Tribology

The goal of the UC Merced Tribology program in the Martini Research Lab is to fundamentally understand what happens at the interfaces of components moving in relative motion to enable design of efficient and long-lasting mechanical interfaces.

Mechanical components rely in some way on the moving surfaces between solid materials and, in general, the function of all mechanical systems depends on tribology, i.e. friction, wear and lubrication. Therefore, tribology is societally and technologically relevant because of its potential to improve the energy efficiency and useful lifetime of things that move. Tribology is also scientifically fascinating because it is inherently interdisciplinary, including mechanical engineering, materials engineering, physics and chemistry. Further, tribology is multi-scale: contacting bodies elastically deform on a macroscopic scale; relative movement between them is resisted by the interlocking of micro-scale surface features; and, at a very fundamental level, friction is due to atomic and molecular interactions at the nanoscale. Understanding the interrelationship between these phenomena will enable development of design tools with which friction and other interface behaviors can be controlled and then explicitly prescribed for a given application.

Some of our current research projects are in the following areas:

 

Tribochemistry:

Low friction in lubricated mechanical components is in part enabled by protective films that form in sliding interfaces during operation. These films, called tribofilms, are formed through chemical reactions between additive molecules in the lubricant and the surfaces, where the reactions are driven by mechanical force exerted by the sliding bodies, i.e. tribochemistry. Despite the presence of tribofilms in most moving components, the mechanisms of film formation are still poorly understood, primarily because the process occurs inside a moving contact and so cannot be directly interrogated experimentally. Experimental approaches are typically limited to pre- and post-sliding surface analyses. An alternative approach, and one we apply in our research, is molecular dynamics simulations that explicitly model the additive molecules and surfaces at the atomic scale. Specifically, we use a reactive force field so the simulations can capture the formation and breaking of covalent bonds that are necessarily part of the tribofilm formation process. In general, this research may enable design of new lubricant additives that yield faster forming and longer lasting protective tribofilms in moving components.

  1. Khajeh A, He X, Yeon J, Kim SH and Martini A, (2018) "Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear", Langmuir, 34, 5971. DOI: 10.1021/acs.langmuir.8b00315.
  2. Yeon J, He X, Martini A and Kim SH (2017) "Mechanochemistry at Solid Surfaces: Polymerization of Adsorbed Molecules by Mechanical Shear at Tribological Interfaces", ACS Appl. Mater. Interfaces, 9, 3142. DOI: 10.1021/acsami.6b14159.
     

Polymeric Viscosity Modifiers:

Viscosity modifiers are polymers that are added to most lubricating oil formulations to minimize the change of viscosity with temperature. These polymer additives ensure that the oil is not too thick when cold or too thin when hot, so they enable moving components to function efficiently across a wide range of operating conditions. Viscosity modifiers are used in nearly all motor oils and most hydraulic fluids. However, design of new polymers that might further improve mechanical efficiency is limited by a lack of understanding of their fundamental mechanisms. We use molecular dynamics simulations to explore the fundamental mechanisms by which polymers perform their critically important function in lubricating oils. Once these mechanisms are understood, they can be leveraged in future viscosity modifier designs. Such optimized viscosity modifiers can become the enabling technology for current trends in mechanical design, such as the use of ultra-low viscosity oils for improved fuel efficiency.

  1. Martini A, Ramasamy UR and Len M, (2018) "Review of Viscosity Modifier Lubricant Additives", Tribology Letters, 66, 58. DOI: 10.1007/s11249-018-1007-0.
  2. Ramasamy US, Lichter S and Martini A (2016) "Effect of Molecular-Scale Features on the Polymer Coil Size of Model Viscosity Index Improvers", Tribology Letters, 62, 23. DOI: 10.1007/s11249-016-0672-0.
     

Tribometry:

Friction and wear are important properties of sliding interfaces because they quantify the energy efficiency and useful lifetime of moving mechanical components. Therefore, developing surfaces or lubricants that provide low friction and wear is critical to minimizing wasted energy and materials. We measure these properties in the lab using tribometers. The tribometers in our lab measure friction during linear reciprocating motion or unidirectional rotation, and wear is characterized after a friction test from optical microscopy or white light interferometry of the resultant worn region on the surfaces. We are using these tools to study novel materials/surface treatments and cutting edge lubricants that may be able to provide lower friction and wear in mechanical components. Current projects include investigation of a Nickel-Titanium alloy for use in space applications, ionic liquid lubricant additives to provided extended life in loss-of-lubricant situations for air vehicles, and novel surface treatments for use in safe and longer lasting biomedical applications.

