Study On Structure And Properties Of Fe-based Amorphous Wear Resistant And Anticorrosive Coatings

Amorphous materials have attracted much attention due to their excellent properties in wear and corrosion resistance fields,and been widely used materials.However,the preparation of large-size and complex-shaped amorphous alloys are greatly limited at present.The existing amorphous alloy preparation technology has the problems of harsh conditions,high cost and unsuitability for large-scale production,etc.Therefore,the development of an advanced amorphous alloy preparation technology becomes the current research focus.Thermal spraying technology to prepare amorphous powder into coating is an effective method to solve the above problems.This technology can not only give full play to the excellent characteristics of amorphous materials,but also reduce production costs and increase economic benefits.In this paper,commercial Fe-Cr-Mo-C-B-P amorphous powder was used as raw material to prepare Fe-based amorphous coating by atmospheric plasma spraying technology.The microstructure,wear resistance and corrosion resistance of the coating and the influence of heat treatment on the microstructure and properties of the coating were systematically studied.The coatings with different amorphous contents were obtained by vacuum heat treatment.The phase composition and microstructure of the coatings were characterized by XRD,SEM and TEM.The crystallization mechanism of the coating was studied by differential scanning calorimetry（DSC）.The wear mechanism and corrosion behavior of the coatings were analyzed by ring-disk friction and electrochemical corrosion methods.The specific conclusions are as follows:（1）Solid powder with spherical or nearly spherical powder was used as the spraying rawmaterial,with good fluidity and the particle size distribution was approximately normaldistribution and D₅₀=25μm.The deposited Fe-based amorphous coating was tightlybound to the substrate,presented a typical layer-to-layer overlapping lamellar structurewith the thickness about 300μm.The coating had compact structure,low porosity,vickers hardness of HV_0.3about 7.54GPa,the coating was mainly amorphous phase andamorphous content about 89%.（2）The as-sprayed Fe-based amorphous coating after heat-treated for 2h at 773K,973K and1173K under the protection of argon gas,The phase structure and properties of thecoating was changed.With the increase of thermal temperature,the coating undergoescrystallization and solid solution,and the higher the temperature,the greater degree ofcrystallization and solid solution.During this process,the precipitation and dispersionstrengthening of nanocrystals,accompanied with the growth of nanocrystals,filled inthe original pores of the coating,porosity decrease and hardness first decrease and thenincrease.（3）The crystallization law of Fe-based amorphous coating was that the coating has twoprocesses of crystallization and solid solution at the heating stage.With the increase oftemperature,the degree of crystallization and solid solution both increase.The phases ofthe coating are mainly crystalline phase(Cr_15.58Fe_7.42C₆)and solid solution phase(（Cr,Mo）_15.58Fe_7.42C₆).The local activation energy decreased with the increase ofcrystallization volume fraction during the whole crystallization process,while the localAvrami index showed a trend of increased first and then decreased.Crystallization wasdifficult to dissolve in solid solution and nucleation was difficult to grow,and thetransition from three-dimensional growth phase nucleation to surface nucleation.（4）The worn failure mechanism of Fe-based amorphous coating was that plasticdeformation occurred on the coating surface first under thermal coupling,and theformed deformation layer could slow down the stress concentration phenomenon of thecoating.Subsequently,the single layer fell off during the friction process,and thecoating showed abrasive wear.The wear amounts of the four coatings before and afterheat treatment are listed as follows:HT1<HT0<HT2<HT3,The order of surfaceroughness changes of wear marks before and after wear:HT0<HT1<HT3<HT2,The stability order of friction coefficient:HT3<HT2<HT0<HT1,which indicatesthat the sprayed and 773k heat treated coatings have better wear resistance,i.e.theamorphous coatings had better wear resistance than the crystalline coatings.（5）In 3.5wt%Na Cl solution,compared with the conventional Fe-based crystallizedcoating,the Fe-based amorphous coating had significantly smaller corrosion current andcorrosion hole size,larger polarization resistance and better corrosion resistance.Thiswas mainly related to the passive films formed on the surfaces of the two coatings.Thedestruction of the passive films on the surfaces of the two coatings was mainly due to local pitting corrosion.However,amorphous coating with extremely short linear regions are more prone to passivation and at the same time had stronger repair ability for passive films,thus having higher pitting corrosion resistance.