Carbon Preforms Research Articles

High strength and self-lubrication graphite/SiC composites

For the reaction-formed graphite/SiC composites, the presence of cracks and poor mechanical properties seriously affect their reliability as wear resistant or sealing materials. In this work, a porous carbon preform was pressure formed first by carbon-composite-powder (PFC@G) and carbon black, impregnated with resin and carbonized subsequently, which can heal the micro-cracks, increased the strength of porous carbon preforms and finally reduce the reactivity of liquid Si and carbon during the reaction forming process. The results show that the modified porous carbon preform has a 4.8 % reduction in porosity and a 3.98 times increase in compressive strength. After reactive melt infiltration, the prepared graphite/SiC composites had a compact and isotropic structure and obtained the highest carbon content of 66.18 vol%. In addition, the coupling effect of the PFC@G size on the carbon content and mechanical properties of composites was studied. With the decreasing of the PFC@G size, the mechanical properties of the composites increased and the carbon content decreased. The sample with a carbon content of 48 vol% and a bending strength of 137 MPa exhibited excellent self-lubrication properties.

Experimental investigation and tomography analysis of Darcy-Forchheimer flows in thermal protection systems

In thermal protection systems (TPS), Darcy’s law or Darcy-Forchheimer’s law is employed to model the pyrolysis gas flow within the anisotropic porous ablator depending on the flow regime considered. A key challenge with using these laws is first, the knowledge of the validity domain of each flow regime in terms of a critical Reynolds number (Rec). Secondly, the lack of data on macroscopic properties, namely, the permeability and Forchheimer tensors is particularly challenging for the relevance of the models. The objective of this work is to contribute to overcoming these challenges by performing experimental and X-ray tomographic image-based characterization of Calcarb, a commercial carbon preform used for manufacturing TPS. For this purpose, fluid flow within Calcarb was studied experimentally in the Through-Thickness (TT) and the In-Plane (IP) directions for Reynolds numbers of 0.05 to 10.46 -representative of the TPS application. Tomography image-based micro-scale simulations, involving the direct resolution of the Navier–Stokes equations under both flow regimes, were also performed. Experimental results exhibit the anisotropic nature of Calcarb, namely through Rec values, corresponding to the Darcy flow regime limit, slightly different in the two directions (Rec of 0.31 and 0.43) with measured permeability values of 1.248×10−10 m2 and 1.615×10−10 m2 for TT and IP directions respectively. In the Forchheimer regime, experimental Forchheimer coefficients β were 2.0010×105 m−1 (TT) and 1.4948×105 m−1 (IP). During the simulation process, a numerical strategy was defined to obtain the permeability tensor yielding values within 8% of the experimental ones. The comparison of experimental results vs simulation results in the Forchheimer regime was performed through the analysis of the pressure gradients as functions of Re in the x, y, and z directions. The numerical estimations were compared successfully with experimental measurements, with a discrepancy of 5.2%, for Re values up to 2.4.

Microstructures and formation mechanism of continuous carbon fiber-reinforced (TiZrHfNbTa)C high-entropy ceramic composites fabricated via high-entropy alloy reactive melt infiltration

Using TiZrHfNbTa high-entropy alloys as cation sources and carbon matrix in carbon fiber-reinforced carbon (Cf/C) preforms as carbon sources, this study presents rapid construction of dense, continuous Cf-reinforced (TiZrHfNbTa)C high-entropy ceramic (Cf/HEC) composites based on reactive melt infiltration (referred to as Cf/HECRMI composites). The results consistently indicate the uniform distribution of elements in the ceramic phase. Based on X-ray diffraction patterns, the ceramic phase was inferred as a (TiZrHfNbTa)C HEC phase. Cf/HECRMI composites can form because the TiZrHfNbTa high-entropy alloy reacts with the carbon matrix to form uniform ceramic monomers, which spontaneously assemble into the (TiZrHfNbTa)C HEC phase under high-temperature conditions. Owing to the dense microstructures, the high strength of the (TiZrHfNbTa)C phase, high strength and toughness of the residual TiZrHfNbTa alloy phase, and Cf reinforcement, Cf/HECRMI composites exhibit excellent mechanical properties with a bending strength of 584.2 MPa. These composites also possess outstanding ablation resistance, with linear and mass rates of 0.6 μm s−1 and 0.9 mg s−1, respectively, under an ablation time of 180 s and ablation temperature maintained at ∼2000 °C. The high ablation resistance can be attributed to dense, stable, and highly viscous complex oxides (HfZrO4, TiNbTaO7, Hf6Ta2O17, Nb2Zr6O17, ZrO2, and HfO2) formed on the sample surfaces.

