1 338。728167 15。035357 303。346580 6。042821 179。062912

2 332。787120 15。011525 295。240009 6。334121 172。617683

3 336。256805 15。023848 300。926200 6。125806 180。479285

4 330。748593 15。043730 294。454490 6。378125 179。388427

5 324。098987 15。014199 288。076245 6。620412 183。959844

3。2。 Microhardness and elastic modulus of the  coatings

Fig。 6 shows the load–depth curves for the automatically prepared composite coating subjected to nanoindentation。 The average analysis results of five tested points are listed in Table 3。 One may see that the load–depth curves of the specimen are very close because of its homogeneous surface。 This result indirectly implied that composite coating prepared automatically were relatively uniform。 The micro- hardness values for the sample appearing in Table 4 range from a low of 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 composite coating is given by:

unloading so as to correct for any thermal drift in the system。 At  least  5  indents  were  made  for  each  specimen,  with the

adjacent indents separated by 10 μm。

3。Results and discussion

3。1。 Morphology of the coatings

Surface morphologies of the Ni/nano-Al2O3 composite coatings prepared automatically and manually are shown in Figs。 2 and 3。 It was observed that the specimen prepared by automatic brush plating had a smaller crystalline microstructure than that of the specimen prepared by conventional brush plating。 The former is much more dense, smooth, and uniform than the latter。 This result may be attrib- uted to the continuous supply of fresh solution, appropriate contact pressure 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 composite coatings 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 content of particles in the composite coating。 Attempts to increase the incor- poration of co-deposited particles using various methods have been reported by many researchers。 This result demonstrated that automatic brush plating can increase the volumetric content of particles in the composite coating。 High vol。% of Al2O3 in automatically prepared coating may be mainly attributed to the continuous supply of fresh solution。

Table 3

Average analysis results of five points

where E and E′ and ν and ν′ are the modulus and Poisson's ratios of the 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 composite coating E = 191。59415 GPa。 This value is higher than that (E = 161 GPa) of the manually plated coating reported in Ref。 [14]。 This result may be attributed to the dense microstructure and the high vol。% of Al2O3  particles in the composite coating。