Molecular Structure and Environment Dependence of Shear-Driven Chemical Reactions: Tribopolymerization of Methylcyclopentane, Cyclohexane and Cyclohexene on Stainless Steel

Tribochemistry, which is another name for mechanochemistry driven by shear, deals with complex and dynamic interfacial processes that can lead to surface wear or formation of beneficial tribofilms. For better mechanistic understanding of these processes, we investigated the reactivity of tribopolymerization of organic molecules with different internal ring strain (methylcyclopentane, cyclohexane, and cyclohexene) on a stainless steel (SS) surface in inert (N 2 ), oxidizing (O 2 ), and reducing (H 2 ) environments at room temperature. On the clean stainless steel surface, precursor molecules were found to physisorb with a broad range of molecular orientations. In inert and reducing environments, the strain-free cyclohexane showed the lowest tribochemical activity among the three molecules tested. Compared to the N 2 environment, the tribochemical activity in H 2 was suppressed. In the O 2 environment, only cyclohexene produced tribofilms and methylcyclopentane while cyclohexane did not. When tribofilms were analyzed with Raman spectroscopy, the spectral features of diamond-like carbon (DLC) or amorphous carbon (a-C) were observed due to photochemical degradation of triboproducts. Based on infrared spectroscopy, tribofilms were found to be organic polymers containing oxygenated groups. Whenever polymeric tribrofilms were produced, wear volume was suppressed by orders of magnitudes but not completely to zero. These results support previously suggested mechanisms which involve surface oxygen as a reactant species in the tribopolymerization process. Graphical Abstract