A multiscale modeling for predicting the thermal expansion behaviors of 3D C/SiC composites considering porosity and fiber volume fraction

Coefficients of thermal expansion (CTEs) are an essential design criterion of the three-dimensional carbon fiber reinforced SiC matrix composites (3D C/SiC). Representative volume element (RVE) models of microscale, void/matrix, and mesoscale developed in this work were used to investigate the CTEs of these composites. A coupled temp-displacement steady-state analysis step was created for assessing the thermal expansions behaviors of the composites by applying periodic displacement and temperature boundary conditions. Three RVE models of cuboid, hexagonal and fiber random distribution were respectively established to comparatively study the influence of fiber package pattern on the CTEs at microscale. Similarly, the effects of different void size, locations, and shapes on the CTEs of the matrix are comparatively analyzed by the void/matrix models. The prediction results at mesoscale corresponded closely to the experimental results. The effect of the porosities on the CTEs was studied by the void/matrix RVE models. The voids were effective in lowering the CTE of the 3D C/SiC composites. Furthermore, the effect of fiber volume fractions on the CTE were also taken into consideration. Equal in-plane and out-of-plane CTEs were realized by selecting appropriate fiber volume fractions for the different directions. The multiscale models developed in this work can be used to predict the thermal expansion behaviors of other complex structure composites.     

Floating catalyst chemical vapor infiltration of nanofilamentous carbon reinforced carbon/carbon composites – Integrative improvement on the mechanical and thermal properties

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were produced by floating catalyst chemical vapor infiltration with ferrocene content ranging 0–2.0?wt%. The NFCs and increased graphitization degree led to an improvement on the mechanical and thermal properties. An excellent combination of high strength and thermal conductivity (TC), and low coefficient of thermal expansion (CTE) was reached by adding 0.5–0.8?wt% catalyst. When the content exceeded 0.8wt%, the strength and TC were decreased by the limited NFC growth and matrix transited from rough laminar to isotropic pyrocarbon. After the treatment of 2500?°C, the strength and CTE decreased whereas the TC was increased. With the catalyst contents at 0.5–0.8?wt%, the flexural and shear strength retention ratios achieved a high value of 73.1–74.5 and 79.1–79.4%, respectively, and the in-plane and out-of-plane TCs exhibited maxima of 339.1 and 72.5?W/(m?K). Relatively low CTE was obtained at 2.0?wt% catalyst owing to the increased amount of cracks and pores.     

In-plane thermal expansion behavior of dense ceramic matrix composites containing SiBC matrix

In order to reveal the effect of matrix cracks resulted from thermal residual stresses (TRS) on the thermal expansion behavior of ceramic matrix composites, SiBC matrix was introduced into C_f/SiC and SiC_f/SiC by liquid silicon infiltration. The TRS in both two composites were enlarged with incorporating SiBC matrix which has higher coefficients of thermal expansion (CTEs) than SiC matrix. Due to the relatively high TRS, matrix cracks and fiber/matrix (f/m) debonding exist in C_f/SiC-SiBC, which would provide the space for the expansion of matrix with higher CTEs. For SiC_f/SiC, no matrix cracking and f/m debonding took place due to the close CTEs between fiber and matrix. Accordingly, with the incorporation of SiBC matrix, the in-plane CTE of C_f/SiC between room temperature to 1100 °C decreases from 3.65 × 10⁻⁶ to 3.19 × 10⁻⁶ K^-1, while the in-plane CTE of SiC_f/SiC between room temperature to 1100 °C increases slightly from 4.97 × 10⁻⁶ to 5.03 × 10⁻⁶ K^-1.     

Floating catalyst chemical vapor infiltration of nanofilamentous carbon reinforced carbon/carbon composites – Densification behavior and matrix microstructure

Nanofilamentous carbon (NFC) reinforced carbon/carbon composites were prepared by floating catalyst film boiling chemical vapor infiltration from xylene pyrolysis at 1000–1100 °C using ferrocene as a catalyst. The influence of the catalyst content on the densification behavior and matrix microstructure of the composites was studied. Results showed that the deposition rate of pyrocarbon (PyC) was enhanced remarkably by the catalyst. The density of the composites deposited at a catalyst content of 0–2.0 wt% decreased along both the axial and the negative radial directions. Rough laminar (RL) PyC matrix was formed at 0–0.8 wt% catalyst content by heterogeneous nucleation and growth. A hybrid matrix consisting of RL and isotropic (ISO) PyCs appeared at a catalyst content of 1.2–2.0 wt%. The reasons for this ISO PyC formation were attributed to the deposition of carbon encapsulated iron particles and homogeneous nucleation. A reinforcing network composed of NFCs and vapor grown carbon fibers was formed on the fiber/matrix interface and within the matrix in this floating catalyst process. The structure of NFC transformed from nanotube to nanofiber when the catalyst content was over 0.5 wt%, around which composites of a high density of 1.75 g/cm³ and uniform RL PyC matrix were produced rapidly.     

