•The HVOF spray parameters were optimized using the Taguchi method.•Effects of hardness and porosity on the cavitation erosion resistance were studied.•Hardness was the main factor for cavitation erosion behavior compared to porosity.•Delamination was the main cavitation erosion fracture of the coating.Fe-based amorphous/nanocrystalline coatings were prepared on the AISI 321 steel substrate by the high-velocity oxygen-fuel (HVOF) thermal spraying technology. The effect of selected parameters (oxygen flow, kerosene flow and spray distance) on the cavitation erosion resistance (denoted as Rc) of the coating were investigated by using the Taguchi method. Statistical tools such as design of experiments (DOE), signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were used to meet the expected objective. It was concluded that the kerosene flow had greater influence on the Rc of the coating and followed by the spray distance and the oxygen flow, respectively. The optimum spray parameters (OSP) were 963 L/min for the oxygen flow, 28 L/h for the kerosene flow, and 330 mm for the spray distance. The Rc of the coating increased with the increase of hardness or the decrease of porosity, and the hardness had a greater influence on Rc than the porosity. The Fe-based coating deposited under the OSP exhibited the best cavitation erosion resistance in distilled water. The cracks initiated at the edge of the pores and the interfaces between the un-melted or half-melted particles, and finally leaded to the delamination of the coating.

•WC–10Co–4Cr and FeCrSiBMn coatings were prepared by HVOF spraying process.•Synergistic effect of cavitation erosion and corrosion of the coatings was studied.•Contribution of corrosion component to total volume loss rate was different.•Mechanical effect was the main factor for cavitation erosion–corrosion behavior.The high-velocity oxygen-fuel (HVOF) spraying process was used to fabricate conventional WC–10Co–4Cr coatings and FeCrSiBMn amorphous/nanocrystalline coatings. The synergistic effect of cavitation erosion and corrosion of both coatings was investigated. The results showed that the WC–10Co–4Cr coating had better cavitation erosion–corrosion resistance than the FeCrSiBMn coating in 3.5 wt.% NaCl solution. After eroded for 30 h, the volume loss rate of the WC–10Co–4Cr coating was about 2/5 that of the FeCrSiBMn coating. In the total cumulative volume loss rate under cavitation erosion–corrosion condition, the pure cavitation erosion played a key role for both coatings, and the total contribution of pure corrosion and erosion-induced corrosion of the WC–10Co–4Cr coating was larger than that of the FeCrSiBMn coating. Mechanical effect was the main factor for cavitation erosion–corrosion behavior of both coatings.

•Fe-based nanocrystalline coatings were obtained by crystallization of amorphous coatings.•The grain size of the annealed Fe-based coatings was significantly increased as the pre-annealing time prolonged.•The cyclic oxidation resistance of the annealed coatings was unexpectedly improved as the increase in grain size.•The parabolic rate constant kp value of the coating pre-annealed at 700 °C for 100 h was as low as 1.7 × 10− 11 g2 cm− 4 s− 1.•The enhanced oxidation resistance of these coatings was attributed to the elimination of cracks and splat boundaries.In this study, Fe-based nanocrystalline coatings were obtained by crystallization of amorphous coatings through heat treatment with different annealing time of 0.5 h, 5 h, 10 h and 100 h, respectively. Cyclic oxidation behaviours of the Fe-based amorphous coating as well as these annealed ones at 750 °C in still air were investigated and compared. The results showed that the grain size of the annealed Fe-based coatings was significantly increased as the pre-annealing time prolonged. However, the cyclic oxidation kinetic curves indicated that the cyclic oxidation resistance of the annealed Fe-based coatings was unexpectedly improved as the increase in grain size. The coating, which was annealed at 700 °C for 100 h, exhibited the lowest accumulated weight gain of 2.8 mg/cm2 and parabolic rate constant kp values of 1.7 × 10− 11 g2 cm− 4 s− 1, indicating the highest oxidation resistance. The enhanced oxidation resistance of the annealed coatings was mainly attributed to the elimination of cracks, pores and splat boundaries caused by the intersplat sintering during heat treatment. Cracks and splat boundaries were the primary diffusion paths for the oxygen, resulted in the different diffusion models of oxygen or metal atoms in Fe-based nanocrystalline coatings.