Experimental investigation on the validity of the local thermal equilibrium assumption in ablative-material response models

Thermal Protection Systems (TPS) material response models rely on the assumption of local thermal equilibrium (LTE) between the solid phase and the gas phase. This assumption was challenged and investigated by several authors but a sufficiently precise knowledge of heat transfer coefficients in TPS materials was lacking to reach final conclusions. The objective of this work is to contribute to filling this gap by providing a literature review of available data in other communities (thermal energy storage, heat exchangers) and by performing an experimental characterization of Calcarb, a commercial carbon preform used for manufacturing thermal protection systems. Heat transfer within Calcarb was studied experimentally in the Through-Thickness and in the In-Plane directions for Reynolds numbers of 1 to 4 - representative of the TPS application - using the transient single-blow technique. Numerical parameter estimation was performed using the Porous material Analysis Toolbox based on OpenFoam (PATO) and the Design Analysis Kit for Optimization and Terascale Applications (DAKOTA). The heat transfer coefficient hv is found to be greater than or equal to 108 W/(m3K) and the LTE assumption is shown to be valid in the conditions of the experiment. To assess the validity of the LTE assumption for other conditions, the above bound of hv may now be used in combination with a local thermal non-equilibrium model.

Oxidation studies of SiC-coated 2.5D carbon fibre preforms

Applying SiC coatings to carbon fibre (Cf) preforms can be very helpful for controlling the fibre/matrix interface characteristics and hence for the successful development and application of continuous Cf-based ceramic matrix composites. The present work focuses on two aspects of this, viz. i) the chemical vapour infiltration (CVI) coating of SiC onto 2.5D Cf preforms and ii) the subsequent oxidation studies of the resulting preforms in the temperature range 1300–1700 °C. With respect to the latter, thermogravimetric analysis (TGA) was used to study the continuous oxidation behaviour up to 1400 °C, with the results being compared to those for uncoated samples, whilst isothermal oxidation studies were also carried out in the temperature range from 1300 to 1700 °C for 4 h to understand the transition from passive to active oxidation of the SiC. Phase identification and detailed microstructural characterisation of the coated preforms and oxidised samples were carried out using XRD, SEM-EDS and TEM-SAED-EDS in order to understand the oxidation mechanism. Improved oxidation resistance was observed compared to other coating methods from the literature; this is believed to be due to the 20 nm particle size of the SiC deposited and the thicker coating at the surface compared to the interior tows of the carbon preforms. A transition from passive to active oxidation of the SiC was seen in the temperature range of 1400–1600 °C, whilst the glassy layer formed cracked, presumably on cooling due to thermal expansion differences, at temperatures >1500 °C.

Instant flow distribution network optimization in liquid composite molding using deep reinforcement learning

Carbon fibre reinforced plastic (CFRP) manufacturing cycle time is a major driver of production rate and cost for aerospace manufacturers. In vacuum assisted resin transfer molding (VARTM) where liquid thermoset resin is infused into dry carbon reinforcement under vacuum pressure, the design of a resin distribution network to minimize fill time while ensuring the preform is completely full of resin is critical to achieving acceptable quality and cycle time. Complex resin distribution networks in aerospace composites increase the need for quick, optimized virtual design feedback. Framing the problem flow media placement in terms of reinforcement learning, we train a deep neural network agent using a 3D Finite Element based process model of resin flow in dry carbon preforms. Our agent learns to place flow media on thin laminates in order to avoid resin starvation and reduce total infusion time. Due to the knowledge the agent has gained during training on a variety of thin laminate geometries, when presented with a new thin laminate geometry it is able to propose a good flow media layout in less than a second. On a realistic aerospace part with a complex 12-dimensional flow media network, we demonstrate our method reduces fill time by 32% when compared to an expert designed placement, while maintaining the same fill quality.

