Qiangliang Yu

Thermoreversible Gel Lubricants through Universal Supramolecular Assembly of a Nonionic Surfactant in a Variety of Base Lubricating Liquids

The present paper investigates a new type of thermoreversible gel lubricant obtained by supramolecular assembly of low-molecular-weight organic gelator (LMWG) in different base oils. The LMWG is a nonionic surfactant with polar headgroup and hydrophobic tail that can self-assemble through collective noncovalent intermolecular interactions (H-bonding, hydrophobic interaction) to form fibrous structures and trap base oils (mineral oils, synthetic oils, and water) in the as-formed cavities. The gel lubricants are fully thermoreversible upon heating-up and cooling down and exhibit thixotropic characteristics. This makes them semisolid lubricants, but they behave like oils. The tribological test results disclosed that the LMWG could also effectively reduce friction and wear of sliding pairs compared with base oils without gelator. It is expected that when being used in oil-lubricated components, such as gear, rolling bearing, and so on, gel lubricant may effectively avoid base oil leak and evaporation loss and so is a benefit to operation and lubrication failure for a long time.

Supramolecular Gel Lubricants Based on Amino Acid Derivative Gelators

This work presents an amino acid-derived urea as the low molecular weight gelators (LMWGs) to solidify a variety of base-lubricating oils as well as different organic solvents. LMWGs self-assemble to form fibrous structures by intermolecular hydrogen bonding and van der Waals forces, and the gelation mechanism was proposed according to the NMR analysis. The obtained gels can liquefy upon heating and gelate again after cooling down, showing reversible temperature-induced phase transition. They showed good thixotropic characteristics so that they became mobile under shearing. Therefore, it a great advantage of these supramolecular gels to be used as potential high-performance semisolid lubricants, which can guarantee both good lubrication and the ability to restrain base oil creeping, oil permeation, or leakage, especially for the gears and rolling bearings. The tribological testing results suggest that these gel lubricants could effectively reduce friction and wear of sliding pairs compared with the corresponding blank base oils and greases.

Ibuprofen-Based Ionic Liquids as Additives for Enhancing the Lubricity and Antiwear of Water-Ethylene Glycol Liquid

Ibuprofen-based (L-Ibu) halogen-free ionic liquids (ILs) were synthesized and evaluated as water-based lubricant additive. In contrast to the conventional ILs with fluoro-containing anions, the present L-Ibu ILs have no corrosive attack to steel and cast iron even in the water environment and exhibit extremely stable hydrolytic stability in water–ethylene glycol (W–EG) system. In addition, the Ibuprofen ILs as the water-based lubricant additives have better friction-reducing, extreme-pressure and antiwear properties than a commercial antiwear water-based lubricant additive of Hostagliss L4. The mechanism for excellent lubricating characteristics is investigated by combining electrical contact resistance measurement, X-ray photoelectron spectroscopy and interfacial adsorption behaviour by quartz crystal microbalance. The results indicate that a formed protective film on the contact surface prevents the direct contact of sliding pairs and contributes to the friction reduction and antiwear properties. More importantly, the prepared ILs provide a comparatively green alternative to the traditional halogenated ILs for the water-based lubrication application owing to halogen-, sulphur-, and phosphorus-free. So, these ILs are expected to develop into a highly efficient lubricant additive applied in water-based hydraulic fluids.Graphical Abstract

Supramolecular ionogel lubricants with imidazolium-based ionic liquids bearing the urea group as gelator

A new class of ionic liquid gels (ionogels) is prepared through the supramolecular self-assembly of imidazolium-based ionic liquids (ILs) bearing the urea group as gelators in normal ILs. The ILs gelator can self-assemble through hydrogen bonding and hydrophobic interaction to form analogous lamellar structures and solidify base ILs. The obtained ionogels exhibit superior anticorrosion and conductivity characteristics. Moreover, ionogels show fully thermoreversible and favorable thixotropic characteristics, such that they can be used as high-performance semisolid conductive lubricants. The tribological tests reveal that these ionogels lubricants can effectively reduce the friction of sliding pairs effectively and have better tribological performance than the pure ILs under harsh conditions. Ionogel lubricants not only maintain the excellent tribological properties and conductivity of ILs, but also prevent base liquids from creeping and leakage. Therefore, ionogel lubricants can be potentially used in the conductive parts of electrical equipments.

Fluorinated Candle Soot as the Lubricant Additive of Perfluoropolyether

In order to improve the tribological properties of perfluoropolyethers (PFPE), fluorinated candle soot is adopted as the lubricant additive because of their special onion-like structure. The candle soot particles (CSP) are modified by 1H,1H,2H,2H-perfluorooctanol (CSP-PFHE nanoparticles), and after the fluorination, they exhibit good dispersivity in PFPE. The mixtures composed of CSP-PFHE nanoparticles and PFPE possess better tribological performance than neat PFPE under different test conditions including variable temperature, the irradiation of atomic oxygen and extreme pressure. The reason can be attributed to that the graphene layers are exfoliated from the surfaces of nanoparticles and adhere onto the steel surfaces to form the tribofilm, which can protect the sliding pairs surfaces from friction and severe wear. Meanwhile, the redundant nanoparticles act as the rolling bearing between the sliding surfaces to decrease the wear and some are packed into the corrosion pits generated by PFPE to prevent further erosion in the process of friction. At the end, the lubricating mechanism of CSP-PFHE nanoparticles as additives of PFPE is proposed based on the test results of scanning electron microscope, contact electrical resistance and X-ray photoelectron spectroscopy.Graphical Abstract

Tribological behavior of laser textured steel impregnated with supramolecular gel lubricant

The friction and wear properties of supramolecular gel lubricant impregnated into the laser surface texturing steel fabricated by laser micromachining were investigated, with that impregnated with commercial oil PAO10 as a comparison. Scanning electron microscopy characterization shows the gel lubricant have been successfully pumped into the pores of laser surface texturing steel from liquid state and solidified afterwards via supermolecular assembly. The friction was measured on an Optimal SRV-IV oscillating reciprocating friction and wear tester. It is shown that laser surface texturing steel impregnated with gel lubricant exhibited outstanding friction reduction and antiwear performance than impregnated with PAO10. The gelator molecules act not only to solidify lubricating oil, but also contribute to boundary lubrication by strong adsorption on substrate surface. The instant self-assembly of gelator to solidify lubricating oils makes impregnation of lubricants easier and helps to keep lubricating oils retention for long-term functioning.