Comparative ablation behaviors of C/SiC-HfC composites prepared by reactive melt infiltration and precursor infiltration and pyrolysis routes

Carbon fiber reinforced silicon carbide-hafnium carbide (C/SiC-HfC) composites were prepared by reactive melt infiltration (RMI) and precursor infiltration and pyrolysis (PIP) routes. The ablation behaviors of the two composites were investigated and compared under an oxyacetylene torch flame. The C/SiC-HfC composites prepared by PIP showed a better ablation resistance than those synthetized by RMI. Microstructural observations revealed an island distribution of HfC for the sample prepared by RMI, which resulted in SiC being directly oxidized during the ablation process. In contrast, the PIP-prepared sample showed a uniform distribution of HfC, which resulted in SiC being oxidized via the Knudsen diffusion mechanism under ablation. The Knudsen diffusion of oxidants retarded the oxidation process, thereby increasing the ablation resistance of the C/SiC-HfC composites prepared by PIP.     

Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology

Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiC_W/SiC). Granulated needle-shaped SiC_W was spray dried into SiC_W spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiC_W preform with two-stage pores using the SiC_W spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiC_W/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiC_W/SiC are discussed.     

High-temperature properties and interface evolution of C/SiBCN composites prepared by precursor infiltration and pyrolysis

C/SiBCN composites with a density of 1.64 g/cm³ were prepared via precursor infiltration and pyrolysis and the bending strength and modulus at room temperature was 305 MPa and 53.5 GPa. The precursor derived SiBCN ceramics showed good thermal stability at 1600 °C and the SiC and Si₃N₄ crystals appeared above 1700 °C. The bending strength of the composites was 180 MPa after heat treatment at 1500 °C, and maintained at 40 MPa-50 MPa after heat treatment for 2 h at 1600 °C–1900 °C. In C/SiBCN composites, SiBCN matrix could retain amorphous up to 1500 °C and SiC grains appeared at 1600 °C but without Si₃N₄. The reason for no detection of Si₃N₄ was that the carbon fiber reacted with Si₃N₄ to form an interface layer (composed of SiC and unreacted C) and a polycrystalline transition layer (composed of B and C elements), leading to the degradation of the mechanical properties.     

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

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

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

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

Degradation of boron nitride interfacial coatings fabricated by chemical vapor infiltration on SiC fibers under ambient air/room temperature conditions

Hexagonal boron nitride (h-BN) interfacial coatings were deposited on SiC fibers by chemical vapor infiltration (CVI) and their degradation behavior under ambient air/room temperature conditions was studied with time. Degradation of the interfacial coatings with time was investigated by characterizing the morphology and microstructure of these materials with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Thermogravimetry coupled with differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical reactions and phase transitions taking place in the BN coatings. The results showed that the as-deposited BN interfacial coatings fabricated by CVI were compact and well bonded to the SiC fibers. BN coatings remained relatively stable under ambient air/room temperature conditions for 50?h, while severe degradation was observed after 500?h of exposure. The degradation of BN interfacial coatings was mainly caused by two factors, namely, reaction with atmospheric air to produce boric oxide and amorphization of the hexagonal structure. The degradation observed under ambient air/room temperature might be due to incomplete crystallinity of BN interfacial coatings. Presence of water vapor may accelerate degradation of the coatings. The results of this degradation test can be used as a reference for the storage of BN coatings fabricated by CVI.     

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

Microstructure and ablation properties of La2O3 modified C/C-SiC composites prepared via precursor infiltration pyrolysis

Effects of La₂O₃ modification on the microstructure, mechanical and ablation properties of C/C-SiC composites were investigated. Experimental results show that a new La₁₀(SiO₄)₆O₃ phase was generated during heat treatment process. The presence of the La-compounds, namely La₂O₃ and La₁₀(SiO₄)₆O₃, had an important impact on the structure of reinforced skeleton and the molten oxide film, and thus strongly affected the mechanical and ablation properties of the composites. Excessive La addition induced the structural damage of the reinforced skeleton, resulting in weakened mechanical and ablation properties. The C/C-SiC composites with 25.65 wt.% La₂O₃ addition displayed better mechanical properties and the best ablation resistance. The La₁₀(SiO₄)₆O₃ phase could react with molten silica to form a viscous glass during ablation. The transformation of La-compounds into La₂Si₂O₇ can reduce the ablation of SiO₂ and enhance the glass film, so as to protect the composites from further ablation.     

