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

Abstract

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), Ni 4 Ti 3 , Ni and NiTiO 3 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.

Electrochemical impedance spectroscopy of HVOF-sprayed NiTi intermetallic coatings deposited on AISI 1045 steel

“…The formation of Ni 4 Ti 3 phase as a metastable precipitate has been previously reported during air plasma spraying of NiTi alloys [15]. The Ni phase is formed either due to crystallization of amorphous phase or preferential oxidation of Ti element during the spraying process [12,15]. Formation of Ni has been previously reported in HVOF spraying of NiTi intermetallic powders [11,12].…”

“…1. A broad diffraction peak at the Bragg angles of 41-45 • indicates the presence of an amorphous/nanocraystalline phase in feedstock powders [12][13][14][15]. The studies of HVOF coating revealed that the peaks belonging to NiTi (austenite phase), Ni 4 Ti 3 , Ni and also NiTiO 3 exist in the respective XRD pattern.…”

“…The Ni phase is formed either due to crystallization of amorphous phase or preferential oxidation of Ti element during the spraying process [12,15]. Formation of Ni has been previously reported in HVOF spraying of NiTi intermetallic powders [11,12]. It is speculated that NiTiO 3 is likely to have formed due to the oxidation of NiTi particles during the spraying process [12,13].…”

“…Formation of Ni has been previously reported in HVOF spraying of NiTi intermetallic powders [11,12]. It is speculated that NiTiO 3 is likely to have formed due to the oxidation of NiTi particles during the spraying process [12,13].…”

“…According to Tafel polarization tests, they concluded that HVOF NiTi coatings prepared from atomized powders showed fairly good corrosion resistance. On the other hand, it has been reported that the polarization behavior of the HVOF NiTi coating prepared from mechanically alloyed powders can be similar to that of the coating prepared from atomized powders [12]. Verdian et al have investigated the corrosion performance of HVOF NiTi coatings on 316L stainless steel substrate [13].…”