Review of Recent Development in Copper/Carbon Composites Prepared by Infiltration Technique

The liquid metal infiltration of carbon preformed with copper and its alloys is already an established and well-known process. It is extensively used by the electronic industry to produce heat sinks of power electronics and electric contacts and sliding electric contacts. The advantage of the process is its ability to produce near net shape components with high volume fractions of carbon at a relatively low price. The process is carried out in a vacuum and with low applied pressure. However, a strong dependence on the temperature of infiltration and its precise control is significant for the sound final product. For certain pair carbon matrix–copper alloys, different results could be obtained according to the infiltration temperature. If the temperature is too low, the solidification may occur prior to complete infiltration (high final porosity). When the temperature is too high, undesirable reactions may occur at the fiber–matrix interface (e.g., corrosive carbides). Therefore, there are still a lot of scientific papers pushing this technology to new directions and over old limits. Publications inside scientific journals within this field deal with composite materials for sliding electrical contact and electrical contact materials, sealing materials, parts of brake disks, pantograph strips for high-speed railways, other electric and mechanical applications and even for wall surface shields in future fusion devices. The present paper reviews used carbon preforms, copper alloys, technological parameters, properties of prepared composites prepared via infiltration during the last 12 years. It can be stated that 1/3 of the papers were published within the last 3 years. Moreover, renewed interest in this low-cost technique could be expected within the next few years due to climate programs and increasing prices of the energy resources.

Synthesis and thermomechanical behavior of SiC/Si compounds derived from wood waste

The traditional method of manufacturing SiC compounds is associated with a serious environmental problem, mainly due to the need for large amounts of energy (generally derived from oil) to reach processing temperatures (typically above 2500 ºC). In addition, the chemical reaction that gives rise to the formation of SiC has CO and CO2 as by-products. Therefore, in this work an alternative method to manufacture SiC/Si composites using waste from the wood industry as the main raw material was developed. SiC/Si composites were fabricated by infiltration of molten silicon into carbon preforms at 1500 °C. The carbon preforms were obtained by pyrolysis (in an inert Ar atmosphere) of four types of resin-carbon mixtures. The carbon used in the mixtures was obtained by pyrolysis of sawdust powder.The mechanical and thermomechanical behavior in uniaxial compression was studied at a constant compression rate of 0.05 mm/min at different temperatures (ambient, 1100 °C and 1400 °C). The maximum resistance values found were in the range of 58 and 384 MPa, while the Young's modulus values were between 40 and 120 GPa. The porosity found in the materials was between 1 and 4%. Finally, the fabricated compounds presented a homogeneous microstructure of interconnected silicon carbide in gray contrast and dispersed and unconnected whitish phases of uniformly distributed silicon.

Anisotropic, biomorphic cellular Si3N4 ceramics with directional well-aligned nanowhisker arrays based on wood-mimetic architectures

Inspired by the transport behavior of water and ions through the aligned channels in trees, we demonstrate a facile, scalable approach for constructing biomorphic cellular Si3N4 ceramic frameworks with well-aligned nanowhisker arrays on the surface of directionally aligned microchannel alignments. Through a facile Y(NO3)3 solution infiltration into wood-derived carbon preforms and subsequent heat treatment, we can faultlessly duplicate the anisotropic wood architectures into free-standing bulk porous Si3N4 ceramics. Firstly, α-Si3N4 microchannels were synthesized on the surface of CB-templates via carbothermal reduction nitridation (CRN). And then, homogeneous distributed Y−Si−O−N liquid phase on the walls of microchannel facilitated the anisotropic β-Si3N4 grain growth to form nanowhisker arrays. The dense aligned microchannels with low-tortuosity enable excellent load carrying capacity and thermal conduction through the entire materials. As a result, the porous Si3N4 ceramics exhibited an outstanding thermal conductivity (TC, kR ≈ 6.26 W·m−1·K−1), a superior flexural strength (σL ≈ 29.4 MPa), and a relative high anisotropic ratio of TC (kR/kL = 4.1). The orientation dependence of the microstructure-property relations may offer a promising perspective for the fabrication of multifunctional ceramics.

