Tribological Characterization of Friction-induced Phase Transformations in Binary Nitinol

Abstract The binary alloy of nickel and titanium, commonly known as Nitinol, is widespread in the biomedical industry such as vascular stents. Recently, Nitinol’s application potential has expanded, including as actuating materials in ultrasonic transducers and robotic systems. In such cases, the temperature-dependent elastic moduli of Nitinol can be controlled via mechanical load or temperature triggers, between those for compliant martensite and stiff austenite. However, we have limited the understanding of the contact mechanics, predominantly the surface reactions and wear rate, of binary Nitinol in contact with other materials. The importance of this is understanding how Nitinol can be integrated in a wider range of industrial applications. Here, tribological investigations are undertaken to assess the phase transformation and contact mechanics of Nitinol under friction conditions, for stainless steel and titanium, chosen as common interfacing materials in actuation systems such as ultrasonic transducers. Experimental results demonstrate the phase transition from martensite to austenite via changes of elasticity in shape memory Nitinol rubbing against stainless steel under a dry condition. During these interactions, Nitinol exhibits a mass reduction of 0.00091 g, with a wear depth of 1.7 µm. Thermal analysis revealed that the austenite start temperature (As) is very close to ambient room temperature, at 25ºC. A multiple-load cycle indentation using a three-sided diamond tip was employed to evaluate the response of Nitinol to localized applied forces, where martensitic Nitinol has experienced a maximum force of 490 mN. Microscopic observations confirm the formation of oxide layers on Nitinol’s surface resulting from titanium exposure to oxygen. Meanwhile, the nonlinear hardness for martensitic Nitinol has been exhibited with increasing depth of penetration, exhibiting a maximum of 4.5 GPa. The novelty of the research is centered on both the tribological and mechanical performance of Nitinol-based actuating systems for a wide range of medical and industrial applications.