Thermal and tangential-momentum accommodation coefficients for N 2 colliding with surfaces of relevance to disk-drive air bearings derived from molecular beam scattering

Molecular beam scattering techniques have been used to probe the dynamics of energy transfer for N₂ colliding with various surfaces of relevance to the disk-drive air bearing. These surfaces include bare and lubricated sputtered carbon films and a lubricated Pt(111) surface, as well as sections of a glass disk used in fly-height testing and of an actual 3.5 in disk. In each case, velocity distributions have been obtained for a wide range of scattering angles for different energies and angles of the incident N₂. These distributions have been analyzed to yield effective thermal and tangential momentum accommodation coefficients. Analysis indicates that N₂ is essentially fully accommodated at the test surfaces for energies characteristic of room temperature gas, with accommodation coefficients close to unity. One important exception is the case of scattering from sputtered carbon films, where the tangential momentum accommodation coefficient appears to exceed unity. A simple model is described to account for this behavior. In this model, a stream of molecules directed at a rough surface at non-normal incidence cannot strike parts of the surface in the shadow of high points. In the absence of diffusion, desorption then occurs preferentially from regions of the surface facing the incident stream, leading to net back-scattering