•Effects of cavitation on cathodic and anodic reactions of the coating were different.•Corrosion resistance of WC–10Co–4Cr coating degraded with the cavitation time.•Cavitation–corrosion was controlled by cathodic reaction and mechanical impact.The effect of ultrasonic cavitation erosion on electrochemical corrosion behavior of high-velocity oxygen-fuel (HVOF) sprayed near-nanostructured WC–10Co–4Cr coating in 3.5 wt.% NaCl solution, was investigated using free corrosion potential, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in comparison with stainless steel 1Cr18Ni9Ti. The results showed that cavitation erosion strongly enhanced the cathodic current density, shifted the free corrosion potential in the anodic direction, and reduced the magnitude of impedance of the coating. The impedance of the coating decreased more slowly under cavitation conditions than that of the stainless steel 1Cr18Ni9Ti, suggesting that corrosion behavior of the coating was less affected by cavitation erosion than that of the stainless steel.

The cyclic oxidation and sulfates-induced hot corrosion behaviors of a Ni-43Cr-0.3Ti arc-sprayed coating at 550-750 °C were characterized and compared in this study. In general, all the oxidation and hot corrosion kinetic curves of the coating followed a parabolic law, i.e., the weight of the specimens showed a rapid growth initially and then reached the gradual state. However, the initial stage of the hot corrosion process was approximately two times longer than that of the oxidation process, indicating a longer preparation time required for the formation of a protective scale in the former process. At 650 °C, the parabolic rate constant for the hot corrosion was 7.2 × 10−12 g2/(cm4·s), approximately 1.7 times higher than that for the oxidation at the same temperature. The lower parabolic rate constant for the oxidation was mainly attributed to the formation of a protective oxide scale on the surface of corroded specimens, which was composed of a mixture of NiO, Cr2O3, and NiCr2O4. However, as the liquid molten salts emerged during the hot corrosion, these protective oxides would be dissolved and the coating was corrupted acceleratedly.

Taguchi method was used to optimize the parameters of the high velocity oxygen fuel (HVOF) spray process and obtain the high corrosion-resistant Fe-based coatings. Based on the signal-to-noise (S/N) ratio and the analysis of variance, the significance of spray parameters in determining the porosity of the coatings was found to be in the order of spray distance, oxygen flow, and kerosene flow. Thus, the optimal parameters for the porosity of the HVOF sprayed Fe-based coating were determined as 280 mm for the spray distance, 963 scfh for the oxygen flow, and 28 gph for the kerosene flow. The potentiodynamic polarization and EIS tests indicated that the Fe-based coating prepared with the optimal parameters exhibited a higher corrosion potential (Ecorr) of −196.14 mV, a lower corrosion current density (icorr) of 0.14 μA/cm2, and a higher coating resistance (Rc) of 2.26 × 106 Ω cm2 than those of the hard chromium coating in 3.5% sodium chloride solution. This superior corrosion resistance could be attributed to the dense structure with low porosity and partially amorphous phases of the Fe-based coatings.

A WC-10Co-4Cr coating was prepared by high-velocity oxygen-fuel (HVOF) thermal spraying process. The cavitation erosion (CE) characteristics of the coating as well as the stainless steel 1Cr18Ni9Ti were investigated in 3.5 wt% NaCl solution. The coating exhibited higher CE resistance than that of the stainless steel 1Cr18Ni9Ti. After being eroded for 20 h, the CE volume loss of the stainless steel 1Cr18Ni9Ti is 3.22 times to that of the coating. The removal mechanism for the coating was erosion of the binder phase first, followed by brittle detachment of hard phases as a result of the action of corrosion and mechanical effect. The cracks were found to initiate at the carbide-binder interface and the edge of the pores, leading to craters on the surface and accelerating the damage of the coating. Fatigue and plastic deformation were found to be the material removal mechanism for the substrate steel 1Cr18Ni9Ti.

