Hertzian Indentation Behavior of Electroless Ni-P-Ti Composite Coatings

Ni-P-Ti composite coatings have been prepared by co-depositing Ni-P and nano-Ti particles which were then annealed. Tensile tests were conducted on standalone coatings to acquire the mechanical properties of the coatings without the effect of the steel substrate. The Hertzian indentation tests were performed on coating-steel substrate bilayer systems to investigate the indentation behavior of the coatings. After annealing, Ni3Ti and superelastic NiTi phases are identified in composite coatings by XRD and EDS. Compared to the as-deposited Ni-P coatings, the toughness of the annealed composite coatings improved significantly. The superelastic effect of NiTi particles was observed in the Hertzian indentation behavior of the annealed composite coatings. The strengthening mechanisms as well as toughening mechanisms such as crack bridging, crack deflection, crack arresting, and transformation toughening were discussed. Recovery ratio (eta) values as well as Hertzian indentation stress distribution were computed to analyze and explain the indentation behavior of the as-deposited and annealed coatings.