Chemical Vapor Infiltration Technique Research Articles

Effect of void defects on mechanical behavior and failure features of C/C honeycomb structure

The combination of honeycomb structure and carbon/carbon (C/C) composites offers unique high load-bearing and light-weight advantages in satellite-bearing platforms. However, the defects generated by the imperfect preparation process are inevitable. This paper employs the chemical vapor infiltration (CVI) technique to fabricate novel C/C honeycomb structures and considers the presence of internal void defects. Based on micro-computerized tomography (μ-CT) images, the void distribution and porosity as well as the micro-structure characteristics are acquired. Meso-scale representative volume element (RVE) models is established to predict the performance of honeycomb wall material. A secondary development of ABAQUS is performed to establish a model of honeycomb structure containing void defects. Multi-scale models are developed for investigating the mechanical performance of the C/C honeycomb structure. Experimental methods and finite element simulations are utilized to examine the effects of porosity and different weave structures on the compression and shear properties, as well as damage modes of C/C honeycomb structures in detail. The results demonstrate that the presence of low porosity ρ = 1 % severely diminishes the load-bearing capacity of the honeycomb and affects its damage pattern. Twill-2 weave C/C honeycomb inherits the advanced performance of the composites and exhibits superior comprehensive properties compared to plain weave and twill-1 weave honeycomb structures.

ZrC-SiC closed-cell ceramics with low thermal conductivity: Exploiting unique spherical closed-cell structure through tape casting and CVI techniques

Porous ultra-high temperature ceramics (UHTCs) are recognized as novel candidates for fulfilling the requirements of thermal protection systems of hypersonic aircrafts, as they possess excellent high-temperature resistance and low thermal conductivity. Currently, the reported porous UHTCs predominantly exhibit an open pore structure. By contrast, closed-cell UHTCs, formed by employing ceramic hollow microspheres (HMs) as pore-forming agents, hold great potential for achieving superior thermal insulation performance. Unfortunately, the implementation of this strategy has been hindered by the scarcity of raw materials and preparation techniques.In this paper, ZrC-SiC closed-cell ceramics were first successfully prepared through a combination of tape casting and chemical vapor infiltration (CVI) techniques, utilizing the self-developed ZrC HMs as the primary raw material. The morphology, microstructure, and thermal insulation properties of the obtained ZrC-SiC closed-cell ceramics were investigated. The results indicate that when the content of ZrC HMs is 30 vol.%, the density of the prepared porous ceramics is 2.09 g cm–3, with a closed porosity of 14.05% and a thermal conductivity of 1.69 W (m K)–1. The results clearly prove that the CVI process can successfully convert ZrC HMs into closed pore structures within porous ceramics. The introduction of ZrC HMs suppresses the contribution of free electrons to thermal conductivity and brings about a large number of solid-gas interfaces, which increases the interfacial thermal resistance and significantly reduces the phonon thermal conductivity. Consequently, the as-prepared ZrC-SiC closed-cell ceramics show excellent thermal insulation properties. This study provides a new idea and method for the development of porous UHTCs and offers a more reliable material choice for thermal protection systems.

In situ growth of SiC wires on CVI densification of SiC foam

AbstractSilicon carbide (SiC) foam prepared by polymer infiltration and pyrolysis (PIP) process was further densified with β‐SiC by chemical vapor infiltration (CVI) technique. Scanning electron microscopy and high‐resolution transmission electron microscopy images confirmed the presence of highly entangled and branched in situ grown SiC wires of uniform diameter (∼500 nm) over the struts of open‐cell SiC foam. A uniform rate increase in diameter from nanometer to micron range (∼11 μm) was observed with an increase in the CVI reaction period. X‐ray diffraction results showed the formation of highly crystalline β‐SiC structure along the <111> direction with stacking faults. The formation of SiC wires was explained by the vapor–liquid–solid mechanism and evenness of the surface and uniform growth rate of SiC confirmed the homogeneous concentration of gaseous species during CVI reaction. The compressive strength increased with relative density, with maximum values of 5.5 ± 1.26 MPa for ultimate SiC foam (ρ = 400 kg/m3) prepared by hybrid PIP/CVI technique. The thermo‐oxidative stability of the resultant foam was evaluated up to 1650°C under air and shows excellent thermal stability compared to SiC foam prepared by PIP route. The densified SiC foam can find potential applications in the field of hot gas filters, catalyst supports, microwave absorption properties, and heat insulation for high‐temperature applications.

