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Unformatted text preview: P. L. Dickrell W. G. Sawyer University of Florida, Mechanical Engineering Department, Gainesville, FL 32611, USA A. Erdemir Argonne National Laboratory, Energy Technology Division, Argonne, IL 60439, USA Fractional Coverage Model for the Adsorption and Removal of Gas Species and Application to Superlow Friction Diamond-Like Carbon The frictional behavior of diamond-like carbon (DLC) films varies with environmental condition. One theory asserts that the cause of variations in the frictional performance is environmental contaminants adsorbing onto the DLC film surface. Testing of the frictional performance of DLC films in a pin-on-disk contact has mapped the transient behavior of the friction coefficient. A model for fractional coverage, based on the adsorption of envi- ronmental contaminants and their removal through the pin contact, is developed. The rate of adsorption is taken from Langmuirs model , which is combined with the removal ratio from Blanchet and Sawyer . The coefficient of friction is based on the average fractional coverage under the pin contact. The model also gives a closed-form expression for the steady-state fractional coverage. Model calculations compared favorably to the time progression of the friction coefficient for a series of earlier experiments on a super- low friction DLC coating , when the fractional removal term was allowed to increase with increasing sliding speed. @ DOI: 10.1115/1.1739408 # Introduction The unique properties of diamond-like carbon ~ DLC ! films are characterized by excellent wear resistance, biocompatibility, and chemical inertness. These films can potentially be used in a wide range of applications, such as bearings, cutting tools, submersible parts, and biomedical applications @ 14 # . Testing of DLC films as solid lubricant coatings has shown them to possess low coeffi- cients of friction, spanning the range m 5 0.001 to 0.6 @ 36 # , al- though the testing conditions and type of DLC film ~ i.e., hydro- genated versus hydrogen-free ! have a strong influence on its frictional behavior. The frictional behavior of the superlow fric- tion DLC film, recently investigated at Argonne National Labora- tory @ 4,7 # , has been theorized to be dependent on the amount of contaminants adsorbed on the rubbing surface @ 7 # . Some attempts have been made to model the frictional behavior of DLC and other carbon films in various environments @ 3,8,9 # . A hypothesis is that the variations in friction coefficient with sliding speed in a pin-on-disk contact are the result of competitive rate processes that involve an interaction of a contaminant gas species with the surface of the diamond-like carbon, such as the adsorp- tion of water, and the removal of these species under the contact of the pin. Much like vapor-phase lubrication @ 1016 # , such pro- cesses have a strong dependence on the gas pressures of the con- taminants, available areas for adsorption, time exposed to the en- vironment, and temperature. This paper outlines the developmentvironment, and temperature....
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This note was uploaded on 08/22/2011 for the course EGM 4313 taught by Professor Mei during the Spring '08 term at University of Florida.
- Spring '08
- Mechanical Engineering