CVI Process Research Articles

Processing and performance of ultra high temperature ceramic matrix composite (UHTCMCs) using radio frequency assisted chemical vapour infiltration (RF-CVI)

Ultra-high temperature ceramic matrix composites (UHTCMCs) have been produced using a radio frequency assisted chemical vapour infiltration (RF-CVI) process. The composites were based on 2.5D carbon fibre preforms with a 0 / 90° (out of plane) fibre orientation and containing 23 % fibre volume fraction. These were initially impregnated with zirconium diboride (ZrB2) powder in the form of a slurry and then, after solvent removal, the majority of the porosity filled with pyrolytic carbon (PyC) using the RF-CVI process at 1273 K and 0.5 kPa chamber pressure. The latter resulted in a uniform rough laminar texture with good interfacial bonding. As intended, an inverse temperature profile was achieved using the RF heating, enabling uniform densification of the preform from the inside out, with no entrapped porosity and achieving 90 % of theoretical density in only 24 h, at least a tenfold reduction in processing time compared to the conventional CVI process and a fivefold reduction compared to other modified CVI processes such as forced flow or pressure gradient CVI. The resulting UHTCMCs displayed good mechanical strength and thermo-ablative behaviour.

Fabrication of bamboo-inspired continuous carbon fiber-reinforced SiC composites via dual-material thermally assisted extrusion-based 3D printing

Ceramic matrix composites (CMCs) structural components encounter the dual challenges of severe mechanical conditions and complex electromagnetic environments due to the increasing demand for stealth technology in aerospace field. To address various functional requirements, this study integrates a biomimetic strategy inspired by gradient bamboo vascular bundles with a novel dual-material 3D printing approach. Three distinct bamboo-inspired structural configurations Cf/SiC composites are designed and manufactured, and the effects of these different structural configurations on the CVI process are analyzed. Nanoindentation method is utilized to characterize the relationship between interface bonding strength and mechanical properties. The results reveal that the maximum flexural strength and fracture toughness reach 108.6 ± 5.2 MPa and 16.45±1.52 MPa m1/2, respectively, attributed to the enhanced crack propagation resistance and path caused by the weak fiber-matrix interface. Furthermore, the bio-inspired configuration enhances the dielectric loss and conductivity loss, exhibiting a minimum reflection loss of −24.3 dB with the effective absorption band of 3.89 GHz. This work introduces an innovative biomimetic strategy and 3D printing method for continuous fiber-reinforced ceramic composites, expanding the application of 3D printing technology in the field of CMCs.

Effect of CVI SiC interphase on tensile properties of SiC fibers with different surfaces

The microstructure, and tensile properties of as-received SiC fiber and SiC coated fiber with different surfaces were investigated. The surface microstructure and composition of the fibers were analyzed through the AFM, SEM, Raman, AES, and TEM analysis. The SiC fibers were or not desized firstly, and then BN interphase or SiC interphase was deposited by CVI process. The tensile properties of the fibers before and after the deposition of the SiC interphase were evaluated, and the influence of the surface composition of SiC fibers on its tensile properties was studied. The results showed that the fiber surface is rich in free carbon and oxygen. The fibers without interphase showed the best tensile properties than those with coated fibers. After depositing SiC interphase, the tensile strength of SiC monofilament without sizing agent and BN interphase reduced mostly.

Local anti-ablation modification of uneven-density C/C composites with the ZrC-SiC composite ceramics

To lower the cost and preparation period of C/C-ZrC-SiC composites, the uneven-density C/C composites (with a high density in the inner region and low density in the surface region) are designed and prepared by thermal gradient CVI process, followed by local modification of ZrC-SiC composite ceramics by precursor infiltration-pyrolysis (PIP) mothed to improve the anti-ablation performance. The ablation tests show that the SiC contents have a great influence on the microstructure of ZrO2 grains at the ablation center by adjusting the ablation temperature and the melting point of ZrO2 grains. Results prove that about 20 wt% SiC is optimum for the ZrC-SiC composite ceramics to perform best during the ablation and modify the uneven C/C composites prepared in this work. Besides, the dense C/C matrix would stop the liquid ZrO2-SiO2 phase from running off, and in return, the dense ZrO2-SiO2 phase would sink into the surface within the ablated region protecting the material beneath, which could protect the carbon fibers of the surface region and the dense C/C matrix beneath.

