Thermal Gradient Chemical Vapor Infiltration Research Articles

Preparation and long-term ablation behavior of Cf-reinforced ZrC-SiC coated C/C-ZrC-SiC composite

To improve the long-term ablation resistance of the C/C composite, the Cf-reinforced ZrC-SiC coated C/C-ZrC-SiC composite is fabricated by thermal gradient chemical vapor infiltration (TGCVI) technology and the subsequent polymer infiltration and pyrolysis (PIP) process. The coating shares the same carbon fiber felt with the matrix, which can strengthen the bond between the coating and the matrix. Results show that the composite has excellent long-term anti-ablation performance (the linear ablation rate of the composite is −0.64 µm/s after ablated for 120 s (3 × 40 s), and the coating can withstand ablation for 160 s (4 × 40 s) without destruction). With the reinforcement of carbon fibers, the coating is peeled layer by layer rather than destructed in the whole coating, and the coating still works in the subsequent ablation cycles (the linear ablation rate of the composite is 2.40 µm/s after ablated for 280 s (7 × 40 s)).

Ablation behavior of C/C-ZrC-SiC composite with gradient distribution of ZrC-SiC ceramics along the thickness

A novel C/C-ZrC-SiC gradient composite (GGC/C-ZS) is prepared via thermal gradient chemical vapor infiltration (TGCVI) technology combined with polymer infiltration and pyrolysis (PIP). And ablation tests are carried out to evaluate the thermal shock and ablation resistance. Results show that the GGC/C-ZS exhibits excellent anti-ablation performance (linear ablation rate is −2.12 μm/s) after ablation for 2 × 40 s (under 2500 °C). The high ZrC-SiC content in the surface region of GGC/C-ZS ensures a dense ZrO2 scale of the ablation center and helps withstand the high thermal stress induced by the ablation. During the ablation, the compact ZrO2 scale can block heat transfer from the surface to the matrix of GGC/C-ZS because of its low thermal conductivity, which can relieve the thermal etching and the massive loss of SiO2 beneath the ZrO2 scale. Besides, the gradient structure can also reduce heat mismatch between the ZrC-SiC ceramics and the C/C matrix, which can avoid the peeling off of the ZrO2 scale after a second ablation process.

A High-Efficiency Technology for Manufacturing Aircraft Carbon Brake Discs with Stable Friction Performance

A binary C/C brake disc (i.e., the test brake disc) was prepared with a C/C (pyrolytic carbon/resin carbon) matrix using modified natural gas as the carbon source through the isothermal chemical vapor infiltration (ICVI) process with a directed flow and the pressure impregnation carbonization (PIC) process with liquid-phase furfural acetone resin. The microstructural, mechanical, thermal, friction and wear properties of the test brake disc were comprehensively analyzed and compared with commercial ones. The results showed that the production efficiency of the test brake disc was 36% higher than that of the commercial ones, which were manufactured through a thermal-gradient chemical vapor infiltration (TCVI) process. The favorable mechanical and thermal properties of the test brake disc were comparable to the commercial ones. While the test brake disc had a more consistently rough laminar microstructure on the worn surface of the brake disc than the commercial ones, this avoided the annular grinding grooves on the worn surface after the braking tests. In addition, the test brake disc had a stable friction coefficient with a low dispersion coefficient of 3.90%, which would improve the friction stability of C/C brake discs used in aircrafts.

Dependence of mechanical properties on microstructure of high-textured pyrocarbon prepared via isothermal and thermal gradient chemical vapor infiltration

In order to explore the dependence of mechanical properties on the microstructure of high-textured (HT) pyrolytic carbon (PyC), the microstructure and properties of HT PyC specimens prepared by isothermal isobaric chemical vapor infiltration (ICVI) and thermal gradient chemical vapor infiltration (TCVI) processes were investigated. The HT PyC of ICVI was uniform and exhibited a more distinct long-range order compared to that of TCVI. Besides, a spatial gradient in texture existed within the HT PyC of TCVI, demonstrating the micro-nonuniformity of the whole matrix. The orientation angle (OA) of HT layers in TCVI increased gradually from the fiber surface to outside, whereas the OA of HT layers in ICVI with better microcrystalline was nearly similar. According to the results of three-point bending and micro-indentation, the flexural strength, interfacial fracture strength, and micro-hardness of TCVI specimens were slightly higher than those of ICVI specimens. Furthermore, the residual thermal stress distribution of TCVI specimens was complex owing to the spatial gradient in temperature of TCVI process, while the residual thermal stress distribution of ICVI specimens was relatively uniform. This work provides meaningful guidance for the process selection and industrial applications of C/C composites.

