It has long been recognized that the angular deflection of an atomic force microscope (AFM) cantilever under “normal” loading conditions can be profoundly influenced by the friction between the tip and the surface. It is shown here that a remarkably quantifiable hysteresis occurs in the slope of loading curves whenever the normal flexural stiffness of the AFM cantilever is greater than that of the sample. This situation arises naturally in cantilever-on-cantilever calibration, but also when trying to measure the stiffness of nanomechanical devices or test structures, or when probing any type of surface or structure that is much more compliant along the surface normal than in transverse directions. Expressions and techniques for evaluating the coefficient of sliding friction between the cantilever tip and sample from normal force curves, as well as relations for determining the stiffness of a mechanically compliant specimen are presented. The model is experimentally supported by the results of cantilever-on-cantilever spring constant calibrations. The cantilever spring constants determined here agree with the values determined using the NIST electrostatic force balance within the limits of the largest uncertainty component, which had a relative value of less than 2.5%. This points the way for quantitative testing of micromechanical and nanomechanical components, more accurate calibration of AFM force, and provides nanotribologists access to information about contact friction from normal force curves [1].

1. J. Appl. Physics 107, 044305 (2010), doi:10.1063/1.3284957

The frictional properties of surface grown zwitterionic polymer brushes: poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) have been characterized using friction force microscopy (FFM) in different liquid media.

1. Effect of molecular weight and solvent on the frictional properties

For brushes thicker than 192 nm, the coefficient of friction decreased with increasing brush thickness, while for brush layers with smaller thicknesses, the coefficient of friction varied little with molecular weight. It is suggested that water molecules bound to PMPC chains act as an interfacial lubricant; as brush thickness increases, the amount of bound water increases and the coefficient of friction decreases. This hypothesis is supported by comparative studies of the approaching parts of force-displacement plots acquired for PMPC brush samples with different molecular weights under water. In particular, it was found that thicker brushes exerted a greater repulsive force to the AFM probe. Gold-coated probes were used throughout this part to avoid any complication might be caused by tip surface chemistry. FFM has also been used to investigate the cononsolvency behaviour of PMPC. Friction force was measured for PMPC brushes with a dry thickness of 307 nm while immersed in alcohol/water binary mixtures with different compositions. A distinct increase was observed in the coefficient of friction at an ethanol-water ratio of 90:10, and a 2-propanol-water ratio of 70:30, but not for methanol/water mixtures. This result is attributed to the conformational change of the polymer brush, which induced the loss of hydration layer.

2. Influence of solvent and tip chemistry on the contact mechanics

To study the contact mechanics of tip-sample interactions in FFM, AFM probes were chemically functionalized by deposition of three different types of self-assembled monolayer of dodecanethiol (C₁₁CH₃) or mercaptoundecanoic acid (C₁₀COOH), or cysteamine (C₂NH₂). In alcohol solvents, friction force acquired using acid- or CH₃- functionalized tip has a linear relationship with the applied load, but nonlinear for amine-terminated tip. It is also noted that the coefficient of friction is highest in 2-proponal for all three types of probe, which again suggests that conformation of PMPC brush is one of the key factors. In aqueous medium, the friction-load relationships were nonlinear and characterized by the Derjaguin-Muller-Toporov model of contact mechanics. Coefficient of friction measured by amine-functionalized probes were greater than that of acid-functionalized probes, and than CH₃- ones, which was attributed to the interaction between polymeric chains and probes.

Nanoscale friction and wear are primary limitations for small-scale devices such as atomic force microscopy (AFM) probes and micro- or nano-electronic mechanical systems with contacting surfaces, and is also relevant to understanding friction and wear in larger-scale contacts. We first present studies that quantify the nanoscale volume loss in sliding wear using AFM and periodic ex-situ transmission electron microscopy (TEM) imaging. Novel carbon-based AFM tip materials, including ultrananocrystalline diamond and diamondlike carbon, exhibit superior wear resistance compared to conventional materials (silicon and silicon nitride)^1-3. We then present results from wear tests performed inside of the TEM using modified in-situ indentation techniques. This permits real-time visualization of the contact geometry and shape evolution of a single asperity with sliding over a countersurface. This allows us to measure wear with a higher degree of precision than previously possible. Insights comparing the wear resistance of carbon-based and Si-based materials, particularly in the context of atom-by-atom wear processes, will be discussed⁴. Finally, we will discuss how nanoscale friction in graphene and other atomically-thin sheets is governed by the high flexibility intrinsic to the atomic scale⁵.

