Interações de van der Waals na superlubricidade de filmes nanoestruturados e hidrogenado

Abstract

Friction is originated by energy dissipation events owing to the lost work of non-conservative forces. The nanotribological mechanisms through which this dissipation can occur include molecular deformation and thermal, phononic, electronic (van der Waals) and magnetic effects. Despite its fundamental importance, these physic-chemical mechanisms that govern friction are poorly understood. Although the literature provides a wealth of information on the state of ultra-low friction, termed structural superlubricity, some associated predictions could only be confirmed quantitatively in a few systems under specific conditions. Carbon-based nanostructured systems, such as thin film of hydrogenated amorphous carbon (a-C:H), have a unique combination of properties with technological-industrial potential for mechanical and electromechanical applications. In this thesis we try to understand the role of the electric interactions and the mechanisms involved in the contact mechanics at atomic-molecular level (fundamental laws) of nanostructured films. Here, we report that the surface potential and frictional forces arising at the sliding interface are proportional to the hydrogen contents of the outermost nanostructured surface atomic layers of a-C:H thin films in the without wear regime. The electromagnetic fluctuations of friction signal with different [H]/[C] contents by nanoindentation followed unidirectional sliding (NUS) and lateral or friction force microscopy (LFM/FFM) depend of the molecular deformation and polarizability, respectively. A higher hydrogen-to-carbon ratio reduces the surface potential, directly affecting frictional forces by a less effective long-term interaction. The superlubricity structural is attributed to a lower polarizability at the outermost nanostructured layer of a-C:H thin films due to a higher hydrogen density, which weakens the atomic-molecular interactions (van der Waals forces), in particular London dispersion forces. From the topological point of view, it is proposed the formation of multipolar nanodomains by hydrogen on the surface of a-C:H to achieve superlubricity. The understanding of damping mechanisms and the control of almost frictionless systems aim to assist in the elaboration of new strategies for applications in micro and nanoelectromechanical devices, such as the reduction of environmental costs and impacts to energy efficiency issues on a global scale.