TLT: What is the definition of tribochemistry, and how is it different from the typical reactions we learn?
Kim: I have been investigating fundamental reaction mechanisms of chemical processes occurring at tribological interfaces that are relevant to lubricant additives as well as solid lubricants in recent years. I also extended tribology to silicate-based functional glasses such as soda lime float panels used for buildings or aluminosilicate panels used for displays, where chemical reactions involving water molecules adsorbed from the surrounding environment (like humid air) play critical roles in wear or mechanical damage of their surfaces. All chemical reactions involved in these systems are collectively called tribochemistry. This is a subcategory of mechanochemistry in general.

General audiences might not be familiar with these terminologies, although they know quite well or at least are familiar with terms like thermal reactions, photochemistry and electrochemistry. For many chemical reactions that do not occur readily at room temperature, we know that if we heat the system, then reactions can proceed readily. These are thermal reactions or, more accurately, thermally activated reactions.

For example, when we go out on sunny days, we often apply sunscreen lotions to protect our skin from sunburn. The reason is because ultraviolet rays in the sunlight can cause malignant chemical reactions in our skin. Chemical reactions involved in such process are called photochemistry. Photosynthesis by plants to produce glucose from carbon dioxide and water also is a photochemical process. In the old days, we used to take pictures using films. Such films are made of chemicals that undergo certain reactions upon exposure to the light. That is another example of photochemistry. When you turn the ignition key in your car, the engine starts. That’s because electricity is driven out of the battery and turns the starting motor. When the car engine runs, the alternator generates electricity, which is then stored in the battery. These are electrochemical reactions. When ferrous metals are exposed to humid air or aqueous solutions, corrosion occurs; this also is electrochemistry. The molecular phenomena or mechanisms involved in these systems mentioned above are extensively covered and taught in college classes. Then, what are mechanochemistry and tribochemistry?

There are many chemical reactions that can occur or be facilitated due to the influence of mechanical energy. Probably the most familiar example would be to make fire by rubbing pieces of wood against each other, creating friction and, hence, heat. When crystalline solids are subjected to high pressure, polymorphic transition of crystal structure can occur due to changes in thermodynamics stability of specific phases under pressure. Recently, there are growing interests in solvent-free “green” organic synthesis methods, where raw chemicals are put in a ball-milling machine, and then mechanical collisions between balls induce chemical reactions. All of these are examples of chemical reactions induced by mechanical force or stress applied to the system.

Tribochemistry is a special case where the mechanical force or stress is applied by shear at the sliding interface. Good examples of such reactions are degradation of lubricant oil and polymeric additives in it under repeated shear, which is the main reason that we need to change engine oils regularly. Another example is the zinc dialkyldithiophosphate (ZDDP)antiwear additive. ZDDP itself in the oil might work as an antioxidant (which is the original purpose that this compound was developed), but, upon shear at the tribological interface, it decomposes and forms surface films that protect the engine parts from frictional wear.

In summary, all chemical reactions are initiated or driven by different forms of energy input. If that energy comes from the difference in thermodynamic potential energy, it is thermochemistry. If it is from electronic excitation upon absorption of photon energy or electron flow under electrical bias, it is called photochemistry or electrochemistry. When the energy input is through mechanical force and stress, then the reaction is mechanochemistry. If that mechanical action is caused by interfacial shear, then we are now dealing with tribochemistry.


All chemical reactions are initiated or driven by various forms of energy input. If that energy input is through interfacial shear, then it is called tribochemistry. If one acquires molecular understanding of tribochemical mechanisms, it could greatly enhance our capability to design new, environmentally friendly boundary and solid lubrications.

