Mesoscale friction anisotropy revealed by slidingless tests
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Yanfei Gaoa) Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Graham L.W. Crossb) School of Physics and Center for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College, Dublin 2, Ireland
Erik G. Herbert Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996
Barry N. Lucas Fast Forward Devices, LLC, Knoxville, Tennessee 37931 (Received 23 March 2011; accepted 5 August 2011)
Using a recently developed multidimensional nanocontact system designed for a quantitative measurement of lateral contact stiffness in the 10–106 N/m stiffness range (or 10–1000 nm contact size), we found a crystallographic-orientation-dependent lateral-stiffness reduction relative to the elastic prediction at contact sizes around 50 nm for polished Ni single crystal surface in air. The slidingless measurement is enabled by a frequency-specific, continuous stiffness measurement technique. Based on an interface microslip model and an anisotropic elastic contact analysis, the resulting friction stress is found to increase monotonically when the tested lateral direction rotates away from the closely packed direction.
The development of new fabrication and testing methods at small length scales provides both challenges and opportunities to the understanding of fundamental deformation and failure mechanisms. A number of instruments are dedicated to the measurements and characterization of frictional behavior, including the atomic force microscope (AFM),1–4 the multidimensional nanocontact system,5–7 the surface force apparatus (SFA),8,9 and other dedicated tribological instruments include scratch testers. The typical measured ranges of lateral contact stiffness for AFM and SFA are less than about 10–102N/m and larger than 106N/m, respectively, so it is often difficult for them to be used for the study of mechanical behavior at intermediate or mesoscopic length scales (10–1000 nm contact size). A recently developed multidimensional nanocontact system gives the stiffness range in between (i.e., 10–106N/m), and its usefulness for evaluating the tangential contact behavior at contact sizes ranging from tens of nanometers to submicrons has been recently demonstrated.5–7 This system is comprised of three independent, orthogonal nanoindentation
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.270 J. Mater. Res., Vol. 26, No. 18, Sep 28, 2011
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actuators which can be used to make quantitative depthsensing indentation and stiffness measurements along each axis. A small harmonic force is applied in any of the x-, y-, or z-axis and can be used to determine the stiffness of the contact. A vibration analysis can be used to deduce the contact stiffness from the harmonic response of the displacement and the phase an
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