Chemical Vapor Infiltration Process Research Articles

Preparation and characterization of carbon felt/carbon composites by chemical vapor infiltration process

Preparation and characterization of carbon felt/carbon composites by chemical vapor infiltration process

Synthesis, structure, and properties of carbon/carbon composites artificial rib for chest wall reconstruction

In this work, braided carbon fiber reinforced carbon matrix composites (3D-C/C composites) are prepared by chemical vapor infiltration process. Their composite structure, mechanical properties, biocompatibility, and in vivo experiments are investigated and compared with those of traditional 2.5D-C/C composites and titanium alloys TC4. The results show that 3D-C/C composites are composed of reinforced braided carbon fiber bundles and pyrolytic carbon matrix and provide 51% open pores with a size larger than 100 μm for tissue adhesion and growth. The Young’s modulus of 3D-C/C composites is about 5 GPa, much smaller than those of 2.5D-C/C composites and TC4, while close to the autogenous bone. 3D-C/C composites have a higher tensile strength (167 MPa) and larger elongation (5.0%) than 2.5D-C/C composites (81 MPa and 0.7%), and do not show obvious degradation after 1 × 106 cyclic tensile loading. The 3D-C/C composites display good biocompatibility and have almost no artifacts on CT imaging. The in vivo experiment reveals that 3D-C/C composites artificial ribs implanted in dogs do not show displacement or fracture in 1 year, and there are no obvious proliferation and inflammation in the soft tissues around 3D-C/C composites implant. Our findings demonstrate that 3D-C/C composites are suitable for chest wall reconstruction and present great potentials in artificial bones.

Safety assessment of the film boiling chemical vapor infiltration (FB-CVI) process through a system-theoretic accident model and process (STAMP)

Film boiling chemical vapor infiltration (FB-CVI) is considered as one of the fastest process methodologies for manufacturing carbon-carbon (C–C) composite products and possesses various advantages compared to conventional methodologies. However, there are safety concerns associated with this process for large-scale manufacturing, mainly owing to the intrinsic nature of the precursor and the process conditions. Considering the multifunctional interactions of the various systems during the process, a system-theoretic process analysis (STPA)/system theoretic accident model and process (STAMP) model is used to perform a safety analysis of the hazardous states of the FB-CVI process at the system level. As a case study, the FB-CVI process equipment employed for the manufacturing of C–C composites is considered. The safety constraints present in the system are assessed for adequacy through a hazard analysis by STPA/STAMP. The analysis through STPA/STAMP demonstrated the capability to create proactive strategies for the design and realization of process equipment that can be employed to manufacture C–C composite products through the FB-CVI process.

Development of C/SiC Fasteners for High-Temperature Applications

Continuous carbon fiber reinforced silicon carbide (C/SiC) ceramic matrix composites are the candidate materials for high-temperature structural applications owing to their high specific strength, low coefficient of thermal expansion, and moderate thermal conductivity. C/SiC based fasteners provide reliability with oxidation resistance up to 1,650°C for joining such C/SiC structural components used in reusable launch vehicles for space applications. The present study describes the methodology to realize C/SiC based M8 fasteners by isothermal-isobaric chemical vapor infiltration process using 2D-stitched carbon fabric preforms. The realized fasteners exhibited a density of 2.10 g/cm3, a tensile strength of 191 ± 3 MPa, a shear strength of 230 ± 69 MPa in non-threaded and 150 ± 86 MPa in threaded regions of shank at room temperature, a high-temperature tensile strength of 170 ± 12 MPa, and a tensile modulus of 70 ± 8 GPa at 1,100°C in the atmospheric conditions. The tensile failure of the C/SiC bolts was observed to depend upon the relative dimensions of fillet radius and thread root radius, wherein a small fillet radius was observed to be detrimental, as it resulted in failure at the head-shank interface of the bolts. The scanning electron microscope images of tensile tested C/SiC bolts indicate extensive fiber pullout, characteristic of quasi-ductile failure of bolts. It is clearly evident from the studies that the C/SiC fasteners are a promising alternative over the currently used molybdenum alloy fasteners.

