Effects of superelasticity and shape memory ability of NiTi-based alloys on deposition efficiency of ultrasonic-assisted coating

NiTi-based alloys are well known for their superelasticity and shape memory effects, which are attributed to the austenitic and martensitic phases of the alloys, respectively. The effects of NiTi-based alloys with different phases on the deposition efficiency and coating adhesion were investigated. The HA-containing coatings were deposited on Ni51.4Ti48.6 (NiTi) in the austenitic phase and Ni49.75Ti48.25Ta2 (NiTiTa) in the austenitic and martensitic phases by ultrasonic mechanical coating and armoring. The coating on the NiTiTa has a higher thickness (3.3 µm) than that on the NiTi (0.7–1.5 µm). The particles within the coating on NiTiTa with a size of 20–40 nm which was smaller than those on NiTi with a size of 50–400 nm. In addition, the particles near the NiTiTa substrates were deformed to an elliptical shape; however, on NiTi, the particles maintained a spherical shape. The relatively low deposition efficiency and poor coating adhesion of NiTi are due to its superelasticity, which cannot be well deformed by ball bombardment. Further, ball bombardment induced phase transformation, and increased the phase transformation temperatures of the alloys. Furthermore, the HA-containing coatings on the alloys improved the corrosion resistance and bioactivity of NiTiTa and NiTi.