Microstructure And High Temperature Oxidation Properties Of Laser Cladding NiCoCrAlY Coating Under The Action Of High-current Pulsed Electron Beam

Aiming at the problems of high surface roughness of laser cladding NiCoCrAlY coating and uncontrolled growth of thermally grown oxide(TGO)during high temperature service,this paper uses laser cladding technology to fabricate dense microstructure,homogeneous and Matrix metallurgically bonded NiCoCrAlY high temperature protective coating.In order to further improve the high temperature protection characteristics of the coating,high current pulsed electron beam(HCPEB)technology was used to polish and purify the surface of the laser cladding layer.The microstructure and phase composition of NiCoCrAlY coating under 5,15 and 25 pulses were studied respectively.On this basis,the mechanical properties and high temperature oxidation resistance of the coating before and after pulsed electron beam were studied.The main research contents and results are as follows:(1)The evolution of the microstructure and phase composition of the NiCoCrAlY cladding layer before and after high-current pulsed electron beam irradiation was studied.According to the results,the NiCoCrAlY coatings irradiated with different pulse times included face-centered cubic?/?'phase and?-Ni Al phase.As the number of pulses increases,the?-Ni phase transforms into the?-Ni Al phase,resulting in an increase in the content of the?-Ni Al phase.Meanwhile,with the increase of irradiation times,the XRD diffraction peaks of?/?'phase with crystal plane indices of(111)and(200)gradually shifted to higher angles.After 5pulses of high-current pulsed electron beam irradiation,many bumps with a size of about 30-80?m appeared on the surface of the cladding layer.When the number of pulses was increased to 15,the pores were further filled with remelted particles,forming a denser surface than the initial bonding layer,when the number of pulses continues to increase to 25 pulses,the coating surface gradually becomes flat and smooth.The number of crater-like defects in 25 pulses is reduced by 53.23%compared with 5 pulses.The impurity content on the surface of the cladding layer is obviously reduced,and the purification effect of irradiation is more obvious.(2)The microhardness and friction and wear properties of the NiCoCrAlY cladding layer before and after high-current pulsed electron beam irradiation were studied.According to the results,the surface hardness of the coating after irradiation with different pulse times was higher than that of the original coating.After 5 pulses of irradiation,the surface microhardness of the coating is the largest,which is 216HV_0.2.The surface microhardness of the original NiCoCrAlY cladding layer is the smallest,which is 198 HV_0.2.After 5 pulses of irradiation,the microhardness of the cladding surface increases by 9%.The friction and wear coefficient and wear amount of the coating surface after high-current pulsed electron beam irradiation are lower than those of the original coating.The average friction coefficient of the original cladding layer is 1.04,and the average friction coefficient after 5 pulses of irradiation is 0.70,the average friction coefficient after 15 irradiations is 0.89,the average friction coefficient after 25irradiations is 0.85.Compared with the original coating,the friction coefficient of the coating decreased by 33.7%after 5 irradiations,and the wear volume after 5 irradiations decreased by66.9%compared with the original coating.(3)The high-temperature oxidation resistance of NiCoCrAlY cladding layers before and after high-current pulsed electron beam irradiation at 850°C was studied at 20h,100h,and 200h.According to the results,under the high temperature environment,the NiCoCrAlY cladding layer before and after irradiation formed a thermally grown oxide layer(TGO).The surface of the original coating in the early stage of oxidation formed loose and porous TGO with microcracks,and its composition and thickness were not uniform.Point-like internal oxidation appeared in the cladding layer,and spinel clusters appeared in the cladding layer in the middle stage of oxidation.In the later stage of oxidation,mixed oxides are mainly used,and accompanied by severe internal oxidation,large-scale cracking and deep peeling appear on the surface,the substrate in local areas is exposed,and the oxide film completely fails.The oxidation mechanism of the coating after HCPEB irradiation changes with the change of the number of pulses.The oxide film of the coating after 5 pulses of irradiation is mainly Al₂O₃ in the environment of 850?.After 15 and 25 pulsed electron beam irradiation,the protective oxide film of the coating is mainly Cr₂O₃,and the protective oxide film grows continuously with time.After 15 pulses of pulse irradiation,the oxidation weight gain of the coating is relatively stable throughout the oxidation cycle,and the surface of the cladding layer forms a fairly flat and continuous TGO,and the TGO mainly has a double-layer structure,including Cr₂O₃ in the surface layer and Al₂O₃ in the subsurface layer.With the progress of the oxidation reaction,the TGO layer gradually thickens and grows uniformly,which effectively prolongs the service life of the coating at high temperature.