Effect Of La₂O₃ On Microstructure And Elevated Temperature Performance Of Nickel Based Alloy Coatings Deposited By Plasma Spray Welding

The sprayed coatings adding different La₂O₃ in Ni35,Ni40 and nickel-coated aluminum were prepared on the surface of 20 steel by plasma spray welding.The coatings microstructure and phase constitution were studied by optical microscopy,scanning electron microscopy equipped with EDS and X-ray diffraction.Micro-hardness tester,thermal shock test and wear test at elevated temperature were used to test the properties of sprayed coatings.The results showed that the Ni35 and Ni40 sprayed coatings were both composed of dendritic proeutectic solid solution γ-Ni（Fe,Cr）and several forms of eutectics similar to petals.The eutectic compounds mainly consisted of Cr23C6 and Cr7C3.Some acicular brittle carbides were found in the interface between sprayed coating and substrate.The contents of eutectics and compounds in Ni40 sprayed coating showed much higher than Ni35 coatings,and the eutectic compounds tended irregularly polygon shape.The hardness of sprayed Ni40 coating stayed at HV426-HV475,while that of Ni35 coating adding0.5%La₂O₃ stayed at HV370 to HV401.Both of the sprayed coatings hardness could satisfy the need of industry.In thermal shock test,the results showed Ni40 sprayed coatings has the same thermos-resistance with Ni35 sprayed coatings.Both of them have better thermos-resistance in 600 ℃.Because the addition of La₂O₃ could refine and homogenize the eutectic compounds with eliminating the elongation needle-like compound and weakening the direction of dendrites,the thermos-resistance temperature in Ni35 sprayed coating adding 0.5%La₂O₃ improved to 800 ℃.Adding 0.5%La₂O₃ in Ni40 sprayed coating improved the hardness of work layer to HV503-HV551.However,it did not reinforce the thermo-resistance in Ni40 sprayed coatings.This maybe relate to the high contents of hard-brittle phase in sprayed coatings.And the higher hardness of sprayed Ni40 coatings decreased the effect of rare earth oxide on refining,homogenization and toughening.The microstructures of nickel-coated aluminum plasma sprayed layer were dendrites.And the dendrites were some lath α-（Fe,Ni）martensite groups.Besides,the intergranular reticular phase was γ-（Fe,Ni）,which made the nickel-coated aluminum plasma spraylayer still maintain good resistance of thermal shock at 750 ℃.The addition of 0.5% La₂O₃ improved the hardness of the nickel-coated aluminum plasma sprayed layer from HV316-HV338 to HV375-HV400 but did not have obvious effect on the resistance of thermal shock,due to the decrease of γ-（Fe,Ni）with better toughness and the increase of lath martensite with strong hardness,which counteract the effect of refining and homogenization on coatings improvement in toughness and resistance of thermal shock.The results of friction wear test of coats at 500 ℃ showed the spray layer of 0.5%La₂O₃/Ni40 had better high temperature resistance of wear than that of 0.5% La₂O₃/Ni35 because its higher C,Cr,B content produced more effective enhancement than Ni35.And due to its intergranular γ-（Fe,Ni）with better toughness enfolded lath martensite,which is not easy to be peeled in the process of wear,the nickel-coated aluminum plasma sprayed layer kept the best high temperature friction stability and high temperature wear performance.However,when the addition of La₂O₃ reached 1.0%,the hardness,resistance of thermal shock and resistance of high temperature friction and wear of the three kinds alloy coatings decreased,compared with the addition of 0.5%.