Preparation And Performance Study Of Iron-Based Amorphous Coatings

Due to their high yield strength,fracture strength,excellent wear and corrosion resistance,and relatively low raw material costs,Fe-based amorphous alloys have received widespread attention.However,due to their poor toughness and size/thickness limitations in direct casting,these alloys cannot be widely used as bulk structural materials.Preparing Fe-based amorphous alloys as coatings through appropriate processes can not only solve the problems of size and brittleness,but also make use of their excellent wear and corrosion resistance,thereby expanding their application range.Currently,laser cladding and high-velocity oxygen-fuel spraying are the most mature processes for preparing iron-based amorphous coatings.However,these processes still have issues such as coating cracking and high porosity,which limit the application of Fe-based amorphous cladding and spraying.In this study,the Fe-Cr-Mo-C-B-Si amorphous system was used as the main research object,and Fe-based amorphous cladding layer and coating were prepared on the surface of Q235 steel substrate using laser cladding and high-velocity oxygen-fuel spraying processes.The effects of different processes and parameters on the amorphous phase content,microstructure,wear resistance,and corrosion resistance of the cladding layer and coating were studied through X-ray diffraction（XRD）,differential scanning calorimetry（DSC）,scanning electron microscopy（SEM）,dry sliding friction and wear,and electrochemical corrosion experiments.The key factors affecting cracking in the cladding layer and porosity in the coating were revealed,and improvement measures were proposed.The main research findings are as follows:（1）The Fe-based amorphous cladding layer mainly consists of amorphous phase,bcc-Fe,solid solution Fe-Cr,and hard phase Fe₃B.However,the coating itself has very low toughness,and there is a large difference in coefficient of thermal expansion between the coating and Q235 steel,resulting in significant thermal stress during laser cladding,which makes the coating prone to cracking.Increasing the laser cladding power and significantly increasing the scanning speed of the laser can reduce the temperature difference between the coating and the substrate,thereby reducing the thermal stress in the coating and minimizing the tendency of cracking in the coating;（2）In the laser cladding process,with the increase of energy density,the content of amorphous phase in the coating shows a trend of first increasing and then decreasing.When the energy density is 125 J/mm²,the coating with the highest content of amorphous phase（66.06%）is obtained.The coating has a high microhardness(with the highest microhardness values exceeding 1000 HV_0.5),and the microhardness,wear resistance,and corrosion resistance also increase with the increase of amorphous phase content in the coating.The main wear mechanisms of the coating are oxidative wear and abrasive wear,accompanied by slight delamination wear.The coating obtained at an energy density of 125 J/mm² has the highest self-corrosion potential（-363 m V）and the lowest self-corrosion current density(1.78×10^-7 A/cm²),indicating the best corrosion resistance;（3）In the high-velocity oxygen-fuel spraying process,as the oxygen-to-fuel ratio decreases,the amorphous phase content in the coating shows an initial increase followed by a decrease,while the porosity exhibits the opposite trend.With the increase in amorphous phase content and the decrease in porosity,the coating’s microhardness,wear resistance,and corrosion resistance are correspondingly improved.The predominant wear mechanisms of the coating are oxidation wear and abrasive wear.The coating with an oxygen-to-fuel ratio of 2.07 exhibits the highest self-corrosion potential（-603 m V）and the lowest self-corrosion current density(2.83×10^-5 A/cm²),indicating the best corrosion resistance;（4）The corrosion resistance of Fe-based amorphous coatings prepared by both processes is superior to that of Q235 steel substrate.After electrochemical corrosion experiments,only a few localized black corrosion phenomena were observed on the surface of laser cladding coatings,while extensive black corrosion was observed on the surface of HVOF coatings,with most of them distributed around the pores.This indicates that the corrosion resistance of the coatings is jointly influenced by the content of amorphous phase and porosity,with porosity having a greater impact.The presence of pores forms channels for Cl^-to infiltrate into the coating,increasing the amount of Cl^-infiltration and the degree of corrosion of the coating.