**2.2. The evolution of the sol solution in the electrolyte**

with desired properties are widely used in military, aerospace, automotive, electronics and

Much effort has been made to improve coating properties and design new nanocomposite coatings in the past decades. One of the focuses is how to achieve better "nano-dispersion". It was well known that the properties such as strength, hardness and wear resistance of coatings can be greatly improved by a good dispersion of the second-phase particles in the matrix. Generally, the nanocomposite coatings are synthesized by directly adding nano-size solid powders into the plating solution. The nanoparticles can be incorporated into the metal matrix during the deposition process. However, it is difficult for the second phase nanoparticles to achieve a good dispersion in the matrix because of their large surface area [3]. The nanoparticles tend to agglomerate because of their high surface energy. In order to achieve a good dispersion of the second-phase particles in the coating matrix, the powder suspension has to be main‐ tained in the electrolyte solution by vigorous agitation, air injection, ultrasonic vibration, or

However, it is always difficult for the nanoparticles to achieve a good suspension because they have very large surface area, especially when the particle size is in a nanometer level. The high surface energy leads to the agglomeration of the nano-particles even though combinations of the above methods are used to reduce the particle agglomeration. There‐ fore, it has been a challenge to explore new techniques to produce highly dispersive nanoparticles reinforced composite coatings, which can take the advantage of the unique properties of the nano-particles to develop nanocomposite coatings with superior mechani‐

We have developed a novel technique: sol-enhanced composite plating, to synthesize highly dispersive oxide nano-particle reinforced composite coatings. In this new method, transparent sol solution containing desirable oxide components is directly introduced into the electrolyte solution at a controlled concentration and/or speed. 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 signif‐ icantly improved mechanical properties [6-10]. In this paper, we will introduce the basic theory of this method, report the current results and discuss the strengthening mechanism. We will also describe the dopant technology that is derived from this novel technique. The potential

Sol-gel processes are versatile solution processes which have been widely applied to synthesize nano-particles, nano-composites, thin films, fibers and ceramics. They have shown consider‐ able advantages, including excellent chemical stoichiometry, compositional homogeneity and low crystallization temperature due to the mixing of liquid precursors on a molecular level [5].

many other industries [1-2].

106 Electrodeposition of Composite Materials

cal and other properties [4-5].

adding surfactants and other types of stabilizers, etc.

industrial applications of these technologies are also discussed.

**2. The basic theory of sol-enhanced method**

**2.1. Brief introduction of sol-gel process**

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 TiO2 sol in electroplating bath to explain the general electrochemical process of the sol-enhanced composite plating.

The transparent liquid TiO2 sol was made by using metal alkoxide tetrabutylorthotitanate (Ti(OBu)4) as the precursor. This metal alkoxide is dissolved into a mixed solution of ethanol and diethanolamine (DEA). It then formed a colloidal suspension under a series of hydrolysis and polymerization reactions. 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].

**Figure 1.** Size distribution of nano-particles in: (a) TiO2 sol and (b) Ni electrolyte after adding TiO2 sol.

The hydrolysis reaction and condensation process of TiO2 sol has been widely investigated. Generally the stereo-hindrance and solvation effects are the main factors to make the sol solution stable. Before TiO2 sol is added into the electrolyte, the sol can be regarded as a stable system without solid liquid interface. Under neutral and basic conditions, the condensation process of Ti macromolecule ions started before the completion of hydrolysis; and the formation of an ordered structure was hindered.

After TiO2 sol is added into the electrolyte, water in electrolyte aggravates the hydrolysis reaction and breaks up the dynamic balance. The sol system becomes unstable and the interface between solid and liquid emerges. Thus the amorphous TiO2 nano-particles formed in situ [5, 9-10]. Fig. 1 presents the size distribution of nano-particles in the sol and the Ni electrolyte after adding the sol. The particle size was characterized by a laser diffraction particle analyzer (Malvern Mastersizer Hydro 2000S). The particle size of TiO2 sol was distributed in the range of 1-10 nm, with a mean value of 2.5 nm as shown in Fig. 1a. The size distribution of sol added electrolyte keeps in a same level with the TiO2 sol. The mean value of the particle size was increased to 7.4 nm and the particle size distribution was in the range of 3-20 nm which means no significant agglomeration occurs (Fig. 1b). The size measurement results are consistent with the HRTEM observation as shown in Fig. 2. The TiO2 nano-particle got from electrolyte has an amorphous structure with a size of ~10 nm.

**Figure 2.** TEM bright field image of TiO2 nano-particles separated from the Ni electrolyte with TiO2 sol addition

#### **2.3. Deposition process of sol-enhanced plating**

The overall sol-enhanced deposition process can be typically divided into several steps. These steps describe the process of particles from the solution to their incorporation in the metal matrix. The first step is the in-situ generation of nano-particles after adding the sol into the electrolyte. Once the nano-particles formed in the electrolyte, some of them are immediately physically adsorbed onto the freshly deposited surface based on the Martin-Williams model. Some of them were immediately adsorbed by hydrate metal ions due to their large surface areas based on the Whithers model [10]. Correspondingly, they were highly dispersed in the electrolyte as shown in Fig. 1b. The organic solvents probably also contribute to the dispersion of the ions-adsorbed nano-particles. Under the combination effect of migration, diffusion and convection, the coating matrix grows up with fine particles, and finally forms a highly dispersive nano-particle reinforced metal-based composite coating as shown in Fig. 3 [5, 9-10].

**Figure 3.** Sketch map of the sol-enhanced plating deposition process
