Session L32: Focus Session: Tribophysics -- Sliding Friction

Cryolubricity of YBCO powder deposits

Motivated by recent reports of superconductivity-dependent friction [1] in macroscopic pin-on-disk measurements of steel on YBCO, [2] we have investigated the tribological properties of YBCO powder deposits on metal electrodes of a quartz crystal microbalance (QCM). Measurements are performed as a function of temperature over the range 80 - 300K, by monitoring the frequency and amplitude of the QCM both in the presence and absence of adsorbed nitrogen film layers. A pulsed magnetic field was applied to isolate the effect of superconductivity at and around the transition temperature. The powder deposits produce negative shifts in the QCM fundamental frequency, an indication of the strength of their attachment to the surface. The shifts exhibit structure as the temperature passes through the superconducting transition temperature, but the presumed drop in friction is not so large as to produce a decoupling effect which would lead to positive shifts. [3] Measurements on alternate QCM electrodes in the presence of adsorbed film layers are ongoing and will be reported on. Funding provided by NSF DMR. \\[4pt] [1] Highland, M. and Krim, J. \textit{Phys. Rev. Lett.} \textbf{2006, }96, 226107.\\[0pt] [2] Ding, Q. et al. \textit{Wear} \textbf{2008, }265, 1136.\\[0pt] [3] Dybwad, G.L. \textit{J. Appl. Phys.} \textbf{1985}, 58, 2789.   

Cryolubricity in Atomic Scale Friction on YBCO Surface

The unique capabilities of ultra high vacuum AFM have been used to characterize temperature dependent friction on YBCO surface. We observed a significant decrease of friction upon cooling of YBCO samples to low temperatures. Our observation indicates the crucial role of electronic and phononic mechanisms of friction in solids at low temperatures. We will discuss the possible mechanisms of cryolubricity in high temperature superconductors.   

Tribo-induced Melting Transition at a Sliding Asperity Contact

Observation of a tribo-induced transition from solid to liquid-like behavior is reported for a Scanning Tunneling Microscope (STM) tip in sliding contact with an indium electrode of a Quartz Crystal Microbalance (QCM). [1] In particular, at sufficiently high asperity sliding speed (about 1 m/s) and/or sample temperature, a change in the contact mechanics is observed that is consistent with melting in terms of both the QCM response and an energy analysis. The results confirm that the surface, rather than bulk, melting point temperature is the more relevant quantity for tribological considerations. Ongoing studies of similar studies for Au-Ni alloys and the impact of lubricants on the melting transition will also be reported on. [1] B.D. Dawson, S.M. Lee and J. Krim, PRLvol. 103 (2009)   

Simulated sliding of surface deposited islands and nanoclusters: from static pinning to high speed ballistic friction

The physics of friction of surface adsorbed molecules, nanoclusters, and rare gas islands is incompletely understood. We present two case studies aimed at improving this state of affairs. The static friction of rare gas islands, as revealed, e.g., in Quartz Crystal Microbalance experiments, is known to persist, unexpectedly, even on incommensurate and defect-free crystal surfaces, where sliding should theoretically be free. Through atomistic simulations mimicking Kr islands on Au(111), we show that surprisingly the island edges may be the ultimate culprits. An edge-originated energy barrier blocks the motion of solitons (tiny density and corrugation modulations with the beat periodicity between adsorbate and surface), keeping the whole island pinned. As the critical static friction force is reached, the barrier vanishes at one point on the edge, and here solitons enter and sweep the island, which becomes depinned. Surface smoothness and high temperature facilitate edge depinning. But the island's thermal expansion also leads to changeable commensurability upon heating, giving rise to the possibility of re-entrant static friction. On a different note, we address the high speed kinetic friction of sliding metal clusters. Simulating the motion of kicked Au clusters on graphite, we characterize a novel ballistic friction regime at high speed, and demonstrate the crossover from this to frictional drift at low speed. Consistent with simple theory, we find that the temperature dependence of the cluster slip distance and slip time is opposite in these two regimes. Crucially to both regimes, rotations and translations are shown to be correlated in slow drift but anti-correlated in fast sliding. This interplay, besides turning the kicked cluster trajectories from straight to curved, controls the crossover between the two sliding regimes. Despite the deep difference with drift, the speed dependence of ballistic friction is, like drift, viscous.   