Influence of CVI process time on the C-ring strength of hybrid SiCf/SiC composites fabricated by CVI–LSI

Influence of CVI process time on the C-ring strength of hybrid SiCf/SiC composites fabricated by CVI–LSI

Carbon@SiC(SiCnws)-Sc2Si2O7 ceramics with multiple loss mediums for improving electromagnetic shielding performance

Multiple loss media can be applied, not only to improve the performance of microwave absorption materials, but also to promote the electromagnetic (EM) shielding efficacy of materials. In this study, multiple loss mediums (C, SiC, and SiCnws) were introduced into Sc2Si2O7 ceramics to synthesise carbon@SiC(SiCnws)-Sc2Si2O7 ceramics by H2 reduction and CVI technology. After 1–3 CVI process cycles, the open porosity decreases from 34.7 %±0.5 % to 8.4 %±0.3 % filled with multiple loss media. The results show that CH3SiCl3 provides the raw material for synthesising SiC(SiCnws) and carbon in ceramics, resulting in adjustable EM shielding properties. Furthermore, the EM shielding mechanism consists mainly of the combined synergistic effect of C, SiC, and SiCnws, which contributes to the reflective shielding efficacy, and promotes the absorption shielding effectiveness, including dipole polarization loss, and interface polarization loss. Subsequently, carbon@SiC(SiCnws)-Sc2Si2O7 ceramics with a multiple loss medium of 31.2 wt% ±0.22 wt% achieve a total EM shielding effectiveness value of 31.8 dB (99.92 % electromagnetic waves are attenuated) with an X-band thickness of 2 mm. Therefore, carbon@SiC(SiCnws)-Sc2Si2O7 ceramics are a promising novel composite, applicable in future EM shielding in high temperature environments.

Processing of Cf/SiC Composites through Two-Channel Temperature-Control CVI: I, Modeling

A two-channel temperature-control CVI scheme was proposed to fabricate thicker and denser composites. The two-channel structure helps to densify a thick preform, and a precise temperature control will guarantee a low and uniform porosity distribution. Validation simulations containing hydrodynamics, mass transfer, heat transfer and pore structure evolution were first carried out. Modeling results confirm that a two-step densification based on the new scheme can work well: At step I, all gases pass through the preform and the high-temperature bottom-preform is densified; At step II, by altering the outlet, temperature and infiltration time, part of gases are sucked into the preform and the remaining coarse preform is densified. The scheme can fabricate tick, uniform and dense composite, it can also avoid huge pump pressure thus protecting fibers from cracking. It is hoped to enlighten the CVI processing of ceramic matrix composites.

Experimental study and numerical modeling of pulse CVI for the production of organomorphic C/SiC composites

AbstractDuring the recent years, considerable attention has been given to the development of efficient and environmentally friendly technologies for the production of ceramic matrix composites. However, little research has been carried out on use of methylsilane (MS) as a halogen‐free precursor in the CVI process. The goal of this work was therefore to study the production of SiC‐matrix composites by pulse chemical vapor infiltration (PCVI) method with MS as a precursor and recently developed organomorphic frameworks as preforms. Experimental research was combined with numerical modeling to get deeper insight into the physical mechanisms underlying the technological process. Results of the study demonstrated that the PCVI allows performing the process with higher densification rate compared to continuous‐flow modifications of CVI. It is shown that optimization of PCVI providing a compromise between the utilization efficiency of the precursor, the densification uniformity, and the process time requires a proper choice of duration of the deposition stage and the process temperature. Numerical modeling proved to be a helpful tool in research and development of PCVI technology.

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.

Novel, hierarchical SiC nanowire-reinforced SiC/carbon foam composites: Lightweight, ultrathin, and highly efficient microwave absorbers

The novel hierarchical SiC nanowire-reinforced SiC/carbon foam composites (SnwSCF) were prepared for the first time by carbonization, CVD, and CVI processes. Morphological results demonstrated that the hierarchical structure, composed of carbon foam with a SiC coating and SiC nanowires, was fabricated successfully. The hierarchical structure significantly improved the microwave-absorbing performance of the carbon foam. The optimal reflection loss value of SnwSCF achieved −31.216 dB at 15.76 GHz with a 1.5 mm absorber thickness. The corresponding effective absorption bandwidth was 4.1 GHz. Furthermore, the microwave-absorption mechanism of SnwSCF is discussed in depth. The incident EM waves were trapped in the hierarchical structure and experienced multiple microwave energy attenuation processes including multiple reflections, interface polarization, and dipole polarization. As a result, SnwSCF is believed to be a superior candidate to its competitors as an ultralight, ultrathin, and highly efficient microwave absorber.

