Xiaofang Bi

Investigation of the failure mechanism of thermal barrier coatings prepared by electron beam physical vapor deposition

Abstract Two-layer structure thermal barrier coatings (TBCs) (NiCoCrAlY [bond coat]+[6–8 wt.%] Y2O3-stabilized ZrO2 [YSZ top coat]) were deposited by electron beam physical vapor deposition (EB-PVD) on a Ni-base superalloy. Pre-treatments were carried out in a vacuum to improve the oxidation resistance of the bond coat, and the thermal cyclic life of the TBC system was investigated through thermal cyclic tests. It was found that the pre-treatments in the vacuum at 1273 K increased the thermal cyclic life of the TBCs by improving the oxidation resistance of the bond coat. The results of thermal cyclic tests indicated that the TBCs prepared by EB-PVD were degraded mainly at the interface between the bond coat and ceramic coat, due to the spallation of the YSZ top coat from the bond coat. According to the results of microstructure observation and composition analysis, the mechanism of failure was proposed as follows: micro-cracks are first initiated in the YSZ top coat along columnar boundaries, and then propagate through the whole top coat. The cracks formed at the thermal growth oxide (TGO) layer, due to the growing micro-cracks, are considered to cause abnormal thermally-grown oxides of the bond coat beneath the cracks, and consequently, the build-up stresses due to the volume increase, which are significant in weakening the interface combination of the top coat and bond coat. During the following cooling process, compressive stresses are introduced, which tend to separate the YSZ top coat from the bond coat, and finally cause the occurrence of the failure of TBC system.

Preparation of Al2O3–YSZ composite coating by EB-PVD

Abstract An Al2O3–YSZ thermal barrier coating was prepared by co-deposition of Al2O3 and YSZ (8 wt.% Yttria Stabilized Zirconia) onto NiCoCrAlY bond coat by means of electron beam physical vapor deposition (EB-PVD). Analysis of SEM/EDXS showed that the structure and composition distribution across the thickness of the coating changed continuously. Simultaneously, a gradient micro-porous microstructure formed in the coating, which resulted in micro-hardness of the coating increasing gradually towards its surface. Additionally, the Al2O3–YSZ coating exhibited a lower thermal conductivity than traditional YSZ coating, which was caused by the micro-porous microstructure in the coating and the vacancies appearing in the ZrO2 lattice.

Change of coercivity of magnetic thin films with non-magnetic layers and applications to spin valve

Co and NiFe thin films were deposited on various buffer layers such as Ta, Cu and Cr to study dependence of coercivity on the buffer layers. It has been found that coercivity of the magnetic layers could be favorably reduced when grown on Ta buffer layer. In contrast, the coercivity was enhanced for the films with Cr buffer layer. X-ray diffraction and small-angle reflection analysis revealed that the change of coercivity with various buffer layers was closely linked with structure of the magnetic films. In addition, magnetoresistance of the sandwiches with the structure of Co/NM/NiFe on different non-magnetic buffer layers has been investigated. The result indicated that spin valve effect for the sandwiches without exchange-biased field can be obtained by selecting appropriate non-magnetic layer.

Preparation of FeSi/Cu nano-multilayer materials and their magnetic properties

Abstract The FeSi/Cu multilayer materials (MLM), as well as the as-deposited FeSi thin film, were prepared by the electron beam physical vapor deposition (EB–PVD) technique. The as-deposited FeSi had a b.c.c (α-Fe) structure with a Si concentration of approximately 6.1 wt.%. The lattice spacing of FeSi in MLM increased with a decrease in FeSi layer thickness. The saturation magnetization (4π Ms) of FeSi/Cu MLM depended on the thickness of the FeSi layer and was found to be 1.75 T when the FeSi layer was thicker than 70 nm, showing a similar value to that of FeSi thin film. Specific saturation magnetization per mg (σs) of FeSi increases sharply with decreasing FeSi layer thickness and even exceed that of pure Fe when the FeSi layer thickness is lower than 40 nm. The lowest value of coercivity appeared when the FeSi layer thickness was 116 nm. Resistivity was proportional to FeSi layer thickness, and was twice as great as that of Fe-6.5 wt.%Si alloy when the layer thickness is 174 nm.

Study on Electronic Structure and Magnetic Properties of Ni 3 Fe Ferromagnetic Layer Adjacent to Cu

The effect of a Cu layer on the electronic structure and magnetic properties of Ni 3 Fe was studied by employing the discrete-variational method in the framework of density-functional theory. Three models were established for Ni 3 Fe(6), Ni 3 Fe(3)/Cu(3)/Ni 3 Fe(3), and Ni 3 Fe(3)/Cu(3). The charge transfer, magnetic moment, and spin exchange split at the Fermi level were obtained for Fe and Ni atoms in a Ni 3 Fe layer. The related characterizations of Ni 3 Fe were estimated from those of Fe and Ni atoms. It was discovered that the magnetic properties of the Ni 3 Fe layer improved when adjacent to Cu layer due to the improvement of the corresponding properties of the Fe atoms in the Ni 3 Fe. However, the magnetic moment and the spin exchange split of the Ni atoms in the Ni 3 Fe decreased when the Ni 3 Fe was adjacent to a Cu layer.