Carnegie Mellon University

MSE Seminar Series

Friday, November 19, 2021 @12:20pm
*Remote course - Zoom link will be provided

Yip-Wah Chung,
Northwestern University

Fracture Toughness of Hard Coatings and Nature of Carbon Tribofilms: Two Case Studies of Accepting What We Read

We will discuss two case studies, one on hard coatings used for enhancing wear resistance and one on carbon films generated in lubricated wear experiments for improved friction and wear performance. In the first case, it is recognized that hardness is not everything when it comes to wear-resistant coatings; coatings must also be fracture-resistant. However, it is difficult to measure the fracture toughness of coatings. About 20 years ago, some researchers argued that certain ratios of hardness H to elastic modulus E (H/E and H3/E2) are good proxies for fracture toughness, although no quantitative data were presented. The use of these proxies has been embraced by the community ever since, as it is so much easier to measure these ratios than actual fracture toughness for coatings. However, is such a correlation valid? In the second case, when certain materials rub against each other in a hydrocarbon environment, it was found that some carbon-containing films were formed at the interface. Early researchers performed Raman analysis on these carbon films and observed that they had the signatures of diamond-like carbon films. The formation of these carbon-containing films coincided with the reduction of friction and wear, which are well known attributes of diamond-like carbon coatings normally deposited on machine components by chemical or physical vapor deposition methods. Suddenly, everyone reported seeing diamond-like carbon films in these friction experiments. Is this true? We will unpack these two case studies, navigate through the data, and see what we can learn.

Yip-Wah Chung

Dr. Yip-­‐Wah Chung is Professor of Materials Science and Engineering and Mechanical Engineering and Associate Director of the Center for Advanced Materials Manufacturing. Yip-Wah obtained his PhD (physics) from University of California at Berkeley in 1977 and served as Director of the Center for Engineering Tribology at Northwestern (1987-1992), Department Chair (1992-1998), program officer at the US National Science Foundation (2003-2005). He was elected Fellow ASM, Fellow AVS, and Fellow STLE. He won the Innovative Research Award from the ASME Tribology Division and the Technical Achievement Award from the National Storage Industry Consortium. He has published over 240 papers, ten patents, two textbooks, one monograph, and was the co-editor of a six-volume Encyclopedia of Tribology. He serves on the editorial boards of Journal of Materials Processing Technology, Materials Science and Engineering R, Surface and Coatings Technology, and Tribology Letters. His current research includes surface science and tribology, thin films, and high-performance alloys. In his leisure time, he is seen doing recreational flying all over the Midwest.