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1. Enhanced Scratch Resistance of Graphite Coating Using a Polydopamine Adhesive Underlayer

   https://doi.org/10.3390/coatings15060690  

   Abe, Adedoyin; Zou, Min (June 2025, Coatings)

   Graphite has great potential as a solid lubricant due to its low friction properties, but its poor adhesion to metal substrates limits its durability unless modified. This study explores the use of polydopamine (PDA), a bioinspired adhesive polymer, as an underlayer to enhance the adhesion and scratch resistance of graphite coatings applied to stainless steel (SS) substrates. Progressive load scratch tests were performed using a stainless steel ball counterface under normal loads ranging from 0.5 to 18 N. The PDA-modified coatings demonstrated significantly improved adhesion and durability, withstanding contact pressures up to 1.6 GPa without delamination or failure. In contrast, graphite-only coatings showed early coating loss, severe material transfer, and wide wear tracks. The PDA underlayer enhanced graphite flake compaction, reduced porosity, and preserved structural integrity under high contact stress. These findings demonstrate that PDA reinforcement enables robust, oil-free lubrication suitable for high-stress environments. 

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2. Exploring the Impact of Spray Process Parameters on Graphite Coatings: Morphology, Thickness, and Tribological Properties

   https://doi.org/10.3390/coatings14060714  

   Abe, Adedoyin; Goss, Josue A; Zou, Min (June 2024, Coatings)

   This study explores, through a full factorial experimental design, the effects of graphite concentration and spray flow rate on the morphology, thickness, and tribological performance of graphite coatings for potential tribological applications. Coatings were applied to rough substrates using varying concentrations and flow rates, followed by analysis of their morphological characteristics, roughness, thickness, coefficient of friction (COF), and wear behavior. The results revealed distinct differences in the coating morphology based on flow rate, with low-flow-rate coatings exhibiting a porous structure and higher roughness, while high-flow-rate coatings displayed denser structures with lower roughness. A COF as low as 0.09 was achieved, which represented an 86% reduction compared to uncoated steel. COF and wear track measurements showed that thickness was influential in determining friction and the extent of wear. Flow rate dictated the coating structure, quantity of transfer film on the ball, and the extent of graphite compaction in the wear track to provide a protective layer. SEM and elemental analysis further revealed that graphite coatings provided effective protection against wear, with graphite remaining embedded in the innermost crevices of the wear track. Low flow rates may be preferable for applications requiring higher roughness and porosity, while high flow rates offer advantages in achieving denser coatings and better wear resistance. Overall, this study highlights the importance of optimizing graphite concentration and spray flow rate to tailor coating morphology, thickness, and tribological performance for practical applications. 

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3. Tribocatalytically-activated formation of protective friction and wear reducing carbon coatings from alkane environment

   https://doi.org/10.1038/s41598-021-00044-9  

   Shirani, Asghar; Li, Yuzhe; Eryilmaz, Osman Levent; Berman, Diana (December 2021, Scientific Reports)

   Abstract Minimizing the wear of the surfaces exposed to mechanical shear stresses is a critical challenge for maximizing the lifespan of rotary mechanical parts. In this study, we have discovered the anti-wear capability of a series of metal nitride-copper nanocomposite coatings tested in a liquid hydrocarbon environment. The results indicate substantial reduction of the wear in comparison to the uncoated steel substrate. Analysis of the wear tracks indicates the formation of carbon-based protective films directly at the sliding interface during the tribological tests. Raman spectroscopy mapping of the wear track suggests the amorphous carbon (a-C) nature of the formed tribofilm. Further analysis of the tribocatalytic activity of the best coating candidate, MoN-Cu, as a function of load (0.25–1 N) and temperature (25 °C and 50 °C) was performed in three alkane solutions, decane, dodecane, and hexadecane. Results indicated that elevated temperature and high contact pressure lead to different tribological characteristics of the coating tested in different environments. The elemental energy dispersive x-ray spectroscopy analysis and Raman analysis revealed formation of the amorphous carbon film that facilitates easy shearing at the contact interface thus enabling more stable friction behavior and lower wear of the tribocatalytic coating. These findings provide new insights into the tribocatalysis mechanism that enables the formation of zero-wear coatings. 

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4. Quality Control Metrics to Assess MoS2 Sputtered Films for Tribological Applications

   https://doi.org/10.1007/s11249-022-01642-y  

   Babuska, Tomas F.; Curry, John F.; Dugger, Michael T.; Jones, Morgan R.; DelRio, Frank W.; Lu, Ping; Xin, Yan; Grejtak, Tomas; Chrostowski, Robert; Mangolini, Filippo; et al (December 2022, Tribology Letters)

   Abstract Pure molybdenum disulfide (MoS 2 ) solid lubricant coatings could attain densities comparable to doped films (and the associated benefits to wear rate and environmental stability) through manipulation of the microstructure via deposition parameters. Unfortunately, pure films can exhibit highly variable microstructures and mechanical properties due to processes that are not controlled during deposition (i.e., batch-to-batch variation). This work focuses on developing a relationship between density, hardness, friction, and wear for pure sputtered MoS 2 coatings. Results show that dense films ( ρ  = 4.5 g/cm 3 ) exhibit a 100 × lower wear rate compared to porous coatings ( ρ  = 3.04–3.55 g/cm 3 ). The tribological performance of high density pure MoS 2 coatings is shown to surpass that of established composite coatings, achieving a wear rate 2 × ( k  = 5.74 × 10 –8 mm 3 /Nm) lower than composite MoS 2 /Sb 2 O 3 /Au in inert environments. 

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5. Effect of Cu and Ni Inclusion on Tribological Performance of Tribocatalytically Active Coatings in Hydrocarbon Environments

   https://doi.org/10.3390/coatings14010061  

   Al_Sulaimi, Rawan; Eskandari, Mohammad; Shirani, Asghar; Macknojia, Ali Zayaan; Miller, Wesley; Berman, Diana (January 2024, Coatings)

   Protective coatings are important for enhancing tribological behavior, preventing surface degradation, and reducing friction-induced energy losses during the operation of mechanical systems. Recently, tribocatalytically driven formation of protective carbon films at the contact interface has been demonstrated as a viable approach for repairing and extending the lifetime of protective coatings. Here, we study the effect of catalytic metals, specifically their composition and amount, on the tribocatalysis process. To achieve this, we test the tribological performance of electro-deposited amorphous CoNiP and CoCuP coatings in different hydrocarbon-rich environments. Our results indicate that the tribocatalytic repair of wear-induced damage is optimal when Ni and Cu are included in the Co-P matrix at 5 wt% Ni and 7 wt% Cu, respectively. Characterization of the wear tracks suggests that among the considered samples, the tribofilms formed on the surface of Co7CuP have the highest concentration of graphitic carbon, leading to a more significant reduction in the COF and wear rate. The carbon tribofilm formation was more pronounced in decane and synthetic oil than in ethanol, which is attributed to the difference in the length of the hydrocarbon molecules affecting viscosity and the lubricant film thickness during boundary lubrication sliding. 

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