Research On Forming Mechanism And Wear Resistance Of Ni-based Coatings Prepared By Pulsed Magnetic Field Assisted Supersonic Plasma Spraying

As an important means of surface strengthening and damage repair,supersonic plasma spraying technology has broad application prospects in large process equipment,civil equipment,oil pipeline and other equipment manufacturing fields,especially in the preparation of anti-friction and wear-resistant coatings on the surface of various key parts and components.However,with the development of modern industrial technology,various equipment parts need to face increasingly harsh working conditions,which brings new challenges to the existing coating preparation technology.Therefore,this paper takes the Ni-based wear-resistant coating on the surface of equipment parts as the research object.A pulsed magnetic field-assisted supersonic plasma spraying method was proposed to prepare Ni-based protective coatings.The mechanism of the effect of pulsed magnetic field on the coating quality was revealed from the aspects of different magnetic powders,particle flight spread solidification characteristics,coating remelting post-treatment and the improvement of coating tribological behavior.（1）In this paper,a pulsed magnet with a center aperture of 100mm and a maximum magnetic field intensity of 7 Tesla is designed and developed based on the theoretical calculation of the pulse magnet field intensity distribution.A pulsed magnetic field assisted supersonic plasma spraying equipment is built by combining the pulsed magnetic field power supply with the control and protection system.It has the advantages of high field strength,no contact,and obtaining frequency stable magnetic field in a short time.Through orthogonal test,the pore and bonding strength of the coating were compared,and the optimal process parameters of pulsed magnetic field-assisted supersonic plasma spraying（5 Tesla magnetic field-assisted）were obtained.（2）Two kinds of magnetic powder coatings were prepared by pulsed magnetic field assisted supersonic plasma spraying equipment under the optimal process.Two kinds of powder in the weak ferromagnetism is referred to as A,and the strong ferromagnetism is referred to as B.The pore,bonding,strength,hardness and wear resistance of coating A are better than that of coating B after the magnetic field strengthening,which is related to the better comprehensive performance of the coating dominated by Ni.The roughness and residual stress properties of B coating are better than those of A.This is related to the formation of magnetic poles by magnetic fields.When Fe element materials solidify,they show ferromagnetism and are more easily attracted by magnetic poles,thus increasing the downward movement trend.The results show that the pulse magnetic field can significantly improve the porosity,roughness,residual stress,bonding strength,hardness,and wear resistance of the Ni-based coating.（3）In this paper,the flight-spreading solidification process of supersonic plasma spraying particles assisted by pulsed magnetic field is simulated by mathematical software,and the influence mechanism of pulsed magnetic field is given by combining with the experimental results.The results show that the trajectory of coating particles is affected by the pulsed magnetic field,that is,the trajectory of coating particles converges to the center,resulting in prolonged time of particles in the center of flame flow and better melting state of particles.In the process of particle spreading,the position and spreading mode of spraying particles impact the matrix are affected by the pulsed magnetic field,which makes the impact position more concentrated,and the morphology is dominated by the circular boss structure.The pulse magnetic field also affects the solidification behavior of the coating through electromagnetic agitation and electromagnetic shock,which directly produces pulse bombardment effect on the molten coating.The internal pores are dispersed and escaped,and the crystal nuclei are also shaken down and re-enter the melt to refine the grains.At the same time,the lamellar structure distribution inside the coating is affected,and the clean area is increased and the strengthening phase distribution is more uniform.（4）Pulse magnetic fields with different gradients were applied during the coating remelting process to explore the effect of magnetic field on the coating remelting process.Among them,5 Tesla magnetic field enhanced coating remelt sample performance is the best,coating porosity decreased,magnetic domain structure ordered arrangement,and the number of nanocrystalline grains increased.The mechanical properties of the coating are obviously improved and the residual stress distribution is more concentrated.The bonding strength increased from 30MPa to 51.6 MPa;Hardness increased from 583 to 751 HV_0.2.This is related to the fact that the coating is in the molten state for a long time in the remelting process,the electromagnetic stirring and electromagnetic oscillation generated by the pulsed magnetic field are more obvious,which makes the upward movement trend of pores in the coating increase,the magnetic domain distribution is more ordered,and the effect of introducing more crystal nuclei and refining grains is obvious.（5）Three types of tribological properties of the coating are analyzed,and the experimental results show that the pulsed magnetic field can significantly improve the tribological properties of the coating.The mean value of the scratch critical load of the coating is increased from 2.974 N to 6.468 N.The average COF of reciprocating wear decreased from 0.426 to 0.365.The contact fatigue limit of the coating is increased from 976.0 MPa to 1116.8 MPa.The mechanism is as follows:the pulse magnetic field changes the force form of the pores in the coating,and the Lorentz force generated accelerates the escape of the pores.Secondly,after pulsed magnetic field,the domain distribution of the coating is more uniform and orderly,and its internal stress is more uniform.Finally,the pulsed magnetic field provides a lot of nucleation work for the Ni-based grains,resulting in the formation of many nanocrystalline and amorphous structures.These three factors work together to improve the tribological properties of the coating.