Li Hejun

Mullite-Al2O3-SiC oxidation protective coating for carbon/carbon composites

In order to exploit the unique high temperature mechanical properties of carbon/carbon (C/C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al2O3-mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C/C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C/C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface.

The numerical simulation of thermal-gradient CVI process on positive pressure condition

Normally, carbon/carbon (C/C) composites are manufactured by negative pressure CVI process. The products manufactured by this process may have higher density, but it takes a very long time and the reaction gases cannot be utilized adequately. The experimental results indicated that the densification rate of C/C composites can be greatly increased and the reaction gas can be utilized more adequately by applying positive pressure chemical vapor infiltration (CVI) process. However, the density homogeneity of the products manufactured by this process is relatively lower. To solve this problem, numerical simulation technology is applied to analyze the characters and feasibility of thermal-gradient CVI (TCVI) process on positive pressure condition. The result indicates that the thermal-gradient CVI process can increase the density homogeneity and densification rate of C/C composites greatly.

The effect of applied stress on damage mode of 3D C/C composites under bend-bend fatigue loading

The bend-bend fatigue behavior of 3D integral braided carbon/carbon composites (3D C/C) was examined. Fatigue test was conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 203 MPa (92% of the static flexural strength), the lag loops of fatigue load-displacement were transformed from elasticity to anelasticity and the flexibility of specimens were enhanced with increase in applied stress. It is revealed that the interfacial sliding abrasion played an important role in the fatigue failure process, and the extent and speed of sliding abrasion were controlled by the level of applied stress.

Modeling of CVI process in fabrication of carbon/carbon composites by an artificial neural network

The chemical vapor infiltration (CVI) process in fabrication of carbon-carbon composites is very complex and highly inefficient, which adds considerably to the cost of fabrication and limits the application of the material. This paper tries to use a supervised artificial neural network(ANN) to model the nonlinear relationship between parameters of isothermal CVI(ICVI) processes and physical properties of C/C composites. A model for preprocessing dataset and selecting its topology is developed using the Levenberg-Marquardt training algorithm and trained with comprehensive dataset of tubal C/C components collected from experimental data and abundant simulated data obtained by the finite element method. A basic repository on the domain knowledge of CVI processes is established via sufficient data mining by the network. With the help of the repository stored in the trained network, not only the time-dependent effects of parameters in CVI processes but also their coupling effects can be analyzed and predicted. The results show that the ANN system is effective and successful for optimizing CVI processes in fabrication of C/C composites.

TaSi2 Oxidation Protective Coating for SiC Coated Carbon/Carbon Composites

Abstract In order to improve the oxidation protective property of carbon/carbon composites at high temperature, TaSi 2 coating was prepared on the surface of SiC coated carbon/carbon composites by pack cementation technique. The crystal structure and morphology of the as-obtained coating was analyzed by XRD and SEM with EDS. The isothermal oxidation test was carried out in air at 1773 K. The results show that the as-prepared coating is composed of SiC, Si and TaSi 2 . The multilayer coating is about 200 μm in thickness, and there are no cracks in it. The results of EDS indicate that TaSi 2 can effectively fill the holes of SiC coating. There is no visible interface between the TaSi 2 outer coating and the SiC inner coating. The isothermal oxidation curve of the TaSi 2 /SiC coated carbon/carbon composites reveals that the coating can effectively protect carbon/carbon composites for 233 h at 1773 K.

Dominant Factors of Metal Jet Breakup in Micro Droplet Deposition Manufacturing Technique

Abstract For the solutions of random variations of metal jet breakup and difficulties in controlling and predicting the process parameters (e.g. jet length) in micro droplet deposition manufacturing technique, experimental methods combining with theoretical analyses have been developed. The jet formation, jet length and their dominant factors (oxygen concentration and disturbance frequency, etc.) are discussed. The statistical law of jet length is found that the probability density function (PDF) of jet length is a log-normal distribution. The results show that the formation and size accuracy of metal jet breakup are improved by adjusting the gas pressure and optimizing the disturbance frequency. Under this circumstance, the jet length and morphological deviation can be minimized, which provides a stable droplet stream for the subsequent manufacturing process.

