diameter ranging between 1 and 100 μm。 There is now a trend to using nano-sized particles as the additional ingredient of the

nickel composite。 Du et al。 [11,12] prepared nano-Al2O3/Ni composite coatings by means of brush plating and found that n-Al2O3/Ni composite coatings show improved tribological properties under abrasive contaminant lubrication。 Xu et al。 [13] reported that the brush-plated nickel silica nano-composite coatings possess finer crystal grains and the bonding between the matrix metal Ni and the SiO2 ceramic particles is compact。 Wang et al。 [14] studied the micro mechanical properties of nano-Al2O3/Ni composite coatings, such as hardness, elastic modulus and indentation creep property by nanoindentation and found that the mechanical properties are basically identical within the whole coating and the hardness is 2。4 times higher than that of steel。 Kolasa et al。 [15] reported that the Al2O3 nanopowder addition into the plating solution increased the coating microhardness, particularly for nickel based coatings。 It can be seen from the above brief review of literature that brush plating has been successfully employed for the prepara- tion of coatings and/or composite coatings。 This attributes to not only its flexibility but also deposit quality, which is achieved and maintained by controlling current density, solu- tion flow, anode design, anode covering material, relative anode to cathode speed, contact pressure between anode (plat- ing tool) and cathode (part), etc。 “Correctly trained and experi- enced operators are essential to ensure that all these important parameters are observed” [1]。 Therefore, deposit quality largely depends on the personal experience。 However, high and stable deposit quality is needed for applications。 So, it is necessary to

realize precise control of brush plating process。

In this present work, an automatic brush plating system was developed。 Ni/nano-Al2O3 composite coatings were prepared with this system。 For the reference, Ni/nano-Al2O3 composite coatings were also prepared by conventional manual plating。 The surface morphology, microstructure, microhardness, elastic modulus and wear resistance of these two coatings were inves- tigated comparatively。

2。Experimental

In order to fully eliminate the disadvantages resulting from the human nature, we developed an automatic brush plating system, which has characteristics as follows:

(1)Anode (plating tool)/cathode (part) movement is precise- ly controlled by using step-motor。

Table 1

Characteristics and parameters of five kinds of solutions

253–255 100–110 55–56 22–23 0。1–0。2

(2)Contact pressure between anode and cathode is adjusted by a specially designed mechanism。

(3)The plating solution is continuously recirculated to the work area by means of a pump, which provides fresh solution and speeds up the plating process since time is not wasted while dipping for solution。 This method has been reported in Refs。 [1–3]。 However, our plating tool was specially made, which employed active thermal man- agement。 During plating process, one part of solution is transported through the tool to the anode with a series of small, uniformly spaced holes。 The other part of solution is transported through the tool back to the reservoir in order to cool the tool during plating process。

(4)Quick change from one step to the other step during the whole plating process is achieved。 Because there are five or six steps in plating process and the corresponding solution can not be mixed, a real challenge to realize automation of brush plating is how to change from one step to the next step quickly。 Five or six tools, pumps, reservoirs and pipes may meet the demands, but this is very complicated。 So, a specially designed apparatus was used in this automatic brush plating system。 It took no more than 3 s to change one step to the other。