In Situ Optical Tribometry Studies of Nanocomposite Coatings
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In Situ Optical Tribometry Studies of Nanocomposite Coatings Richard R. Chromik1,2, Colin C. Baker1,3, Andrey A. Voevodin3 and Kathryn J. Wahl2 1 North Carolina State University, Department of Physics, Raleigh, NC 27695 2 U.S. Naval Research Laboratory, Code 6176, Tribology Section, Washington, DC 20375 3 U.S. Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, OH 45433 ABSTRACT Six nanocomposite coatings of yttria-stabilized zirconia, Au, diamond like carbon and MoS2 have been studied by in situ tribometry. The coatings all had a nominal MoS2 content of between 14 and 18 mol%, while the concentrations of the other components were varied. Steady state friction coefficients in both dry (0.04–0.05) and wet sliding conditions (0.06–0.08) were similar for all coatings. However, by in situ tribometry, wear mechanisms were found to differ depending on coating composition and test environment humidity. Friction spiking in high YSZ coatings was identified with local plowing and scoring of the track followed by an extrusion of transfer film material. Trends in coating hardness and modulus are correlated to composition and coating wear processes. INTRODUCTION Demanding tribological applications, such as those where temperature and humidity excursions are expected, have spurred the development of specialized coatings having friction adaptation properties. Among these tribologically “smart” materials, nanocomposite coatings including multiple solid lubricant components, have shown low friction coefficients in various environments [1,2]. In addition, due to their nanoscale structure, improvements in hardness and wear resistance have also been observed [1-3]. In a previous investigation on nanocomposite coatings combining yttria-stabilized zirconia (YSZ), Au, diamond-like carbon (DLC) and MoS2 [1], friction adaptation to humidity changes was found to be optimal for mole fractions near 0.2 for both C and MoS2. Also, coatings with a concentration of Au ranging from 10 to 20% were found to survive as many as 10,000 cycles at 500 °C. Thus, these nanocomposite coatings were found to be tribologically robust over a wide range of operating conditions. In this study, we use a custom built in situ tribometer [4] to examine, friction, velocity accommodation and wear mechanisms for YSZ/Au/DLC/MoS2 nanocomposite coatings as a function of coating composition and test environment. Coating stoichiometry has been varied over a wider range than previously studied, where the mole fractions of three components (Au, YSZ and C) were varied while the MoS2 content was kept about the same. EXPERIMENTAL Nanocomposite coatings of YSZ/Au/DLC/MoS2 were prepared in a vacuum chamber using simultaneous magnetron sputtering and pulsed laser deposition (MSPLD) [5]. Laser ablation used for deposition of YSZ, carbon and MoS2, and magnetron sputtering for Au. A circular target for laser ablation was assembled with two quarter sections of YSZ, one quarter section of MoS2, and one quarter sec
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