Self-lubricating particles that were impossible to characterize… until now

Understanding the real role of the third body in friction

Reducing friction is a central challenge in many mechanical systems, whether in industrial applications or in devices operating in extreme environments. A significant share of global energy losses is directly linked to it, making it a major lever for improving performance and reducing energy impact.

In this context, solid lubricant materials such as molybdenum disulfide (MoS₂) play an important role. Yet the way they actually work is not limited to the intrinsic properties of the deposited material. When a contact is subjected to sliding, particles form and accumulate in the interface. These materials, known as the “third body”, then become the actual contact medium and largely govern the frictional behavior.

Understanding their mechanical properties is therefore essential to explain and control tribological performance.

A long-underestimated experimental barrier

Despite their decisive role, third-body particles remain particularly difficult to study. Their micrometer size, complex three-dimensional geometry and heterogeneous internal structure make them difficult to handle. On top of this comes their fragility, which severely limits the use of conventional mechanical characterization techniques.

In practice, these constraints long prevented any direct measurement of their behavior. Available knowledge relied mainly on indirect observations, leaving considerable uncertainty about the mechanisms actually at work in the contact interface.

Direct characterization inside the electron microscope

A recent advance has overcome this barrier by developing an experimental approach capable of characterizing these particles directly.

The method relies on integrating robotic micromanipulation systems inside a scanning electron microscope (SEM). This configuration makes it possible to isolate particles taken from friction zones, manipulate them individually and apply controlled forces to them while observing their deformation.

The tests involve forces ranging from a few micronewtons to several hundred micronewtons, for deformations ranging from a few tens of nanometers to more than one micrometer. This approach thus enables fine measurement of the mechanical behavior of objects that were previously inaccessible.

Revealing mechanical behaviors

The results reveal marked differences depending on the nature of the particles studied.

Particles from a third body formed from pure MoS₂ exhibit a very stiff behavior. Their deformation remains limited and their response is essentially elastic, reflecting a low capacity to accommodate mechanical stresses.

Conversely, particles from a tantalum-enriched MoS₂ system show a pronounced elasto-plastic behavior. They deform significantly under relatively low loads, with cracks appearing at forces of just a few tens of micronewtons. This deformability reflects an ability to accommodate stresses within the interface.

Raw MoS₂ particles, for their part, show an intermediate behavior, marked by internal reorganization phenomena related to their lamellar structure.

A direct link with lubrication performance

These observations establish a direct link between the mechanical properties of the particles and the tribological performance of the system.

Stiff, poorly deformable particles have a limited capacity to absorb and redistribute stresses, which can lead to faster degradation of lubrication conditions. Conversely, particles able to deform plastically can adapt to the contact, promoting more stable sliding and a lasting reduction in friction.

This result provides experimental validation of hypotheses that had until now been largely qualitative, by linking third-body mechanics to its functional performance quantitatively for the first time.

Towards a new approach to tribology

Beyond the specific case of MoS₂, this approach opens up broader prospects for the characterization of materials at small scales. It gives access to complex objects taken directly from their operating environment, without modifying or standardizing them.

By making it possible to measure these atypical systems, it contributes to a finer understanding of the physical mechanisms at work in contact interfaces. It also offers new possibilities for guiding the development of higher-performance materials and lubrication solutions.

This ability to measure the inaccessible represents a paradigm shift in the study of friction phenomena at small scales.