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 (Y 2 O 3 , YF 3 and YCl 3) on phase composition, surface oxidation and mechanical properties were studied. NiTi, NiTi 2 and the impurity phase titanium nitride (TiN) were observed in the original and Y 2 O 3 modified coating. In addition, incompletely melted Y 2 O 3 exists in the Y 2 O 3 -modified coating. The coatings with YCl 3 and YF 3 were composed of NiTi and NiTi 2 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, Y 2 O 3 , YCl 3 and YF 3 -modified coatings. Furthermore, the R-phase was characterised in the YF 3 -modified coatings. The average microhardness values of the original, Y 2 O 3 , YCl 3 and YF 3 -modified coatings were 669.68 HV 0.3 , 755.1 HV 0.3 , 579.1 HV 0.3 and 390.47 HV 0.3 , respectively, of which the microhardness of the YF 3 -modified coating was the lowest. Although RE compounds did not significantly change the tribological properties, the Y 2 O 3 -modified coating exhibited the best tribological properties (∼0.886 mm, 2.6 mg). The results show that YF 3 can isolate pollutants from the air in the coating and protect the coating in a semi-open environment. • NiTi alloy coating was prepared by laser cladding in a semi-open environment. • The TiN impurity phase disappeared inside the coating after the addition of YF 3. • The coating with YF 3 addition has the lowest microhardness. • The YF 3 prevents the Ti element in coatings from being contaminated by N 2 in the air. [ABSTRACT FROM AUTHOR]