Preparation and Tribological Properties of Covalent Organic Framework Nanomaterials

Covalent organic framework materials are a rising class of two-dimensional layered porous nano-materials, where the atoms in-plane are connected by covalent bonds and van der Waals forces are dominated between the layers. Because of such unique features, covalent organic framework materials have posed a promising potential in lubricant additives. In this work, cyanuric chloride and 2,5-diamino-benzo-dithiazole were served as raw materials, we prepared a novel covalent organic framework nanomaterial (TTC) by a facile solvothermal reaction. The analyses showed that TTC had a good crystallinity and a flexible two-dimensional layered structure with the slight wrinkling.　Using water and polyethylene glycol 400 mixed fluid as the base lubricant, when as-synthesized nanomaterials (TTC) introduced into it, a good dispersion could be obtained with the assistance of ultrasonication, due to the formation of hydrogen-bonds between TTC and glycol moieties. Subsequently, the tribological performances of TTC as the additive of mixed fluid, under a load of 100 N, were investigated using an SRV-V friction and wear tester as well as a three-dimensional profiler. As the concentration increased, the friction coefficient firstly decreased and then increased. When the addition amount of TTC was up to 0.5%, the friction coefficient (COF) and wear volume of the lower disk can be dramatically reduced (COF reduced by 26% and wear volume decreased by 90%), as compared to that of the control lubricant. Next, the scanning electron microscopy images of the worn surface of the lower disks lubricated with the base fluid and the blends with the different contents of TTC additive were obtained. Lubricated by the base fluid, the worn surface displayed a moderate adhesive and abrasive wear, because of the base fluid with the low viscosity, which hardly produced a stable lubricating film. Besides, under the high load, the direct contact between the asperities on the surface of the friction pair caused a cold weld, then adhesive wear was triggered by the shear moving. TTC can be effectively adsorbed onto the disk surface to form a protective film, hence the adhesive wear can be impeded remarkably. However, at lower concentrations, TTC cannot feed a complete lubricating film, and the unprotected part was plastically deformed due to continuous sliding. Unexpectedly, as concentration of TTC increased, the distinct corrosion pits were observed on the surface of lower disk. In order to further understand the friction performance and the evolution of worn surface, the element and chemical states of the worn surface lubricated with the base fluid and blend with 0.5% addition were analyzed by X-ray photoelectron spectroscopy. The analysis revealed that TTC additives not only adsorbed physically onto the disk surface, but also reacted chemically with the metal ions to produce a tribofilm, which was responsible for the corrosion dots.　Finally, tribological mechanism was proposed that the high π electron density of the triazine and benzothiazole rings caused a strong interaction between TTC and metal iron atoms/ions to form a stable adsorbed-lubrication film, also the high affinity of sulfur atoms to metal iron atoms further boosted this effect. Synergistically, the weak van der Waals force between TTC layers reduced the transverse shear strength, and the intra-layer covalent bonds ensured the mechanical strength of the lubrication film.