Softening under membrane contact stress due to ultra-thin Ru coatings on Au films

This study reveals that ultra-thin, single-layer metal coatings can be used to tailor the deformation resistance of metal thin film surfaces, even when the penetration depths of the indents extend well beyond the coating layer. Nanoindentation of ultra-thin (≤20 nm) Ru and Pt coatings on thick Au films (350 nm) was used to illustrate how ultra-thin layers affect both the elastic and plastic deformation behavior. In spite of their (generally) higher hardness compared to Au, the Ru and Pt coating systems reduced the deformation resistance of the underlying Au, even though the maximum depth was on the order of the coating thickness. The effects of coating thickness, mechanical properties, and residual stress on the transitions between membrane- and substrate-dominated behavior were established by controlling the deposition conditions of the layers. The results show that the transition from ultra-thin coating- to underlying film-dominated response occurs when the indent penetration depth to coating thickness ratio is less than ∼0.5.