Ni Coating Fabricated by Cathodic Plasma Electrolytic Deposition with Different Deposition Parameters
Ni coating has engendered considerable interest as an attractive candidate for functional coating,which has good oxida-tion,excellent corrosion,and wear resistance.Ni coatings are widely used in aeronautical coatings,biomedical devices,and marine engineering.Thermal spraying,hot dip,and other methods have been used to manufacture Ni coatings.Those technical means are complicated in process,strict in equipment requirements,and high in cost,which restricts the development and application of Ni coat-ing.Cathodic plasma electrolytic deposition(CPED)has attracted more and more attention in coating preparation and surface modifica-tion in recent years due to the ability to produce effectively clean surfaces,good coating adhesion,high deposition rate,relatively sim-ple and environmentally friendly.The quality of CPED-prepared coatings is affected by various parameters,such as electrolyte parame-ters,treatment time,electrolyte temperature,and power supply parameters.To realize the controlled processing of CPED technology,it is necessary to deeply study the effect of these deposition parameters and optimize the coating preparation process.Pure Ni coatings were prepared using the cathodic plasma electrolytic deposition technique.The effects of deposition parameters on the deposition pro-cess of Ni coatings in both single deposition and cyclic deposition modes were systematically investigated using the field emission scan-ning electron microscope(FESEM),the energy dispersive spectrometer(EDS),and the laser confocal scanning microscopy(LCSM).In addition,the deposition mechanism of Ni coatings prepared by CPED was summarized and discussed.The results showed that in the single deposition mode,an appropriate increase in deposition voltage in the range of 100～110 V helped to smooth the surface morphol-ogy of the coatings.The arithmetic mean deviation(Sa)and maximum height of profile(Sz)of the coatings gradually decreased with the increase in voltage.With the increase of voltage,the diameter of spherical particles in Ni coatings decreased,which could be attribut-ed to the increase of the nucleation rate due to the shortening period of the bubble generation and collapse.When the voltage exceeded 110 V,the intensity and density of the plasma discharge were intensified,causing holes and defects.In the single deposition mode,the nodules morphology alternated with the flat melt morphology after more than 3 min of deposition time.The appearance of the two al-ternating morphologies could be attributed to the nodules induced a distortion of the local electric field,which became the dominant re-gion of the plasma discharge under the action of the tip discharge.Under the action of partial high-density plasma discharge,the nod-ule gradually melted with the appearance of typical melt morphology.With the increase of deposition time,the coating thickness first increased and then tended to stabilize.The coating thickness was maximum(～10 μm)at a deposition time of 10 min.In addition,the coatings were flat and dense with good coverage when deposition time were less than 10 min.With deposition time over 10 min,the de-fects of the coatings became obvious,and even the substrate was exposed.In the cyclic deposition mode,when the lifting speed was 1200 µm·s-1,the area of cathode instantaneous conduction was relatively large,and the establishment process of gas film was easily interfered with by large bubbles,which led to abnormal deposition.At a lifting speed of 600 μm·s-1,the deposition time at the same point of the cathode was relatively long,which increased the possibility of forming dendritic morphology,thus affecting the uniformity of the coating.Sz and Sa of Ni coatings first decreased and then increased with the increase of the lifting speed.The coating prepared at a lifting speed of 900 μm·s-1 had the flattest and most uniform surface(Sa=2.11 μm,Sz=21.13 μm)with a coating thickness of 5.62μm.An increase in the immersion time led to more pronounced melt state morphology and smoother coating surface in the cyclic depo-sition mode.Compared with the coating prepared at an immersion time of 10 s,Sz and Sa of the coatings obtained at 30 s decreased by 40.7％and 52.9％,respectively,while the coating thickness increased by over 18％.Increasing the depositions passes resulted in the preferential growth of the dendrites,which could be attributed to the vapor gaseous envelope periodically underwent build-up and change,and the tip discharge was significantly weaker than single long-time deposition.As the deposition passes increased,Sz and Sa continuously increased,and the porosity of the coating gradually increased,forming a three-dimensional porous rough morphology.Compared with the coating obtained by depositing 5 passes,Sa and S,of the coating obtained by depositing 20 passes increased about 5.5 times and 2.8 times,respectively.There was no significant increase in the thickness of the dense zone by increasing the depositions passes,and the increase in coating thickness was almost entirely contributed by the growth of the dendritic loose zone.The thickness of the coating obtained at 20 deposition passes was up to 24.24 μm,but the densification was poor,with pores of about 2 μm in diameter between the dendrites.
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