Ni/nano-TiO₂ Composite Electrocoatings: Correlation Between Structural Characteristics Microhardness and Wear Resistance

Abstract

Nanocomposite coatings were obtained by electrochemical codeposition of TiO₂ nano-particles (mean diameter 21 nm, Degussa P₂₅) with nickel, from an additive-free Watts type bath. Pure Ni and composite Ni-TiO₂ coatings were electrolytically deposited under both direct and pulse current conditions and an extended region of electrolysis conditions (pH, current density, TiO₂ loading in the bath). Pure Ni deposits were produced under the same experimental conditions for comparison. The aim of this study was to correlate the observed structural characteristics of the coatings (crystallographic orientation and grain size of nickel matrix,) and incorporation percentage with the resulting microhardness values and tribological behavior. Overall, the data have demonstrated that when attributing the observed strengthening effect of composites, not only grain refinement and dispersion strengthening mechanisms, but also preferred crystalline orientation should be taken into consideration. The variation of the wear rate is directly associated with the microhardness variations and consequently, governed by the same parameters that determine the synergistic strengthening mechanism proposed. Pure and composite coatings with [110] crystalline orientation exhibited the highest microhardness values and wear resistance.