Atomic Layer Deposited Al2O3 Coatings on NiTi Alloy

Surface treatment of NiTi alloys to improve their biocompatibility is of interest. In this study, TiO₂ and/or Al₂O₃ layer is deposited on the surface of NiTi thin plates as a protection layer by using atomic layer deposition (ALD). The self-limiting reaction mechanism of ALD can allow extremely conformal coating on 3D surface with a thickness control in atomic scale. Titanium tetrachloride (TiCl₄) and trimethylaluminum (TMA) are used as the precursors of Ti and Al, respectively, while water is applied as an oxidant. In a typical ALD reaction cycle, 0.04 s pulses of metal precursor and water are separated by 5 s purge of mass amount of N₂ to prevent unwanted chemical vapor deposition. During the ALD reaction, the temperature is kept at about 40^oC to prevent any heat treatment of NiTi. Deposition rates of TiO₂ and Al₂O₃ on silicon substrates are 0.65 and 1.3 Å/cycle, respectively. The coarse surface of NiTi plates is smoothened by adding the Al₂O₃ layer.

          Various ALD cycle numbers are chosen to test the thickness effect on corrosion resistance. The coated surface of NiTi is observed using scanning electron microscopy before and after tensile mechanical loading. An attention is paid to some negative phenomena such as delaminating of the coated layers. The corrosion resistance of ALD thin films is studied using potentiodynamic scan and impedance spectroscopy techniques in Hank’s and borate buffer solution at 37^oC. Character of interfacial reactions is described using point defected model. We expect to see positive effect of uniform and conformal passive barrier layers on the resistance against corrosion attack accompanied by hydrogen absorption.