There is considerable interest in discovering novel mechanochemical reaction pathways for synthesizing materials. Mechanochemical synthesis is often faster, less energy-intensive, does not require toxic solvents, and can provide superior product selectivity and yield. However, developing a fundamental understanding of reaction mechanisms requires precise quantification of forces involved. For example, in mechanochemical reactors like ball mills, when the reactants experience forces at contacts where balls collide with each other or with the walls, there can be a combination of normal forces as well as shear forces on the reactants. The relative amount of these forces depends on the location of contact with respect to the relative velocity of the balls, and on the degree of relative rotation of the balls. In this work, we seek to examine how different degrees of sliding and rolling influence the kinetics of mechanochemical processes. Three different processes are studied as example systems to demonstrate the potential of this approach: (i) tribosintering of metal oxide nanocrystals, (ii) tribochemical reactions of a phosphorus-based reactive anti-wear additive, and (iii) Diels-Alder reactions. We performed these studies in a mini-traction machine (MTM), which consists of a rotating ball in contact with an independently rotating disc which permits testing over a wide range of slide-to-roll ratios. The experimental setup permits the compressive force and temperature to be controlled during the test and the resultant traction (shear) force to be measured. This provides broad control over the reaction conditions. To study the progression of reactions <i>in situ</i>, we use an optical interferometry technique known as spacer layer imaging, which is used to visualize and quantify thin films of products that build upon the surfaces in the sliding/rolling contact. We also perform ex-situ morphological and chemical evaluation of reacted surfaces by white light interferometry and time-of-flight secondary ion mass spectrometry techniques to analyze the products. We will discuss how these processes depend on slide-to-roll ratio, applied force, temperature, and reactant concentration. This work demonstrates the application of the MTM system as a novel approach to study different classes of mechanochemical reactions under well-defined forces at concentrated contacts, providing mechanistic insights into the roles of compression and shear in mechanochemical processes.

Studying mechanochemical reactions at mixed sliding-rolling contacts

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