Microstructural and hardness studies of ZrO2 reinforced NiP nano-composite coatings for anti-erosion and anti-wear applications

Electroless NiP coatings are widely employed in energy production industry to protect compressor components from corrosion, erosion and wear. Coating modification introducing ceramic nano-particles as reinforcing phase leads to the improvement of matrix hardness, enhancing performances in terms of reliability and durability. This study aims to produce NiP-ZrO2 nanocomposite coatings capable of achieving a synergistic improvement of properties through the optimization of composition and microstructure. Coatings with medium P content (MP, 5-6 wt%) and high P content (HP, 10-12 wt%) reinforced with different amounts of nano-ZrO2 are studied to evaluate hardness according to compositional variations. The incorporation, dispersion and distribution of nanoparticles are assessed by coupling SEM imaging with original procedure for image analysis implemented in MatLab. Evolution of properties upon microstructural changes were evaluated via XRD analysis and microhardness measurements after heat treatments at 200°C, 340°C, 400°C and 600 °C for 1h and at 340°C and 400°C for 0.5h to 8h. Results demonstrated that a concentration gradient of embedded nanoparticles is present across the thickness of MP coatings, with higher %vol of agglomerated nano-ZrO2 near the substrate. Higher microhardness values correspond to areas with higher %vol of well-dispersed nanoparticles. Differently, a homogeneous distribution of nano-ZrO2 was found on HP coatings, associated to uniform hardness values. Analysis after 1h thermal treatments demonstrated that hardness increases with increasing treatment temperature up to 400°C in both plain and nanocomposite samples, whereas higher temperature leads to oxidation and decreasing in mechanical properties. No microhardness gradient was registered for reinforced coatings after thermal treatment. Tuning temperature and duration of heat treatment allows control over crystallinity and grain growth by kinetics means, allowing management of coating properties.