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC f /SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated. Comparing the microstructures of SiC f /C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC f /SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC f /SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process. In addition, among the SiC f /SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC f /SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.

SiC-Si composite part fabrication via SiC powder binder jetting additive manufacturing and molten-Si infiltration

High hardness and superior thermal stability of silicon carbide (SiC) are advantageous for engineering parts but they make it difficult to manufacture complex products with full density. Reaction-bonded SiC are effective solutions for the paradox. SiC-Silicon (Si) composite materials are manufactured by molten Si infiltration through porous SiC and/or Carbon (C) preforms owing to superior wetting of molten Si on SiC and reactive wetting of molten Si on C. Cold isostatic pressing of mixed powders feedstock, green machining and joining are a typical manufacturing pathway for green-body parts. Multiple processes with tools and significant materials loss are regarded as wastes from the viewpoint of lean manufacturing principle. In the present study, feasibility of binder jetting additive manufacturing (BJT AM) technology was assessed for green-body part manufacturing stage in the course of SiC/Si parts production. Adoption of BJT AM showed opportunities for process savings, tool savings, materials savings and design freedom. However, strength of brown-body parts as well as density of as-built green-body parts are challenges. To overcome the limitations, phenolic resin solution impregnation through as-built green parts was introduced. It is highlighted that green-body strength, brown-body strength and infiltrated-body strength are modified with evolutionary phase transformations from cured phenolic resin to reaction-synthesized SiC by way of decomposed carbon through the conventional post-AM densification manufacturing pathway.

Hybrid processing of TiC/TiCu/C composites with tailored hardness

This work reports the effect of TiC volume fractions on hardness of TiC/TiCu/C composites synthesized through in situ reactive infiltration of Ti-Cu alloy into porous carbon preforms prepared by 3 D-printing. Reactive melt infiltration of the alloy at 1100 °C under argon into C-preforms with different porosities resulted in materials composed of TiC, Cu-rich intermetallic matrix (Ti3Cu4) and residual carbon. The microstructure consists of TiC grains distributed along the Cu-Ti/C boundary. The hardness of TiC/Ti3Cu4/C composites could be tailored by the TiC volume fraction and the distribution in the composites, which are determined by the processing parameters and the initial porosity of the carbon templates in 3 D-printing stage. The hardness of the produced composites ranges from 350 HV to 570 HV and could be tailored by the TiC volume fraction and distribution in the composites, which are determined by the processing parameters and the initial porosity of the carbon-preforms.

Effect of Y2O3 Additive Amount on Densification of Reaction Bonded Silicon Carbides Prepared by Si Melt Infiltration into All Carbon Preform

The conversion of all carbon preforms to dense SiC by liquid infiltration can become a low-cost and reliable method to form SiC-Si composites of complex shape and high density. Reactive sintered silicon carbide (RBSC) is prepared by covering Si powder on top of 0.5-5.0 wt% Y2O3-added carbon preforms at 1,450 and 1,500°C for 2 hours; samples are analyzed to determine densification. Reactive sintering from the Y2O3-free carbon preform causes Si to be pushed to one side and cracking defects occur. However, when prepared from the Y2O3-added carbon preform, an SiC-Si composite in which Si is homogeneously distributed in the SiC matrix without cracking can be produced. Using the Si + C = SiC reaction, 3C and 6H of SiC, crystalline Si, and Y2O3 phases are detected by XRD analysis without the appearance of graphite. As the content of Y2O3 in the carbon preform increases, the prepared RBSC accelerates the SiC conversion reaction, increasing the density and decreasing the pores, resulting in densification. The dense RBSC obtained by reaction sintering at 1,500 °C for 2 hours from a carbon preform with 2.0 wt% Y2O3 added has 0.20% apparent porosity and 96.9% relative density.

