KEY CONCEPTS
• High-energy quantum beams are powerful tools used to study the nature of materials. 
• In tribology, X-ray analyses have been used to study solid surface layers and to understand changes in tribological behaviors, both before and after processes of tribological interest occur at and near solid surfaces. 
• Quantum beam analyses are being used to study tribological processes and show much promise for future use.

The use of high-energy quantum beams began in the mid-20th Century in the field of quantum physics. As quantum beams required large scale accelerator facilities, beam analysis was initially limited in applications outside of fundamental physics. However, the synchrotron accelerator is more accessible today and is used as a powerful tool in a variety of areas, including materials science, medicine, earth science and nano-fabrication, among other fields.

STLE Life Member Stephen Hsu, a professor at George Washington University in Washington, D.C., notes, “Quantum beam refers to high-intensity, ultra-bright beams of light in X-rays, synchrotron radiation and neutrons, electrons, positrons, muons, protons, ions and photons.” These beams are generated using particle accelerators, synchrotron radiation and high-power laser equipment. Initially, these beams were used to research the nature of forces in nature and matter. Currently, these beams are being used to characterize and understand materials to advance technological frontiers.

Tribological applications
In tribology, X-ray analyses have been used in a number of applications to study solid surface layers, as well as adsorbed species and chemical products on solid surfaces to understand changes in tribological behaviors, both before and after processes of interest occur. Researchers began using high-energy beam facilities to obtain information on tribo-interfaces at higher resolution, high signal to noise (S/N) ratio and in situ. Quantum beam analyses have the potential to be powerful and promising tools for understanding chemistry and the structure of materials, as well as their transformation in tribological processes occurring at and near solid surfaces.

Some analytical methods with synchrotrons include X-ray absorption fine structure (XAFS), X-ray fluorescence analysis (XRF), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). X-ray absorption near edge structure (XANES) and near edge X-ray absorption fine structure (NEXAFS) have been used to study the self-assembled monolayer of hydrocarbon films, the molecular orientation of polymers and the carbon atomic bonding state on diamond-like carbon (DLC) surfaces, among other topics.

Synchrotron XRD has been used to observe deformation, structural changes and phase changes in metals and coatings, including steel and DLC surfaces, that lead to scuffing and other tribo-failures. Neutron reflectometry (NR) has been used to observe the growth of boundary lubrication films in additives, while neutron small angle scattering (SANS) has been used to study behaviors of viscosity index improvers.

These methods have been combined with Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and XPS to provide more comprehensive understanding of tribological issues. Quantum beam analyses use high-energy beams, which means they can be performed in situ and may be developed into direct operando analysis of model machine elements in the near future.

Quantum beam analyses are occurring around the globe with researchers exploring different techniques and applications to understand tribological topics. The following sections provide more specific examples of this work.

Neutron reflectometry
Neutron reflectometry involves shining a beam onto a flat surface and measuring the intensity of the reflected radiation to understand the structure of thin films. Tomoko Hirayama, mechanical engineering professor at Kyoto University in Kyoto, Japan, is currently using neutron reflectometry to study the precise structure of a lubricant/substrate interface. While Hirayama also has used synchrotron X-ray analyses and neutron analyses to understand tribological phenomena, her work with neutron reflectometry is most pertinent to this article.

In the early 2000s, Hirayama notes that neutron diffraction was being used in other applications, and the suggestion was made to use it to directly analyze the friction interface. Neutron reflectometry can now reproducibly identify the thickness and density of additive absorbed layers at the lubricant/substrate interface. Hirayama and colleagues “have now developed the reflectometry system to the point where they can perform analyses in various situations, such as temperature change, pressure change and interface structure analysis in shear fields.”¹

Hirayama used neutron reflectometry with her own analysis cell “for the analysis of the lubricant/substrate interface under a static condition…where the shape and structure of the cell has evolved to allow for environmental changes such as temperature changes, pressure changes and shear fields.”² Neutron beam analysis “can make a clear distinction between base oil and additive molecules by deuterating either of them” (i.e., replacing one or more hydrogen atoms with deuterium, a hydrogen isotope). Base oils and additive molecules are generally both hydrocarbon based, which makes them difficult to distinguish clearly by light or X-rays. Hirayama notes, “Neutrons, on the other hand, can clearly distinguish between the hydrogen and deuterium, allowing the structure of the additive at the interface to be clearly extracted.”

