AVS 50th International Symposium
    Nanometer Structures Wednesday Sessions
       Session NS-WeA

Paper NS-WeA10
Nanotribology - Tribochemical Wear of Muscovite Mica in Aqueous Solution

Wednesday, November 5, 2003, 5:00 pm, Room 308

Session: Nanotribology
Presenter: J.M. Helt, The City University of New York College of Staten Island and the Graduate Center
Authors: J.M. Helt, The City University of New York College of Staten Island and the Graduate Center
J.D. Batteas, National Institute of Standards and Technology
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Part I: Atomic force microscopy (AFM) is used to probe defect nucleation, prior to gross wear, of muscovite mica under aqueous environments. Nucleation is found to present itself initially as surface charging due to stress-induced tribochemical scission of the terminating surface bonds. As the surface bonds are continually cleaved, an ensemble of defects contribute to the ~5.2 Å to ~ 3 Å crystal lattice reconstruction observed in AFM topography and frictional force micrographs. Following lattice restructuring, abstraction of mica surface materials ensues, yielding visible wear scars ranging from ~ 2 - 10 Å in depth. Environmental [OH-] profoundly affects the efficacy of wear events, which is illustrated by the acceleration or inhibition of wear with adjustment of pH under identical load and scan conditions. Part II: In AFM wear trials the area scanned is defined by the length of the slow (L@sub sscan@) and fast scan axis. The ratio of L@sub sscan@ to image resolution (res, lines/image) defines the magnitude of the line step (LS = L@sub sscan@/res) . A contact radius - LS relationship indicates that overlap of successive scans will result if the contact radius - line step ratio (CRLS) is < 1/2. Consequently, the history associated with a single image scan is not equivalent at various loads due to the contact radius being proportional to the load@super 1/3@. Theoretical & experimental analysis on muscovite mica with a Si@sub 3@N@sub 4@ tip illustrate the effects of scan overlap. The CRLS model, derived from the Hertz contact mechanics theory, shows that scan overlap is considerable. AFM wear trials on mica under pH 5 conditions legitimize this development, with CRLS readily predicting the additional scan history from scan overlap for the mild wear regime. CRLS theory, however, consistently underestimates severe wear trends, which is to be expected in light of Hertzian principles employed.