Characterization of Ni-P based poly-alloy and composite coatings involving nanoindentation and nanoscratch tests

Tungsten and titanium dioxide-based alloy and composite coatings are customarily used in tribological applications. On the contrary, copper-based alloy coatings are preferable for anti-corrosion applications. Thus with an aim to develop poly-alloy and composite coating for tribological applications, two different quaternary coatings viz., Ni-P-Cu-W (poly-alloy) and Ni-P-Cu-TiO2 (composite) were synthesized and studied. Hardness and elastic modulus of the coatings were studied by nanoindentation tests. These properties are the primary indicators of the quality of the coatings and are also useful in the modelling of micro or nano-scale friction and wear. Nano-tribological properties of these coatings were investigated by nano-scratch tests. The impact of annealing temperature on the nano-mechanical, and nano-tribological properties of these coatings was investigated. The results revealed that the hardness and elastic modulus of Ni-P-Cu-W coating reached a maximum value of ~10.48 GPa and ~249.07 GPa respectively after annealing at 500 °C due to grain enlargement and Ni3P phase formation. Consequently, the lowest wear rate (~5.00 × 10−14 m2N−1) and coefficient of friction (~0.29) were obtained after annealing at 500 °C. Regarding Ni-P-Cu-TiO2 coating, maximum hardness (~0.79 GPa) and elastic modulus (~98.05 GPa) were obtained at 400 °C annealed condition. As expected the lowest wear rate (~2.16 × 10−11 m2N−1) and coefficient of friction (~0.08) were observed after annealing at this temperature. In composite coating heating at 400 °C caused grain growth and formation of stable Ni3P phase, but at the further higher temperature it transformed into metastable phase and those properties deteriorated. Pile-up formation also decreased due to the increase of strain hardening exponent with annealing temperature. Heat treatment improved the properties more significantly in alloy coatings compared to composite coatings.