Microstructure formation and properties of HVOF sprayed NiTi coatings prepared from amorphous/nanocrystalline NiTi powders

NiTi alloys were sprayed on AISI 1045 steel substrate using high velocity oxyfuel process. In this regard, amorphous/nanocrystalline NiTi powders were used as feedstock. The phase composition and microstructure of the coatings were examined by X-ray diffraction and scanning electron microscopy. The results indicated that B2-NiTi (austenite phase), Ni₄Ti₃, Ni and NiTiO₃ were present in the coatings. In addition, all sprayed coatings showed high microhardness values (>588 HV0·3). Further investigations showed that with the increase in fuel/oxygen ratio, the oxide content increased and porosity level decreased. Hereon, a dense coating with porosity level of 0·8% and oxide content of 5% was obtained as the optimum coating. Polarisation tests were employed to study the corrosion behaviour of the optimum coating in 3·5%NaCl solution. These tests indicated that the corrosion current density of the optimum coating is comparable to that of NiTi alloys reported in the literature.