Synthesis of Nanostructured Ni–TiO[sub 2] Composite Coatings by Sol-Enhanced Electroplating

Abstract

A Ni-TiO 2 composite film was prepared by electrodeposition containing Ti precursor sol ͑sol-enhanced deposition͒. The electrochemical process, microstructures, and properties of the sol-enhanced and traditional composite coatings were studied and compared. The sol-enhanced Ni-TiO 2 composite coating possessed a smooth surface and a compact microstructure and showed higher mechanical properties ͑430 HV 100 ͒ compared with the traditional coatings ͑360 HV 100 ͒. It is believed that the strengthening effects resulted from the high dispersion of TiO 2 nanoparticles. The sol added coatings also showed slower growth of Ni grains along the ͓220͔ direction but did not change the orientation. It is suggested that the sol addition reduced the thickness of the diffusion layer and increased the limited current density. Therefore, the polarization control in the sol-enhanced process changed from the traditional concentration polarization to electrochemical polarization, avoiding the formation of loose and dendritic structures at high current density.Since the Watts solution was formulated in 1916, Ni electroplating has been a commercially important and versatile surface coating/ finishing process. 1,2 Ni or Ni alloys have been widely deposited on the surfaces of working parts to improve their corrosion and wear resistance or modify magnetic and other properties. 2-4 To achieve better properties, the electrodeposited Ni or Ni alloys were modified by codepositing second-phase particles in the matrix called composite coatings. The traditional composite coating method is a solid particle mixing process: The second-phase particles are suspended in the Ni electroplating solution, and then both the particles and the Ni ions codeposit onto the specimens/parts to form composite coatings. 4 The Ni-based composite coatings were fabricated earlier from suspensions of relatively large ͑typically micrometer level͒ particles of carbides, 5 oxides, 6 diamond, 7 and Teflon. 8,9 More recently, there has been increasing emphasis on codepositing Ni ions and superfine or nanosized particles to synthesize a new structure, i.e., nanocomposite coatings. The superfine/nanoparticles are dispersed into the Ni matrix, providing significantly improved properties, such as hardness and wear resistance. [10][11][12][13][14] The strengthening mechanisms for nanocomposite coatings can be interpreted based on the dislocation model such as the Orowan theory. 15 In this theory, the critical condition for a dislocation to bypass the particles in its glide plane is to bend the dislocation to a semicircle between the particles. The dislocation with its dipoles annihilated can move forward while dislocation loops are left behind, surrounding each particle. 15-17 Orowan's criterion indicates that mechanical properties of composite coatings increase with both decreasing mean planar interparticle spacing and particle size.Based on the above theory, the incorporation of second-phase superfine/nanoparticles can be much more effective than microsized particles in reinfor...

Au-Ni-TiO2Nano-Composite Coatings Prepared by Sol-Enhanced Method

“…The formation and movement of the nano-particles is much easier than the solid particles with the traditional mixing method. 20 When the TiO 2 sol solution is added into the electrolyte, the large quantity of Au and Ni ions in the solution will lead to the polymerization reaction of sol, which forms the well dispersed TiO 2 nanoparticles under the effect of ethanol and DEA. During the electroplating process, the TiO 2 nano-particles are embedded into the alloy matrix under the charge attraction.…”

“…[16][17][18][19][20][21][22][23][24] 8.68 ml of tetrabutylorthotitanate [Ti (OBu) 4 ] was dissolved into the mixture solution of 35 mL ethanol and 2.82 mL diethanolamine (DEA). After magnetic stirring for 2 h, it was hydrolyzed by adding a mixture of 0.45 mL deionized water and 4.5 mL ethanol dropwise under magnetic stirring.…”

Nanocomposite Coatings Deposited by Sol-Enhanced Electrochemical Methods

“…The evolution of the sol solution in the electrolyte was analyzed from two parts: (1) the formation of nano-particles, and (2) the size evolution of nano-particles before and after adding into electrolyte [9][10]. Considering the comparability of different kinds of sol, we hereby illustrate the behavior of TiO 2 sol in electroplating bath to explain the general electrochemical process of the sol-enhanced composite plating.…”

“…The small particles in sol with the charge on the surface cannot grow up for the covered layer of solvent molecules. Different small particles can be well suspended in the solvent, subjecting to the combined effect of charge, van der Walls force and gravity [9][10][12][13]. The hydrolysis reaction and condensation process of TiO 2 sol has been widely investigated.…”

“…Nano-particles with a size of below 25 nm will be in-situ generated and then incorporated into the coating matrix. This method can lead to a highly dispersive distribution of desired nanoparticles in the coating, resulting in significantly improved mechanical properties [6][7][8][9][10]. In this paper, we will introduce the basic theory of this method, report the current results and discuss the strengthening mechanism.…”