Diffusion control reducing friction of nanocomposites

Recent achievements in the development of nanostructured thin films with outstanding mechanical properties and thermal stability allow to protect surfaces of working tools to operate at high speeds with improved lifetime. Challenging applications where tools operate without lubricants under extreme conditions such as high loads, high temperatures and aggressive oxidative and corrosive environments, however, require development of new class of materials with even better properties. One of the approaches to enhance performance of tools for environmentally friendly and cost effective dry machining, is based on an incorporation of lubricious agents such as vanadium, which tend to form Magnéli-phase oxides during operation. Easy slipping shear planes of such oxides made them lubricious under pressure and temperature, subsequently reducing friction and wear. However, it is very challenging to release the lubricious agents in a controllable way. New approaches have thus to be established to avoid rapid depletion of the agents during their spontaneous diffusion towards film surface. We propose here atomistic simulation-based design concept of a new class of nanocomposite thin films with controlled long-term release of solid lubricants. This concept relies on the encapsulation of nanograins of lubricious metals such as vanadium and/or silver by quasi-amorphous Si-N matrix, which allows their diffusion and agglomeration at the film surface in tribological contact, formation of lubricious phases and thus reduction of friction and wear. These processes will be simulated by a combination of first-principles density functional theory and molecular dynamic methods to search for an optimum structure and properties of interfaces between individual phases to controllably transport the species to the film surface. The ultimate ambition of the project is to synthesize coatings with simulated structural features, validate their structure and high-load high-temperature tribological performance by advanced characterization methods and propose new concepts to control diffusion in nanoscale materials by sophisticated material design. These general concepts will allow for further development of high- performing self-lubricating coatings for advanced dry machining applications of hard-to-cut materials at high speeds.

Hard nanocomposite coatings are established as an effective protection of tools against wear at temperatures, reaching up to 1000C. The use of lubricants is, however, often required to extend the lifespan of coated tools due to high contact friction, especially when cutting hard-to-process materials. This reliance on lubricants has significant environmental implications, particularly when large quantities are needed. To address this challenge, vanadium is commonly introduced into the coatings to form VOx solid lubricant at temperatures exceeding 700C. Although vanadium in the coating effectively reduces both contact friction and wear, its rapid out-diffusion towards the coating surface during operation significantly hinders the long-term performance. The collaborative project "Diffusion Control of Friction-Reducing Nanocomposite Materials" conducted by the Austrian Montanuniversität Leoben and the Czech Technical University in Prague brought together leading theoretical and experimental researchers to develop new methods to control the sustained release of vanadium from TiSiVN nanocomposite coatings by manipulating their microstructure and composition based on theoretical ab-initio calculations. The research focused primarily on understanding the diffusion mechanisms within TiN crystallites, SiN amorphous matrix and across TiN/V/SiN interphases. Advanced calculations, with a particular emphasis on vanadium transport from the bulk of the TiSiVN coatings to their surface, guided the synthesis of coatings, which in turn exhibited longer operational lifespans during dry cutting compared to tools coated with conventional TiSi(V)N coatings. The complexity of the problems addressed in this project revealed the importance of optimizing calculation routines and employing novel approaches. Machine learning-based calculations of interatomic potentials are one of them. The potential, accuracy and transferability of these advanced methods have been validated through atomistic modelling of amorphous silicon nitride, a mechanically anisotropic component of the TiSi(V)N coating system. The research also significantly contributed to the understanding of processes governing the formation of the solid VOx lubricant, its interaction within the contact zone at various operation temperatures and subsequent reduction of contact friction. Furthermore, a novel type of TiSiN-based coatings with biomimetic architectures has been designed to address the strength-ductility trade-off of generally hard yet brittle TiSiVN coatings to ensure both high mechanical and thermal stability. The concept relies on mimicking the microstructure of nacre ensuring effective crack deflection upon mechanical loading and thus enhanced fracture toughness. Theory-guided development of the TiSiVN-based nanocomposite coatings with complex microstructures and optimized compositions ensuring sluggish vanadium diffusion and thus controlled long-term formation of friction-reducing solid VOx lubricant represents a novel and promising strategy to enhance the lifetimes of coated tools operating in harsh environments at reduced reliance on liquid lubricants. Application of these innovative approaches in the development of protective coatings has the potential to make processes in steel, automotive and aerospace applications more effective and to contribute to sustainable machining practices.