Effects of Al2O3 coating induced structural and mechanical changes of C/C composites

Different C/C composites, identified as Cf/C, CAl2O3/C, C1600/C and C1600-Al2O3/C, were fabricated by condensation of different treated 3D needle-punched carbon fiber preforms through a thermal gradient chemical vapor infiltration (TG-CVI) method. Effects of high temperature treatment and Al2O3 coating of the carbon fiber preforms on matrix texture and mechanical properties of C/C composites were investigated. SL PyC is obtained for Cf/C and C1600/C, which preforms are without Al2O3 coating. Whereas the texture of carbon matrix for CAl2O3/C, and C1600-Al2O3/C changes to RL under the same deposition conditions, indicating that Al2O3 coating layer significantly affects the texture of carbon matrix. Heat treatment of the carbon fiber preforms at 1600 °C for 4 h obviously increase the toughness of the C/C composites. The fracture toughness increased by 226% from 0.938 MPa/m2 for Cf/C to 3.061 MPa/m2 for C1600-Al2O3/C. The significant toughness increasing is attributed to the unique fracture mode for C1600-Al2O3/C, where a hollow “cylindrical” PyC matrix is pulled out and a corresponding big hole is left with a central fiber in it. The flexural strength, however, is mainly affected by the final density of the C/C composites.

Experimental and Modeling Studies on the Chemical Vapor Infiltration with Respect to the Effects of Thermal Gradient and Other Operating Parameters

Effects of operating parameters in the thermal gradient chemical vapor infiltration of propane such as thermal gradient, diffusion, infiltrations time, and concentration of propane were studied by focusing on the visualizations of the intrinsic effects of these parameters. A uniform deposition in the preform was obtained with a gradually increasing temperature along the gas flow. The uniformity of deposition through the preform got improved with increasing deposition time. Results of numerical modeling estimated the experimental data very well when the pre-exponential factor of the overall rate of carbon deposition from propane reported by Vaidyaraman[1] was multiplied by 4. The average density of a preform increased by about 3 times from 0.38 to 1.15 g/cm3 after 60 hr deposition with a thermal gradient under the conditions of 3% propane in nitrogen and 840 to 900 °C.

Microstructure and flexural properties of C/C-Cu composites strengthened with in-situ grown carbon nanotubes

Carbon nanotubes (CNTs) reinforced carbon/carbon-copper (C/C-Cu-CNTs) composites were fabricated by thermal gradient chemical vapor infiltration (TGCVI). The effect of copper and CNTs on the microstructure and flexural properties of carbon/carbon (C/C) composites were investigated. Results showed that the flexural strength of C/C-Cu-CNTs composites was larger than that of pure C/C composites and C/C-Cu composites. Copper particles coated on carbon fibers reduced the contact area between pyrolytic carbon matrix and carbon fibers, leading to the decrease of flexural strength by 17.34% in direction Z and 22.44% in direction X, respectively. However, CNTs catalyzed by copper particles linked adjacent pyrolytic carbon layers together as a bridge and made propagation resistance of cracks increased, which strengthened the matrix. The flexural strength of C/C composites was improved by 33.61% in direction Z and 23.94% in direction X, respectively.

