Abstract
Electrodeposited CrAl 2O 3 composite coatings could be produced only when the Al 2O 3 particles were either coated with nickel or dry ground before mixing in the conventional hexavalent chromium plating bath. The nickel-coated Al 2O 3 particles were, however, unstable in the plating bath. Using dry ground Al 2O 3 powder it was shown that increasing the bath load of Al 2O 3 increased its content in the coating at all current densities. At all bath loads, maximum codeposition took place at 35 A dm −2. When the bath temperature was increased above 40 °C the Al 2O 3 content of the coating decreased. At 40 °C the coating was very uneven and cracked. The microhardness of the composite coating increased with increasing Al 2O 3 content and it was always larger than that of the pure chromium coating. The two-step adsorption mechanism proposed by Guglielmi does not hold in this system. It is proposed that the mechanism of codeposition consists of three processes, i.e. adsorption, transportation by hydrodynamic and electrophoretic forces and entrapment in the chromium matrix.
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