Effects of Ti Content and Annealing on Fracture Toughness and Scratch Resistance of Electroless Ni-P-Ti Coatings

Ni-P coatings exhibit good corrosion resistance; however, they have not been used in applications where high toughness and wear resistance are required. Therefore, to enhance the toughness and wear resistance of Ni-P coatings, superelastic NiTi particles were incorporated within the Ni-P coatings. However, due to the high cost of the superelastic NiTi compared to Ti powder, Ti was co-deposited with Ni-P and subsequently annealed to produce Ni-P-NiTi coating. The fracture toughness and scratch resistance of the coatings were measured by scratch tests. The effects of Ti content and annealing on coatings’ microstructure were investigated using XRD and laser confocal microscope. The relationship between the microstructure and the fracture toughness and scratch resistance was also studied. It is found that the formation of superelastic NiTi particles within the annealed Ni-P-Ti coating significantly improves the coating’s fracture toughness and scratch resistance. The average grain sizes of Ni and Ni3P decrease with an increase in Ti content and decrease in annealing temperature and time. The reduction in grain size improves both fracture toughness and hardness (or strength), which gives rise to promoted scratch resistance.