Study On Microstructure And Properties Of CoCrFeNi High-entropy Alloy-based Coatings Prepared By APS

In this study,Cu,Si,and TiC reinforcing phase were added into the CoCrFeNi alloy system.CoCrCuFeNi,CoCrCuFeNiSi and CoCrCuFeNiSi/TiC feedstock powders and coatings were prepared by mechanical alloying and plasma spraying,respectively.The effects of Si element and TiC reinforcing phase on the mechanical alloying process and microstructure,microhardness,wear properties and high-temperature oxidation resistance of the coatings were studied.In addition,the effect of ultrasonic impact treatment on the microstructure and mechanical properties of CoCrCuFeNi coating with good plasticity was also explored.The results show that in the process of mechanical alloying,with the increase of milling time,the particle size of CoCrCuFeNi,CoCrCuFeNiSi and CoCrCuFeNiSi/TiC powders increases first and then decreases,and the particle size tends to be uniform;The internal structure of the powder is gradually fine and the element distribution is gradually uniform;Compared with CoCrCuFeNiSi powder,the alloying process of CoCrCuFeNiSi/TiC powder is accelerated due to the effect of ball milling medium of TiC.Feedstock powders with good fluidity were prepared by screening the ball-milled powders.CoCrCuFeNi powder is composed of BCC and FCC phases;CoCrCuFeNiSi powder is composed of BCC,FCC and a small amount of Si;CoCrCuFeNiSi/TiC composite powder is composed of BCC,FCC and TiC phases.CoCrCuFeNi,CoCrCuFeNiSi and CoCrCuFeNiSi/TiC high-entropy alloy-based coatings with uniform thickness,dense structure and good mechanical bonding were deposited by plasma spraying.Metal oxides are found at the interface between particles in CoCrCuFeNi coating due to the atmospheric environment of plasma spraying;The higher oxygen affinity of Si element significantly reduces the metal oxides at the interface between CoCrCuFeNiSi coating particles;The distribution of TiC ceramic particles in CoCrCuFeNiSi/TiC composite coating is uniform.CoCrCuFeNi high-entropy alloy coating is mainly composed of FCC1 and FCC2.The addition of Si promotes the formation of BCC phase in CoCrCuFeNiSi coating,and CoCrCuFeNiSi/TiC composite coating is composed of BCC,FCC and TiC phases.The average microhardness of the as-sprayed CoCrCuFeNi coating is 309HV0.2.Due to the formation of BCC phase,the average microhardness of CoCrCuFeNiSi coating is improved（321HV0.2）.With the addition of TiC ceramic particles,the average microhardness of CoCrCuFeNiSi/TiC composite coating is increased to 493HV0.2.The volume wear rates of CoCrCuFeNi coating at room temperature and 700^oC are0.46×10^-4mm³·N^-1·m^-1 and 0.27×10^-4mm³·N^-1·m^-1,respectively,which are far lower than 316stainless steel matrix material under the same wear conditions.Due to the formation of BCC phase in CoCrCuFeNiSi coating,its room temperature wear resistance is improved,and the volume wear rate is reduced to 0.35×10^-4 mm³·N^-1·m^-1.The room temperature volume wear rate of CoCrCuFeNiSi/TiC composite coating is 0.22×10^-4 mm³·N^-1·m^-1,and compared with CoCrCuFeNiSi coating,the wear resistance is significantly improved.In addition,the wear resistance of the three coatings in this study is better than the traditional Ni Cr BSi coating,which is expected to become a new generation of high-entropy alloy-based wear-resistant coating.The high-temperature oxidation test of CoCrCuFeNiSi/TiC composite coating at 1000^oC shows that the phase on the surface of the coating is mainly spinel composite oxide in the oxidation process of 10-100 h.With the increase of oxidation time,spinel grains grow and finally form a dense oxide film on the surface of the coating.The oxidation weight gain curve of the coating shows a parabolic law,K_p is 3.06 mg²·cm^-4·h^-1,while the 316 stainless steel matrix material shows a linear law,K_p=4.48 mg·cm^-2·h^-1,indicating that the coating has good oxidation resistance at 1000^oC and can effectively protect the matrix.After ultrasonic impact treatment,the surface roughness of CoCrCuFeNi coating decreased first and then increased.With the decrease of coating thickness,the internal pores are significantly reduced and the density is greatly improved.With the increase of impact strength,the grain size decreases and the lattice strain increases significantly.After ultrasonic impact treatment,the average microhardness of the coating reaches 397HV0.2.In addition,compared with the as-sprayed coating,the room temperature volume wear rate of the impact coating is reduced to 0.18×10^-4mm³·N^-1·m^-1,and the wear resistance is further improved.