ICMCTF 2026 Session MC1-2-FrM: Friction, Wear, Lubrication Effects, & Modeling II

Two-dimensional materials (2DMs) exhibit extraordinary properties due to their dimensionality. Specifically, Titanium Carbide (Ti₃C₂T_x) of the MXene family, has been explored in tribology as additives to lubricants or as solid lubricant. [1] However, Ti₃C₂T_x is often synthesized by employing hazardous chemicals, limiting a true industrial implementation of MXenes. Sustainable methods to produce MXenes are being explored. Electrochemistry is the most promising, but it is often limited by a low effective yield.

We proposed for the first time a novel electrochemical method (pulsed voltammetry) to produce electrochemical MXenes (EC-MXene).[2] EC-MXene express tribological performances, which we detail by combining extensive surface analytics with DFT calculations.[3]

Extensive surface analytics (Raman, TEM, EELS, AFM, LEIS, XRD, and XPS) are used to confirm that the product is Ti₃C₂T_x. The same techniques are employed to characterize the wear tracks formed on an EC-MXene coated steel sample against three different counterbodies (Si₃N₄, Al₂O₃, and Steel). DFT calculations are used to extract the work of adhesion of EC-MXene against a model of the three counterbodies.[3]

To summarize,surface nanobubbles formed on the working electrode are able to reactivate the MAX interface, improving the etching efficiency and delivering MXenes with less -F termination and more -O presence. EC-MXenes present tribological performances like the one reported for classical MXene, while demonstrating a positive environmental trade-off. The tribological mechanisms relies on the transfer of an EC-MXene film onto the counterbody resulting in a shift of the sliding plane from a counterbody/coating to an EC-MXene/EC-MXene configuration.

[1] P. G. Grützmacher et al. “ The Promise of Solid Lubricants for a Sustainable Future.” Adv. Mater. (2025): doi.org/10.1002/adma.202500867

[2] Ostermann, M., Piljevic´, et al., Pulsed Electrochemical Exfoliation for an HF-free sustainable MXene Synthesis. Small. (2025) 21, 2500807

[3] Piljević M., et al., Electrochemically synthesized MXenes as sustainable solid lubricants: Mechanistic insights into tribofilm formation and interfacial dynamics. Carbon, 2026, 248

Sputter-deposited molybdenum disulfide (MoS₂) coatings have been used for decades in aerospace applications due to their ultra-low steady-state coefficients of friction (µ_ss < 0.05). Developing MoS₂ coatings for demanding applications with predictable and reliable performance over time (i.e., high-quality) requires tuning the coating microstructure through process variations. In this work, we explore process-structure-property-performance relationships of pure MoS₂ solid lubricant coatings where coatings are sputter deposited using different process gases. Helium, kypton, neon, argon and xenon are used to sputter deposit MoS₂ of varying morphologies, and the impact on critical performance traits such as initial friction, run-in, and aging resistance are studied. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

In recent years, demand for lightweight materials in the automotive and aerospace industries has increased, leading to a growing need for machining aluminum alloys. In aluminum alloy machining, Diamond-Like Carbon (DLC) coatings—especially hydrogen-free tetrahedral amorphous carbon (ta-C) coatings—are widely used due to their excellent wear resistance and low friction, which help suppress material adhesion and tool wear caused by hard Si particles in the alloy.

However, under more severe machining conditions, further improvements in coating performance are required to extend tool life, especially in terms of wear resistance and delamination resistance. One of the representative approaches for such performance enhancement is the addition of transition metal elements to DLC coatings, and numerous studies have been reported in this area. Among these, tantalum (Ta) is known to form strong covalent bonds with carbon and is expected to achieve both mechanical strength and improved adhesion strength through the reduction of residual compressive stress.Nevertheless, studies on its influence on machining performance remain limited.

In this study, tantalum-doped ta-C (ta-C:Ta) coatings with varying Ta contents were fabricated, and the correlation between Ta content and coating properties, as well as its effect on the drilling performance of aluminum alloy (ADC12), was systematically evaluated.

For each coating, microstructural analysis and residual stress measurements were conducted, along with ball-on-disk friction tests and scratch tests. Additionally, aluminum alloy cutting tests were performed to evaluate wear resistance and cutting force. As a result, the friction coefficient and specific wear rate tended to increase with higher Ta content in the friction tests. On the other hand, the scratch tests showed an increase in critical load, and a correlation between critical load and residual compressive stress was confirmed. Observations of the scratch marks revealed that ta-C:Ta coatings exhibited smaller delamination areas compared to undoped ta-C coatings. The dispersed structure of TaC nanocrystals observed in the ta-C:Ta coatings is suggested to suppress delamination propagation and contribute to improved toughness.

