LSU Historical Dissertations and Theses

1997

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Science (Interdepartmental Program)

E. I. Meletis

Abstract

Diamond-like carbon (DLC) coatings have high potential for developing advanced tribological systems and the overall objective of the present research is to provide a framework to theoretically design and experimentally develop and study such systems of interest. The present research concentrates on the following specific aspects: the fundamental wear mechanism of DLC coatings; finite element (FE) modeling to understand the stress distribution in coating/substrate system under indentation and friction; and the influence of graded interface on the tribological behavior of DLC coatings. The experimental results showed that DLC films possess low friction coefficient ($f\sb{init}$: 0.12-0.20, $f\sb{fin}$: 0.06-0.08) and low wear rate $(1.6\times 10\sp{-9}$mm$\sp3$m$\sp{-1}$N$\sp{-1}).$ The friction behavior of DLC coatings includes three distinct regimes: a break-in period, an intermediate constant friction plateau and a steady-state stage. The intermediate plateau coincides with the formation of a carbon-rich transfer layer on the counter-surface, and the steady-state stage with graphitization of the DLC structure. A wear-induced graphitization mechanism is proposed based on the experimental evidence. Operational and environmental parameters have significant effects on the friction process of DLC coatings through their influence on graphitization. The results of FE modeling showed that coating thickness, contact stress and the ratio of coating/substrate properties affect the yielding of coating/substrate system. The calculations also revealed that a graded interface can significantly decrease the size of the plastic zone in the substrate during indentation/friction and thus can improve coating behavior. An innovative concept is the advanced coating systems with a functionally-graded interface (FGI). This concept was experimentally studied by developing DLC coatings with FGI based on the theoretical FE predictions. It was found that the durability of the DLC film is affected by the presence of the FGI and the loading. Under low loading, FGI has a small effect due to the limited yielding occurring in the substrate. Under high loading, the presence of FGI produced significant improvements ($\sim$80% increase in coating lifetime) by reducing the plastic zone size and preventing yielding at the coating/substrate interface, which is consistent with the FE predictions.

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