Processes And Properties Of CeO₂ Nano-particles Modified Cr₃C₂-NiCr Coatings Prepared By Activated Combustion High Velocity Air Fuel Spraying

In recent years,thermal spraying technology become more and more widely used in aerospace,nuclear,mechanical and thermal power fields.With the development of thermal spraying technology and its application fields,the performance of thermal spraying materials has been highly demanded,especially for the high temperature anti-oxidation and corrosion resistance coating.For the high-temperature coatings,Cr₃C₂-NiCr coating has good resistance to high-temperature oxidation and high-temperature corrosion and can meet the needs of many high-temperature working conditions,but even in the severe conditions,such as boiler tubes,garbage incinerator and so on,it often exists oxidation,abrasion and corrosion damage,since shortening the life of the coating.Therefore,it is an effective way to improve the high-temperature performance of Cr₃C₂-NiCr coating by doping rare-earth oxide CeO₂.Cr₃C₂-NiCr coatings doped nano-CeO₂ powders with content of 0,2,4,6 and 8wt.%were prepared on T91 steel substrate by activated combustion high velocity air fuel?AC-HVAF?technology,respectively.Structure and microstructure of the coatings and powders were analyzed by XRD,SEM and other analytical methods.Effects of the nano-CeO₂ with different doping content on the porosity,bonding strength and microhardness of Cr₃C₂-NiCr coatings were investigated.The friction,abrasion,corrosion resistance in 3.5 wt.%NaCl solution,high temperature oxidation and high temperature corrosion resistance of Cr₃C₂-NiCr coatings with nano-CeO₂ were studied emphatically.The main conclusions are as follows:?1?By analyzing the contrast between the morphology,microstructure,porosity,microhardness and bonding strength of the modified Cr₃C₂-NiCr coatings with different content of nano-CeO₂?0,2,4,6 and 8 wt.%?.Effects of doping nano-CeO₂ on the microstructure of Cr₃C₂-Ni Cr coating were preliminarily discussed.The results showed that nano-CeO₂ particles were dispersed in the coating.The addition of nano-CeO₂content can refine the structure,reduce the porosity,the oxidized inclusions and micro-cracks in the coatings.The average hardness of the coatings become larger with the content of nano-Ce O₂ increasing to 4 wt.%.The bonding strength is also enhanced when the content of nano-CeO₂ is 4 wt.%.As the results,the Cr₃C₂-NiCr coating with doping 4 wt.%nano-CeO₂ has the best properties.?2?By using the orthogonal test,the processes of activated combustion high velocity air fuel?AC-HVAF?were optimized for preparing Cr₃C₂-NiCr coating with 4wt.%CeO₂.The influence factors of porosity and hardness were as follows:fuel I pressure>powder feeding rate>spraying distance>fuel II pressure.The best processing parameters were powder feeding rate of 20%,fuel I pressure of 117 psi,fuel II pressure of 108 psi,spraying distance of 320 mm.At this time,the porosity of coating was 0.57%and the hardness was 1239.15 HV_0.3.?3?The addition of nano-CeO₂ can obviously improve the abrasion resistance of the coatings.The friction coefficient and the abrasion rate of W3 coating was 0.435 and6.52 mm³/N·m,respectively.The abrasion mechanism was abrasive abrasion and adhesion abrasion.While,the friction coefficient and the abrasion rate of W1 coating was 0.542 and 15.87 mm³/N·m,respectively.The abrasion mechanism changed to abrasive abrasion and slight oxidation abrasion.It indicates the addition of nano-CeO₂can refine the microstructure of Cr₃C₂-NiCr coating and enhance the microhardness and density of the coating.?4?The addition of nano-CeO₂ can obviously improve the corrosion resistance of coatings.When adding 4 wt.%nano-CeO₂,the corrosion current of the coating was0.354?A·cm^-22 and the self corrosion potential was-191.23 mV.The corrosion mechanism is pitting corrosion with shallow pit.The corrosion current and self corrosion potential of original Cr₃C₂-NiCr coating were 3.014?A·cm^-2 and-301.40 mV,respectively.Compared with the corrosion current and self corrosion potential of the T91 steel matrix were 6.051?A·cm^-22 and-434.80 m V,respectively,it showed that the addition of nano-CeO₂ can significantly reduce the number of pores,oxidized inclusions,melted and unmelted particles and micro-cracks in the coatings.Therefore,it prohibited the corrosion fluid entering the coating channels.?5?After 800°C cyclic oxidation for 100 h,the oxidation weight gain of Cr₃C₂-NiCr coating with 4 wt.%nano-CeO₂ was 0.704 mg·cm^-2,while,the original Cr₃C₂-NiCr coating was 0.762 mg·cm^-2.The results showed that the addition of nano-CeO₂can help to reduce the oxidation rate of metal elements and produce dense Cr₂O₃ and NiCr₂O₄ spinel,which improve the resistance of high-temperature oxidation.?6?Cr₃C₂-NiCr coating with 4 wt.%nano-CeO₂ showed good high-temperature corrosion resistance in Na₂SO₄-25 wt.%NaCl at 800°C,the oxidized layer produced by high temperature corrosion was continuous and dense.The particles were refined and the corrosion products showed obvious layered distributions.The results showed that the Ce elements decomposed by nano-CeO₂ at high temperature can produce CeS,which existed in the grain boundary and reduced the corrosion rate.