Preparation of UHTCMCs by hybrid processes coupling Polymer Infiltration and Pyrolysis with Hot Pressing and vice versa

Carbon fibre reinforced ultra-high temperature ceramic (UHTC) matrix composites were fabricated coupling water-based powder slurry infiltration, Polymer Infiltration and Pyrolysis (PIP) and Hot Pressing (HP) techniques. This study aims to identify the best sequence of consolidation techniques to better integrate the carbon fibre cloths into an ultra-refractory sintered ceramic matrix of ZrB₂-SiC. Infiltrated preforms with UHTC powder slurry were densified via: a) a pre-sintering step by HP followed by two PIP cycles with polycarbosilane, and vice versa, b) two PIP cycles followed by a cycle of HP. Flexural strengths at room temperature and at 1500 °C (167 MPa and 592 MPa, respectively) were found to be significantly higher for composites obtained by the second route, suggesting that sintering of polymer-derived SiC during HP improves the structural properties of C_f/ZrB₂-SiC composites. This work presents an effective method for UHTCMC manufacturing in a shorter time than traditional PIP process.     

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.     

Characterization of carbon fiber-reinforced ultra-high temperature ceramic matrix composites fabricated via Zr-Ti alloy melt infiltration

Carbon fiber-reinforced ultra-high temperature ceramic matrix composites (C/UHTCMCs) were fabricated via Zr-Ti alloy melt infiltration (Zr-Ti MI) using carbon-carbon composite (C/C) preforms and alloys with three different compositions. Alloys were successfully infiltrated into C/C to form solid solutions of TiC and ZrC, with melting temperatures > 2900 °C. Notably, residual alloys were not observed after MI occurred at 1750 °C. Bending strength and fracture toughness of the C/UHTCMCs at room temperature and 1500 °C in air/Ar revealed that mechanical properties of the composites were similar to those of the C/C preform. During arc wind tunnel tests at 2000 °C, a recession of C/UHTCMCs fabricated using Ti-rich alloys was observed; however, this behavior was not observed for the composites prepared using Zr-rich alloys owing to the formation of a ZrO₂ solid solution. Accordingly, Zr-Ti MI is a viable method for preparing C/UHTCMCs without degrading the mechanical properties of the C/C preform, while increasing the ablation resistance.     

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™-iBN and Hi-Nicalon™-S fibers

Isothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in-plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™-iBN or Hi-Nicalon™-S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi-Nicalon™-S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures >1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.     

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB₂ ultra‐high temperature ceramic (UHTC) particulate filled C_f/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc‐jet testing of the UHTC composites highlighted the excellent ultra‐high temperature oxidation performance of these materials.     

Materials characterisation and mechanical properties of Cf-UHTC powder composites

Ultra-high temperature ceramic composites based on carbon fibre, Cf, preforms impregnated with hafnium diboride, HfB₂, powder and then densified with carbon by chemical vapour infiltration, CVI, have been mechanically tested to measure the room temperature flexural, interlaminar shear, compressive and tensile strengths. The latter was also measured at 1000 °C. All the composites suffered a degree of delamination during the different mechanical tests but the strength values obtained were at least equal to, or better than, those previously reported in the literature for ultra-high temperature ceramic (UHTC)-based composites. Importantly, in spite of the oxidation of the tensile samples tested at 1000 °C, similar tensile strength values were obtained at both temperatures, suggesting that the materials can resist elevated temperatures. The samples tested at higher temperature did show greater evidence of fibre pull out, possibly due to a weaker fibre-matrix interface as a result of oxidative degradation. The results also suggested that the 0° orientation plies in the Cf preform structure offered greater resistance to mechanical stresses; this suggests that composites can now be designed to offer even greater strength values.     

Mechanical properties and interfacial characteristics of 2.5D SiNOf/BN wave-transparent composites

2.5D SiNO_f/BN wave-transparent composites were fabricated by borazine infiltration and pyrolysis route at 800 °C?1400 °C. The fracture behavior of the composites was investigated on the basis of the retained fiber strength, in-situ fiber and matrix mechanical properties, and fiber/matrix bonding strength. Nano-indentation were employed to determine the in-situ elastic modulus and hardness of the fiber and BN matrix, and single-fiber push-out experiments were performed to quantify the fiber/matrix bonding strength. The interfacial characteristics of the 800 °C?1200 °C fabricated composites were further studied in terms of physical bonding and chemical reaction. Physical bonding was resulted from thermal mismatch between the fiber and matrix, which induced compressive radial stress at the interface. The radial stress increased continuously with increasing fabrication temperature. Meanwhile, the TEM analysis confirmed chemical diffusion at the fiber/matrix interface, which further improved the interfacial bonding strength. The chemical reaction mechanism was proposed.     

