TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation

ABSTRACT The martensitic phase transformation in Ti40.4Ni48Hf11.6 shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO2/Si composites. The shape memory properties of magnetron sputtered Ti40.4Ni48Hf11.6 films annealed at 635°C – 5 min are influenced by film thickness and the underlying substrate. Decreasing TiNiHf film thickness from 21 μm to 110 nm results in the reduction of all characteristic transformation temperatures until a critical thickness is reached. Particularly, Ti40.4Ni48Hf11.6 thin films as low as 220 nm show transformations above room temperature when deposited on SiO2 buffer layer, which is of great interest in nano-actuation. In comparison, 220 nm films on Si substrates are austenitic at room temperature, and thus not suitable for actuation. Thermal fatigue tests on TiNiHf/SiO2/Si bimorphs demonstrate better functional fatigue characteristics than freestanding films, with an average reduction of 15°C after 125 cycles, with temperature stabilization subsequently. Experimental bi-directional actuation results are promising in the development of bistable actuators within a PMMA/TiNiHf/Si trimorph composite, whereby the additional PMMA layer undergoes a glass transition at 105°C. With the aid of constitutive modeling, a route is elaborated on how bistable actuation can be achieved at micro- to nanoscales by showing favorable thickness combinations of PMMA/TiNiHf/Si composite. Graphical abstract