Thermal shock behavior of 3-dimensional C/SiC composite

The thermal shock behavior of a three-dimensional carbon fiber reinforced SiC matrix fabricated by chemical vapor infiltration (CVI) technique was studied using the air quenched method. Damage to composites was assessed by a destructive technique of measuring mechanical properties using three-point flexure and SEM characterization. C/SiC composites displayed good resistance to thermal shock, and retained 83% of the original strength after quenching from 1300 to 300°C 100 times. The critical ΔT of C/SiC in combustion environment was 700°C. The critical number of thermal shocks for the C/SiC composite was about 50 times. When the number of thermal shocks was less than 50 times, the residual flexural strength of C/SiC composites decreased with the increase of thermal shock times. When the number of thermal shocks of C/SiC was greater than 50, the strength of C/SiC did not further decrease because the crack density was saturated.     

Mechanical and ablation properties of 2D-carbon/carbon composites pre-infiltrated with a SiC filler

In order to improve the mechanical and ablation properties of 2D-carbon/carbon composites, a SiC filler was added to a 2D-preform before isothermal chemical vapor infiltration densification by using a powder infiltration technique. Backscattered electron images showed that the SiC filler was mainly concentrated between the fiber bundles and between the layers. The tensile and flexural strengths of the composites were improved by the addition of the SiC filler because of the increase of interfacial surface areas between the bundles and between the layers, the less residual open porosity, and also the strong bonding between the SiC particles and the pyrocarbon matrix. The composites with filler experienced a 15.2% lower thickness erosion rate and a 51.7% lower mass erosion rate, compared to those C/C without filler. This was attributed to the low oxygen permeability of the SiO₂ shielding the exterior inter-bundle pores as well as to a thermal barrier effect.     

Environment-friendly deposition of SiCN interlayer for reinforcing carbon fibers in C/SiC composites

Several C/SiC composites with no interlayer, single pyrocarbon (PyC) interlayer and PyC/SiCN interlayer were fabricated by polymer infiltration and pyrolysis process. The microstructure and mechanical properties were investigated. The results verified that SiCN interlayer was formed on carbon fibers. Both bulk density and flexural stress of C/SiC composite with PyC/SiCN interlayer were slightly higher than composite fabricated with single PyC interlayer. When the weight fraction of SiCN interlayer in the composite was about 18 wt%, the flexural stress of the composite was enhanced to 416 MPa from 352 MPa for composite with single PyC interlayer. The observations of pulled-out fibers on fracture surfaces revealed non-catastrophic fracture features for PyC/SiCN deposited C/SiC 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.     

Fabricating thick-section carbon fiber/silicon carbide composites by machining-aided chemical vapor infiltration

Chemical vapor infiltration (CVI) is a prominent process for fabricating carbon fiber/silicon carbide (C/SiC) composites. However, the preparation of enclosed-structure or thick-section C/SiC composites/components with CVI remains a challenge, since the difficulty of densification increases. Here, machining-aided CVI (MACVI) is designed, in which infiltration-assisting holes are utilized (machined) to increase matrix deposition. To validate the approach, thick-section (10 mm thick) C/SiC composites were fabricated by MACVI. Porosity analysis and microstructure characterization were performed on the fabricated MACVI C/SiC composites and their CVI counterparts, showing a density increase up to 12.7% and a porosity decrease up to 32.1%. The mechanical behavior of the fabricated MACVI C/SiC composites was characterized, showing an increase of flexural strength by a factor of 1.72 at most. Besides, the toughness also largely increases. Both the porosity decrease and the strength and toughness increase brought by MACVI demonstrate its effectiveness for fabricating stronger and tougher enclosed-structure or thick-section ceramic matrix composites/components.     

Design of a C/SiC functionally graded coating for the oxidation protection of C/C composites

To reduce the residual thermal stress between the carbon fiber-reinforced carbon (C/C) composites and the SiC coating layer, functionally graded materials (FGM) consisting of a C/SiC compositionally graded layer (C/SiC interlayer) were adopted. After designing the compositional distribution of the C/SiC interlayer which can relieve the thermal stress effectively, the deposition conditions of the entire compositional range of the C/SiC composites were determined using a thermodynamic calculation. According to the design and calculation the C/SiC interlayer and the SiC outer layer were deposited on the C/C composites by a low pressure chemical vapor deposition (LPCVD) method at deposition temperatures of 1100 and 1300 °C. The stress calculation and the experimental results suggested that the SiC-rich compositional profile in the FGM layer is the most effective for relieving the thermal stress and increasing the oxidation resistance.     

The influence of thermal gradient on pyrocarbon deposition in carbon/carbon composites during the CVI process

A thermal gradient CVI process was investigated. A graphite heater in the center of a carbon felt disk preform was heated by Joule heating to a temperature of 900 °C, the carbon felt had a low thermal conductivity, and the rapid natural gas flow cooled the exterior surface of the preform. The rate constant of the chemical vapor deposition reaction increased exponentially with increasing temperatures with pyrocarbon being formed only in the designated deposition zone. Plugging of surface pores in the preforms, which often occurs in the isothermal CVI technology was unusual in this thermal gradient CVI process. As the deposition process went on, the deposited zone moved progressively towards the outside surface of the preform. The electrical resistance between two electrodes decreased gradually while the power of the thermal gradient CVI furnace increased non-linearly with the progressive densification. The temperature distribution in the thermal gradient furnace changed non-linearly with time and position. The relationship between temperature and position in the deposition zone followed the classical Fourier law. The microstructure of pyrocarbon at different positions was discussed.     