TLT: Why is tribochemistry important to tribological studies? Where can we find it in our daily lives?
Kim: A well-known example of tribochemistry to lubrication engineers is the boundary lubrication where lubricant additives included in motor oils react with the contacting surfaces under sliding conditions to modify their chemical composition, which consequently modifies adhesion and resistance to sliding. ZDDP is one of such additives widely used in engine oils. Another important example is the effect of humidity on solid lubricants. For example, graphite is a good solid lubricant in humid air, but it loses the efficiency in dry conditions. Hydrogenated diamond-like carbon (H-DLC) is an excellent coating material that can provide ultralow friction in dry or inert conditions, but it loses its superlubricity in humid air. Although the molecular or atomistic origin of these behaviors is still unclear, it is believed that tribochemical reactions involving water molecules impinging from the gas phase play critical roles. If one acquires molecular understanding of tribochemical mechanisms, it could greatly enhance our capability to design new, environmentally friendly boundary and solid lubrications. Our knowledge of reaction kinetics and thermodynamics on open surfaces (such as heterogeneous catalysts) is insufficient to achieve this goal because those reactions are often driven thermally, photochemically or electrochemically, while tribochemical reactions are initiated or facilitated by mechanical actions in buried (sliding) interfaces. Currently, it is not well understood how the mechanical force or energy is channeled into chemical reaction coordinates, driving specific chemical reactions at the sliding or tribological interfaces.

TLT: How do researchers investigate tribochemistry, and what current studies are you working on? What will tribochemistry look like in the future?
Kim: The influences of interfacial chemical reactions on the sliding, fretting and rolling wear of materials have been known for a long time. Such influences have been exploited in the formulation of lubricants where extreme-pressure and antiwear additives react with the tribological surfaces to deposit protective coatings (often called tribofilms). Such reactions also are involved in wear processes. The key question that has been elusive in the tribochemistry field is, what really drives chemical reactions at the sliding interface? Temperature rise due to frictional heat or emission of high energy particles upon wear have been proposed and verified in some limited systems, but there are many tribochemical reactions still occurring when the frictional heating is negligible and surface wear is completely suppressed.

In the fundamental mechanistic study of tribochemistry, there are so many technical challenges that make such study difficult. First of all, the frictional contact area is often too small to produce a large quantity of reaction products for separation and purification for further characterization. Thus, the exact chemical structure of tribochemical reaction products are often unknown or poorly characterized. Because chemical reactions occur in the buried and dynamic interface, it is extremely difficult to monitor them in situ or in operando conditions. Thus, most tribochemical reaction studies rely on post-reaction analysis. But, without being able to isolate and purify them, it is not possible to find out the exact chemical composition or molecular structure; then, it is extremely difficult to pinpoint the reaction mechanism.

In my research, I circumvent such complications using the vapor phase lubrication (VPL) approach. The VPL concept was originally devised by the late professor Elmer E. Klaus of Penn State University for lubrication of internal combustion engines with organophosphate vapors. So, my group is continuing that legacy of VPL in model molecular tribology and tribochemistry. We applied the VPL concept to lubrication of micromechanical systems (MEMS) and demonstrated its efficacy for reliable operation of MEMS devices. Now, we are using the VPL approach as a platform to study tribochemical reaction mechanisms. In VPL, it is possible to completely suppress surface wear and keep frictional heating insignificant. Since no solvent is used in VPL, any reaction intermediates or products with vapor pressure lower than the initial reactant supplied to the surface will remain in the sliding track (or around the sliding track if they are squeezed out by shear action). By controlling various reactions parameters in VPL—molecular structures of vapor supplied, partial pressure of reacting molecules in the vapor, contact pressure of the sliding contact, chemistry of substrate materials and compositions of carrier gas—my group is shedding light into key chemical and physical parameters involved in the rate-limiting step of tribochemical reactions.

My work showed that the oxidative chemisorption of reacting molecules onto one of the shearing surfaces is necessary for effective transfer of mechanical energy from the shearing solid to the molecules being sheared at the interface. The transferred energy is used to physically deform molecules out of their equilibrium geometry with the lowest energy minimum; such distortion of chemisorbed molecules might increase the reactant energy state (making reactant less stable, thus more reactive), lower the transition state energy, or open a new reaction pathway that would not readily occur in thermal, photochemical or electrochemical reactions. This finding provides critical insights into the “shear-assisted” thermally activated reaction models that were originally proposed by Prandtl and Eyring nearly 100 years ago. However, experimental work alone is not possible to unveil all aspects of molecular mechanisms, especially atomistic details of the molecular deformation of intermediate species under reactive conditions. For that reason, my group is in active collaboration with computational simulation groups. I predict synergistic and iterative collaborations among experimental and computational approaches are critically needed for further and deeper understanding of tribochemistry fundamentals.

Seong H. Kim’s research is currently supported by the National Science Foundation (Grant # CMMI-2038494). You can reach him at shk10@psu.edu.