High compression performance of C /SiC-Al composites fabricated by CVI and vacuum pressure infiltration

The strengthening effect of aluminum (Al) alloy as the second phase matrix on C f /SiC-Al composites was investigated, and microstructures and compression performance of the composites were characterized and tested. The results show that compressive strength and modulus of C f /SiC-Al composites were 627±21.4 MPa and 98±2.4 GPa, respectively. Compared with C f /SiC composites prepared by chemical vapor infiltration (CVI) process, the compressive strength increased by 34.8%, and the modulus increased by 8.9%. The introduction of Al alloy by vacuum pressure infiltration process significantly improved the ability of the composites to withstand pressure loads. However, due to the Al alloy filling in the inter-bundle pores inside C f /SiC composites, the space for the fiber to be deformed by force was limited, and the length of the fiber extraction was shortened. • Aluminum alloy was successfully introduced into C f /SiC composites prepared by CVI. • Aluminum alloy can effectively fill the inter-bundle pores inside the C f /SiC. • The compressive strength of the as-prepared composites was significantly improved. • The filling of aluminum alloy changes the fracture mode of the composites.

Effect of pyrolytic carbon interphase on mechanical properties of mini T800-C/SiC composites

The effect of pyrolytic carbon (PyC) thickness on the tensile property of mini T800 carbon fiber reinforced SiC matrix composites (C/SiC) was studied. PyC interphase was prepared by chemical vapor infiltration (CVI) process using C3H6–Ar as gas source, the PyC thickness was adjusted from 0 to 400 nm, and then the SiC matrix was prepared by CVI process using methyltrichlorosilane (MTS)–H2–Ar as precursor and gas source. The results showed that the tensile strength of mini T800-C/SiC increased first and then decreased with the increase of the PyC thickness. When the thickness of PyC was 100 nm, the average strength reached the maximum value of 393 ± 70 MPa. The Weibull modulus increased from 2.0 to 8.06 with the increase of PyC thickness, and the larger the Weibull modulus, the smaller the dispersion, which indicated that the regulation of PyC thickness was conducive to improve tensile properties.

Dissociation mechanisms of CH4 on pristine, N-doped and vacancy graphene by DFT study

Methane is used as the carbon precursor for the fabrication of C/C composites by using chemical vapor infiltration. Demystify the heterogeneous reactions on the carbon surface during the pyrolysis of methane is important for us to understand the formation and growth of the pyrocarbon. In this paper, the dissociation mechanisms from CH4 to C2H2 on pristine graphene, N-doped graphene, and vacancy graphene during the chemical vapor infiltration process are systematically studied using the density functional theory. After the optimized adsorption and co-adsorption configurations are obtained, the transition state calculations are performed to identify the reaction pathways. The adsorption energy, energy barrier, reaction energy, Mulliken atomic charges, and partial density of states of the dissociation processes are analyzed. We revealed that the adsorption of molecules CH4, C2H6, C2H4, and C2H2 on the substrates are physisorption, and the adsorption of CH3/H, 2CH3, C2H5/H, C2H5, C2H3/H, and C2H3 on these substrates are strong chemisorption. In addition, these reactions are easier to take place on the vacancy graphene, yet much harder on the N-doped graphene.

Development of Orthogonal 3-D Woven Amorphous SiC Fiber / SiC/YSi<sub>2</sub>-Si Matrix Composites

For evaluating Low temperature Melt Infiltration (LMI) process, adapted to amorphous SiC fiber with high toughness and low thermal stability, high-manufacturable and high-strength orthogonal 3-D woven amorphous SiC fiber / SiC / YSi2-Si Matrix Composites was developed. Thermophysical properties of candidate molten Si alloys were measured by the electrostatic levitation furnace, Si-Y alloy was selected because of the low melting temperature and superior infiltration ability compared to reference material, Si-Ti alloy. Dense orthogonal 3-D woven amorphous SiC fiber / SiC / YSi2-Si Matrix Composites of which porosity was less than 3vol.% was successfully fabricated by weaving an amorphous SiC fiber coated with CVD-BN and constructing SiC matrix via Chemical Vapor Infiltration (CVI) process and LMI. Process temperature of LMI was 50°C lower than general Melt Infiltration (MI) process with Si. It was demonstrated that the orthogonal 3-D woven amorphous SiC fiber / SiC / YSi2-Si Matrix Composites has equal tensile strength until 1100°C and excellent fatigue properties at 1100°C over 150MPa against to crystalline SiC fiber/SiC-Si Matrix Composites.

