Chemical Vapor Infiltration Process Research Articles

Microstructural characterization and failure analysis of 2D C/SiC two-layer beam with pin-bonded hybrid joints

In this paper, the microstructural characteristics and mechanical behavior of 2D C/SiC two-layer beams with pin-bonded hybrid joints were investigated. The pin joints were incorporated into the two-layer beam during the assembling stage, and the adhesively bonded joints can be introduced by subsequent chemical vapor infiltration (CVI) process. For 2D C/SiC two-layer beam it was confirmed that the distribution of the adhesively bonded joints was random. Based on the partial interaction composite beam theory, a simplified method was proposed to characterize the effect of adhesively bonded joints on the bending stiffness of the 2D C/SiC two-layer beam. The stress distribution around the pin joints was described by finite element modeling. It was found that the radial stress, the hoop stress and the in-plane shear stress around the pin joints were unique in their distributions. Under the action of these stresses, two kinds of failure modes can be initiated: one is the debonding of the pin–hole interface, and the other is the fracture of the 2D C/SiC plate. Based on the stress results, an empirical failure criterion was present to predict the ultimate failure of the 2D C/SiC two-layer beam.

Residual thermal stresses in carbon/carbon–Zr–Ti–C composites and their effects on the fracture behavior of composites with different preforms

Carbon/carbon(C/C)–Zr–Ti–C composites were fabricated using chemical vapor infiltration and liquid metal infiltration processes. The residual thermal stress (RTS) distribution in the C/C–Zr–Ti–C composites was analyzed by Raman spectroscopy and finite element (FE) calculation. The mechanical behaviors of C/C–Zr–Ti–C composites with three types of carbon fiber preforms were tested. The results showed that there was very high compressive RTS in the fibers and high tensile RTS in the pyrocarbon (PyC) in the marginal region near the carbide. However, in the central region away from the carbide, relatively low compressive stress and tensile RTS were found in the fibers and PyC, respectively. The FE analysis showed that the introduction of carbide into the C/C composites caused a significant increase of the RTS in the PyC near the carbide, indicating that the carbide could change the RTS distribution in C/C composites. Moreover, the distribution of the RTS and its release combined with the carbide distribution and fiber architecture led to variations in the mechanical performance of composites with different preforms.

SiCf/SiC 복합체의 화학기상침착 거동에 미치는 권선 구조와 침착 변수의 영향

SiC and its composites have been considered for use as nuclear fuel cladding materials of pressurized light water reactors. In this study, a SiC f /SiC composite as a constituent layer of SiC triplex fuel cladding was fabricated using a chemical vapor infiltration (CVI) process in which tubular SiC fiber preforms were prepared using a filament winding method. To enhance the matrix density of the composite layer, winding patterns, deposition temperature, and gas input ratio were controlled. Fiber arrangement and porosity were the main parameters influencing densification behaviors. Final density of the composites decreased as the SiC fiber volume fraction increased. The CVI process was optimized to densify the tubular preforms with high fiber volume fraction at a high H2/MTS ratio of 20 at 1000℃; in this process, surface canning of the composites was effectively retarded.

화학침착법과 고분자함침 열분해법의 복합공정으로 제조한 SiCf/SiC 복합체의 제조 공정에 따른 파괴거동

SiC f /SiC composites were prepared using the hybrid process of chemical vapor infiltration (CVI) and polymer impregnation and pyrolysis (PIP). Before the application of PIP, partially matrix-filled preform composites with different densities were fabricated by control of chemical vapor infiltration time and temperature. The changes of the final density of the SiC f /SiC composites had a tendency similar to that of preform composites partially filled by CVI. Composites with lower density after the CVI process had a larger increment of density during the PIP process. Three types of microstructures were observed on the fractured surface of the composite: 1) well pulled-out fibers and lower density, 2) slightly pulled-out fibers and higher density, and 3) only bulk SiC. The different fractions and distributions of the microstructures could have an effect on the mechanical properties of the composites. In this study, SiC f /SiC composites prepared using a hybrid process of CVI and PIP had density values in the range of 1.05 ~ 1.44 g/㎤, tensile strength values in the range of 76.4 ~ 130.7 MPa, and fracture toughness values in the range of 11.2 ~ 13.5MPa·m 1/2 .

