Microstructure And Properties Investigation Of Mo₂FeB₂ Ternary Boride Coating Deposited By Laser Cladding

With the continuous transformation of the international manufacturing industry,the process of manufacturing and service life improvement of manufactured components are facing severe challenges.The high-performance components used in upscale and extreme equipment manufacturing are widely affected.Hence,this study selected 45 steel as the substrate because of its widely application in aerospace,military,medical,shaft,gear,and other fields.The Mo₂FeB₂ ternary boride coating on the 45 steel substrate was prepared via laser cladding.The preparation of Mo₂FeB₂ coating provides the guidance for repairing and remanufacturing of high-performance components in the upscale and extreme equipment manufacturing industry.The results reveal the relationship between the laser cladding parameters and the forming quality of Mo₂FeB₂ coating,and also reveals the correlation between the materials and the properties.This research significantly impacts the theoretical and application for improving the surface modification,repair,and remanufacturing of ternary borides.To investigate the influence of Mo₂FeB₂ powder sizes and laser cladding parameters on the forming quality and coating properties.First,results of the coatings with different powder sizes showed that the coating obtained with fine powder had excellent forming quality.The microstructure agglomeration phenomenon was increased with the increase of powder size.The coating obtained with～6μm powder size had the optimal microhardness and fracture toughness.Then,to provide the guidance for optimization of parameters.An empirical model between the parameters and forming quality of coating was established.The laser power positively affected the coating width and dilution rate.The scanning speed had a negligible effect on the height and dilution rate.The pre-placed thickness had a positive influence on the height and width.The results of sensitivity analysis showed that the sensitivity of height to pre-placed thickness was the highest;the sensitivity of width to laser power was the highest;and the sensitivity of dilution rate to laser power and pre-placed thickness had significant impacts.Finally,the results of laser densities showed that the coating obtained with the laser energy density of 90 J.mm^-2and175 J.mm^-2 had the optimal microhardness and fracture toughness,respectively.The optimal processing parameters were laser power 2100 W,scanning speed 3 mm/s and pre-placed thickness 1 mm.To enhance the microhardness and wear resistance of Mo₂FeB₂ coating,the in-situ synthesized carbides reinforced Mo₂FeB₂ coating was fabricated.Mo₂FeB₂/WC composite coating had optimal morphology,and the eutectic structure was formed in the Mo₂FeB₂/WC composite coating.The residual stress in the coating,heat affected zone（HAZ）,and substrate were all tensile stress in the Mo₂FeB₂/Nb C,Mo₂FeB₂/WC,and Mo₂FeB₂/Ta C composite coating.The microhardness,residual stress,coefficient of friction（COF）,and wear rate were 1543.6 HV_0.5,298 MPa,0.38,3.90×10^-5 mm³/Nm,respectively.The wear mechanism was typical adhesive and abrasive wear.Cracks were observed in the composite coatings when the in-situ synthesized WC content was 1 wt.%and 9 wt.%.With the increase of in-situ synthesized WC content,the eutectic structure changed from lath-like to lamellar and dendritic.The composite coating presented the best comprehensive properties when the in-situ synthesized WC content was 7 wt.%,when the microhardness,residual stress,COF,and wear rate were 1593.1 HV_0.5,285MPa,0.32,3.42×10^-5 mm³/Nm,respectively.The results implied that an appropriate in-situ synthesized WC content effectively improved the comprehensive properties of Mo₂FeB₂ coating,which was conducive to improving the service life.To improve the toughness of Mo₂FeB₂/WC composite coating,the effect of La₂O₃contents on the toughness and tribological properties of Mo₂FeB₂/WC composite coating were investigated.The results showed that La₂O₃ had no significant effect on the phases of the composite coatings.The microstructure was initially refined and then coarsened with an increase of La₂O₃content,La was found to exist at the Mo₂FeB₂ phase grain boundaries.The microhardness of 0.3 wt.%La₂O₃ coating showed a decrease of only5.7%compared to that of Mo₂FeB₂/WC composite coating,while the fracture toughness showed a increase of 3.8%.The COF,wear rate,of the 0.3 wt.%La₂O₃ coating were 0.31,3.13×10^-5 mm³/Nm,respectively.The crack resistance reached the highest in the Mo₂FeB₂/WC/0.3 wt.%La₂O₃ coating.The wear mechanism of the composite coatings containing La₂O₃ was typical abrasive wear.The composite coating exhibited an evident brittle fracture morphology,and the deformation resistance increased in La₂O₃ added coating.Therefore,the toughness of Mo₂FeB₂ coating can be improved by adding appropriate rare earth.To fabricate the anti-friction ternary boride coating,the anti-friction effect of self-lubricating materials on the Mo₂FeB₂/WC composite coating was discussed.The results indicated that the addition of WS₂,MoS_2,and h-BN increased the bonding properties of the coating.The Mo₂FeB₂/WC/h-BN coating exhibited a fine dendritic structure.Compared with the Mo₂FeB₂/WC coating,the microhardness,COF,and wear rate of the Mo₂FeB₂/WC/WS₂,Mo₂FeB₂/WC/MoS_2,and Mo₂FeB₂/WC/h-BN composite coatings were decreased by 15.44,13.34,11.06%;9.09,15.15,30.30%;10.80,19.03,30.97%,respectively.The wear mechanisms of the self-lubricating composite coating were abrasive and oxidative wear.The addition of WS₂,MoS_2,and h-BN had an anti-wear effect on the Mo₂FeB₂ coating,and the effect resulted from h-BN was optimal.The microhardness of the coatings decreased with an increase in h-BN content.Also,by increasing the heat treatment temperature,the microhardness was initially increased and then decreased.The best wear properties was obtained in the composite coating containing 10 wt.%h-BN;meanwhile,the heat treatment promoted the distribution of elements.The coating heat-treated at 600℃showed optimal comprehensive properties:the microhardness and wear rate were 1416.8 HV_0.5,1.84×10^-5 mm³/Nm,respectively.The wear mechanism was typical abrasive wear.The anti-friction effect can be realized by preparing self-lubricating composite coatings,enhancing the service life of wear-resistant components.