Microstructure and Frictional Wear Behavior of Fe-Based Amorphous Coatings
During the last few decades,Fe-based amorphous alloys had attracted much interest owing to their high strength and hard-ness,excellent soft magnetic properties,superior wear and corrosion resistance,relatively low material cost,and other advantages that allow the application scope of these alloys to be substantially extended.However,Fe-based amorphous alloys were usually fabricat-ed in the form of ribbons and rods owing to the limitation of glass forming ability(GFA)and their intrinsic brittleness,which signifi-cantly limited the range of possible applications.To avoid these disadvantages,the route of spraying amorphous coatings had been rec-ognized as an attractive and effective way,and was extremely viable candidates as surface protective coatings for applications in ad-verse environmental conditions involving high wear and corrosion,such as in power stations,and ships.Although the previous works have demonstrated that Fe-based amorphous coatings show high hardness as well as good wear resistance at room temperature,the in-formation concerning the wear behavior and related wear mechanism of Fe-based amorphous coatings at relatively high temperature was relatively limited.Here,the Fe57Cr15Nb4B20Si4 amorphous coatings were designed and deposited by commercial high velocity oxygen fu-el(HVOF)and atmospheric plasma spraying(APS)technologies with varied parameters onto AISI 4032 aluminum alloy,respective-ly,denoted as A1,A2,A3,H1,H2,and H3 coating.X-ray diffraction(XRD)and differential scanning calorimeter(DSC)were ap-plied to clarify the phase structure and the amorphous nature.Scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and transmission electron microscopy(TEM)were employed to identify the morphology,microstructure,porosity,and ele-ments distribution of the as-sprayed coatings.Microhardness measurement and mechanical testing machine were adopted to detect the hardness and bonding strength of the coatings.The dry sliding wear tests were conducted using a ball-on-plate mode with the counter-part of GCr15 steel at room temperature,200 and 400 ℃,respectively.The worn surface after frictional wear test were studied to in-vestigate the wear mechanism.The results showed that this Fe-based alloy system had a high process robustness during spraying.All coatings deposited by both APS and HVOF processes exhibited dense and lamellar structure,and bonded well with substrates.The similar phase structure composition was found in all coatings,whereas the coatings prepared by APS process showed higher amorphous content compared to HVOF coatings,which was mainly related to the characteristics of APS process.On the one hand,higher plasma beam temperature and lower particle flight speed made the powder melt more completely;and on the other hand,higher particle tem-perature increased the temperature gradient of cooling solidification,which were conducive to improving the uniformity of the coating structure and obtaining high amorphous content.Few crystal diffraction peaks corresponding to M2B phase were observed in all coat-ings,indicating that the prepared coating exhibits certain crystallization phenomena.However,the content of crystalline phase was af-fected by the spraying parameters,in particular APS spraying.The crystal diffraction peak intensity in coating significantly decreased with increasing APS power and spraying distance,demonstrating that the amorphous phase content of the coating increased gradually.With respect to the coatings prepared by HVOF spraying,the three coatings displayed extremely low porosity and oxide content values which were hardly affected by the spraying parameters.The good adaptability of Fe-based amorphous powder to the HVOF spraying pro-cess was of great significance for its practical engineering applications.In contrast,the porosity and oxide content in the APS-sprayed coatings were relatively high,and obviously changed with the increase in APS power and spraying distance.This was because the plas-ma flame flow temperature of APS was higher but the particle flight speed was lower compared with the HVOF process,which caused that the spreading deformation ability of molten particles was weakened,and the in-flight time was too long,further declining the fill-ing capacity and increasing the oxidation probability of molten particles.The flighting speed and time were determined by the APS pa-rameters,thus the porosity of coating decreased while oxide content increased with the increase in APS power and spraying distance.In addition,the bonding strength and microhardness of HVOF-sprayed coatings were much higher than that of APS-sprayed coatings,reaching to 40.1 MPa and HV 1230.6,respectively.There were significant differences in the frictional wear behavior of Fe-based amor-phous coatings prepared by different spraying processes.The friction coefficients of H-2 coatings prepared by HVOF process were rela-tively stable,while A-3 coatings prepared by APS exhibited obviously unstable fluctuation characteristics,though the overall friction coefficient remained between 0.7 and 1.3.The increased temperature accelerating the tribological reaction,leaded to the friction coeffi-cient and wear rate of both HVOF-sprayed and APS-sprayed coatings increasing at 200 ℃ and decreasing at 400 ℃.At room tempera-ture,abrasive wear and fatigue wear dominated the wear mechanism,whereas oxidative wear prevailed in relatively high temperature.In comparison to APS-sprayed coating,HVOF-sprayed coating had relatively lower values of friction coefficient and wear rate at all temperatures,due to the higher dense structure and microhardness.The obtained results provided experimental bases for the prepara-tion,wear performance,and failure mechanism analysis of Fe-based amorphous coatings,and formed a basis for future works aiming to shed further light on the wear-resist protective coating under various harsh service conditions.
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