The effect of thickness on the martensitic transformation in Ti–Ni films studied by internal friction measurements

Ti–Ni films with thicknesses of 2 and 8 μm were deposited at 823 K onto Si substrates. Internal friction measurements using frequencies near 2 kHz were performed during heating and cooling cycles between 223 and 393 K. Upon heating, we observe two loss peaks that can be correlated with the martensite ( M )→ R phase → austenite ( A ) phase transformations. Upon cooling, only one peak related to the A→R transformation is observed. The height of the loss peak for the M→R transformation increases with decreasing film thickness. In contrast, the height of the RA peak decreases. In conjunction with the internal friction measurements, the stress evolution during heating and cooling was examined using the wafer curvature method. The stress in the films due to phase transformations and thermal misfit between film and substrate increases with decreasing film thickness. It can be shown that the behaviour of the martensitic transformation depends on the constraint imposed by the substrate and on film thickness.