Isothermal Chemical Vapor Infiltration Research Articles

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.

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.

Mechanical properties and ablation resistance of HfC nanowire modified carbon/carbon composites

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.

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.

A Numerical Study of Densification Behavior of Silicon Carbide Matrix Composites in Isothermal Chemical Vapor Infiltration

We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the reaction. Thereby, we established mathematical models on two scales, respectively, preform and reactor. These models were used for the numerical simulation of the process of ceramic matrix composites densified by isothermal chemical vapor infiltration (ICVI). The models were used to carry out a systematic study on the influence of process conditions and the preform structure on the densification behaviors. The most important findings of our study are that the processing time could be reduced by about 50% without compromising the quality of the material, if the processing temperature is 950–1 000 °C for the first 70 hours and then raised to 1 100 °C.

Thermo-mechanical characterization of carbon fiber reinforced silicon carbide composites having boron nitride as an interphase material

The carbon fiber reinforced silicon carbide composites were prepared by an isothermal chemical vapour infiltration process. In order to achieve the required density, the carbon fiber preforms in the form of rectangular panels were infiltrated by silicon carbide (SiC) matrix. Prior to the matrix infiltration, a thin coating of boron nitride, as an interphase, was applied on the fiber preform. The test samples were subjected to seal coating of silicon carbide by chemical vapour deposition process. The effect of protective SiC seal coating was examined by testing (3-point bend test) the uncoated and the seal coated samples at different temperatures. Higher value of the flexural strength was observed for the seal coated samples as compared to the uncoated samples, when got tested at high temperature (up to 1400 °C). The detailed analysis of the fractured surfaces of the tested samples was carried out.

Enhanced anti-ablation performance of carbon/carbon composites modified with ZrC-SiC

The ZrC-SiC inhibited carbon/carbon composites (C/C-ZrC-SiC) were fabricated by combining precursor infiltration and pyrolysis (PIP) of ZrC and reactive melt infiltration (RMI) of ZrSi2. The achieved C/C-ZrC-SiC composites exhibited an unusual microstructure. The dense ZrC-SiC matrix formed by RMI was distributed in the fiber webs. The ZrC-C matrix developed by PIP and isothermal chemical vapor infiltration (ICVI) was uniformly distributed in the non-woven layers. After ablation under oxyacetylene torch, C/C-ZrC-SiC composites exhibited a desirable ablation resistance. The formed ZrO2 with a framework structure acted as the barrier to effectively shield the ablation heat and withstand mechanical denudation of high speed flame stream. In the brim region of the ablated surface, the SiO2 layer was capable of further reducing ingress of oxygen into the materials below.

Preparation and thermal conductivity of C/C composites filled with diamond particles

In order to improve the thermal conductivity of C/C composites, micro-scale high thermal conductive diamond particles were filled into unidirectional carbon fiber reinforced high texture (HT) pyrolytic carbon (PyC) matrix composites to generate C/C-diamond composites by isothermal chemical vapor infiltration (ICVI) method. Microstructure of C/C-diamond composites was characterized by polarized-light microscope, X-ray diffraction, scanning electron microscope and Raman spectroscopy. HT PyC can nucleate surrounding the surface of diamond and grow in laminar structure, and has a higher graphitization degree. The effects of diamond particles with different volume fraction on thermal diffusivity and thermal conductivity of C/C-diamond composites were discussed. The thermal conductivity of C/C-diamond composites with the diamond particle content of 2 vol% was 19.6% higher than that of C/C composites. However, with the further increase of diamond particle content, HT PyC laminar structure became disorganized and the interfacial pores between diamond and PyC increased in turn, resulting in the increase of interfacial thermal resistance. Therefore, the thermal conductivity of C/C-diamond increased at first and then followed downward tendency with the increase of diamond particle content. A model related to the heat conduction of C/C-diamond composites was proposed.

Microstructure and interlaminar shear property of carbon fiber-SiC nanowire/pyrolytic carbon composites with SiC nanowires growing at different positions

In order to improve the interlaminar shearing strength of carbon fiber/pyrolytic carbon (Cf/PyC) composites, SiC nanowires (SiCNWs) growing at different positions were introduced into carbon fiber/pyrolytic carbon composites to generate carbon fiber-SiC nanowire/pyrolytic carbon (Cf-SiCNWs/PyC) composites. Cf-SiCNWs/PyC composites were prepared by sol-gel and isothermal chemical vapor infiltration (ICVI) method. The morphology, microstructure and compositions of composites were investigated by SEM, TEM, XRD and XPS. The interlaminar shearing strength was tested and the effect of SiCNWs growth positions on the interlaminar shearing strength was investigated. The results showed that SiCNWs were consisted of perfect single crystalline structure of β-SiC with diameter of 160–200 nm. The SiCNWs could grow at four kinds of positions to combine with carbon fibers to form multi-scaled reinforcements (micro-scaled carbon fibers and nanoscaled SiCNWs). The interlaminar shear strength of Cf-SiCNWs/PyC composites were increased by 78% compared with Cf/PyC composites without SiCNWs. The improvement of interlaminar shear strength was attributed to bridging and pull-out of multi-scaled reinforcements composed of carbon fibers and SiCNWs as well as the enhancement of fiber/matrix interface bonding generated by SiCNWs growing at different positions.

