Zhaobing Cai

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.

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.

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.

Remanufacture of Zirconium-Based Conversion Coatings on the Surface of Magnesium Alloy

Brush plating provides an effective method for creating a coating on substrates of various shapes. A corroded zirconium-based conversion coating was removed from the surface of a magnesium alloy and then replaced with new coatings prepared via brush plating. The structure and composition of the remanufactured coating were determined via x-ray photoelectron spectroscopy, x-ray diffraction, and Fourier transform infrared spectroscopy. The results revealed that the coatings consist of oxide, fluoride, and tannin-related organics. The composition of the coatings varied with the voltage. Furthermore, as revealed via potentiodynamic polarization spectroscopy, these coatings yielded a significant increase in the corrosion resistance of the magnesium alloy. The friction coefficient remained constant for almost 300s during wear resistance measurements performed under a 1-N load and dry sliding conditions, indicating that the remanufactured coatings provide effective inhibition to corrosion.

Effect of surface nanocrystallisation on the microstructure and wear resistance of vacuum carburised alloy

ABSTRACT In this work, a nanocrystallisation surface layer with an average grain size of 29 nm was fabricated on the 12Cr2Ni4A steel by the supersonic fine particles bombarding (SFPB). The vacuum carburising process was carried out on the original and the SFPB pre-treatment samples. X-ray diffraction, scanning electron microscopy and hardness tester were employed to study the phase constituents, grain size, hardness and residual stress of the two carburised samples. Experimental results showed that compared to the coarse-grain carburised layer (without SFPB), the carburised layer with SFPB pre-treatment has smaller martensite and carbide, and its thickness, hardness, compressive residual stress and wear resistance have been significantly improved. The dominant wear mechanisms of the two carburised samples were both abrasive wear and fatigue wear.