PyC Layer Research Articles

Ablation resistance and mechanical properties of C/C-HfC-SiC composites with a bulletproof-like layer

Two structural designs are based on the arapaima fish scale structure to enhance the performance of composites. A double-layer SiC/PyC coating was deposited to modify C/C-HfC-SiC composites by chemical vapor deposition. The introduction of PyC coating reduced the porosity and roughness of composites. Ar-O2-Al2O3 plasma ablation system was used to assess the composites. The composites with arapaima fish scale structure showed good flexural and ultra-temperature particle erosion resistance. The samples with the introduction of PyC layer showed higher flexural strength of 220 MPa, which is 40.13 % higher than that of without. After ablation 40 s, the linear ablation rates decreased from 5.95 μm/s to −4.6 μm/s. This study provides new ideas to develop C/C composites with improved ablation-resistant and mechanical properties.

Ultra-broadband absorbance of nanometer-thin pyrolyzed-carbon film on silicon nitride membrane

Fifty percents absorption by thin film, with thickness is much smaller than the skin depth and optical thickness much smaller than the wavelength, is a well-known concept of classical electrodynamics. This is a valuable feature that has been numerously widely explored for metal films, while chemically inert nanomembranes are a real fabrication challenge. Here we report the 20 nm thin pyrolyzed carbon film (PyC) placed on 300 nm thick silicon nitride (Si3N4) membrane demonstrating an efficient broadband absorption in the terahertz and near infrared ranges. While the bare Si3N4 membrane is completely transparent in the THz range, the 20 nm thick PyC layer increases the absorption of the PyC coated Si3N4 membrane to 40%. The reflection and transmission spectra in the near infrared region reveal that the PyC film absorption persists to a level of at least 10% of the incident power. Such a broadband absorption of the PyC film opens new pathways toward broadband bolometric radiation detectors.

Interfacial fracture behavior and adhesive strength in tensile and shear loading of SiC-PyC-SiC composites by micro-scale specimens

Micromechanical tests were performed to understand the local fracture behavior and adhesive strength of a fiber-reinforced ceramic matrix composite (silicon carbide fiber reinforced silicon carbide matrix with the pyrolytic interphase in between, SiCfiber/PyC/SiCmatrix) and to determine the mode-dependent interfacial fracture toughness. A combined approach of pico-second laser ablation and focused ion beam milling was used to fabricate notched and unnotched micro-cantilever and in-plane micro-shear specimens for mode I and II testing. Due to the complexity of the sandwich system, finite element simulations were implemented, which took into account the elastic heterogeneity, the influence of PyC thickness, Young's modulus and beam thickness. This allowed us to obtain mode-specific geometry functions and to increase the accuracy of the obtained stress intensity factor. In all cases, a failure at the SiCfiber/PyC interface was observed. While straight through notched specimens exhibit a systematic overestimation of fracture toughness due to the less accurate alignment of notch root and PyC layer, curved notched specimens show a very low interfacial fracture toughness of 0.24 ± 0.02 MPa√m and 0.17 ± 0.06 MPa√m in mode I and mode II, respectively. Combined with tests on unnotched specimens, a critical flaw size for the present microstructure of the PyC phase of ca. 21 nm was identified. The data underlines the important role of the PyC interlayer in damage resistant ceramic composites and the developed methodology, which can be used for systematic studies of their properties as a function of process, geometry, and boundary conditions.

Effect of ultra-high temperature treatment on microstructures and mechanical properties of TRISO particles

