Preparation Of High-entropy Alloy Coatings By APS And Study On Their Microstructure And Properties

As a kind of promising material,high-entropy alloys?HEAs?attracted more and more attentions in recent decades.With the advantages of high strength,high hardness,good wear resistance,good thermal stability,good corrosion and oxidation resistances,HEAs can be used as tools,moulds and so on.Until now,some advanced methods such as vacuum melting,laser cladding and magnetron sputtering have been used to prepare HEA bulk materials and coatings.However,the application of these processing methods was restricted because of the waste of materials and high cost.Based on the advantages of concentration of energy,high deposition efficiency and low cost,atmospheric plasma spraying?APS?became an ideal technology to prepare HEA coatings.Besides,as a novel method,pulsed electric field assisted APS can increase the density and the properties of the coatings by the in-flight particle refinement.Therefore,in this study,AlCoCrFeNiTi and AlCoCrFeNiSi feedstock powders were prepared by mechanical alloying,and the powder characteristics such as phase structure,microstructure,surface morphology,particle size distribution and microhardness were investigated.APS was employed to deposit AlCoCrFeNiTi,AlCoCrFeNiTi/NiCrBSi and AlCoCrFeNiSi coatings on 316 stainless steel substrates using mechanically alloyed powders as feedstocks.And the microstructure and mechanical properties of the coatings were investigated.The microstructure of both the feedstock powder and the as-sprayed coating were analyzed.The bonding strength and the microhardness of the as-sprayed coating were tested,and the wear behaviour of the coatings at 25,500,700 and 900 oC was investigated.Besides,in-flight particle characteristics such as temperature and velocity during pulsed electric field assisted APS were measured.Results show that with the prolonging of the milling time,the particle size first increased and then decreased.After milling for 30 h,the AlCoCrFeNiTi powder was composed of BCC phase with a particle size range of 20-60 ?m.For the AlCoCrFeNiSi powder,besides simple BCC and FCC phases,Si element can still be detected,and it exhibited a particle size range of 1-30 ?m.AlCoCrFeNiTi and AlCoCrFeNiSi HEA coatings were mainly composed of matrix BCC phase,minor FCC phase and ordered BCC phase.And the grain size of the coatings was 42.3 and 30.1 nm.The average microhardness of the as-sprayed AlCoCrFeNiTi and AlCoCrFeNiSi HEA coatings was 642 HV and 611 HV,respectively,and they showed good wear resistance.At the wear temperature of 25 oC,the volume wear rates were 0.77±0.01×10^-4 and 0.38±0.08×10^-4mm³·N^-1·m^-1,respectively.And as the wear temperature increased to 700 oC,the volume wear rate decreased to 0.23±0.02×10^-4 and 0.29±0.06×10^-4mm³·N^-1·m^-1.Besides,due to the addition of NiCrBSi,the AlCoCrFeNiTi/NiCrBSi composite coating showed a much higher microhardness of 676 HV,and the composite coating exhibited better wear resistance.During the pulsed electric field assisted APS and the normal APS,with the increase of the spray distance,the particle temperature first increased and then decreased.And the descent velocity of the particle temperature was lower in pulsed electric field assisted APS.Besides,in the spray distance range from 80 to 100 mm,the in-flight particle velocity decreased with the increase of the spray distance,and the velocity in pulsed electric field assisted APS was higher than that in normal APS.An HEA coating with high density and high properties may be prepared by pulsed electric field assisted APS.