Load-dependent high temperature tribological properties of silver tantalate coatings

Silver tantalate coatings were produced by reactive unbalanced magnetron sputtering from silver and tantalum sources as potential high temperature solid lubricants. The films were wear-tested at 750 °C under normal loads of 1, 2, 5, or 10 N against a Si3N4 counterface. These sliding tests revealed that the friction monotonically increased as the load was increased. A systematic investigation of the surface and sub-surface regions of the wear track, using techniques such as Scanning Auger Nanoprobe, Atom Probe Tomography, and cross-sectional transmission electron microscopy, revealed the following trends with increasing load: (1) a decrease in the amount of Ag on the surface of the wear track; (2) a decrease in the thickness of the mechanically mixed layer that forms as a result of the reconstruction of AgTaO3 to form Ta2O5 and Ag; and (3) the formation of a porous structure throughout the tribofilm as a result of the segregation and migration of Ag from the original AgTaO3 matrix. These results were complemented by molecular dynamics simulations, which confirmed the increase of friction with load. Further, the simulations support the hypothesis that this trend can be explained in terms of decreased presence of Ag clusters near the sliding surface and the associated decreased porosity.