Tristructural-isotropic (TRISO) fuel particles have long been widely used in high temperature gas-cooled reactors (HTGR), where their high temperature mechanical integrity and ability to retain fission products even in the accident conditions are the critical safety issues. In the present work, TRISO particles were subjected to ultra-high temperature thermal treatments at 1900, 2000, and 2200 °C, respectively, which were much higher than the projected accident temperature (1620 °C). Phase compositions and microstructures of the samples before and after thermal treatments were studied by X-ray diffraction (XRD), Raman spectroscopy, and Scanning Electron Microscopy (SEM). Nano hardness and Young's modulus of the SiC layer in TRISO particles were studied by nano indentation technique, and crushing strength of the TRISO particles was evaluated by mechanical crush tests. The results show that, there were no obvious temperature effects on phase composition and microstructure, except for SiC layer in the sample thermal treated at 2200 °C, in which many pores caused by SiC decomposition were observed. The nano hardness and Young's modulus values of SiC layer in the as-received and thermally treated TRISO particles were similar within the experimental error. On the other hand, the crushing strength of the thermally treated TRISO particles decreased greatly as compared with the as-received particles, but the crack propagation mode changed from through-the-particle to deflection inside the buffer PyC layer or between buffer PyC and inner PyC (IPyC) layers. The change of crack propagation mode could probably due to the shrinkage of PyC layers due to anisotropy at high temperature and ordering of graphitization of IPyC layers, which probably weakened the interfacial bonding between the IPyC and the buffer PyC layers. The results of this study provide a database reference for the performance of TRISO particles under ultra-high temperature operation and even accident conditions.

Morphologies evolution and mechanical behaviors of SiC nanowires reinforced C/(PyC-SiC) multilayered matrix composites

• The bionic (PyC-SiC) n multilayered structure was used to improve flexural properties. • The SiC nanowires formed in-situ on the surface of multilayered structure played an important role in micro-zone reinforcements. • The (PyC-SiC) n multilayered structure and SiC nanowires were prepared at the same time, and they could form a multiscale reinforcing system. • The flexural strengths and flexural modulus of modified composites were up to 62.36% and 102.20% respectively higher than C/C composites. SiC nanowires reinforced C/(PyC-SiC) n multilayered matrix composites (SM-CS for short) were prepared by combined with sol-gel and chemical vapor infiltration (CVI) method. Firstly, (PyC-SiOC) n multilayered structure was formed by cycles of impregnation and deposition. Then SiOC was transformed into SiC by heat-treatment, and (PyC-SiC) n multilayered structure would be obtained. At the same time, the PyC layer which was designed as the outmost layer could decrease gas supersaturation to form in-situ tubular SiC nanowires on the surface of multilayered structure. The results of three-point bending test showed that the maximum force of SM-CS composites was increased by the number of cycles of the preparation process, which were up to enhanced by 74.38% compared with C/C composite materials. The fracture surface showed that the improvement was due to the multiscale reinforcing system of (PyC-SiC) n multilayered structure and SiC nanowires. Multilayered structure can protect carbon fibers and release stress concentration by induction of cracks. And the mechanical interlocking effect of SiC nanowires could reinforce bonding force of the remaining matrix.

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 Hf1-xZrxC 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 Hf1-xZrxC coating were studied. HfCnw/PyC-reinforced coatings exhibited equiaxial crystal structure. After incorporating HfCnw/PyC, ablation property of Hf1-xZrxC 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 Hf1-xZrxC coating with PyC layer thickness of about 50 nm exhibited the best ablation property.

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf1-xZrxC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf1-xZrxC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf1-xZrxC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, Kc and bonding strength of Hf1-xZrxC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, Kc and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher Kc of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiCf)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiCf using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiCf/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiCf/PyC/SiCNWs. Three mini-composites, SiCf/PyC/SiC, SiCf/PyC/SiCNWs/SiC, and SiCf/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiCf/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiCf/SiC mini-composites were investigated.

Comparative study of the effect of fiber surface treatments on the flexural and interlaminar shear strength of carbon fiber-reinforced composites

The aim of this study was to achieve enhanced flexure and interlaminar shear strength of carbon fiber-reinforced composites by grafting MWCNTs and depositing PyC layer on the fiber using CVD technique. The surface morphology of fiber prior and after treatment was examined by FESEM and TEM and the composites fabricated with surface modified fiber were tested by three-point bend test for comparative flexure and interlaminar shear behavior. A remarkable improvement of 32 % and 54 % was observed in the flexure strength of composites made of CNTs grafted and PyC-coated fiber respectively as compared to reference composites whereas significant augmentation of 35 % and 53 % respectively was found in the ILSS of composites. In-depth fractographic investigation was conducted to investigate the mechanisms responsible for this enhancement in flexure strength and ILSS of composites. The mechanical test results were supported and validated by the fractographic analysis. Individual CNT pullout, CNTs bundle pullout and crack bridging by CNTs were evidently observed as dominant mechanisms of fracture in these composites in addition to conventional interface debonding and fiber pullout.

