| At present,thermal spraying technology has been widely used.Normally,the thickness of the thermal sprayed coating is 300 ?m-400 ?m.Some studies have shown that,within a certain range,the thick coating owns better wear resistance and fatigue property than the thin coating.However,thick coating is likely to peel from the substrate during the preparation process due to the large internal stress and other factors.Therefore,thick coating is difficult to be applied in production practice.Therefore,it is necessary to prepare a thick coating and further study its microstructure and properties.In the present work,the NiCrBSi coating with thickness of 800 ?m was prepared on the AISI 1045 steel substrate via plasma spraying.Subsequently,the thick coating was remelted by gas tungsten arc(TIG)process.The microstructure and phase composition of the thick coating before and after remelting were investigated by scanning electron microscopy,transmission electron microscopy and X-ray diffractometer.The microstructure evolution of the as-sprayed coatings after remelting was investigated analyzed.The friction and wear test of the two coatings before and after treatment was carried by a sliding wear apparatus.Finally,the failure mode and failure mechanism of as-sprayed and remelted coating are studied by means of fatigue testing machine.Finally,TiN particles were added to the coating to study the effect of TiN particles on the microstructure,mechanical property and wear properties of the coating.The results showed that:(1)TIG remelting eliminated most of the defects(such as microcracks,pores,unmelted particles and so on)in the thick as-sprayed coating.The porosity of the coatings decreased from 5.6%to 0.2%,and the compactness of the coatings was improved significantly.(2)The main phases in the thick as-sprayed coating are changed from ?-Ni,Cr7C3 and Cr2B to ?-(Ni,Fe),(Fe,Cr)23C6,CrB,Ni3B,Fe3C and Ni3Si.The remelted coating is mainly composed of fine dendrite y-(Ni,Fe)and thin interdendritic lamellar eutectic?-(Ni,Fe)+((Fe,Cr)23C6,CrB).Moreover,there are some submicron particles Ni3B in the grain interior of y-(Ni,Fe),and a large number of nanocrystallines CrB and Fe3C in the grain boundary of y-(Ni,Fe).The microstructure of the solid solution y-(Ni,Fe)was completely coherent at the interface after remelting,and the coating changed from mechanical bonding to metallurgical bonding.(3)TIG remelting can significantly improve the mechanical properties of the thick coating.The elastic modulus of remelted coating is 84.4%higher than that of the as-sprayed coating.And the fracture toughness of the remelted coatings is 19.5 times than that of the as-sprayed coating.The average hardness of the remelted coating is increased by 42.8%compared with the as-sprayed coating.(4)TIG remelting can significantly improve the wear properties of the thick coating.Under the condition of dry friction and wear,the wear mass of remelted coating is only 36.2%of that of the as-sprayed coating under the same load and frequency.In addition,the wear mechanisms of the two coatings are also different.The wear mechanisms of the as-sprayed coatings are mainly delamination wear,adhesion wear and oxidation wear.While,the wear mechanisms of the remelted coatings are mainly adhesion wear,micro-abrasive wear and oxidation wear.(5)TIG remelting can significantly improve the fatigue properties of the thick coating.TIG remelting affects the fatigue life and dispersion of the coating.Under the same test conditions,the contact fatigue life of remelted coating is higher than the as-sprayed coating;and the life dispersion of the remelted coating is lower than the as-sprayed coating.The contact fatigue failure modes of as-sprayed coatings include pitting failure,spalling failure,delamination failure.The main failure modes of remelted coating are pitting and spalling.In addition,the p-N curves of the two coatings under contact fatigue were established to predict the failure probability of the as-sprayed and remelted coatings under different contact fatigue life values.(6)The effect of TiN particles on the microstructure,mechanical properties and wear property of NiCrBSi remelted coating is as follows:the hardness of NiCrBSi+TiN remelted coating is 29.1%higher than that of NiCrBSi remelted coating,the elastic modulus is increased by 10%,and the fracture toughness decreased by 27.1%.Moreover,the average friction coefficient decreased by 25.4%.Therefore,a Ni-based thick coating with good wear resistance can be obtained by plasma spraying and remelted technique. |
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