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.     

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.     

Microstructure and tribological behaviors of C/C–BN composites fabricated by chemical vapor infiltration

In this paper carbon fiber reinforced carbon–boron nitride binary matrix composites (C/C–BN) were prepared by chemical vapor infiltration (CVI). The infiltration of BN in the CVI process was controlled by the diffusion of BCl₃, and BN matrix was distributed homogeneously in the porous carbon fiber reinforced carbon matrix composites (C/C) due to the good infiltration ability of BN. The as-received C/C–BN composites were composed of 92 vol% C and 8 vol% BN. Both the friction coefficient and wear rate of C/C composites decreased significantly by the incorporation of BN. After heat-treated at 1600 °C, the interlayer spacing of CVI BN decreased to 3.36 Å, and CVI BN with high crystalline degree displayed the excellent lubricating effect, leading to the decrease of friction coefficient and wear rate. The improvement of the tribological properties also was partially attributed to the improved oxidation resistance and the formation of friction film by the incorporation of BN matrix.     

Effects of infiltration conditions on the densification behavior of carbon/carbon composites prepared by a directional-flow thermal gradient CVI process

Multiple quasi three-dimensioned carbon fibre preforms with disk-like shape were simultaneously densified by a directional flow thermal gradient CVI process. The effects of infiltration conditions, including temperature (ranging from 850 to 1050 °C), temperature gradient (5 and 10 °C/mm), pressure (2.5, 5.0, 7.5 and 9.5 kPa) and the type of carrier gas (N₂ or H₂), on the densification behavior of the resultant carbon/carbon composites were investigated. The results showed that lower temperatures (below 900 °C), a larger temperature gradient and higher pressure are favorable for higher average bulk density and homogeneous infiltration. Carbon/carbon composites disks with an average bulk density of 1.78 g/cm³ were achieved in one CVI cycle at a total pressure of 9.5 kPa. It was also found that adding N₂ carrier gas has no pronounced influence on the densification of the preforms. As compared to N₂, H₂ had positive effects on the densification of the preforms for temperatures above 900 °C, but it had negative effects on the densification when the control temperature was as low as 850 °C.     

Oxidation behavior and protection of carbon/carbon composites prepared using rapid directional diffused CVI techniques

The oxidation kinetics of carbon/carbon (C/C) composites prepared using a rapid directional diffused (RDD) CVI process were studied. The results showed that the Arrhenius curve for the RDD CVI C/C composites consists of two straight lines, the intercept of which is at about 700 °C at the linear oxidation stage. The oxidation rates are controlled by the surface reaction at 600-700 °C, and the corresponding activation energy is 121 kJ/mol. Between 700 and 800 °C, the oxidation rates are dominated by chemical reaction and diffusion, and the relevant activation energy is 80 kJ/mol. SEM investigation showed that the oxidation starts with original pores on the C/C composite surface with the carbon fiber and matrix oxidized simultaneously. An inexpensive and easily pasted coating containing epoxy organic silicon resin, borates, refractory particulates, etc. was developed. After isothermal temperature, thermal cycle and immersion water oxidation tests, the coating was demonstrated to exhibit good oxidation-resistance properties. The oxidation-resistant mechanism of the coating is discussed.     

Microstructure and elastic properties of individual components of C/C composites

Carbon fiber reinforced carbon matrix (C/C) composites are often used for structural and frictional applications at a wide range of temperatures due to their excellent mechanical and thermal properties. Tailoring of mechanical properties through optimization of microstructure is critical for achieving maximum composite performance. This article addresses the evolution of the fiber and matrix microstructure and related nano-mechanical properties in two different C/C composites after being subjected to heat treatment at temperatures between 1800 and 2400 °C. Microstructure and corresponding nano-mechanical properties of C/C composites were studied using Polarized Light Microscopy (PLM), High-Resolution Transmission Electron Microscopy (HRTEM) and nanoindentation techniques. Increased heat treatment temperature (HTT) led to formation of a better-organized microstructure of fiber and matrix and also to formation of thermal cracks. The elastic modulus of rough laminar CVI pyrocarbon decreased from 18 to 12 GPa with increased HTT. In contrast, the isotropic CVI pyrocarbon and charred resin matrix displayed only a slight change of elastic modulus. The elastic modulus of PAN fiber increased from 18 to 34 GPa, indicating the development of a better-organized microstructure in the fiber-axial direction.     

