Tailoring the Surface of NiTi Alloy Using PIRAC Nitriding Followed by Anodization and Phosphonate Monolayer Deposition

Abstract

Nickel titanium (NiTi, nitinol) shape memory alloy was nitrided using a powder immersion reaction assisted coating (PIRAC) method to modify its surface properties. PIRAC nitriding is based on annealing the samples in an atmosphere of highly reactive nitrogen supplied by decomposition of unstable nitride powders or, alternatively, by selective diffusion of atmospheric nitrogen to the sample surface. Since it is not line of sight limited, PIRAC nitriding of nitinol alloys allows uniform treatment of complex shapes (e.g., surgical implants). It results in the formation of a Ni-free TiN film and considerably improves the corrosion behavior of the underlying NiTi alloy. The usefulness of PIRAC nitrided nitinol could be significantly enhanced by attaching to its surface a phosphonate anchored self-assembled monolayer (SAM). Phosphonate-anchored SAMs on PIRAC nitrided nitinol and on thin films of TiN sputtered onto a silicon wafer have been characterized by FTIR, contact angle measurements, and XPS analysis. The...