The Characterization of Fibrous Material in CVI Graphite Cylinder Wall

By isothermal chemical vapor infiltration (ICVI) preparation of carbon/carbon composites, We found fibrous materials with silver metallic luster in CVI furnace graphite cylinder outer wall, fiber diameter is 1 ~ 2 mm, length is 4 ~ 15 mm. The characterization of fibrous material have been systematically studied by scanning electron microscopy (SEM),element analysis, energy dispersive X-rays spectroscopy (EDS).After 2300 degrees heat treatment, the fibrous material were characterized by ultra-violet laser Raman spectroscopy. The SEM examination shows that these fibrous materials have a spherical top, the cross-section reveals a unique structure in which layers like growth rings lie concentrically on top of each other. The EDS analysis show the main element of fibrous material is carbon and a small amount of metallic element. Raman spectra show after 2300°C high temperature treatment ,the carbon fibrous material transformed from layer structure to graphite structure.

High temperature oxidation behaviour of CVD β-SiC seal coated SiC f /SiC composites in static dry air and combustion environment

Abstract A composite with SiC matrix and 40 vol% of SiC fibres with a β-SiC coating (SiC f /SiC) was prepared by the isothermal chemical vapour infiltration process. Oxidation of the SiC f /SiC composite was investigated at 1200 °C, 1300 °C and 1400 °C, for up to 100 h in static dry air, and for up to 24 h in combustion environment. In static dry air, the composite showed mass gain with nearly parabolic oxidation kinetics. The uniform oxide scale developed cracks as oxidation progressed. Oxidation in combustion environment was performed using an oxy-acetylene flame containing CO 2 and H 2 O gases. The chemical erosion by the reaction of H 2 O with oxide scale was the likely ablation mechanism. At 1200 °C and 1300 °C, the oxidation rate was higher than that of ablation and the composite showed a net mass gain. However, at 1400 °C, significant ablation was observed. Localised attack by the combustion gases caused pitting of the β-SiC seal coating. The present study shows that β-SiC seal coating remains intact at relatively lower temperatures (below 1300 °C) for a shorter period of time (~24 h) and is lost it at higher temperatures (~1400 °C) by ablation.

Homogenization for chemical vapor infiltration process

Multi-scale modeling and numerical simulations of the isothermal chemical vapor infiltration (CVI) process for the fabrication of carbon fiber reinforced silicon carbide (C/SiC) composites were presented in [Bai, Yue and Zeng, Commun. Comput. Phys., 7(3):597-612, 2010]. The homogenization theory, which played a fundamental role in the multi-scale algorithm, will be rigorously established in this paper. The governing system, which is a multi-scale reaction-diffusion equation, is different in the two stages of CVI process, so we will consider the homogenization for the two stages respectively. One of the main features is that the reaction only occurs on the surface of fiber, so it behaves as a singular surface source. The other feature is that in the second stage of the process when the micro pores inside the fiber bundles are all closed, the diffusion only occurs in the macro pores between fiber bundles and we face up with a problem in a locally periodic perforated domain.

Preparation of co-deposited C/C-ZrC composites by CLVD process and its properties

Carbon/carbon composites doped with zirconium carbide were prepared by CLVD using a three-step method. Carbon fiber felts were first densified by chemical liquid-vapor deposition (CLVD) process from precursor pyrolysis, followed by heat treatment at 1773 K. Finally, the C/C KrC composites were densified by isothermal chemical vapor infiltration (ICVI). Microstructure, mechanical properties and ablation resistance of the composites were investigated. The results showed that, the pyrolytic carbon and ceramics are co-deposited on the carbon fibers and the CLVD processing time is much less than that of PIP process. The flexural strength of CLVD-processed composites was enhanced, from which it can be deduced that co-deposition of the ceramics and pyrocarbon improved the interface bonding strength between carbon fibers and the matrix. In addition, C/C-ZrC composites exhibited excellent ablation resistance and ZrO2 protective layer formed after ablation.

