Effect of diffusion-assisting holes on the tensile properties of a thick-section two dimensional C/SiC composite fabricated by chemical vapor infiltration

The concept of diffusion-assisting holes (DAHs) has been developed to increase matrix deposition in the middle layers of the thick-section ceramic matrix composites (CMCs) that are fabricated by chemical vapor infiltration (CVI). However, the effect of DAHs on the tensile properties of CMCs has not been studied. Here, the tensile properties and the state of matrix deposition of a 10-mm-thick two dimensional (2D) C/SiC with DAHs are investigated. Results showed, with DAHs, a zone of increased deposition with a radius of ca. 1.4 mm around a hole was introduced and the net-section strength of the 10-mm-thick 2D C/SiC was increased by 48.1%. In addition, the tensile load bearing capacity was also increased by 34.1%, although the load bearing section decreased with DAHs. The increased net-section strength and tensile load bearing capacity of the C/SiC are attributed to the increased matrix deposition in the middle layers of the thick-section composite.     

Strengthening the thermal Negative Poisson's ratio structures by SiC chemical vapor infiltration

Negative Poisson's ratio structures exhibit adjustable thermal expansion behavior as the thermal stress can be dispersed or offset by torsion, bending, and tension of the struts. However, the structural stability under cyclic thermal stress significantly determines the long-term durability. Strengthening the Negative Poisson's ratio structure can ensure high thermal and mechanical reliability. The work designed a heat-induced torsional Negative Poisson's ratio structures and fabricated it by 3D printing. For efficient strengthening, the preforms were further densified by chemical vapor infiltration (CVI) of SiC to enhance the reliability. Pores and gaps in the preforms were homogeneously covered and filled by the SiC, enhancing the surface finish and mechanical performance. The heat induced torsion of the structures dispersed the heat flow in one single direction, reducing the thermal stress concentration. The independent thermal expansion change of the structural unit can offset or consume the heat dissipation stress, and further improve the reliability and thermal stability through the densification process. As a result, the 120° twisted structure exhibited an average coefficient of thermal expansion (CTE) of 6. 12 × 10^?6/K from room temperature (RT) to 500 °C, and the instantaneous CTE reached the minimum value of 4.01 × 10^?6/K at 125 °C. Meanwhile, the load-bearing capacity strengthened significantly, exhibiting the optimized strength of 11.31 MPa and Young's Modulus of 36.44 GPa, revealing a significant improvement than those of preforms, promising for high load-bearing and low expansion application of structure-function integrated low expansion material.     

Effect of a diffusion-enhancing hole on the densification of a thick-section 2D C/SiC composite

Diffusion-enhancing holes (DEHs) have been used to mitigate the large density gradients that are formed in the thick-section ceramic matrix composites (CMCs) fabricated by chemical vapor infiltration (CVI). However, the densification characters of the thick-section CMCs with DEHs through vapor infiltration remain a concern. Here, the densifications of a10-mm-thick two dimensional (2D) C/SiC composite with or without DEHs are investigated by experiments and calculations. Results showed both the measured densities, the predicted final densities, and the density growth rates (DGRs) for the composite with DEHs (diameters of 2 or 4 mm) are higher than those of the counterpart without DEHs, due to the forming of dense rings (DRs) around DEHs and the increased infiltration in the large pores (diameter > 52 μm). In addition, the diffusion increase in infiltration with DEHs is attributed to the increase of Knudsen diffusion resulted from the reopening of the blocked/sealed pores by DEH-machining.     

Analytical and numerical study of the densification of carbon/carbon composites by a film-boiling chemical vapor infiltration process

The film-boiling chemical vapor infiltration (CVI) process is a fast process developed for composite material fabrication, and especially carbon/carbon composites. In order to help define optimal conditions, a local 1D model has been developed to study the densification front which establishes itself during the processing of a carbon/carbon fibrous preform. The model features heat conduction, precursor gas diffusion, densification reactions and structural evolution of the porous medium. The effects of total mass flux, Thiele modulus, porous medium geometry on front behavior parameters such as width, velocity and residual porosity are presented as semi-analytical correlations. An existence criterion is produced, which involves a minimal heat flux. Comparison between process-scale experiments and simulation is then possible by inserting the semi-analytical results achieved in the local study of the front into a light numerical model involving the entire preform. The model has been validated with respect to previous experimental and numerical data.     

