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

The global energy transition, driven by the rapid electrification of mobility, wind power systems, and other advanced energy technologies, demands a deeper understanding of how electrical loading influences friction, wear, and tribochemical film formation in electrified contact interfaces. Bearings, gears, and electrical contacts in these systems are increasingly exposed to stray currents that accelerate surface degradation, lubricant breakdown, and coating failure. To address these challenges, traditional tribometers have been redesigned into electro-tribometers that integrate precise control of mechanical, electrical, and environmental parameters. This presentation highlights different platforms developed to study materials and lubricants under electrical conditions: (1) the electrified four-ball test, used to evaluate current-induced wear and lubricant degradation in concentrated point contacts; (2) the electrified pin-on-disk test, operated under controlled gas atmospheres (air, nitrogen, argon, or humidified conditions) to investigate electro-tribochemical reactions and film growth with high reproducibility; (3) the electrified Mini-Traction Machine (MTM), designed to examine traction and mixed-lubrication behavior under simultaneous rolling, sliding, and current flow; (4) the block-on-ring test, adapted to simulate and study the combined effects of mechanical loading, sliding contact, and electrical current on tribological interfaces; and (5) the electrified ball-cratering (micro-abrasion) test, adapted for localized wear and debris characterization of materials and coatings under electrical bias. Together, these experimental routes establish a unified framework for probing electro-tribochemical mechanisms, coating durability, and lubricant stability, paving the way toward standardized testing of materials, coatings, and lubricants for electromobility, wind power, and other energy conversion systems.

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.