Xiufang Cui

Effects of film thickness and microstructures on residual stress

The accurate measurement of stress is an essential requirement of residual stress studying. In this paper, Cu thin films with different thickness were prepared by magnetron sputtering. The micromorphologies of the films were observed using atomic force microscopy, and the microstructures were analysed by transmission electron microscopy and X-ray diffraction. The residual stress distributions of the films were measured by the curvature and nanoindentation methods. The results show that Cu films present polycrystalline state, and the grains size grew up with increasing film thickness. The residual stress calculation results from nanoindentation tests using the Suresh model were consistent with the curvature method, the residual compressive stress changed into residual tensile stress and the D values (the difference between the maximum and minimum stress value) decreased with increasing film thickness, and the stress distribution became uniform. The interface stress of the films weakened with the film thickness increasing because the crystalline grains grew larger.

Effect of Magnetic Field on Properties and Element Distribution of Ni-Based WC Composite Coatings

The Ni-based WC coatings enhanced by WC particle were fabricated on FV520B by plasma cladding device. The influence of magnetic force on the microstructure and performance of the coating was investigated. If the magnetic field does not exist, the microstructure of coating is a cluster of block-shaped structures; it is observably different from the dendritic and crumbling snowflake-like structures formed under transverse and longitudinal magnetic fields. The WC particles were distributed at the grain boundary. With the effect of longitudinal magnetic field, wear resistance and erosion resistance of coatings improved markedly. When axial magnetic field intensity came to 38 mT, the microhardness of coatings reached a maximum value, 720 HV0.2. Electron probe microanalyzer (EPMA) indicates the metallurgical combination at the interface and element interdiffusion happened between the coating and substrate.

Study of gadolinium based protective coating for magnesium alloys

Abstract Gadolinium based conversion coating, a new chemical protective coating for magnesium alloys, was prepared and its microstructure and corrosion resistance were investigated. The micromorphology, composition and elemental chemical states were observed by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) respectively. The corrosion resistance was evaluated by a potentiodynamic polarisation curve and electrochemical impedance spectroscopy. Nanomechanics integrated test system was used to represent the nanomechanical property and residual stress. The results indicate that the morphology of the coating is a cracked mud structure. The EDS and XPS results reveal that the coating is primarily made of magnesium and gadolinium oxides. Gadolinium coating forming mechanism can be generally divided into two periods: the dissolution of the substrate, the codeposition of Gd(OH)3 and Mg(OH)2. The potentiodynamic polarisation curve and electrochemical impedance spectroscopy (EIS) show that the coating can improve the corrosion resistance of magnesium alloys.

Influence of rare earths addition on residual stress of Fe-based coating prepared by brush plating technology

Abstract The effect of rare earths (La, Ce and Pr) addition on residual stress in iron coatings prepared by brush plating was investigated. The results showed that the addition of rare earth transformed the residual stress in the coating from tensile to compressive. To relieve the residual stress, on the one hand, RE elements segregated at the grain boundaries which restricted the coalescence of the grains and provided more capability of grain deformation. On the other hand, RE elements could purify detrimental element and absorb hydrogen atoms in the coating. Among the three rare earths, elements lanthanum showed the most significant effect on surface morphology and residual stress.

Corrosion resistance of organic coating pretreated by phytic acid

Abstract In this study, phytic acid pretreatment is used as a substitute for toxic chromate pretreatment to improve the corrosion resistance of organic coating on AZ91D magnesium alloys. The microstructures of phytic acid conversion films are observed by a transmission electron microscope. The role of phytic acid pretreatment on the corrosion resistance of organic coating is investigated using salt spray test and electrochemical impedance spectroscopy. The experimental results show that the phytic acid conversion film exhibits a dense amorphous structure and phytic acid pretreatment can effectively improve the corrosion resistance of organic coating on magnesium alloys.