Friction performance of C/C composites prepared using rapid directional diffused chemical vapor infiltration processes

A technology used to prepare C/C composites using a rapid directional diffused (RDD) chemical vapor infiltration process has been investigated. General RDD technologies were explored, and optimal parameters were determined. The friction and wear properties of this material were researched. The results showed that in the RDD process, propylene and nitrogen were rapidly and directionally diffused into the carbon preforms enabling carbon deposition to occur from the inside of the preform to the outside. This method prevents the formation of an outer crust on the surface of preforms and facilitates uniformity of densification. With the RDD process no surface machining was required between chemical vapor infiltration (CVI) cycles thereby enabling continuous densification and reducing the CVI cycle times. The optimum processing conditions for RDD CVI were as follows; furnace temperature 950 °C; and furnace pressure 6.7 kPa. The C/C composites produced using RDD CVI processing exhibited good friction performance. Their curves of the brake moment with the velocity are stable under dry conditions, and their wet brake moment is greatly reduced. The average thickness wear is decreased to 9.5×10⁻⁴ mm/surface/stop.     

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·m^1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si₃N₄ matrix composites (SiC_f/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 SiC_f/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?m^1/2).     

Preparation and properties of 2D C/SiC-ZrB2-TaC composites

Two-dimensional (2D) C/SiC-ZrB₂-TaC composites were fabricated by chemical vapor infiltration (CVI) combined with slurry paste (SP) method. 2D laminate was prepared by stacking carbon cloth that was pasted with a mixture of polycarbosilane-ZrB₂-TaC slurry. A small amount of carbon fiber tows were introduced into the preform in the vertical direction. After heat-treated at 1800 °C, the 2D laminate was densified with SiC by CVI to obtain 2D C/SiC-ZrB₂-TaC composites. Properties including flexural strength, interlaminar shear strength, and thermal expansion of the composites were investigated. The ablation test was carried out under an oxyacetylene torch flame. The morphologies of the ablated specimens were analyzed. The results indicate that the adding vertical fiber tows and heat-treatment at 1800 °C can greatly improve the mechanical properties of the composites. The co-addition of TaC and ZrB₂ powders into C/SiC composite effectively enhance its ablation resistance.     

Highly conductive and high-strength carbon nanotube film reinforced silicon carbide composites

Film formed by carbon nanotubes is usually called carbon nanotube film (CNT_f). In the present study, CNT_f fabricated by floating catalyst method was used to prepare CNT_f/SiC ceramic matrix composites by chemical vapor infiltration (CVI). Mechanical and electrical properties of the resulting CNT_f/SiC composites with different CVI cycles were investigated and discussed, and the results revealed that the CNT_f has a good adaptability to CVI method. Tensile test demonstrated an excellent mechanical performance of the composites with highest tensile strength of 646 MPa after 2 CVI cycles, and the strength has a decline after 3 CVI cycles for an excessively dense matrix. While, the elastic modulus of the composite increased with the CVI cycles and reached 301 GPa after 3 CVI cycles. Tensile fracture morphologies of the composites were analyzed by scanning electron microscope to study the performance change laws with the CVI cycles. With SiC ceramic matrix infiltrated into the CNT_f, enhanced electrical conductivity of the CNT_f/SiC composite compared to pure CNT_f was also obtained, from 368 S/cm to 588 S/cm. Conductivity of the SiC matrix with free carbon forming in the CVI process was considered as the reason.     

Ring compression properties of SiCf/SiC composites prepared by chemical vapor infiltration

SiC fiber reinforced SiC matrix (SiC_f/SiC) composites prepared by chemical vapor infiltration are one of promising materials for nuclear fuel cladding tube due to pronounced low radioactivity and excellent corrosion resistance. As a structure component, mechanical properties of the composites tubes are extremely important. In this study, three kinds of SiC_f preform with 2D fiber wound structure, 2D plain weave structure and 2.5D shallow bend-joint structure were deposited with PyC interlayer of about 150–200?nm, and then densified with SiC matrix by chemical vapor infiltration at 1050?°C or 1100?°C. The influence of preform structure and deposition temperature of SiC matrix on microstructure and ring compression properties of SiC_f/SiC composites tubes were evaluated, and the results showed that these factors have a significant influence on ring compression strength. The compressive strength of SiC_f/SiC composites with 2D plain weave structure and 2.5D shallow bend-joint structure are 377.75?MPa and 482.96?MPa respectively, which are significantly higher than that of the composites with 2D fiber wound structure (92.84?MPa). SiC_f/SiC composites deposited at 1100?°C looks like a more porous structure with SiC whiskers appeared when compared with the composites deposited at 1050?°C. Correspondingly, the ring compression strength of the composites deposited at 1100?°C (566.44?MPa) is higher than that of the composites deposited at 1050?°C (482.96?MPa), with a better fracture behavior. Finally, the fracture mechanism of SiC_f/SiC composites with O-ring shape was discussed in detail.     

