Insights on Silver Migration Mechanisms and their Influence on the Wear Behavior of Thermally Sprayed Self-lubricating Coatings Up to 350 °C

Coatings containing silver (Ag) as a solid lubricant are used in a wide variety of applications. These are usually produced by physical vapor deposition (PVD) or by thermal spray, and silver reportedly act reducing friction when tested at temperatures up to 400 °C. The mechanisms of solid lubrication of silver in these PVD coatings are well understood, consisting of silver providing a thin layer with low shear strength capable of accommodating the velocity between the sliding parts, thus reducing friction. However, the mechanisms of silver migration towards the surface, particularly for thermally sprayed coatings, are not yet fully understood. In this study, two coatings containing silver (PS304 and PS400) were sprayed and tested against alumina at temperatures up to 350 °C. Diffusion of silver was observed in this temperature range, with evidence of silver going through grain boundaries. A higher concentration of silver was observed at the wear track of both coatings tested at higher temperatures, accompanied by a significant reduction in friction, but no evidence of a layer of silver at the surface of the track was observed. Instead, a multilayered structure was observed on the PS304 while a smooth tribolayer was observed being formed on the PS400, with a smooth transfer film formed at the alumina counterface.