Hirayama also has been examining the combined effects of multiple additives. She notes, “By deuterating each additive one by one, in turn, we can quantify the density of the deuterated additive at the interface.” This allows for the effects of multiple additives to be identified as synergistic or competitive.

Hirayama notes, “The only drawback of neutron analysis is that it requires the use of a large quantum beam facility for the neutron beam.” Currently, Hirayama is using the reflectometer called SOFIA in the J-PARC MLF facility in Tokai, Ibaraki Prefecture, Japan. SOFIA has a very high intensity of neutron radiation, making it “possible to follow the formation of the adsorption layer over time on a time scale of minutes,” which offers advantages in understanding tribological phenomena.

NEXAFS spectroscopy
STLE member Filippo Mangolini, assistant professor at The University of Texas at Austin, has used NEXAFS spectroscopy “to gain insights into the surface chemical and structural transformations occurring in DLC films upon tribological testing.” Mangolini notes that while DLC coatings have unique mechanical properties and show excellent tribological performance, they also have limits, including “insufficient thermal stability, high reactivity in oxidizing environments and strong dependence on the friction and wear responses in environmental conditions.” These limits mean that DLC coating cannot be used in harsh environments, “including as protective coatings for aerospace components, automotive components, advanced manufacturing tools and next-generation magnetic storage devices.”

One way to improve DLC coatings is to alloy the DLCs with metals and metalloids. In particular, the addition of silicon and oxygen has been shown to increase DLC thermal stability. Mangolini says that NEXAFS spectrometry has been used to explain “the origins of the promising lubricating performance and superior thermal stability of a class of alloyed DLC, namely silicon- and oxygen-containing DLC.” Imaging NEXAFS measurements have shown that “these surface chemical changes could be correlated with the dependence of the friction response of alloyed DLC with gas pressure.”³

Mangolini has “also developed new methodologies for quantifying the local carbon bonding configuration using NEXAFS.” The contribution of a thin substrate covered by a surface layer has been studied using NEXAFS spectra in a non-destructive way.

The National Synchrotron Light Source II at the Brookhaven National Laboratory in Upton, N.Y., and the Advanced Light Source at the Lawrence Berkeley National Laboratory in Berkeley, Calif., were used for this research. The Advanced Photon Source at Argonne National Laboratory in Lemont, Ill., and the Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory in Menlo Park, Calif., also were used.

While focusing on NEXAFS spectroscopy, Mangolini also has used extended X-ray absorption fine structure (EXAFS) measurements in evaluating the local atomic structure of materials. His team plans to explore the use of resonant soft X-ray scattering (RSoXS) to address open questions in nanocomposite materials in the future.

Mangolini finds X-ray absorption spectroscopy’s major advantage to be its ability to provide unique insights into the local chemistry and structure of materials. NEXAFS, in particular, can provide quantitative information on the hybridization state of carbon, which is a key factor that determines the mechanical and tribological properties of carbon-based materials. However, the requirement of a photon source with tunable energy and high photon flux means a research laboratory cannot perform this type of work. As synchrotron facilities offer limited beam time, the use of X-ray absorption spectroscopy for tribological studies is limited. The complexity of data acquisition and interpretation also limits these efforts. However, Mangolini notes, “Recent instrumental advancements have resulted in the availability of hard X-ray systems that can allow NEXAFS (or XANES) measurements to be performed in the laboratory.” This development is still in process but is a step toward making X-ray absorption spectros copy more widely available.