Aligned carbon nanotube-reinforced silicon carbide composites produced by chemical vapor infiltration

A chemical vapor infiltration (CVI) technique was used to overcome most of the challenges involved in fabricating exceptionally-tough CNT/SiC composites. Nanotube pullout and sequential breaking and slippage of the walls of the CNTs during failure were consistently observed for all fractured CNT/SiC samples. These energy absorbing mechanisms result in the fracture strength of the CNT/SiC composites about an order of magnitude higher than the bulk SiC. The CVI-fabricated CNT/SiC composites have an strongly-bonded tube/matrix interface and an amorphous, crack-free SiC matrix, enabling the composites to withstand oxidization at 700–1600 °C in air.     

Tough and wear-resistant carbon fiber reinforced TiC-based composite prepared by alloyed melt infiltration at low temperatures

To improve the toughness and friction properties of carbon fiber reinforced ceramic matrix composite, a Cu alloy modified carbon fiber reinforced TiC based ceramic matrix composite was designed and prepared by TiCu alloy melt infiltration at low temperatures up to 1100 °C. The as-produced composite was mainly composed of carbon, TiC, Ti₃Cu₄, TiCu₄ and Cu phases. Due to the ductile Cu alloy introduced into the matrix, the composite showed good mechanical performance especially the fracture toughness. The flexural strength reached about 248.36 MPa while the fracture toughness was up to 15.78 MPa·m^1/2. The high toughness of the composite was mainly attributed to the fiber bridging, fiber pull-out, interface debonding, crack propagation and deflection. The tribological performance of the as-produced composite was measured using SiC and 440C stainless steel balls as counterparts, respectively. The as-prepared composite exhibited good wear resistance and the wear mechanism was discussed based on the microstructural observations.     

Fabrication of carbon/carbon composites by an electrified preform heating CVI method

Carbon/carbon composites are manufactured using the electrified preform producing directly heat CVI process. The preforms are prepared by laminating the carbon fiber felts with crossply reinforcement, and infiltrated with carbon using natural gas or propylene as a reactant, with nitrogen as diluent at atmospheric pressure. The relations between the resistivity of samples and infiltration time are determined under the operating conditions. The results indicate that the preforms have gained a high infiltration rate by this technology, and the samples have higher densities using natural gas rather than propylene. Their highest average bulk densities are up to 1.71 g/cm³ after the preforms of 1100×500×35 mm size have been densified for 80 h using natural gas. The carbon fibres in the preforms have not been damaged by this technology as yet, and the composites prepared have sufficiently high flexural properties. As the brake angular velocity is increased with the constant brake moment inertia and specific pressure, the average coefficient of friction for the composites prepared using natural gas is linearly and greatly decreased, but the variations of the brake moment inertia have a slight influence on the average coefficient of the friction when the brake angular velocity and specific pressure are kept constant. Their average thickness wear is 13×10⁻⁴ mm/surface per stop.     

Chemical vapor deposition and infiltration processes of carbon materials

The chemical vapor deposition (CVD) and the chemical vapor infiltration (CVI) processes of carbon materials are reviewed starting from the historical aspects and including the latest developments in the preparation of C/C composites. Our presentation is based on an analysis of the different types of reactors, of the composite materials with different types of pyrocarbon as matrices and a comparison between the different processes. In particular, the classical isothermal-isobaric technique and temperature or pressure gradient reactors, which lead to a higher deposition efficiency, are compared. A complementary aspect is the structural and physical analysis of the deposited pyrocarbons: they are considered as reproducible metastable phases which are obtained under non-equilibrium thermodynamic conditions. The final relevant point concerns the relationship between the process parameters and the type of pyrocarbon. In particular, the so-called rough laminar microstructure, useful for most composite applications, is described.     

Tensile behavior of ceramic matrix minicomposites with engineered interphases fabricated by chemical vapor infiltration

Ceramic matrix composites (CMCs) exhibit quasi-ductile behavior beyond the initial elastic region driven by a weak fiber-matrix interface that can be further engineered by introducing a finite thickness interphaseleading to enhanced strength and toughness. The current work explores the engineering of interphases in CMCs by a controlled variation of fabrication process parameters. C/BN/SiC minicomposite configurations have been fabricated by chemical vapor infiltration (CVI) with the intent of varying interphase thickness and constituent volume fractions by varying the interphase and matrix infiltration durations. The effect of processing durations on the resulting microstructure, tensile response, and damage mechanisms up to and during ultimate failure of CMC minicomposites have been investigated. The presented results highlight the significant influence of processing duration on the tensile and failure behavior of CMC minicomposites thereby providing an insight into the processing-microstructure-tensile response relationship in CMCs.     

