Spotlight Presentations

Spotlight presentations are featured talks that have been identified by the TFC Planning Committee of primary interest in each of the following  tribology research topics. Spotlight presentations have been given extra time from the traditional 20-minute presentation to 40-minute presentations.

Biotribology

3221833: Evolutionary Routes to Damage Tolerant Materials: Unique Microstructure and Fracture Properties of Enamel in the Grinding Dentition of the Hadrosaurid Dinosaur  Brandon Krick, Lehigh University, Bethlehem, PA, Gregory Erickson, Florida State University, Tallahassee, FL, Tomas Babuska, Tomas Grejtak, Lehigh University, Bethlehem, PA, Stephen Kuhn-Hendricks, Florida State University, Tallahassee, FL, Mark Norell, American Museum of Natural History, New York, NY, Soumya Varma, Yi Lee, Siddhartha Pathak, University of Nevada, Reno, Reno, NV. 
bakrick@lehigh.edu.

Abstract:
Effective energy storage and release is required for survival of insects. Elaterid beetles, known familiarly as ‘click’ beetles or ‘skipjacks,’ respond to physical stimulus is by an audible and tangible snap, executed within the hinge between their body segments with no external leverage required. This is similar to the explosive power release of the mantis shrimp and the trapjaw ant. The aim of this study is to use recorded images of the snap maneuver to identify the function of the friction latch at the hinge. Scanning electron microscopy (SEM) was used to identify the geometry of the anatomy of the interface. A peg protrudes from the prothorax, and mates against a lip on the mesosternum in a ‘brace’ position just before the snap initiates. High-speed x-ray video reveals a rotation of the peg locally against the lip just before snap, which appears to redistribute the reaction forces. The slight angle change simultaneously orients the force more laterally to overcome friction, and decreases the normal force at the interface, allowing for slip. This new understanding allows for more sophisticated modeling of the contact area and local contours which support the load throughout the critical slip event.

3200311: Contact Mechanics and Articulation of the Frictional Latch Mediating Function of the Snap Maneuver in Elaterid Beetles 
Alison Dunn, Lihua Wei, Ophelia Bolmin, Aimy Wissa, University of Illinois, Urbana, IL, Marianne Alleyne, University of Illinois at Urbana-Champaign, Urbana, IL
acd@illinois.edu

Abstract:
Effective energy storage and release is required for survival of insects. Elaterid beetles, known familiarly as ‘click’ beetles or ‘skipjacks,’ respond to physical stimulus is by an audible and tangible snap, executed within the hinge between their body segments with no external leverage required. This is similar to the explosive power release of the mantis shrimp and the trapjaw ant. The aim of this study is to use recorded images of the snap maneuver to identify the function of the friction latch at the hinge. Scanning electron microscopy (SEM) was used to identify the geometry of the anatomy of the interface. A peg protrudes from the prothorax, and mates against a lip on the mesosternum in a ‘brace’ position just before the snap initiates. High-speed x-ray video reveals a rotation of the peg locally against the lip just before snap, which appears to redistribute the reaction forces. The slight angle change simultaneously orients the force more laterally to overcome friction, and decreases the normal force at the interface, allowing for slip. This new understanding allows for more sophisticated modeling of the contact area and local contours which support the load throughout the critical slip event.

3225042: Cells Sense and Respond to Frictional Shear Stress 
Angela Pitenis, University of California, Santa Barbara, Santa Barbara, CA
apitenis@ucsb.edu

Abstract:
The body’s first line of defense against the external world is primarily composed of epithelial cells. These cells and the soft tissues they comprise often bear significant stresses under large strains, conduct complicated mass and heat transport functions, and continuously restructure, remodel, and rebuild in response to physical challenges, insults, and injuries. The body manages these challenges through high water content gel layers, which are continuously secreted by all moist epithelial cells. These gel networks may be thought to act as mechanical fuses under shear forces, thereby protecting the underlying epithelia. The tribology of mucinated cellular monolayers has revealed that cells are capable of producing strong pro-inflammatory signaling molecules, or cytokines, following excessive shear stresses. Recent studies used a series of progressively stiffer hydrogel probes (by decreasing water content) to impart shear stresses of 40, 60, and 80 Pa under identical normal forces against cell monolayers. This work revealed that higher shear stresses under identical normal force conditions led to an increase in apoptotic cells in the monolayer. These findings suggest that mitigating frictional shear stress could be a viable design strategy for soft implantable devices.