Cf/C composites: correlation between CVI process parameters and Pyrolytic Carbon microstructure

Chemical Vapour Infiltration (CVI) technique has been long used to produce carbon/carbon composites. The Pyrolytic Carbon (Py-C) matrix infiltrated by CVI could have different microstructures, i.e. Rough Laminar (RL), Smooth Laminar (SL) or Isotropic (ISO). These matrix microstructures, characterized by different properties, influence the mechanical behaviour of the obtained composites. Tailoring the process parameters, it is possible to direct the infiltration towards a specific Py-C type. However, the factors, influencing the production of a specific matrix microstructure, are numerous and interconnected, e.g. temperature, pressure, flow rates etc. Due to the complexity of the physical and chemical phenomena involved in CVI process, up to now it has not been possible to obtain a general correlation between CVI process parameters and Py–C microstructure. This study is aimed at investigating the relationship between infiltration temperature and the microstructure of obtained Py-C, for a pilot - sized CVI/CVD reactor. Fixing the other process parameters and varying only the temperature, from 1100°C to 1300°C, the Py-C infiltration was performed on fibrous preforms. Polarized light microscopy, with quantitative measurements of average extinction angle (Ae), and Raman spectroscopy were used to characterize the obtained Py-C microstructures.

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm3 and 82 W/(m K), respectively.

Hydrogen transfer reaction in polycyclic aromatic hydrocarbon radicals.

Density functional theory calculations have been successfully applied to investigate the formation of hydrocarbon radicals and hydrogen transfer pathways related to the chemical vapor infiltration process based on model molecules of phenanthrene, anthra[2,1,9,8-opqra]tetracene, dibenzo[a,ghi]perylene, benzo[uv]naphtho[2,1,8,7-defg]pentaphene, and dibenzo[bc,ef]ovalene. The hydrogen transfer reaction rate constants are calculated within the framework of the Rice-Ramsperger-Kassel-Marcus theory and the transition state theory by use of the density functional theory calculation results as input. From these calculations, it is concluded that the hydrogen transfer reaction between two bay sites can happen almost spontaneously with energy barrier as low as about 4.0 kcal mol(-1), and the hydrogen transfer reactions between two armchair sites possess lower energy barrier than those between two zigzag sites.

Effects of dip-coated BN interphase on mechanical properties of SiCf/SiC composites prepared by CVI process

BN interphase was successfully synthesized on SiC fiber fabrics by dip-coating process using boric acid and urea as precursors under N2 atmosphere. The morphology of BN interphase was observed by SEM, and the structure was characterized by XRD and FT-IR spectra. The SiCf/SiC composites with dip-coated BN interphase were fabricated by chemical vapor infiltration (CVI) process, and the effects of BN interphase on the mechanical properties of composites were investigated. The results show that the SiC fibers are fully covered by BN interphase with smooth surface and turbostratic structure (t-BN), and the thickness is about 0.4 μm. The flexural strengths of SiCf/SiC composites with and without BN interphase are about 180 and 95 MPa, respectively. Compared with the as-received SiCf/SiC composites, the composites with BN interphase exhibit an obvious toughened fracture behavior. From the microstructural analysis, it can be confirmed that the BN interphase plays a key part in protecting the fibers from chemical attack during matrix infiltration and weakening interfacial bonding, which can improve the mechanical properties of SiCf/SiC composites remarkably.

Effects of different loading methods on thermal expansion behaviors of 2D cross-ply carbon/carbon composites from 850 °C to 2300 °C

2D cross-ply carbon/carbon (C/C) composites were prepared by isothermal chemical vapor infiltration (ICVI) process. Thermal expansion behaviors of 2D cross-ply C/C composites and effects of static loading and fatigue loading conditions on coefficient of thermal expansion (CTE) of the C/C composites from 850°C to 2300°C were studied. In static test cases, different damage levels were produced inside samples by displacement-controlled three-point bending loading. Meanwhile, flexural fatigue experiments were conducted under stress level of 70%, cycling numbers (N) of 105 and 106 at a sinusoidal loading frequency of 15Hz. Afterwards, the thermal expansion behaviors were investigated. It showed that CTE of the C/C composites after static loading and fatigue loading had a decrease compared with CTE of the as-prepared C/C composites. After static loading, there was a larger decrease in CTE for the C/C composites with more severe damage levels. Moreover, the influence of damage on CTE reached to the greatest at around 1550°C. The largest decline percentage of the C/C composites after static loading and fatigue loading was 30% and 85% in contrast to the as-prepared C/C composites, respectively. In addition, weakening interfacial strength of the C/C composites was the main effect on the large decrease of CTE after the fatigue loading.