Densification kinetics and mechanical properties of carbon/carbon composites reinforced with carbon nanotubes produced in situ

Carbon nanotube reinforced carbon/carbon composites were fabricated via in situ growth of carbon nanotubes directly on needle-punched felts following direct resistive heating thermal gradient chemical vapor infiltration. The infiltration kinetics, mechanical properties and microstructure of carbon nanotube reinforced carbon/carbon composites were studied. The results show that rough laminar pyrocarbon is obtained owing to CNTs favoring the formation of high-textured pyrocarbon. When nickel nitrate content is 0.10 ml/L, carbon nanotubes with the diameter about 30 nm are obtained together with continuous network on the carbon fibers. The flexural strength and interlaminar shear strength of the composites obtained with 0.1 mol/L nickel nitrate contents are increased by nearly 44.2% and 40.7%, respectively, compared to that of the composite containing no carbon nanotubes.

Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750K) sublimation to high-temperature (2467K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.

Directional thermophysical, ablative and compressive behavior of 3D carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites with an apparent density higher than 1.84g/cm3 were fabricated using multiple carbon fiber tows oriented in four directions and densified using a combined process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by pitch impregnation/carbonization cycles carried out under high pressure. The samples were machined along X-, Y- and Z-axis of the preform architecture denoting the weakest to the strongest direction in the composite respectively. The Y-directional sample showed the highest thermal diffusivity and the lowest CTE values. The directional ablative behavior was studied using plasma arc testing on a Huels type arc-heater. The Z-directional sample showed the lowest ablation and erosion rates while the other two showed a discrepancy in the linear and calculated erosion rates. To further investigate it, the samples after the test were analyzed in the heat-affected zone for a reduced density. Based on empirical correlation, a new term called “corrected linear ablation rate” has been formulated which also helped in the calculation of surface roughness after the ablation testing. Compressive tests were performed before and after plasma testing. After plasma testing, the compressive strength and modulus were almost the same except for the X-directional sample whose modulus reduced to 50%. The X-directional sample, being the weakest direction, showed a double shear type failure, while the other two failed with end crushing, resulting from the different fiber architecture.

Three-Dimensional Heat Transfer Analysis of a TG-CVI Reactor

Thermal-gradient chemical vapor infiltration (TG-CVI) is an alternative process to the classical CVI by involving a temperature gradient to obtain uniform densification. It allows fabricating C/SiC and C/C composites starting from a fibrous preform and gasses precursor. The main interest of these processes is increasing the density homogeneity and the densification rate. In the proposed TG-CVI reactor, a heater with a constant temperature (1323 K) is placed in the core of the reactor and the cool gas flows from the outside to the center. Therefore, a continuous radially moving densification from inside to the outside of porous preform can be achieved. A numerical model was developed in order to investigate temperature distribution and velocity profile in the TG-CVI reactor. The commercial CFD software CFD-ACE+ was used. The obtained three-dimensional results were compared with experimental data. Three reference points were defined to measure the temperature in the preform. The computed results of TG-CVI showed a good agreement with the measured data. The simulation results of this study are important for the optimization of the densification process in the preform in The TG-CVI reactor.

Microstructure and Mechanical Properties of C/C-ZrC Composites

采用ZrOCl 2 溶液浸渍法把锆化合物引入碳纤维预制体, 经热处理、热梯度化学气相渗透致密化和高温石墨化工艺制备了C/C-ZrC复合材料。性能测试结果表明, C/C复合材料的弯曲强度和模量随ZrC含量的增加而增大, ZrC含量为12.08wt%时, 其强度和模量分别为42.5 MPa 和9.6 GPa, 比未改性试样分别提高了70.0%和43.3%。基体中结合较弱的微米级ZrC颗粒的存在不利于碳基体强度的提高, 但其对材料最终性能的影响是次要的, 碳基体中亚微米/纳米级ZrC颗粒的存在和良好的ZrC-C界面结合, 提高了碳基体的强度和模量, 进而提高了复合材料的最终性能。