Interpretation of Surface Dynamics and Bond Strengths of Polystyrene Microspheres on a Quartz Crystal Microbalance by Analysis of Decoupling Curves

In this study we have analyzed the behavior of 5$\mu $m diameter polystyrene spheres deposited from aqueous solution on the electrode of a quartz crystal microbalance (QCM). By varying the driving voltage of the QCM, the samples exhibit a frequency response with respect to the crystal amplitude (``decoupling curve''). Decoupling curves provide information about the friction, via changes in the coupling between the spheres and the QCM surface as the oscillation amplitude is swept forward and backward. Qualitative information in regards to the change in coupling and surface dynamics can be backed out using known QCM frequency response models.\footnote{Dybwad, G.L. J. Appl. Phys. \textbf{1985}, 58, 2789}$^,$\footnote{Flanigan, C.M.; Desai, M.; Shull, K.R. Langmuir \textbf{2000}, 16, 9825} Optical observations show that the particle motion is dependent on the oscillation direction and surface topology of the QCM. Optically observed sliding motion as well as preliminary data on detection of microsphere transfer from adjacent QCMs will also be reported on.   

Motion of Sb nanoparticles on HOPG: simulation study of frictionless behavior

Using DFT total energy techniques adapted to accommodate van der Waals dispersion forces, we have simulated the behavior of the recently studied motion Sb nanoparticles on HOPG surface with the quest to elucidate the experimentally~observed frictional duality [1, 2]. Both frictionless and ``normal'' behavior, which scales with apparent contact area was identified experimentally. Several different nanoparticle geometries, including atoms, small to medium size~clusters, and surfaces were included in the simulations. The Sb-HOPG interface is found to be primarily van der Waals bonded. From the simulations the frictionless behavior can only be accounted for by a self-lubricity mechanism. Therein the smallest structurally stable Sb tetrahedra, attached to the contact plane of the Sb nanoparticle, yield essentially uncorrugated potential energy surfaces and hence, frictionless sliding over HOPG. [1] D. Dietzel, Phys. Rev. Lett. \textbf{101}, 125505~(2008). [2] A. Schirmeisen, U.D. Schwarz, ChemPhysChem \textbf{10}, 2373 (2009).   

Scaling laws in superlubric sliding of metallic nanoparticles

If an interface between two incommensurate surfaces is atomically clean, a state of virtually frictionless sliding is anticipated, often referred to as ``superlubricity.'' Theory predicts that the lattice mismatch at the interface causes a decrease of shear stress with increasing contact area, ultimately leading to vanishing friction. Analyzing the contact area dependence of superlubric friction should therefore confirm the concept of superlubricity. To measure the interfacial friction we have manipulated metallic nanoparticles of different size on atomically flat surfaces by contact mode atomic force microscopy techniques. An optimized experimental setup allowed us to quantify friction of nanoparticles which previously appeared to be sliding frictionless [1]. As theoretically expected, interfacial friction showed a nonlinear contact area dependence with a shear stress decreasing with contact area. This confirms the superlubric sliding of the nanoparticles under investigation. \\[4pt] [1] Dietzel et al., Phys. Rev. Lett. 101, 125505 (2008).   

Layered atomic structures of silver vanadate compounds for low shear strength at high temperatures

The aerospace industry has been a strong driving force for the creation of new and effective wear-resistant and lubricious materials at high temperatures (T $>$ 500 \r{ }C). Solid lubricants (SLs) such as graphite and molybdenum disulfide oxidize and, hence, degrade rapidly at T $>$ 350 \r{ }C. The selection of oxides is a clear viable alternative for the choice of SLs when confronting the problem of oxidation. Double metal oxides of the form Me$_{x}$TM$_{y}$O$_{z}$, where Me is a noble metal and TM a transition metal, were found to exhibit relatively low coefficients of friction in the 500 to 700 \r{ }C range ($\mu $ = 0.1-0.3) . Very recently, our group has undertaken to understand the friction properties of a silver vanadate, which was shown to be an effective lubricant up to 1000 \r{ }C. We show, using \textit{ab-initio }calculations within the density functional theory framework, that the layered atomic structure of silver vanadate with weak inter-planar bonds that facilitate sliding, resulted in a low coefficient of friction.   