Mechanical and electromagnetic shielding properties of SiCf/SiC composites fabricated by combined CVI and PIP process

2.5D KD-I SiCf/SiC composites were fabricated by combined chemical vapor infiltration (CVI) and precursor infiltration-pyrolysis (PIP) processes for electromagnetic interference (EMI) shielding applications. The effects of PyC interphase and Al2O3 filler on mechanical and EMI shielding properties of SiCf/SiC composites were studied. The PyC interphase effectively improves the accommodation of the fibers and matrix, and Al2O3 filler compensates for matrix shrinkage and acts as a strengthening phase to improve the capability of bearing force for the composites. Thus, the composites with PyC interphase and Al2O3 filler possess the maximum flexural strength and fracture toughness with the values of 313MPa and 12.7MPam1/2. Owing to the existence of excess carbon and PyC interphase, the composites also maintain the highest SET and SEA values of 30 and 20dB, respectively, indicating its advantages as a kind of structural and functional materials. The superior SEA value is mainly attributed to the repeated reflection and dissipation of electromagnetic wave between the interfaces of PIP-SiC matrix and Al2O3 particles.

Fabrication and Characteristic of Nextel 720 Fiber‐Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration Process

Most current processes for fiber‐reinforced silicon nitride composites are conducted at very high temperature, which is not possible to use oxide fiber as reinforcement. Here, low‐temperature process of chemical vapor infiltration (CVI) was utilized to fabricate Nextel 720 oxide fiber tow‐reinforced silicon nitride matrix composite with PyC as interphase. The tensile strength was analyzed by Weibull distribution. The microstructure showed that there were two types of interface bonding. The strong interface bonding determined the unexpected low strength of the composites. This indicated that the suitable interface design is the urgent issue for oxide fiber‐reinforced silicon nitride composite by CVI.

Thermal expansion behaviors of unidirectional carbon/carbon composites from 800 to 2500 °C

Unidirectional carbon/carbon (C/C) composites were prepared by the isothermal chemical vapor infiltration (ICVI) process. Thermal expansion behaviors of the composites from 800 to 2500°C were investigated along the longitudinal and vertical directions. Meanwhile, the thermal expansion curves of carbon fibers (CF) and pyrolytic carbon (PyC) from 800 to 2500°C were investigated individually. It was shown that coefficients of thermal expansion (CTEs) of untreated unidirectional C/C composites measured in the longitudinal and vertical directions gently increased with the increase of temperature from 800 to 1200°C. But in the temperature range of 1200–1600°C, longitudinal CTE of the untreated composites decreased sharply till reaching the lowest point at around 1600°C, and increased linearly as the temperature increased. While vertical CTE of the untreated ones increased rapidly over 1200°C until arriving the peak value at around 1600°C and fell rapidly in the temperature range of 1600–2500°C. On the other hand, for treated (2500°C, 2h) unidirectional C/C composites, CTEs in both directions increased slowly as the temperature increased from 800 to 2500°C, which varied from 1.68 to 3.07×10−6K−1 in the longitudinal direction and from 6.78 to 8.04×10−6K−1 in the vertical direction.

Effect of thermal cycling of SiCf/SiC composites on their mechanical properties

SiCf/SiC composites are class of high temperature structural materials being developed for use in nuclear fusion and fission reactor systems because of their superior high temperature mechanical properties, low radiation damage and low induced radioactivity. Two types of 2D SiCf/SiC composites were made through isothermal and isobaric chemical vapor infiltration process using eight harness satin-woven ceramic-grades Nicalon™ fibers with boron nitride (BN) interface, namely: one with lower interface thickness and a second type with higher interface thickness. The BN interface was applied to the fiber prior to SiC matrix addition to modify the interfacial bond strength leading to better toughness and improved oxidation resistance. The density achieved was around 2.6g/cc. The composite specimens were subjected to thermal cycling treatment using an in-house furnace. The mechanical properties such as tensile strength, fracture toughness and interfacial bond strength were also studied for all the composites before and after thermal cycling. It is seen from the results that both composites withstood thermal shocks and thermal cycling treatment. It was also concluded from the present work that good balance between load transfer and crack arresting was established.

S042014 繰返しMI法によるC/SiC複合材料の高速緻密化プロセス([S042]セラミックスおよびセラミックス系複合材料)

A rapid and low-cost manufacturing process of a carbon-fiber-reinforced SiC matrix (C/SiC) composite was examined in this study. In this process, the space between fibers in a 3D carbon fiber preform was infiltrated by chemical vapor infiltration(CVI) and silicon melt infiltration (Si-MI). In the CVI process, pyrolytic carbon interfacial layer and SiC barrier layer were deposited on the carbon fiber surface. Then, remaining unfilled space in the pre-coated preform was filled with carbon matrix by infiltration of carbon black slurry and/or polymer impregnation and pyrolysis (PIP) of phenolic resin. The carbon filled preform was infiltrated with silicon melt at 1450℃.Then,they were further densified by several cycles of Si elimination at 1650℃ and the PIP of phenolic resin followed by the Si-MI. Through these processes, residual Si was reduced and the pores in the composite were filled by SiC. Then, the microstructure and flexural strength of the C/SiC composite at room temperature and 1200, 1400, 1600℃ were evaluated. Experimental results revealed that those composites possessed dense SiC matrix and exhibited the high flexural strength of approximately 500MPa at 1600℃.

