Shouyang Zhang

Preparation and mechanical properties of carbon/carbon composites with high textured pyrolytic carbon matrix

Short carbon fiber felts with an initial porosity of 89.5% were deposited by isobaric, isothermal chemical vapor infiltration using natural gas as carbon source. The bulk density of the deposited carbon/carbon (C/C) composites was 1.89 g/cm3 after depositing for 150 h. The microstructure and mechanical properties of the C/C composites were studied by polarized light microscopy, X-ray diffraction, scanning electron microscopy and three-point bending test. The results reveal that high textured pyrolytic carbon is deposited as the matrix of the composites, whose crystalline thickness and graphitization degree highly increase after heat treatment. A distinct decrease of the flexural strength and modulus accompanied by the increase of the toughness of the C/C composites is found to be correlated with the structural changes in the composites during the heat treatment process.

Interface-like fracture mechanism in pyrolytic carbon matrix-based carbon–carbon composites

Abstract Interface fracture mechanism is specially important for the properties of composites. In the present research of carbon–carbon composites, an interface-like fracture mechanism in the pyrolytic carbon matrix is first reported. The interface-like fracture can provide carbon–carbon composites with significantly improved strength and toughness, which has the effects similar to the appropriate interface fracture mechanism between fibers and matrix. This special behavior can be considered as a useful supplement for the traditional conception of interface fracture of composites.

Characterisation of wear particles from biomedical carbon/carbon composites with different preforms in hip joint simulator

A hip joint simulator was employed to predict the clinical wear behaviour of carbon/carbon (C/C) composites with needled carbon cloth preform and carbon felt preform. Wear particles generated from the two kinds of C/C composites were isolated and characterised by the size distribution and morphology. The evolvement of wear particles in the hip joint simulator was proposed. The results show that the wear particles from two kinds of C/C composites have a size ranging from submicron to tens of micrometers. The wear particles have various morphologies including broken fiber, fragment fiber, slice pyrolytic carbon and spherical pyrolytic carbon. C/C composites with needled carbon cloth preforms have larger size range and more broken fiber particles and slice pyrolytic carbon particles in comparison with C/C composites with carbon felt preforms. The evolvement of pyrolytic carbon particles is caused by surface regularization, whereas, the evolvement of carbon fiber particles is related to stress direction in the hip joint simulator.

Potential field emitters: HfC nanorods sheathed with a HfO2 nanoshell

Large-scale hafnium carbide (HfC) nanorods sheathed by a hafnium oxide (HfO2) nanoshell were fabricated using a vacuum catalytic chemical vapor deposition (CVD) method in a HfCl4–C3H6–H2–Ar system containing a small amount of oxygen impurities. The microstructures and morphologies of the products were characterized by an X-ray diffractometer, field-emission scanning electron microscope, transmission electron microscope, and energy-dispersive X-ray spectrometer. The results show that the synthesized needle-like nanostructures, with a diameter of ~100 nm and a length below 2 μm, consist of a HfC core and a HfO2 shell with a thickness of ~5 nm surrounding the core. According to two comparative experiments, a growth mechanism is proposed to explain the formation of the nanorods through a combination of an oxygen-assisted growth process and a root-type vapor–liquid–solid process. The HfC nanorods exhibit excellent field emission (FE) properties with a low turn-on field of 3.2–3.4 V μm−1, high field enhancement factor of 2335, and stable emission current density with a fluctuation below 5% over two hours.

Effect of stress level on fatigue behavior of 2D C/C composites

Abstract Laminated carbon fiber clothes were infiltrated to prepare carbon fiber reinforced pyrolytic carbon (C/C) using isothermal chemical vapor infiltration (CVI). The bending fatigue behavior of the infiltrated C/C composites was tested under two different stress levels. The residual strength and modulus of all fatigued samples were tested to investigate the effect of maximum stress level on fatigue behavior of C/C composites. The microstructure and damage mechanism were also investigated. The results showed that the residual strength and modulus of fatigued samples were improved. High stress level is more effective to increase the modulus. And for the increase of flexural strength, high stress level is more effective only in low cycles. The fatigue loading weakens the bonding between the matrix and fiber, and then affects the damage propagation pathway, and increases the energy consumption. So the properties of C/C composites are improved.

A molecular-level analysis of gas-phase reactions in chemical vapor deposition of carbon from methane using a detailed kinetic model

This work describes a modeling study of methane pyrolysis in chemical vapor deposition (CVD). The model consists of a detailed chemical kinetic model, which includes 241 species and 909 gas-phase reactions for methane pyrolysis mechanism, and a plug-flow model, which describes the transport conditions in CVD. Reasonably good agreements were obtained between the simulation results and the experimental results of methane pyrolysis in CVD of pyrocarbon in a vertical hot-wall deposition reactor without any artificial adjustments. The mole fractions of hydrogen, acetylene, ethylene, and benzene increased with a decreasing growth rate as the residence time and the initial methane pressure increased. Sensitivity analysis and reaction paths were conducted to identify the crucial reaction steps and explain how they impact in this pyrolysis process. Results showed that methane pyrolysis had an incubation stage to form a necessary gas atmosphere for the pyrolysis to move forward and C3 species were the main direct source for benzene formation. These results should be useful to understand methane pyrolysis at a molecular level in CVD, as well as the relationship between the gas species and the pyrocarbon.

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

Abstract 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.

Microstructural characterization of non-equilibrated 4D carbon/carbon composites in ultrahigh temperature environment:

A comparative study has been carried out on microstructure, ultrahigh temperature tensile properties, and erosion behavior of non-equilibrated four-directional reinforced carbon/carbon composites, prepared using polyacrylonitrile based carbon fibers as the reinforcement and coal-tar pitch as the matrix, to assess the effect of microstructural changes on the properties of the composites in an ultrahigh temperature environment. X-ray diffraction and scanning electron microscopic analyses suggest that the carbon matrix is highly graphitized, which is composed of mosaic and domain structures. The composites retain superior tensile properties with a visco-plastic behavior at ultrahigh temperature, which is consistent with the observation of mixed microstructural changes, crack propagation, and deflection as well as fiber debonding. The ablation behavior of non-equilibrated four-directional reinforced carbon/carbon composites is mainly dominated by thermo-chemical ablation and mechanical erosion, and different ...

Evolution of solidification microstructure and dynamic recrystallisation of Inconel 625 during laser solid forming process

This study investigated the evolution of solidification microstructure and dynamic recrystallisation (DRX) during the laser solid forming of the Ni-based Inconel 625 superalloy. The as-deposited microstructure mainly showed epitaxially grown columnar grains with fine equiaxed grains between them. These fine equiaxed grains were formed by the discontinuous DRX (DDRX) and continuous DRX (CDRX) processes, which were induced by the cyclic thermal stress resulting from the repeated laser deposition. The bulging of pre-existing grains and sub-grain rotation were the main mechanisms of the DDRX and CDRX phenomena, respectively. Additionally, after the occurrence of DRX, the dislocations were released and there was no distortion in the recrystallised grains. Coarse equiaxed grains were present in the top zone of the deposit; these grains were formed by the columnar-to-equiaxed transition during the solidification of the molten pool after the end of the laser re-melting and deposition process.