An imaging technique using rotational polarization microscopy for the microstructure analysis of carbon/carbon composites.

A novel image analysis technique was proposed for microstructure investigation of carbon/carbon (C/C) composites. The rotational polarization microscopy was developed to meet the special imaging requirements. The samples of C/C composites were observed in reflection polarized light microscope, where the analyzer was rotated instead of the stage, and the polarizer was taken out. The bireflectance of like‐graphite negative uniaxial crystal was analyzed. It was the theoretic foundation of image collection and data processing. The analyzer was rotated through 36 × 10° intervals without any movement of the specimen. The polished cross‐section of C/C composites took micrographs at each analyzer orientation. All image data collected from the same field of view were processed by image registration and image fusion. The synthesized images were obtained by calculating the maximum and minimum gray values and their differences at each point of the million pixels at 18 orientations of the analyzer. They are unique and quite reliable to be applied to analyze the microstructure of C/C composites. Subsequently, image segmentation was performed, and the feature parameters of each component were calculated. Good agreement was found between the results from image analysis and experimental data. Microsc. Res. Tech., 2012.

The rapidly solidified aging copper alloy by BP neural network

Rapid solidifiation is a kind of new process for enhancing the hardness and electrical conductivity of Cu−Cr−Zr copper alloy. The use of BP neural network (NN) is presented to model the non-linear relationship between parameters of age hardening processes and the mechanical and electrical properties of rapdily solidified Cu−Cr−Zr alloy. The improved model is developed by the Levenberg-Marquardt training algorithm and the good generalization performance is demonstrated. So, an important foundation has been laid for optimisticaly controlling the rapidly solidified aging processes of Cu−Cr−Zr alloy.

Hydroxyapatite coating on C/C with graphene oxide interlayer

ABSTRACT Hydroxyapatite (HA) coatings were synthesised on carbon/carbon composites (C/C) with a graphene oxide (GO) interlayer. The morphology and microstructure of the GO interlayer and the HA coating were researched. The effect of applying GO interlayer on the morphology and microstructure of HA coating was investigated. The in vitro bioactivity of HA coating for C/C with GO interlayer was investigated. The results indicated that the GO interlayer was spread on the surface of C/C substrate uniformly. The morphology of the HA coating on original C/C was uneven composed of HA grains with a length 10–50 μm and nanoplate structure HA grains. The morphology of the HA coating with GO interlayer was uniform composed of single nanoplate structure HA grains. The formation process of the HA coating on C/C with GO interlayer was discussed. In addition, the simulated body fluid immersion tests showed that the HA coating with GO interlayer exhibited good bioactivity.

Joining of C/C Composites and GH3128 Ni-based Superalloy with Ni-Ti Mixed Powder as an Interlayer

Abstract A modified SiC coating was prepared on the surface of C/C composites by a pack cementation process. Ni-Ti mixed powder was used as an interlayer to join SiC coating modified C/C composites and GH3128 Ni-based high-temperature alloy (GH3128) by a vacuum hot-pressing diffusion process. The chemical element distribution, microstructure and mechanical properties of the joint between C/C composites and GH3128 were investigated by X-ray diffractometry, scanning electron microscopy, energy-dispersive spectrometry and material universal testing machine. Results show that the SiC modified coating on the surface of C/C composites not only improves fully the wettability of C/C composites to the Ni-Ti intermediate layer, but also alleviates effectively the thermal stress in the joint due to mismatch thermal expansion. Under the same joining condition, the shear strength of the joint with SiC modified coating at room temperature is up to 22.50 MPa, whereas that of the joint without SiC coating modification is almost 0.