Collaborative Research: Understanding Run-In and Superlubricity of Diamond-Like Carbon Coatings from a Tribochemical Perspective

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Smartphone screens are an impressive example of how new technologies often rely on surface engineering, such as the ability to create durable, hard and slippery surfaces. Many key economic sectors can benefit from minimized wear and friction of surfaces, including the automotive, medical device, computer component and defense sectors, so that this research directly and positively impacts the economic welfare and national security of the United States. Recently, a hard surface coating known as diamond-like carbon (DLC) has been found to achieve an extreme level of slipperiness called "superlubricity". This desirable property is, however, very sensitive to environmental conditions such as the amount of water or hydrogen in the surrounding atmosphere. Sensitivity to environmental conditions limits the effectiveness of the coating for many potential applications. This collaborative research aims to understand the chemical reactions that govern superlubricity in DLC and to discover ways to extend its high performance to a wider range of conditions. The collaborators at Penn State and St. Olaf College specialize in measuring friction using different state-of-the-art methods that complement each other. The broader impacts of this study extend to training the next generation of scientists in the United States by supporting student participation in the research. Educational activities will include outreach for underrepresented minority recruitment at Penn State and undergraduates at St. Olaf, instruction of undergraduates in friction-related physics at St. Olaf, instruction in DLC surface properties for a graduate course at Penn State, surface characterization tutorials in professional conferences, and student participation in international collaborations. Due to its amorphous nature, the carbon atoms in DLC have very broad distributions in bond length and angle. The covalent bonds in DLC are formed during the high-energy deposition process and cannot be relaxed or rearranged without annealing at extremely high temperatures. This means that the lengths and angles of many bonds in DLC deviate significantly from the minimum-energy structure of ideal sp2 and sp3 hybridization. Those bonds are weaker and more reactive compared to the bonds with parameters close to the ideal structures. Such broad distributions in bond parameters and reactivities are intrinsic parameters specific to each DLC coating and its deposition conditions. The main thesis of this study is that the presence of highly-distorted carbon-carbon bonds facilitates a mechanochemically-induced polymorphic transition to graphitic domains at the shearing interface. Under this hypothesis, the run-in process can be attributed to the shear-induced mechanochemical transformation of the distorted carbon networks to graphitic (or graphene-like) domains. The transformation process will also be affected by reactions with molecules impinging from the gas phase. These surface reactions with surrounding gas molecules are extrinsic parameters affecting the run-in and superlubricity of DLC. A mechanically-assisted thermal activation (Arrhenius-type) model will be combined with structural characterization to study how the intrinsic and extrinsic parameters facilitate or hamper the shear-induced mechanochemical transformation of the DLC interface to graphitic domains with ultra-low shear resistance.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.