Energy/Environmental/Manufacturing

3221703: The Influence of Material Properties on the Formation of Subsurface Bearing Failures with Microstructural Alterations
Benjamin Gould, Aaron Greco, Nicholaos Demas, Argonne National Laboratory, Lemont, IL
bgould@anl.gov

Abstract:
White etching cracks (WECs) have been identified as a dominant mode of premature failure within wind turbine gearbox bearings. Though WECs have been reported in the field for over a decade, the conditions leading to this failure, and the process by which this failure culminates, are both highly debated. Because of this, the development of benchtop tests capable of accurately recreating these failures at an accelerated rate are difficult to come by. Recent work has identified inclusions containing both an aluminum oxide component as well as a manganese sulfide component as preferential initiators of these failures. These inclusions are prevalent in larger bearings but sparse in standard benchtop test samples. The present work investigates the formation of WECs using special samples manufactured from a WT bearing, and shows the cleanliness of the test specimen plays a drastic role in the formation of these failures.

3221746: Tribology in the Triboelectric Nanogenerator: Assessing Durability Issues 
Anne Neville, Abdel Dorgham, Joshua Armitage, Leeds University, Leeds, United Kingdom
a.neville@leeds.ac.uk

Abstract:
The triboelectric effect (or contact electrification) has long been understood as the process of electrical charge transfer resulting from the mechanical contact of material surfaces [1]. The underlying mechanisms that contribute have been the subject of much research [2,3] and applications have been developed in order to harness and control this charge transfer for a variety of purposes [4]. This paper will focus on the development of quantitative methodologies for characterising the triboelectric properties of materials on the macroscale and assessing the topography and surface chemistry effects of materials as a function of time as their surfaces evolve [5,6]. The methodology in question involves the development and construction of a novel test apparatus, capable of replicating and analysing the electrical output and tribological properties of a scaled-up triboelectric nanogenerator (TENG). This allows for tribological parameters to be correlated with triboelectric properties such as contact potential difference, induced current and charge polarity for different material pairings, both insulating and conductive. The change in surface topography and the development of transfer films will be the focus of this paper; assessing the “tribology” aspects of TENGs and the evolution of triboelectric properties with time.

Fluid Lubrication

3221522: Comparison of Dry and Lubricated Traction in Rolling Contacts - A Review 
Gerhard Poll, Leibniz University Hannover, Hannover, Lower Saxony, Germany
poll@imkt.uni-hannover.de

Abstract:
In dry rolling contacts, slip is known to be the result of elastic deformations of the contacting solid bodies when subject to tangential forces. The Coulomb friction law is commonly applied to determine the maximum traction. Thus, solid contact mechanics are governing the traction characteristics. In contrast, in lubricated contacts, fluid rheology is thought to prevail. In reality, solid body elastic deformations as a result of tangential forces also exist in lubricated contacts. and should be considered at high pressures. Also, the lubricants tend to solidify into a glassy state with increasing pressure and then need to be regarded as solid interfacial layers with elastic properties. On the other hand, the surfaces in dry contacts are often not “clean” and covered by layers with specific rheological properties. Given the fact that discussions in the scientific community have re-intensified recently, the authors intend to review the state of research regarding this topic.

3212737: The Rheological Assumptions of Classical EHL: What Went Wrong? 