  1. Song J, Chen C, Zhu S, Zhu M, Dai J, Ray U, Li Y, Kuang Y, Li Y, Quispe N, Yao Y, Gong A, Leiste UH, Bruck HA, Zhu JY, Vellore A, Li H, Minus ML, Jia Z, Martini A, Li T and Hu L, (2018) "Processing bulk natural wood into a high-performance structural material", Nature, 554, 224. DOI: 10.1038/nature25476.
  2. Walters N and Martini A (2018) "Friction Dependence on Surface Roughness for Castor Oil Lubricated NiTi Alloy Sliding on Steel", Tribology Transactions, Published Online. DOI: 10.1080/10402004.2018.1520949.
     

Atomic-Scale Friction:

At the macro-scale, frictional sliding is extremely complicated because it occurs at the many surface features, or asperities, of the contacting surfaces. To explore the fundamental mechanisms of friction, we therefore take a heuristic approach of studying a single unit of friction, i.e. a single nanoscale asperity. At nanoscale asperity interfaces, the position and dynamics of discrete atoms determine frictional resistance to sliding, so their sliding resistance is called atomic friction or atomic-scale friction. Further, friction patterns measured using atomic force microscopy can be directly correlated to the atomic lattice of the contact bodies. Here we complement atomic force microscope experiments with molecular dynamics simulations of single asperity sliding to understand the physical and chemical origins of friction at the nanoscale. This approach can provide quantitative information about the frictional resistance of the surface, energy dissipation, and correlations between sliding resistance and the atomic structure of the substrate surface, all of which can ultimately lead to a fundamental understanding of friction itself.

  1. Baykara MZ, Vazirisereshk MR and Martini A (2018) "Emerging superlubricity: A review of the state of the art and perspectives on future research", Applied Physics Reviews, 5, 041102. DOI: 10.1063/1.5051445.
  2. Dong Y, Li Q and Martini A (2013) "Molecular dynamics simulation of atomic friction: A review and guide", Journal of Vacuum Science and Technology A, 31, 030801. DOI: 10.1116/1.4794357.
  3. Li Q, Dong Y, Perez D, Martini A and Carpick RW (2011) "Speed dependence of atomic stick-slip friction in optimally matched experiments and molecular dynamics simulations: The role of dynamics vs. energetics", Physical Review Letters, 106, 126101. DOI: 10.1103/PhysRevLett.106.126101.

Nanocontacts:

Understanding and predicting the size of a nanocontact as it forms, evolves, and then separates under load has significant implications for many engineering applications, including in materials characterization, nanomanufacturing and nanodevices. From a continuum mechanics perspective, the definition of “contact” is a location at which the distance between two bodies is exactly zero. However, for nanoscale interfaces between bodies, the concept of contact area is difficult to precisely measure or even define. Experiments performed using an atomic force microscope can provide measurements of proxies for contact area, including adhesion, friction and conduction. In situ transmission electron microscopy can add to this with 2D images of the contact. However, only simulations provide access to the 3D contact area and the atomic-scale details within the contact itself. We use atomistic models of the apex of an atomic force microscope probe that are designed to capture the exact details of the experiment. The model is then used as a tool with which to explore the meaning of contact at the nanoscale.

  1. Jacobs TDB and Martini A, (2017) "Measuring and Understanding Contact Area at the Nanoscale: A Review", Applied Mechanics Reviews, 69, 061101. DOI: 10.1115/1.4038130.
  2. Hu X, Nanney W, Umeda K, Ye T and Martini A (2018) "Combined Experimental and Simulation Study of Amplitude Modulation Atomic Force Microscopy Measurements of Self-Assembled Monolayers in Water", Langmuir, 34, 9627. DOI: 10.1021/acs.langmuir.8b01609.

 

The research is being led by the Tribology Group (PI: Dr. Ashlie Martini).

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ATTN: Graduate Division
5200 N. Lake Road, Ste KL 227
Merced, CA 95343

 

 
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