Isothermal Chemical Vapor Infiltration Research Articles

Probabilistic physics-integrated neural differentiable modeling for isothermal chemical vapor infiltration process

Chemical vapor infiltration (CVI) is a widely adopted manufacturing technique used in producing carbon-carbon and carbon-silicon carbide composites. These materials are especially valued in the aerospace and automotive industries for their robust strength and lightweight characteristics. The densification process during CVI critically influences the final performance, quality, and consistency of these composite materials. Experimentally optimizing the CVI processes is challenging due to the long experimental time and large optimization space. To address these challenges, this work takes a modeling-centric approach. Due to the complexities and limited experimental data of the isothermal CVI densification process, we have developed a data-driven predictive model using the physics-integrated neural differentiable (PiNDiff) modeling framework. An uncertainty quantification feature has been embedded within the PiNDiff method, bolstering the model’s reliability and robustness. Through comprehensive numerical experiments involving both synthetic and real-world manufacturing data, the proposed method showcases its capability in modeling densification during the CVI process. This research highlights the potential of the PiNDiff framework as an instrumental tool for advancing our understanding, simulation, and optimization of the CVI manufacturing process, particularly when faced with sparse data and an incomplete description of the underlying physics.

Compression and thermophysical properties of carbon fiber-reinforced high textured pyrocarbon and SiC dual matrix composites

C/C–SiC composites with high textured pyrocarbon (HT PyC) were fabricated by isothermal chemical vapor infiltration and reactive melt infiltration. The evolution of HT PyC before and after heat treatment at 2100 °C as well as the internal defects of the composites, such as pores and cracks, were characterized. The compression and thermophysical properties of the C/C–SiC composites with 2100 °C heat-treated HT PyC (C–S-21) and C/C composites with 2100 °C heat-treated HT PyC (C–C-21) were studied. The results showed that the compression strength and modulus of C–S-21 were 323.19 ± 11.52 MPa and 4.49 ± 0.13 GPa, which were increased by 58.94% and 125.63%, respectively, compared with those of C–C-21 owing to the smaller number of pores and the higher modulus of SiC and Si, respectively. However, fewer pores and strong interfacial adhesion resulted in the brittle fracture of C–S-21. The thermal conductivity of C–S-21 was 73.56 W/(m·K) and 44.94 W/(m·K) in the XY and Z directions, respectively, which was attributed to the fewer defects and better orientation of the graphitized layer in HT PyC as well as the smaller number of pores.

Simultaneously enhancing mechanical and tribological properties of carbon fiber composites by grafting SiC hexagonal nanopyramids for brake disk application

Extensive attention has been drawn to the development of carbon fiber composites for their application in brake disks due to the increasing demand for brake disks with high mechanical strength and better tribological properties. Herein, we design SiC hexagonal nanopyramids modified carbon/carbon (SiCNPs-C/C) composites, in which SiCNPs are radially grafted on the carbon fibers by the combined sol-gel and carbothermal reduction method, and pyrolytic carbon (PyC) matrix is deposited on nucleation sites including carbon fibers and SiCNPs by isothermal chemical vapor infiltration (ICVI). Benefiting from the special structure, SiCNPs-C/C composites exhibit superior mechanical and frictional performance. Compared with C/C composites, SiCNPs-C/C composites have 147%, 90.3%, 70.6%, and 117.9% improvement in the hardness, interlaminar shear strength, and out-of-plane and in-plane compressive strength, respectively, which is attributed to the optimized fiber/matrix (F/M) interfaces bonding and the enhanced cohesion strength of PyC matrix. In addition, the friction coefficient of SiCNPs-C/C composites increases by 25.5%, and the wear rate decreases by 38.0%. This work provides an optional design thought for the nanomaterials and enlightens the mechanical and frictional modification of composites in the field of the brakes.

Structural and mechanical changes by adding graphene to C/C composites

The structure and mechanical properties of C/C composites prepared by isothermal isobaric chemical vapor infiltration (ICVI) were investigated to evaluate the influence of introducing graphene. The results indicated that adding graphene significantly improved the texture of pyrocarbon. The pure pyrocarbon was primarily composed of smooth laminar (SL) structure, while the pyrocarbon in composites containing graphene was dominated by regenerative laminar (ReL) structure. Furthermore, with 0.66 wt% addition of graphene, the compression performance of the C/C composites was prominently improved by 70.9%, increasing from 107.37 MPa to 183.45 MPa, while the bending strength of the C/C composites was slightly decreased. The variation of mechanical properties was attributed to graphene-induced weakening of interfacial bonding and improved texture of pyrocarbon, resulting in a more compact arrangement of carbon layers and better integrity of the matrix. This work contributed to the understanding of the improved properties and structure of C/C composites with graphene addition.

Synchronously promoted mechanical and biotribological properties of carbon fiber composites by constructing Si3N4 nanowires@pyrolytic carbon intertwined network

Constructing strong interaction architecture is the key to achieve high performance of composite materials, where the combination of high mechanical strength and excellent biotribological properties are required. Herein, Si3N4 nanowires@pyrolytic carbon core-shell structure (SNs@PyC) are designed and constructed into carbon fiber-carbon-hydroxyapatite composites (CHC) for improving the mechanical and biotribological performances. SNs@PyC enhanced CHC (SNs@PyC-CHC) are fabricated by a combined method composed of precursor impregnation-pyrolysis, chemical vapor deposition, pulse electrochemical deposition and isothermal chemical vapor infiltration. SNs@PyC are distributed inside CHC uniformly with intertwined network structure. SNs@PyC induces the uniform deposition of hydroxyapatite matrix, favors the infiltration of the carbon matrix, enhances the interface between carbon matrix and Si3N4 nanowires, and promotes the cohesion strength of the carbon matrix. Compared with CHC, the flexural strength, compressive strength and shear strength of SNs@PyC-CHC are increased by 63%, 47% and 300%, respectively. In addition, the friction coefficient and wear rate of SNs@PyC-CHC are decreased by 55% and 84%, respectively. SNs@PyC-CHC may have a promising application in biomedical.

A High-Efficiency Technology for Manufacturing Aircraft Carbon Brake Discs with Stable Friction Performance

A binary C/C brake disc (i.e., the test brake disc) was prepared with a C/C (pyrolytic carbon/resin carbon) matrix using modified natural gas as the carbon source through the isothermal chemical vapor infiltration (ICVI) process with a directed flow and the pressure impregnation carbonization (PIC) process with liquid-phase furfural acetone resin. The microstructural, mechanical, thermal, friction and wear properties of the test brake disc were comprehensively analyzed and compared with commercial ones. The results showed that the production efficiency of the test brake disc was 36% higher than that of the commercial ones, which were manufactured through a thermal-gradient chemical vapor infiltration (TCVI) process. The favorable mechanical and thermal properties of the test brake disc were comparable to the commercial ones. While the test brake disc had a more consistently rough laminar microstructure on the worn surface of the brake disc than the commercial ones, this avoided the annular grinding grooves on the worn surface after the braking tests. In addition, the test brake disc had a stable friction coefficient with a low dispersion coefficient of 3.90%, which would improve the friction stability of C/C brake discs used in aircrafts.

Influence of fiber/matrix interface on the texture evolution and fracture toughness of C/C

This study was conducted to obtain a more in-depth insight into the role of fiber/matrix interface on the texture and mechanical performance of carbon fiber reinforced carbon composites (C/C). In this study, a series of isothermal chemical vapor infiltration (ICVI) experiments were performed on the deposition of pyrocarbon (PyC) onto carbon fibers (CFs) and pre-oxidized fibers (PFs). The results showed that the graphitization degree and orientation angle (OA) of the fiber and matrix of the PF-SL were relatively low compared with the CF-SL. Most importantly, the OA of CF-SL between the interfaces was much higher than that of PF-SL. Besides, a low texture transition layer with a width of tens of nanometers was observed between the fiber/matrix interfaces, which may reduce the lattice distortion and interfacial energy. In addition, the fracture toughness of CF-SL was higher than that of PF-SL, which was attributed to interfacial differences. Further, PF-SL had a coarser interface than CF-SL due to the carbonization reaction during ICVI, which induced gas escape and PF shrinkage and bulges on the surface.

The ablation and mechanical behaviors of C/(SiC-ZrC)n multi-layer structure matrix composites by chemical vapor infiltration

Continuous carbon fiber reinforced (SiC-ZrC)n multi-layer structural matrix composites were prepared by isothermal chemical vapor infiltration method. The microstructure, bending and ablation behavior of the composites with different number (n, n = 1, 2, 4) of SiC-ZrC layer were studied. The fracture morphology was step-like fracture. With the increase of n, the number of interfaces doubled, benefiting that the cracks kept deflecting and branching at the interface to consume lots of energy. The bending strength increased from 127.63 MPa (n = 1) to 276.85 MPa (n = 4). After ablation, among the prepared composites, C/(SiC-ZrC)4 composite not only had the highest residual bending strength (169 MPa), but also had the best ablation resistance, which the mass and linear ablation rates were 1.17 ± 0.19 mg/s and 7.50 ± 0.56 µm/s, respectively.

Design, Fabrication, and Mechanical Properties of T-700TM Multiaxial-Warp-Knitting-Needled-C/SiC Composite and Pin.

In this paper, the 12k T-700TM Multiaxial-Warp-Knitting–Needle (MWK–N) C/SiC composite and pin were designed and fabricated using the isothermal chemical vapor infiltration (ICVI) method. The composite’s microstructure and mechanical properties were examined by subjection to tensile and interlaminar shear tests. Three types of double-shear tests were conducted for C/SiC pins, including shear loading perpendicularly, along, and at 45° off-axial to the lamination. The fracture surface of the tensile and shear failure specimens was observed under scanning electronic microscope (SEM). The relationships between the composite’s microstructure, mechanical properties, and damage mechanisms were established. The composite’s average tensile strength was σuts = 68.3 MPa and the average interlaminar shear strength was τu = 38.7 MPa. For MWK–N–C/SiC pins, the double-shear strength was τu = 76.5 MPa, 99.7 MPa, and 79.6 MPa for test types I, II, and III, respectively. Compared with MWK–C/SiC pins, the double-shear strength of MWK–N–C/SiC pins all decreased, i.e., 26.7%, 50.8%, and 8% for test types I, II, and III, respectively. The MWK–N–C/SiC composite and pins possessed high interlaminar shear strength and double-shear strength, due to the needled fiber in the thickness direction, low porosity (10–15%), and high composite density (2.0 g/cm3).

Construction of sandwich-structured C/C-SiC and C/C-SiC-ZrC composites with good mechanical and anti-ablation properties

Four kinds of sandwich-structured C/C-SiC and C/C-SiC-ZrC composites with or without a SiC interphase deposited by isothermal chemical vapor infiltration (ICVI), were designed and fabricated by a joint process of electromagnetic coupling chemical vapor infiltration (ECVI) and precursor infiltration and pyrolysis (PIP). The fabricated composites are macroscopically nonhomogeneous materials with low density, high strength and low ablation rate. The interphase and matrix constituents had remarkable effects on the mechanical and ablation properties of these composites. The C/C-SiC composites with an ICVI-SiC interphase exhibited the highest flexural strength of 306.5 MPa. While the C/C-SiC-ZrC composites with the interphase showed the best anti-ablation performance with low linear and mass ablation rates of 0.37 μm/s and 0.04 mg/cm2·s, respectively, after the ablation for 500 s under an oxyacetylene flame test at around 2000 °C.

Synthesis and characterisation of Cf/SiC composites with modified silicon carbide matrix

Cf/SiC composites with multilayer [(PyC/SiC)n; n = 4] interface and modified silicon carbide matrix were prepared through isothermal isobaric chemical vapor infiltration (ICVI) technique and characterised room temperature mechanical properties. The monotonic tensile properties were studied at elevated temperature (i.e. 1250 °C, and 1350 °C) and compared with room temperature tensile properties. The tensile strength at RT, 1250 °C, and 1350 °C of the composite was found to be 193 ± 10 MPa, 238 ± 14 MPa, and 189 ± 20 MPa respectively. This paper also focusses the effect of thermal exposure of composites on residual tensile strength. The enhanced strength and toughness behaviour observed for the prepared composites because of the presence of multi-layer interface and modified matrix (SiC/Si-B-C/SiC). SEM microstructural studies carried out on tested specimens and cross section of prepared composites revealed the evidence for the self-healing behaviour from the modified matrix.

Ablation and heat insulation performances of nose-shaped ZrC-C composites with gradient pore structure

Nose-shaped ZrC-C composites with gradient pore structure (ZrC-CGS) were successfully prepared by isothermal chemical vapor infiltration using ZrC skeleton as preform. Pyrolytic carbon with medium texture was deposited around ZrC skeleton. From the top to back, the porosity of the ZrC-CGS composites increased gradually from 0% to 62.9%. Compared with the ZrC-C composites with dense structure (ZrC-CDS), the mass ablation rate of the ZrC-CGS composites decreased by 15.3% and the heat resistance capacity increased by 45.6%, and their coefficients of thermal expansion decreased obviously in the temperature range of 200–1500 °C. The simulation results showed that the residual thermal stress and back temperature of the ZrC-CGS composites are lower than those of the ZrC-CDS composites. This work provides an effective strategy for designing and preparing ultra-high temperature composites having both ablation and heat insulation performances by controlling the pore structure and distribution.

Effect of methane and acetaldehyde precursor on the microstructures of pyrocarbon films grown on quartz substrates

The microstructure of pyrolytic carbon significantly affects the performance of aircraft brake discs. In this study, to explore the effect of precursor on the microstructure of pyrocarbon, two types of pyrocarbon films were grown on the quartz glass substrates by isothermal isobaric chemical vapor infiltration using methane and acetaldehyde as precursors. The microstructures of both the films and their formation mechanisms were investigated and compared. The thicknesses of the pyrocarbon films from methane and acetaldehyde were approximately 860 nm and 986 nm, respectively. The film from acetaldehyde comprised quite a few carbon nanoflakes with sizes in the range of 100–200 nm. However, the film from methane exhibited lots of small and dense carbon layers, with a large number of carbon particles of about 20 nm size. Compared to the methane deposited film, the lattice fringes of the carbon layers in the acetaldehyde deposited film were more ordered and had a better orientation. Moreover, the acetaldehyde deposited film had fewer defects owing to the higher sp2/sp3 ratio than that of the methane deposited film. This study may provide new insights into the selection of precursors for preparing pyrocarbon.

Microstructure and mechanical performance evolution of C/C–ZrC composites exposed to oxyacetylene flame

C/C–ZrC composites were prepared by isothermal chemical vapor infiltration (ICVI) combined with reactive melt infiltration (RMI). The ablation behavior of the C/C–ZrC was investigated using an oxyacetylene flame. The effect of ablation time on the microstructure and mechanical property evolution of the composite was studied. The results showed that as the ablation time prolonged, the linear and mass ablation rates of the composite increased firstly and then stabilized. After 15 s ablation, the flexural strength and modulus of the C/C–ZrC were interestingly increased by 141.8% and 40.9%, which reached 138.42 MPa and 6.45 GPa, respectively. During ablation, the preferential oxidation effect of ZrC could mitigate the oxidation of pyrolytic carbon (PyC) and carbon fibers, and the volume change induced by the ZrC →ZrO2 phase transformation could weaken its bonding with PyC, which was beneficial for releasing the internal residual stresses of the C/C–ZrC and then contributed to the mechanical performance improvement.

Zirconium carbide skeleton reinforced pyrocarbon composites with tunable mechanical strength, thermophysical properties and ablation resistance

Zirconium carbide skeleton reinforced pyrocarbon (ZrC-C) composites with different weight ratio of ZrC/PyC were designed and prepared by hot-pressing sintering and isothermal chemical vapor infiltration. Their thermal expansion coefficients at 200–1500 ℃ were controlled in the range of 4.95 × 10−6–7.34 × 10−6 ℃−1. As the weight ratio of ZrC/PyC increased from 1.4 to 2.4, the compressive strengths of ZrC-C composites along the Z direction increased from 188.0 ± 6.9–252.6 ± 11.2 MPa. After ablation for 60 s using oxyacetylene torch with a heat flux of 2.4 MW/m2, the surface temperatures of the samples increased from 2177 to 2272 ℃, but their linear and mass ablation rates decreased from 5.290 ± 1.267 to −0.766 ± 0.587 μm/s and 3.504 ± 0.375 to 0.451 ± 0.138 mg/s, respectively. High-content ZrO2 skeleton formed by the oxidation of ZrC could further restrain the composite consumption. Finite element simulation results showed that the thermal stresses increased with the increase of the weight ratios of ZrC/PyC.

