Effects of polyacrylic acid molecular weights on V 2 C-MXene nanocoatings for obtaining ultralow friction and ultralow wear in an ambient working environment.

Two modified V 2 C-MXene nanocoatings are prepared through different molecular weights of polyacrylic acid (polyacrylic acid with ∼4 50 000 is marked as LPAA, and polyacrylic acid with ∼4 000 000 is marked as HPAA) and two-dimensional V 2 C-MXene. Their properties are characterized using a ball-on-disc tribometer, three-dimensional white-light interferometry topography images, optical microscope, Raman spectrometer, focused ion beam/scanning electron microscope and high-resolution transmission electron microscope/energy dispersive X-ray spectrometer (HRTEM/EDS). As a result, an ultralow friction ( μ ≈ 0.073 ± 0.024) and an ultralow wear (3.41 × 10 -7 mm 3 N -1 m -1 for ball scar, and 7.49 × 10 -8 mm 3 N -1 m -1 for disc track) are achieved for the LPAA@V 2 C vs. steel ball system tested under 4 N in the air through tribo-physicochemical interactions. During the rubbing process, the LPAA@V 2 C nanocoating is transferred onto counter-bodies to form mixed-phase lubricative tribofilms. Monitoring via a HRTEM/EDS, the mixed-phase lubricative tribofilms are found to be mainly composed of amorphous carbon phases containing O and V and layered nano-debris along the sliding surface. The tribofilm's stable structure is the key to realizing ultralow friction and ultralow wear through the LPAA modification. These findings disclose that MXene-based nanomaterials can be applied for material engineering and mechanical engineering under common working conditions.