Scott Bair, Georgia Tech, Atlanta, GA
scott.bair@me.gatech.edu

Abstract:
The field of elastohydrodynamic lubrication (EHL) now has two very different approaches to the problem. In the first, the classical approach, the pressure dependence of viscosity has from the beginning been quite different from that which is measured in viscometers. This rather odd situation resulted from a desire to analyze the response of the liquid film without the complication and labor of viscometer measurements. Instead, the viscosity was extracted from the observed behavior of the film itself. Two assumptions regarding the pressure, temperature and shear dependence of viscosity have been essential to the way that classical EHL developed over the last forty years. 1. The liquid in the inlet zone responds in Newtonian fashion. 2. The shear stress versus shear rate relationship of the liquid has the same functional form as the average shear stress versus average shear rate obtained from a traction curve. The new, quantitative, approach employs transport properties measured in instruments (such as viscometers) which do not rely upon these assumptions. There has been a rapid succession of advances in understanding of film forming and of friction under the new approach. This paper compares the assumed viscosities with accurate measurements and explores the history of and possible motivations for the efforts to reject primary measurements.

3221151: Base Oil Friction Prediction under Severe Conditions from Molecular Dynamics Simulations 
Nicolas Fillot, Alejandro Porras-Vazquez, Laetitia Martinie, Philippe Vergne, INSA-LaMCoS, Villeurbanne, France
nicolas.fillot@insa-lyon.fr

Abstract:
Film thickness prediction in elastohydrodynamic (EHD) lubrication regime benefits nowadays from efficient semi-analytical formula. This is still not the case for EHD friction because several physical behaviors interact simultaneously (fluid rheology, thermodynamics, shear heating, etc.). To identify the key mechanisms responsible for lubricated friction at the molecular scale, Molecular Dynamics simulations are proposed. The material studied is a commercial base oil of which the chemical composition is determined. At equilibrium (without shear), the thermodynamic state of the lubricant is identified. Under pressure, temperature and shear, friction evolves in different ways according to the thermodynamics state of the lubricant, from shear rate-dependent to shear rate-independent friction. The calculated friction is then confronted to experimental results obtained from a ball-on-disc apparatus. As shown by [1] and [2] on pure liquids, independently of surface effects, the numerical prediction, though at very high shear rate, proves to be a very good extrapolation of experimental results also for this commercial base oil.

Surfaces & Interfaces

3218530: Measuring Friction at a Single Interface with Two Independent Microtribometers: A Model Study with Alumina Spheres on Gold or Single-Crystal MoS2 
Brian Borovsky, St. Olaf College, Northfield, MN, Nikolay Garabedian, University of Delaware, Newark, DE, Gabriel McAndrews, Raymond Wieser, St. Olaf College, Northfield, MN, David Burris, University of Delaware, Newark, DE
borovsky@stolaf.edu

Abstract:
Fundamental studies of micro/nanoscale friction often employ scanning probes operating at relatively low speeds (< 1 mm/s). These approaches typically rely on compliant spring-based mechanisms whose deflections are sensed electrically or optically. Over the last decade, several groups have developed an alternative approach using quartz crystal microbalances (QCM) to quantify frictional forces at solid-on-solid contacts. QCM-based techniques achieve high sliding speeds (~1 m/s) that are relevant to most practical devices. By reciprocating a probe-on-flat interface at MHz frequencies, the QCM detects lateral forces in a regime where the probe can be considered infinitely rigid. We have integrated spring-based and QCM-based measurements into a single system in order to perform well-controlled comparisons of these two distinct techniques. The interfaces examined use two model materials, polycrystaline gold and single-crystal MoS2, against alumina microspheres. We show how integrating these complementary approaches can help bridge the gap between fundamental and practical micro/nanotribology studies.