Microstructure and properties of needle punching chopped carbon fiber reinforced carbon and silicon carbide dual matrix composite

Chopped carbon fiber preform reinforced carbon and SiC dual matrix composites (C/C–SiC) were fabricated by chemical vapor infiltration (CVI) combined with liquid silicon infiltration. The preform was fabricated by repeatedly overlapping chopped carbon fiber web and needle punching technique. A geometry model of the pore structure of the preform was built and reactant gas transportation during the CVI was calculated. The microstructure and properties of the C/C–SiC composites were investigated. The results indicated that the CVI time for densification of the preform decrease sharply, and the model showed the permeability of the preform decreased with the increase of its density. The C/C–SiC exhibited good mechanical characteristics, especially excellent compressive behavior, with the vertical and parallel compressive strength reached to 359(±40) MPa and 257(±35) MPa, respectively. The coefficient of friction (COF) decreased from 0.60 (at 8 m/s) with the increase of sliding velocity, and finally stabilized at ~0.35 under the velocity of 20 m/s and 24 m/s, and the variations of COF were not sensitive to the sliding distance. The wear rates were between 0.012 cm³/MJ and 0.024 cm³/MJ under different velocities. These results showed that the chopped carbon fiber preform reinforced C/C–SiC are promising candidates for high-performance and low-cost friction composites.     

Microstructure difference between core and skin of T700 carbon fibers in heat-treated carbon/carbon composites

The core and skin microstructure of T700 carbon fibers in carbon/carbon composites, prepared with chemical vapor infiltration (CVI) at 1000 °C and heat treated at 2300 and 2800 °C, have been studied by means of high-resolution transmission electron microscopy (HRTEM). The orientation angles of the graphitic basal planes obtained from selected-area electron-diffraction patterns show that the structure difference between the core and skin is a change of the degree of preferred orientation of the graphitic basal planes which decreases gradually from the skin region to the core region after CVI at 1000 °C. With increasing heat-treatment temperature, the basal planes orient parallel to the fiber axis, first in the skin region and then in the core region. In addition, the diameter of the core region decreases considerably from about 3.3 to 2.2 μm after heat treatment at 2800 °C. HRTEM lattice-fringe images show that the graphitic crystallite size increases significantly both in the core and skin, but more in the skin. Moreover, with increasing crystallite size, pores of nanometer scale start to form in the fiber.     

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.     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

High density carbon fiber/boron nitride matrix composites: Fabrication of composites with exceptional wear resistance

This paper describes the fabrication of high density (ρ ∼ 1.75 g/cc) composites containing a hybrid (carbon and boron nitride), or complete boron nitride matrix. The composites were reinforced with either chopped or 3D needled carbon fibers. The boron nitride was introduced via liquid infiltration of a borazine oligomer that can exhibit liquid crystallinity. The processing scheme was developed for the chopped carbon fiber/boron nitride matrix composites (C/BN) and later applied to the 3D carbon fiber reinforced/boron nitride matrix composites (3D C/BN). The hybrid matrix composites were produced by infiltrating the borazine oligomer into a low density 3D needled C/C composite to yield 3D C/C-BN. In addition to achieving high densities, the processing scheme yielded d₀₀₂ spacings of 3.35 Å, which afforded boron nitride with excellent hydrolytic stability. The friction and wear properties of the composites were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/BN composites outperformed both the previously reported C/C-BN and chopped fiber reinforced C/C. The high density 3D C/BN performed as well as both the 3D C/C and the C/BN. The 3D C/C-BN provided outstanding wear resistance, incurring nearly zero wear across the entire testing spectrum. The coefficient of friction was relatively stable with respect to energy level, varying from 0.2 to 0.3.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.     

高温热处理对C/C复合材料刹车盘尺寸的影响

研究了用CVI工艺制备的二维C／C复合材料刹车盘在进行高温热处理时的尺寸变化。第一次热处理时刹车盘直径增大，厚度减小；第二次、第三次热处理时刹车盘直径和厚度均增加。随着热处理次数的增加，刹车盘尺寸变化越来越小，趋于稳定。刹车盘尺寸的变化与C／C复合材料中碳原子的有序化重新分布和不同炭相产生应力裂纹等因素有关。     

Thermophysical properties of densified pitch based carbon/carbon materials—II. Bidirectional composites

Bidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN based carbon fabric as reinforcement and coal tar and synthetic pitches as matrix precursors. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties i.e. thermal conductivity and specific heat have been evaluated both at room temperature and between 40 and 300 °C. The temperature dependence of thermal diffusion, specific heat and thermal conductivity has been studied and correlated with microstructure of carbon/carbon composites. It is found that the specific heat of carbon/carbon composites shows increase with temperature with an inverse slope in the temperature range of 150-200 °C. Accordingly, though the thermal conductivity decrease with temperature due to increased phonon interactions, it shows reversible action between 150 and 200 °C.     