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.     

Fabrication of highly dense three-layer SiC cladding tube by chemical vapor infiltration method

Three-layer silicon carbide (SiC) cladding architectures are considered to be promising materials for current light-water nuclear reactors. Herein, a novel processing approach was proposed to fabricate dense three-layer SiC tubes by introducing SiC nanowires (NWs) on the graphite rod, which resulted in change in the valley-peak structure of SiC_f tubular preform. A dense three-layer-NWs SiC cladding tube, consisting of a chemical vapor infiltration (CVI)-SiC inner layer, a CVI-SiC_f/SiC composite layer, and a CVI-SiC outer layer, was obtained through CVI process. Microstructure and hoop strength of the as-obtained three-layer-NWs SiC cladding tube were systematically investigated. By avoiding delamination of the layers and reducing the pores, the three-layer-NWs SiC cladding tube exhibited an average hoop strength of 316.6 MPa with a Weibull modulus of 11.55.     

The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP

Traditionally, SiC components with complex shapes are very difficult or even impossible to fabricate. This paper aims to develop a new manufacturing process, combining selective laser sintering (SLS), cold isostatic pressing (CIP) and polymer infiltration pyrolysis (PIP), to manufacture complex silicon carbide parts and improve the mechanical properties of silicon carbide ceramic parts. The density and porosity of SiC/SiC composites were measured. Furthermore, the mechanical properties of the specimens with cold isostatic pressing and the specimens without cold isostatic pressing were compared. The bending strength of the specimens with cold isostatic pressing was 201?MPa, and the elastic modulus was 1.27?GPa. And, the bending strength of the specimens without cold isostatic pressing was 142?MPa, and the elastic modulus was 0.88?GPa. Increasing the density of SiC/SiC can enhance the mechanical properties of SiC/SiC composites.     

Low-surface-temperature jump behavior of C/SiC composites prepared via precursor impregnation and pyrolysis in high-enthalpy plasma flows

The response of C/SiC composites prepared via precursor impregnation and pyrolysis was investigated in a 1 MW plasma wind tunnel. Under a considerable aero heating of up to 26.2 MJ/kg of specific total enthalpy, the samples were exposed to heat fluxes exceeding 5.7 MW/m² and low pressures of 4.5–6.6 kPa. The samples were able to withstand low heat fluxes and low stagnation pressures, and their carbon-rich nature improved the thermal conductivity, presenting a low steadystate surface temperature. However, a spontaneous jump in the surface temperature at around 1700 °C was observed at high heat fluxes and high stagnation pressures. The jump temperature was lower compared with that reported in previous studies, and was found to increase rapidly to temperatures above 2000 °C. This low-temperature jump phenomenon was associated with the evolution of microstructure during testing, and the underlying mechanism was revealed through the use of thermodynamics analysis.     

Fabrication of SiC ceramic architectures using stereolithography combined with precursor infiltration and pyrolysis

Stereolithography based additive manufacturing provides a new route to produce ceramic architectures with complex geometries. In this study, 3D structured SiC ceramic architectures were fabricated by stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis (PIP). Firstly, photosensitive SiC slurry was prepared. Then, stereolithography was conducted to fabricate complex-shaped green SiC parts. Polymer burn-out was subsequently performed, and porous SiC preforms were produced. After that, precursor infiltration and pyrolysis was used to improve the density and strength. Finally, 3D-structured SiC ceramic architectures with high accuracy and quality were obtained. It is believed that this study can give some fundamental understanding for the additive manufacturing of SiC ceramic structures.     

