Nanostructured hydroxyapatite coatings on NiTi shape memory alloys by ultrasonic mechanical coating and armouring

Mechanical energy-induced deposition on NiTi shape memory alloys (SMAs) is difficult to accomplish due to the pseudoelasticity of NiTi SMAs. In this study, a high-frequency ball-bombardment method called ultrasonic mechanical coating and armouring (UMCA) is introduced to synthesize a hydroxyapatite (HA)-containing coating on a NiTi SMA. The effects of treatment duration, Ti addition, and ball-to-powder ratio on the HA content of the coating are investigated. Results show that the HA content of the coating largely increased during the initial 10 s and decreased with further ball-bombardment. The addition of Ti powder into the chamber assisted HA anchoring on the NiTi SMA surface. Using a ball-to-powder ratio of 1:1 induced the highest HA content on the NiTi SMA as compared to the 1:2 and 2:1 ratios. Coating consisting of 10–50 nm nanoparticles of HA and Ti formed under localized compressive forces were induced through shot collisions. The bombardment also created a local amorphous structure of the NiTi SMA near the coating/NiTi SMA interface. Laser annealing induced oxidation of the coating, which improved the coating adhesion and corrosion resistance of the NiTi SMA in Hank's balanced salt solution.