Oxidation and microstructure of SiCf/SiC composites in moist air up to 1600°C by X‐ray tomographic characterization

AbstractSiCf/SiC composites that possess PyC or BN interface layers were fabricated and then oxidized in moist air at 1000, 1200, 1400, and 1600°C. High‐resolution CT was used for capturing 3D images and quantifying the SiC phase, mesophase, and voids. The oxidation behavior and microstructural evolution of SiCf/SiC with PyC or BN interface are discussed in this study. The microstructure of the SiCf/SiC with a PyC layer was seriously damaged in moist air at high temperature, whereas the BN interface layer enhanced the oxidation resistance of the SiCf/SiC. These results are also confirmed by using XRD, oxidation mass gain, tensile testing, and SEM measurements. The results of the oxidation behavior and microstructural evolution for SiCf/SiC oxidized in dry air are also compared with the results of this study. Comparing the SiCf/SiC with a PyC interface layer, the composite with a BN interface layer oxidized in moist air exhibits a high void growth rate and a low SiO2 grain growth rate from 1000 to 1600°C. This work will provide guidance for predicting the service life of SiCf/SiC for multiscale damage rate models of materials at a local scale and will also provide guidance on the life service design of SiCf/SiC materials.

X-ray tomography study on the crushing strength and irradiation behaviour of dedicated tristructural isotropic nuclear fuel particles at 1000 °C

Two types of dedicated Tristructural isotropic (TRISO) nuclear fuel particles, PyC-1 (Kernel/Buffer/PyC) and PyC-2 (Kernel/Buffer/SiC/PyC) from PYCASSO (Pyrocarbon irradiation for creep and swelling/shrinkage of objects) neutron irradiation experiments, were studied. For unirradiated particles, crushing experiments using a unique hot cell, combined with in situ X-ray computed micro-tomography (XCT) imaging, were conducted at room temperature (RT) and at 1000 °C. Although the SiC layer on the particles is presumed to provide ‘mechanical stability’ to the TRISO particles, results showed a remarkable reduction (~45%) in the crushing strength of the PyC-2 particles at 1000 °C compared to RT. The fracture patterns of the two types of particles, both at the contact zone and on subsequent propagation, differ significantly at RT and 1000 °C. Further, irradiated particles (irradiation temperature: 1000 ± 20 °C; irradiation doses: 1.08–1.23 dpa and 1.49–1.51 dpa) were imaged by XCT; 250 PyC-1 particles and 223 PyC-2 particles were studied in total and the change in radius/layer thickness in each type was examined. It was found that the buffer densification was lower in PyC-1 particles compared to PyC-2 particles, and the PyC layer shrank in the PyC-I particles, whereas it expanded in PyC-2. Results are discussed in terms of how the residual stresses can impact the high-temperature and post-irradiation behavior of these particles.

Preparation and Properties of Cf/SiC-ZrC Ultrahigh Temperature Ceramic Matrix Composites

Ultra-high temperature materials have many excellent properties, such as high melting point, high specific strength, high thermal conductivity, high conductivity, corrosion resistance and good chemical stability, which make them become high temperature structural materials for aircraft in extreme environments.Cf/SiC-ZrC ultra-high temperature ceramic matrix composites were fabricated by the combined process of chemical vapor deposition and precursor infiltration pyrolysis (CVI-PIP) in this study. The phase composition and microstructure features of the composites were characterized using X-ray diffraction, scanning electron microscopy. A PyC layer was observed as the interfacial layer between carbon fiber and the ultrahigh temperature ceramic matrix. It can protect the carbon fiber from damage during the cyclic PIP process. Nano crystalline ZrC particles were observed in the composites. The ablation mechanisms of the composites were tested in arc-jet wind tunnel and it exhibited good ablation-resistant properties. The ablation mechanism of composite materials in wind tunnel environment is discussed.

