Structural Evolution And Property Of Laser-annealed Electroless Ni-P/Ni-Mo-P Duplex Coatings

Electroless plating technology was widely used in mechanical,aerospace,petrochemical and other industries because of its simple process,easy operation,good wear resistance,corrosion resistance,uniformity of coating thickness and high density.However,due to the influence of working conditions,the coating may be exposed to high pressure,friction and corrosive environment,while the as plated coating performance is not enough to support the more complex working environment,so it needs to be further improved by heat treatment.At the same time,with the expansion of the application of electroless plating,single coating can not meet the requirements of practical application,and gradually towards the development of double coating.In this paper,two kinds of Ni-P/Ni-Mo-P duplex coatings were prepared.With the assistance of XRD,EDS,SEM,JADE analysis software and electronic hardness test technologies,the microstructure characteristics of the inner and outer coatings before and after laser crystallization were compared,and the influence of different laser treatment processes on the phase composition,microstructure and hardness of the Ni-P/Ni-Mo-P duplex coatings was studied,which provided technical support for laser treatment technology in the field of material surface treatment.The main conclusions were as follows:?1?As-plated:Two kinds of Ni-P/Ni-Mo-P double alloy coatings prepared by experiments:Ni-9.19P/Ni-4.11Mo-6.5P?medium phosphorus?and Ni-9.19P/Ni-4.63Mo-12.39P?high phosphorus?duplex coatings.The outer layer of medium phosphorus is of mixed crystal structure with a few holes on the surface,while the outer layer of high phosphorus is of amorphous structure with almost no holes.Their inner layer is amorphous,but there are holes in it.The thickness of the inner and outer coatings of the two alloys is about 30m,and the thickness of the inner and outer coatings is about the same.The crystallinity of high phosphorus outer layer is lower than that of middle phosphorus outer layer.?2?The effect of laser scanning treatment on the microstructure of the coating:Under the condition of 14-6mm/s laser treatment,the holes appeared on the surface of the two kinds of duplex coatings when Ni₃P phase transformation took place,but with the further decrease of laser scanning rate,the holes disappeared.At the same laser scanning speed,the crystallinity of the high phosphorus outer layer is slightly lower than that of the middle phosphorus outer layer,the crystallization temperature of Ni Mo phase and Ni₃P phase is lower than that of the middle phosphorus outer layer,the mass fraction of Ni3P phase in the high phosphorus outer layer is always higher than that of the middle phosphorus outer layer,and the grain size of Ni/Ni Mo and Ni3P phase is higher than that of the middle phosphorus outer layer.?3?The effect of the second laser scanning treatment on the microstructure of the coating:a.The primary laser scanning speed is 12mm/s and the secondary laser scanning speed is variable?9-6mm/s?:The surface morphology of Ni-9.19P/Ni-4.11Mo-6.5P?medium phosphorus?duplex coatings is similar,and there is no hole.In the process of secondary laser crystallization,Ni₃P phase precipitated in the outer layer,but Ni-Mo phase only appeared at the secondary scanning rate of 6mm/s,and the mass fraction of Ni/Ni-Mo phase was always higher than that of Ni₃P.At the same laser scanning speed,the second laser treatment can improve the crystallization degree and the mass fraction of Ni₃P phase,but the crystal size is slightly larger than that of the first laser treatment.b.The primary laser scanning speed is 14-10mm/s and the secondary laser scanning speed is fixed?6 mm/s?:The surface morphology of Ni-9.19P/Ni-4.11Mo-6.5P?medium phosphorus?duplex coatings is similar,and there is no hole.In the second laser crystallization process,the crystallization characteristics of the inner and outer coating are basically the same,which are slightly higher than the crystallization degree of the first laser scanning coating of 6mm/s.The grain size of the inner and outer Ni/Ni-Mo phase is larger than that of Ni3P phase,and the outer layer has the formation of Ni-Mo phase.For the different laser treatment states in this study,the Ni-9.19P/Ni-4.11Mo-6.5P?medium phosphorus?duplex coatings treated with 12mm/s one time scanning and 6mm/s two time scanning laser has better microstructure than other laser treatment states.?4?The effect of laser scanning on the hardness of the coating:For Ni-P/Ni-4.63Mo-12.39P?high phosphorus?duplex coatings,in the laser crystallization process of 14-6 mm/s,the hardness of the inner and outer coating reaches the maximum value at the scanning speed of 10 mm/s and 8 mm/s respectively.The hardness of Ni-4.63Mo-12.39P coating is higher than that of Ni-9.19P coating at the same scanning rate except that the scanning rate is 10 mm/s.