Anti-fracture performance of micro-arc oxidization films on TiNiSi shape memory alloy after elongation, bending and shape recovery

Abstract The adhesion of a coating on a shape memory alloy (SMA) as an implant is very crucial, because the coating may break when the implant undergoes shape change and recovery during implantation. This study investigated parameters for micro-arc oxidation (MAO) applied to Ti 50 Ni 49 Si 1 SMA from the viewpoint of coating adhesion. Four kinds of adhesion tests were utilized to evaluate the adhesion of a coating: pull out, scratch, tension-recovery, and bending-recovery tests. Ti 50 Ni 49 Si 1 SMA is a martensitic phase alloy, and it has higher transformation temperatures (M s  = 48.83 °C and A f  = 112.19 °) than the equiatomic Ti 50 Ni 50 SMA. MAO produced an oxide film with a porous surface. Increasing the anodic voltage caused the pore size, the film thickness, and the surface roughness to increase, while the shape recovery rate was deceased. The oxide film formed by applying an 80 V voltage contained small amounts of nanopores. The application of 120 V or 150 V produced micropores and nanopores that were distributed in the middle of the film and film/substrate interface, respectively. The adhesive strength of the MAO films formed by 80 V, 120 V, and 150 V were 33.8 ± 6.4, 42.6 ± 4.8, and 78.9 ± 4.2 MPa, respectively. The oxide films did not fail in the pull out evaluation. The critical load of the MAO films was increased with the anodic voltage: its values for the oxide films formed at 80 V, 120 V, and 150 V were 71.2 ± 2.3, 90.9 ± 2.8, and 112.7 ± 2.4 mN, respectively, and they were measured by scratch tests. However, the use of a low anodic voltage or short duration is beneficial to MAO film adhesion when plastic deformation is imposed to the substrate. Specimens anodized at 80 V remained unbroken, but the films were detached at 120 V and 150 V after their elongation by tensile force and subsequent recovery by heating. The bending and recovery behaviours caused most of the MAO films to crack, except for the specimens anodized at 80 V for 3 and 5 min.