Study On Microstructure And Properties Of Al-ceramics Composite Coatings Prepared By Supersonic Plasma Spraying

Magnesium alloy has the advantages of high specific strength,strong thermal and electrical conductivity,shock absorption performance and good processing performance,and has a large number of applications in aerospace,transportation,national defense industry,electronic communications and other fields.However,the chemical properties of magnesium alloy are active,the oxide film is loose and porous,the corrosion resistance is poor,and the absolute strength of magnesium alloy is not high,and the wear resistance is poor,which limits its wider application in the industrial field.In order to improve the wear resistance and corrosion resistance of magnesium alloy surface,three aluminum-ceramic composite coatings were prepared on the surface of AZ61 magnesium alloy by supersonic plasma spraying technology:Al-Al₂O₃（AA）,Al-Al₂O₃-YSZ（AAZ）,and Al-Al₂O₃-Cr₂O₃（AAC）.The surface and cross-sectional morphology of each coating were observed,the phase composition and microhardness of the coating were analyzed,and the sliding friction and wear properties and corrosion resistance of the coating were mainly studied.The surfaces of AA coatings present a wave-like morphology formed by droplet spreading and stacking,and the cross-sectional morphology of the coating has a layered structure,the sheet size is large,and the microstructure uniformity is poor.There are obvious pores and a small number of cracks in the coatings,and the porosity of AA70coating is high,reaching 6.71%.The microhardness of the coatings increased with the increase of Al₂O₃ content,and the AA70 coating reached 1.32GPa.The wear resistance of the coating was higher than that of AZ61 magnesium alloy,and the wear resistance of AA70 coating was increased by 62.22%compared with the magnesium alloy substrate.In electrochemical tests,coatings have lower dissolution activity and corrosion rate than magnesium alloy substrate,showing better electrochemical resistance.In the salt spray test,a large number of corrosion products and cracks are produced on the surface of AZ61magnesium alloy,and the corrosion gain is higher than those of the coatings.AA30 and AA50 coatings have a large number of cracks in the coating due to the accumulation of corrosion products,and galvanic corrosion occurs at the interface between the coating and the substrate.Eventually,the coatings peel off.AA70 coating has better corrosion resistance.Compared with AA coatings,the cross-sectional structure of AAZ coatings has been refined,the three-phase microstructure is more uniformly mixed,the size of Al₂O₃ phase in the microstructure is refined,the appearance of pores is reduced,the YSZ phase is dispersed in bands and spherical particles,and the overall porositys of AAZ coatings is about 3%.The microhardness of the coating increases with the increase of ceramic phase content,and the AAZ70 coating reaches 2.07GPa.The sliding friction wear test results show that the increase of hardness of AAZ coatings reduces the degree of adhesive wear of the coating.The wear resistance of AAZ50 coating is lower than that of AAZ30 and AAZ70 coatings,which may be due to the porous structure of the coating reducing the bonding strength of the metal Al phase and ceramic particles,and the exposed ceramic particles finally peel off in the wear marks after the Al phase is gradually worn.In the salt spray test,the cracks of AAZ coatings are less,no peeling phenomenon occurs,the weight gains of the coatings are less than those of AA coatings,about 50%of AZ61magnesium alloy,the corrosion degree of the coatings is related to the porosity,the porosity of AAZ30 coating is low,the corrosion products on the surface are less,and the higher porositys of AAZ50 and AAZ70 coatings aggravate the corrosion inside the coatings.The spray sizes on the surface of AAC coatings are smaller than those of AA coatings and AAZ coatings,and the melting quality of the powder is higher.The cross-sectional structure of the coating corresponds to the surface morphology,the powder has a better melting state and flattening after impact,which increases the contact point and fusion area between the droplets,the interlayer bond is tight,the structure is uniform and fine,Cr₂O₃ is distributed in the coatings in strips and sub-micron punctate dispersion,and the porosity of the coating is about 1%.The hardness of the coatings increases with the increase of ceramic content,and the AAC coatings are higher those that of AAZ coatings and AA coatings,and AAC70 coating reaches 2.36GPa.AAC coatings uniform and fine structure,easy to form a continuous friction film in friction wear,reduce friction resistance,and high hardness makes the coating show the best wear resistance.In the salt spray test,the corrosion degree of AAC30 coating was the smallest,and the corrosion mainly occurred on the surface of the coating,and no cracks and other defects were found in the substrate,indicating that AAC coatings had a good protective effect on the magnesium alloy substrate.