where different deflections and branches of propagating cracks and fiber pullout patterns were observed. Moreover, induced fugitive PyC interface conditions have great effects on the density, microstructure and mechanical properties of the resultant composites.

Comparative Analysis of Low‐Cycle Fatigue Behavior of 2D‐Cf‐PyC/SiC Composites in Different Environments

A carbon fiber‐reinforced silicon carbide matrix composite with pyrolytic carbon interface (Cf‐PyC/SiC) and a protective coating was prepared by isothermal low pressure chemical vapor infiltration. Low‐cycle fatigue behavior of this material system was investigated at high temperatures up to 1800°C in a combustion environment and at room temperature in air, respectively. The combustion environment includes thermal mechanical loading, high temperatures, and oxidizing atmosphere. Low‐cycle fatigue tests were conducted at a maximum stress level of 180 MPa but at various temperatures and fatigue cycles. The residual strength variation of fatigue‐survived samples was due to different damage mechanisms in different environments.

Interlaminar shear fatigue of a two-dimensional carbon fibre reinforced silicon carbide composite

Interlaminar shear fatigue properties of a two-dimensional carbon fibre reinforced silicon carbide composite were investigated at room temperature (RT) and 900°C in air. The interlaminar shear strength (ILSS) of the survived specimens was determined to reveal the damage mechanisms. The composite presents excellent resistance to the fatigue at RT. The fatigue limits at 900°C are much lower than that at RT. Moreover, ILSS can be enhanced for the survived composite to some extent. The damage involves the matrix cracking and interfacial debonding. Such damage offers the channels for the oxidation of the pyrolytic carbon interface and carbon fibres, and leads to the decrease in the fatigue limits at 900°C.

Molecular dynamics study of cascade damage at SiC/C interface

Continuous silicon carbide (SiC) fiber-reinforced SiC (SiCf/SiC) composites have been considered to be used as structural materials in advanced nuclear reactors for its excellent properties. Their mechanical properties have been greatly improved during the last decade. But the radiation damage at the SiC and pyrolytic carbon interface would degrade the mechanical integrity of the composites, while the mechanism of degradation is remaining unknown at present. In this study, molecular dynamics simulations have been used to model the irradiation cascade of five SiC/C composite systems. According to the angle between the graphite layer and the interface, the models are marked as M0, M28, M56, M77 and M90, in which the number represents the angle. Forty primary knock-on atoms (PKAs) at different positions in each composite system are used to bombard the interface. In each run a collision cascade may be initiated by giving one of the 40 atoms 1.5 keV kinetic energy. The relationships between the distribution of defects and simulation time and PKA position are systematically studied, and compared with those in bulk SiC, which are marked as MW. Results show that the radiation damage resistance of SiC/C interface is significantly lower than bulk SiC, and the interface structure has an impact on the number of defects. Radial distribution function (RDF) is employed to examine the coordination of interfacial atoms. The results show that the higher the density of graphite atoms in the interface, the larger impact the irradiation on the RDF and coordination.