Self-healing conversion coating with gelatin–chitosan microcapsules containing inhibitor on AZ91D alloy

ABSTRACT The microstructure of cerium-based conversion coatings with and without microcapsules containing Ce(NO3)3 or La(NO3)3 on magnesium alloys were investigated and the corrosion resistance was evaluated as well. The gelatin–chitosan microcapsules were prepared by complex coacervation. Microcapsules can effectively avoid the traditional porous state, namely, by releasing active and repairing material immediately after changes in coating integrity. Microcapsules distribute in the tiny cracks of coating deposited on magnesium alloys. The electron probe micro-analyser results characterise the element distribution of coating. The potentiodynamic polarisation curve and impedance spectra show that the conversion coating with microcapsules can enhance the corrosion resistance of magnesium alloys. The microcapsules containing La(NO3)3 show a better corrosion resistance so as to promote a self-healing performance.

Effect of magnetic field on brush plating Ni–Co alloy

Ni–Co electro-brush plating on stainless steel FV520(b) surface was conducted in a perpendicular magnetic field with different intensities. The effects of the applied magnetic field on surface morphology, alloy microstructure, tribological properties and residual stress were investigated by scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, mechanical tests and a nanoindentation technique. The results showed that a smoother and more uniform Ni–Co alloy coating layer with less defects could be obtained in magnetic field of high intensity. Furthermore, by increasing the intensity of the magnetic field, the content of Co in the alloy coating and the wear resistance of the coating layers increased, and the residual stress decreased. In addition, the magnetic field had a significant influence on the valence change of Ni and Co. In particular, when the intensity of magnetic field was 0.2 T a coating with best integrated performance was obtained.

Microstructure and thermal stability of a Ni-Cr-Co-Ti-V-Al high-entropy alloy coating by laser surface alloying

A Ni-Cr-Co-Ti-V-Al high-entropy alloy (HEA) coating with a BCC phase and (Ni, Co)Ti2 compounds was synthesized successfully by laser surface alloying on a Ti-6Al-4V substrate. The microstructure of as-synthesized coatings is typical, namely, the microstructure from the coating to the substrate changes from equiaxed grains to columnar grains. After remaining at 900 °C for 8 h, the constituent phases remain unchanged. However, owing to the unceasing dissolution of the Ti element, the lattice parameter of the BCC HEA phase changes from 3.06 Å to 3.16 Å. The thermoanalysis results show that the oxidation film on the Ni-Cr-Co-Ti-V-Al HEA coating is mainly composed of TiO2, V2O5, and NiO. The oxidation resistance of this HEA coating may be due to the existence of NiO and the alloying elements Al, Cr, and Co; the oxidation phenomenon should be responsible for the mass increase in the thermogravimetry process. The differential scanning calorimetry and the dynamic differential scanning calorimetry curves show that the synthesized HEA coating is stable below 1005 °C.

The mechanical and corrosion resistance properties of Ti/TiN multi-layer films produced by physical vapor deposition

Purpose – The aim of the present paper is to investigate the mechanical performance of multi-layer films. With the wide application of optic and electronic thin-films, membrane materials and membrane technology have become one of the most active fields of research in contemporary materials science (Dumont et al., 1997). Multi-layer films have evolved as candidates for these applications because of their unique properties. TiN and Ti/TiN multi-layer films were fabricated using the DC magnetron sputtering method. A nano-indentation tester and electronic film distribution tester were utilized to evaluate the mechanical properties and residual stress of the films. The existence of interface effects on the mechanical properties and corrosion resistance of the films were analyzed. Design/methodology/approach – In this study, the Ti/TiN multi-layer films were fabricated using the DC magnetron sputtering method. The films were deposited on polished 45# steels. Ti was used as the sputtering target. Ar and N2 were ...

Performance Evolution of Phytic Acid Conversion Film in the Forming Process

To improve conversion film techniques, control film properties and improve quality of following techniques, in this study, the environment-friendly phytic conversion films were deposited on AZ91D magnesium alloy. The performance evolution of the film during the forming process such as mechanical property, residual stress, corrosion resistance, micromorphology, composition, and roughness was investigated by nanomechanical testing system, electrochemical workstation, scanning electron microscope, Auger electron spectroscopy, and atomic force microscope. The results indicate that, in the forming process, the changes of micromorphology, roughness, mechanical property, and corrosion property are closely related to the variation of elements and residual stress. With the prolonging of film formation time, the difference between film elements and substrate elements gradually increases, and the residual stress first increases then decreases. When the film formation time is about 30 min, the film begins to crack and the residual stress is released, which leads to the decrease of mechanical property and corrosion resistance of the film.