Computer Modeling Chemical Vapor Infiltration of SiC Composites

A novel multiphase field model is formulated to simulate the complex microstructure evolution during chemical vapor infiltration (CVI) process, which is widely used technique to produce SiC matrix composites reinforced by SiC... | Find, read and cite all the research you need on Tech Science Press

Upscaling the Chemical Vapor Infiltration Process of Activated Carbon with TMS

Activated carbons are important adsorbents covering a broad range of applications from gas to air purification. In order to improve their mechanical stability and their resistance against oxidation they are infiltrated here with an inert material, SiC or SiO 2 . For this process the chemical vapor infiltration technique with tetramethylsilane as precursor is used. The process is designed for the infiltration of larger quantities (up to 50 g) which allows further analysis of the materials produced: in comparison to the uncoated activated carbon the novel adsorbent have an increased breaking strength and the BET-surface areas and pore volumes are on high levels. The possibility of using the novel material for solvent adsorption is demonstrated for acetone.

Numerical Modeling Chemical Vapor Infiltration of SiC Composites

The multiphase field model for chemical vapor infiltration (CVI) of SiC/SiC composites is developed in this study, thereby to reproduce the microstructure evolution during CVI process and to achieve better understanding of the effect of process parameters (e.g., temperature, pressure, etc.) on the final product. In order to incorporate the thermodynamics of methyltrichlorosilane (MTS) pyrolysis into phase field model framework, the reduced chemical reaction mechanism is adopted. The model consists of a set of nonlinear partial differential equations by coupling Ginzburg‐Landau type phase field equations with mass balance equations (e.g., convection‐diffusion equation) and the modified Navier‐Stokes equations which accounts for the fluid motion. The microstructure of preferential codeposition of Si, SiC under high ratio of H2 to MTS is simulated and the potential risk of blockage of the premature pores during isothermal CVI process is predicted. The competitive growth mechanism of SiC grains is discussed and the formation process of potential premature pore blockage is reproduced.

Carbon Deposition from Biotar by Fast Pyrolysis Using the Chemical Vapor Infiltration Process within Porous Low-Grade Iron Ore for Iron-Making

This paper presents a technology for iron-making using biomass and a low grade iron ore by implementing chemical vapor infiltration (CVI) for the tar carbonization process. In this process, tar vapor from pyrolysis of biomass was infiltrated within a porous ore and carbon deposition occurred on the pore surface. For preparation, ore sample was heated in an electric furnace to decompose combined water in the sample, creating nanosized pores. In the experiments, the traditional slow pyrolysis was compared with fast pyrolysis to determine which condition is better for maximizing carbon deposition. Tar vapor from the pyrolysis process was introduced into the porous ore and trapped inside it, where tar decomposed and carbonized within the pores. The product of this process which is a carbon−magnetite composite with close arrangement of iron ore and carbon is useful for reduction of iron by carbon and is able to lower the temperature needed for reduction of iron to occur, as compared to that in steelworks. The purpose of this research is to compare the effects of slow pyrolysis and fast pyrolysis processes on the amount of carbon deposited within iron ore by the tar carbonization process and to observe the reduction reactivity of the carbon-deposited iron ore.

Modeling of Pore Structure Evolution Between Bundles of Plain Woven Fabrics During Chemical Vapor Infiltration Process: The Influence of Preform Geometry

A three‐dimensional model combining the nonconservative level set method and the steady‐state diffusion equation is presented for studying evolving pore structure between bundles of plain woven fabrics during chemical vapor infiltration process. Parametric studies have been conducted to determine the sensitivity of the evolution of structural properties and effective gas diffusion coefficients to the geometric parameters of the preform. Based on our results, some analytical expressions are proposed to estimate the evolution of structural properties and the tortuosity. The prediction of the presented models is well consistent with the existing data of experiments.

Rapid densification of C/SiC composite by incorporating SiC nanowires

To obtain a dense carbon fiber reinforced SiC matrix (C/SiC) composite by a chemical vapor infiltration (CVI) technique within a short time, SiC nanowires were introduced into the fiber preform before CVI process. The SiC nanowires were produced on the fiber fabrics by impregnating the mixture of polycarbosilane (PCS) and ferrocene catalyst into the fiber fabrics and pyrolysis of them at high temperatures. The preform that had been obtained by stacking these fiber fabrics was densified by CVI in 200h, about half time less than the conventional CVI process. It was found that the incorporated SiC nanowires reduced the pore size and provided extra surfaces to accelerate the CVI process. The C/SiC composite with nanowires shows higher density and better flexural strength than the conventional C/SiC composite.