The effect of defect structure on the dynamic frictional force at sliding Al/Al interfaces

For moderate sliding velocities, less than velocities which induce structural or phase transformations, the defect and dislocation densities determine the local plastic flow characteristics near a sliding interface. We present the results of large-scale NEMD simulations for the tangential frictional force at an Al/Al interface in the velocity range 10 - 500 m/s for pressures of 15 GPa. The defect structures in the initial state are similar to those used in NEMD simulations to characterize the effect of defect densities on the high strain rate branch of the flow stress for Cu and Al (R.Ravelo, B.L.Holian and T.C.Germann, SCCM-2009). We discuss the contribution to dissipation of pre-existing defects and dynamically generated defects and their influence on the resulting steady state frictional force.   

Measuring contact area at the nanometer scale

The real area of contact between surfaces A$_{real}$ plays a central role in macroscopic theories of friction, which often assume friction is proportional to A$_{real}$. The meaning of contact and A$_{real}$, as well as their connection to friction, become unclear when considering atomic interactions at nanometer scales. We use molecular dynamics simulations to compare and contrast different definitions of contact. The geometries considered include a flat or rough elastic substrate and a rigid surface that is flat or spherical. Both adhesive and non-adhesive interactions are considered. Contact area is measured by counting atoms within some cutoff distance or from the time averaged pressure on the opposing surfaces. The effects of cutoff distances or pressure thresholds, temperature, load, and measurement time are described. Many definitions lead to undesirable results, such as strong load dependence of A$_{real}$ between atomically flat surfaces. These results are related to the statistical distributions of contact times and probabilities, which exhibit a surprisingly universal dependence on the mean force at each temperature.   

Macroscopic and microscopic friction at low temperatures: heat transfer at an AFM tip and the effect of superconductivity on friction

Temperature is an important parameter in both macroscopic and microscopic (single-asperity) friction. Temperature dependent measurements can help discriminate between phononic and electronic friction mechanisms. At cryogenic temperatures, phonons freeze out, and electronic states change dramatically in the vicinity of phase transitions such as superconductivity. Models of single asperity friction focus on thermally activated hopping processes. We report measurements using a variable temperature AFM and a macroscopic tribometer to study friction at low temperatures. Preliminary results on macroscopic sliding block tribometer with a niobium track show no change in friction through the superconducting transition near 9K. Our analysis of the thermal environment of a variable temperature AFM shows that the temperature difference between the tip and the substrate can be several hundred Kelvin, which severely complicates comparison between theory and experiment. Current work on an isothermal AFM that avoids these problems will also be discussed.   

Transverse Shear Microscopy: Probing Elastic Anisotropy at Organic Surfaces

We demonstrate that an unconventional mode of lateral force microscopy can distinguish between sliding friction and elastic shear deformation at the surfaces of molecular single crystals. Specifically, when the scan vector is perpendicular to the cantilever axis, as in case of friction force microscopy, the cantilever twists due to torque on the tip resulting from friction forces at the tip-sample interface. However, aligning the scan vector \textit{parallel} with the cantilever axis while still monitoring cantilever twist, a mode we term the transverse shear microscopy, affords improved sensitivity to elastic shear deformation at the crystal surface. Scanning along particular crystallographic directions in the transverse shear mode generates a cantilever torque that can be related quantitatively to the elastic modulus tensor of the crystal. The velocity and temperature dependencies of both the transverse shear and friction signals confirm that the transverse shear response has a fundamentally different physical origin than friction. The ability to image elastic anisotropy at high resolution is useful for microstructural characterization of soft materials, and for relating other physical properties ($e.g$., optical, thermal or electrical anisotropy) to bonding anisotropy in such systems.   

Tens of micro-scale wavy crack propagation in silicon nitride films

Microscale wavy crack propagation of silicon nitride thin film was found during heating process, which is the first discovery for a silicon nitride film, since Yuse and Sano found the oscillating crack propagation of a glass sheet. The wavy crack characteristics were investigated by changing the metal, metal line widths, metal thickness and silicon nitride thickness. The paths of this crack can be controlled by a metal pattern, which allows decohesion between silicon nitride and SiO2 substrate so that the silicon nitride can buckle. The crack initiates at the boundary of a buckled area and then propagates towards the strained area on the low reactive metal patterning for relieving strain energy. The width of the wavy crack can be controlled down to submicron scale, hence can be directly applied for nanofabrication and microfluidic techniques. We discussed the difference between Yuse' and Sano' experiment and our findings. This study provides a bridge to broaden the discussion between the physics and mechanics community that focuses on fracture theory and other communities researching in nanofabrication.