Discovery of High-Density Interfacial Nanofilms for Lubrication

Reducing friction and wear under extreme conditions, such as high temperatures, pressures, and sliding velocities, is a grand challenge in tribology. This thesis aims to address these challenges via development of nanoscale lubricants and understanding the fundamental mechanisms enabling superlubricity in these demanding environments. Utilizing high-throughput atomistic simulations and computational analyses, we uncover atomic-scale phase transformations leading to the formation of interfacial high-density lubricating phases. Our research addresses key tribological objectives, including mitigating challenges posed by extreme conditions and identifying the atomic-scale mechanisms behind on-demand lubricant formation. This thesis consists of several chapters that delve into various aspects of extreme-condition lubrication. The initial chapters focus on reducing friction across diverse surfaces using nanoscale blends engineered to diminish friction by considering the crucial connection between surface adhesion and friction. We also examine the structural deformations of single and multi-layer graphene which have experimentally shown a lot of promise as a nanoscale lubricant for stainless steel surfaces. These chapters benchmark our atomistic simulations approach and lay the foundation for further inquiries into the lubricating properties of various nanoscale materials. We next explore cryogenic lubrication by investigating the formation of high-density ice phases and examine their tribological characteristics, providing insights into realization of superlubricity in cryogenic environments. Detailed characterization of the interfacial structural and dynamical properties traces the origin of superlow friction to the enhanced mobilities and dramatically lowered adhesion energies. We also present a detailed dynamic phase diagram for ice, which highlights the regime of applied load, temperature, and shearing velocity where superlubricity is achieved. Building on our success with high-density ice-based lubricants, we aim to explore similarities in material properties among tetrahedral solids to discover high-density amorphous/liquid lubricants for high-temperature, high-load applications. We construct dynamic phase diagrams for Silicon and Germanium, revealing the capability to achieve exceptionally low friction across a wide range of temperatures and applied loads through operando phase transformations into high-density amorphous phases. Our research emphasizes the intriguing phase transformations occurring within tetrahedral coatings during shearing, which can be harnessed to enhance lubrication. The universality of high-density tetrahedral lubricant phases holds great promise for substantial reductions in friction and wear under diverse extreme tribological conditions. Finally, we explore prospective avenues for the design and development of novel high-density amorphous lubricants, specifically tailored for extreme conditions. Our research work introduces the exciting potential of integrating various single and multi-elemental tetrahedral coatings to tackle lubrication challenges under extreme conditions by leveraging in-situ phase transformations during shearing processes.