The Fabrication And Electro-crystallization Mechanism Of Ni-Co/Y₂O₃ Composite Coatings

In acid sulfamate bath,Ni-Co/Y₂O₃ composite coatings were prepared at ultrasonic-assisted electro-deposition by using nickel sulfamate and cobalt sulfamate as main salts and solid Y₂O₃ nanoparticles with an average particle size of 50 nm as second phase.The properties and micro-structure of electrodeposited composite coatings were tested by using XRD,SEM and AFM to explore the optimum technological conditions?cathodic current density,ultrasonic power,and the concentration of Y₂O₃ nanoparticles in the bath?.The effects of Y₂O₃ nanoparticles on the Ni-Co co-deposition process were investigated by a series of electrochemical testing techniques containing cyclic voltammetry?CV?,chronoamperometry?CA?,and electrochemical impedance spectroscopy?EIS?.The kinetics parameters of the co-deposition process were calculated by fitting experimental curves,which aims to reveal the electrocrystallization mechanism of the composite coatings.The results show that:?1?The composite coatings with the bath temperature 40 ^oC and pH 4,ultrasonic power of 250 W,the Y₂O₃ concentration of 9 g/L in the bath and co-deposited at cathode current density of 4 A/dm² reported the optimized micro-hardness,wear and corrosion resistance.And the compact structure and finer grains could be seen in the deposits.?2?The electro-deposition process of Ni-Co/Y₂O₃ composite deposits could be illustrated by Guglielmi's two-step adsorption model.The cathodic current density of the Ni–Co alloy clearly increased at–0.95 V,the reduction potential of Ni-Co/Y₂O₃composite was about at-0.80 V.The addition of Y₂O₃ nanoparticles in the bath shifted initial deposition potential of Ni²⁺and Co²⁺to more positive potential and decreased cathodic polarization.?3?For both coatings,the nucleation/growth process approximately agreed with the Scharifker–Hill instantaneous nucleation model.The effect of solid nanoparticles on the electrochemical reduction reaction on the cathode surface was different at different step potentials.At lower potentials?–1.05 V^–1.10 V?,Y₂O₃ nanoparticles promoted the nucleation of Ni-Co alloy,Ni-Co/Y₂O₃ composite coating had higher nucleation rate.At higher potentials?–1.15 V^–1.20 V?,the promotion effect of Y₂O₃nanoparticles on the nucleation of Ni-Co alloy was weakened.?4?The Nyquist plots of Ni-Co and Ni-Co/Y₂O₃ contained a complete capacitance loop at high frequency?10⁴ Hz¹0 Hz?and an inductive loop at low frequency?10 Hz¹0^-11 Hz?on the cathode surface in deposition process.Fitted calculation showed that Y₂O₃ nanoparticles had no obvious effect on the double layer at the electrode/electrolyte interface,but the charge transfer resistance was decreased.