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Rough surfaces in sliding contact: A combined atomistic/continuum investigation of topography evolution

Final Report Summary - TOPOGRAPHY EVOLUTION (Rough surfaces in sliding contact: A combined atomistic/continuum investigation of topography evolution)

Mechanical systems have moving parts which dissipate energy at surfaces sliding with respect to each other. Energy efficient devices need interfaces that are tailored to consume little to no energy. This requires a fundamental understanding of processes during the contact of two surfaces, most of which are rough over a wide range of scales. A single roughness protrusion, also called asperity, can experience extreme pressure and shear. This microjunction can interact via non-bonded (van-der Waals adhesion), cold-weld and adhere, yield plastically or fracture. This project aim at identifying the contribution of these processes to friction using theoretical methods from atomistic to continuum scales.

Within the project, significant progress was made towards a fully seamless coupling of an atomistic system to an elastic boundary. Such coupling is necessary to span the scales from nanometers at the atomic-scale to millimeters that we can see with our bare eyes. An efficient Green's function technique was developed and integrated into a state of the art molecular dynamics code. This technique is now ready to be used to study adhesion and plasticity of realistic systems.

First, the contact of adhesive rough surface was investigated. Macroscopic objects rarely stick together although the van der Waals interactions between surface atoms produce attractive pressures that are orders of magnitude larger than atmospheric pressure. This “adhesion paradox” has been linked to surface roughness, which reduces the area of intimate atomic contact to summits on the rough landscape. The atomic-scale calculations culminated in a parameter-free theory that captures the interplay between elasticity, interatomic attraction, and surface roughness. It predicts how adhesion changes contact area and when surfaces are sticky. The results offer a simple explanation for why tape sticks to our desktops but a sheet of paper does not, and may aid in the design of adhesives and in engineering surface roughness to enhance or eliminate adhesion. In the non-sticky limit, the equations offer an explanation for the widespread success of Amonton’s friction law that makes the simplifying assumption of ignoring adhesive contributions.

Second, the plasticity of atomic solids with self-affine roughness was studied. First results indicate that plasticity is largely confined to the topmost layers of the atomic solid. Atomic geometry and plasticity modifies pressure distribution and contact geometry on the smallest scales only. Macroscopic relations, such as the dependence of contact area on load are largely unaffected and require only minor renormalization of the (dimensionless) prefactor.

Third, fundamental plastic properties of bulk amorphous carbon and amorphous carbon asperity-asperity junctions were studied. The phenomenology of plastic deformation in this simple network glass is similar to bulk metallic glasses: Deformation occurs in discrete shear transformation events that are spatially localized. These events localize into shear bands that confine the shear into a damaged portion of the material. It has been found that this shear localization, unwanted in many structural applications, is the origin of the superior wear resistance of amorphous carbon coatings: It localized damage at the interface and prevents extensive subsurface deformation and hence damage in the coating material.

The results obtained in this project have potential to be used to optimize surfaces and coatings for certain friction and wear requirements. This makes devices more reliable and dissipate less energy - and will eventually help to reduce energy consumption and CO2 emission of mechanical devices.