Self-adaptive low-friction coatings

The reduction of friction continues to be a hot-topic in mechanical engineering. Lower friction between mechanical parts in contact diminishes the energy consumption, vibrations, noise, contact temperature, and wear. Among many ways to fight the friction, solid lubricants in the form of thin films deposited by physical vapor deposition methods are widely used in industrial applications. Transition metal dichalcogenides (TMD) are suitable as solid lubricants due to their anisotropic layered structure, where the adjacent lamellae with strong covalent bonding interact through relatively weak van der Waals forces. Pure sputtered TMD films are almost friction-less at ultra-high vacuum sliding conditions; however, the films are sensitive to environmental attack, particularly in the presence of oxygen and water, which limits their mechanical properties and wear resistance. Alloying of TMD with other element has improved their properties, such as adhesion, hardness and load bearing capacity. Nevertheless, the high sensitivity to environmental attacks still remains the main restriction for full the industrial use of TMD-based coatings as self-lubricants. We deposit and analyze the tribological behavior of different TMD coatings alloyed with carbon (WSC, WSeC, MoSC and MoSeC systems) deposited by magnetron sputtering. Three TMD microstructures have been prepared: i) randomly oriented platelets in amorphous carbon matrix, ii) nanograins of TMD in amorphous carbon matrix and iii) mixture of carbide and TMD nanograins embedded into the carbon matrix. Special attention has been paid to the analysis of the frictional and wear mechanisms under different operating conditions, such as contact pressure, air humidity or temperature. Nanoscale analysis of the wear track revealed the formation of a thin tribolayer exclusively consisting of TMD platelets oriented to exhibit the lowest friction. In some cases, the depth reorientation of the originally randomly oriented TMD platelets as a reaction to the sliding process has been observed. Such self-adaptation explains the extremely low friction coefficient together with a high load-bearing capacity; moreover, the films are much less sensitive to environmental conditions compared to pure TMD. The project could be considered as a fundamental research; however, some coatings (W-S-C and Mo-Se-C systems) have been recently tested by several industrial partners with promising results.

High temperature tribology

The group pioneers high temperature tribology of hard protective coatings. Our aim is to simultaneously analyze oxidation resistance, thermal stability and sliding properties of different coatings at elevated temperature. We demonstrated that tribological behavior at elevated temperature cannot be estimated using oxidation tests and room temperature sliding. We observed unexpected failures of hard films at elevated temperature suggestion hot-adhesion problems bringing a new factor to be evaluated in the field of high-temperature coating industry. Our work is well acknowledged by industrial partners - we have published more than 10 papers with coating producers!

Biocompatible coatings

We participate in large industrial project focused on development of coating technology for medical implants, which will be used by national implant producers. From many potential bio-applications, two with the largest market impact have been selected: orthopedic and dental implants. I case of orthopedic implants, we develop and apply chemically stable coating decreasing friction and, particularly, the production of the wear debris and corrosion products, which limits functionality of present non-coated implants. The main issue of dental implants is interaction of implant surface with surrounding tissue. Carbon-based coating with optimized metal content significantly increases surface bioactivity and enhances tissue healing; moreover, the coating provides long-term protection against corrosion attacks. The role of AMG in the project is to test the wear resistance of the films in laboratory scale including knee simulator; the main issue is to optimize the polymer/coating contact.

Interface design of crystalline materials with improved radiation damage

The overall objective of the proposed project is to develop a multiscale predictive modelling framework able to identify suitable strategies for optimising the properties of existing NMMC systems and for identifying new ones with improved radiation damage resistance and mechanical properties, thus providing a suitable guideline to correlated experimental approaches. AMG will prepare specific coating and test their structural and mechanical properties before and after radiation exposure. Experimental results will be used to validate theoretical simulations.

Ab initio modeling

Transition metal dichalcogenides (TMDs) exhibit many exploitable properties, such as variable electronic behaviour - e.g. insulating, metallic or semiconducting - and diverse phase transitions as a function of external parameters. The substantial band gap of semiconducting TMDs suggests high on/off-current ratios are achievable (unlike in e.g. gapless graphene) and so digital electronics based on 2D materials are a possibility. Moreover, with increasingly sophisticated fabrication techniques, low-dimensional MX2 (primarily molybdenum disulphide, MoS2) structures are becoming readily achievable. It is likely that we will observe the emergence of unique properties in these low-dimensional materials, as we have transition metal d-type orbitals in the valence and conduction bands; Mo 4d and S 3p orbitals are presumed to play a decisive role in dictating the properties of the material. With a view to expanding the range of properties, research is not only focused on pure TMDs, but also: doped and alloyed species, thickness and strain effects, surface contaminants/adsorbates and TMD/TMD heterostructures. Their hybridization produces materials of novel functional composition, with the aim of tuning their chemical and electronic properties for optimal performance. Using the state-of-the-art Vienna Ab-initio Simulation Package we are able to study and gain a better understanding of the electronic structure of TMD materials, corroborating experimental findings whilst facilitating the design of new advanced materials.

We are investigating the microscopic origin of the tribological properties of MX2 TMDs. Ab initio based calculations are used to capture the atomic contribution to the macroscopic friction. We combine structural and dynamic information from group theoretical analysis and phonon band structure calculations with the characterisation of the electronic features using non-standard methods. Moreover, we formulated a new lattice dynamics descriptor to disentangle the electro-structural features responsible of the macroscopic friction has been. The outcomes produced a new protocol to engineer the friction at the nanoscale. We are exploiting the formulated protocol to design new tribological materials with enhanced frictional properties.

Classical methods

On the other hand, if the goal is to study not only perfectly crystalline structures (i.e. including dislocations, defects, effects on the edges of truncated systems, etc) and/or to follow the dynamical evolution of nonequilibrium processes (e.g. sliding, heat transfer, etc) for more than few picoseconds, one is obliged to resort to classical techniques, such as atomistic molecular dynamics simulations. The key point of these methods is the empirical force fields (which represent the particle-particle interactions) employed for the calculation of the atomic forces during the simulations. For TMD materials, several force fields are present, although most force fields are still very system specific and contain limitations in reliability and applications towards realistic setups.

Classical force fields could then be used for simulating experimentally observed features of TMD materials, like for example the structural rearrangement that occurs during the sliding under load of as-deposited (i.e. essentially amorphous) material to form layered structures. Moreover, with the everyday increasing computational power, we are getting closer to match theoretical simulations to experimental studies. The ultimate aim is to tackle the problem of understanding the mechanisms behind frictional behavior in order to improve existing near-frictionless TMD-based materials and the design of new materials.

Activities in collaboration with CAP

AMG researchers are also active members of the Centre of Advanced Photovoltaics (CAP), which was established in 2016 to bring together experts in the field of photovoltaics from Czech Technical University in Prague and overseas institutions. CAP provides a unique opportunity to connect researchers from diverse fields including materials engineering, physical sciences and architecture. Simulations within the project are focused on a bottom-up approach of novel materials and interfaces using advanced atomistic simulations. Firstly, we explore 2D materials based on TMDs, focusing on their structure and corresponding electronic properties, progressing to the investigation of the effects of e.g. selective doping. Ultimately, we will use our results for the design of optimized van der Waals (hetero)structures for use in PV devices. An alternative line of research deals with the optimization of an interface between organic-inorganic hybrid perovskite materials with oxide scaffolds.

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