Densification Behavior of Thermal Gradient CVI of Large-scale C/C Composites

Using integral carbon felt as reinforced perform, large-scale C/C composites were prepared by thermal gradient-chemical vapor infiltration (TG-CVI) with two temperature controlling ways, namely keeping constant temperature of the inner wall or outer wall of the samples. The results indicated that, for keeping constant tempera- ture of the outer wall, the composite density was merely 0.64 g/cm 3 , showing high-low-high distribution across the wall. Meanwhile, its structure comprised rough laminar and smooth laminar. For keeping constant temperature of the inner wall, the density of the sample showed evenly distribution across the wall and increased to 0.98 g/cm 3 , and densification efficiency was 73.79%, higher than that of the former. Under this temperature controlling way, the composite comprised only rough laminar carbon with superior properties. Compared to the outer wall temperature controlling, the composite prepared by the inner wall temperature controlling, needed higher temperature and more appropriate temperature gradient during densification, which was consistent well with the ideal densification model. Moreover, the inner wall temperature controlling way enabled the large-scale C/C composite densification from in- ner side to outside wall, and achieved uniform density distribution, high densification efficiency and excellent mi- crostructure.

Effect of high temperature heat treatment on the ablation of SiC–ZrB2–ZrC particles modified C/C composites in two heat fluxes

Effect of high temperature heat treatment on the ablation of carbon/carbon (C/C) composites modified by SiC–ZrB2–ZrC particles was investigated under oxyacetylene torch. The composites were prepared through thermal gradient chemical vapor infiltration at 900–1100°C and precursor infiltration and pyrolysis at 1400–1800°C. The SiC–ZrB2–ZrC particles modified C/C composites with and without heat treatment at 2100°C were named as C/C-ZSA and C/C-ZSB, respectively. Compared with C/C-ZSB, C/C-ZSA had four kinds of cracks, higher thermal conductivity, lower ablation rates in heat flux of 4.18MW/m2 and promoted ablation in 2.38MW/m2. Different thermal conductions induced various rises of surface temperature. The surface temperature and these four cracks determined local thermal stress and infiltration of oxidizing species, which dominated mechanical erosion of outer layer and oxidation of sub-layer. Compared with C/C-ZSB, the existence of these four cracks and lower surface temperature of C/C-ZSA reduced mechanical erosion in 4.18MW/m2 but resulted in enhanced oxidation in 2.38MW/m2.

Electrophoretic deposition of carbon nanotubes onto carbon fiber felt for production of carbon/carbon composites with improved mechanical and thermal properties

Carbon nanotubes (CNTs) were introduced into carbon fiber felts by electrophoretic deposition (EPD) and then pyrocarbon (PyC) was infiltrated in the felts by thermal gradient chemical vapor infiltration using methane as precursor to prepare carbon nanotube (CNT) reinforced carbon/carbon composites (CNT-C/C composites). The effects of CNTs by EPD on microstructure, mechanical and thermal properties of carbon/carbon (C/C) composites were investigated. Results showed that CNTs had complicated effects on the microstructure of PyC in CNT-C/C composites: PyC next to carbon fiber (CF) was isotropic texture and PyC far from CF showed medium texture, while PyC in C/C composites without CNTs (REF-C/C composites) presented low texture. Compared with REF-C/C composites, compressive strength and modulus of CNT-C/C composites were increased by 37% and 19%, respectively, and fracture mode of the composites changed from pseudo-plastic fracture to brittle fracture; coefficient of thermal expansion of CNT-C/C composites in the direction perpendicular to carbon fiber plies was decreased by 30%.

The Experiment and Numerical Simulation of Composite Countersunk-head Fasteners Pull-through Mechanical Behavior

Fasteners made of the anisotropic carbon/carbon (C/C) composite material have been developed for joining C/C composite material components in the high-temperature environment. The fastener specimens are fabricated from the C/C composites which are made from laminated carbon cloths with Z-direction carbon fibers being punctured as perform. Densification process cycles such as the thermal gradient chemical vapor infiltration (CVI) technology were repeated to obtain high density C/C composites fastener. The fasteners were machined parallel to the carbon cloths (X-Y direction). A method was proposed to test pull-through mechanical behavior of the countersunk-head C/C composite material fasteners. The damage morphologies of the fasteners were observed through the charge coupled device (CCD) and the scanning electron microscope (SEM). The internal micro-structure were observed through the high-resolution Mirco-CT systems. Finally, an excellent simulation of the C/C composite countersunk-head fasteners were performed with the finite element method (FEM), in which the damage evolution model of the fastener was established based on continuum damage mechanics. The simulation is correspond well with the test result . The damage evolution process and the relation between the countersunk depth and the ultimate load was investigated.