Investigation of the formability behaviour of optimized tufted and un-tufted multi-layer carbon preforms during the stamping process

The originality of this work consists of studying the stamping behaviour of tufted and un-tufted multi-layer carbon preforms. Several tufted preforms with different stratifications have been manufactured. The stamping test was carried out using a hemispherical punch and conducted at two blank-holder pressures (0.05 and 0.2 MPa). The experimental data show that the addition of tufting yarn, the number of layers and the blank-holder pressure significantly affected the forming behaviour: the tufted preform presents a higher punch force, lower material drawin and shear angles with significant structural defects than the un-tufted preform. The increase of the blank-holder pressure increases all these characteristics and emphasizes the structural defects on the fibrous reinforcements. Similarly, the transition from two layers to four layers lamination at the same blank-holder pressure is followed by an increase of the punch force, reducing the material draw-in and the shear angles especially those measured at the transient zone, and causes more structural defects on all stamped preforms. Therefore, two localized tufting configurations, Right Localized Tufted and Inclined Localized Tufted, at the stamping transition area have been proposed. The results show that these two configurations present a minimum punch force and a maximum material draw-in similar to those measured on the un-tufted structure. The shear angles are much greater than those recorded on the conventionally (fully) tufted preform. Thus, the localized tufting in the most stressed areas proves to be the most suitable solution for the stamped preforms.

RBSC Prepared by Si Melt Infiltration into the Y<sub>2</sub>O<sub>3</sub> Added Carbon Preform

RBSC Prepared by Si Melt Infiltration into the Y<sub>2</sub>O<sub>3</sub> Added Carbon Preform

A new process for minimizing residual silicon and carbon of reaction-bonded silicon carbide via chemical vapor deposition

In order to reduce the amount of excess silicon and carbon, reaction-bonded silicon carbide was fabricated by liquid silicon infiltration using a porous carbon preform prepared by chemical vapor deposition. A carbon preform was fabricated at 1100−1400 ℃ at 6.6 kPa for 1 h using acetylene. The deposited carbon preform has a porous structure with nano-sized carbon particles and uniform pore size distribution. Depending on the deposition temperature of the carbon preform, the porosity, the pore size, and the size of the carbon particles were controlled. As the deposition temperature increased, the porosity and average pore size increased, but the average carbon particle size decreased. The residual silicon of the prepared reaction-bonded silicon carbide was 2.7 % when the porosity of the carbon preform deposited at 1100 °C was 51.1 %. The hardness increased up to 21.7 GPa as the amount of the residual silicon reduced.

Investigation of the formability behaviour during stamping of tufted and un-tufted carbon preforms: towards localized reinforcement technologies

The originality of this work consists in the study of the stamping behaviour of tufted and un-tufted carbon preforms. Several preforms by varying the tufting thread orientations (0°, 90° and 0°/90°) and with different stratifications ([0°/90°,-45°/+45°] and [0°/90°,-45°/+45°]2) were manufactured in order to study their out-of-plane deformability. The stamping test was carried out using a hemispherical punch and conducted at two blank-holder pressures (0.05 and 0.2 MPa). The experimental data highlight the influence of tufting threads on the forming force, maximum material draw-in, shear angle mapping and wrinkling phenomenon during the forming process. Furthermore, the orientations of tufting thread, the number of layers and the pressure of the blank-holder significantly affected the forming behaviour: all tufted preforms present a higher forming force, lower maximum material draw-in and shear angles and notable structural defects than the un-tufted preform. The increase in the pressure of the blank-holder from 0.05 to 0.2 MPa increases all these characteristics and emphasizes the structural defects on the fibrous reinforcements. Similarly, the transition from two layers to four layers in lamination at the same blank-holder pressure is followed by an increase of the forming force required for stamping operation, reducing the material draw-in and the shear angles especially those measured at the transient zone, and causes more structural defects on all stamped tufted and un-tufted preforms. For all these reasons, two localized tufting configurations, Right Localized Tufted (RLT) and Inclined Localized Tufted (ILT), at the stamping transition area are proposed. The stamping results show that these two original configurations present a minimum punch force and a maximum material draw-in similar to those measured on the un-tufted structure. The shear angles are higher than those recorded on the conventionally tufted preforms (over the entire surface).