Numerical simulation of chemical vapor infiltration of propylene into C/C composites with reduced multi-step kinetic models

A parallel-consecutive reaction model of chemistry and kinetics is proposed to simulate homogeneous gas-phase reactions of propylene pyrolysis in CVI processes. An improved bipore model is also suggested to describe the changes of the pore topology with densification. The competition between the heterogeneous reactions of pyrolytic carbon deposition and the homogeneous reactions is analyzed by a numerical simulation method. Numerical simulation shows that continuous higher density region occurs early in a certain depth of the substrate, which blocks precursor transport into the deeper region. Changing processing parameters can alter when and where the continuous higher density region takes place. Inside-out densification is an inherent characteristic for CVI processes, while premature surface crusting is an apparent phenomenon. According to the concentration ration between C₂H_x and C₆H_y, the textures of pyrolytic carbon are successfully predicted. The present model is validated by comparing predicted with observed densities.     

Ablation behavior of C/C-ZrC and C/SiC-ZrC composites fabricated by a joint process of slurry impregnation and chemical vapor infiltration

C/C-ZrC and C/SiC-ZrC composites were fabricated by a joint process of slurry impregnation and chemical vapor infiltration, in which ZrC matrix was obtained by slurry impregnation process, while C or SiC matrix was introduced by chemical vapor infiltration process. The as fabricated C/C-ZrC and C/SiC-ZrC composites have densities of 1.67 g cm^?3 and 1.91 g cm^?3 respectively. Tensile strength is 89.4±8.4 MPa and 182.2±14.0 MPa respectively for the as prepared C/C-ZrC and C/SiC-ZrC. Ablation behavior of the C/C-ZrC and C/SiC-ZrC composites under air plasma was studied and compared in detail. Due to different oxidation resistance and heat transfer capacity of the matrix, these two ZrC based composites showed various ablation behavior. The linear erosion rate is 48 µm s^?1 and 39 µm s^?1 respectively for C/C-ZrC and C/SiC-ZrC composites.     

Textures of pyrolytic carbon formed in the chemical vapor infiltration of capillaries

Capillaries, 1.1 mm in diameter and 17.0 or 32.5 mm in length, were infiltrated at a temperature of 1100 °C and methane pressures from 5 to 30 kPa. Layer thickness and carbon texture were determined at cross-sections of 2, 16 and 32 mm from the open end of the capillaries using polarized light microscopy. Average deposition rates, determined from layer thickness and infiltration time, as a function of methane pressure indicate a rate increase up to a saturation adsorption at pressures between 10 and 15 kPa (range 1) and a strong rate increase above these pressures (range 2). This result implies carbon formations based on the growth mechanism in range 1 and the nucleation mechanism in range 2. The carbon texture shows a maximum in range 1 and a minimum in the transition from range 1 to range 2 followed by a clear increase in range 2. The maximum in range 1 corresponds to the particle-filler model describing formation of various textures of carbon by the ratio of aromatic species to C₂ species. Increasing texture degrees in range 2 suggest that the nucleation mechanism may lead to high textured carbon provided that the residence time for intramolecular rearrangments of polycyclic aromatic hydrocarbons is sufficient.     

Design of a pilot-scale microwave heated chemical vapor infiltration plant: An innovative approach

A hybrid Microwave-assisted Chemical Vapor Infiltration (MW-CVI) pilot plant to produce silicon carbide-based Ceramic Matrix Composites was designed, built and setup, as a part of the European project HELM. Being different from the existing lab-scale MW-CVI equipment, this pilot plant was designed with the idea of a further industrial scale-up.In order to enable the infiltration of the large samples of interest in industrial applications, the reactor was designed with the internal microwave cavity acting as an overmoded resonator at the frequencies of interest. The designed pilot plant allowed proper microwave heating of cylindrical samples of diameter doubled with respect to typical lab-scale preforms, with reproducible operating conditions in terms of transmitted/reflected power. First infiltration trials resulted in an average reaction efficiency of 25 % with the desired inside-out silicon carbide infiltration. The main steps of the design and the results of the first infiltration tests are discussed in this paper.     

以环己烷为前驱体制备炭/炭复合材料

以环己烷为前驱体利用化学液气相沉积工艺,采用针刺炭纤维毡为预制体,制备了具有光滑层和粗糙层结构的炭/炭复合材料。利用金相显微镜、高分辨扫描电子显微镜进行了材料的微观组织结构的分析,分析了在不同位置不同织构热解炭的形成机理。同时阐述了化学液气相沉积工艺原理。实验结果表明,通过调整工艺参数,利用化学液气相沉积工艺可以制备具有不同微观组织结构的炭/炭复合材料。     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.