Wear Resistance of Cu/Ag Multilayers: A Microscopic Study

The microscopic wear behavior of copper-silver multilayer samples was studied by performing sliding wear tests using a tribo-indenter. Multilayers with an average composition of Cu₉₀Ag₁₀ and Ag layer thicknesses ranging from 2 to 20 nm were grown by magnetron sputtering. For reference, a homogeneous Cu₉₀Ag₁₀ solid solution film was similarly grown. The thin films were subjected to two-dimensional wear tests by rastering a cono-spherical diamond indenter under loads of 100-400 μN for 1-20 consecutive passes or cycles. The wear volumes were determined by atomic force microscopy. Characterization of the specimens employed nanoindentation, nanoscratch, and transmission electron microscopy (TEM). Wear rates were found to reach steady state after five cycles or less. The hardness values of the as-grown and worn samples both increased with decreasing thickness of the Cu and Ag layers, whereas the steady-state wear rates decreased. Notably, the wear resistance increased faster than the corresponding increase in indentation hardness, indicating a deviation from the Archard's law. An inverse relationship between the wear rate and hardness was, however, recovered when using scratch hardness, suggesting that scratch hardness is a better predictor of wear resistance. Characterization of subsurface wear microstructures by TEM revealed that forced chemical mixing and dissolution of layers occur to a depth of ≈40 to 50 nm, stabilizing a chemically homogeneous solid solution below the wear surface. Comparative wear tests on thicker multilayers revealed that Cu/Ag interfaces reduced the wear rate significantly, thus helping to rationalize the high wear resistance of thin multilayers.