Abstract
Dynamic friction occurs not only between two contact objects sliding against each other, but also between two relative sliding surfaces several nanometres apart. Many emerging micro- and nano-mechanical systems that promise new applications in sensors or information technology may suffer or benefit from noncontact friction. Herein we demonstrate the distance-dependent friction energy dissipation between the tip and the heterogeneous polymers by the bimodal atomic force microscopy (AFM) method driving the second order flexural and the first order torsional vibration simultaneously. The pull-in problem caused by the attractive force is avoided, and the friction dissipation can be imaged near the surface. The friction dissipation coefficient concept is proposed and three different contact states are determined from phase and energy dissipation curves. Image contrast is enhanced in the intermediate setpoint region. The work offers an effective method for directly detecting the friction dissipation and high resolution images, which overcomes the disadvantages of existing methods such as contact mode AFM or other contact friction and wear measuring instruments.
Highlights
Friction is very common in the macro world, but the explanation of the origin is still scarce and controversial [1,2,3,4,5]
The first atomic-scale feature of a graphite surface was observed as the stick-slip phenomenon in the friction force microscopy (FFM), and some other two-dimensional materials were proved to be in superlubricity state and exhibit excellent lubricant properties as additives at microscale [14,15,16]
From the equations of the bimodal motion, the second order flexural phase φ2, the first order torsional amplitude Atr, and torsional phase φtr at two resonance frequency peaks can be imaged in Fig. 2, where the matrix is PS and the circular area is low-density polyethylene (LDPE), respectively
Summary
Friction is very common in the macro world, but the explanation of the origin is still scarce and controversial [1,2,3,4,5]. Atomic force microscopy (AFM), an innovative technique invented in 1986, has become a multifunctional and powerful apparatus for imaging nanometer resolution surface structures and friction measurement in various environments [6,7,8,9,10,11,12,13]. The resolution has reached a sub-atomic level, this technique is only capable to measure either the lateral or the normal force [18]. How to measure two-directional interaction by the tip is still a challenge issue in the development of AFM technology. An optional force sensor is a microcantilever beam, which can flexibly detect the two directional interaction by theoretically exciting both the flexural and torsional vibration modes [19, 20].
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