Effect of heat treatment on cracking and strength of carbon/carbon composites with smooth laminar pyrocarbon matrix

Carbon fiber preforms were pre-heat treated at 2100°C and then filled with smooth laminar (SL) pyrocarbon matrix by isothermal chemical vapor infiltration. The microstructure and flexural properties of carbon/carbon (C/C) composites after heat treatment from 1800 to 2500°C were investigated. The microstructure investigation indicated that heat treatment did not affect the extinction angle, but resulted in distinct interfacial cracks and concentric cracks in the SL pyrocarbon. Three-point bending tests revealed that the flexural strength of the C/C composites decreased rapidly from 221 to 108MPa after treated at 2100°C. This can be explained that the multiple interfacial debonding and concentric cracks lead to poor stress transfer capabilities and high stress concentration. The flexural strength increased back to 126MPa when the composites were heat treated under a higher temperature at 2500°C. This phenomenon was attributed to cracks bridging in the debonding region and the thin high-textured layer at the fiber-matrix interface.

Preparation of high texture three-dimensional braided carbon/carbon composites by pyrolysis of ethanol and methane

A three-dimensional (3D) carbon/carbon (C/C) composite was prepared using the gaseous mixture of ethanol and methane as the precursor by isothermal chemical vapor infiltration. The preform was infiltrated at 1100°C with the gas pressure from 2 to 10kPa. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Texture and fracture morphology of the pyrolytic carbon matrix were observed using a scanning electronic microscope. After 150h infiltration, the average bulk density was up to 1.72±0.02gcm−3. 3D C/C composites with high texture pyrolytic carbon matrix were obtained by pyrolysis of ethanol and methane. The average flexure and tensile strengths of the composites are 362MPa and 116MPa, respectively.

Electromagnetic interference shielding effectiveness of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)n matrix (C/(PyC–SiC)n) composites were fabricated by alternately depositing PyC and SiC inside preforms via isothermal chemical vapor infiltration. A four-layer structure consisting of alternate PyC and SiC was formed compactly on the surface of carbon fibers. The electromagnetic interference shielding effectiveness (SE) in the frequency range of 8.2–12.4GHz of the composites was investigated. The composites showed a high SE value of about 40dB in the whole frequency band due to the repeated reflection and dissipation of electromagnetic wave between the interfaces of SiC layers and PyC layers, PyC layers and carbon fibers. Therefore, C/(PyC–SiC)n composites exhibited great potential as functional materials.

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.

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Abstract Carbon fiber reinforced multilayered (PyC–SiC)n matrix (C/(PyC–SiC)n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)n composites with a density of 1.43 g/cm3 are 204.4 MPa and 3028 kJ/m3 respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm3. The enhanced mechanical properties of C/(PyC–SiC)n composites are attributed to the presence of multilayered (PyC–SiC)n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)n composite is a promising structural material with low density and high flexural strength and toughness.

Microstructures and Mechanical Properties of Zirconium Dioxide Modified Carbon/Carbon Composites

To improve the preparation technology and mechanical properties of carbon/carbon composites, a kind of Zirconium dioxide modified Carbon/carbon (C/C–ZrO2) composites were prepared by Sol-Gel Process combined with isothermal chemical vapor infiltration (ICVI) method. This method could shorten preparation cycle greatly to reduce the cost. The microstructures and mechanical properties of the C/C–ZrO2 composites were investigated. The results indicate that the ZrO2 modified phase is uniformly distributed in the carbon matrix, the bending strength and modulus of C/C–ZrO2 composites are much higher than unmodified C/C composites due to its particle toughening and fiber toughening.

Compressive and interlaminar shear properties of carbon/carbon composite laminates reinforced with carbon nanotube-grafted carbon fibers produced by injection chemical vapor deposition

Carbon nanotubes (CNTs) were grown in situ on the surface of carbon fiber cloths by injection chemical vapor deposition using ethanol as a carbon source and ethylenediamine as a promoter. The nanotube-grown cloths were stacked to form multilayered preforms, and then densified by isothermal chemical vapor infiltration to prepare CNT reinforced carbon/carbon (CNT–C/C) composite laminates. The effect of CNTs on microstructure and mechanical properties of carbon/carbon (C/C) laminates were investigated. Results show that the existence of CNTs not only contributes to refine the pyrocarbon (PyC) grain size but also inhibits matrix cracking, resulting in high cohesion of PyC matrix. The grown CNTs possess high surface area thus modifying the fiber surface condition, which in turn improves the fiber/matrix (F/M) interface. Both the high matrix cohesion and strong F/M interface bonding give rise to the powerful interaction between adjacent layers in the CNT–C/C laminates. The measured out-of-plane, in-plane compressive strength and interlaminar shear strength of these laminates show a significant enhancement of 32%, 115% and 108%, respectively compared with the laminates without CNTs. Out-of-plane and in-plane compressive modulus of these laminates are also improved by 11% and 46%, respectively.