Mechanical and Tribological Behavior of Carbon Nanotube Solid Lubricants: From Structural Design to <i>in Situ</i> Characterization

Robust solid lubrication systems are crucial to the reliability, efficiency, and lifetime of advanced mechanical assemblies operating in demanding environments. Carbon nanotubes (CNTs) have attracted extensive attention as emerging solid lubricants due to their exceptional mechanical strength, high aspect ratio, thermal stability, and intrinsic self-lubricating capability. However, the coupled relationship between mechanical reinforcement and the tribological response in CNT-based coatings remains insufficiently understood, limiting their optimized deployment across metal, polymer, and ceramic substrates. This review provides a comprehensive assessment of CNT solid lubricants with a particular emphasis on the synergistic interactions among structural characteristics, mechanical behavior, and frictional performance. The discussion encompasses the roles of CNT geometry, chirality, dispersion state, defect density, and interfacial bonding in governing deformation, crack propagation, load transfer, lubrication modes, and wear resistance across different composite systems. Furthermore, advances in <i>in situ</i> scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are summarized, highlighting their ability to reveal real-time deformation, interfacial evolution, nanoscale contact mechanics, superlubricity transitions, and fracture mechanisms. The review compares the dominant strengthening and lubrication mechanisms observed in different material platforms, including grain refinement, crack bridging, Hall-Petch strengthening, Orowan looping, interlayer sliding, rolling dynamics, interfacial filling, and transfer film formation. Finally, key challenges and outlooks are outlined, including the need for controlled interfacial modulation, scalable dispersion strategies, stimulus-responsive lubrication, and multiscale mechanistic integration under extreme operating conditions. This review establishes a cohesive framework linking CNT structures, mechanical properties, tribological functionality, and advanced characterization, providing guidance for the rational design of next-generation CNT-based solid lubrication systems.