Self-lubricating nanoscaled metal matrix composites

Friction and wear are ubiquitous phenomena in mechanical components subjected to relative motion. The usual approach to reduce friction and wear has been for decades the use of fluid lubricants. The increasing complexity of mechanical systems and their progressively demanding operational environ- ments require new engineering solutions for their proper functioning and extended duty life. For exam- ple, the application of fluid lubricants has been always severely restricted by the environmental condi- tions during operation, resulting mainly in their deployment in applications at, or near room temperature, so as to diminish their degradation. Furthermore, another challenge of fluid lubricants is the replenish- ment during operation, resulting sometimes in maintenance stops that affect the smooth operation. An alternative to these drawbacks is the use of solid lubricants in self-lubricating systems, since it over- comes the most critical issues in a straightforward way. This approach, though already explored in the literature, still has a wide span of open questions that are critical for their extensive application. The first concerns the type of lubrication mechanism that is most suitable. Current solid lubricants present two main lubrication modes, being strictly different from each other. In layered lubricants, the mechanism is based on the interfacial shear and in fibre-like lubricants, the mechanism is a mix of rolling and glid- ing. The second open question is related to the integration of the lubricant to the containing technical metal. In this case, chemical and physical reactivity between both phases has to be explored for each particular system. Finally, the third main question lays on the possibility of finding an all-rounder, which might be able to operate in the most diverse and extreme conditions, without being significantly degraded and maintaining the required lubricity. This project aims at providing a first integral and thorough analysis of self-lubricating composites by combining an innovative manufacturing technology (high pressure torsion) and advanced microstruc- tural and chemical characterization techniques. The chosen matrix materials are Ni-based superalloys, which find their application niche in extreme environments like, for example, turbine blades. As solid lubricants, traditional layered materials will be tested (graphite, MoS2 and WS2) and contrasted to novel solid lubricants (carbon nanotubes and graphene) that lack the usual operational limits observed in the former. After manufacturing, the composites will be extensively characterized before and after being subjected to sliding conditions in diverse environments (temperature and humidity). The main objective of the project is to obtain a self-lubricating composite that may function in a broad set of conditions.

The project "Self-lubricating nanoscaled metal matrix composites" centers on the development and analysis of self-lubricating metal matrix composites designed for high-performance components operating under demanding conditions. These composites, which incorporate solid lubricants, are specifically tailored for environments where conventional liquid lubricants are ineffective or fail, such as high temperatures, heavy loads, and oxidative atmospheres. The primary goal was to create a robust material concept that combines exceptional mechanical strength and microstructural stability with reliable lubrication functionality. The Ni-based superalloy Inconel 718 was chosen as the matrix material due to its proven performance in high-temperature applications, offering excellent thermomechanical properties and corrosion resistance. Solid lubricant reinforcements investigated included traditional layered materials (graphite, MoS, WS) as well as advanced nanostructured carbon materials like carbon nanotubes (CNTs) and graphene nanopowder. A key innovation in the project was the use of an integrated powder processing approach, combining colloidal mixing (CM) to enhance particle dispersion with high-pressure torsion (HPT) for material consolidation and microstructure refinement. This method enables full densification at room temperature while producing nanocrystalline structures that significantly enhance mechanical properties. Together, CM and HPT facilitated the production of nanostructured Inconel 718 composites with optimized particle distribution, ensuring consistent mechanical and tribological performance. Mechanical testing demonstrated remarkable strength levels. Nanocrystalline Inconel 718 achieved ultimate tensile strengths exceeding 2.1 GPa, while graphite-reinforced composites reached values as high as ~2.4 GPa, all while maintaining ductile fracture behavior. High-temperature studies, conducted using in-situ X-ray diffraction and complementary techniques, revealed complex temperature-dependent processes. In systems reinforced with nanocarbon materials, carbide formation (e.g., NbC and Cr-rich M23C6) was observed at elevated temperatures, effectively suppressing -phase formation, recovery, recrystallization, and thermal expansion. Additionally, the composites exhibited exceptional thermal microstructural stability, with the nanocrystalline state remaining intact up to 700 C after short-term annealing and grain sizes staying below 500 nm even after prolonged exposure at 900 C. Tribological evaluations highlighted the critical role of lubricant type. Conventional solid lubricants showed limited self-lubrication in Inconel 718, whereas CNTs achieved a five-fold reduction in friction and mild wear, likely due to the formation of a stable lubricating layer. These findings establish a solid foundation for the development of durable, low-maintenance components suitable for aerospace, energy systems, and high-temperature engineering applications.