THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, November 20, 2020
1:00 PM 

via 

Bluejeans Video Conferencing 

 https://bluejeans.com/216995024

will be held the

DISSERTATION DEFENSE

for

Chuchu Zhang

"Wear Rate and Mechanism Maps for Stainless Steel at High Temperature"

Committee Members: 

Prof. Richard Neu, Advisor, MSE/ME

Prof. Preet Singh, MSE

Prof. Joshua Kacher, MSE

Prof. Jeffrey Streator, ME

Prof. Shreyes Melkote, ME

Abstract: 

Contacting interfaces moving in a back and forth way relative to each other at high temperature is found in mechanical systems like internal combustion engines, aerospace propulsion systems, energy generation systems and metalworking equipment. The working temperature for components in such mechanical systems, for example, pivot-vanes/ring in variable geometry turbocharger (VGT, also called variable-nozzle turbine, VNT) for diesel engines can vary from ambient temperature to 700, and even higher for gasoline engine. Materials selection for low friction, low wear in high temperature applications is a complex problem because of the many influencing factors and coupling effects especially when friction and wear control methods like lubricating cannot be used.

It was found that a compacted and sintered wear protective layer, namely “glaze layer” can be formed in many Co, Ni and Fe alloys under certain circumstances, and it is possible to take advantage of this glaze layer formation to reduce friction and wear at high temperature. This dissertation is aimed for better understanding of glaze layer and its role in the severe-to-mild wear transition observed as temperature increases, and utilize this knowledge to push forward development of practical methodologies to build a robust high temperature wear map that benefits high-temperature tribo-pair design. 310S austenitic stainless steel is chosen as a sample system to generate a temperature-frequency wear mechanism map. This map covers considerably large temperature and frequency in its working condition than has previous been considered. It has been found that the environmental temperature plays the overall dominant role in determination of the wear mechanism, although in the transition zone, a coupling effect between temperature and frequency has been observed. The experimental data is also used to validate the applicability of a newly proposed critical cycle (NGL) model for glaze layer formation.

In addition, we proposed a novel way to understand the role of glaze layer using computer vision algorithms. Two workflows, one for quantitative glaze layer identification and the other for image alignment, have been developed. For glaze layer identification, we used computer vision concepts that considers the color and reflectiveness of glaze layer under optical microscope (OM).  For image alignment, we developed a strategy to conduct pixel-to-pixel alignment of images acquired by multiple techniques (e.g., OM, scanning electron microscopy, 3D optical profilers) with sub-pixel error. As such, the correlation between the height map and locations of the glaze layer within the wear scar can be readily determined. The proposed methodology is applied to analysis the worn surface of 310S after high temperature fretting test, and the glaze layer is found to always occupy relatively high locations within wear scar. With temperature rise, the projected coverage of glaze layer follows the same increasing trend with three distinguishable stages, and the threshold temperature of the three stages matched with severe-to-mild wear transition. These results provide evidence that severe-to-mild wear transition resulted from spreading of glaze layer coverage, and glaze layer may reduce friction and wear by reducing real contact area.