Effect of C/SiC Volume Ratios on Mechanical and Oxidation Behaviors of Cf/C–SiC Composites Fabricated by Chemical Vapor Infiltration Technique

Effect of C/SiC Volume Ratios on Mechanical and Oxidation Behaviors of Cf/C–SiC Composites Fabricated by Chemical Vapor Infiltration Technique

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.

Fabrication of SiCf/SiC composites through hybrid processing via chemical vapor infiltration, electrophoretic deposition, and liquid silicon infiltration

ABSTRACT The present work demonstrates the effectiveness of a novel hybrid process comprising chemical vapor infiltration (CVI), electrophoretic deposition (EPD), and liquid silicon infiltration (LSI) techniques for the successful fabrication of low-porosity SiCf/SiC composites. For this purpose, fiber/matrix interphase dual coating layers of BN and SiC were coated onto SiC fabrics using CVI. A ceramic matrix consisting of SiC and carbon black nanoparticles was then infiltrated into the fine voids of the fabrics using EPD. Finally, LSI was performed to obtain dense microstructures with low porosities by filling the remaining small gaps and reacting Si with C to form SiC. Microstructural results observed by scanning electron microscopy revealed a dense structure with no damage to the fibers. The experimental density was found to be 2.62 g/cc, with an open porosity of 0.55. The room-temperature flexural strength was evaluated to be 111 MPa, and the composites displayed little fiber pull-out.

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.

The Influence of Si3N4w Volume Fraction on the Microstructure and Properties of Si3N4w/Si3N4 Composites via CVI Technique

Wave-transparent ceramic composites with excellent comprehensive performances are of particular significance for the development of space vehicles. In this study, Si3N4w was prepared by the gel-casting process and then combined with chemical vapor infiltration (CVI)-obtained Si3N4 to yield Si3N4w/Si3N4 composites with five Si3N4w volume fractions. The influence of the Si3N4w volume fraction on the microstructures, mechanical properties, and dielectric characteristics of the Si3N4w/Si3N4 composites were all studied. The results showed that Si3N4w/Si3N4 composites possess A sandwich structures. Higher Si3N4w volume fractions led to less incorporated amounts of the Si3N4 matrix at the same depth. The 25 vol% Si3N4w/Si3N4 composites integrated relatively higher Si3N4w volume fraction and load transfer ability of the matrix, leading to better flexural strengths reaching up to 41.37 MPa. The dielectric constants of the five prepared Si3N4w/Si3N4 composites ranged from 2.25 to 4.0. The dielectric losses were all lower than 0.005, demonstrating the good dielectric properties of the Si3N4w/Si3N4 composites.

Effect of initial density on thermal conductivity of new micro‐pipeline heat conduction C/SiC composites

AbstractC/SiC composites prepared by chemical vapor infiltration technique (CVI) have been regarded as thermal structural materials widely. However, these composites still suffer from poor functional properties like low thermal conductivity, especially in thickness direction of the composites, limiting their large‐scale applications. Herein, mesophase pitch based carbon fiber (MPCF) and continuous wave laser machining were utilized to construct highly effective heat conductive micro‐pipelines within CVI C/SiC composite. The effect of initial density on the final density and thermal conductivity of the as‐obtained MPCF‐C/SiC composites were investigated. The results revealed that higher initial density would directly enhance the thermal conductivity and reduce the negative impact of the bottle‐neck effect. At temperatures between 100°C and 500°C, MPCF‐C/SiC composites preserved more than threefold of the thermal conductivity (340%) when compared to reference C/SiC composites. This work provides a highly effective route for enhancing the thermal conductivity of C/SiC, which would broaden their future applications.