Effects of initial density during laser machining assisted CVI process and its influence on strength of C/SiC composites

Novel laser-assisted chemical vapor infiltration (LA-CVI) technique has been used to improve the density and strength of carbon fiber reinforced SiC composites (C/SiC). Initial density of C/SiC before laser machining played an important role in determining the final density and strength of composites. Results show that final density and bending strength of lower initial density composites were better than that of higher initial density samples after LA-CVI process, while the porosity exhibited opposite behavior. Micro-CT and COMSEL simulation results verify that after LA-CVI process, dense band width of C/SiC with initial density of 1.5 g/cm3 was twice as that of C/SiC with initial density of 1.8 g/cm3. This led to crack propagation bypassing the micro-hole. In conclusion, low initial density when laser machining was carried out resulted in better properties of final composites.

Microstructure and Dielectric Property of 3D BNf/Si3N4 Fabricated by CVI Process

As potential wave-transparent materials applied at high temperatures, 3D BNf/Si3N4 ceramic matrix composites were prepared by low pressure chemical vapor infiltration or deposition (LPCVI/ CVD) process from SiCl4-NH3-H2-Ar gas precursor at 800 °C. The densification process, microstructure and dielectric properties of 3D BNf/Si3N4 composites were investigated. The results indicated that 3D BNf/ Si3N4 was successfully fabricated by LPCVI/CVD, with final open porosity of 2.37% and density of 1.89 g/ cm3. Densification kinetics of 3D BNf/Si3N4 is a typical exponential pattern. The Si3N4 matrix was uniformly infiltrated into porous BNf preform. The deposited Si3N4 matrix was amorphous by XRD analysis. Introduction of BN fiber into Si3N4 ceramic lowered the permittivity of Si3N4. The fabricated BNf/Si3N4 composites possess low permittivity of 3.68 and low dielectric loss of lower than 0.01, which are independent of temperature below 400 °C. Transmission coefficient of BNf/Si3N4 composite is 0.57 and keeps stable below 400 °C. BNf/Si3N4 can be fabricated at low temperature and may be candidates for the microwave transparent materials.

Effects of channel modification on microstructure and mechanical properties of C/SiC composites prepared by LA-CVI process

Carbon fiber reinforced SiC matrix composites (C/SiC) with four different deposition channel sizes were fabricated via a novel laser-assisted chemical vapor infiltration (LA-CVI) method. Effects of infiltration channel sizes on microstructure and mechanical properties of C/SiC composites were investigated. The results showed that increasing the size of channels could expand infiltration passages and densification bands, which was consistent with theoretical calculations. Due to the presence of channels, the flexural strength of C/SiC composite increased by 14.47% when the channel diameter was 0.3 mm, compared to C/SiC composites prepared via conventional CVI process. Characteristics of matrix cracking and crack propagation on fracture surface were analyzed by using scanning electron microscopy. LA-CVI C/SiC composites displayed significantly improved damage-tolerant fracture behavior. Thus, findings of this work demonstrate that LA-CVI fabricated C/SiC composites are promising for a wide range of applications, particularly for enclosed-structure and thick-section C/SiC composites.

Influence of the Si–O–C interplayer on the flexural behaviors of C/Si–C–N composites

Carbon fiber reinforced Si–C–N matrix composite(C/Si–C–N) with a Si–O–C interlayer (C/Si–O–C/Si–C–N) was fabricated via CVI and PIP process. The flexural behaviors of C/Si–O–C/Si–C–N were investigated using the three-point-bending method and the SEM technique. The results indicated that the flexural strengh of the C/Si–O–C/Si–C–N increases with increasing temperature and the modulus of the composite is essentially unchanged. The strength of C/Si–O–C/Si–C–N is comparable to that of C/PyC/Si–C–N, and the role of Si–O–C interlayer in C/Si–C–N can rival that of the PyC interlayer. The weaker interfacial bonding and the larger thickness of Si–O–C interlayer make a contribution to this at RT while the thinner interlayer and unstable structure of Si–O–C interphase do it above 1300 °C.

