Interactions Balancing Competition and Cooperation between Covalent–Organic Framework Additives and PEG Base Oil toward Advanced Lubrication

Abstract

Severe competition between nanolubricant additives and polar lubricating oil molecules for the formation of lubricant films has been hindering the progress of green and advanced lubricants. In this work, based on the tautomerism of cyanuric acid molecule (trione and triol configurations), two kinds of triazine-based covalent−organic frameworks (COFs), that is, Ton-COFs and Tol-COFs, were synthesized as additives of the polar PEG 400 oil, realizing compromise between them by providing delicate interactions. The triazine matrix bonding with intense polar groups in the framework of additives offers more powerful interactions to competitively form the adsorbed lubricant film on the surface of the metal substrate over PEG 400 oil and also bolts PEG 400 oil molecules by the hydrogen bonding inside the pore of the framework to cooperatively bear against the load. Molecular quantum chemical calculations further confirm that Ton-COFs can produce a more intense interaction with Fe atoms in the form of coordination and ions•••π than Tol-COFs, far beyond PEG 400, and the cross-sectional profile of the worn surface definitely exhibits a protective lubricant film only composed of Ton-COFs. Consequently, at the low concentration of 0.3 wt %, the excellent friction reduction (41.2%) and antiwear property (97.4%) are achieved for the Ton-COFs compared to pure PEG 400 oil; moreover, 28.6% and 79.0% for Tol-COFs at the essential concentration of 0.7 wt % are achieved. This finding provides a novel insight from molecules to materials into guiding the development of additives for advanced lubricants.

Mechanochemical Synthesis of Thiadiazole Functionalized COF as Oil-Based Lubricant Additive for Reducing Friction and Wear

“…First, the Thdz@COF-F nano-particles with sufficient surface area are easy to deposit or absorb on the friction interface, repairing the pits induced by wear and forming an adsorption film. 45 In this procedure, the iron atoms of friction pairs are positively charged after emitting electrons under friction heat and shear force, so that the electron-rich triazine and thiadiazole groups interact electrostatically with the friction pairs, contributing to the adsorption and repairing effect of Thdz@COF-F. 46 Afterward, the COF adsorption film endures complex tribochemical reaction with the friction pair material and the air under high frequency, heavy load and local high temperature, constructing a more uniform and robust tribo-film. 47 As a result, it is the tough protective tribo-film that averts the direct contact of friction pairs and plays the significant role of antifriction and antiwear.…”

“…On the basis of tribological test results and XPS analysis, the lubrication mechanism of Thdz@COF-F is proposed in depth and depicted in Scheme . First, the Thdz@COF-F nanoparticles with sufficient surface area are easy to deposit or absorb on the friction interface, repairing the pits induced by wear and forming an adsorption film . In this procedure, the iron atoms of friction pairs are positively charged after emitting electrons under friction heat and shear force, so that the electron-rich triazine and thiadiazole groups interact electrostatically with the friction pairs, contributing to the adsorption and repairing effect of Thdz@COF-F .…”

Fortified Lubricating Response to Sustainable PEG System from Protic Ionic Liquid and Their Strong Hydrogen Bonding Network

“…Of course, various types of additives in the lubricating system have distinct interaction forces on the interfacial behavior, such as the van der Waals force, physical and/or chemical adsorption, coordination, or hydrogen bonding. − Among them, the common materials mainly include nanomaterials, − hybrid materials, , gel, functional modification materials, , covalent/metal–organic framework materials, , ionic liquids (ILs) , , deep eutectic solvent, etc. Based on highly designable molecular structures and tunable properties characteristic, highly polar ILs are superior in competitive adsorption for polyethylene glycol base systems, laying the foundation for the lubricating system with high-performance, multifunctional properties, and service stability .…”