**1. Introduction**

Coatings are widely used on the surface of different substrates in order to improve the durability of working parts by increasing their resistance against wear, erosion, and corrosion. Comparing with the general coatings and conventional composite coatings, the nanocompo‐ site coatings possess much improved properties because of the "nanosize dispersion effect". It contains nanoparticles of a wide range of materials including oxides, carbides, metals, silicates and other ceramics with a size range of 1 to 100 nm. With the development of nanotechnology, the nanocomposite coatings are not only used as a surface protection material but also for developing special functional properties. Nowadays, the nanostructured coatings

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with desired properties are widely used in military, aerospace, automotive, electronics and many other industries [1-2].

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 adding surfactants and other types of stabilizers, etc.

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‐ cal and other properties [4-5].

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 industrial applications of these technologies are also discussed.