Texture-inducing effect of SiC nanowires and their influence on thermal conductivities of carbon/carbon composites up to 1900 °C

SiC nanowire-modified C/C (SiCNWs-C/C) composites were prepared. The effects of SiC nanowires on the microstructures of pyrolytic carbon matrix (PyC) were illuminated and the thermal conductivities of SiCNWs-C/C composites were investigated from room temperature to 1900 °C. The results showed that SiCNWs exhibited a strong inductive effect for the formation of a high texture PyC layer in C/C composites. And the thermal conductivities of SiCNWs-C/C composites were higher than those of C/C composites in both parallel and perpendicular directions to the fiber axis. Especially, the thermal conductivity perpendicular to the fiber axis was significantly increased, which could be ascribed to the improvement of microstructures of pyrolytic carbon matrix.

Micro-mechanical evaluation of SiC-SiC composite interphase properties and debond mechanisms

Abstract SiC-SiC composites exhibit exceptional high temperature strength and oxidation properties making them an advantageous choice for accident tolerant nuclear fuel cladding. In the present work, small scale mechanical testing along with AFM and TEM analysis were employed to evaluate PyC interphase properties that play a key role in the overall mechanical behavior of the composite. The Mohr-Coulomb formulation allowed for the extraction of the internal friction coefficient and debonding shear strength as a function of the PyC layer thickness, an additional parameter. These results have led to re-evaluation of the Mohr-Coulomb failure criterion and adjustment via a new phenomenological equation.

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) Cf/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the Cf/ZrC–SiC composites without and with a thin PyC interlayer of 0.15µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2µm. The highest flexural strength of 272.3±29.0MPa and fracture toughness of 10.4±0.7MPam1/2 were achieved for the composites with a 1.2µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the Cf/ZrC–SiC composites slightly and the ZrO2 scale acts as an anti-ablation component during the testing.

Microstructure, mechanical properties and oxidation behavior of carbon fiber reinforced PyC/C-TaC/PyC layered-structure ceramic matrix composites prepared by chemical vapor infiltration

A new kind of carbon fiber reinforced PyC/C-TaC/PyC layered-structure ceramic matrix composites (Cf/C-TaC composites) was prepared by chemical vapor infiltration. The middle C-TaC ceramic layer was a co-deposited layer of PyC and TaC phases, in which its structure can also be divided into three different zones, from amorphous near the inner PyC interface to equi-axial and then needle-like structures. Nano-indentation tests show that the hardness and elastic moduli of the C-TaC co-deposited ceramic layer are higher than the values of PyC layer, with the results of a higher flexural strength, fracture toughness of the as-prepared composites when compared with C/C composites. Cf/C-TaC composites show a complex relationship of mass gain/loss with oxidation temperature. The mass loss increases at 900–1300 °C; and begins to loss at 1400 °C, when the temperature reaches to 1500 °C, the mass losses acutely. XRD results show that the residual oxide products on the surface of Cf/C-TaC composites at 900–1500 °C are all of Ta2O5 phase but with different crystal structures and morphologies, including hexagonal (δ) structure with an amorphous morphology at 900 °C, orthorhombic (β) structure with a hexagonal-block or oval-shaped crystallite morphology at 1000–1300 °C, tetragonal (α) structure with square rectangular shape morphology, monoclinic (α’) structure with triangular shape morphology and triclinic (α’’) structure with rhombic/long-strip-shape morphology at 1400–1500 °C, which may have great influence on the oxidation-resistance of Cf/C-TaC composites. The formation mechanism of β-Ta2O5 phase had been analyzed by TEM. The oxide contains an amorphous matrix, in which crystallized β-Ta2O5 phase was nucleated and grown. The nucleation and growth process of crystallized β-Ta2O5 are both controlled by the rearrangement of lattice at the crystallite/amorphous matrix interface.

