Nanoindentation and nanoscratch behavior of electroless deposited nickel-phosphorous coating

• Electroless NiP coating was annealed at 600 °C in an argon atmosphere.
• Electroless NiP coating was observed to follow inverse Hall-Petch relation.
• Hardness, strain hardening and elastic modulus increased on annealing the coating.
• Increased scratch hardness in annealed coating leads to reduction in the COF.
• Direct correlation was found between nanoindentation and nanoscratch results.

Electroless nickel-phosphorous (NiP) coating was deposited on steel substrate followed by annealing at 600 °C in an argon atmosphere. Nanoindentations and nanoscratch experiments were performed on the as-deposited and annealed coating. Improvement of 32% in hardness was found on annealing the coating, which might be due to the two major phenomena. Firstly, presence of Ni3P precipitates which accelerated the bowing of dislocations and observed to be the dominant mechanism in increased hardness. Secondly, diffusion of iron (Fe) and chromium (Cr) atoms from the substrate to the coating. Scherrer technique and TEM analysis showed the increase in Ni grain size on annealing. Annealed coating showed an enhancement of 31% in elastic modulus compared to as-deposited coating. Increased elastic modulus could be due to the reduction in triple junction and grain boundary as a result of increased grain size. Strain hardening exponent was found higher for annealed coating (0.33) compared to as-deposited coating (0.25) indicating lower pile-up formation in annealed coating during indentation. Higher scratch hardness resulted lower scratch width and depth for the annealed coating. Coefficient of friction decreased from 0.30 for as-deposited coating to 0.23 for annealed coating, resulting 58% reduction in calculated wear rate of the scratch of annealed coating. Direct correlation was observed between nanoindentation and nanoscratch results of as-deposited and annealed NiP coatings.

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