Microstructures and tribological properties of carbon/carbon-boron nitride composites fabricated by powdered additives and chemical vapor infiltration

The carbon fiber reinforced/carbon-boron nitride (C/C-BN) dual matrix composites were fabricated via adding hexagonal boron nitride (h-BN) powders into the needled carbon felt and subsequent chemical vapor infiltration (CVI) process. An experimental investigation was performed to study the influences of BN volume content on the microstructures and tribological properties of C/C-BN composites. The results indicate that the pyrolytic carbon (PyC) in the C/C-BN composites is regenerative laminar (ReL) due to the inducement of BN powders during CVI process, whereas the PyC in the C/C composite is classic smooth laminar. Additionally, the friction coefficients of C/C-BN composites with three different BN contents in volume fractions (4.5, 9 and 13.5 vol%) are all higher compared to the reference C/C composite (0.22). Note that the highest coefficient of friction (0.29) is obtained when the BN volume content in the C/C-BN composite is 9 vol%. Moreover, the linear and mass wear rates of C/C-BN composites as well as the 30CrSiMoVA counterparts are significantly decreased with the increase of BN volume content. The favorable friction and wear properties of C/C-BN composites are attributed to the synergistic effect induced by the ReL PyC and BN. The microstructural variation of C/C composites modified by h-BN could improve the compatibility between the C/C-BN composites and 30CrSiMoVA counterpart, resulting in an enhanced adhesive attraction between the wear debris and the surface of 30CrSiMoVA counterpart. Furthermore, the investigations concerning the friction surfaces indicate that the formation of sheet-like friction films with large areas are more easily to occur on the surfaces of 30CrSiMoVA counterparts mating with the C/C-BN composites rather than mating with the C/C composite.     

Carbon/carbon-boron nitride composites with improved wear resistance compared to carbon/carbon

This paper describes the fabrication of a carbon fiber reinforced/carbon-boron nitride (C/C-BN) hybrid matrix composite for possible use in aircraft brakes. These composites were fabricated via liquid infiltration of a liquid crystalline borazine oligomer into a low-density carbon fiber/carbon matrix (C/C) composite. The friction and wear properties of the C/C-BN were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/C-BN composites with densities of 1.55 g/cc displayed wear rates 50% lower than values observed with C/C samples with densities of approximately 1.75-1.8 g/cc. This includes the near elimination of wear from 300 to 600 kJ/kg, which represents the normal landing regime for aircraft brakes. This encouraging behavior is attributed in part to the improved oxidation resistance of the BN at high energy levels and the ability of the BN to facilitate formation of a stable wear film at lower energy levels. The coefficient of friction, while being slightly lower than the values for C/C, appeared much less sensitive to changes in energy level.     

炭布高温预处理对C/C复合材料摩擦磨损性能的影响

采用炭布叠层为坯体，经等温ＣＶＤ工艺制备Ｃ／Ｃ复合材料，并对其摩擦磨损性能进行初步探讨，结果表明，炭布高温预处理能改善Ｃ／Ｃ复合材料的摩擦磨损性能，随着刹车压力的增大，试样的磨损率增大而摩擦系数则减小。     

Preparation and characterization of C/SiC–ZrB2 composites by precursor infiltration and pyrolysis process

Ultra-high temperature ceramic matrix composites (C/SiC–ZrB₂) are prepared by slurry and precursor infiltrations and pyrolysis method. C/SiC–ZrB₂ composites with ZrB₂ volume content from 10% to 24.6%, have balanced performance of fracture toughness (17.7–8.1 MPa m^1/2), flexural strength at room temperature (367–163 MPa) and at high temperature (strength retention 74% at 1800 °C and over 32% at 2000 °C), better oxidation and ablation resistance under oxyacetylene torch environment (recession rate 0.01 mm/s).     

Erosion behavior of SiC–WC composites

Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 °C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m^1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 30° to 90°, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm³/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated.