The Synthesis?Micro-characterization And Properties Of CoCrFeNi(Cu,W,W_0.5Mo_0.5,Mo,WC) High Entropy Alloy Coatings

The surface modification of materials has been a research focus of material preparation and property development due to the fact that it can not only recycle the application of substrate material,but also improve the surface properties.It is also in accordance with the development perception of “green economy and cyclic development”.High entropy alloys?HEAs?not only possess unique structure,but also exhibit excellent performance,which can be used as coating material.Besides,it can realize surface modification and open up new channels of HEA powder applications.In this thesis,Co?Cr?Fe?Ni elements are selected as the major research object,and CoCrFeNi(Cu,W,W_0.5Mo_0.5,Mo,WC)HEA powders are fabricated by mechanical alloying?MA?process through adding W,Mo,W_0.5Mo_0.5 and WC.The CoCrFeNi based high-entropy alloy coatings?HEACs?are prepared by vacuum hot pressing sintering?VHPS?technique on Q235 steel substrate.The influence of elements or compounds addition and milling time on microstructure evolution of CoCrFeNi-based HEA powder has been studied.Moreover,the microstructure,microhardness,wear resistance and corrosion resistance in 3.5% NaCl solution of HEACs have been also systematically investigated.The purpose of this work is to obtain HEACs with comprehensive properties,and provide theoretical and practical basis for the extensive application of HEACs in the practical engineering.The detailed results are shown as below.1.Investigation of CoCrFeNi and CoCrFeNiCu HEA powders and their HEACs: Face-centered cubic?FCC?solid solution phase appears in both HEA powders after 200 h of MA,while Cu addition can induce coarsen grain size and enhance the melting point obviously.Coatings are dense without any other defects,such as porosity and present continuous transition boundary between the coating and substrate.CoCrFe Ni coating still remains one single FCC solid solution phase with uniformly distributed principal composition.In addition to FCC phase?matrix?,Cu-rich FCC solid solution is observed afte Cu addition.The microhardness coatings is significantly higher than that of the substrate?160 HV?,and CoCrFeNi coating possesses higher microhardness?450 HV?than that with Cu addition.Besides,friction coefficients of both coatings are much lower than that of substrate,showing the obviously improved wear resistance.Moreover,wear resistance of CoCrFeNi coating is much better than that with Cu addition.Corrosion tests in 3.5% NaCl solution show that both coatings have better global corrosion resistance than substrate as reflected by the lower corrosion current density and more positive corrosion potential as well as the wider passive region.Among both the HEACs,CoCrFeNi coating also has the best global corrosion resistance with the lowest corrosion rate.2.Investigation of CoCrFeNiW?CoCrFeNiW_0.5Mo_0.5 and CoCrFeNiMo HEA powders and their HEACs: The coexistence of FCC and body-centered cubic?BCC?solid solution phases exhibits for these three HEA powders after 60?80 and 100 h of milling time,respectively.Prolonging milling time to 200 h,all the phases still remain stable.W addition induces the short formation time of FCC solid solution phase and obvious grain refinement,and leads to the enhanced melting point.CoCrFeNiMoxW1-x HEACs are compact without any other defects,such as porosity.It presents the continuous transition boundary between the coating and substrate.Compared with coating powders,phase transformations of VHPS-coatings are found,and two FCC solid solution phases,minor NiW-type and ?-CoCr intermetallic compounds occur.The microhardness the tested coatings are significantly higher than that of the substrate,and CoCrFeNiW possesses the betst microhardness of 669 HV.The coating friction coefficients are much lower than that of the substrate,showing the improved wear resistance,especially the CoCrFeNiW coating reveals the best wear resistance.Electrochemical corrosion in 3.5% NaCl solution shows that coatings have better global corrosion resistance than substrate,moreover,Mo addition plays the important role in enhancement of corrosion resistance.3.Investigation of CoCrFeNi?WC?HEA powders and their HEACs: Single FCC solid solution and WC phases coexist in CoCrFeNi HEA powders with different percentage of WC?10 wt.%?30 wt.%?after MA time of 200 h.HEACs are compact without any other defects,such as porosity,and they still reveal FCC solid solution phase and WC phases.Microhardness of CoCrFeNi HEACs with 10 wt.% and 30 wt.% WC addition is significantly higher than that of the substrate,reaching 475 and 531 HV,respectively.Wear resistance of both coatings are much better than that of the substrate.Besides,for the HEAC with 30 wt.% of WC,friction coefficient declines remarkably,indicating the enhanced wear resistance.It shows that coatings have better global corrosion resistance than substrate in 3.5% NaCl solution during electrochemical corrosion process.HEAC with 30 wt.% of WC presents declined global corrosion resistance,which is mainly related to the decrease of the contents for Co,Cr and Ni elements.4.Relevance between microstructures and properties: By comprehensive analysis of CoCrFeNi(Cu,W,W_0.5Mo_0.5,Mo,WC)coatings in this thesis,it is summaried that the CoCrFeNiW coating possesses the higher microhardness and greatly enahanced wear resistance as well as the corrosion resistance.It is related to the higher contents of NiW-type and ?-CoCr intermetallic compounds as well as the precipitated phase with uniform size and regular morphology.Besides,the CoCrFeNi coating exhibits good global corrosion resistance in 3.5% NaCl solution,which is due to the higher contents of good corrosion resistance element of Cr and the existence of single phase structure in sintered product.