SUBJECT: Ph.D. Proposal Presentation
BY: Farzad Sadeghi-Tohidi
TIME: Thursday, April 3, 2014, 9:00 a.m.
PLACE: Love Building, 210
TITLE: Investigation of the Effects of Large Stress Gradients on Fatigue Behavior of Nickel Thin Films
COMMITTEE: Dr. Olivier Pierron, Chair (ME)
Dr. Richard Neu (ME)
Dr. Hamid Garmestani (MSE)
Dr. Antonia Antoniou (ME)
Dr. Samuel Graham (ME)


In recent years, thin films technologies have gained growing interest in the development of stretchable electronics, flexible displays, and a wide range of microelectromechanical systems (MEMS) application. Using the movable parts and protective layers under long-term cycling load in the above mentioned application could result in fatigue failure of these devices. Hence the investigation of fatigue degradation mechanisms of thin films could address some of major reliability concerns in the widespread application for micro- and nanotechnologies. The extreme stress gradients cannot be obtained at the bulk scale and happens for notched micro-components or in the case of micro-component under bending loading. Consequently, a micro-components-based fatigue test is needed to recreate the extreme stress gradients and study the fatigue life of thin films under relevant loading condition. Previous research on the fatigue of Ni thin films under extreme stress gradients revealed a fatigue crack initiation mechanism (cyclic slip band oxide thickening mechanism) that does not occur at the bulk, along with the formation of non-propagating fatigue cracks for stress amplitudes as large as 40% of the specimens’ tensile strength. In the proposed research, a microresonator-based technique is presented to investigate the effect of extreme stress gradient on the fatigue properties of LIGA Ni films in both mild and harsh environments. Specifically, a wide range for normalized stress gradient (η equal to 0.06, 0.13, 0.175, and 0.36 µm^(-1)) will be considered to investigate the stress gradient effects on fatigue crack initiation and microstructurally small crack propagation. It is expected that the results of this proposed research will bring significant insight regarding the reliability concerns of metallic thin film components in stretchable and flexible electronics as well as those of MEMS/NEMS devices.