Hard films including carbon nitride (CNx), diamond-like carbon (DLC), and titanium nitride (TiN) were respectively deposited on the biomedical NiTi alloy substrate by using reactive direct current magnetron sputtering. A Ti transition layer was pre-deposited in order to strengthen the adhesion between the hard films and NiTi alloy substrate. The tribological properties of the deposited hard films were systematically investigated by using scratch tester and friction wear tester. It was shown that the tribological performance of NiTi alloy substrate was improved effectively after the deposition of gradient films. The deposition of Ti transition layer benefited in increasing the adhesion strength between the NiTi substrate alloy and the inorganic hard films, especially for the CNx film. Among the three types of gradient films, Ti/CNx film possessed the best tribological performance (with the minimum friction coefficient of 0.092 and the maintenance of smooth and intact surface after the friction and wear tests). (C) 2011 Elsevier B.V. All rights reserved.