A comparative study on Cf/PyC/SiC minicomposites prepared via CVI process for hypersonic engine application

AbstractCarbon fiber reinforced silicon carbide (SiC) minicomposites were prepared from three variants of commercially available carbon fibers, viz., T300‐3K, T300J‐3K, and T300‐6K. The SiC matrix infiltration was done via chemical vapor infiltration process using methyltrichlorosilane as the precursor. Minicomposites were characterized for the composition and morphology of the matrix material deposited. It was found that the matrix contains 2H‐SiC along with the major phase 3C‐SiC. Cyclic tensile tests were carried out on the composites to understand the damage mechanism and load bearing characteristics under cyclic loading conditions. The dependence of peak and residual strains on the fiber volume fraction was studied. Oxidation of the CMCs in air at 1500°C was studied and the result was explained based on a five part process.

Electroplating chromium on CVD SiC and SiCf-SiC advanced cladding via PyC compatibility coating

Electroplating Cr on SiC using a pyrolytic carbon (PyC) bond coat is demonstrated as an innovative concept for coating of advanced fuel cladding. The quantification of coating stress, SEM morphology, XRD phase analysis, and debonding test of the coating on CVD SiC and SiCf-SiC is shown. The residual tensile stress (by ASTM B975) of electroplated Cr is > 1 GPa prior to stress relaxation by microcracking. The stress can remove the PyC/Cr layer from SiC. Surface etching of ∼20 μm and roughening to Ra > 2 μm (by SEM observation) was necessary for successful adhesion. The debonding strength (by ASTM D4541) of the coating on SiC slightly improved from 3.6 ± 1.4 MPa to 5.9 ± 0.8 MPa after surface etching or machining. However, this improvement is limited due to the absence of an interphase, and integrated CVI processing may be required for further advancement.

Materialographic Microstructural Analysis during the Process Development for the Gas Phase Synthesis of Silicon Carbide

Abstract Silicon carbide is a superior material for the manufacturing of semiconductors. This is where the project on the gaseous phase synthesis of SiC components made of carbon precursors comes in. On the basis of graphite substrates and a reaction with thermally evaporated silicon monoxide in the CVI process (chemical vapor infiltration), silicon carbide with strongly process dependent structural composition is formed. The process can be used for the coating of C substrates as well as for the manufacturing of complex components. With the help of new approaches in process development and via detailed materialographic microstructural analysis, the understanding of gaseous phase synthesis was promoted. The transition of different carbon structures in equivalent silicon carbide structures was verified.

Study on the Critic Ablation Property of 2D Carbon Reinforced Silicon Carbide (C/SiC) Laminated Composites via CVI Process

Two dimensions (2D) C/SiC laminated composites is the material with isotropic properties in laminated sheets, which is considered as a promising thermal skin for aircrafts. There are intense thermal flux and thermal impact at the local interference region during the flight of the aircrafts. Therefore, mastering ablation and mechanical properties of 2D C/SiC laminated composite under extreme environments become the guild lines for the designs of the flight corridor and the aircraft security. This paper presents the experimental results of the ablation and thermal impact of C/SiC composites under different thermal environments (thermal flux ~5 MW/m2), which were carried out with the equipments of free-jets and conduct pipes. The effects on the ablation and mechanical properties of the C/SiC composites are studied, including gas pressure, thermal temperature, and the rates of temperature increasing and decreasing. The results show that the active oxidation and ablation behaviors of 2D C/SiC laminated composites under the thermal flux 5 MW/m2 consist with that of theoretical simulations. The critical failure conditions of 2D C/SiC laminated composite is also provided for the enveloping designs of the whole composites lightweight aircrafts.

Microstructural-property correlation of CVI processed Cf/SiC composites and property enhancement as a function of CVD SiC seal coating

Carbon fiber reinforced silicon carbide composites were prepared by an isothermal chemical vapour infiltration (ICVI) process. The test samples were seal coated by SiC by the chemical vapour deposition process. The nature of pyrocarbon coating (as an interphase) was studied before the infiltration of SiC matrix by a CVI process. The effect of protective SiC seal coating was examined by testing the uncoated and the seal coated samples for flexural properties, coefficient of thermal expansion (CTE) and pore size distribution. The microstructure-property correlation of the flexural test samples showed that the seal coating protected the carbon fibers from oxidation, which resulted in the significant improvement in the flexural strength value at the test temperatures as compared to the uncoated samples. Relatively lesser value of the coefficient of thermal expansion was observed for the seal coated sample.

Utilization of Low Grade Iron Ore (FeOOH) and Biomass Through Integrated Pyrolysis-tar Decomposition (CVI process) in Ironmaking Industry: Exergy Analysis and its Application

Utilization of Low Grade Iron Ore (FeOOH) and Biomass Through Integrated Pyrolysis-tar Decomposition (CVI process) in Ironmaking Industry: Exergy Analysis and its Application