Spotlight Presentations

Spotlight presentations are a special category of talks being presented during the Tribology Frontiers Conference featuring comprehensive longer (40-minute) presentations that have been chosen by the Tribology Frontiers Conference Planning Committee, with notable principal investigators (PIs) and researchers showcasing their most compelling tribology research to a broader audience.

Monday, November 13

Session 1B - Lubricants I

9:20 am - 10:00 am
3950002: SPOTLIGHT: Self-Assembly and Chemistry of Molecules in Oil Environment 
Hitoshi Washizu, Kyosuke Kawakita, Tomoya Hasegawa, Riku Araki, Tomohito Horio, Takehiro Kobayashi, Ryuichi Okamoto, Kosar Khajeh, University of Hyogo, Kobe, Japan; Yoshiki Ishii, Kitasato University, Sagamihara, Japan; Natsuko Sugimura, National Institute of Technology, Kagoshima College, Kirishima, Japan

Abstract:
Self-assembly of solute molecules in oil solvent is important since many of the lubricant are used in this condition. Molecular simulation is useful to treat these phenomena and recently we applied to self-assembled system such as liquid crystals and polymer solution. Here, we report our recent research on molecular simulations of additives and thickener in oil environment with and without metal surfaces. Although carbonate oiliness additives adsorb physically on the charged metal surface relatively quickly, extreme pressure additives such as organophosphate make aggregation structure in oil and do not easily adsorb on the surface. The dependence of the molecular structure to the stability is simulated. After adsorption to the surface, the pathway of charge transfer on the surface is different on the nascent surface and oxidized surface. In order to understand the grease, formation and breakage dynamics of self-organized structure of reverse-micelle under shear is analyzed using dissipative particle dynamics, and the dynamics is related to the experimentally found dynamics of grease thickener. The simulation method is further extended to treat wider time and space dynamics, such as multi-physics approach of coarse-grained molecules coupled with fluid dynamics, or molecular dynamics using AI based potential to treat huge chemical reactions. These new approaches are reveling the tribochemistry of lubricants on the surface from molecular level.

Session 1C -Tribochemistry I

9:20 am - 10:00 am
3959444: SPOTLIGHT; Interplay of Mechanics and Chemistry Governs Wear of Diamond-Like Carbon Coatings Interacting with ZDDP-Additivated Lubricants 
Michael Moseler, Fraunhofer IWM, Freiburg, Germany 

Abstract:
Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimized by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear. 

Session 2A - Materials Tribology II

2:00 pm - 2:40 pm
3952065: Spotlight: New Contact Temperature Model for Polymer Contacts, Considering Tribo-System Geometry: A Step Toward Generalization 
Mitjan Kalin, University of Ljubljana, Ljubljana, Slovenia

Abstract:
Contact temperatures are clearly an important contact parameter in tribology design. However, we still lack a generally accepted and broadly used model allowing for easy and fast estimation of the contact temperature. This is true for steels and various other metals, and so much more for polymers. Namely, polymers are significantly more sensitive to contact temperatures due to their poorer mechanical and thermal properties. Therefore, the inaccuracy in predicting the contact temperatures in polymer contacts may result in notable variation of expected surface conditions and detrimental tribological behavior. In this work we present a ready-to-use temperature model for polymer/steel contact, preferentially designed for pin-on-disc studies, which also considers tribo-system geometry (volume, surface). Moreover, a further development of a more generalized model for various tribological systems and their geometries is presented and discussed, as well as the effect of different contact material properties. 

3:00 pm - 3:40 pm
3963437: Spotlight: Effect of Counterbody Material on Tribofilm Formation of PTFE-Chromium Composites
Mark Sidebottom, Faysal Haque, Miami University, Oxford, OH; Sifat Ullah, Rensselaer Polytechnic Institute, Troy, NY

Abstract:
Different filler particles (e.g. α-Al2O3, activated carbon nano fillers etc.) composited with PTFE reduce wear 
by~10,000x when slid against 304 stainless steel. These composites are known as ultralow wear materials. Recently, three new composites (PTFE-Cr, PTFE-Ti, PTFE-Mn) achieved ultralow wear when slid against Brass 260. However, these composites showed mixed performance against 304 SS. In this study, four different counterbody materials (Cu 110, Zn-galvanized steel, 304 SS, and brass 260) were tested against PTFE-Cr to identify how counterbody material properties affected wear and friction performance. The tests revealed high variation in friction coefficient (µ~0.15-0.28) and wear rate (8x10-9 mm3/Nm < K < 1x10-6 mm3/Nm). To understand the evolution of wear, the tribosystem was analyzed using optical microscopy, profilometry, and surface energy measurements. Variance in transfer film morpohology was dependent on the counterbody material the PTFE-Cr composite slid against.

Session 2B -Lubricants II

2:00 pm - 2:40 pm
3962263: SPOTLIGHT; CNTs to Enhance Heat Transfer of EV Fluids 
Jun Qu, Chanaka Kumara, Wenbo Wang, Hsin Wang, James Haynes, Oak Ridge National Laboratory, Oak Ridge, TN; Ning Ren, Jacob Bonta, Edward Murphy, Roger England, Valvoline Global Operations, Lexington, KY

Abstract:
The e-motor in an EV rotates 15,000 rpm and potentially goes up to 30,000 rpm in the future. The copper winding on the e-motor gets very hot, which is cooled by the lubricant. Increasing heat transfer efficiency currently is a primary challenge for e-motor oils to allow higher currents for higher torque output. The remarkable thermal conductivity (2800–6000 W/mK, 10X higher than copper) of carbon nanotubes (CNTs) present another opportunity for being used as additives for the e-motor fluids to improve the heat transfer capacity. However, CNTs have poor suspendability in fluids and tend to aggregate and precipitate. This study innovated covalent functionalization for CNTs to enable stable suspension and uniform dispersion of CNTs in both polar and non-polar fluids. Initial research has added 0.1 wt.% of modified CNTs into an EV base oil, resulting in 10% higher thermal conductivity and 10% higher volumetric heat capacity with <4% viscosity increase, resulting in 8% improved heat transfer efficiency (Mouromtseff Number). The effects of CNT type, diameter, length, and concentration on the fluid’s thermal and rheological properties are being investigated. Research sponsored by the Vehicle Technologies Program, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE). 

