Tribochemistry, tribocatalysis, and the negative-ion-radical action mechanism

Terms such as tribochemistry, catalysis, and tribocatalysis are often used to account for various tribological findings, especially those relating to boundary lubrication. The major goal of this article is to present/discuss tribochemistry and tribocatalysis mostly in terms of the negative-ion-radical action mechanism (NIRAM). This article also briefly overviews most recent books and book chapters and discusses selected physical and chemical phenomena in the boundary friction contact concerning tribochemistry and tribocatalysis. A catalyst provides a new and easier pathway for reactant molecules to be converted into product molecules. What exactly does the term tribocatalysis mean? Is there any relationship between tribochemistry and tribocatalysis? This article attempts to answer these questions and will demonstrate the importance of the NIRAM approach and the concept of NIRAM-hard and soft acids and bases in tribochemistry, catalysis, and tribocatalysis. It is demonstrated that in both tribochemical reactions and some heterogeneous catalytic reactions, intermediate reactive species are formed by the same mechanism mostly governed by the NIRAM approach. Emphasis is on the role of thermal electrons (thermionic emission) on catalysis and both thermal electrons and exo-electrons on tribocatalysis. Some examples of tribocatalytic reactions are also introduced. The relationship between adhesive wear, especially severe-mild wear transition, is discussed from the point of view of tribochemical reactions between environmental gases and a worn surface.