Study on the Mechanical Properties of Heat-Treated Electroless NiP Coatings Reinforced with Al2O3 Nano Particles

Abstract

This work reports the effects of electroless co-deposition of Al2O3 nanoparticles and NiP to obtain a NiP-Al2O3 coating on the structure and mechanical properties of the composite coatings. The effects of annealing heat treatments at 373 K, 473 K, 573 K, and 673 K (100 °C, 200 °C, 300 °C, and 400 °C) on the structure and properties of the coatings were evaluated. The as-deposited coatings are a mixture of crystalline and amorphous phases that tend to crystallize during heat treatment. Heat treatment at higher temperatures causes the precipitation of the Ni3P phase. The mechanical properties of as-deposited and heat-treated NiP-Al2O3 coatings were evaluated using depth-sensing indentation tests performed at loads of 200 mN. The incorporation of Al2O3 nanoparticles induces strengthening of the NiP coating by dispersion. Heat treatment of the NiP-Al2O3 coatings induced crystallization of the amorphous phase with the formation of nanosized grains and the precipitation of Ni3P. Consequently, there is an increase in the hardness and Young’s modulus of the coatings to 15.4 ± 0.5 and 227 ± 2.8 GPa, respectively, in a combined hardening effect induced by dispersion of the Al2O3 nanoparticles and crystallization and precipitation during heat treatment.