Research On Preparation Technology And Properties Of Ni-Co-Nanooxide Composite Coatings Fabricated With Ultrasonic Assisted Electrodeposition

Ni-Co alloy has good chemical stability and mechanical properties.As a kind of coating material,Ni-Co alloy can make the metal surface have various excellent properties.The Ni-Co alloy coating with low cobalt content has high hardness,excellent wear resistance and corrosion resistance.It is widely used as the wear-resistant parts or the corrosion-resistant parts in the industry.The Ni-Co alloy coating with high cobalt content has excellent high temperature performance.It is extensively used in high temperature working environments,such as the inner coating of continuous caster crystallizer.The wear resistance and corrosion resistance of the copper material are poor,and the high temperature oxidation resistance is also insufficient.Ni-Co alloy coating on the surface of copper can significantly improve the surface properties of copper.It can meet the requirements of different service conditions,and significantly improve the working life of copper.The second phase nano-particles are dispersed in Ni-Co alloy coatings by ultrasonic-asisted composite co-deposition technology,which can meet the requirements of high hardness,high wear resistance and high toughness of the coatings.At the same time,it can reduce the cobalt content in the coatings,further reduce the manufacturing cost of Ni-Co alloy coatings,and improve the working life of the coatings.In this paper,Ni-Co alloy coatings containing nano-Al₂O₃ and nano-CeO₂ particles were prepared by ultrasonic-assisted codeposition technology.The effects of ultrasonic field and addition of nano-oxide particles on the mechanical properties,high temperature oxidation resistance and corrosion resistance of Ni-Co alloy coatings were compared.The electrocrystallization mechanism of Ni-Co-nanooxide composite codeposition was also studied in order to provide a theoretical reference for the life extension technology and further popularization of Ni-Co-based alloy coating on copper surface.In this dissertation,by optimizing the ultrasonic assisted electrodeposition technological parameters,the process conditions for preparing the Ni-Co-Al₂O₃ composite coating on the surface of copper substrate were determined as follows,i.e.,the bath temperature was 60℃,the ultrasonic power was 240W,the current density was 3A/dm²,the concentration of nano-Al₂O₃ was 10g/L,the content of cobalt salt was 3.5g/L,and the pH value of the bath was4.8.The process conditions for preparing the Ni-Co-CeO₂ composite coating were determined as follows,i.e.,the bath temperature was 40℃,the ultrasonic power was 160W,the current density was 2A/dm²,the nano-CeO₂ concentration was 1g/L,the cobalt salt content was 3g/L,and the pH value of the bath was 4.8.The observation and analysis of the micro-morphology and phase composition of the coatings show that under the action of external ultrasonic field,nano-oxide particles are uniformly dispersed on the surface of the coatings and in the matrix metals.The surface of the composite coating is smooth,the structure is compact and the thickness is uniform.The grain size of the coatings are reduced and the crystal grains are refined.The thickness and bonding strength of Ni-Co-nanooxide composite coatings were determined.The results show that the application of ultrasonic field in the composite electrodeposition process can increase the thickness and bonding strength of Ni-Co-nanooxide composite coatings.Ultrasound cavitation can significantly increase the content of nano-oxide particles in the coating.The Ni-Co-nanooxide composite coating can obtain the best mechanical properties,the higher the hardness and modulus of elasticity.The coatings have a significantly reduced coefficient of friction and excellent wear resistance.The research on high temperature oxidation resistance shows that the composite coating of Ni-Co-nanooxide deposited by ultrasonic assisted electrodeposition has the smallest parabolic rate constant and the best high temperature oxidation resistance under the condition of oxidation at 800℃for 20h.Ultrasound cavitation promotes the dispersion of nano-oxide particles in the coating.The dislocation movements of the coating are hindered by nano-oxide particles and the grains are refined.A denser oxide film is formed on the surface of the coating,which improves the high temperature oxidation resistance of the Ni-Co-nanooxide composite coating.XRD analysis shows that the high temperature oxidation products of Ni-Co-Al₂O₃coating are NiO and CoO,while those of Ni-Co-CeO₂ coating are NiO,CoO,CuO and Cu₂O.According to the high temperature oxidation products and the high temperature oxidation resistance of Ni-Co-Al₂O₃ and Ni-Co-CeO₂ coatings at different temperatures,it is concluded that the high temperature oxidation resistance of Ni-Co-Al₂O₃ coatings is better than that of Ni-Co-CeO₂ coatings.The results of electrochemical corrosion test show that the Ni-Co-nanooxide composite coatings prepared under the optimum process conditions have lower self-corrosion current density and excellent corrosion resistance.Static immersion corrosion tests of Ni-Co-nanooxide composite coatings were carried out in 10wt.%NaOH,3.5wt.%NaCl and5wt.%H₂SO₄ solutions,respectively.The results show that after 240 hours of static immersion,the coatings have no obvious weight loss in NaOH and NaCl corrosive solution,and the corrosion resistance is excellent,but the weight loss is large and the corrosion resistance is poor in H₂SO₄ corrosive solution.The research on the electrocrystallization behavior during electrodeposition reveals that in the initial stage of electrodeposition,the addition of nano-oxide particles in the plating solution causes the codeposition initiation potential to shift negatively,and the cathodic polarization effect increases,which inhibites the Ni-Co electrodeposition process.After the ultrasonic field is applied,with the increase of ultrasonic power,the codeposition initiation potential gradually shiftes positively,and the cathodic polarization effect gradually decreases,which promotes the electrodeposition of Ni²⁺and Co²⁺on the cathode surface.In the process of metal ion nucleation,adding nano-oxide particles to the plating solution and applying ultrasonic field can increase the peak current density of the cathode and shorten the relaxation time of metal nucleation.The nucleation rate of metal ions increases and the electrocrystallization nucleation of metal ions on the cathode surface is promoted.In the electrodeposition process,the addition of nano-oxide particles and the application of ultrasonic wave don’t change the initial nucleation mode of Ni-Co electrocrystallization.The electrocrystallization behavior of Ni-Co and Ni-Co-nanooxide coatings conformes to Scharifker-Hill three-dimensional instantaneous nucleation model.