AFM probe for measuring ∼10−5 ultra-low friction coefficient: Design and application

Abstract

Superlubricity provides a novel approach to addressing friction and wear issues in mechanical systems. However, little is known regarding improving the atomic force microscope (AFM) friction coefficient measurement resolution. Accordingly, this study established the theoretical formula for the AFM friction coefficient measurement and deduced the measurement resolution. Then, the formula was applied to the AFM probe with a rectangular cross-section cantilever. The measurement resolution is associated with the dimensional properties of the AFM probe, the mechanical properties of the cantilever material, the properties of the position-sensitive detector (PSD), and probably the anti-vibration performance of the AFM. It is feasible to make the cantilever as short as possible and the tip as high as possible to improve the measurement resolution. An AFM probe for measuring an ultra-low friction coefficient was designed and fabricated. The cantilever’s length, width, and thickness are 50, 35, and 0.6 µm, respectively. The tip height is 23 µm. The measurement resolution can reach 7.1×10−6 under the maximum normal force. Moreover, the AFM probe was applied to measure the superlubricity between graphene layers. The friction coefficient is 0.00139 under 853.08 nN. This work provides a promising method for measuring a ∼10−5 friction coefficient of superlubricity.

AFM probe with the U-shaped cross-sectional cantilever for measuring the ultra-low coefficient of friction of 10−6

“…An AFM probe primarily comprises a cantilever and a tip installed at the front part of the cantilever. The theoretical COF measurement resolution Δμ can be expressed as follows [17]:…”

“…According to our previous study, under the Friction 3 www.Springer.com/journal/40544 | Friction maximum normal force F N, max , Δμ can be rewritten as follows [17]:…”

“…According to the measurement principle of the lateral and normal forces by AFM, Equation ( 2) can be derived as follows [17]:…”

“…According to our previous research, given that the length of the laser spot is 20-50 μm and the silica microsphere tip used in the superlubricity research is of micron magnitude in general, l and h tip are set as 50 μm and 23 μm, respectively. They are nearly pushed to their boundaries [17]. 4) I t and I x C are related to the geometrical properties of the cantilever, such as the cross-sectional shape and dimensions.…”

“…Our previous research evaluated the theoretical COF measurement resolution Δμ using the AFM probe with the solid rectangular cross-sectional cantilever on an advanced AFM (MFP-3D, Asylum Research). The limit of Δμ was ~10 -5 [17]. Therefore, it is critical to reducing Δμ further to enable more accurate COF measurements, contributing to the superlubricity research.…”