Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99465

99465 - A Modified Opto-Acoustic Method for Measuring Thickness-Dependent Material Property Gradients 

While dealing with bi-material interfaces of thin films on substrates, it is known that material properties of thin films can vary with the thickness of the film. This material property gradient occurs due to the chemical interactions of thin film with substrate and geometrical confinement. Currently thin films are used in flexible electronics, MEMS and NEMS devices, and soft robotics. To optimize reliability and performance of thin films it is important to have a fundamental understanding of material property variance in selected thin films. This would allow the scientists and engineers to know the true strength and impact resistance of thin films before implementing them for specific applications.

 

Current methods of measuring material properties have some disagreement in their results. Some studies suggest that the elastic modulus should decrease with decreasing thickness of thin films while others suggest the opposite. However, even within the theory of a decreasing elastic modulus with film thickness, different measurement methods seem to have varying sensitivity to the slope of the elastic modulus reduction.

 

Our group has conducted experimental and numerical analysis on how decreasing the thickness of polymer thin films affects their elastic modulus. Polystyrene and PMMA polymers were studied at different molecular weights to see the effect it has on the slope of the elastic modulus decrease in the thin film thickness region. The experimental method utilized a three-dimensional Laser Doppler Vibrometer (LDV) system to measure the signal propagation in the substrate and the thin film. This system is advantageous in the way that it contains three integrated LDV’s with one emitting laser being able to simultaneously measure velocities or displacements in three dimensions. Additionally, elastic modulus of the thin film samples was measured using the surface wrinkling method, as this method has been verified before and could be used as a baseline. The numerical model simulated the transient response of the substrate and the thin film to mimic the response of the substrate and the thin film seen with the LDV system.

 

The experimental results with the LDV show general agreement with the numerical model in the film thickness region below 60 nm. However, in the thickness region above 60 nm, the experimental and numerical models seem to have opposing results, indicating that there is a difference in the mechanical behavior and properties of polymer films in the thin film and bulk polymer regions. This method could be further advanced to non-invasively measure the mechanical properties of polymer thin films.

Presenting Author: Anastasia Timofeeva North Carolina State University

Presenting Author Biography: Anastasia Timofeeva is a graduate student at North Carolina State University, pursuing a PhD degree in Mechanical Engineering. Previously Anastasia has received a Bachelor of Science degree in Engineering Science from the Pennsylvania State University in 2019 as well as a Bachelor of Science degree in Physics and a Bachelor of Arts degree in Languages and Cultures with a concentration in Chinese from the Bloomsburg University of Pennsylvania in 2018. <br/><br/>Anastasia’s research interests include structure health monitoring, signal processing, and machine learning. Her current projects focus on studying the mechanical property gradients in polymer thin films and the occurrence of stress concentrations in polymer fibers during production.

Authors:

Anastasia Timofeeva North Carolina State University
Runqiao Song North Carolina State University
Brendan O'connor North Carolina State University
Kara Peters North Carolina State University