Study On Characterization And Mechanisms Of Ni-ZrO₂ Nanocomposite Coatings By Pulse Electroplating Technology

Electroplating nanocomposite coatings have attracted considerable interestbecause of their superior mechanical and tribological properties. Recently, as a newtype of electroplating nanocomposite technology called pulse electroplating (PE).nanocomposite technology has been developed. This technique can combine bothadvantage of pulse electroplating technology and electroplating nanocompositetechnology. The most commonly employed industrial technique for the electroplatingof metals involves the application of direct current electroplating (DC). However,fewer results have been reported on PE nanocomposite coatings. The study on therelationship between PE nanocomposite process and characterization ofnanocomposite coatings is still inadequate. So, it results in that PE nanocompositetechnology hasn't been brought into play adequately.Nanoparticles agglomerated very easily in the plating bath because of its highsurface energy;the agglomeration of these particles in the plating bath wouldcorrespondingly result in enhanced amount of agglomerated particles in compositecoatings, which also results in the unique properties of nanocomposite coatingsdisappear. Therefore, dispersion of these particles in the plating bath and coatings isone of the key steps toward achieving optimal mechanical properties forElectroplating nanocomposite coatings.In order to understand the characteristic of PE nanocomposite technology and toobtain excellent nanocomposite coatings, the effect of PE nanocomposite process onthe property and microstructure of nanocomposite coatings has been investigated.Furthermore, in order to reduce the aggregation of nanoparticles during electroplatingprocess, ZrO₂ nanoparticles are uniformly codeposited into a nickel matrix byelectroplating of nickel from a Ni bath containing particles in suspension which aremonodispersed with dispersant.The influence between ZrO₂ nanoparticles and Ni bath each other is discussed.Dispersant PEDA (Polyelectrolyte dispersant containing aromatic rings)is used todisperse ZrO₂ nanoparticles into Ni bath and laser light-scattering,TEM and FITRtechniques are used to investigate the behaviors of ZrO₂ nanoparticles in Ni bath.Studies show that the inert particles ZrO₂ are significantly agglomerated in the bathwhen dispersant absence, however, the addition of dispersant PEDA could indeeddecrease the agglomeration of ZrO₂ particles in the bath and the particle size is about63 nm. Ni-ZrO2 nanocomposite coatings containing uniformly and monodispersedZrO2 nanoparticles are prepared from the composite plating bath with monodispersednanoparticles, and the characterizations of the nanocomposite coatings are analyzedby SEM,XRD,hardness test,cathodic polarization curves for hydrogen evolutionreaction. The results show that the performances of Ni-ZrO2 nanocomposite coatingswith monodispersed ZrO2 nanoparticles are improved remarkably.The Ni-ZrO2 nanocomposite coatings are prepared by PE and the nanocompositecoatings are analyzed by SEM,EDX and XRD. Compared to DC electroplating, theresults show that PE nanocomposite technology offers several advantages such as theavailability of additional process parameters which can be varied independently andpossibility of achieving excellent coatings;PE nanocomposite technology can resultin a higher particle deposit concentration compare to DC electroplatingnanocomposite technology;the surface morphology of PE nanocomposite coatings aremore compact and finer grains than direct DC nanocomposite coatings;PEnanocomposite technique demonstrate that ZrO2 nanoparticles in the bath can beselectively codeposited with the nickel film and ZrO2 particles are fairly uniformlydistributed and less agglomerated throughout the Ni matrix, the particle size is about60 nm.The hardness and wear behavior of PE nanocomposite coatings have beenstudied and the wear track of all kinds nanocomposite coatings have been analyzed bySEM. The wear mechanisms of pure Ni coatings,DC nanocomposite coatings and PEnanocomposite coatings have been discussed. The results reveal that PEnanocomposite coatings have a high hardness and excellent wear resistant property.This wear behavior could be attributed to that the effective dispersion of ZrO2provided by PE process would significantly decrease the agglomeration of ZrO2particles. It is possible that the bonding strength between the uniformly dispersedparticles and the nickel matrix was strong as compared to that between theagglomerated particles. Therefore, during the wear process of PE nanocompositecoatings, abrasive wear and fatigue wear is reduced.The corrosion behavior of different coatings have been analyzed bypotentiondynamic polarization curves for coatings in different medium;The thermalstability and oxidation resistance of different coatings have been examined byhardness testing and XRD analyzing for coatings before and after heat treatment. Theresults show that the corrosion resistance of pure Ni coatings is superior to that ofnanocomposite coatings but the corrosion resistance of PE nanocomposite coatings isbetter than that of DC nanocomposite coatings. The thermal stability and oxidationresistance of PE nanocomposite coatings is best among all kinds of coatings.The interaction between the ZrO2 nanoparticles and Ni deposit is studied by XPS.The results show that it is possible that there is chemical bond between ZrO2nanoparticles and nickel. The XPS date indicates that d-orbitals of the nickel mayestablish interaction with the O element of the ZrO2 surface. Under the experimentalcondition, there is no interaction between ZrO2 nanoparticles and nickel in the DCnanocomposite coatings, on the contrary, the interaction exist in the PEnanocomposite coatings.