The atomic-scale origins of friction: A nanotribological study of diamond surfaces

This thesis is a product of an exploration of the fundamental mechanisms of friction at the nanoscale. In particular, the effect of phonons was investigated using atomic force microscopy (AFM) and two separate approaches for sample preparation: (1) by varying the bulk isotope concentration of diamond, and (2) by controlling the mass of the surface-terminating species. This work also represents the first study of the tribology of the hydrogen-terminated diamond surface with regard to surface orientation and sliding direction in the wearless, nanoscale regime. These results provide useful insight into the role of surface and sliding orientations and the surface termination for nanoscale friction and adhesion. Surface properties determine the resistance to shear and the subsequent energy dissipation during sliding through the work of adhesion and shear strength at the nanometer length scale. This research is important both from a basic scientific perspective and for applications in small devices with contacting, sliding surfaces. This work was performed using AFM in both 1 atmosphere of dry nitrogen and in ultra-high vacuum (UHV). Specific effort was made to produce reliable and quantitative AFM results and to measure and maintain the quality of the tip and sample. This includes the development of new calibration and data acquisition methods for friction measurements.; Using this methodology, friction and adhesion measurements for hydrocarbon-coated AFM tips on H-terminated (001)- and (111)-oriented microcrystalline diamond grains demonstrated no dependence for shear strength on surface orientation or sliding direction. However, the work of adhesion was approximately doubled on the (001) surface. Thus, while the surface orientation of diamond affects adhesion (and therefore contact area), it typically has no effect on the intrinsic resistance to sliding. In addition, measurements comparing hydrogen- vs. deuterium-terminated diamond (001)(2x1) surfaces suggest that the mass of the surface atoms has a significant impact on the shear strength, both in air and UHV. This work represents the first experimental observation connecting a purely phononic property to the intrinsic tribological properties of a nanoscale contact. These results, including related issues regarding the breakdown of continuum methods that arise during the analysis of the results, are discussed, and future directions highlighted.