In the cutting tests, the coating containing 1.1 at.% Ta demonstrated the best wear resistance and lowest cutting force by significantly suppressing chipping while maintaining resistance to abrasive wear. These results suggest that controlling residual stress through appropriate Ta addition and enhancing toughness via fine TaC structures are effective strategies for improving tool life in aluminum alloy machining.

Plasma electrolytic oxidation (PEO) is an advanced surface treatment method used to produce stable barrier oxide coatings on light alloys. This coating technique is advantageous due to its simplicity and environmental benefits compared to relative methods such as anodising and promising for applications across many sectors including aerospace, automotive and biomedical. However, conventional PEO coatings have limitations to many areas of performance as they can only do so much as a passive protective layer. For this reason, it is important to further engineer the coating system to enhance the most beneficial behaviours and improve coating functionality.

An area that PEO is often used is for enhancement of wear resistance of aluminium alloys in sliding contact applications. PEO coatings must be modified to adapt tribological behaviour to changing environment, temperature and loading conditions. In these situations the main option is to add adaptive lubricants to the oxide coating to reduce friction and improve the overall performance of the component, some common examples being graphite or molybdenum disulphide based solid lubricants. These are capable of vastly reducing the friction coefficients [1] but encounter limitations under increased loading as they can delaminate due to weak interlayer bonding [2], with detrimental effects on coating longevity.

To address this problem, we investigate the application of MXene based solid lubricants on PEO coated Al alloys. MXenes are an innovative class of 2D inorganic materials with increased bonding strength between the layers of transition metal compounds making up their structure. To improve the compatibility between the PEO base and MXene top layer, in the first part of our work, we optimise the surface characteristics and properties of the PEO coating, adapting the surface texture and porosity to allow for better adherence and retention of the dry lubricant in sliding tribological contacts. Subsequent experiments include reciprocating sliding wear tests against steel and ceramic substrates, under hertzian contact pressures up to 1 GPa in dry and wet environments. The tribological behaviour and wear mechanisms are investigated by confocal microscopy, Raman spectroscopy and cross-sectional microanalysis. The key factors influencing the adaptive tribological performance of MXene-functionalised PEO coating systems are discussed with recommendations on further research directions made.

References

[1] A. Shirani, et. al., Surface and Coatings Technology, 2020, 397,126016.

[2] M. Lin, et. al., Tribology International, 2021,154, 106723.

Carbon films can form under tribological conditions through tribochemical reactions, which are chemical reactions assisted by mechanical stresses that promote the formation and dissociation of carbon-based molecules present at the interface, as well as their recombination at the interface.

In the past, we have shown—through both ab initio molecular dynamics (MD) simulations—that graphene films can form from methane due to the dissociation of the molecule and the formation of carbon chains that interconnect, initially generating an amorphous film. Subsequently, under the combined action of load and shear, rehybridization of the carbon atoms occurs, leading to the formation of graphene films [1].

Similarly, we have demonstrated that, starting from graphene flakes or from aromatic molecules of vegetal origin, graphene films can be obtained through the polymerization of these molecules under tribological conditions [2].

In both cases, the formation of the tribofilm is accompanied by a rapid decrease in friction as well as wear.

With the advent of machine learning potentials —which makes it possible to significantly increase the computational efficiency of MD simulations while maintaining their accuracy—we have been able to perform large scale simulations of sliding interfaces and identify key atomistic mechanisms of tribofilm formation screening different types of carbon-based molecules [3].

[1] G. Ramirez, O. L. Eryilmaz, G. Fatti, M.C. Righi, J. Wen, and A. Erdemir, Tribochemical Conversion of Methane to Graphene and Other Carbon Nanostructures: Implications for Friction and Wear, ACS Applied Nano Materials 3, 8060 (2020).

[2] Y. Long, A. Pacini, M. Ferrario, N. Van Tran, S. Peeters, B. Thiebaut, S. Loehlé, J.M. Martin, M.C. Righi, and M.I. De Barros Bouchet, Superlubricity from mechanochemically activated aromatic molecules of natural origin: A new concept for green lubrication, Carbon 228, 119365 (2024).

[3] L. Razzolini, A. Pacini, M. Ferrario and M. C. Righi, article in preparation based on the master thesis Degradation of hydrocarbon oil at sliding iron interfaces, University of Bologna (2025).