Tribology of adaptive nanocomposite yttria-stabilized zirconia coatings containing silver and molybdenum from 25 to 700°C

The effect of different types and concentrations of metal additions on the tribological properties of yttria-stabilized zirconia-based coatings was investigated in an effort to develop nanocomposite chameleon coatings exhibiting low friction in air throughout a broad temperature range. A hybrid process of magnetron sputtering and pulsed laser deposition was used to grow nanocrystalline yttria-stabilized zirconia (YSZ) imbedded in an amorphous YSZ/metal matrix. Silver was effective as a lubricant at 500 degrees C and lower, while molybdenum formed a soft lubricious oxide at higher temperatures. YSZ-Mo composites exhibited cracks and fissures after deposition, which were enlarged after heating. Cracking was eliminated in YSZ-based coatings containing both silver and molybdenum. The YSZ-Ag-Mo coatings also exhibited moderately low friction coefficients across the entire 25-700 degrees C range. Characterization of the coatings after wear testing revealed that the development of a continuous silver layer protected the underlying YSZ-Mo material from oxidation, provided lubrication up to 500 degrees C, and allowed for the controlled formation of lubricious molybdenum oxide compounds in the wear track at higher temperatures. High-temperature adaptive lubrication mechanisms resulting from noble metal diffusion and soft oxide formation in areas affected by wear are described. (c) 2006 Elsevier B.V. All rights reserved.