3222456: Non-Linear Dynamical Effects in Frictional Energy Dissipation for Atomistic Friction Model 
Motohisa Hirano, Hosei University, Koganei, Tokyo, Japan
hirano@hosei.ac.jp

Abstract:
The non-linear dynamics in friction is studied from an atomistic point of view. The study of the Frenkel-Kontorova model with kinetic energy terms has found the two distinct different regimes appear in the parameter space specifying the model: the superlubricity and the friction regimes. Depending on the parameters such as initial velocity and the interfacial potential amplitude, we have found the novel frictional property in which the mean sliding velocity initially does not change for a while, but it suddenly drops at unexpected time and later the sliding speed recovers for some period, consequently the mass center sliding velocity shows Brownian motion. This paper discusses the mechanism of the sliding speed’s sudden drop, i.e., the catastrophe of the breakdown of the superlubricity from the viewpoints of nonlinear oscillation. The problem of what triggers the catastrophe is studied by examining whether the chaotic vibration exists in the dynamic Frenkel-Kontorova model.

3253437: A Near-Surface Microstructure Evolution Map for CuNi Alloys under Sliding Obtained from Large-Scale Molecular Dynamics
Stefan Eder, Vienna University of Technology, Vienna, Austria, Ulrike Cihak-Bayr, Manel Rodríguez Ripoll, AC2T research GmbH, Wiener Neustadt, Austria, Daniele Dini, Imperial College London, London, United Kingdom, Carsten Gachot, Vienna University of Technology, Vienna, Austria
stefan.j.eder@tuwien.ac.at

Abstract:
In this work, we study the microstructural response of five FCC CuNi alloys subjected to sliding with molecular dynamics simulations featuring tens of millions of atoms. The initial grains average 40 nm in size to ensure that plasticity is not dominated by grain boundary sliding, so our polycrystalline aggregate exhibits dislocation pile-up, twinning, and grain refinement analogous to polycrystals with much larger grains. By analyzing the depth-resolved time development of the grain size, twinning, shear, and the stresses in the aggregate, we produce a microstructure evolution map for CuNi alloys. This captures the predominant microstructural phenomena occurring for a given composition and normal pressure, and aids engineers in optimizing materials/surfaces to work within a required operating range. We compare tomographic visualizations of our atomistic model with focused ion beam images of the near-surface regions of real CuNi alloys that were subjected to similar loading conditions.

Tribochemistry

3221348: Probing and Understanding Elementary Steps in Tribochemical Reactions 
Wilfred Tysoe, UW Milwaukee, Milwaukee, WI
wtt@uwm.edu

Abstract:
Tribology invesigates the effects of mechanical forces during sliding. The effect of stress on tribochemical reaction rates is described using the Bell model but obtaining a molecular understanding of the way in which an external force modifies reaction rates requires knowing the elementary steps for a simple model reaction pathway. This is illustrated for the gas-phase lubrication of copper by dimethyl disulfide where two stress-activated elementary-step reactions are identified. The first is the tribochemical decomposition of adsorbed methyl thiolate species to form gas-phase hydrocarbons and chemisorbed sulfur. In a second stress-induced reaction, surface sulfur is transported into the subsurface region of the copper. The first effect is investigated theoretically using quantum calculations and the results are compared with those for the decomposition of methyl thiolate species on a Cu(100) single crystal substrate measured by atomic force microscopy in ultrahigh vacuum.

Materials Tribology

3221362: Reliability of Microswitches Subject to Cold and Hot Switch Cycling 
Maarten de Boer, Changho Oh, Carngie Mellon University, Pittsburgh, PA, Robert Carpick, University of Pennsylvania, Philadelphia, PA
mpdebo@andrew.cmu.edu

Abstract:
A key problem limiting progress in microelectronics is that when transistors are scaled down in size, they leak current and as such are highly energy inefficient. A potential solution is a nanoswitch, in which a gap is physically opened so that leakage is eliminated. However, much work is needed to demonstrate that such switches reliably meet a cycles-to-failure requirement. Using Pt-coated microswitches, we explore materials, process parameters, cycling conditions and environments to improve the cycle count. We report on process parameters that repeatedly achieve 300 million cycles when cold switched (contact voltage on only when switch is closed). Such switches are then tested under hot switching conditions (contact voltage always on) as a function of voltage and contaminant level. Lower voltage improves lifetime in a clean environment, but degrades it in a contaminated environment. This is attributed to a graphitic film that builds up but does not break down electrically.