Effect of thermal residual stress on the tensile properties and damage process of C/SiC composites at high temperatures

Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.     

Mechanical, thermal and ablative properties of interply continuous/spun hybrid carbon composites

The mechanical and thermal properties of interply hybrid carbon fiber (continuous and spun fabric)/phenolic composite materials have been studied. Hybrid carbon/phenolic composites (hybrid CP) with continuous carbon fabric of high tensile, flexural strength and spun carbon fabric of better interlaminar shear strength and lower thermal conductivity are investigated in terms of mechanical properties as well as thermal properties.Through hybridization, tensile strength and modulus of spun type carbon fabric reinforced phenolic composites (spun CP) increased by approximately 28% and 20%, respectively. Hybrid CP also exhibits better interlaminar shear strength than continuous carbon fabric/phenolic composites (continuous CP).The in-plane thermal conductivity of hybrid CP is 4-8% lower than that of continuous CP. As continuous filament type carbon fiber volume fraction increases, the transversal thermal conductivity of hybrid CP decreases.The erosion rate and insulation index were examined using torch test. Spun CP has a higher insulation index than continuous CP and hybrid CP over the entire temperature range. Hybrid CP with higher content of spun fabric exhibits higher insulation index as well as lower erosion rate.     

Microstructure-induced thermal stresses in pyrolytic carbon matrices at temperatures up to 2900 °C

Carbon/carbon composites produced by chemical vapor infiltration consist of carbon fibers embedded in a pyrolytic carbon matrix with a cylindrically layered structure at the microscale. Each coating layer has a different texture and different mechanical properties that depend on temperature. Stress distributions in such carbon matrices subjected to thermal loading and their possible failure scenarios are analyzed. A two-scale numerical model is developed. At the nanoscale, material properties of each layer are determined using a methodology based on the Eshelby's theory for continuously distributed inclusions. The resulting material parameters for each layer are then used in the finite element modeling at the microscale. Calculations are conducted for composites with different matrix structures for several cases of thermal loading. Calculated stress distributions show zones of maximal stress concentration and provide information on possible failure regions which correspond well with experimentally identified failure regions.     

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

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.     

高性能C/C复合材料制备工艺研究

对制备C/C复合材料的化学气相渗透工艺进行了系统的实验研究,着重分析了热解碳的沉积过程。研究表明,在化学气相渗透的初始阶段,热解碳主要在碳纤维表面沉积,并与碳纤维之间形成了界面结合;随后,热解碳的沉积继续填充碳纤维预制体内部的气孔。这一过程有助于缓解纤维与陶瓷基体之间的界面应力。研究表明,通过调节热解碳的沉积时间可以得到具有一定密度梯度的C/C复合材料。     

Effects of silanization of C/C composites and grafting of h-BN fillers on the microstructure and interfacial properties of CVI-based C/C-BN composites

A low-density carbon/carbon (C/C) composite/silane coupling agent/hexagonal boron nitride (h-BN) hybrid reinforcement was prepared by grafting polyethyleneimine (PEI)-encapsulated modified h-BN fillers onto a carbon fiber surface using 3-aminopropyltriethoxysilane (APS) as the connection to improve the distribution uniformity of h-BN fillers in quasi-three-dimensional reinforcements and the interfacial properties between the fibers/pyrocarbon (PyC) in the C/C-BN composites obtained after densification by chemical vapor infiltration (CVI). The microstructure and chemical components of the hybrid reinforcement were investigated. The transmission electron microscopy (TEM) sample was prepared using a focused-ion beam (FIB) for the h-BN/PyC interfacial zone. The interlaminar shear strength (ILSS) and impact toughness were analyzed to inspect the composites’ interfacial properties. The results show that APS and h-BN are uniformly grafted on the fiber surface in the chopped fiber web inside the C/C composite without a density gradient, and agglomeration occurred and significantly increasing the fiber surface roughness. The highly ordered h-BN basal plane may affect the order degree of PyC near the h-BN/PyC interface. The addition of h-BN reduces the PyC texture near it, causing the annular cracks to disappear gradually. The lower PyC texture and the rougher fiber surface strengthen the interfacial bond of the fiber/matrix. Consequently, the ILSS strength of the C/C-BN composites first increases and then decreases as the h-BN filler content increases and is always higher than that of the C/C composite, while the addition of h-BN fillers weakens its impact toughness. When the h-BN content in the C/C-BN composite is 10 vol%, the ILSS of the C/C-BN composites was 15.6% higher than that of the C/C composites. However, when the h-BN content is excessive (15 vol%), the densely grafted h-BN will bridge each other, reducing the subsequent CVI densification efficiency to form a loose interface, causing a decrease in the shear strength.     

Effects of needle-punched felt structure on the mechanical properties of carbon/carbon composites

The effects of needle-punched felt structure, including mass ratio of non-woven cloth to short-cut fiber web, PAN-based carbon fiber types of non-woven cloth and thickness of unit (one layer of non-woven cloth and short-cut web was named as a unit), on the flexural properties of C/C composites from pressure gradient CVI are discussed. Results show that flexural strength and modulus increase when mass ratio of non-woven cloth to short-cut fiber web changes from 7:3 to 6:4 and that PAN-based carbon fiber types of non-woven cloth strongly influence the flexural properties. The strength of C/C composites is not linear with the strength of non-woven cloth carbon fiber because of the important interface between carbon fiber and matrix carbon. It is suitable to choose T300 or T700 as reinforcing carbon fiber for C/C composites in the present study. An optimum unit number per cm of the needle-punched felts for higher flexural properties exists.     

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.     

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.     

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.