Investigation of the effects of extreme stress gradients on fatigue behavior of nickel microbeams

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Sadeghi-Tohidi, Farzad
Pierron, Olivier N.
Garmestani, Hamid
Antoniou, Antonia
Graham, Samuel
Neu, Richard W.
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Thin films technologies continue to play a key role in the development of stretchable electronics, flexible displays, and a wide range of microelectromechanical systems (MEMS) applications. Movable components can be exposed to cyclic loading in these applications, which can result in fatigue failure. Hence the investigation of fatigue degradation mechanisms of thin films is required to address some of these reliability concerns. Particularly, extreme stress gradients can occur for notched or unnotched micro-components in bending mode. In this research, a microresonator-based technique is presented to investigate the effect of extreme stress gradient (normalized stress gradients of 17 and 36%.m-1) on the fatigue properties of 20-m-thick electroplated Ni microbeams in both mild (30°C, 50% RH) and harsh (80°C, 90% RH) environments. The technique relies on the measured evolution of the resonance frequency throughout the fatigue test and finite element models to calculate a fatigue life corresponding to the nucleation and growth of a ~2-micrometer-long crack. In addition, the growth rates of these microstructurally small cracks were estimated also based on the measured resonance frequency evolution. The results of this dissertation highlight that the fatigue life of nickel microbeams under extreme stress gradients is dominated by the ultraslow growth of microstructurally small cracks, which is a strong function of the applied stress gradient. The calculated initial crack propagation rates are ~10 times larger for  = 17%.m-1 compared to  = 36%.m-1 for a between 400 and 450 MPa. The discrepancy is even larger with increasing crack size from 0 to 2 m. For  = 36%.m-1, the initial rates decrease with increasing a, whereas for  = 17%.m-1, the crack propagation rates do not decrease with increasing a. These effects result in significant difference in fatigue lives by orders of magnitude: at a ~ 450 MPa, the fatigue life is 1000 times larger for  = 17%.m-1 (Nf = 105 cycles) than for  = 36%.m-1 (Nf = 108 cycles). The stress-life fatigue curves exhibit low Basquin exponents, b, varying from -0.039 to -0.023 for stress gradients increasing from 17% to 36%.m-1. Consequently, larger endurance limits (50% of the tensile strength) are associated with the steeper stress gradients. Little differences were observed in the fatigue response (stress-life fatigue curves, crack propagation rates) between the two investigated environments (30°C, 50% RH vs 80°C, 90% RH) with only slightly shorter fatigue lives in the harsher environment. However, scanning electron microscope images of the fatigued specimens and energy dispersive spectroscopy results from the sidewalls and fracture surfaces of the fatigued microbeams highlight an environmental component in the fatigue process, in the form of oxide formation at the location of the extrusions and along the crack flanks of these microstructurally small cracks. The results of this research bring significant insight regarding the reliability concerns of metallic microbeams.
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