Session: 13-06-01: Applied Mechanics and Materials in Micro- and Nano-Systems

Paper Number: 145746

145746 - Shear Properties to Fracture of Sub-Mm-Thick Commercially Fabricated Liga Ni-Fe and Ni-Co Films 

Motivation: Accurate and comprehensive mechanical properties of thin- and thick films with well-understood uncertainties are needed for reliable structural design and failure analysis of microelectromechanical systems (MEMS).

Problem and Objective: Over the past three decades there have been many experimental studies of various mechanical properties of electrodeposited metal films, such as those fabricated through the LIGA (Lithographie, Galvanoformung, Abformung) process. But their shear properties have received relatively little attention. The objective of this work was to help fill this gap by investigating the in-plane shear stress vs shear strain behavior, through yielding to fracture, of two commercially fabricated sub-mm-thick LIGA nickel alloys.

Background: Unlike silicon and ceramic films, electrodeposited metal films are ductile and capable of undergoing extensive plastic deformation before fracture. There is an extensive body of research reporting LIGA nickel alloys’ tensile, bending, fatigue, and creep properties. However, their shear properties have received relatively little attention despite the importance of shear properties to structural applications. The few studies on shear properties tended to focus on fabricating LIGA micro torsional actuators which were then actuated or displaced out of the plane of the substrate. While micro torsional tests are useful for measuring the shear modulus, they present challenges in accurate strain measurement, particularly at large deformations, which can limit the accuracy of shear stress vs shear strain curves. They are also not amenable for shear testing to large deformations and thus are typically not done to fracture on ductile materials such as LIGA films.

Methods: Instead of micro torsional specimens, we designed planar simple-shear specimens. This allows for more straightforward loading of the specimens to fracture, at different strain rates, and full field strain measurement via digital image correlation (DIC). The freestanding planar shear specimens were fabricated from sub-mm-thick LIGA Ni alloys in a commercial wafer fabrication process. We then used miniature tabletop and benchtop tensile testing load-frames to load the shear specimens to fracture and DIC of optical microscope images to measure displacements. We developed a method to measure the shear strains by fitting the measured displacements to a hyperbolic tangent curve. In this way, we obtained experimental shear stress vs shear strain curves at two strain rates. We also characterized the chemistry of the specimens using electron dispersive spectroscopy, and we used scanning electron microscopy to characterize the microstructure and to investigate the fracture mechanisms. Finally, we modeled the shear tests using finite element analysis to further investigate the LIGA materials’ shear behavior just before fracture.

Results: We show experimental shear stress vs simple-shear strain curves, until fracture, for two commercial LIGA Ni alloys: a Ni-Fe alloy, and a Ni-Co alloy, and at two strain rates (0.001/s and 1/s). We report measurements of the shear strength, shear modulus, and an engineering yield stress in simple-shear that is analogous to the 0.002 offset yield in a tensile test. We observed rate effects on the shear strength and the true strain to fracture. Fractography showed microvoid coalescence in the Ni- Co material and more brittle fracture in the Ni-Fe alloy. From finite element analysis we obtained the equivalent plastic strain and stress triaxiality just before fracture for the Ni-Fe alloy.

Impact: As shear properties, especially to fracture, have been under-reported in the literature for electrodeposited metal films, this work will further enable MEMS designers to predict the mechanical behavior of LIGA MEMS more accurately, especially in applications which might result in extensive deformation and fracture. Future work involves using these data to support the development of material constitutive models for structural failure predictions of LIGA MEMS.

 

[UNCLASSIFIED. Distribution Statement A: Approved for public release. Distribution unlimited.]

 

 

Presenting Author: Li-Anne Liew National Institute of Standards and Technology

Presenting Author Biography: Li-Anne Liew is a research scientist in the Applied Chemicals and Materials Division at the National Institute of Standards and Technology. Her research interests include microsystems and MEMS design, fabrication and testing for applications such as materials testing, electronics cooling and cryogenic applications. Her current research focuses on MEMS for cryogenic applications and micromechanical behavior of metal alloys for MEMS structural applications.

Authors:

Li-Anne Liew National Institute of Standards and Technology
David Read National Institute of Standards and Technology
May Martin National Institute of Standards and Technology
Peter Bradley National Institute of Standards and Technology
Todd Christenson HT MicroAnalytical, Inc
Jeffrey Smyth U.S. Army Combat Capabilities Development Command Armaments Center
John Geany U.S. Army DEVCOM AC