Effect of polyurethane coating on mechanical response of shape memory alloy wires

Abstract

In micro-electro-mechanical systems, the risk of short circuit arises when shape memory alloy (SMA) wire actuators mistakenly come into contact with other metal parts. Coating the SMAs with insulating layer can solve this problem. However, unlike traditional conductive wires that experience minimal length changes with current, SMA wires undergo significant length variations during electrical execution. This can affect the bonding between the coating and the SMA wires, as well as the mechanical response of the actuators. Therefore, this study investigated the effect of an insulating coating on the mechanical response of SMA wires. The results show that a layer of polyurethane coating provides long-term insulation to the SMA wires. While this coating reduces the recoverable strain of SMA wires under passive heating, its effect on actuation strain under electrical heating is relatively minor. Additionally, the coating enables the SMA wire to work smoothly by relieving overshooting and vibration phenomena during execution, and helps the wire work in a wider current range than an uncoated wire. Moreover, the coating enhances the response speed of SMA wires during power-on but reduces the response speed during power-off, this influence weakens as driving currents increase. These distinct mechanical responses can be primarily attributed to the coating’s influence on heat transfer behavior of the SMA wires.