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.

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.

Mechanically strong and bioactive carbon fiber-SiC nanowire-hydroxyapatite-pyrolytic carbon composites for bone implant application

Carbon fiber-SiC nanowire-hydroxyapatite-pyrolytic carbon composites (CHS) composed of carbon fiber, hydroxyapatite (HA), SiC nanowires (SiCnws) and pyrolytic carbon (PyC) were prepared by sol-gel, heat treating, electrochemical deposition (ECD) and isothermal chemical vapor infiltration (ICVI) for bone implant application. The morphology, microstructure and the composition of CHS were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The mechanical properties of CHS were tested and compared with the traditional bone implant materials. The in-vitro bioactivity was evaluated by testing the cell morphology, cell proliferation and cell differentiation. The results show that SiCnws and HA are distributed uniformly inside the CHS. SiCnws are attached on the carbon fibers. The diameter of the SiCnws is around 250 nm and the length is more than 100 μm. HA could grow on the surface of both SiCnws and carbon fibers, forming a brush shape. The flexural strength, shear strength, out-of-plane and in-of-plane compressive strength of CHS are 127 MPa, 90 MPa, 169 MPa and 213 MPa, respectively. The elastic modulus of CHS is 7.2 GPa. The mechanical properties of CHS are similar to those of cortical bone. In-vitro cell test shows that CHS has excellent cell proliferation and cell differentiation. CHS possessing good mechanical and biological performances may have potential to be applied for bone implant materials.

Mechanical and thermal conduction properties of carbon/carbon composites with different carbon matrix microstructures

Four carbon/carbon composites with densities of 1.75–1.81 g/cm3were produced with matrix carbons derived from (a)coal tar pitch and furfural acetone resin by impregnation-carbonization, (b) natural gas by isothermal chemical vapor infiltration and (c) xylene by film-boiling chemical vapor infiltration. Theirmechanical properties and thermal conductivities (TC) were compared and correlated to the matrix carbon microstructures. Results showed that the strength of the composites decreased as thecarbon matrix changed from natural gas pyrocarbon (PyCN), to resin-derived carbon, to xylene PyC (PyCX), and to pitch-derived carbon. The highest flexural and interlaminar shear strengths of 208.7 and 26.4MPa, respectively, were obtained for the PyCN matrix. The large amount of fiber pull-out and step-like matrix fracture contributed to a high toughness of composites with PyCN and PyCX matrices. Relatively low strength and toughness were obtained for the pitch-derived carbon. The change of TC with the matrix type was consistent with the graphitization degree and apparent crystallite height. The highest in-plane and out-of-plane TC reached 148.2 and 75.4 W/(m·K), respectively, for the PyCX matrix, which was due to its carbon layers having the highest preferred orientation. The chaotic structure and large number of defects of the resin-derived carbon produced a relatively low TC. PyCN was the most suitable matrix for composites with excellent mechanical properties. PyCXshould be used to improve the TC and toughness of the low-cost composites.

Vapour phase synthesis and characterisation of Cf/SiC composites with self-healing Si-B-C monolayer coating

Carbon fibre reinforced CVI-SiC matrix (Cf/SiC) composite is well known for its superior properties such as low density, high specific modulus, high fracture toughness, and high temperature mechanical properties. In the present work, 2.5-Directional Cf/SiC composites with (PyC/SiC) n=4 multilayer interface having two different thicknesses with a density of ~2.1 g cm-3 are prepared through isobaric isothermal chemical vapour infiltration technique. High temperature tensile properties of the prepared composites with and without Si-B-C seal coating are studied and the results are presented. Samples prepared without seal coat exhibited a KICof ~ 30 MPa m1/2, and tensile strength of ≥200 MPa at room temperature. Si-B-C seal coated Cf/SiC composites has shown significant increase (28%) in high temperature tensile strength at 1200 °C and 1500 °C respectively compared to uncoated composites. Microstructural observations, XRD, and XPS studies support the observed thermomechanical behaviour of these composites at 1200 °C and 1500 °C.