X-ray photoelectron spectroscopy (XPS) is a powerful surface analysis technique widely employed in tribology and wear studies to investigate chemical changes on lubricated surfaces, transfer layers, and worn materials. The method provides elemental composition, chemical state information, and bonding environments of the top few nanometers of a surface, making it indispensable for understanding friction-induced transformations, oxidation processes, and additive interactions in tribological systems.

In tribology, XPS is particularly valuable for analyzing lubricant films and their degradation under mechanical stress. Lubricants often contain additives such as zinc dialkyldithiophosphates (ZDDP), anti-wear agents, and friction modifiers that form protective tribofilms on contacting surfaces. XPS identifies the chemical states of elements like phosphorus, sulfur, and zinc in these films, revealing how they decompose and react under shear. For example, ZDDP-derived tribofilms show characteristic XPS peaks for phosphate and sulfate species, indicating thermal and mechanical breakdown of the original compound. The P 2p and S 2p spectra provide insights into the formation of polyphosphates and sulfide phases, which contribute to wear protection.

Transfer layers, formed when material from one surface adheres to another during sliding contact, are another critical area where XPS excels. These layers often consist of a mixture of worn material, reaction products, and decomposed lubricant additives. XPS can distinguish between metallic, oxidized, and chemically reacted species within the transfer layer. For instance, in steel-on-steel sliding contacts, the Fe 2p spectrum reveals the presence of metallic iron, iron oxides (FeO, Fe₂O₃, Fe₃O₄), and iron sulfides or phosphates if sulfur- or phosphorus-containing additives are present. The relative intensities of these peaks help quantify the extent of oxidation and tribochemical reactions.

Worn surfaces exhibit complex chemical modifications due to friction-induced heating, plastic deformation, and environmental interactions. XPS detects these changes by analyzing shifts in binding energies, which correlate with chemical state alterations. For example, carbonaceous materials in lubricants or coatings may graphitize under shear, leading to changes in the C 1s spectrum. The appearance of carbide or adventitious carbon peaks indicates structural reorganization due to tribological stress. Similarly, oxygen-containing functional groups increase when surfaces oxidize under friction, detectable through O 1s spectra.

Oxidation is a major wear mechanism that XPS effectively characterizes. The technique differentiates between native oxides, mechanically induced oxides, and thermally grown oxides. In aluminum alloys, for instance, the Al 2p peak shifts to higher binding energies when aluminum oxidizes to Al₂O₃. The thickness of oxide layers can also be estimated using XPS depth profiling or by analyzing the relative intensities of metal and oxide peaks. This is crucial for understanding how oxide layers influence friction and wear behavior.

Additive decomposition is another key focus area. Anti-wear and extreme pressure additives undergo complex reactions during sliding, forming protective films that reduce wear. XPS tracks these reactions by monitoring changes in elemental speciation. For example, sulfur-containing additives decompose to form sulfides or sulfates, detectable through S 2p spectra. Phosphorus-based additives generate phosphates or phosphides, identifiable via P 2p spectra. The chemical evolution of these films correlates with their tribological performance, allowing researchers to optimize additive formulations.

While XPS provides detailed chemical information, it is often complemented by time-of-flight secondary ion mass spectrometry (ToF-SIMS) for a more comprehensive wear analysis. ToF-SIMS offers higher surface sensitivity and can detect molecular fragments and low-concentration species that XPS may miss. Together, these techniques provide a complete picture of tribochemical processes. For example, XPS identifies the oxidation state of iron in a wear scar, while ToF-SIMS detects organic fragments from degraded lubricants or transfer films.

A typical workflow in tribological studies involves initial surface characterization with XPS to establish baseline chemistry, followed by tribological testing, and post-test XPS analysis to identify chemical changes. Depth profiling may be employed to examine subsurface modifications. Comparing unworn and worn surfaces reveals reaction pathways and degradation mechanisms.

XPS is also used to study boundary lubrication, where thin films prevent direct metal-to-metal contact. The chemical composition of these films, including adsorbed additives and reaction products, is critical for their performance. XPS can differentiate between physically adsorbed and chemically reacted species, providing insights into film formation mechanisms.

In summary, XPS is an essential tool in tribology for investigating lubricant films, transfer layers, and worn surfaces. Its ability to identify chemical states and quantify surface composition makes it indispensable for understanding wear mechanisms, additive functionality, and oxidation processes. When combined with ToF-SIMS, it offers a robust approach for comprehensive tribochemical analysis, aiding in the development of advanced lubricants and wear-resistant materials.