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2.4 times higherthan that of steel. Kolasa et al. [15] reported that the Al2O3nanopowder addition into the plating solution increased thecoating microhardness, particularly for nickel based coatings.It can be seen from the above brief review of literature thatbrush plating has been successfully employed for the prepara-tion of coatings and/or composite coatings. This attributes tonot only its flexibility but also deposit quality, which isachieved and maintained by controlling current density, solu-tion flow, anode design, anode covering material, relativeanode 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 importantparameters are observed” [1]. Therefore, deposit quality largelydepends on the personal experience. However, high and stabledeposit quality is needed for applications. So, it is necessary torealize precise control of brush plating process.In this present work, an automatic brush plating system wasdeveloped. Ni/nano-Al2O3 composite coatings were preparedwith this system. For the reference, Ni/nano-Al2O3 compositecoatings were also prepared by conventional manual plating.The surface morphology, microstructure, microhardness, elasticmodulus and wear resistance of these two coatings were inves-tigated comparatively.2. ExperimentalIn order to fully eliminate the disadvantages resulting fromthe human nature, we developed an automatic brush platingsystem, which has characteristics as follows:(1) Anode (plating tool)/cathode (part) movement is precise-ly controlled by using step-motor. (2) Contact pressure between anode and cathode is adjustedby a specially designed mechanism.(3) The plating solution is continuously recirculated to thework area by means of a pump, which provides freshsolution and speeds up the plating process since time isnot wasted while dipping for solution. This method hasbeen reported in Refs. [1–3]. However, our plating toolwas specially made, which employed active thermal man-agement. During plating process, one part of solution istransported through the tool to the anode with a series ofsmall, uniformly spaced holes. The other part of solutionis transported through the tool back to the reservoir inorder to cool the tool during plating process.(4) Quick change from one step to the other step during thewhole plating process is achieved. Because there are fiveor six steps in plating process and the correspondingsolution can not be mixed, a real challenge to realizeautomation of brush plating is how to change from onestep to the next step quickly. Five or six tools, pumps,reservoirs and pipes may meet the demands, but this isvery complicated. So, a specially designed apparatus wasused in this automatic brush plating system. It took nomore than 3 s to change one step to the other.With this automatic brush plating system, Ni/nano-Al2O3composite coatings on 45 steel substrates were prepared froman electrolyte containing 20 g/l nano-Al2O3 particles. The plat-ing procedure includes electroclean, activate 2, activate 3, pre-plate, and plate. Rinse is needed before next step. Thecharacteristics and parameters of these five kinds of solutionsare listed in Table 1.
The composition of electric brush Nisolution is shown in Table 2. Each experiment was carried outwith a fresh solution. The surface to be plated is the outside ofcylinder with 48 mm in diameter. The Ni/n-Al2O3 compositeplating layer with a deposit thickness of 50 μm was obtained. Electric brush plating was operated at a working voltage of 12–14 V. The initial temperature of this series of experiments wasroom temperature. The temperatures of five solutions beforeplating were the same, 20 °C. It should be noted that thetemperature of the solution will rise during plating process.Apart from nickel plating, the other four steps were performedin no more than 2 min. Therefore, temperatures of these foursolutions rose slightly. For nickel plating, the deposit time washalf an hour. The bath temperature rose from 20 to 30 °C.During nickel plating, we found that the current rose from 4to 8 A with the bath temperature increasing. The contact areabetween the anode and the part was 0.02 dm2. So, the averagecurrent density varied from 200 to 400 A/dm2. The temperatureof contact area measured by an infrared detector was below50 °C, which was well controlled by our cooling method. The samples used for wear tests consisted of two parts, i.e. a lowerring and an upper block. The lower ring was made from a 45steel with outside diameter 40 mm, inside diameter 16, andwidth 10 mm, respectively. The upper blocks with dimensions30×30×10 mm were also made from 45 steel and thenquenched to achieve hardness HRC 53-55. Ni/nano-Al2O3composite coatings were plated onto the surface of the lowerring specimens. For comparison, manually plated Ni/nano-Al2O3 composite coatings on the surface of ring were alsotested. The wear tests were carried out on an MM-200 testingmachine under lubricant condition, where No. 20 machine oilwas used (see Fig. 1).
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