Solvent-free covalent MXene nanofluid: A new lubricant combining the characteristics of solid and liquid lubricants

[Display omitted] •A novel solvent-free covalent MXene nanofluids is proposed for application in lubrication.•The MXene nanofluids perform superior tribological performances under harsh conditions.•The MXene nanofluids exhibit a synergistic effect of solid lubricant and liquid lubricant.•The good lubrication is ascribed to the formation of thick tribofilms. A sole liquid lubricant or solid lubricant cannot cater to the demands of high-efficiency lubrication performance. From the standpoint of fusion of solid and liquid lubrication, solvent-free nanofluids are expected to be promising lubricants. In this work, solvent-free covalent MXene nanofluids were synthesized by grafting MXene with organosilane (KH560) and polyether amine oligomer (M2070) via covalent linkage. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, thermogravimetric analysis and rheological characterizations demonstrated that the MXene nanofluids were successfully synthesized and presented macroscopic fluidity at room temperature. Importantly, when used as a lubricant, the nanofluids perform well in friction-reduction and anti-wear properties even at high velocity (50 Hz), elevated temperature (100 °C), and heavy load (1100 N). The excellent tribological properties are ascribed to the nanofluids integrating the advantages of significantly improved load-bearing capacity and lubrication performance of the core nanomaterials, as well as liquid-like fluidity and self-healing characteristics of the organic shell. Moreover, the formation of 120–180 nm thick tribofilm on the worn steel surface was identified, which is significant in improving the lubrication performance. The above results suggest that the solvent-free MXene nanofluids should be a quite excellent candidate lubricant for integrating the advantages of solid and liquid lubricants and provide theoretical guidance for structural design of high-performance solvent-free nanofluid lubricants.