Improved nanoindentation phase transformation in functional structure of NiTi SMA and graphene

Graphene layers were deposited on the surface of NiTi shape memory alloy (SMA) to enhance the spherical indentation depth and the phase transformed volume through an extra nanoscale cooling. The graphene-deposited NiTi SMA showed deeper nanoindentation depths during the solid-state phase transition, especially at the rate dependent loading zone. Larger superelastic deformation confirmed that the nano-scale latent heat transfer through the deposited graphene layers allowed larger phase transformed volume in the bulk and, therefore, more stress relaxation and depth can be achieved. During the indentation loading, the temperature of the phase transformed zone in the stressed bulk increased by ~12-43°C as the loading rate increased from 4,500 μN/s to 30,000 μN/s. The layers of graphene enhanced the cooling process at different loading rates by decreasing the temperature up to ~3-10°C depending on the loading rate.