Abstract:For the first time, functionally electroless nickel plated ZrO2 (NCZ) graded Ni-NCZ composite coating has been successfully co-electrodeposited from a bath with gradually increasing of stirring rate. Studies showed that co-electrodeposition in a bath with stirring rate of 250 r/min results in the maximum co-electrodeposited particle content and the best particle distribution. To produce NCZ graded Ni-NCZ composite coating, the stirring rate was continuously increased from 0 to 250 r/min. The electroplated coating had a continuous gradient increasing of co-electrodeposited NCZ content from substrate towards the surface. The results showed that with increasing the co-electrodeposited NCZ particles content in Ni matrix, microhardness increases from interface towards the surface of the coating. Little crystallite size of Ni matrix and higher co-electrodeposited hard particles content were recognized as the reasons of microhardness increasing. Bend test revealed that the functionally graded composite coating shows more excellent adhesion to the substrate compared with the ordinary distributed Ni-NCZ on the same substrate. This result is attributed to lower mechanical mismatch between coating and substrate in the functionally graded composite coating with respect to the uniformly distributed one. The results of wear resistance measurements reveal that wear resistance of functionally graded Ni-NCZ is higher than that of ordinary distributed composite coating.

Key words: co-electrodeposition; functionally graded composite coating; microhardness; wear resistance; adhesion