I. Introduction
UNIQUE shape-memory effect, superelasticity, and excellent biocompatibility are displayed by NiTi. However, it contains a large amount of Ni, and its use in subjects with high Ni sensitivity and long-term stability is questionable, though most studies have shown that NiTi is highly stable in the body with minimal Ni release [1]–[3]. Currently, much work has been done to modify the surface property of NiTi alloy [4]–[6]. As surface coating is an effective method to improve the durability of the materials used in the aggressive environments, therefore by selecting proper coating methods and coating materials, we may simultaneously prolong the service life of the substrate materials and increase the commercial value of the products.