Precise graphenization of ultra-thin diamond coatings achieved without substrate damage
image: Illustration of the laser-induced transformation of diamond into planar-oriented graphite, with interfacial thermal stress control, followed by mechanical cleavage into graphene, ultimately achieving precise graphenization of ultra-thin diamond coatings. The resulting surface exhibits significantly enhanced frictional performance and good toughness at the nanometer scale.
Credit: By Bo Yan, Ning He, Ni Chen, Matthias Weigold, Huiwen Chen, Shuchen Sun, Yang Wu, Shiyang Fu, Liang Li and Eberhard Abele
Researchers from Nanjing University of Aeronautics and Astronautics (NUAA) have successfully overcome the thickness limitations of diamond coatings and enabled the precise in situ conversion of diamond to graphene, all while keeping interfacial thermal stress below critical thresholds and maintaining control over coating thickness and surface roughness.
The work, reported in the International Journal of Extreme Manufacturing, demonstrates that the obtained diamond coatings significantly outperform the original versions. The surfaces exhibited a 63%–72% reduction in friction coefficients, all of which were below 0.1, with a minimum of 0.06, and a 59%–67% decrease in specific wear rates. Moreover, adhesive wear in the friction counterpart was significantly inhibited, resulting in a reduction in wear by 49%–83%.
Although the conservation of tools and reduction in friction energy consumption could contribute to lowering the carbon footprint, controlling the graphenization process poses significant challenges due to the extremely low thickness and substantial physical property differences between diamond coatings and metal substrates.
"Compared to thick diamond films, diamond coatings exhibit distinct characteristics such as extremely small thickness (< 10 μm), significantly different physical properties of their metal substrates, and smaller grain sizes. These differences lead to several key issues in the graphenization of diamond coatings, including sudden delamination, difficulty in precise depth control, challenges in achieving uniform graphenization, and the need to improve surface roughness." said Prof. Ning He and A/Prof. Ni Chen, co-authors of this study.
Diamond coatings offer exceptional properties, including extremely high hardness, excellent thermal conductivity, and the ability to be applied to complex-shaped substrates, making them ideal for high-performance applications. However, untreated diamond coatings often suffer from uneven thickness and high surface roughness, leading to poor interfacial friction conditions. These drawbacks hinder their ability to meet the strict demands of advanced engineering applications, limiting their broader adoption. While many studies have explored ways to overcome these challenges, a widely effective solution has yet to be achieved.
Researchers at NUAA previously enhanced the surface properties of thick diamond films by generating covalently bonded graphene through laser induction and mechanical cleavage. Yet, applying this graphenization process to diamond coatings presents additional challenges due to their distinct characteristics.
To tackle these obstacles, the team introduced the concept of thermal stress control during laser induction processes, and developed a finite element numerical model based on temperature and thermal stress fields for laser-induced graphitization of diamond coatings.
By combining experiments with theoretical calculations, they established two power limits––graphitization threshold power and critical delamination power––along with corresponding empirical formulas, clarifying the mechanisms and control strategies for rapid graphitization and interfacial thermal stress in diamond coatings. Ultimately, they achieved precise graphenization of the diamond coating surface below the interfacial thermal stress threshold, allowing control over the coating's thickness and roughness without damaging the metal substrate.
"This work effectively and cost-efficiently enhances the performance of diamond coatings, producing surfaces with excellent mechanical properties and structural stability. Under heavy loads and dry friction conditions, the surface friction coefficient of the coating is less than 0.1, reaching a minimum of 0.06." said co-author Bo Yan.
The researchers are further exploring applications of this approach, aiming to improve the operational performance of existing sliding surfaces while contributing to energy savings and emission reduction. They also seek to extend these findings to broader applications, including de-icing and microwave absorption.
About IJEM:
International Journal of Extreme Manufacturing (IF: 16.1, consecutive 1st in the Engineering, Manufacturing category) is a multidisciplinary and double-anonymous peer-reviewed journal uniquely publishing original articles and reviews of the highest quality and impact in the areas related to extreme manufacturing, ranging from fundamentals to process, measurement, and systems, as well as materials, structures, and devices with extreme functionalities.
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