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Diamond-like carbon coatings (DLCs) held, and continue to hold, great promise for the improvement in friction and wear resistance of engine parts. The DLC tribofilm is generated by breaking down the molecules of the lubricating oil, regenerating itself as it is worn away. Successfully analyzing the properties inherent in these chemical reactions to enable maximum benefit can be complex.
Raman spectroscopy is typically used to determine the vibrational mode of individual molecules enabling scientists to identify each one. By identifying each molecule mode of a DLC tribofilm, scientists and engineers can synthesize the most effective tribological solutions for things like car engines.
The Raman Technique Used to Analyze DLC Tribofilms
Raman analysis has been used for many years. Back in 2009, scientists were experimenting with DLC tribofilms. Hydrogenated DLC films were deposited on flat samples and engine components using an industrial-scale reactor. The goal was to try and further reduce mechanical energy loss caused by tribological wear and tear.
The structure of the DLC coating allowed scientists to validate its application on the engine parts due to the high hardness and adhesion the DLC achieved using sub-layers. Raman analysis was used on both the flat samples and on the coated engine parts that had a complex shape. The Raman method was used because engineers knew it would allow them to identify and characterize the different values of the structured organization.
Raman signals allowed the team to look at the sub-layers of the film, identifying where a coating thickness decrease occurred due to wear. This method is still commonly used by motorsport engineering teams as many of the cars have complex mechanical gears and other moving parts that can benefit from Raman analysis. Being able to identify where wear is likely to occur and adjust the DLC accordingly can mean the difference between a race win or loss.
In the last decade, Raman spectroscopy has been used to analyze DLC tribofilms elsewhere. In 2014, hydrogenated DLC was doped with tungsten and the films were deposited by a reactive magnetron sputtering from graphite and composite graphite with tungsten in an argon-methane atmosphere. The microstructure of the DLC was studied using dozens of different scanning techniques, including Raman spectroscopy.
The DLC coatings were tested in both lab and practical conditions with the worn surfaces of the engine components analyzed by both Raman spectroscopy and secondary ion mass spectroscopy. Raman analysis was able to determine that the DLC offered excellent wear resistance. The worn surface components were still covered by a very thin tribolayer that had been formed from oil additives during road testing.
Different Forms of Raman Spectroscopy on DLC Tribofilms
The two most common forms of Raman analysis are Surface-Enhanced Raman Scattering (SERS) and Tip Enhanced Raman Scattering (TERS). They are primarily used because they offer a fast, convenient and non-destructive molecular detection technique. This is essential in analyzing the tribological properties of DLC films.
SERS and TERS are both able to provide quantitative and qualitative information on the molecular orientation, chemical properties, order states and conformational changes of DLC tribofilms, making Raman spectroscopy an essential resource for tribological engineers and scientists.
Indeed Far-field Raman spectroscopy is versed in helping teams of scientists investigate the chemistry of worn surfaces as the method allows them to obtain the structural information of a DLC tribofim. Once each molecule effect is identified, the tribofilm can then be applied in the most appropriate manner.
Far-field Raman spectroscopy is not without flaws. However, low sensitivity and micron depth resolution makes it challenging to analyze all the molecules and variations on DLC tribofilms.
SERS Raman Spectroscopy on DLC Tribofilms
SERS is the preferred analysis on DLC tribofilms as it can solve the analytical barrier the far-field technique throws up. The surface-enhanced technique enables the molecular scanning of DLC tribofilm modes affording tribological engineers everywhere the ability to further reduce the wear and tear of contact mechanics.
References and Further Reading
Self-healing, self-lubricating tribofilm – Tech Briefs - 2017 https://www.techbriefs.com/component/content/article/tb/techbriefs/materials/27697
DLC-W coatings tested in combustion engine — Frictional and wear analysis – Science Direct – 2014 https://www.sciencedirect.com/science/article/abs/pii/S0257897214006288
Raman analysis of DLC coated engine components with complex shape: Understanding wear mechanisms https://www.sciencedirect.com/science/article/pii/S0040609009015727
Understanding the Friction Reduction Mechanism Based on Molybdenum Disulfide Tribofilm Formation and Removal – Langmuir – 2018 https://pubs.acs.org/doi/pdfplus/10.1021/acs.langmuir.8b02329?src=recsys#
Surface- and Tip-Enhanced Raman Scattering in Tribology and Lubricant Detection—A Prospective – MDPI – 2019 https://www.mdpi.com/2075-4442/7/9/81/htm