Understanding Nanoscale Surface Roughness and its Effect on Macroscale Adhesion

Surface roughness affects the functional properties of surfaces, including adhesion, friction, and wear. However, experimental investigations to quantify these links are often inconclusive, primarily because surfaces are fractal and the values of measured roughness parameters depend on measurement size. The objectives of this dissertation research were two-fold: first, to establish a new way to characterize multi-scale topography; and second, to demonstrate how it can be used to understand the effect of topography on surface properties. In the present research, the topography of rough surfaces was characterized beyond the limits of conventional surface measurement techniques. Using transmission electron microscopy, surface features were measured down to the Ångström-scale. This small-scale topography information was combined with conventional larger-scale surface characterization to achieve a comprehensive surface description spanning eight orders of magnitude in size. Data from various length scales were combined using the power spectral density (PSD), and this was used to compute scale-independent roughness parameters. This approach was applied to four different types of polycrystalline diamond films to interrogate differences between materials with similar surface chemistry but different surface topography. Then, this comprehensive description of topography was used to understand the topography dependence of soft-material adhesion. Specifically, adhesion measurements with in situ observation of contact size were performed using soft elastic polydimethylsiloxane hemispheres (modulus ranging from 0.7 to 10 MPa) on the polycrystalline diamond films of varying roughness. The results showed that the apparent work of adhesion when coming into contact was reduced below the intrinsic value by the energy required to achieve conformal contact. Further, the total energy lost during contact and removal is equal to the product of the intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively understand the dependence of soft-material adhesion on surface roughness