An experimental study has been performed to learn about the physical and chemical mechanisms of self-lubrication of coatings that comprise nanostructured composites of yttria-stabilized zirconia (YSZ), silver, and molybdenum. These and other YSZ-based nanocomposite coatings have received increasing attention in recent years because they offer a combination of hardness, toughness, resistance to wear, and low-friction C properties that make them attractive for reducing wear and friction and increasing the lifetimes of hot, sliding components of mechanical systems. In addition to the excellent mechanical and thermal stability of the basic YSZ ceramic material, the nanocomposite structures of these coatings, consisting of combinations of amorphous and crystalline phases, provide a “chameleon” surface adaptation, in which different phases turn into lubricants in response to different test environments, contact loads, sliding speeds, and temperatures. Moreover, proper sizing of nanocrystalline grains can restrict crack sizes and create large volumes of grain boundaries, thereby increasing the toughness and contact-load-bearing capabilities of these coatings.

The YSZ-Ag-Mo Coating Is Covered with a TiN surface layer containing a random array of pinholes. This layer retards the diffusion of Ag to the surface, thereby reducing the rate of depletion of lubricant in order to prolong the high-temperature wear lifetime.
The YSZ-Ag-Mo composite coatings for the present experimental study were deposited on steel and nickel-alloy substrates in a hybrid process that included deposition of 100-nm-thick titanium adhesion layers by use of a filtered titanium arc plasma, followed by pulsedlayer deposition of YSZ from a YSZ target. Ag and Mo were added to the coatings by magnetron sputtering from Ag and Mo targets. All of the coatings were grown to a thickness of about 2 μm. In some cases, the coating was formed as two 1-μm-thick YSZ-Ag-Mo layers, and a 100-nm-thick TiN diffusion-barrier layer was deposited between the YSZ-Ag-Mo layers by introducing a flow of nitrogen into the deposition chamber during operation of the filtered titanium arc plasma source. In other cases, TiN barrier layers containing pinholes were deposited on the surfaces of YSZ-Ag-Mo coatings to limit through-the-thickness diffusion of silver (see figure).

In the study, the focus was on the evolution of microstructure, diffusion of silver, and contact surface oxidation processes in sliding contact at high temperature in air. The microstructures of the coatings were determined by x-ray diffraction and transmission electron microscopy. Coefficients of friction were measured by use of a high-temperature ball-on-disk tribometer. The wear scar surfaces and coating cross-sections were studied by use of scanning electron, transmission electron, scanning transmission electron, and focused ion beam microscopes, which provided the information on spatial distributions of chemical compositions, including distributions of silver and molybdenum along with microstructural features.

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


This Brief includes a Technical Support Package (TSP).
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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This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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