NiTi Wires Coated by Nanomultilayers – A Solution for Self-healing?

One of the causes that can lead to the replacement or repair of metallic components is the propagation of cracks, which limits their lifetime. In this context, it is critical to develop self-healing systems for repairing cracks locally [1]. A possible strategy for this purpose consists in the incorporation of nitinol (NiTi) wires coated with nanoscale reactive multilayer (ML) thin films. These multilayers react releasing heat in a highly localized way, and in certain circumstances the reaction can become self-sustained. Therefore, reactive MLs are promising for a variety of applications, including joining [2]. The heat released after reaction can be used to melt a repairing material (low melting temperature) which will serve as a mobile phase to fill the crack. The coated material constitutes a self-healing system because the shape memory NiTi wires themselves can be used as sensors for the detection of cracks. This paper aims at coating nitinol wires with reactive ML thin films in order to use the heat released by the self-propagating reaction of the MLs for repairing cracks in metallic components. The Ni-Al system was selected because the equiatomic Ni/Al multilayer films exhibit a good compromise between cost and heat of reaction. Given the lack of studies on ML coating of wires, firstly it is necessary to study the possibility of depositing ML thin films onto the curved surface of the wires.