Ultrahigh superelastic damping at the nano-scale: A robust phenomenon to improve smart MEMS devices

Micro and nano pillars of Copper-based shape memory alloys (SMAs) with feature sizes between about 2 μm and 250 nm are known to exhibit ultra-high mechanical damping due to the nucleation and motion of stress-induced martensite interfaces during superelastic straining. While this behavior could be extremely useful to protect micro electro-mechanical systems (MEMS) against vibrations in aggressive environments, a fundamental question must yet be answered in order to envisage further applications, namely, whether this damping is reproducible and stable over long times and many cycles, or whether the damping is a signal of accumulating damage that could compromise long-term usage. In the present paper this crucial question is answered; we show that micropillar arrays of Cu-Al-Ni SMAs exhibit a completely recoverable and reproducible superelastic response, with an ultra-high damping loss factor η > 0.1, or even higher for sub-micrometer pillars, η > 0.2, even after thousands of cycles (>5000) and after long times spanning more than four years. Furthermore, the first high-frequency tests on such nanoscale SMAs show that their superelastic response is very fast and relevant to ultra-high damping even at frequencies as high as 1000 Hz. This paves the way for the design of micro/nano dampers, based on SMAs, to improve the reliability of MEMS in noisy environments.