Molecular Mechanisms of Tribochemistry and Lubrication
CECAM-HQ-EPFL, Lausanne, Switzerland
Many important tribological phenomena can ultimately be described as chemical reactions occurring in the presence of mechanical forces, i.e., tribochemical reactions. One example is boundary lubrication, where lubricant additives included in motor oils react with the contacting surfaces under sliding conditions to modify their chemical composition with consequent modification of adhesion and resistance to sliding. Another important example is the effect of humidity on solid lubricants; It is well documented that the lubricity of MoS2 is adversely affected by air humidity, while graphite and
diamond/diamond-like carbon (DLC) coatings depend on moisture for good lubricating properties. The microscopic origin of these behaviors is still unclear.
In order to design new, environmentally friendly, solid and boundary lubricants is necessary to acquire microscopic understanding of tribochemical mechanisms. Our knowledge of the reaction kinetics and thermodynamic driving forces at the open surface is not sufficient to accomplish this goal because the tribological conditions often induce reactions that are not seen in ordinary situations. Tribological conditions include additional effects such as frictional heating, molecular confinement, and high shear stresses and contact pressures. Increases in reaction rates are generally observed at tribological interfaces, a phenomenon that provides a very appealing approach for the synthesis of chemicals without the use of solvents.
Addressing these issues will require both novel computational approaches and the use of simplified model systems that nevertheless mimic real tribological systems. This CECAM workshop will, therefore, include both theorists and experimentalists to allow experimental systems to be developed that are sufficiently simple that they are amenable to being simulated under conditions encountered in the experiment to provide a robust comparison between theory and experiment. As a consequence, the workshop will focus on the following themes:
1. Molecular mechanisms and computational strategies in tribochemistry
2. Lubricant Additives
3. Solid Lubrication; Layered Materials
4. Solid Lubrication; Coatings
Each theme will include both invited and contributed talks by both theorists and experimentalists to address the open issues and challenges in each area. Some are listed in the following:
• Which is the best computational strategy to face the complex tribochemistry problem? Can we identify key tribochemical mechanisms to be studied on different time/length scales and a rational way to combine them in a multiscale description?
• Can we define a computational protocol for the in-silico design of lubricant additives?
• Layered materials represent clean, well-defined interfaces where simulations can combine with experiments to answer fundamental questions such as: what is the effect of load compared to the effect of shear on reaction kinetics? What are the effects of molecular confinement? Is transition-state theory applicable to frictional processes?
• Wear-resistant, low-friction coatings such as diamond and diamond-like carbon (DLC) represents a very efficient way to increase the life of the operating systems and reduce the energy losses by friction. However, the functionality of the carbon-based films is deeply influenced by tribochemistry. We wish to identify strategies to control the molecular-level mechanisms to facilitate the widespread use of these materials at the macroscale such as automotive and biomedical applications.
M. Clelia Righi (University of Modena and Reggio Emilia and CNR-Nano) - Organiser
Chris Lorenz (King's College London) - Organiser
Eddy Tysoe (University of Wisconsin-Milwaukee) - Organiser & speaker