Development of the Low-Density Phenolic-Based Fibrous Ablator ZURAM-K

ZURAM is an ablative carbon–phenolic thermal protection material, developed by the DLR, German Aerospace Center (DLR) in cooperation with the Institute of Space Systems, University of Stuttgart. In addition to building up a basic competence in the sector of ablative thermal protection materials, an objective of the development was to provide a nonclassified ablative material for university research. ZURAM is comparable to similar low-density ablative thermal protection materials with respect to the material density and general ablative performance. It is based on a rigid, highly porous carbon fiber preform, which is filled with a nano-porous matrix derived from a phenolic resin. In the context of the ongoing material development, possibilities to improve the material were investigated with the aim to lower its thermal conductivity and to enhance the adaptability to varying mission profiles. Preliminary results indicate that by substituting the carbon preform with a phenolic fiber preform, a significant reduction of the thermal conductivity can be achieved while recession rates are comparable to the ones of standard ZURAM. The scope of the present work is the development of ZURAM-K based on phenolic Kynol fibers. Furthermore, results from tests in the arc-heated DLR facilities L2K and L3K, as well as data on the thermal conductivity and heat capacity, will be presented.

A new strategy of reusing abandoned carbon fiber reinforced plastic: Microstructures and properties of C/C composites based on recycled carbon fiber

A new strategy of recycling and reusing abandoned carbon fiber reinforced plastics (CFRP) is proposed: CFRPs are first fully carbonized to CF reinforced carbon (C/C) preforms, and then are manufactured into high value-added C/C composites. The results showed that the carbon residue rate of epoxy-resin (EP) matrix was fully recovered as the decomposition route of EP matrix was changed by charring agent. The recycled CF (rCF) was not markedly oxidized or thermally damaged, and possessed comparable properties with those of the virgin CF (vCF) after pyrolysis. The pyrolytic char had no obvious negative effect on the densification efficiency of the rCF reinforced carbon (rCF/C) composites. Both of the rCF/C and vCF reinforced carbon (vCF/C) composite bodies were quite dense, and exhibited almost no difference in their microstructures. The rCF/C and vCF/C composites therefore had quite close interface bonding strength (12.6 MPa and 13.0 MPa, respectively), and bending strength (106.4 MPa and 111.5 MPa, respectively). Furthermore, the rCF/C composites possessed comparable ablative rate with that of the vCF/C composites. The rCF/C composites derived from abandoned CF/EP composites present a great potential to be used as substitutes for vCF/C composites owing to their indistinguishable properties.

New method of obtaining heat-resistant biomorphic composites with carbide reinforcement

In this work, an original method of obtaining biomorphic composites of the Me – MeCx type, consisting of several stages, has been proposed. One of the steps is the Infiltration through Nonreactive Solvent (INS) method, which consists in converting biomorphic carbon preforms into porous microchannel MeCx ceramics by transferring a carbide-forming element to carbon through a solvent that does not react with carbon. The interaction of biomorphic alder-based carbon templates with melts of Cu – Ti and Sn – Ti systems has been investigated. It has been shown that as a result of the interaction of biocarbon templates with Cu-10Ti and Cu-20Ti alloys, carbon walls turn into TiC grains of micron and nanometer sizes, which partially repeat the template microstructure. The mechanism of formation of such a structure has been proposed. It has been demonstrated that, upon the interaction with Sn – Ti system, TiC and Ti2SnC layers are formed on the surface of the microchannels with good preservation of the initial template structure.