航天飞行器热防护系统技术综述

综述表明,C/C和C/SiC复合材料是宇宙输送系统飞行器前端部位热防护系统的最佳材料选择,多层抗氧化涂层、超高温陶瓷(UHTC)涂层、UHTC基体改性是提高其高温长期使用的有效途径。指出多层UHTC涂层、纳米级UHTC颗粒、火花等离子浇结(SPS)及碳气凝胶填充碳泡沫新型热防护结构等在高温热防护材料方面已显现出实际应用方向。     

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.     

Behavior of two-dimensional C/SiC composites subjected to thermal cycling in controlled environments

Thermal fatigue behavior of two-dimensional carbon fiber reinforced SiC matrix composites fabricated by chemical vapor infiltration technique was investigated using an on-line quench method in controlled environments which simulated an aero-engine gas. A system of damage information acquisition (SDIA) was developed to study changes in electrical resistance of the C/SiC composites during their damage in dynamic testing. Damage to composites was assessed by the ultimate tensile strength (UTS) and SEM characterization. The results showed that: (1) under different atmosphere, the 2D-C/SiC composites subjected to thermal cycling behaved very differently and the most sensitive atmosphere was the wet oxygen; (2) external load could accelerate the degradation of the composites and changed the oxidation regimes of fibers; (3) the electrical resistance of the specimen could be detected on-line, stored in real time and analyzed reliably by the newly-developed SDIA; (4) 2D-C/SiC composites had an excellent thermal fatigue resistance in different environments.     

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

High‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon?) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon? fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 10⁵ cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Effects of SiC content on mechanical,thermal and ablative properties of carbon/phenolic composites

Silicon carbide (SiC) particles were utilized to improve the mechanical, thermal and anti-ablative properties of carbon/phenolic (C/Ph) composites. SiC–C/Ph composites were fabricated with different weight percentage of SiC by vacuum impregnation method. The mechanical and thermal properties were characterized by compression tests, thermal conductivity tests, and thermogravimetric analysis; meanwhile, ablation resistance was investigated using plasma wind tunnel tests and scanning electron microscopy. Experimental results showed that 5 wt% SiC modified C/Ph composites owned the optimum properties. Moreover, introducing SiC particles could result in an obvious decrease of compression strength, but an increase of thermal stability, thermal conductivity and anti-ablative performance. Notably, the ablation rate reached its the lowest point at 5% the SiC content in resin matrix composites.     

Effects of zirconium carbide content on thermal stability and ablation properties of carbon/phenolic composites

Ceramic particles were utilized to improve thermal stability and ablation properties of carbon/phenolic (C/Ph) composites. In this study, zirconium carbide (ZrC) modified C/Ph composites were fabricated by vacuum impregnation method, and effects of ZrC content on thermal stability and ablation properties were investigated by thermogravimetry analysis and plasma wind tunnel test. Moreover, morphological characterization was carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Experimental results showed that increasing ZrC content could lead to an evident increase in char yield, but an observable reduction in linear ablation rates and back-face temperatures because of the formation of ZrO₂ layer on the ablation surface. The work provided an effective way to improve thermal stability and ablation properties of C/Ph composites.     

Construction of sandwich-structured C/C-SiC and C/C-SiC-ZrC composites with good mechanical and anti-ablation properties

Four kinds of sandwich-structured C/C-SiC and C/C-SiC-ZrC composites with or without a SiC interphase deposited by isothermal chemical vapor infiltration (ICVI), were designed and fabricated by a joint process of electromagnetic coupling chemical vapor infiltration (ECVI) and precursor infiltration and pyrolysis (PIP). The fabricated composites are macroscopically nonhomogeneous materials with low density, high strength and low ablation rate. The interphase and matrix constituents had remarkable effects on the mechanical and ablation properties of these composites. The C/C-SiC composites with an ICVI-SiC interphase exhibited the highest flexural strength of 306.5 MPa. While the C/C-SiC-ZrC composites with the interphase showed the best anti-ablation performance with low linear and mass ablation rates of 0.37 μm/s and 0.04 mg/cm²·s, respectively, after the ablation for 500 s under an oxyacetylene flame test at around 2000 °C.     

