Annealing behavior of electrodeposited Ni-TiO2 composite coatings

The properties of nickel-ceramic composite electrodeposits are greatly influenced by the type and amount of ceramic particles in the deposit and the microstructure of the nickel matrix. In the present study, Ni-TiO2 coatings were electroplated in the nickel sulfamate bath containing 50 g l−1 TiO2. Results show that TiO2 codeposited in nickel led to the formation of nanocrystalline nickel grains surrounding the TiO2 particle and the increase in crystalline defects such as twins, dislocations and grain boundaries. These defects, on one hand, enhanced the hardness of the Ni-TiO2 coating and, on the other hand, reduced the recrystallization temperature of nickel. Nevertheless, codeposited TiO2 effectively inhibited the growth of recrystallized nickel grains. And yet, the hardness of the composite coating dropped drastically after annealing at temperatures higher than 600 °C due to the relatively large voids formed by the reaction of TiO2 particles with nickel.