Effect of environment on microstructure evolution and friction of Au-Ni multilayers

We present results from a systematic investigation of environmental effects on the frictional behavior of Au-Ni multilayer films of varying interlayer spacing. The current results, sliding against ruby spheres in a dry N2 atmosphere, are compared to prior work on the tribological behavior of these materials under ultra-high vacuum (UHV) [Cihan, E. et al., Sci. Rep. 9, 9480: 1-10 (2019)]. Under both conditions, there is a regime of high friction when the interlayer spacing is large and a regime of low friction when the spacing is small. The low friction regime is associated with a critical grain size below which grain boundary sliding is expected to be the dominant mechanism of deformation. A shear-induced alloy formation (60-65 at% Ni in Au) and a concomitant low friction coefficient was observed with multilayer spacings of 20 nm and lower under UHV. A distinct microstructure was found in dry N2, and is attributed to different interfacial characteristics due to adsorbed species; rather than mixing between Au and Ni layers, only the uppermost Au layers were affected by shearing. These observations are coupled with the friction and wear behavior of multilayer samples sliding under different environments.