tribofilms

Tribofilms are ultrathin protective films that form on contacting surfaces during tribological sliding, as a result of chemical and physical processes in the contact zone. They are typically confined to the immediate contact region and range from a few nanometers to a few hundred nanometers in thickness. Tribofilms can be inorganic, organic–inorganic hybrids, or purely organic, and may originate from lubricant additives, solid lubricants, or a combination of mechanisms.

Formation mechanisms rely on tribochemical reactions at asperity contacts under elevated pressure, temperature, and shear. Additives such as zinc dialkyl dithiophosphate (ZDDP), sulfur- and phosphorus-containing compounds, and chlorinated or fluorinated species decompose and react with exposed metal surfaces to produce protective layers of phosphates, sulfides, oxides, or mixed phases. Solid lubricants such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), graphite, or graphene can also deposit from the lubricant to form low-shear boundary layers. The resulting tribofilm may be glassy, crystalline, or amorphous and its composition depends on materials, lubrication, and operating conditions.

Functionally, tribofilms reduce direct metal–metal contact, lower friction, and slow wear by providing a sacrificial or low-shear interface. They are dynamic and can reform when lubricant conditions change, but excessive growth or spallation can lead to performance losses or surface damage. Formation and stability depend on surface materials, roughness, load, speed, temperature, and environmental factors.

Tribofilms are studied using surface analysis techniques such as X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, secondary ion mass spectrometry, and in situ tribometry, to relate composition and thickness to tribological performance. They are particularly important in engines, transmissions, and heavy machinery where lubricant additives play a central role.