Mechanical stability under sliding contact of thin silver film embedded in brittle multilayer

We have investigated the mechanical stability of a coating consisting of a single 10 nm thin silver film embedded in an otherwise brittle multilayer under sliding contact. Using in situ monitoring of the contact zone during sliding, we show that for large friction coefficients the steady state scratch propagation is driven by the 3D plastic deformation of the thin silver layer at the front of the contact. As a side effect, the top brittle layer spalls away. We also demonstrate that initiation of this damage mechanism results from the brittle failure of the film. This initial failure is shown to predominantly occur at the rear half but well inside the contact zone and then propagate to the front of the contact. This observation demonstrates that a combination of tensile and shear stresses is necessary to drive the initial interface crack. This initiation mechanism qualitatively differs from hertzian cracking where friction-induced shear stresses are necessary to enhance in plane tensile stresses. In relation with this initiation mechanism, we discuss the key parameters for stability.