Effects of different Y compounds on the microstructure and properties of laser cladding NiTi coatings in a semi-open environment

In recent years, nickel–titanium (NiTi) coatings have shown excellent tribological performance due to their unique phase transition characteristics. However, in the laser cladding of NiTi coatings in a semi-open environment, there are still issues with nitrogen and oxygen impurities arising in the coatings due to their exposure to air. In this study, three rare earth (RE) compounds were used to modify NiTi coatings in a semi-open environment to obtain coatings without any impurities. The effect of different RE compounds (Y2O3, YF3 and YCl3) on phase composition, surface oxidation and mechanical properties were studied. NiTi, NiTi2 and the impurity phase titanium nitride (TiN) were observed in the original and Y2O3 modified coating. In addition, incompletely melted Y2O3 exists in the Y2O3-modified coating. The coatings with YCl3 and YF3 were composed of NiTi and NiTi2 phase and the TiN impurity phase was not observed in either coating. In addition, the oxidation phenomenon at the top of the coating gradually weakened in the original, Y2O3, YCl3 and YF3-modified coatings. Furthermore, the R-phase was characterised in the YF3-modified coatings. The average microhardness values of the original, Y2O3, YCl3 and YF3-modified coatings were 669.68 HV0.3, 755.1 HV0.3, 579.1 HV0.3 and 390.47 HV0.3, respectively, of which the microhardness of the YF3-modified coating was the lowest. Although RE compounds did not significantly change the tribological properties, the Y2O3-modified coating exhibited the best tribological properties (∼0.886 mm, 2.6 mg). The results show that YF3 can isolate pollutants from the air in the coating and protect the coating in a semi-open environment.