Behavior of carbon matrix in carbon fiber reinforced composites (CFRC) synthesized through the film boiling chemical vapor infiltration (FB-CVI) process

Film boiling chemical vapor infiltration (FB-CVI) technique is considered as one of the fastest process to manufacture carbon fiber reinforced carbon (CFRC) composite products. In the present work, the characteristics of the carbon matrix deposited through FB-CVI process under deposition and infiltration are investigated. A novel reactor with unique features was developed, to simulate the process conditions. Utilizing the indigenously build reactor, pyrocarbon (PyC) was deposited on graphite substrates. The kinetics of the process is investigated within temperature range of 1100 °C–1150 °C and characteristics of carbon matrix was investigated. The nature of the carbon deposited and effect of graphitization on the microstructure of the deposited PyC was examined. The role of preform architecture on densification through FB-CVI process and characteristics of the composites were further studied. The mechanical behavior of the FB-CVI based CFRC material were investigated through nanoindentation technique. The present study has provided process guidelines for densification of 2D continuous fibers based preform. Structure property relationship is evolved to optimally develop CFRC composite products for aerospace applications.

Homogenization of a discrete network model for chemical vapor infiltration process

Homogenization of a discrete network model for chemical vapor infiltration process

Investigation of flexural behavior and microstructure of Amosic-3 silicon carbide composites under neutron irradiation

SiC/SiC composites prepared by the chemical vapor infiltration process have been investigated under neutron irradiation at approximately 40 °C to doses of 0.03 and 0.2 displacements per atom (dpa) in the China Mianyang Research Reactor. Irradiation effects were characterized by three-point flexural tests, micro-Raman spectra and transmission electron microscopy observations. The results demonstrated that the ultimate flexural stress and flexural modulus decreased remarkably with irradiation doses up to 0.2 dpa. Micro-Raman spectra of the Amosic-3 fiber and SiC matrix revealed that both reduction of Si–C bonds and enhancement of Si–Si bonds were increased from 0.03 to 0.2 dpa. Microstructural observation revealed that the Amosic-3 fiber suffered from grain fragmentation and amorphization. Meanwhile, matrix amorphization is generated preferentially at the site of stacking fault structures. In addition, it suggested that in the current work, up to 0.2 dpa, full amorphization was not completed in the Amosic-3 fiber, PyC and SiC matrix.

Influence of sintering temperature and CVI time on mechanical properties of 3D-printed alumina ceramics

Recently, complex-shaped ceramic components are being fabricated by using 3D printing and subsequent chemical vapor infiltration (CVI) processes. The results indicate that both sintering temperature and CVI time significantly influence mechanical properties of 3D-printed ceramic components. At the sintering temperature of <1500 °C, the pores are reduced during sintering and the ceramic components exhibit significant shrinkage, whereas, at the sintering temperature of 1500 °C, the shape of grains is changed from spherical to cylindrical, which increases the pore size and expands the ceramic parts. Moreover, the ceramic parts are severely densified after sintering at >1500 °C due to high sintering temperature. As 3D-printed ceramic parts render low strength after sintering at 1500 °C, the subsequent CVI process is carried out to increase the flexural strength (2 times) by introducing SiC, and in turn, improving the density of ceramics. One should note that the CVI-induced SiC is deposited on the surface of Al2O3 grains, improving the strength of the ceramics. The proposed sintering mechanism consists of the deformation of grains and subsequent densification, leading to the deformation of ceramic components.

Development of C/PyC/SiC CMCs by CVI Process for Hypersonic Vehicle Application

Hot structures are essentially required for the hypersonic vehicle applications owing to their operation in the dense flight environments. This paper aims at the development and testing of ceramic matrix composite components for air breathing hypersonic vehicle application. In this regard, 2.5D C/PyC/SiC composites were prepared by isothermal Chemical vapour infiltration (CVI) process. X-ray CT analysis was employed to understand the extent of CVI densification in the ceramic matrix composites laminates, and it was under stood that that the intra-bundle infiltration precedes the inter-bundle infiltration with an edge (high) to center (low) gradient. The composite specimens showed an average tensile strength of 206 ± 21 MPa and an average flexural strength of 405 ± 25 MPa, respectively, at room temperature. Thermal conductivity of composite specimens shows an increase in trend with temperature. The composite laminates were successfully tested for use in the air intake ramps of hypersonic air breathing engines.

