Mechanical Fatigue Analysis of AlN/SUS430/AlN Heterolayered Foils in Micromachined Devices

Abstract

The combination of piezoelectric aluminum nitride (AlN) films with flexible substrates made of stainless steel (SUS) foils has been applied in low-frequency and robust microdevices, especially in piezoelectric vibration energy harvesters (PVEHs). To find guides in the design of PVEHs, the fatigue properties of micromachined specimens fabricated using SUS430 foils and AlN/SUS430/AlN heterolayered foils were investigated in this work. In particular, the thickness dependence of AlN films on the fatigue behavior of AlN/SUS430/AlN heterolayered foils was investigated. AlN films with different thicknesses of 1 and 2 μm were deposited on 50-μm-thick SUS430 substrates. The specimens for fatigue tests were prepared as cantilevers by microfabrication processes. First, Young's moduli were measured. Fatigue tests were subsequently conducted by resonant bending. The fatigue behavior is discussed in this paper on the basis of both the stress-number of cycle (S-N) curves and the fatigue failures on the surfaces of the cantilevers. The fatigue limits of SUS430 (50 μm), AlN (1 μm)/ SUS430 (50 μm)/AlN (1 μm), and AlN (2 μm)/SUS430 (50 μm)/AlN (2 μm) were estimated to be 264, 291, and 368 MPa, respectively. The larger stiffness of the foils with a heterolayered AlN/SUS430/AlN structure indicated their larger fatigue limit than the SUS430 foil. Fatigue failure in the form of cracks was observed on the surface of SUS430 for AlN (1 μm)/SUS430/ AlN (1 μm) when vibrating at the maximum stress of 488 MPa for 2.9 × 10 7 cycles. Surface defects of intrusions and slip bands rather than cracks were observed on the SUS430 substrate for the heterolayered AlN (2 μm)/SUS430/AlN (2 μm) structure when vibrated at the maximum stress of 605 MPa for 2.9 × 10 7 cycles, resulting in its higher fatigue limit than that of AlN (1 μm)/SUS430/AlN (1 μm). No surface defects on AlN films were observed after undergoing the aforementioned long-cycle vibration.