(Ag,Cu)−Ta−O Ternaries As High-Temperature Solid-Lubricant Coatings

       Ternary oxides have gained increasing attention due to their potential use as solid lubricants at elevated temperatures. Most conventional solid lubricants, such as graphite and MoS₂, fail at elevated temperatures (>≈300 °C) because temperature-induced oxidation and the associated irreversible chemical changes result in a failure to provide the desired lubricity Considering the shortage of available candidates for high-temperature applications, developing effective solid lubricants that remain thermally stable and lubricious over a wide range of temperatures is imperative. Silver tantalate (AgTaO₃), possesses a layer-like structure; can form a soft, metallic (silver) phase when subjected to sliding; and is highly resistant to oxidation. The coefficient of friction (CoF) for AgTaO₃ was reported to decrease with temperature down to 0.06 at 750 °C. This result was attributed to the thermal and shear-induced surface reconstruction, in which a mechanically mixed layer, including nanocrystalline Ag, Ta₂O₅, and AgTaO₃ phases, was observed to form at the sliding interface.15 It was proposed that the aggregated silver, in the form of “clusters”, facilitates sliding and is responsible for the low friction at the sliding interface. However, the high mobility of silver suggests that it will also diffuse rapidly on the surface, leading to the relatively low resistance to wear observed for AgTaO₃.

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