Improvement in Thermo-oxidative Stability and Mechanical Strength of Carbon Foams for Thermal Management of Reusable Launch Vehicles

Carbon foams were prepared and coated with silicon carbide (SiC) by chemical vapour infiltration (CVI) technique and tested for improvement of oxidation stability and compressive strength, compared to virgin carbon foams. Microstructural and morphological characterisation of SiC-coated carbon foam samples proved the formation of stoichiometric high strength, crystalline β-SiC over amorphous carbon which enables the ceramic foam to take high compressive loads. The final ceramic foam was successfully tested for oxidation stability up to a maximum temperature of 1650 °C under air atmosphere. The weight loss of SiC-coated carbon foam was found to depend on the final density, which depends on the SiC coating thickness. The functional property of reducing the back wall temperature of ceramic matrix composites (CMC) based light weight thermal protection systems (TPS) using SiC-coated carbon foam as the core material was validated by carrying out the plasma testing under atmospheric re-entry conditions. The process is noted for the cost-effective method of imparting desired ceramic property of SiC to less-expensive carbon foam structure.

Effect of carbon matrix on mechanical and tribological properties of Cf/Cu/C composites

Using fibre web/copper mesh integrated fabric as the preform, carbon fibre and copper fibre fabric reinforced carbon matrix (Cf/Cu/C) composites with different types of carbon matrix were prepared by chemical vapor infiltration (CVI) technique with C3H6 + N2 and impregnation/carbonization (I/C) technique with furan resin. The microstructures, mechanical and tribological properties of Cf/Cu/C composites with resin carbon, pyrolytic carbon (PyC) and PyC/resin carbon matrix were characterized. The results show that Cf/Cu/C composites with PyC matrix possess the highest flexural strength and the lowest hardness. Cf/Cu/C composites with resin carbon matrix have the highest wear rate and friction coefficient, and the failure mechanism includes both abrasive wear and delamination wear. For Cf/Cu/C composites with PyC matrix, the wear process is mainly controlled by abrasive wear. It is easier for PyC matrix to form a lubricant film.

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.

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 EMW absorbing properties of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures

SiC nanowire/siliconboron carbonitride-Silicon nitride (SiCnw/SiBCN-Si3N4) ceramics were prepared via a low-pressure chemical vapor deposition and infiltration (LPCVD/CVI) technique. The as-prepared ceramics were annealed at varying temperatures (1200–1600 °C) in a N2 atmosphere, and their crystallization mechanism and absorbing properties were subsequently studied. The absorbing properties of the SiCnw/SiBCN-Si3N4 ceramics improved with the annealing temperature up to a certain value and decreased thereafter. Among the samples tested, the SiCnw/SiBCN-Si3N4 ceramics annealed at 1300 °C showed the highest permittivity (real and imaginary parts) and dielectric loss values in the X-band (ca. 5.34, 2.55, and 0.47 respectively), and this could be attributed to the precipitation of carbon and SiC nanocrystals. The sample treated at 1300 °C decreased its minimum reflection coefficient (RC) from −12.0 to −59.68 dB (compared with the as-received SiCnw/SiBCN-Si3N4 ceramics) and the effective RC (below -10 dB) in the whole X-band could be achieved when the thickness was set to 3–3.5 mm. These results revealed that the absorbing performance was significantly improved after the heat treatment at 1300 °C.

Effect of heat treatment temperature on microstructure and electromagnetic shielding properties of graphene/SiBCN composites

Three-dimensional (3D) graphene/SiBCN composites (GF/SiBCN) were prepared by depositing SiBCN ceramics in 3D graphene foam via the chemical vapor infiltration technique. The effect of the heat treatment temperature on the microstructure, phase composition, and electromagnetic properties of the GF/SiBCN composite was investigated. The SiBCN ceramics maintained an amorphous structure in the composite below 1400 °C above which the crystallinity of the free carbon phase gradually increased. While the Si3N4 and B4C phases started to crystallize at 1500 °C and their crystallinity increased with temperature, SiC was observed at 1700 °C. The electromagnetic shielding effectiveness of GF/SiBCN increased with the heat treatment temperature.

