High-frequency Pulse Electrodeposition And Properties Of Ni-Co/ZrO₂ Nanonanocomposite Coatings

Ni-Co/ZrO2 nanocomposite coatings were produced by high-frequency(above 20 kHz)pulse electrodeposition in a modified Watt's bath containing ZrO2 particles,which have high microhardness,excellent high temperature oxidation resistance and good corrosion resistance.The surface morphology and microstructure of the coatings were characterized by scanning electron microscopy(SEM)with energy dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD).It was found that the good dispersion of ZrO2 particles in the matrix of coatings should be associated with excellent properties of the coatings,and that the best dispersion was achieved by using 0.2 g/L SDS.After a series of experiments,the best plating process was obtained:solution pH 5,plating temperature 45?,current density,-2?6 A/dm2,duty cycle 20%?60%.If the experiments conditions were kept constant,the grain size of Ni-Co/ZrO2 nanocomposite coatings was 16.6 nm,however,the grain size of Ni-Co alloy coating was 24.5nm.According to the SEM and XRD results of Ni-Co/ZrO2 nanocomposite coatings fabricated by different conditions,it could be found that higher frequencies(above 100 kHz)and higher duty cycle(above 40%)contributed to the coatings' compact and smooth structure and reduced the relative intensity ratio I(200)/I(111).The EDS results revealed that the ZrO2 content in the coatings was affected by the duty cycle,average current density,pulse frequency and the content of ZrO2 in the plating solution.The maximum content of ZrO2 in the coatings(11.6 wt.%)was obtained in the following conditions:duty cycle of 40%,average current density of 5 A/dm2,pulse frequency of 100 kHz,the ZrO2 content in plating solution of' 10 g/L.The microhardness of Ni-Co/ZrO2 nanocomposite coatings initially increased and then decreased with the increase of pulse frequency.When the pulse frequency was 100 kHz,the microhardness reached the maximum.When the duty cycle was increased from 20%to 40%,the microhardness of the coatings also increased;when the duty cycle was increased from 40%to 60%,the microhardness began to decline.As the current density increased,the microhardness of the coatings firstly increased and then decreased.The high temperature oxidation resistance of the coatings was closely related to ZrO2 content in the coatings.The oxidation weight curve obeyed the parabolic law.The Ni-Co/ZrO2 nanocomposites with higher ZrO2 content had better oxidation resistance.The corrosion resistance ability of Ni-Co/ZrO2 nanocomposite coatings was related to pulse frequency and duty cycle,and the ZrO2 content and microstructure of the coatings could be controlled by adjusting frequency and duty cycle.ZrO2 particles can prevent the corrosion of the metal matrix and reduce structural defects.In addition,the coatings with higher packing density planes have better corrosion resistance.The(111)plane has higher packing density than the(200)plane,and the frequency and duty cycle can affect the relative intensity ratio I(200)/I(111).