Effect of substrate bias voltage on the mechanical properties and deformation mechanisms in the nanostructured Ti-22Nb-10Zr coating
Abstract
The manufacturing of future implants based on NiTi shape memory alloys, like stents, orthodontic archwires, intracranial aneurysms..., clips aims to eliminate the cytotoxicity and allergic problems associated with the release of nickel ions. In this context, the design of beta-rich Ti-22Nb-10Zr (mass.%) coating by magnetron sputtering, with a non-linear elastic behavior and tunable mechanical properties, offers a novel approach for the development of a new biocompatible implants with self-adjusting mechanical properties.
The XRD and TEM analyses reveal that microstructure consisting of hexagonal close-packed (alpha-phase), prevalent body-centered cubic (beta-phase) and orthorhombic (alpha ''-phase) nanograins, which can be tuned during the deposition by a single parameter - applied bias voltage as a result of the activation of a stress-induced martensitic transformation (beta -> alpha ''). We found that the minimum stress value to trigger the SIM transformation must be higher than 712 MPa (bias -63 V).
The application of higher bias voltage values alters the main deformation mechanism, from a combination of reversible SIM transformation and mechanical twinning to dislocation slip, causing the compressive residual stresses to decrease from 712 to 120 MPa, the hardness increases from 2,1 to 4,1 GPa, and the coating stops showing low Young's modulus (<50 GPa) and non-linear elastic behavior.