Structural Characterization of Intermetallic NiTi Coatings Obtained by Thermal Spray Technologies

Abstract

NiTi alloy is known to have a potential interest specially for its superelasticity and shape memory effect (SME). Most of the research undergone up to now investigates such properties in bulk materials. The few works related to overlays (coatings) use magnetron sputtering for thin films and Vacuum Plasma Spray (VPS) for thicker coatings. VPS is the mostly used thermal spray technique regarding the Ti oxidation problems which considerably affect the so desired SME. The present work compares the microstructure of VPS NiTi coatings with those of High Velocity Oxygen Fuel HVOF and a liquid cooling device coupled to a Atmosphere Plasma Spraying (APS+Q), with optimized spraying conditions trying to minimize Ti oxidation. Apart from their characterization by Scanning Electron Microscopy, the main purpose is to further study how particles are retained within the coating so that Transmission Electron Microscopy is also employed. This has allowed the observation of amorphous and nanocrystalline phases as well as to the change from cubic B2 NiTi lattice to metastable lattices.