Session 2C - Tribochemistry II

2:00 pm - 2:40 pm
3771581: SPOTLIGHT; A New Friction Modifier Mechanism Based on Pressure-Induced Hydrogen Bonding
Thomas Reddyhoff, James Ewen, Pushkar Deshpande, Imperial College London, South Kensington, London, United Kingdom; Mark Frogley, Diamond Light Sources, Didcot, United Kingdom; Mark Welch, Wren Montgomery, Natural History Museum, London, United Kingdom

Abstract:
We present new research into the use of n-alcohols as “traction-modifier” additives that can be blended with oils in order to reduce elstohydrodynamic friction (traction) without impacting film thickness. This is based on a recent discovery that neat n-alcohols can self-assemble under pressure to form layered structures that provide liquid superlubricity, (i.e., a friction coefficient below 0.01, inside lubricated contacts [1]). This occurs within the central, high-pressure region within a contact so that film thickness is unaffected. Furthermore, similar beneficial behaviour occurs even after n-alcohols have been diluted by a hydrocarbon base oil. These performance gains are supported by ball-on-disc tribometer friction and film thickness data, while insights into the mechanism are provided by FTIR measurements made on lubricants samples within a high-pressure diamond anvil cell. The link between molecular structure and friction reducing performance is discussed along with the practicalities of implementing such additives in practice.

2:40 pm - 3:20 pm
3757795: SPOTLIGHT: Understanding the Effect of Forces on Tribochemical Reaction Rates 
Wilfred Tysoe, University of Wisconsin-Milwaukee, Milwaukee, WI

Abstract:
The effect of applied stress σ on the rates of tribochemical reactions is described using the Bell model, where the rate varies as exp(σΔV‡/kBT), where ΔV‡ is the activation volume. Strategies for measuring reaction pathways are illustrated using the gas-phase lubrication of copper by dimethyl disulfide (DMDS) where the rate of reaction of on a Cu(100) single crystal substrate is measured by exerting the force using an atomic force microscopy tip. The measured angular dependence of the methyl thiolate decomposition rate suggests that the kinetics can be analyzed using quantum mechanical methods that are used to analyze thermal reaction rates and is confirmed by measuring the effect of a normal stress on the reaction rate which is excellent agreement with values calculated using quantum theory. This approach is extended to studying shear-induced methyl thiolate decomposition which occur more rapidly and on investigating the tribochemical decomposition of carboxylates on Cu(100). 

Tuesday, November 14

Session 3A - Biotribology I

9:40 am - 10:20 am
3972425: Spotlight: Slippery Physics 
Angela Pitenis, Allison Chau, Conor Pugsley, Madeleine Miyamoto, Yongkui Tang, Claus Eisenbach, Thomas Mates, Craig Hawker, Megan Valentine, University of California Santa Barbara, Santa Barbara, CA

Abstract:
Hydrogels are three-dimensional, crosslinked networks of hydrophilic polymers swollen in water. This class of soft materials is used in many industries (e.g., biomedicine, agriculture, wastewater management) due to their high water content, capacity for water retention, and tunable mechanical properties. Polyacrylamide hydrogels have also attracted scientific interest in the tribology community due to their fascinating energy-dissipation properties. In this study, we observe that increasing solution pH reduces the friction coefficient of polyacrylamide hydrogels in sliding contact with hemispherical probes of either borosilicate glass or polytetrafluoroethylene (PTFE). We propose that the mechanisms of pH-dependent polyacrylamide friction may be a combination of electrostatics and hydrolysis.

Session 3C - Fluid Lubrication I

9:20 am - 10:00 am
3957120: SPOTLIGHT: Soft Lubrication: Beyond Classical EHL Theory
Carmine Putignano, Polytechnic University of Bari, Bari, Italy

Abstract:
Assessing the main peculiarities of lubrication between soft solids is a crucial issue that has only recently raised the attention of the lubrication science community. Indeed, in the last decades, massive research efforts have been dedicated to understanding the role of non-Newtonian lubricants, but very little has been done to determine what occurs when the lubricated solids are not linearly elastic, and are instead characterized by a different rheology, including viscoelasticity and porosity. In this work, an innovative numerical methodology is presented to analyze the lubrication regimes between linear viscoelastic and porous layers. In detail, an explicit finite difference scheme is coupled to a Boundary Element solver: this enables the study of the viscoelastic lubrication without any limitation in terms of material properties, geometry and viscosity. All this is validated by means of experiments specifically developed to deal with soft matter. To measure the film thickness, an innovative technique based on interferometry for soft solids has been employed.

The following step deals with the analysis of squeeze conditions, where the fluid is interposed between the rigid punch and a viscoelastic substrate. Results show the presence of pressure peaks the contact edges and very peculiar trends for the central and the minimum thickness values. This confirms the necessity of ad hoc developed numerical methods for soft lubrication.


Session 4C - Fluid Lubrication II