THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, May 3, 2021

1:00 PM

via

Blue Jeans Video Conferencing

https://bluejeans.com/826977245

will be held the

DISSERTATION DEFENSE

for

Jonathan Leung

“Mechanisms of White Etching Matter Formation Under Rolling Contact Loading”

Committee Members:

Prof. Richard Neu, Advisor, ME/MSE

Prof. David McDowell, ME/MSE

Prof. Arun Gokhale, MSE

Prof. Shreyes Melkote, ME

Prof. Jeffrey Streator, ME

Abstract:

During rolling contact fatigue (RCF), microstructure transformed regions known as white etching matter (WEM) can form adjacent to subsurface cracks and debonded non-metallic inclusions in bearing steels. WEM is nanocrystalline, compared to the coarse-grained matrix of bearing steels. As WEM is commonly found in prematurely failed bearings, it is hypothesized that the WEM accelerates bearing failures. The current qualitative understanding is that repeated rubbing and beating of these internal interfaces drives WEM formation. These interfaces consist of either subsurface cracks or the interface between Al2O3/MnS inclusions and the matrix. Current gaps exist in quantitatively predicting WEM formation and relating this theory to different microstructure features, bearing operating conditions, and the characteristics of the interface.

A new implementation of the Ruiz fretting damage parameter (FDP) quantifies the rubbing and beating of the interfaces. The locations of maximum FDP correspond with the experimentally observed areas, interface conditions, and loading conditions attributed to WEM formation demonstrating the utility of the FDP to describe the mechanism for WEM formation at subsurface interfaces. The formation of radial cracks from non-metallic inclusions is captured by either the Ruiz fretting fatigue damage parameter (FFDP) or Smith-Watson-Topper (SWT) critical plane parameter. Both parameters indicate that the maximum tangential stress around the inclusion drives crack formation in the matrix. The mechanical properties of WEM have also been measured using spherical nanoindentation. The characterization shows that the WEM is elastically soft with a low yield strength and exhibits a low shear resistance. This low shear resistance behavior and reduced yield strength can explain the widespread dispersion of WEM along crack interfaces and debonded inclusion interfaces.

The agreement between the experimental observations of the locations and density of WEM at subsurface cracks and non-metallic inclusions to the maximum FDP values strengthens the hypothesis that WEM formation is due to the high frictional energy dissipation at the rubbing and beating interfaces. It is anticipated that engineers and scientists can use these damage parameters to quantify the critical conditions that lead to WEM formation.  Predicting the likelihood of WEM formation at various microstructure features and operating conditions will improve microstructure-sensitive RCF predictions and the development of bearing steels.