Mangolini states, “The characterization of materials’ surfaces after tribological tests is traditionally performed ex situ (i.e., outside the tribometer and after the friction test).” In these ex situ tests, “the analysed surface might be significantly altered compared to the surface present in the tribometer experiment.” In contrast, in situ experiments “allow the direct probing of the mechanics and chemistry characterizing the buried sliding interface as they form in their ‘natural state.’” Mangolini notes that in situ instrumentation at synchrotrons will provide researchers with tools that “can not only be critical for advancing our fundamental understanding of the phenomena occurring at sliding interfaces but also enable the development of new products, from oil formulations to lubricious coatings.”

Synchrotron X-ray diffraction: Japan
Kazuyuki Yagi, associate professor at Kyushu University in Fukuoka, Japan, uses synchrotron X-ray diffraction “to investigate in situ variations on crystal grains of the contact surface during friction and an occurrence of scuffing.” Scuffing is a form of wear that occurs in highly loaded lubricated contacts (e.g., piston rings, gear teeth). When boundary lubricants break down, localized solid phase welding can occur, which, when broken, roughens the surface so that the extent of roughening expands and increases, leading to rapid damage. Yagi’s work with “the synchrotron X-ray diffraction system indicated that crystal grains change from martensite of bulk material to austenite because of instantaneous temperature rises by plastic flow during scuffing.”^4,5 Furthermore, Yagi analyzed the in situ XRD spectra and succeeded in indicating variations in temperature rise, orientation, size and dislocation density of crystal grains during tests.

Yagi has used a beam line at SPring-8, a synchrotron radiation facility in Sayo Town, Sayo District, Hyögo Prefecture, Japan. He also has used a special beam line BL33XU at SPring-8 for investigating scuffing phenomena. Yagi designed a friction tester with a visible camera and a near infrared camera to capture the contact area in situ simultaneously with the X-ray detector.

Yagi has found that the incident X-ray beam of a conventional XRD analyzer has low intensity, which means that analysis of in situ phenomenon in the contact area cannot occur. In contrast, the quantum beam provides a much stronger intensity compared to a normal analyzer. This is important as the intensity of the diffracted light is much weaker than the intensity of the incident light. Therefore, quantum beam analysis provides an advantage to in situ analysis of the contact area at a high frame rate.

However, Yagi notes, “The intensity of the quantum beam is still insufficient to analyze chemical reaction films with thicknesses of the nano-meter order.” Establishing stronger quantum beams would allow the in situ structure analysis of boundary films to succeed. Yagi projects that X-ray diffraction analysis will contribute to the development of new materials and surface treatments to improve tribological performances.

Synchrotron X-ray diffraction: Russia
Ivan Bataev, assistant professor at Novosibirsk State Technical University in Novosibirsk, Russia, has worked on the operando analysis of surfaces during dry sliding friction “to see how fast the structure of materials evolves due to the sliding from the original state via some unsteady state to a steady state.” Operando analysis of friction allows the study of product performance by increasing the coefficient of friction and decreasing wear. Industries looking at friction-induced transformation need to find a balance between high wear resistance and low product cost, and include the railroad industry, mining industry and agricultural industry, to name a few.

Bataev explains that the X-ray diffraction patterns are recorded and analyzed using modified Williamson-Hall and modified Warren-Averbach peak profile analysis techniques to “observe how fast the dislocation density increases, what kind of dislocations (i.e., edge or screw) grow faster and when the threshold dislocation density is achieved.” This process permits the observation of “friction-induced phase transformations, oxidation of the friction surface, formation of wear debris and other phenomena.”

During friction, the surface layer structure changes to a new structure, with the new structure responsible for the long-term operation of the friction pair. Bataev states, “The aim of our project is to understand how the original structure of the sample is related to the structure produced by friction.” The goal of this research is to understand how to control the final structure and improve the tribological properties of metallic materials, with the hope that the findings will help in designing a new generation of wear-resistant materials.