Microstructure and thermal expansion behavior of diamond/SiC/(Si) composites fabricated by reactive vapor infiltration

Diamond/SiC/(Si) composites were fabricated by Si vapor vacuum reactive infiltration. The coefficient of thermal expansion (CTE) of composites have been measured from 50 to 400 °C. With the diamond content increasing, CTE of composite decreased, simultaneously, the microstructure of the composites changed from core–shell particles embedded in the Si matrix to an interpenetrating network with the matrix. The CTEs of composites versus temperature matched well with those of Si. The Kerner model was modified according to the structural features of the composites, which exhibited more accurate predictions due to considering the core–shell structure of the composites. The thermal expansion behavior of the matrix was constrained by diamond/SiC network during heating.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Effect of sintering temperature on the thermal expansion behavior of ZrMgMo3O12p/2024Al composite

A 2024Al metal matrix composite with 10?vol% negative expansion ceramic ZrMgMo₃O₁₂ was fabricated by vacuum hot pressing, and the influence of sintering temperature on the microstructure and thermal expansion coefficient (CTE) of alloys was investigated. Experimental results showed that all ZrMgMo₃O_12p/2024Al composites sintered at 500–530?°C had a similar reticular structure and exhibited different linear expansion coefficients at 40–150?°C and 150–300?°C. The addition of 10?vol% ZrMgMo₃O₁₂ decreased the CTEs of 2024Al by ~ 16% at 40–150?°C and by ~ 7% at 150–300?°C. This addition also increased the hardness of 2024Al by ~ 23%. The density of the composites and the content of Al₂Cu in ZrMgMo₃O_12p/2024Al increased as the sintering temperature increased. The CTEs of the composites decreased, whereas hardness increased. Thermal cycling from 40?°C to 300?°C caused the CTEs of the composites to decrease gradually and reach a stable value after seven cycles. The lowest CTEs of 15.4?×?10^?6 °C^?1 at 40–150?°C and 20.1?×?10^?6 °C^?1 at 150–300?°C were obtained after 10 thermal cycles and were reduced by ~ 32% and ~ 17%, respectively, compared with the CTE of the 2024Al. Among the current reinforcements, ZrMgMo₃O₁₂ negative expansion ceramics showed the highest efficiency to decrease the CTE of Al matrix composites.     

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.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

Thermal expansion behavior of Cu/Cu2O cermets with different Cu structures

Cu/Cu₂O cermets were prepared with Cu₂O matrix imbedded with branch-like or spherical Cu powders. The coefficients of thermal expansion (CTE) of them were tested. The CTE curves can be divided into three segments. From 25 °C to 150 °C, CTEs were found to decrease with temperatures. The CTEs were influenced by the structure of the conductor phase. For cermets prepared with spherical Cu, the increase in CTEs was basically linear with temperatures above 150 °C. In contrast, cermets with same Cu content prepared with branch-like Cu had a CTE with an increasing rate as the temperature rose from 150 °C to 900 °C, and the increasing rates in these temperature range are much higher than those prepared with spherical Cu.     

Effect of pyrocarbon texture on the mechanical and oxidative erosion property of SiC coating for protecting carbon/carbon composites

SiC coating was conducted on C/C composites with rough laminar (RL) or smooth laminar (SL) pyrocarbon matrix separately. The residual stress, elastic modulus and microhardness of RL-SiC (RLS) and SL-SiC (SLS) coatings were investigated. The results showed that compared with SLS, RLS coating possessed smaller residual stress and higher hardness and elastic modulus, which was beneficial for its resistance to cracking and then contributed to the anti-oxidation performance improvement. At temperatures of 300–1400 °C, its mass loss was only 2.41%. At 1500 °C, it showed good self-sealing ability and could provide C/C composites against oxidation at least 120 h.     

