ICMCTF 2022 Session E1-2-WeM: Friction, Wear, Lubrication Effects, and Modeling II

Typically, liquid lubricants are introduced between rubbing surfaces in machine elements and mechanical systems, thus minimizing friction and wear. Diminishing oil resources, the need for ever lower frictional losses, as well as higher demands on the lubricants in terms of resistance against extreme conditions such as high temperatures or low environmental pressures push liquid lubricants to their limits. Therefore, focus turns to solid lubricants. Two-dimensional materials with graphene-like structure have gained remarkable attention, because they have demonstrated excellent tribological properties, even when applying only one or a few atomic layers of the material to the contact zone. There are even several studies, which reported friction coefficients (COFs) below 0.01 and, therefore, superlubricious performance. However, the mechanisms of friction reduction and the influence of the materials’ structural and mechanical properties on the latter are still not well understood. One of the newest members of the class of 2D materials are so-called MXene, which were discovered in 2011. MXenes, which are 2D transition metal carbides, nitrides, and carbonitrides, with layers a few atoms thick and, thus, represent an entire class of 2D materials. Single flakes of MXene can be described by the chemical formula M_n+1X_nT_x (n = 1 to 4). MXenes offer an extreme versatility in terms of composition, layer thickness, and surface terminations. This makes them ideal model materials to study the influence of theses parameters on tribological behaviour. Despite a still small number of publications on the tribological prosperities of MXenes the field is rapidly growing. First studies have already shown very promising results in terms of wear resistance and friction reduction, even reaching the superlubric regime.

Manganese Phosphate (MnPh) coatings are nowadays used in rolling bearings applications due to their advantages such as wear resistance, corrosion resistance, improved fatigue life, and anti-fretting performance.

In this work, MnPh coatings with a thickness of about 5 µm were deposited by a chemical conversion process. AISI 52100 steel substrates were placed in a phosphoric acid bath, where an acid-metal reaction took place locally depleting the hydronium (H₃O⁺) ions, raising the pH, and causing a manganese phosphate dissolved salt to fall out of the solution and be precipitated onto the steel surface. Analysis of the surface microstructure and composition of the coatings by X-ray diffraction (XRD), Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Electron Dispersion Spectroscopy (EDS) has revealed a polycrystalline coating with prismatic-shaped crystals, and about 20% content of Mn. The nanomechanical properties, studied by nanoindentation, exhibit a surface with hardness H_IT ~ 1 GPa and Young’s modulus E_IT ~ 50 GPa. A nanoindentation statistical method was used to obtain H_IT and E_IT value frequencies over a large area of the coating. We demonstrate that the nanoindentation frequency peaks can be correlated to the coating crystalline orientation revealed by XRD.

Friction and wear-related failures remain the greatest problems in today’s moving mechanical components, from microelectromechanical devices to automotive assemblies and to biological systems. The critical need to reduce and eliminate the tribological failures constitutes the necessity for continuous search of novel materials and lubrication solutions. In this presentation, we overview recent advances in establishing the fundamental understanding of materials interactions at sliding interfaces and use this knowledge as a guide to developing nanomaterials solutions that enhance reliability and efficiency of tribological systems. We evaluate tribological performance of carbon nanomaterials and demonstrate realization of superlubricity regime at macroscale in carbon-based systems. To extend the lifetime of the tribological materials, we demonstrate tribochemically-driven self-replenishment of carbon-based materials inside the contact interfaces, thus, in addition to the superlubricity, enabling a zero-wear sliding regime.

Overall, the findings have not only allowed us to solve some long-standing puzzles, but could also open a new avenue for the development of new concepts and design strategies for next generation of tribologically efficient materials systems.

This work presents the development of self-lubricating metallic alloys for laser deposition processes. Laser deposition processes such as laser metal deposition or direct energy deposition are additive manufacturing techniques that offer a great flexibility and efficiency compared to traditional subtractive manufacturing processes. However, the extreme thermal conditions during deposition and the rapid cooling times pose great challenges in the alloy design. This work illustrates these challenges using metals, such as iron and nickel-base alloys, incorporating lubricious soft metals and metal sulfides.

The microstructure and phase composition of the deposited self-lubricating alloys are characterized using X-ray diffraction, scanning and transmission electron microscopy, showing the importance of having the soft metal as single phase without forming intermetallic compounds or being in solid solution. Additionally, the role on friction of the metal sulfide composition and stoichiometry formed during the laser deposition process is discussed. Afterwards, their friction and wear performance are evaluated using high temperature tribological tests in air and vacuum.

The results reveal that the self-lubricating laser deposited alloys are able to control friction from room temperature to 600 °C in ambient air and at least until 300 °C in vacuum. In ambient air, the friction reduction mechanism is determined by the soft metal and the metal sulfides. At higher temperatures, the contribution of the soft metal diminishes due to oxidation so that the role of metal sulfides to self-lubrication is dominant. In vacuum conditions, the laser deposited self-lubricating alloys can effectively reduce friction down to 0.25 without the aid of an additional lubricant against martensitic stainless steel at 300 °C. This overall tribological performance makes the presented self-lubricating alloys potential candidates for high temperature forming and space applications.

Nanocomposite coatings consisting of an amorphous carbon matrix (a-C) with nanocrystallites of transition metal dichalcogenides (TMDs) embedded in it can provide protection against friction and wear in different operating environments, from vacuum to humid ambient air conditions. Magnetron sputtered W-S-C coatings are part of this group of coatings which shows good adaptive tribological behaviour. One potential issue is the running-in behaviour of these coatings as the formation of lubricious tribofilms is crucial for a good tribological response. These tribofilms are most often rich in WS₂, a lubricious compound that is an exceptional lubricant in inert environments (dry N₂, vacuum). A way to improve the running-in behavior would be to change the chemical composition of the coatings across their thickness. Therefore, we deposited a W-S-C coating with varied carbon content across its thickness. The bottom layers of the film were rich in carbon which improved the hardness and thus the load-bearing capacity. The carbon content was gradually reduced towards the top-most layers and finally, a pure WS₂ layer is deposited. In comparison, coatings with a constant composition of ~30 at. % and 50 at. % of carbon were also deposited. The characterization of the coating included scanning electron microscopy with wavelength dispersive spectroscopy for morphological and a study of the chemical composition. X-ray diffraction for structural analysis. Nanoindentation and scratch testing for hardness and adhesion studies respectively. Finally, tribological studies were performed in ambient conditions, dry N₂ environment and at elevated temperature. The correlation between the tribological results and the physico-chemical properties of the coatings revealed that every coating has an advantageous sliding environment.