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Unformatted text preview: P. L. Dickrell W. G. Sawyer 1 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611 J. A. Heimberg I. L. Singer K. J. Wahl Naval Research Laboratory, Tribology Section Code 6176, Washington, DC 20375 A. Erdemir Argonne National Laboratory, Energy Technology Division, Argonne, IL 60439 A Gas-Surface Interaction Model for Spatial and Time-Dependent Friction Coefficient in Reciprocating Contacts: Applications to Near-Frictionless Carbon A closed-form time- and position-dependent model for coverage, based on the adsorption of environmental contaminants and their removal through the pin contact, is developed for reciprocating contacts. The model employs an adsorption fraction and removal ratio to formulate a series expression for the entering coverage at any cycle and location on the wear track. A closed-form solution to the series expression is presented and compared to other coverage models developed for steady-state coverage for pin-on-disk contacts, re- ciprocating contacts, or the time-dependent center-point model for reciprocating contacts. The friction coefficient is based on the average coverage under the pin contact. The model is compared to position- and time-dependent data collected on near-frictionless carbon self-mated contacts on a reciprocating tribometer in a nitrogen atmosphere. There are many similarities between the model curves and the data, both in magnitude and trends. No new curve fitting was performed in this paper, with all needed parameters coming from previous models of average friction coefficient behavior. @ DOI: 10.1115/1.1829719 # Introduction Diamondlike carbon ~ DLC ! films are of tribological interest due to their low friction, low wear rate, high hardness, and chemical inertness @ 1,2 # . A class of diamondlike carbon coatings termed near-frictionless carbon ~ NFC ! , developed at Argonne National Laboratory, has been shown to sustain superlow coefficients of friction ( m , 0.003) and wear rates ( K , 3 2 10 mm 3 /Nm) in self- mated contacts @ 3 # . Details on the preparation and characteristics of the films have been addressed in other literature @ 4 # . The tribo- logical behavior of these films is sensitive to the environment, only realizing their low friction coefficient and wear rate in inert, dry, or vacuum environments @ 3,58 # . The NFC films used in this study have a high hydrogen content. When gaseous water is added to the environment, the friction coefficient in NFC self-mated con- tacts rises @ 8 # , suggesting a gas-surface interaction where water molecules disrupt the low friction of the NFC pair. Velocity- dependent friction coefficients of these films in nitrogen atmo- spheres were measured by Heimberg et al. @ 7 # . They hypothesized that the velocity dependence was due to a gas-surface interaction that had longer times to affect the film at slower sliding speeds....
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- Spring '08