Processing and characterization of composite Shape Memory Alloy (SMA)thin film structures for microactuators

摘要

The high work to volume ratio and the stress recovery of Shape Memory Alloy (SMA) thin films with temperaturemakes them an ideal choice for microactuators. However, these materials have not gained widespread acceptance due toissues associated with phase transformation. Primary concerns are rapid change in stress at the transformationtemperature giving the actuator a step function like response and a significant shift in transformation temperatures due toa wide hysteresis. In the present research, TiNiCu (53.59at%Ti, 39.05at%Ni), TiNi (50.32at%Ti) and TiNiHf(39.56at%Ti, 48.63at%Ni) composite SMA thin films that display close to linear stress temperature behavior (slope: 2-7MPa/°C) with high stress recovery (300-550MPa), wide transformation range (60-130°C) and low hysteresis (10-30°C)were fabricated. Properties were achieved through the deposition of SMA thin films with varying composition in alayered (composite) format on Si wafers. The TiNi+TiNiCu composite exhibited a two-step transformation (slopes of 2.5and 3.9 MPa/°C) without a significant impact on stress recovery. Displaying identical recovery stresses, the TiNiHf filmpossessed a 65°C transformation range and the TiNiHf+TiNi composite exhibited a wider range of 120°C A strongcorrelation between deposition conditions, annealing parameters and transformation characteristics was observed for allthe SMA films.