No abstract

Tricresyl phosphate (TCP) forms protective films on moving mechanical components through thermally driven decomposition and interactions with the ferrous surfaces of the components. These reactions are hidden from view in moving interfaces, but are known to be sensitive to both the surface material and the isomeric form of the TCP. Here, temperature-programmed reaction spectroscopy (TPRS) and gas chromatography-mass spectroscopy (GC-MS) are complemented by reactive molecular dynamics (MD) simulations to investigate the thermal decomposition of meta and para isomers of TCP reacting with ferrous materials. Key observations are that the primary decomposition product of TCP is cresol, more cresol is generated on Fe 2 O 3 than on Fe 3 O 4 , and that para-TCP isomers are more reactive than meta-TCP isomers. These trends are explained using the simulations to identify multiple reaction pathways leading to cresol formation. The likelihood of each pathway is quantified and correlated to surface material and TCP isomer trends in terms of energy barriers for the rate-limiting steps in the decomposition reactions.

tbc

Tribochemistry, which is another name of mechanochemistry driven by shear, deals with complex and dynamic interfacial processes that can lead to facilitation of surface wear or formation of bene cial tribo lms. For better mechanistic understanding, we investigated the reactivity of tribopolymerization of organic molecules with different internal ring strain energy (methylcyclopentane, cyclohexane, and cyclohexene) on a stainless steel (SS) surface in inert ( N 2 ), oxidizing (O 2 ), and reducing (H 2 ) environments. On the clean SS 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 tested. Compared to the N 2 environment, the tribochemical activity in H 2 was suppressed. In the O 2 environment, only cyclohexene produced tribo lms and methylcyclopentane and cyclohexane did not. When tribo lms 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, tribo lms were found to be organic polymers containing oxygenated groups. Whenever polymeric tribro lms were produced, wear volume was suppressed by orders of magnitudes but not completely to zero. These results supported the previously suggested mechanisms which involved surface oxygens as a reactant species of the tribopolymerization process.

Two‐dimensional (2D) lamellar materials are normally capable of rendering super‐low friction, wear protection, and adhesion reduction in nanoscale due to their ultralow shear strength between two basal plane surfaces. However, high friction at step edges prevents the 2D materials from achieving super‐low friction in macroscale applications and eventually leads to failure of lubrication performance. Here, taking graphene as an example, the authors report that not all step edges are detrimental. The armchair (AC) step edges are found to have only a minor topographic effect on friction, while the zigzag (ZZ) edges cause friction two orders of magnitude larger than the basal plane. The AC step edge is less reactive and thus more durable. However, the ZZ structure prevails when step edges are produced mechanically, for example, through mechanical exfoliation or grinding of graphite. The authors found a way to make the high‐friction ZZ edge superlubricious by reconstructing the (6,6) hexagon structure to the (5,7) azulene‐like structure through thermal annealing in an inert gas environment. This will facilitate the realization of graphene‐based superlubricity over a wide range of industrial applications in which avoiding the involvement of step edges is difficult.