With this automatic brush plating system, Ni/nano-Al2O3 composite coatings on 45 steel substrates were prepared from an 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。 The characteristics and parameters of these five kinds of solutions are listed in Table 1。 The composition of electric brush Ni solution is shown in Table 2。 Each experiment was carried out with a fresh solution。 The surface to be plated is the outside of cylinder with 48 mm in diameter。 The Ni/n-Al2O3 composite plating layer with a deposit thickness of 50 μm was   obtained。

Fig。 1。 Schematic diagram of block-on-ring testing machine。

Fig。 2。 SEM surface morphology of automatically plated Ni/n-Al2O3 composite coating。

Electric brush plating was operated at a working voltage of 12– 14 V。 The initial temperature of this series of experiments was room temperature。 The temperatures of five solutions before plating were the same, 20 °C。 It should be noted that the temperature of the solution will rise during plating process。 Apart from nickel plating, the other four steps were performed in no more than 2 min。 Therefore, temperatures of these four solutions rose slightly。 For nickel plating, the deposit time was half an hour。 The bath temperature rose from 20 to 30 °C。 During nickel plating, we found that the current rose from 4

to 8 A with the bath temperature increasing。 The contact area between the anode and the part was 0。02 dm2。 So, the average current density varied from 200 to 400 A/dm2。 The temperature of contact area measured by an infrared detector was below 50 °C, which was well controlled by our cooling method。  The

Fig。 3。 SEM surface morphology of manually plated Ni/n-Al2O3 composite coating。

Fig。 4。 EDX spectra of automatically plated Ni/n-Al2O3  composite coating。

samples used for wear tests consisted of two parts, i。e。 a lower ring and an upper block。 The lower ring was made from a 45 steel with outside diameter 40 mm, inside diameter 16, and width 10 mm, respectively。 The upper blocks with dimensions 30 × 30 × 10 mm were also made from 45 steel and then quenched to achieve hardness HRC 53-55。 Ni/nano-Al2O3 composite coatings were plated onto the surface of the lower ring specimens。 For comparison, manually plated Ni/nano- Al2O3 composite coatings on the surface of ring were also tested。 The wear tests were carried out on an MM-200 testing machine under lubricant condition, where No。 20 machine oil was used (see Fig。 1)。 All tests were conducted under load of 300 N for 2 h。 Each test was repeated three times, and the average was adopted as the experimental data。

The surface morphology and microstructure of the compos- ite coatings were investigated using a Quant 200 scanning electron microscopy (SEM)。 The Al2O3 concentration in the coatings was measured using a Genesis 60S energy dispersive spectroscopy (EDS)。 Microhardness of the coatings was deter- mined using a NanoTest 600 Nanoindenter (Micro Materials, Ltd。,  Wrexham,  U。K。)。  Nanoindentation  experiments   were

Fig。 5。 EDX spectra of manually plated Ni/n-Al2O3  composite coating。

Fig。 6。 Indention curves obtained from automatically plated sample。

conducted to a maximum load of P = 15 mN。 The loading and unloading phases of indentation were carried out over a time span of approximately 20 s in all the experiments。 At the maximum load, a dwell period of 5 s was imposed before

Table 4Indentation results of automatically prepared sample

Point Maximum depth (nm) Maximum load (mN)