AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM3
Light vs. Heavy Diamond: an Investigation of the Fundamental Origins of Friction

Thursday, November 18, 2004, 9:00 am, Room 210C

Session: Tribology, Adhesion, and Friction
Presenter: R.J. Cannara, University of Wisconsin-Madison
Authors: R.J. Cannara, University of Wisconsin-Madison
R.W. Carpick, University of Wisconsin-Madison
Correspondent: Click to Email
Mechanisms of friction include wear, bond-breaking and surface chemistry, molecular deformation, electronic dissipation, and vibrational excitations (or phonons). Resolving the individual contribution of each of these mechanisms poses a great challenge. To overcome this obstacle and isolate the role of phonons in frictional energy dissipation, we compare insulating crystals that are identical, except for having different isotopic concentration. We report adhesion, as well as load- and temperature-dependent nano-scale friction measurements, for a tungsten carbide-coated Si tip on the hydrogen-terminated (111) surfaces of three single-crystal synthetic Type IIa diamonds. Each of the specimens is composed of a different bulk mixture of @super 12@C and @super 13@C. Varying the average mass and isotopic disorder in diamond strongly influences its phonon band structure and thermal properties. For example, previous work has revealed that both phonon lifetimes and thermal diffusivity of diamond increase with isotopic purification. Moreover, its thermal diffusivity is dramatically enhanced at low temperatures (down to ~100 K). All of these phonon properties determine the probability that atomic vibrations (initially created by slip processes during sliding) will reduce frictional energy dissipation by aiding subsequent slip events. We discuss isotope, or phonon, contributions to atomic-scale friction measurements performed in nitrogen and ultra-high vacuum at different temperatures. In addition, we present evidence for structural interfacial effects that lead to negative differential friction behavior.