The high-velocity oxygen-fuel (HVOF) spraying process was used to prepare WC–10Co–4Cr coating onto AISI 1045 steel substrate. The microstructure, hardness and dry sliding wear behavior of the coating were investigated and compared with cold work die steel Cr12MoV. The results showed that, the coating had low porosity, high microhardness, and homogeneous distribution of WC particles. The coating was composed of WC and W2C phases. With increase in load, the friction coefficients of the coating and the cold work die steel Cr12MoV decreased. The friction coefficients and wear mass losses of the coating were lower than that of the steel Cr12MoV. The dominant wear mechanism of the coating under both loads (30 and 50 N) was extrusion deformation and abrasion wear. For steel Cr12MoV, the dominant wear mechanism was plastic deformation and oxidation.

This review focuses on the recent development of iron (Fe)-based amorphous/nanocrystalline composite coatings, which have attracted much attention due to their attractive combination of high hardness/strength, elevated abrasive wear resistance, and enhanced corrosion resistance. Accompanying the advancements in various thermal spray technologies, industrial application fields of Fe-based amorphous/nanocrystalline composite coatings are becoming more diverse. In the main part, the typical empirical rules for the design of amorphous alloys with high glass-forming ability are generalized and discussed at first. Then various thermal spray technologies for the fabrication of Fe-based amorphous/nanocrystalline composite coatings, such as high velocity oxygen/air spray (HVOF/HVAF), air plasma spray (APS), low-pressure plasma spray (LPPS), high-energy plasma spray (HPS), and high velocity arc spray (HVAS) processes, are introduced. The microstructures, hardness, wear resistance, and corrosion resistance of Fe-based amorphous/nanocrystalline composite coatings formed using these thermal spray technologies are reviewed and compared. Finally, the existing challenges and future prospects are proposed.

•Spray parameters of nanostructured WC–Co–Cr powder are optimized by Taguchi method.•The important sequence of spray parameters on the coating hardness is suggested.•WC–Co–Cr (OSP) coating has a uniform microstructure with high microhardness.•WC–Co–Cr (OSP) coating exhibits higher wear resistance compared with steel Cr12MoV.In this paper, the Taguchi method was employed to optimize the spray parameters (spray distance, oxygen flow and kerosene flow) to achieve the highest hardness and, in turn, the best wear resistance of the high-velocity oxygen-fuel (HVOF) sprayed nanostructured WC–10Co–4Cr coating by investigating the correlation between the spray parameters and the hardness. The important sequence of spray parameters on the hardness of the coatings is kerosene flow > oxygen flow > spray distance, and the kerosene flow is the only significant factor. The optimal spray parameter (OSP) for the coating is obtained by optimizing hardness (330 mm for the spray distance, 2000 scfh for the oxygen flow and 6.0 gph for the kerosene flow). The coating deposited under the OSP with low porosity and high microhardness consists predominately of WC and a certain amount of W2C phases. The coating deposited under the OSP exhibits better wear resistance compared with the cold work die steel Cr12MoV. The material removal of the coating is the extrusion of the ductile Co–Cr matrix followed by the crack and the removal of the hard WC particles.

•A nanostructured WC–10Co–4Cr coating was fabricated by HVOF spraying.•The amorphous phase and nanoclusters were identified by HRTEM.•The coating had a uniform microstructure with low porosity and good thermostability.•The coating exhibited good corrosion behavior in NaCl solution.A nanostructured WC–10Co–4Cr coating was deposited on the substrate of AISI 1045 steel by means of high-velocity oxygen-fuel (HVOF) thermal spraying process. The detailed microstructures and phase composition of the coating were analyzed by using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), differential scanning calorimeter (DSC) and high-resolution transmission electron microscope (HRTEM). It was revealed that the amorphous phase, nanoclusters and carbides, including W2C and WC were present in the coating. The coating with low porosity of 0.85% had a dense structure and good thermostability. The temperature of the amorphous phase transforming into the nanocrystalline structure was 645 °C. Corrosion behavior of the coatings was investigated by electrochemical measurement. The results showed that the coating exhibited better corrosion resistance than the hard chromium coating in 3.5 wt.% NaCl solution.