Bataev’s group designed a new friction tester that can be used at synchrotron beamlines. While initially the samples used were large (i.e., diameter of 66 mm and thickness of 10 mm), sample sizes have been reduced to a diameter of only 10 mm.

Bataev used the VEPP-3 synchrotron in Novosibirsk, Siberia, Russia, in the early 2000s. Since then, he has used the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and the European X-ray Free-Electron Laser Facility (European XFEL) between the towns of Hamburg and Schenefeld in Schleswig-Holstein, Germany. A new X-ray research facility is under construction in Novosibirsk called SKIF, which is a Russian acronym for Siberian Circular Photon Source.

To understand the synchrotron XRD results, Bataev uses “optical microscopy and scanning electron microscopy to characterize the structure of the subsurface layer” and optical interferometry to reconstruct the topography of the friction surfaces in three dimensions. In addition, he has used “X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy to study the chemical composition after the friction.”

Synchrotron radiation offers obvious advantages, including the ability to carry out in situ and operando experiments and observe the phenomena that accompany friction and wear with a very high time resolution. Bataev notes, “By varying the grazing angle, one can control the penetration depth of the X-ray and, thus, one can probe the structure at a different depth from the surface.”

Limitations of synchrotron radiation include access to synchrotron beamtime and limitations on what can be placed in the path of the beam. In addition, researchers have to bring their own tribometers to experiments or design new equipment.

Conclusion
Bataev expects a rapid increase in this field with testing conditions expanding to become more similar to real operating conditions. Studying the influence of lubricants on sliding friction and rolling friction also will continue to expand.

Hirayama and her associates are “currently collaborating with a number of companies to conduct collaboration research” using neutron reflectometry to “understand the effect of the developed additives from the viewpoint of interfacial structure.” She notes, “There is a relatively good correlation between interface structure and tribological properties.”

Future quantum beam analysis offers new applications to benefit industrial development with many opportunities available for additional tribological research in this field. The primary limitation to this work is the availability of synchrotron beam time, which does push users to carefully plan experiments to achieve meaningful results within the allotted time.

The application of quantum beam analysis in tribology is only beginning.

REFERENCES
1. Hirayama, T., Maeda, M., Sasaki, Y., Matsuoka, T., Komiya, H. and Hino, M. (2018), “Growth of adsorbed additive layer for further friction reduction,” Lubrication Science, 31 (5), pp. 171-178. Available here.
2. Hirayama, T., Torii, T., Maeda, M., Matsuoka, T., Inoue, K., Hino, M., Yamazaki, D. and Takeda, M. (2012), “Thickness and density of adsorbed additive layer on metal surface in lubricant by neutron reflectometry,” Tribology International, 54, pp. 100-105. Available here.
3. Mangolini, F., Koshigan, K.D., Van Benthem, M.H., Ohlhausen, J.A., McClimon, J.B., Hilbert, J., Fontaine, J. and Carpick, R.W. (2021), “How hydrogen and oxygen vapor affect the tribochemistry of silicon- and oxygen-containing hydrogenated amorphous carbon under low-friction conditions: A study combining x-ray absorption spectromicroscopy and data science methods,” ACS Applied Materials & Interfaces, 13 (10), pp. 12610-12621.
4. Yagi, K., Kajita, S., Izumi, T., et al. (2016), “Simultaneous synchrotron x-ray diffraction, near-infrared, and visible in situ observation of scuffing process of steel in sliding contact,” Tribology Letters, 61, Article number 19. Available here.
5. Yagi, K., Izumi, T., Koyamachi, J., et al. (2020), “In situ observation of crystal grain orientation during scuffing process of steel surface using synchrotron x-ray diffraction,” Tribology Letters, 68, Article number 115. Available here.

Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.