Particle size is a critical consideration for many powder coating-related industries since it significantly influences the properties of the produced materials. However, the effect of particle size on the characteristics of plasma sprayed NiCrBSi coatings is not well understood. This work investigates the microstructures, hardness and electrochemical corrosion behavior of plasma sprayed NiCrBSi coatings synthesized using different-sized powders. All coatings mainly consist of Ni, N₃B, CrB, Cr₇C₃ and Cr₃C₂ phases. The coatings produced by small particles (50–75 μm) exhibit lower porosity (2.0 ± 0.8%). Such coatings show a higher fraction (15.5 vol.%) of the amorphous phase and lower hardness (700 HV_0.5) than the counterparts (8.7 vol.% and 760 HV_0.5, respectively) produced by large particles (75–100 μm) with higher porosity (3.0 ± 1.6%). Meanwhile, the coatings produced from smaller particles possess a larger number of non-bonded boundaries, leading to the easier penetration of corrosive medium, as well as a higher corrosion current density (0.254 ± 0.062 μA/cm²) and a lower charge transfer resistance (0.37 ± 0.07 MΩ cm²). These distinctions are attributed to particle size-induced different melting degrees and stackings of in-flight particles during deposition.

The remelting method is introduced to improve the properties of the as-sprayed NiCrBSi coatings. In this work, tungsten carbide (WC) was selected as reinforcement and the as-sprayed and remelted NiCrBSi/WC composite coatings were investigated by X-ray diffraction, scanning electron microscopy, hardness test and tribology test. After spraying, WC particles are evenly distributed in the coating. The remelting process induced the decarburizing reaction of WC, resulting in the formation of dispersed W₂C. The dispersed W₂C particles play an important role in the dispersion strengthening. Meanwhile, the pores and lamellar structures are eliminated in the remelted NiCrBSi/WC composite coating. Due to these two advantages, the hardness and the high-temperature wear resistance of the remelted NiCrBSi/WC composite coating are significantly improved compared with those with an as-sprayed NiCrBSi coating; the as-sprayed NiCrBSi coating, as-sprayed NiCrBSi/WC composite coating and remelted NiCrBSi/WC composite coating have average hardness of 673.82, 785.14, 1061.23 HV, and their friction coefficients are 0.3418, 0.3261, 0.2431, respectively. The wear volume of the remelted NiCrBSi/WC composite coating is only one-third of that of the as-sprayed NiCrBSi coating.

Bio-ceramic coatings were synthesized on ZK60 magnesium alloys by micro-arc oxidation (MAO). The degradation behavior of the ZK60 alloys with and without MAO coating in the simulated body fluid (SBF) was studied. The samples were characterized by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (CLSM), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) was used to study the degradation behavior. The results showed that the porous MAO coating mainly consisted of MgO, Mg₂SiO₄, Mg₃(PO₄)₂, and CaCO₃. The pH values of both coated and uncoated samples increased over time. However, the pH values of the SBF for coated samples always maintained a lower level compared with those for the uncoated samples. Thereby, the coated samples showed a much lower degradation rate. After immersion in SBF for 5 days, corrosion product containing Ca and P was found on both samples, while the deposition was more active on the coated samples. The degradation models for the uncoated and coated samples in the SBF are also proposed and discussed.

A concept of two-steps current-decreasing mode derived from constant current mode was developed to fabricate micro-arc oxidation (MAO) coatings on ZK60 magnesium alloy in a dual electrolyte system. The growth characteristics of coatings were analyzed by voltage-time curves, and the coating microstructures were characterized by means of scanning electron microscopy. Meanwhile, the roughness, corrosion behavior, and microhardness of MAO coatings were investigated. The results show that the MAO coatings exhibit a smooth and compact surface, and have improved hardness, higher thickness, smaller roughness, and uneven distribution of holes. Such positive characteristics result in improved corrosion resistance of MAO coatings. The coating produced under the two-steps current mode of “1.2–0.6A” shows a smaller corrosion rate of 0.1559 g/m²⋅h compared with the one produced under the other mode. The results of nanoscratch tests show that the coating fabricated by “1.2–0.6 A” mode has strong bond strength with the substrate. Under this optimized mode, the MAO process also has the lowest energy consumption of 49.8 W/(dm²⋅μm).

In this work, ZK60 magnesium alloy was employed as the substrate substratum for producing micro-arc oxidation (MAO) coating which contains Ca and P. Electrophoretic deposition (ED) process was conducted on the micro-arc oxidized sample to prepare a hydroxyapatite (HA) layer on the original coating. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and Fourier transform infrared spectroscopy (FT-IR) were used to analyze the phase constituents and microstructures of both MAO and MAO-ED coatings. Corrosion resistance and degradation behavior of the coatings in SBF were investigated by electrochemical tests and simulated body fluid (SBF) immersion tests. The results indicate that a dense HA layer about 5 μm in thickness had been successfully prepared on the MAO coating. After going through the ED process, the porosity of the MAO-ED coating had decreased from 5.63 to 0.81%. The Ca/P ratio of the MAO-ED coating had become 1.60, which indicates a potential biocompatibility of the material. Therefore, the MAO-ED coating can induce the deposition of bioactive products from SBF. During an immersion test of SBF for 10 days, the weight gain of MAO-ED sample continued to increase, showing that the deposition rate of the induced products is always higher than that of corrosion products before experiencing the 10 days of immersion. The deposition of induced products protects the substrate from being corroded so effectively that it ensures the good mechanical properties and biocompatibility during the initial stage of implantation. According to the changes in sample morphologies and the electrochemical measurements in SBF, a relevant degradation model has been suggested and the underlying mechanism for degradation behavior is discussed.

The bond strength is a critical consideration for the plasma-sprayed NiCrBSi coatings. However, the conventional methods for testing the bond strength of NiCrBSi coatings always cost time and money. If there is a simple method that could predict the bond strength of the prepared NiCrBSi coatings without destroying the coatings, it would be significantly beneficial for industrial applications. In this work, a new method was proposed based on the total areas of the interfacial pores for the NiCrBSi coatings. The NiCrBSi coating was prepared by plasma spraying technology and the as-sprayed coating was subsequently remelted by plasma arc using the powers of 20 kW, 25 kW, and 30 kW, respectively. The interfacial microstructures, the size distributions and total areas of the interfacial pores, interfacial hardness, and bond strength of all prepared coating samples were investigated. After remelting, the number and the total area of interfacial pores decrease with increasing the remelting power. Correspondingly, the interfacial hardness and bond strength of coatings increase with increasing the remelting power The bond strength of coatings basically has a linear relationship with the total area of interfacial pores. The built relationship may be used to predict the bond strength of NiCrBSi coatings.