Fabrication of C/SiC composites by siliconizing carbon fiber reinforced nanoporous carbon matrix preforms and their properties

Reactive melt infiltration (RMI) has been proved to be one of the most promising technologies for fabrication of C/SiC composites because of its low cost and short processing cycle. However, the poor mechanical and anti-ablation properties of the RMI-C/SiC composites severely limit their practical use due to an imperfect siliconization of carbon matrixes with thick walls and micron-sized pores. Here, we report a high-performance RMI-C/SiC composite fabricated using a carbon fiber reinforced nanoporous carbon (NC) matrix preform composed of overlapping nanoparticles and abundant nanopores. For comparison, the C/C performs with conventional pyrocarbon (PyC) or resin carbon (ReC) matrixes were also used to explore the effect of carbon matrix on the composition and property of the obtained C/SiC composites. The C/SiC derived from C/NC with a high density of 2.50 g cm^?3 has dense and pure SiC matrix and intact carbon fibers due to the complete ceramization of original carbon matrix and the almost full consumption of inspersed silicon. In contrast, the counterparts based on C/PyC or C/ReC with a low density have a little SiC, much residual silicon and carbon, and many corroded fibers. As a result, the C/SiC from C/NC shows the highest flexural strength of 218.1 MPa and the lowest ablation rate of 0.168 µm s^?1 in an oxyacetylene flame of ～ 2200 °C with a duration time of 500 s. This work opens up a new way for the development of high-performance ceramic matrix composites by siliconizing the C/C preforms with nanoporous carbon matrix.     

Excellent ablation resistance and reusability of silicon carbide fiber reinforced silicon carbide composite in dissociated air plasmas

This work explores the potentials of SiC fiber reinforced SiC matrix composites (SiC_f/SiC) with SiC coating to resist aerodynamic ablations for thermal protection purpose. A plasma wind tunnel is employed to evaluate their anti-ablation property in dissociated air plasmas. The results suggest a critical ablation temperature of SiC coated SiC_f/SiC, ≈ 1910 °C, which is the highest ever reported in literatures. Benefited by ‘all-SiC’ microstructures and relative flat ablated surfaces, the SiC_f/SiC is still ablation-resistant up to ≈ 1820 °C after the occurrence of ablation. This implies an excellent ablation resistance and reusability property of SiC_f/SiC, which surpasses that of traditional carbon fiber reinforced composites. Finally, an ablation mechanism dominated by surface characteristic is proposed. For the SiC coated SiC_f/SiC, ablation is prone to take place at surface cracks formed by thermal mismatch; while for the ablated SiC_f/SiC, ablation is triggered at the exposed fiber bundles which is over-heated in the plasmas.     

Microstructure,mechanical and anti-ablation properties of SiCnw/PyC core-shell networks reinforced C/C–ZrC–SiC composites fabricated by a multistep method of chemical liquid-vapor deposition

C/C–ZrC–SiC composites reinforced by SiC nanowire (SiCnw)/pyrocarbon (PyC) core-shell networks were prepared by a multistep method of chemical liquid-vapor deposition (CLVD). The microstructure, mechanical property and ablation resistance were researched. The investigations presented that the PyC was deposited on the SiC nanowires, and the micro-scale core-shell structures were produced. Moreover, these micro-scale structures not only connected with the fibers and matrices, but also filled the pores in the composites. In contrast with C/C–ZrC–SiC composites, the flexural modulus and strength of SiCnw/PyC-C/C–ZrC–SiC composites increased by 36.91% and 44.53%, and the fracture mode was changed from the brittle to pseudo-plastic fracture. After the oxyacetylene torch ablation at two temperatures for 90s, the composites strengthened by SiCnw/PyC core-shell possessed a better resistant ablation. At ablation temperature of 2300 °C, the mass loss rate and linear reduction rate of the composites with core-shell networks decreased by 66.18% and 57.55% in contrast with the non-reinforced composites, and declined by 56.46% and 57.48% at ablation temperature of 3000 °C. The obvious decrease of ablation rates was ascribed to the dense microstructure, the small coefficient of thermal expansion (CTE), the good thermal conductivity, and the resistant ablation roles of SiCnw/PyC core-shell systems.     

Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life

New promising thermal barrier coatings (TBCs) processed by the sol–gel route are deposited onto NiPtAl bond coated superalloy substrates using the dip and/or spray coating technique. In this study, the optimization of the process, including an appropriate heat treatment prone to densify the yttria-stabilized-zirconia (YSZ) top-coat and leading to the sintering and the development of a resulting crack network, is investigated. In particular, relevant information on internal strain evolution during the heat treatment are obtained using in situ synchrotron X-rays diffraction and confirm a stabilization of the TBC through the occurrence of the micro-cracks that beneficially releases the in-plane sintering stress. Such TBCs are subsequently reinforced using additional material brought within the cracks using sol–gel spray coating. The effect of various process parameters, such as the pre-oxidation of the bond-coat, on the sol gel TBCs consolidation and their cyclic oxidation resistance enhancement, is presented. Reinforced sol–gel TBCs are successfully oxidized up to more than one thousand 1 h-cycles at 1100 °C, without any detrimental spallation.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Investigation of thermal expansion of 3D-stitched C–SiC composites

Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10^?6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10^?6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min.     

Effect of HfC nanowires grown on carbon fibers on the microstructure and thermophysical properties of C/C composites

One-dimensional (1D) hafnium carbide nanowires (HfCnws) were grown in situ on carbon fibers (CFs) via a Ni-assisted pyrolysis method of organometallic polymer precursor. Scanning electron microscopy (SEM), transmission electron microscope (TEM), polarized-light optical microscopy (PLM), and Raman were used to analyze the effect of HfCnws on the microstructure of pyrolytic carbon (PyC). The specific heat capacity (HC), thermal diffusivity (TD), thermal conductivity (TC), and coefficient of thermal expansion (CTE) of HfCnws-C/C composites were also investigated. Results show that HfCnws wrapped by carbon nanosheet were successfully synthesized. The diameter of HfCnws is about 30 nm and the thickness of carbon nanosheet is about 10 nm, which could induce the formation of isotropic (ISO) PyC. After introducing HfCnws, the TD and CTE of HfCnws-C/C composites were increased. Ni2HfCnws-C/C composites show a higher TC and TD, and the CTE increased with the increasing content of HfCnws.     

Preparation of carbon fiber/Mg-doped nano-hydroxyapatite composites under low temperature by pressureless sintering

In order to protect carbon fibers (CF) from oxidation damage during sintering process, rod-like Mg-doped nano-hydroxyapatite (Mg-nHA) with an increased thermal decomposition temperature and reduced sintering temperature was synthesized by hydrothermal method. The synthesized bone-like Mg-nHA with similar composition and morphology to bone apatite was used as the matrix to prepare CF reinforced Mg-nHA composites (CF/Mg-nHA) at a low temperature of 700 °C by pressureless sintering. The increase of temperature slightly influenced the growth of Mg-nHA prepared by hydrothermal method from 160 °C to 200 °C. The Mg-nHA were short and rod-like in structure with a length of approximate 100 nm. When doping 1% magnesium, the decomposition temperature of Mg-nHA increased by 100 °C compared with that of nHA. This can protect CF from oxidation damage which is often encountered when sintering CF reinforced hydroxyapatite composites at high temperature and enhance reinforcing effects of CF. The bending strength of CF/Mg-nHA with 1 wt% CF was 8.51 MPa, which increased by 19.5% compared with Mg-nHA. Alternatively, the rod-like Mg-nHA was prepared on the surface of CF by electrochemical deposition and Mg-nHA coated CF was used to reinforce Mg-nHA, the coefficient of thermal expansion mismatch between CF and HA matrix could be mitigated. The compressive strength of Mg-nHA coated CF reinforced Mg-nHA (CF/Mg-nHA/Mg-nHA) composites with 0.5% CF sintered at 800 °C were 41.3 ± 1.56 MPa, which was attributed to the improved strengthening effect of CF and the good interface between CF and Mg-nHA matrix.     

High temperature oxidation resistance of Y2O3 modified ZrB2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, Y₂O₃ modified ZrB₂-SiC coating was fabricated on C/C composites by atmospheric plasma spraying. The microstructure and chemical composition of the coatings were characterized by SEM, EDS, and XRD. Experiment results showed that the coating with 10 wt% Y₂O₃ presented a relatively compact surface without evident holes and cracks. No peeling off occurred on the interface between the coating and substrate. The ZSY10 coating underwent oxidation at 1450 °C for 10 h with a mass loss of 5.77%, while that of ZS coating was as high as 16.79%. The existence of Y₂O₃ played an important role in inhibiting the phase transition of ZrO₂, thus avoiding the cracks caused by the volume expansion of the coating. Meanwhile, Y₂SiO₅ and ZrSiO₄ had a similar coefficient of thermal expansion (CTE), which could relieve the thermal stress inside the coating. The ceramic phases Y₂SiO₅, Y₂Si₂O₇ and ZrSiO₄ with high thermal stability and low oxygen permeability reduced the volatilization of SiO₂.