Complementary atomic force microscopy (AFM) and molecular dynamics (MD) simulations were conducted to determine the work of adhesion for diamond(111)(1x1) and diamond(001)(2x1) surfaces paired with other carbon-based materials. In the AFM experiments, the work of adhesion between an amorphous carbon tip and individual (001)(2x1)-H and (111)(1x1)-H crystal grains of a microcrystalline diamond sample was determined from pull-off force measurements, and by using fits to friction versus load measurements. Both methods yielded adhesion values that were larger on the (001)(2x1)-H diamond surface by 27-55%, with the magnitude of the difference depending on the measurement method and the tip used. As well, on both surfaces, the work of adhesion for a 150 nm radius tip was found to be ~3-4 times lower than for a 45 nm radius tip. The MD simulations allowed the quantification of the influence on adhesion of variables that are not easily changed in an experiment, including hydrogen coverage, commensurability, and atomic roughness. For self-mated contacts, the average adhesion between two flat diamond(001)(2x1) surfaces calculated from the MD simulations was smaller than for self-mated diamond(111)(1x1) contacts for all hydrogen coverages examined. As well, the relative alignment of the opposing surfaces was found to significantly affect the adhesion, such that incommensurate alignment strongly reduces adhesion. Changing the counterface to an amorphous carbon surface, which more closely modeled the AFM experiment, resulted in adhesion reductions when paired with both diamond(111)(1x1) and (001)(2x1) that were of similar magnitude. Pairing model diamond nanocomposite surfaces with the diamond(111)(1x1)-H sample resulted in even larger reductions in adhesion. These results point to the importance of atomic-scale roughness in determining adhesion, a variable which is not known nor easily controlled in AFM experiments, and could be one factor contributing to the different trend in adhesion for diamond(001)(2x1) versus diamond(111)(1x1) observed in the experiments and simulations. However, the absolute values of the works of adhesion for experiment and simulation are in reasonable agreement. The calculated W values show a modest dependence on hydrogen coverage, whereby an optimal coverage is found which is intermediate to fully terminated and fully exposed. Although, fully H-terminated surfaces have a lower surface energy, removing an optimal number of H atoms reduces the work of adhesion by producing a larger mean separation between the counterface and the topmost atoms, which now include C atoms. Density functional theory calculations performed on hydrogen-terminated, single-crystal diamond surfaces revealed small, C-H bond dipoles on both the diamond samples, with the (001)(2x1)-H surface having the larger dipole, but having a smaller dipole moment per unit area due to the lower surface density of C-H bonds. Charge separation at the surface is another possible source of the difference between the measured and calculated work of adhesion.
Citation: Journal of Adhesion Science and Technology
Pub Type: Journals
adhesion, diamond, nanocomposite diamond, amorphous carbon, diamond-like carbon, nanotribology, work of adhesion, pull-off force, atomic force microscopy, friction force microscopy, molecular dynamics, ab initio, density functional theory