The study revealed different “chameleon”- like high-temperature- adaptive lubrication mechanisms in the nanocomposite coatings. Coefficients of friction of about 0.4 or less were found to be maintained at all temperatures from 25 to 700 °C. The asdeposited coatings were found to include silver nanograins embedded in amorphous/ nanocrystalline YSZ-Mo matrices. At high temperatures, heating-induced diffusion and coalescence of silver were found to result in microstructural and chemical changes that included formation of silver films on surfaces with silverdepleted YSZ-Mo layers left underneath. Crystallization of zirconia matrices was found to occur simultaneously with diffusion of silver to surfaces when the coatings were heated. It was confirmed that the diffusion of silver to, and coalescence of silver on, the surfaces of YSZ-Ag-Mo nanocomposite coatings plays an important part in high-temperature lubrication.

Silver was determined to be an effective lubricant at temperatures below 500 °C, and coalescence of silver on surfaces was found to isolate molybdenum inside the composites from ambient oxygen. At temperatures above 500 °C, the silver surface layers were found to be rapidly removed from wear tracks and, hence, the reactive molybdenum inside the silver- depleted YSZ-Mo layers was exposed to ambient air. Contact tribochemistry was found to result in the formation, in wear tracks, of molybdenum oxides, which provided lubrication at 700 °C.

In the cases of specimens containing the internal TiN barrier layers, these layers were found to preserve lubricants underneath, thereby providing for continuous replenishment of lubricants. The TiN layers were also found to force subsurface silver to diffuse laterally toward wear scars, once the TiN layers were breached by wear. This behavior affords an adaptive response, which includes on-demand supply of lubricant from storage volumes inside YSZ-Ag- Mo composites to surface contact areas. The YSZ-Ag-Mo coatings that contained the internal TiN barrier layers were found to maintain coefficients of friction of approximately 0.4 during more than 25,000 cycles, while the monolithic YSZ-Ag-Mo coatings lasted fewer than 5,000 cycles. The specimens having TiN surface layers with pinholes were found to have wear lifetimes greater than 50,000 cycles.

This work was done by J. J. Hu of the Air Force Research Laboratory, and C. Muratore, and A.A. Voevodin of UES, Inc. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. AFRL-0023

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Self-Lubrication of Hot YSZ-Ag-Mo Nanocomposite Coatings (reference AFRL-0023) is currently available for download from the TSP library.

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