| AVS 60th International Symposium and Exhibition | |
| Tribology Focus Topic | Monday Sessions |
| Session TR+AS-MoM |
| Session: | Bridging Scales and Characterization |
| Presenter: | T.D.B. Jacobs, University of Pennsylvania |
| Authors: | T.D.B. Jacobs, University of Pennsylvania K.E. Ryan, United States Naval Academy P.L. Keating, United States Naval Academy D.S. Grierson, systeMECH, LLC J.A. Lefever, University of Pennsylvania K.T. Turner, University of Pennsylvania J.A. Harrison, United States Naval Academy R.W. Carpick, University of Pennsylvania |
| Correspondent: | Click to Email |
As components in devices and microscopy applications shrink to nanometer length scales, adhesion forces play an increasingly dominant role in the physics of contact. In particular, tip-based approaches for data storage, nanomanufacturing, and nanoelectromechanical systems rely on accurate knowledge and control of adhesion between a sharp asperity and a surface. It is well known that surface roughness affects adhesion at macro- and microscopic scales. However, the atomic-scale roughness of nanoscale tips is rarely measured or accounted for. Here, we characterized the atomic-scale roughness of carbon-based probes, and measured the corresponding effect on adhesion using simulations and experimental techniques.
We have conducted contact and sliding experiments inside of a transmission electron microscope (TEM), using a modified in situ nanoindentation apparatus. Similar experiments were used recently to study wear of nanoscale silicon probes1. In the present work, nanoscale asperities composed of either diamond-like carbon (DLC) or ultrananocrystalline diamond (UNCD) were brought into contact and separated from a flat diamond substrate. The in situ nature of the testing allowed characterization of surface roughness with sub-nanometer resolution immediately before and after contact. Additionally, complementary adhesion simulations were conducted using molecular dynamics (MD) with conditions matched as closely as possible with the experiments (e.g., materials, asperity shape, environment). The RMS roughness for the experimental tips spanned 0.18 - 1.6 nm; for the simulated tips, the range was 0.03 nm (atomic corrugation) to 0.12 nm. Over the tested range of roughness, the measured work of adhesion was found to decrease by more than an order of magnitude as roughness increased, with a consistent trend observed between experimental and simulation results2. The dependence of adhesion upon roughness was accurately described by a simple analytical model.
This combination of simulation and novel in situ experimental methodologies allowed for an exploration of an unprecedented range of tip sizes and length scales for roughness, while also intrinsically verifying consistent behavior between the two approaches. These results demonstrate a high sensitivity of adhesion to interfacial roughness down to the atomic limit. Furthermore, they indicate that present approaches for extracting work of adhesion values from experimental measurements of adhesion forces contain significant uncertainty due to an unmeasured variable – atomic-scale roughness.
1 T. D. B. Jacobs, R. W. Carpick, Nature Nanotech., 8, 108-112 (2013)
2 T. D. B. Jacobs, et al., Tribol. Lett., 50, 81-93 (2013)