Enhancing the tensile properties of SiC composites by a two-dimensional CNT film

Tensile properties of silicon carbide (SiC) ceramics were improved by introducing a two-dimensional carbon nanotube film (CNTf) with excellent mechanical properties. CNTf was first assembled by floating catalyst chemical vapor deposition (CCVD); then CNTf with the pyrolytic carbon interfacial layer (CNTf/PyC) was prepared by chemical vapor infiltration (CVI). Afterwards, SiC was infiltrated into the CNTf and CNTf/PyC preforms by the CVI process, to prevent the uneven dispersion and agglomeration of CNT and damage caused by high temperature sintering. Two composites (CNTf/SiC, CNTf/PyC/SiC) were ultimately prepared, and their microstructures and tensile properties were studied. The results show that the tensile strengths of CNTf/PyC, CNTf/SiC and CNTf/PyC/SiC are 716 ± 60, 153 ± 11, and 155 ± 12 MPa, respectively, which illustrates that SiC ceramic reinforced by CNTf exhibits excellent tensile properties. Besides, the porosities of CNTf/SiC and CNTf/PyC/SiC are 17.4% and 22.6%, respectively. CNTf/PyC/SiC shows higher porosity but better tensile properties, indicating that the PyC interfacial layer enhances the tensile properties of CNTf/SiC composite. In addition, the strengthening and toughening mechanisms are also discussed.

Full-scale multi-physics numerical analysis of an isothermal chemical vapor infiltration process for manufacturing C/C composites

The internal architecture of a CVI reactor significantly influences the gas flow behavior, as well as the complex time-varying chemical reactions, but has been typically ignored in previous CVI models. Herein we developed, validated, and applied a fully three-dimensional (3D) physicochemical CVI model of an industry-scale reactor to simulate an isothermal CVI process for fabricating bulk carbon-carbon composites using methane as a precursor gas and a multi-layered preform consisting of a non-crimp fabric and felt. The flow inside the reactor was modeled using the Navier–Stokes equation, coupled with the convection-diffusion equation, to simulate the dispersive behaviors of the reactive gases inside the porous preform. The interactive molecular diffusion of methane (CH4), ethylene (C2H4), acetylene (C2H2), and benzene (C6H6) were modeled by considering the multi-step hydrocarbon reactions between the species. The hydrocarbon concentration changes, resulting from the carbon deposition on the preform surface, were computed to predict the evolution of the preform density and porosity. The current surface area of the preform was then determined based on the current porosity. The numerical results for the average preform density agreed well with the experimental data. In addition, the present model can provide detailed simulations of the temporal and spatial evolution of the preform density that cannot be experimentally observed. The effectiveness and utility of the developed model could benefit the design of CVI reactors and processes and minimize the need for test runs when processing conditions change.

The control of interfacial bonding state and optimization of mechanical properties of Si3N4f/BN/Si3N4 composites via different synthesis technologies

Silicon nitride fibers-reinforced silicon nitride matrix (Si3N4f/BN/Si3N4) composites with a BN interphase were fabricated by chemical vapor infiltration (CVI) and precursor infiltration pyrolysis (PIP). It is shown that PIP Si3N4f/BN/Si3N4 composites possessed better mechanical properties and achieved effectively interface debonding and fiber pull-out, whose flexural strength and fracture toughness can achieve 189 ± 17 MPa and 5.6 ± 0.6 MPa·m1/2, about twice and 1.5 times as much as those of CVI Si3N4f/BN/Si3N4 composites, respectively. Combining the micro-nano mechanical and macro-mechanical properties investigations, it is revealed that weakening the interfacial bond and achieving proper modulus matching relationship between fibers and matrix by PIP process could remarkably increase the mechanical properties of the Si3N4f/BN/Si3N4 composites, compared with CVI process.