A comparative study of tensile strength of Cf/SiC composites having single layer and multilayer interphases

Tensile behaviour of stitched 2D carbon fiber reinforced silicon carbide (Cf/SiC) composites having variable interphases was studied in detail. Two types of interphases (single layer of pyrocarbon interphase and multilayers of pyrocarbon/silicon carbide) were deposited initially on the carbon fiber preforms followed by silicon carbide matrix infiltration. Multilayer interphase was deposited for two different durations. Cf/SiC composites were prepared by isothermal chemical vapour infiltration (ICVI) technique. The as-cut and silicon carbide seal coated samples were subjected to tensile tests in air at room temperature and at 1200 °C. The effect of seal coating and type of interphase on the tensile properties was studied. It was observed that the composites (in uncoated and seal coated condition) with multilayer interphase showed higher values of tensile strength at both the test temperatures as compared to single layer interphase. Also, the composites fabricated with multilayer interphase deposition for longer duration showed highest values of tensile strength at both the test temperatures. The fractured regions of tensile tested samples were analyzed in detail by SEM in order to understand microstructure-property relationships.

Rotary ultrasonic milling of C/SiC composites fabricated using chemical vapor infiltration and needling technique

Carbon fiber reinforced silicon carbide (C/SiC) has a widespread application in the aerospace industry owing to its superior properties like hardness, wear resistance, low density and heat resistant. However, poor characteristics of conventional machining of C/SiC composite has impoverished its utility in space and military applications. To overcome this problem, a new method of rotary ultrasonic milling has been used for machining the composites with a varying material density which have been prepared using chemical vapor infiltration (CVI) and needling technique available at Indian space research organization. Different response characteristics were studied with different machining parameters namely material density, feed rate, and axial depth of cut. The pragmatic results revealed that maximum average change in the surface roughness is 6.08 μm for 1.74 g cm−3, 50 mm min−1, and 0.01 mm whereas material removal rate decreases by 33.29%, 66.57%, and 66.82% on increasing the material density from 1.74 to 2.46 g cm−3, feed rate from 50 to 200 mm min−1 and axial depth of cut from 0.01 to 0.04 mm respectively. Furthermore, it is apparent from tool wear study that wear reduces as the material density is decreased. Subsequently, morphological examination revealed that SiC was uniformly distributed throughout the sample and very less gap present inside the structure and energy dispersive spectroscopy confirms the presence of carbon (45.77%) and silicon (54.23%) in the fabricated composites.

Fabrication of high‐strength porous SiC‐based composites with unidirectional channels

AbstractCarbon fiber reinforced SiC‐based composites with unidirectional channels were obtained by freeze‐casting and chemical vapor infiltration techniques, and their microstructure and fracture behavior were investigated. The results indicated that the sizes of the unidirectional channels could be controlled in the 15‐70 μm range. The significant pseudo plastic and ductility features of the porous SiC‐based composite were demonstrated during the fracture process. The strains of the composites reached 4 ± 1% and 50 ± 6% during bending and compressive experiments, respectively. The bending and compressive strengths, 123 ± 20 and 99 ± 15 MPa, respectively, were far superior to those of homogeneous ceramics presenting the same porosity. Combining the aforesaid method with 3D printing techniques, a SiC‐based composite part with controlled macro‐micro channels was fabricated, which indicated that this method could be useful for fabricating ceramic parts with complex structure and superior mechanical performance.

Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si3N4 matrix composites (SiCf/Si3N4) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiCf/Si3N4 composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than −7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa▪m1/2).

Effect of chemical vapour infiltration technique on the gradient microstructure and high-temperature strength of gel-cast Al2O3-based ceramic moulds

A new approach for the preparation of Al2O3 ceramic moulds with gradient microstructure using a chemical vapour infiltration (CVI) technique is proposed. A ceramic mould with a gradient microstructure was designed for casting hollow turbine blades. Ceramic moulds with a homogeneous microstructure were formed by gel casting. With the introduction of SiC by CVI, the gradient microstructure was finally formed in the ceramic mould. Owing to its dense interior and loose exterior structure, the ceramic mould resisted the impact of liquid alloys and showed good deformability at high temperature. After CVI and sintering at 1450 °C in air, the high-temperature strength of the ceramic mould was improved via the reduction of porosity and the generation of mullite, a high-temperature strengthening phase. For the ceramic mould with 60 vol% solid loading, the high-temperature strength at 1500 °C increased from 3.61 to 36.9 MPa and the thermal conductivity increased as well. The microstructures and phase compositions of the sintered ceramic samples were investigated by scanning electron microscopy and X-ray diffraction, respectively. The combination of gel-casting and the CVI technique helped fabricate a ceramic mould with a gradient microstructure and good high-temperature properties.