The effect of composition and heat treatment on microhardness of Ni-P and Ni-P-nanoZrO2 coatings

Electroless Ni-P coatings have widespread industrial usage and their composition can be tailored by the introduction of hard ceramic nanoparticles, to create a nanocomposite coating with improved properties. In this work, nanoZrO2 reinforced Ni-P coating with medium P content (MP, ~6 wt.%) and high P content (HP, ~11 wt.%) were produced and heat treated at 200°C, 340°C, 400°C and 600°C, to study the evolution of microstructure and microhardness properties as a function of coating composition and annealing temperature. Results demonstrated that introducing ZrO2 nanoparticles efficiently increases the hardness of both MP and HP nanocomposites without altering their microstructure, either in the as-deposited state or after heat treatments. Conversely, P content strongly affects microstructure and crystallization behavior upon heat treatments: nanocrystalline MP coatings experience Ni3P precipitation with treatment at 340°C, with subsequent controlled grain growth, whereas amorphous HPs undergo crystallization and Ni3P precipitation after treatment at 400°C. Microhardness evolution according to microstructural changes was evaluated and both coatings exhibited the highest HV50 values after treatment at 400°C, at which complete Ni3P precipitation is achieved. Microhardness of nanocomposite coatings is always higher than particle-free ones, demonstrating that the synergistic effect of dispersion hardening (given by nano-ZrO2) and precipitation strengthening (given by Ni3P) can be an effective method to improve properties and meet the demanding need of engineering applications.