Determining the Nanoscale Friction and Wear Behavior of Si, SiC and Diamond by Microscale Environmental Tribometry

Abstract

This paper offers a review of the author’s decade-long attempts to examine the tribochemical changes that occur with various crystallinities of (a) polished silicon (b) unpolished and polished polycrystalline diamond films, and (c) a commercially available version of polished polycrystalline α-SiC. Theory-based model experiments were performed in ∼1.33 × 10 Pa = 1 × 10-5 Torr vacuum (∼93% of the residual gases is water vapor) and some in low partial pressures of hydrogen test atmospheres, at temperatures ranging from lab-ambient to 950°C. The apparatus used was a unique pin-on-oscillating-flat-type scanning electron microscope (SEM) tribometer specially built to fill the gap between an atomic force microscope and a conventional, bench-top friction and wear tester. Its primary purpose has been to examine the changes in the tribological behavior of a variety of bearing materials and solid lubricants, under realistic engineering (Hertzian) contact stresses in the GPa to MPa (from many thousands to hundreds of psi) range, as influenced by elevated temperatures in moderate vacuum and in low partial pressures of inert or reactive gases. The coefficient of friction and wear measurements were occasionally complemented by surface analyses to decipher the footprints of atomic-level surface interactions by the tribological behavior of essentially microscopic (∼50 to 500 μm diameter) Hertzian contacts. All the friction trends indicate that the changes in adhesion (and thus the coefficient of adhesive friction) can be explained by the number of dangling (high-friction), reconstructed (reduced-friction) or adsorbate-passivated (low-friction) surface bonds developing on the counterfaces as a function of temperature and atmospheric environment.