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C/C-HfC composites with a density of 2.14 g/cm3 was successfully prepared using an inorganic hafnium solution as a precursor via chemical vapor infiltration (CVI) and precursor infiltration pyrolysis (PIP) methods, and its microstructure and composition were analyzed. The results show that the HfC ceramics are uniformly dispersed in the pores of the matrix and closely bonded to the matrix. The material's ablation resistance is significantly enhanced by introducing HfC ceramics. Under the same heat flux conditions, when the ablation time is 120 s, the linear and mass ablation rates of the C/C-HfC composite are 6.20×10-2 mm·s-1 and 2.03×10-2 g·s-1, respectively, which are reduced by 48.33% and 40.12%, respectively, compared to those of the C/C composite. During the ablation process, HfC would react with oxygen in the heat flux to generate molten HfO2, which uniformly covers the surface of the matrix to form a protective layer, isolates the heat flux, prevents oxidation of the matrix, and prevents heat transfer. The evaporation of molten HfO2 also takes away some of the surface's heat. With the increase of ablation time, the loss of HfO2 will gradually increase, the protective layer on the surface of the matrix will be gradually destroyed, and the ablation damage of the heat flux on the matrix would become more severe.

Carbon/carbon-polyimide composites were prepared by chemical vapor infiltration and vacuum-pressure impregnation curing process. Firstly, the carbon felt was deposited with pyrolytic carbon by chemical vapor infiltration method to prepare porous carbon/carbon materials with pyrolytic carbon contents of 7.33%(mass fraction, the same below), 14.55% and 22.00%, respectively. Then, through a specific impregnation curing process, the carbon/carbon-polyimide composites P1, P2 and P3 and the control group P0 without pyrolytic carbon were obtained in turn. Through SEM, linear expansion coefficient test and mechanical properties test, the effects of different pyrolytic carbon contents on the microstructure, average linear expansion coefficient and mechanical properties of carbon/carbon-polyimide composites were investigated. The results show that in the temperature range of 20-300℃, the XY-direction average linear expansion coefficients of P0, P1, P2 and P3 are between (0.67-0.79)×10-6℃-1, and the XY-direction average linear expansion coefficients of four groups are basically the same. The Z-direction average linear expansion coefficients of P1, P2 and P3 decrease by 41.6%, 41.8% and 24.1%, respectively compared with P0. Pyrolytic carbon significantly reduces the Z-direction average linear expansion coefficient of materials. The XY-direction compressive strengths of P0, P1, P2 and P3 are 243.91, 244.73, 216.65 MPa and 210.79 MPa, respectively, and the Z-direction compressive strengths are 269.22, 258.80, 246.68 MPa and 219.20 MPa, respectively. The compressive strength of P1 is equivalent to that of P0. The Z-direction bending strengths of P0, P1, P2 and P3 are 92.77, 77.11, 80.71 MPa and 86.06 MPa, respectively. Different pyrolytic carbon contents have different effects on the mechanical properties of materials. The lower pyrolytic carbon content can basically maintain compressive strength of materials, while the higher pyrolytic carbon content can basically maintain bending strength of materials.

Taking the three-directional orthogonal preform with different Z-direction fiber distance as the research object, the carbon/carbon (C/C) composites were prepared by the combination of chemical vapor infiltration and resin impregnation, and the effect of weaving parameters on the microstructure and bending performance of C/C composites were studied. The calculation model was established by taking the smallest repeated structure of three-directional orthogonal preform as a unit, and the relationship between fiber content and weaving parameters of three-directional preform was obtained and verified. The results show that the fiber content of preform is increased with the decrease of Z-direction fiber distance and X, Y-direction fiber layer distance; when the Z-direction fiber distance is increased, the twist deformation of the fiber interweave becomes larger, resulting in the change of the pore structure of the preform; under the same densification process, the change of pore structure affects the composition and distribution of carbon matrix in C/C composites, but has no effect on the morphology of carbon matrix; when the fiber content in X, Y-direction is increased and the distance between fibers in Z-direction is decreased, the flexural strength of the C/C composites is higher.

