Indentation and bending behavior of electroless Ni-P-Ti composite coatings on pipeline steel

Electroless Ni-P coatings have been adopted in many industrial applications due to their excellent wear and corrosion resistance. The unique properties and autocatalytic nature suggest a potential usage as a pipeline inner coating in oil and gas industries. However, low toughness and erosion resistance limit the suitability of electroless Ni-P coatings as pipeline coatings. To improve the toughness of the Ni-P coating, nano-sized titanium particles were used to reinforce Ni-P coating in this study. The effect of titanium particles on Ni-P coatings and the effect of annealing on the structure and mechanical properties were studied. The morphology and structure of Ni-P-Ti coating were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Vickers hardness, three-point bending and Hertzian indentation tests were employed to characterize the mechanical properties of Ni-P-Ti coating. It is found that adding titanium did not change the semi-amorphous microstructure of as-deposited Ni-P coating. The micro-hardness of as-deposited and annealed Ni-P-Ti coating was found to be higher than that of Ni-P coating. Furthermore, the addition of titanium particles increases toughness of the as-deposited coating. Also, it is found that toughness increases with increasing annealing temperature. The annealing also has a significant effect on the indentation behavior of the coatings.