Fatigue-induced thick oxide formation and its role on fatigue crack initiation in Ni thin films at low temperatures

This study highlights an oxidation-assisted fatigue crack initiation mechanism in 20 μm thick electroplated Ni films under loading conditions relevant for a wide range of microelectromechanical systems, such as extreme stress gradients at the surface (50% decrease over the first micrometer from the surface). Microresonators subjected to in-plane bending at ∼8 kHz were fatigued for billions of cycles in humid air, at 30 °C, 50% relative humidity (RH), and 80 °C, 90% RH, for maximum stress amplitudes up to ∼500 MPa (∼55% of the ultimate tensile strength). Transmission electron microscopy (TEM) revealed highly localized thick oxides (∼1 μm) on specimens fatigued for several billions of cycles. These oxides are two to three orders of magnitude thicker than the regular native oxides at these low temperatures, and only form at the location of cyclic slip bands. These oxides appear to be thicker for higher partial pressures of water, based on the TEM comparison of one specimen fatigued at 30 °C, 50% RH to one fatigued at 80 °C, 90% RH. Fatigue microcracks were observed within these highly localized thick oxides. Finite element models were also employed to confirm these results based on the interpretation of the evolution of the devices' resonance frequency. This oxidation-assisted fatigue crack initiation mechanism at low temperatures constitutes a significant departure from the established mechanisms for bulk metals and their environmental effects. A possible explanation for the different governing mechanism is the presence of extreme stress gradients in these microscale components. Under these loading conditions, the classical fatigue crack initiation mechanisms are not operational, allowing this alternative mechanism to become dominant. This study highlights the need to further understand the coupled size and environmental effects on the fatigue of metallic thin films.