1. Prevention of nanoscale wear in atomic force microscopy through the use of monolithic ultrananocrystalline diamond probes. J. Liu, D.S. Grierson, J. Notbohm, S. Li, S.D. O’Connor, K.T. Turner, R.W. Carpick, P. Jaroenapibal, A.V. Sumant, J.A. Carlisle, N. Neelakantan & N. Moldovan, Small, in press (2010).

2. Ultra-low nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like-carbon. H. Bhaskaran, B. Gotsmann, A. Sebastian, U. Drechsler, M. Lantz, M. Despont, P. Jaroenapibal, R.W. Carpick, Y. Chen & K. Sridharan, Nature Nanotechnology 5, 181-185 (2010).

3. Wear resistant diamond nanoprobe tips with integrated silicon heater for tip-based nanomanufacturing. P.C. Fletcher, J.R. Felts, Z. Dai, T.D. Jacobs, H. Zeng, W. Lee, P.E. Sheehan, J.A. Carlisle, R.W. Carpick & W.P. King, ACS Nano, accepted (2010).

4. On the application of transition state theory to atomic-scale wear. T.D. Jacobs, B. Gotsmann, M.A. Lantz & R.W. Carpick, Tribol. Lett., accepted (2010).

5. Frictional characteristics of atomically-thin sheets. C. Lee, Q. Li, W. Kalb, X.-Z. Liu, H. Berger, R.W. Carpick & J. Hone, Science 328, 76-80 (2010).

The contact area dependence of the interfacial friction experienced during the translation of the antimony is studied under different conditions using the tip of an atomic force microscope as a manipulation tool [1]. In vacuum a dual behavior in the friction-area curves is found had been found earlier, characterized by the observation that some particles exhibit friction below the detection limit while other similarly sized particles showed constant shear stress values [2]. New investigations with improved sensitivity confirm the reproducibility of this effect and that neither the particle’s morphology nor their relative orientation towards the substrate lattice change this behavior. In contrast, we find that a temporary exposure to ambient air can lead to a drastic increase in the particle’s friction.

[1] A. Schirmeisen and U. D. Schwarz, ChemPhysChem 10 (2009) 2358

[2] D. Dietzel et al., Physical Review Letters 101 (2008) 125505

Because the structure and properties of diamond-like carbon (DLC) can vary depending upon deposition conditions, the tribological response of DLC (and diamond) is very sensitive to environmental conditions. For instance, the presence of water vapor has been shown to negatively impact the friction performance of hydrogenated DLCs but to improve the performance of nanocrystalline and ultrananocrystaline DLCs. Tribochemical reactions of the water with the DLC are thought to be at the heart of this long-standing puzzle.

With that in mind, we have been working to develop a potential energy function that is capable of modeling DLC in the presence of water. To be realistic, such a potential energy function should be able to model tribochemical reactions that may occur as a result of the sliding. In addition, because H, C, and O have very different electronegativities, the potential energy function must be capable of modeling charges and fluctuating charges that arise from electronegativity differences in a realistic way. This talk will outline our efforts at potential development and present some preliminary results of DLC friction in the presence of water.

**Supported by The Air Force Office of Scientific Research.

The wear and tribological properties of diamond-like carbon (DLC) coatings have been investigated and well documented under various laboratorial and industrial conditions. However, investigations into failure behavior of the coatings when subjected to cyclic impact-sliding loads are scarce. In this study, an inclined ball-on-plate impact-sliding tests were used to evaluate the fatigue cracking and peeling failure behavior of a DLC (a-C:H) coating and a TiN coating as comparison. By adjusting the impact velocity of a steel impacting ball that is connected to and driven by air cylinder, various dynamic impact loads can be obtained. The impact load vs. time curves were recorded and showed three stages, i.e., impact loading stage, vibration stage and quasi-static sliding stage for each impact-sliding cycle. Four loading combinations of impact/static forces (50N/100N, 100N/100N, 50N/200N and 100N/200N) were used in the tests. The test results showed that the DLC coating performed better than the TiN coating under the impact forces but worse under the sliding stages where the quasi-static force was applied by the air cylinder.