NiTi thermal sprayed coatings characterization

Abstract
NiTi alloy has been particularly studied over many years for its shape memory properties and excellent corrosion resistance and biocompatibility, thus making it suitable for many biomedical applications. Then, bulk material properties have been well characterized depending on the nominal composition, manufacturing method and thermal treatments. It has been, however, less studied the use of such alloy in its overlay form, many times just employing Vacuum Spraying (VPS) for thick coatings in order to avoid Ti oxidation. The present study explores and compares the spraying of atomized NiTi powders by VPS as well as by Atmospheric Plasma Spraying (APS) and High Velocity Oxygen Fuel (HVOF). X-ray diffraction studies conducted at –100 and 100°C revealed that the feedstock powder presented a metastable rhombohedral NiTi phase at low temperatures and at room temperatures and higher the result was beta phase. DSC experiments also supported the previous phase transformations. After spraying, according to the thermal history of the particles in each spraying technology, it is observed the retention of the β-NiTi feedstock phase as well as the appearance of metastable phases, amorphization and nanocrystalline areas. Also with XRD at different temperatures and, especially in the case of the HVOF coating, due to the moderate heat input of the process, the final results fairly agreed with those of the original powder.