Au ion irradiation of various silicon carbide fiber-reinforced SiC matrix composites

Silicon carbide fiber-reinforced SiC matrix composites are promising candidates as fuel cladding for several concepts of Generation IV reactors and as structural materials for fusion reactors. The composites used in this study were composed of a SiC matrix obtained by chemical vapor infiltration associated with various fiber types (Tyranno Type-S, Tyranno SA Grade-3 and Hi-Nicalon Type-S) and with a PyC layer as the interphase. 12MeV Au ions were used for irradiation up to 0.05 and 1 displacement per atom (dpa) fluences at room temperature and 800°C. Analysis of both microstructure and composition of composites were performed by scanning electron microscopy (SEM), electron probe microanalysis and Raman spectroscopy. At room temperature and low fluence, Raman spectroscopy results showed that irradiation induces a disordered/distorted state into fibers and matrix. With increasing fluence, a total amorphization of these constituents occurs. The increase in the irradiation temperature leads to a damage recovery and partial recrystallization of samples. Image analysis performed from SEM micrographs highlights no significant change in fiber diameter and shape. However, SEM analysis suggests a longitudinal shrinkage of Tyranno Type-S fibers for the composite irradiated at 1dpa at room temperature and 800°C. These results are in complete agreement with conclusions from neutron irradiations suggesting an appropriate relevance of irradiations with 12MeV Au.

Fiber/matrix interfacial shear strength of C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers

The fiber/matrix (F/M) interfacial shear strength (IFSS) of carbon/carbon (C/C) composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers was investigated. To obtain C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers, a thin layer of PyC was deposited on carbon fibers. After this, TaC and SiC–TaC layer(s) were uniformly deposited on the PyC coated carbon fibers. As an outer-layer, a PyC layer was deposited on these TaC and/or SiC–TaC coated carbon fibers by isothermal chemical vapour infiltration (CVI) and then densified with resin carbon by impregnation and carbonization. Finally, C/C composites with PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers were obtained. The effects of PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers on interfacial shear strength (IFSS) of C/C composites were investigated. Single fiber push-out tests were conducted on the fibers aligned perpendicularly on the thin slices specimen surface using nano-indentation. Results showed that the IFSS of C/C composites decreased with the introduction of PyC–TaC–PyC and PyC–SiC–TaC–PyC multi-interlayers. After heat treatment (at temperatures ranging from 1400 to 2500 °C) of C/C composites with PyC–TaC–PyC multi-interlayers, it was found that the IFSS decreased with the increase in temperature. This decrease in IFSS is explained by taking into account the microstructural variations on heat treatment.

Pressure analysis in the fabrication process of TRISO UO2-coated fuel particle

The pressure signals in the coating furnace are obtained experimentally from the TRISO UO2-coated fuel particle fabrication process. The pressure signals during the coating process are analyzed and a simplified relationship about the pressure drop change due to the coated layer is proposed based on the spouted bed hydrodynamics. The change of pressure drop is found to be consistent with the change of the combination factor about particle density, bed density, particle diameter and static bed height, during the successive coating process of the buffer PyC, IPyC, SiC and OPyC layer. The newly proposed relationship is validated by the experimental values. Based on this relationship, a convenient method is proposed for real-time monitoring the fluidized state of the particles in a high-temperature coating process in the spouted bed. It can be found that the pressure signals analysis is an effective method to monitor the fluidized state on-line in the coating process at high temperature up to 1600°C.

겔침전과 화학증착법에 의한 구형 UO2입자와 TRISO 피복입자 제조

HTGR using a TRISO coated particles as nuclear raw fuel material can be used to produce clean hydrogen gas and process heat for a next-generation energy source. For these purposes, a TRISO coated particle was prepared with 3 pyro-carbon (buffer, IPyC, and OPyC) layers and 1 silicone carbide (SiC) layer using a CVD technique on a spherical UO₂ kernel surface as a fissile material. In this study, a spherical UO₂ particle was prepared using a modified sol-gel method with a vibrating nozzle system, and TRISO coating fabrication was carried out using a fluidized bed reactor with coating gases, such as acetylene, propylene, and methyltrichlorosilane (MTS). As the results of this study, a spherical UO₂ kernel with a sphericity of 1+0.06 was obtained, and the main process parameters in the UO₂ kernel preparation were the well-formed nature of the spherical ADU liquid droplets and the suitable temperature control in the thermal treatment of intermediate compounds in the ADU, UO₃, and UO₂ conversions. Also, the important parameters for the TRISO coating procedure were the coating temperature and feed rate of the feeding gas in the PyC layer coating, the coating temperature, and the volume fraction of the reactant and inert gases in the SiC deposition.