Nanoinfiltration behavior of carbon nanotube based nanocomposites with enhanced mechanical and electrical properties

In this work, carbon nanotube (CNT) based nanocomposites with high mass fraction are proposed by in-situ bridging carbon matrix into CNT paper through optimized chemical vapor infiltration (CVI). Nanoinfiltration behavior of CNTs is basically investigated under the CVI process. The contact between each CNT can be strengthened and the conductive pathways can be established, resulting in the better mechanical and electrical properties. Compared with the pristine CNT paper, the CNT/C composite after pyrolysis process confirms a remarkable advance in tensile strength (up to 310 ± 13 MPa) and Young's modulus (up to 2.4 ± 0.1 GPa). Besides, a notable feature of electrical conductivity also shows an improvement up to 8.5 S/cm, which can be attributed to the mass fraction of CNT (41 wt%) breaking the limits of percolation thresholds and the efficient densification of this sample to establish the conductive pathways. This study has a broad application in the development of the multi-functional electrical and engineering materials.

Influence of BCl3/NH3 flow ratio on growth and microstructure of CVI-processed boron nitride interfacial coatings

Chemical vapor infiltration (CVI) is usually utilized to deposit hexagonal boron nitride (h-BN) interfacial coatings with layered microstructure. The deposition rate of BN coating increased with the increase of ammonia flow, indicating that the nucleation occurs on the surface of fibers rather than the gaseous phase. However, when ammonia flow is increased from 3.0 L min−1 to 7.5 L min−1, the deposition rate exhibits an increase of only 30%, which implies that the deposition of BN coatings is controlled by the boron deposition. Furthermore, the coating samples, prepared with ammonia flow of 3.0–4.2 L min−1, exhibits a relatively smooth surface, whereas the flow rates of 5.4 and 7.5 L min−1 result in strumae with a diameter of >4 μm. The smooth coatings demonstrate a near-stoichiometric ratio of boron and nitrogen, and render excellent thermal stability from room temperature to 1300 °C in argon and air. Moreover, the chemical composition of strumae-containing coatings is far from the desired stoichiometry and both coatings contain a lower amount of hexagonal phase, which can be ascribed to the lower thermal stability in the presence of oxygen and water vapors. The present study provides useful insights into the influence of CVI process parameters on the quality of BN coatings.

Dependence of mechanical properties on microstructure of high-textured pyrocarbon prepared via isothermal and thermal gradient chemical vapor infiltration

In order to explore the dependence of mechanical properties on the microstructure of high-textured (HT) pyrolytic carbon (PyC), the microstructure and properties of HT PyC specimens prepared by isothermal isobaric chemical vapor infiltration (ICVI) and thermal gradient chemical vapor infiltration (TCVI) processes were investigated. The HT PyC of ICVI was uniform and exhibited a more distinct long-range order compared to that of TCVI. Besides, a spatial gradient in texture existed within the HT PyC of TCVI, demonstrating the micro-nonuniformity of the whole matrix. The orientation angle (OA) of HT layers in TCVI increased gradually from the fiber surface to outside, whereas the OA of HT layers in ICVI with better microcrystalline was nearly similar. According to the results of three-point bending and micro-indentation, the flexural strength, interfacial fracture strength, and micro-hardness of TCVI specimens were slightly higher than those of ICVI specimens. Furthermore, the residual thermal stress distribution of TCVI specimens was complex owing to the spatial gradient in temperature of TCVI process, while the residual thermal stress distribution of ICVI specimens was relatively uniform. This work provides meaningful guidance for the process selection and industrial applications of C/C composites.

Adsorption of small hydrocarbons on pristine, N-doped and vacancy graphene by DFT study

In this paper, the adsorption mechanisms between different species (CH3, CH4, C2H2, C2H4, C2H6, and C6H6) and the carbon-based materials (pristine graphene, N-doped graphene, and vacancy graphene) during the chemical vapor infiltration process were systematically studied using the density functional theory. The optimized adsorption configurations were obtained by using Dmol3 code in Material Studio. The adsorption energies, Mulliken atomic charges, density of states, and charge density differences of the adsorption process were analyzed. It showed that the CH3 is chemisorbed, while the other species (CH4, C2H2, C2H4, C2H6, and C6H6) are weak physisorbed on pristine graphene, N-doped graphene and vacancy graphene. In addition, the introduction of nitrogen dopant and vacancy defect into the pristine graphene does not significantly affect the adsorption mechanisms between small hydrocarbons and graphene.