热梯度法化学气相渗温度控制

建立了热梯度法化学气相渗工艺（ＣＶＩ） 模型，计算了圆筒炭毡内的温度分布，给出热梯度法ＣＶＩ 工艺沉积温度的控制方法。利用这种方法，采用热梯度法ＣＶＩ 工艺，在１００ ｈ 之内成功地制备出内径１６０ ｍ ｍ 、高３９０ ｍ ｍ 、厚２０ｍ ｍ 的圆筒炭毡增强Ｃ／Ｃ 复合材料，密度达到１ ．６ ｇ／ｃｍ ３ 。     

Oxidation behavior of C/SiC-SiBCN composites at high temperature

In order to improve the anti-oxidation performance of C/SiC composites at high temperature, C/SiC composites should be modified by self-healing components. SiBCN ceramic is an ideal self-healing component because of excellent oxidation resistance and thermal stability. C/SiC composites were modified by PDC SiBCN ceramic (C/SiC-SiBCN) by using CVI combined with polymer infiltration and on-line pyrolysis (PI-OP). The oxidation behaviors of C/SiC composites fabricated by CVI method and C/SiC-SiBCN composites fabricated by CVI + PI-OP method and CVI + PIP method at different temperatures in air were compared. The results showed that the strength retention ratios of the composites fabricated by the three methods decreased with the increase of temperature. Compared with the samples fabricated by the other two methods, the weight loss of the samples fabricated by CVI + PI-OP method was greater, but the strength retention ratio was higher.     

A new strategy for high-value reutilization of recycled carbon fiber: Preparation and friction performance of recycled carbon fiber felt-based C/C-SiC brake pads

To achieve the high-value reutilization of recycled carbon fiber (rCF), a new strategy of preparing rCF-based C/C-SiC brake pads is proposed in this work. The results show that the rCF possesses crystal structure and tensile strength comparable with those of virgin CF (vCF) exception of pyrolytic char adhering to the surface of rCF after pyrolysis. The rCF was converted into C/C composites through impregnation-pyrolysis. Pyrolytic char was found to have no evident negative effect on the densification rates of the rCF C/C composites. By reactive melt infiltration, the rCF C/C-SiC composites were fabricated based upon the rCF C/C composites. The achieved rCF C/C-SiC composites do not differ markedly from the vCF group control in terms of microstructure and bending strength. Furthermore, the thermal diffusion coefficients of the rCF C/C-SiC composites are very close to those of vCF C/C-SiC composites in the temperature range 25°C-300 °C. The coefficient of friction values of the rCF C/C-SiC composites are as stable as those of vCF control group, both being maintained at approximately 0.4 during friction test, whether at 25 °C or 300 °C. The wear rate of the rCF C/C-SiC composites is 3.8 μm·min⁻¹, nearly indistinguishable from that of the vCF C/C-SiC composites, i.e., 4.5 μm·min⁻¹, further suggesting that the two materials resemble each other closely. The rCF C/C-SiC composites exhibit great potential for use as alternative brake pads to serve auto braking systems. This work opens up a new path for high-value reuse of rCF.     

Oxidation protective SiC-Si coating for carbon/carbon composites by gaseous silicon infiltration and pack cementation: A comparative investigation

To reduce the influence of coating preparation on the mechanical properties of carbon/carbon composites and further improve the antioxidant properties of the coating, a SiC-Si coating was fabricated on carbon/carbon composites by gaseous silicon infiltration (GSI-SiC-Si). For comparison, a SiC-Si coating was also prepared by pack cementation (PC-SiC-Si). A comparative investigation showed the GSI-SiC-Si coating possesses relative lower roughness, better mechanical and anti-oxidation properties. The GSI-SiC-Si coating samples maintained 87.8 % flexural strength of bare C/C composites, while the C/C substrate was severely siliconized in PC-SiC-Si coating samples. The GSI-SiC-Si coating samples could undergo 30 thermal cycles between 1773 K and room temperature and effectively protect C/C composites from oxidation at 1773 K for more than 500 h without weight loss.     

High-Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites

SiC–Mo₅(Si,Al)₃C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three-point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.