Influence of heat treatment temperature on microstructure and thermal expansion properties of 2D carbon/carbon composites

The influence of heat treatment from 1900 to 2650 °C on microstructure and thermal expansion properties of thermal gradient chemical vapor infiltration (TCVI)-infiltrated 2D carbon/carbon composites was investigated. The structure evolution was characterized by polarized light microscopy, X-ray diffractometer, Raman spectroscopy and thermal expansion behavior was studied by thermal dilatometer. The results revealed that with the increasing heat treatment temperature, successive microstructural changes in carbon matrix and fibers occurred, and cracks and pores in the composites increased. These resulted in a 35.6% decrease of CTE in Z direction and a 13.9% decrease in XY direction in the composites heat-treated at 2650 °C compared with as-deposited composites. The CTE mainly depended on the thermal expansion of matrix in Z direction while it relied on that of the fibers in XY direction.

Fracture mechanism of 2D-C/C composites with pure smooth laminar pyrocarbon matrix under flexural loading

Using natural gas as carbon source, 2D needle felt as preform, 2D-C/C composites were prepared by thermal gradient chemical vapor infiltration. Their microstructures were observed under polarized light microscope (PLM) and scanning electron microscope (SEM), and the flexural behaviors before and after heat-treatment were studied with a universal mechanical testing machine. The fracture mechanism of the composites was discussed in detail. The results show that, carbon matrix exhibits pure smooth laminar (SL) characteristic including numerous wrinkled layered structures and some inter-laminar micro-cracks. With the decreasing density, the strength of the composites decreases and the toughness increases slightly; after 2500 °C heat-treatment, the inter-laminar micro-cracks in matrix increase, the strength decreases, and the toughness obviously increases. The fracture mode of the composites changes from brittle to pseudo-plastic characteristic due to more crack deflections in SL matrix.

Microstructure and flexural properties of carbon/carbon composite with in-situ grown carbon nanotube as secondary reinforcement

Carbon nanotubes (CNTs) were in-situ grown in carbon felts using ferric chloride as catalyst and natural gas as carbon precursor via thermal gradient chemical vapor infiltration (TGCVI). Subsequently, the carbon felts were densified to obtain CNT reinforced carbon/carbon (C/C) composites in the same furnace. Effects of CNTs on the microstructure and flexural property of C/C composites were investigated by polarized light microscopy, Raman spectroscopy, scanning electron microscopy and universal mechanical testing machine. The results of PLM observation and Raman analysis showed that CNTs have two-sided effects on the microstructure of pyrocarbon: the pyrocarbons in the region without CNTs show medium texture; while, in the region full of CNTs, the microstructure was low-textured or even isotropic though the TGCVD conditions would lead to the deposition of pure low texture pyrocarbons. Analysis based on stress–strain curves demonstrated that the flexural strength increased first and then decreased with the CNT content increasing. When the CNT content was 5.23wt%, the flexural strength was maximum and had a nearly 35% improvement compared with pure C/C composite. Besides, after adding CNTs, the flexural modulus of the composites decreased and the ductility increased obviously, indicating CNTs can toughen C/C composites.

Anti-oxidation and ablation properties of carbon/carbon composites infiltrated by hafnium boride

To improve the anti-oxidation and ablation properties of carbon/carbon (C/C) composites, they are modified by hafnium boride (HfB2) using a two-step process of in situ reaction and thermal gradient chemical vapor infiltration. X-ray diffraction is used to monitor the composition of the samples. Scanning electron microscope images show that the carbon fibers are uniformly coated by HfB2 particles. The oxidation onset temperature of carbon fibers is greatly increased from 300 to 700°C after HfB2 coating. After modification with HfB2, the linear and mass ablation rates of the C/C composites are decreased by 51.80% and 24.32%. During oxidation and ablation, the interface between carbon matrix and fiber is effectively protected by HfB2 due to the reaction of HfB2 with the oxygen, and the resultant hafnium oxide may form the liquid film to resist the oxygen at high temperature.