AFM nanoindentation-based study of thickness effects on the pseudoelastic behavior of Ni-Mn-Ga thin films

In freestanding form or attached to the substrate, Ni-Mn-Ga Shape Memory Alloys (SMAs) thin films and their near-stoichiometric configurations have attracted interest in recent years for applications in next-generation MEMS technologies. Thin films' capacity to recover stress-induced strain energy and total strain are critical to assess their potential for applications in these technologies. However, these capacities have not been extensively explored in this kind of alloys, especially at the nanoscale. In this work, we report a study of these aspects at the nanoscale in near-stoichiometric Ni₂MnGa thin films by Atomic Force Microscopy (AFM)-assisted nanoindentation technique. Films with thickness ($t$) of 100, 250 and 500 [nm] fabricated on MgO(001) monocrystalline substrates by DC magnetron sputtering were studied. Results show that films exhibit a high capacity to recover stress-induced strain energy (> 70 % of the total strain energy) for relatively high indentation depths (> 0.3$t$). Pseudoelasticity effects were observed under certain film size and indentation depths conditions, which was evidenced by the presence of practically no mechanical hysteresis (plastic strain) concerning maximum strains that are comparable to the films' thicknesses. This behavior was observed together with considerable strain energy dissipation, suggesting the emergence of the pseudoelastic response due to stress-induced martensitic transformation. Our results suggest that the pseudoelastic behavior is strongly dependent on the film thickness, which seems to involve a competition between substrate-induced hardening effects and bulk martensitic transformation.