Microstructure And Friction Wear Properties Of Laser Cladding NiCoCrAlY Coating

Titanium alloys have been widely used in aerospace,medical and military fields due to their excellent physical characteristics such as low density,high strength and good biocompatibility.But titanium alloy has the disadvantages of poor wear resistance,low hardness and poor high temperature stability,which seriously limits the application of titanium alloy in the field of high temperature wear.MCr Al Y（M is Ni,Co or mixed of them）coating has excellent oxidation resistance and flexible addition of elements,which is one of the effective high temperature coating systems of titanium alloy.The Ni Co based coating combines the advantages of Ni based coating and Co based coating,which has excellent high temperature performance.Therefore,NiCoCrAlY powders were selected as cladding material.And the NiCoCrAlY coatings were prepared by laser cladding on TC4 surface with the side axis synchronous powder feeding.The performance of coatings under different laser power were compared,and the laser parameters were further optimized.The effects of nano CeO₂and B₄C with different mass fraction on the wear resistance of NiCoCrAlY coating at high temperature were studied.The SEM,XRD,EDS,microhardness tester,high temperature friction tester and ultra depth of field three-dimensional microscope were used to analyze the microstructure and mechanical properties of coatings surface and wear tracks.The effects of different laser power,powder types and powder mass fraction on the high temperature wear resistance of coatings were analyzed,and the excellent high temperature wear resistance coating was prepared.The NiCoCrAlY coating on TC4 surface was analyzed,found that the surface of NiCoCrAlY coating was smooth.When the laser power is increased to 1400 W,the cracks on the coating-section disappear.When the laser power is increased from 1000 to 1400 W,the friction coefficient of the coating keeps about 0.35,but the wear of the coating first decreases and then increases.In the high temperature wear stage,the wear rate of NiCoCrAlY coating is 27-87 times lower than that of TC4.The addition of Nano-CeO₂ has little effect on the phase composition of NiCoCrAlY coating.The coating consists of Ni Ti₂,Ti₃O,Co O andα-Ti.Nano-CeO₂can improve the nucleation rate of molten pool and refine the grain.When the mass fraction of CeO₂is 2%,the number of dendrites on the coating surface increases.When the mass fraction of CeO₂is 6%,the grain on the coating surface is refined and the hardness is the highest,reaching HV_0.51033.In the high temperature wear stage,the wear rate of the coating decreases by 2.23%,26.9%and 41.6%with the increase of CeO₂mass fraction.The addition of CeO₂can increase the friction coefficient of coatings,which is more than 0.4.Adding B₄C to NiCoCrAlY coating can produce Ti C、Ti B、Ti B₂and Al B₂ceramic phases.With the increase of B₄C mass fraction,the dendrite on the coating surface disappears and black block ceramic phase is formed.The microhardness of coating surface increases with the increase of B₄C mass fraction.When the B₄C mass fraction reaches 15%,the microhardness of coating surface reaches HV_0.51213.The average friction coefficients of coatings with 5,10 and 15%B₄C are 0.3089,0.2931 and 0.2594,respectively.In the high temperature wear stage,the wear rate of coatings containing 5%,10%and 15%B₄C is 65.97%,68.59%and 79.75%,respectively,which are lower than that of coatings without B₄C,respectively.This is because the ceramic phase in the coatings acts as wear-resistant material.However,the fatigue pitting of coating with B₄C during high temperature friction and wear is mainly caused by the ceramic phase falling off.