The issue of water flooding in proton exchange membrane fuel cells (PEMFC) under conditions of humidity levels leads to significant mass transfer losses. To address this challenge, a fiber-structured microporous layer (MPL) with high porosity is developed, and investigates the impact of carbon nanotube (CNT) on its heat treatment process. The inclusion of CNT facilitates the cyclization of polyacrylonitrile in the pre-oxidation process. Thereby reduces the fracture of nanofibers, improved the microstructure and the porosity of the membrane during carbonization. The carbon nanofiber membranes containing CNT shows a higher oriented arrangement of carbon grains after graphitization. The high porosity of the fiber membrane MPL confers exceptional mass transfer capabilities. At 100% RH, the MPL sample CFP-2C improves the maximum power density and a maximum current density of PEMFC by 47.5% and 55.3%, respectively, compares to the conventional carbon black particles MPL. [ABSTRACT FROM AUTHOR]

The surface of carbon fiber is treated with sizing agent containing hydroxylated carbon nanotubes, fullerenols, and graphene oxide, respectively. The effects of carbon nanomaterials and their structures on the surface properties of carbon fiber, the interfacial properties and the mechanical properties of carbon fiber/epoxy composites are investigated. All three types of carbon nanomaterial can improve the interfacial bonding strength and modulus, thereby enhancing the interlaminar shear strength (ILSS) of composites. Moreover, the ILSS increases with the increase of interfacial bonding strength, due to the roughness and the number of active groups. Compared with the composite without carbon nanomaterials, the interfacial bonding strength of the three nanoparticle‐modified composites increased by 14%, 5% and 4%, the modulus increased by 10%, 26%, and 9%, and the ILSS increased by 14%, 8%, and 7%, respectively. The interfacial bonding strength has a more significant impact on the ILSS than the interfacial modulus. Highlights: CNTs, graphene oxide and fullerenols regulate the surface characteristics of carbon fibers.The influence of carbon nanomaterials on the interfacial properties of composites.The relationship between the fiber surface, the interface and the composites. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polyamide composites have outstanding properties, but their applications are significantly affected by the service environment. This work evaluates the effects of temperature (25, 80, 140, and 200°C) and hygrothermal conditions on the mechanical properties of CF/PA6 composite containing delamination damage, and studies its damage behaviors and failure mechanisms. After hygrothermal aging, recrystallization of PA6 improves the heat resistance of CF/PA6 composites, but water absorption reduces the interfacial strength of composites, resulting in the decrease of modulus and compressive strength (24% and 42%, respectively). With the increase in temperature, the compressive strength of CF/PA6 composites gradually decreases (from 132 MPa at 25°C to 24 MPa at 200°C), and the failure mode transitions from fiber fractures, interlayer shear fractures, delamination to kinking. In addition, the compressive failure process of CF/PA6 composites is divided into three stages: buckling, delamination bulging and kinking failure. Highlights: Hygrothermal aging reduces the fiber/resin interface performance.Fiber fracture, interlayer shear and delamination convert to kinking failure.The failure process is divided into buckling, delamination and kinking. [ABSTRACT FROM AUTHOR]

Low‐velocity impact damage is one of the main defects of composite laminates, which seriously affects the bearing capacity and service life of composite laminates. Multiple impact experiments of carbon fiber/epoxy composite laminates were carried out under different impact energies. The finite‐element model was established according to the experimental conditions. The impact damage mechanism of laminated plates was studied by means of simulated damage cloud image, water immersion ultrasonic C‐scan, and computed tomography. The results showed that the matrix damage occurred mainly during the whole impact process, and the tensile damage of the matrix was dominant. As the number of impacts increased, the impact resistance of composite laminates decreased and the damaged area increased, with the second impact having the greatest influence on the degree of damage to the laminates; multiple impacts with low energy were easy to cause delamination at the top of the laminated plate. With the increase in impact energy, the damage at the bottom of the laminated plate was more serious, and the propagation mode of delamination damage in laminates changed from top‐bottom to bottom‐up. Highlights: Establishing a multiple impact finite‐element model for composites.Accurately predicting the impact mechanical response of composites.Revealing multiple impact damage evolution of composites by simulation. [ABSTRACT FROM AUTHOR]

Ceramic matrix composites (CMCs) have become important structural materials of aero-engines under elevated operating temperature while the abradable ceramic coatings matched with CMCs are seldom reported. In this work, the atmospheric plasma spray (APS) technique was used to fabricate and design Yb2Si2O7-based coatings with various contents of CaF2. The phase composition and microstructure of the coatings, as well as Rockwell hardness, were characterized. The wear and abradable behaviors of the coatings including friction coefficients and volume wear rates were evaluated at room temperature and 900 °C, respectively. The results show that the Rockwell hardness and friction coefficients decreased, and the volume wear rates of the coatings at room temperature significantly raised with the increase of CaF2 content. The coatings exhibited quite different behaviors at 900 °C compared with those at room temperature owing to the lubricant effect of CaF2. The morphologies of the coatings after the wear tests were characterized and the possible failure mechanisms were analyzed, and this study may provide guidelines for the application of the abradable coatings suitable for CMCs. [ABSTRACT FROM AUTHOR]