Mechanical and dielectric properties of SiCf/SiC composites fabricated by PIP combined with CIP process

2D KD-1 SiC fiber fabrics were employed to fabricate SiC_f/SiC composites by an improved polymer infiltration and pyrolysis (PIP) process, combined with cold isostatic pressing (CIP). The effect of CIP process on the microstructure, mechanical and dielectric properties of SiC_f/SiC composites was investigated. The infiltration efficiency was remarkably improved with the introduction of CIP process. Compared to vacuum infiltration, the CIP process can effectively increase the infiltrated precursor content and decrease the porosity resulting in a dense matrix. Thus SiC_f/SiC composites with high density of 2.11 g cm⁻³ and low porosity of 11.3% were obtained at 100 MPa CIP pressure, together with an increase of the flexural strength of the composites from 89 MPa to 213 MPa. Real part (ε′) and the imaginary part (ε″) of complex permittivity of SiC_f/SiC composites increase and vary from 11.7-i9.7 to 15.0-i12.8 when the CIP pressure reaches 100 MPa.     

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.     

Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.     

Microstructure and mechanical properties of C/C composites modified by single-source precursor derived ceramics

To improve the mechanical properties of C/C composites, the SiC(N)/TiC ceramic derived from single-source precursors (SSPs) were introduced into the C/C by precursor infiltration and pyrolysis (PIP) at 1100 °C. The shear strength of all modified composites were improved by nearly 2 times compared with the pristine C/C composites (37.4 MPa) due to the increased interfaces by multiphases incorporation. After further annealing at 1500 °C, the SiC(N)/TiC ceramic in C/C composite transferred into the nanocomposites, SiC(N)/TiC-NCs, in which nano-scale TiC particles distributed into the SiC(N) matrix. The modified samples still exhibited about 39% improvement on shear strength. Large numbers of uniform micro-cracks occurred in both above SSPs derived ceramic cases, reducing stress concentration during the shear testing. Moreover, some ring-shaped cracks were observed in the fracture region which can consume large amounts of energy in crack propagation process and then benefit to improve the shear strength.     

化学气相沉积致密针刺C/C复合材料的结构及力学性能研究

以T300炭纤维无纬布、网胎为原材料,层叠针刺成型炭纤维预制体,并采用化学气相沉积工艺对预制体进行致密,制成密度为1.55 g/cm3的针刺C/C复合材料。对针刺C/C复合材料的微观结构进行了观察分析,并对材料力学性能进行了测试。结果表明:化学气相沉积致密的针刺C/C复合材料呈现出以层间大量垂直纤维束为节点的类钉板状网状结构,这种特殊结构使材料层间结合更好,材料整个结构更加紧密;针刺C/C复合材料内部纤维被沉积形成的热解炭所包裹,热解炭的织构类型为光滑层(SL)和粗糙层(RL)并存;针刺C/C复合材料的各项力学性能均达到了较高水平,并且高温力学性能比常温力学性能有了很大幅度的提高。     

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

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

A study of laser machining combined with dynamic chemical etching in micro-hole drilling of 2.5D SiCf/SiC composites

In this study, we used a novel method of laser machining combined with dynamic chemical liquid etching (LMDCE) to drill holes in 2.5D SiC_f/SiC ceramic matrix composites (CMC-SiC). A chemical solution that could quickly remove the recast layer without damaging the substrate was selected. Severe recast layer and microcrack defects were observed when laser machining was performed in the air. The surface of the material was highly carbonized due to the thermal effect of the laser. The effect of different defocus amounts and scanning speeds on the hole taper was studied. A lower scanning speed can ensure that a smaller taper is obtained by the microhole. The bore diameter of the holes processed with a defocusing amount of 0 or −1 mm is more uniform. The results show that with the assistance of a dynamic chemical solution, the fibers break neatly into needle-like shapes, the thermal effect of the laser on the ceramic substrate is significantly weakened, the microhole shows good roundness, and there are no recast layers and oxides on the sample surface. In addition, microcracks are significantly reduced, and high-quality microholes without a heat-affected zone (HAZ) are machined. The method provides a new idea on how to eliminate machining defects and achieve higher-quality micromachining for ceramic matrix composites.