Experimental Investigation On Binary Nanocomposite Coatings By Electrodeposition In An Ultrasonic Field

Nanocomposite coatings, obtained by using composite electrodeposition technology, exhibited high wear resistance, temperature oxidation resistance, corrosion resistance, catalytic properties and so on. The properties of nanocomposite coatings mianly depended on the nature, type and distribution of nanoparticles as well as the process condition of the electrodeposition. Usually, nanocomposite coatings are prepared by adding one kind of nanoparticles into electrolyte using common method. Few researches on nanocomposite containing two kinds of nanopaticles were carried out. Ultrasonic could restrain the agglomeration of nanoparticles, refine grains of coatings and then favor the enhancement in properties of nanocomposite coatings. Thus, composite coatings with better properties are expected to be prepared by adding two kinds of nanoparticles into the electrolyte, combined with the strengthening effect of ultrasonic in electrodeposition process.In this dissertation, the fundamental research of binary nanocomposite coatings prepared by electrodeposition under ultrasonic condition was developed by adding rare earth Y₂O₃ and ceramics ZrO₂ nanoparitcals into the electrolyte. Effects of the process parameters on the nanoparticle content in the composite coatings were investigated. The surface morphology, microstructure of different coatings were observed and analyzed. The high-temperature oxidation resistance, friction and wear properties of different coatings are also examined. The improvement action and mechanism of rare earth and ceramics nanoparticles as well as ultrasonic on the microstructure and properties of the coatings were explored. The main works complete and conclusions are presented as follows:Firstly, effects of the addition content of two kinds of nanoparticles, cathode current density and ultrasonic parameters on the nanoparticle content in the composite coatings were investigated. The optimized process parameters of preparing Ni-Y₂O₃-ZrO₂ nanocomposite coatings prepared by electrodeposition in an ultrasonic field are obtained. The optimized process parameters are as follows: Y₂O₃ nanoparticles concentration 10g/L, ZrO₂ nanoparticles concentration 20g/L, cathode current density 2A/dm², ultrasonic frequency 45kHz and ultrasonic power 300W. Secondly, the cathodic polarization curve and electrochemical impedance spectra of two kinds of nanoparticles and metal nickel ion were studied. The nanoparticles in the electrolyte not only cause negative transfer of the reduction potential, but also change impedance the of the electrode reaction and present two inductive loops at low frequency of the impedance spectra.Thirdly, effects of the process parameters on the surface morphologies and the crystal preferential of different coatings were analyzed. Pure Ni coating has an irregularity surface and loose structure and revealed a (2 2 0) preferred orientation. Nanoparticles and ultrasonic result in refining of the grain size of matrix metal Ni. Nanoparticles reduce the diffraction intensity of crystal face (2 2 0), and change the diffraction strongest peak of the coatings.The Ni-Y₂O₃-ZrO₂ nanocomposite coating prepared under ultrasonic condition presents a refined crystal grain and compact microstructure; the diffraction intensity of crystal face (1 1 1), (2 0 0) and (2 2 0) of the coating are balanced.Fourthly, the microhardness, high-temperature oxidation resistance, friction and wear properties of pure Ni coating, composite coatings and binary nanocomposite coatings were investigated. The Ni-Y₂O₃-ZrO₂ nanocomposite coating prepared under ultrasonic condition exhibits higher microhardness, better high-temperature oxidation resistance and friction and wear properties than those of the other coatings. It is suggested that the coating has excellent properties owing to the cooperative effects of two kinds of nanoparticles and ultrasonic.Fifthly, the factors and mechanism which affected the properties of binary nanocomposite coatings were analyzed. Appropriate matching proportion of two kinds of nanoparticles could promote the codeposition of nanoparticles and the ultrasonic could make these nanoparticles disperse uniformly in nanocomposite coatings, thus refine the grain size and improve the properties of the nanocomposite coatings.The electrodeposition process of Ni-Y₂O₃-ZrO₂ binary nanocomposite coating with excellent properties has been obtained through experimental research in this dissertation. This research would play a guiding role in developing the composite coatings with excellent properties.