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All tests were conducted under load of300 N for 2 h. Each test was repeated three times, and theaverage was adopted as the experimental data.The surface morphology and microstructure of the compos-ite coatings were investigated using a Quant 200 scanningelectron microscopy (SEM). The Al2O3 concentration in thecoatings was measured using a Genesis 60S energy dispersivespectroscopy (EDS). Microhardness of the coatings was deter-mined using a NanoTest 600 Nanoindenter (Micro Materials,Ltd., Wrexham, U.K.). Nanoindentation experiments were conducted to a maximum load of P=15 mN. The loading andunloading phases of indentation were carried out over a timespan of approximately 20 s in all the experiments. At themaximum load, a dwell period of 5 s was imposed beforeunloading so as to correct for any thermal drift in the system.At least 5 indents were made for each specimen, with theadjacent indents separated by 10 μm.3. Results and discussion3.1. Morphology of the coatingsSurface morphologies of the Ni/nano-Al2O3 composite coatingsprepared automatically and manually are shown in Figs. 2 and 3.Itwas observed that the specimen prepared by automatic brush platinghad a smaller crystalline microstructure than that of the specimenprepared by conventional brush plating. The former is much moredense, smooth, and uniform than the latter. This result may be attrib-uted to the continuous supply of fresh solution, appropriate contactpressure between the anode and the surface to be plated, and appro-priate relative anode to cathode speed.Figs. 4 and 5 show EDX spectra of the Ni/n-Al2O3 compositecoatings prepared by automatic and manually brush plating, respec-tively. The Al2O3 concentration in the former was 2.85% as deter-mined by EDS, whereas that of the latter was only 1.81%. Generally,the degree of wear of such a coating depends on the volumetric contentof particles in the composite coating. Attempts to increase the incor-poration of co-deposited particles using various methods have beenreported by many researchers. This result demonstrated that automaticbrush plating can increase the volumetric content of particles in thecomposite coating. High vol.% of Al2O3 in automatically preparedcoating may be mainly attributed to the continuous supply of freshsolution.
3.2. Microhardness and elastic modulus of the coatingsFig. 6 shows the load–depth curves for the automatically preparedcomposite coating subjected to nanoindentation. The average analysisresults of five tested points are listed in Table 3. One may see that theload–depth curves of the specimen are very close because of itshomogeneous surface. This result indirectly implied that compositecoating prepared automatically were relatively uniform. The micro-hardness values for the sample appearing in Table 4 range from a lowof 6.042821 GPa to a high of 6.620412 GPa. The average microhard-ness value is 6.30026 GPa. The Young’s modulus of the compositecoating is given by:1E*¼ ð1−m2ÞEþ ð1−mV2ÞEVð1Þwhere E and E′ and ν and ν′ are the modulus and Poisson's ratios ofthe coating and indenter, respectively. E* is the reduced modulus.Here, we have average E*=179.10163±4.112 GPa, E′=1141 GPa,ν′=0.28, ν=0.31, hence we got the Young's modulus of the compositecoating E=191.59415 GPa. This value is higher than that (E=161GPa) of the manually plated coating reported in Ref. [14]. This resultmay be attributed to the dense microstructure and the high vol.% ofAl2O3 particles in the composite coating.3.3. Wear resistance of the coatingsThe evolution in friction behavior of the composite coatings againstthe quenched 45 steel under lubricant condition is presented in Fig. 7.The coefficient of friction (COF) versus time is plotted for automati-cally plated coating (C1) and manually plated coating (C2). During therunning-in period, COF increased from a low value to a very high value, and thereafter, COF marginally decreased and attained a steadystate value. Comparing the steady state COFs, manually plated coatingshows a higher value of 0.101, whereas it is much lower in the case ofautomatically plated coating. One may see that the fluctuation of COFof automatically plated coating is much smaller than that of manuallyplated coating. This result may be attributed to the uniform and densecoating prepared automatically.4. ConclusionsAn automatic brush plating system was developed to up-grade the conventional brush plating in which the depositquality largely depends on the operator's skill and experience.This system realized precise control of relative anode to cath-ode speed, continuous supply of fresh solution, quick changefrom one step to the other, cooling of plating tool, and contactpressure between anode and surface to be plated. With thissystem, Ni/nano-Al2O3 composite coatings on 45 steel sub-strates were prepared from an electrolyte containing 20 g/l nano-Al2O3 particles. For comparison, Ni/nano-Al2O3 com-posite coatings were also prepared by conventional brush plat-ing. Surface morphology observed by SEM revealed thatautomatically prepared coating were much more dense, smoothand uniform than that prepared by conventional brush plating.The former also exhibited better mechanical properties than thelatter. In the present study, the average microhardenss of theautomatically prepared composite coating was 6.30026 GPa,and its Young's modulus was 191.59145 GPa.
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