Pre-oxidized MXene nanosheets decompose under friction and reconstruct into a dual-phase tribofilm of graphitized carbon and titanium oxides, cutting wear by 91%.
(Nanowerk Spotlight) When two surfaces slide against each other under load, the contact zone becomes a site of intense chemical activity. Frictional heat and mechanical stress break bonds, strip atoms from surfaces, and drive reactions that would not occur under ambient conditions. The thin films that form at these interfaces, often just nanometers thick, determine whether a machine runs smoothly or grinds itself apart.
One approach to engineering these films is to seed the lubricant with nanoscale solid additives that migrate into the contact zone and react under friction. The concept of nanoscale friction has opened a design space where the chemistry of individual particles can be tuned to control what forms at a sliding interface. Two-dimensional materials are particularly suited to this role because their atomically thin layers shear easily past one another.
Among them, MXenes, a family of transition metal carbides first synthesized in 2011, stand out for their strong chemical affinity for metal surfaces. Previous work has shown that MXene coatings can deliver exceptional wear resistance under demanding conditions. But MXenes oxidize readily in air and moisture, and the oxidation progressively destroys their layered structure. This instability has been treated as a fundamental barrier to practical use.
A study published in Advanced Functional Materials (“Controlled Oxidation of MXene for Regulating Tribochemistry at Sliding Interfaces”) inverts this logic. A research team based primarily at Xi’an Jiaotong University deliberately oxidized MXene nanosheets with hydrogen peroxide, tuning the degree of oxidation before dispersing them in synthetic lubricating oil.
Rather than degrading performance, the controlled oxidation transformed the material into a far more effective lubricant additive, one that reconstructs itself into a durable protective coating on steel surfaces during sliding.
Schematic illustration of the synthesis process for oMXene and the proposed friction reduction mechanisms of oxidized MXene (oMXene) (top) vs. premixed graphene/TiO2 (bottom). oMXene undergoes tribochemically driven in situ reconstruction, enabling the formation of a continuous tribofilm that provides sustained friction reduction and wear protection. On the contrary, directly dispersed decomposition-related products suffer from agglomeration during friction, leading to abrasive wear. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The team prepared oxidized MXene by dispersing Ti₃C₂Tₓ nanosheets in dimethyl sulfoxide and adding hydrogen peroxide at carefully varied ratios. This wet-chemical approach introduced titanium oxide species uniformly across the nanosheet surfaces while preserving the layered architecture.
Natural oxidation, by contrast, caused uncontrolled edge degradation and fragmentation. Spectroscopic and diffraction analyses confirmed that the process progressively replaced titanium-carbon bonds with titanium-oxygen bonds and generated anatase-phase titanium dioxide on the surfaces.
The optimal oxidation ratio proved narrow. At a hydrogen peroxide to MXene ratio of 2:1, the material delivered its best performance. Mild oxidation actually raised friction, likely because uneven surface chemistry prevented a uniform protective film from forming.
Excessive oxidation fragmented the layered structure and disrupted film continuity. Only the intermediate degree provided enough chemical reactivity without sacrificing structural integrity.
Performance testing in a ball-on-disk tribometer using GCr15 bearing steel at contact pressures above 800 MPa gave striking results. The optimally oxidized MXene in PAO4 base oil cut the coefficient of friction by 57% relative to the neat oil and by 26% relative to pristine MXene. The wear rate dropped by over 91%, an order-of-magnitude improvement.
Higher sliding speeds amplified the effect by promoting thicker hydrodynamic films and accelerating the tribochemical reactions that build the protective layer.
The composition of the tribofilm explained its effectiveness. Under combined contact stress and frictional heating, the oxidized MXene adsorbed onto the steel surface and decomposed, undergoing tribochemical reconstruction. This generated a dual-phase film roughly 11.5 nm thick. One component consisted of highly graphitized carbon with ordered sp²-bonded networks, providing a low-shear sliding plane. The other consisted of titanium and iron oxides forming a mechanically robust, load-bearing framework.
The contrast with pristine MXene proved critical. Hydrodynamic forces pushed unoxidized nanosheets toward the edges of the wear track rather than allowing them to deposit within it. Any decomposition that occurred produced disordered, defect-rich carbon rather than the organized graphitic phase seen with oxidized MXene. Depth profiling confirmed that the oxidized variant formed a substantially thicker, more titanium oxide-enriched protective layer.
A control experiment ruled out the possibility that simply mixing the decomposition products would replicate the result. A premixed blend of graphene and titanium dioxide particles in the same oil produced higher friction and pronounced ploughing grooves. The particles agglomerated into clusters larger than the oil film thickness, causing abrasive damage rather than protection. This confirmed that the in situ tribochemical reconstruction, not just the presence of carbon and oxide species, drives the performance gains.
The work reframes MXene oxidation from a degradation problem into a design parameter. By controlling the degree of pre-oxidation, the researchers programmed the material to reconstruct itself into a high-performance interfacial architecture once friction activated the process. The strategy offers a rational pathway for designing lubricant additives that exploit chemical instability rather than fight it, turning what was considered a fatal flaw into the mechanism for durable surface protection.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
