Surface topography as a material parameter

Abstract

Abstract: Materials science is about understanding the relationship between a material's structure and its properties-in the sphere of mechanical behavior, this includes elastic modulus, yield strength, and other bulk properties. We show in this issue that, analogously, a material's surface structure governs its surface properties-such as adhesion, friction, and surface stiffness. For bulk materials, microstructure is a critical component of structure; for surfaces, the structure is governed largely by surface topography. The articles in this issue cover the latest understanding of these structure-property connections for surfaces. This includes both the theoretical basis for how properties depend on topography, as well as the latest understanding of how surface topography emerges, how to measure and understand topography-dependent properties, and how to engineer surfaces to improve performance. The present article frames the importance of surface topography and its effect on properties; it also outlines some of the critical knowledge gaps that impede progress toward optimally performing surfaces.

Keywords: Adhesion; Contact mechanics; Friction; Roughness; Surface topography.

Figure 1

Surface topography can range from kilometers to the atomic scale. (a) The Corona Heights Fault in San Francisco, Calif., provides an example of the multiscale nature of roughness, where roughness features exist over length scales from tens of kilometers down to microns. The surface topography shows self-affine fractal-like scaling, here manifested as a power-law $C\left(q\right)\propto q^{-1-2H}$ in the PSD, over this whole range. (b) The topography of ultrananocrystalline diamond, a common wear-resistant coating, deviates from self-affine behavior, but still shows roughness across a wide range of scales from centimeters to Ångströms. TEM, transmission electron microscopy; LIDAR, light detection and ranging. (a) Adapted with permission from Reference . © 2012 American Geophysical Union. (b) Adapted with permission from Reference . © 2018 American Chemical Society.

Figure 2

The surface tetrahedron. We propose to think about surfaces in terms of structure–processing–property relationships. Just like the classical materials tetrahedron (a), these relationships can be described schematically by the surface tetrahedron (b).

Figure 3

Size scales that control behavior, and techniques that can measure at different sizes. (a) A contemporary electric vehicle has many interfaces whose properties are crucial for its function. (b) Most functional surface properties are governed primarily by certain ranges of size scales. Surface-measurement techniques are also confined to certain length scales, such that multiple methods must be combined for comprehensive topography characterization. AFM, atomic force microscopy; RADAR, radio detection and ranging; LIDAR, light detection and ranging. Image of the electric vehicle from Reference , reprinted with permission by L.I.F. Cabrera.