PacSurf2022 Session TF-TuE: Next-generation Protective Coatings and Tribological Applications

Materials exposed to harsh environments continue to face ever-increasing technological, environmental, and economic challenges. Consequently, the field of coating and surface engineering (CSE) technologies has been extremely active, addressing numerous challenges related to the increasingly stringent requirements for the performance of coatings and surface engineering solutions. This frequently includes a controlled combination of several functional properties and long-term environmental stability and durability (multifunctional character of the coatings).

These requirements call for novel thin film fabrication processes and new materials with a synergistic combination of tailored characteristics, while solutions must fit into a sustainable (green) development approach including new “clean” (environmentally friendly) fabrication technologies, and life cycle compatible with economic and environmental constraints.

Protection against materials deterioration is particularly important in the context of aircraft components as operation conditions can vary very widely from severe erosive wear to exposure to hot oxidative gases, extreme thermal loads, or even instant icing of critical surfaces. The reduction of emissions through better engine efficiency can be achieved by using lighter structural materials while preserving the integrity of gas path aerodynamic characteristics; this can be accomplished by surface engineering and application of coatings resulting in the reduction of friction, erosion, wear, and corrosion.

In response, our Functional Coating and Surface Engineering Laboratory (FCSEL, www.polymtl.ca/larfis) has proposed, a comprehensive approach to surface engineering problems. This is based on a simultaneous action of the following key elements: (i) in-depth understanding of the technological problem, (ii) availability of the appropriate metrology tools (testing methods) that allow one to seek appropriate solutions in terms of (iii) nanostructured coating materials, and (iv) their integration in specific coating architectures while applying (v) suitable fabrication processes.

In this presentation, the global approach described above will be illustrated by examples related to the development of protective coating systems against solid particle erosion, high-temperature oxidation, and ice accumulation, as well as our work on the next-generation low-emissivity thermal barrier coatings.

One of the most promising solutions for long term lubrication during dry machining operations are self-lubricant coatings. However, the main problem in these kind of coatings is the swift diffusion of the lubricious element and consequent loss of lubricant properties after short periods of time. By combining the mechanically sound TiSiN coating system with its reported anti-diffusion properties of the amorphous SiN a solution to halt the diffusion of the lubricious element can be found. In this work, triple layer coatings constituted by a Ag doped TiN layer sandwiched between two layers of either TiN or TiSiN were deposited by HiPIMS working in deep oscillation magnetron mode (DOMS) with the objective of studying the diffusion of Ag (the lubricious element) in these two different types of matrices (TiN and TiSiN) when exposed to high temperatures. After annealing treatment of the samples at 600ºC and 800ºC for 2 hours, RBS and TEM analyses allowed to observe that the morphology of the coatings had a big impact in the way Ag diffusion occurred. Open columnar structures facilitate Ag movement through surface diffusion of the columns. While, at first, it seemed that the TiN matrix was a better barrier to the diffusion of the silver, the incorporation of Si₃N₄ cap layers allowed to disregard the effect of structure and confirm that with the right morphology, the TiSiN matrix can completely halt the diffusion, thus demonstrating its ability to be applied in tools for dry machining operations.

Understanding wear phenomena is essential to prevent fatal accidents caused by the destruction of machine systems due to wear. Wear amount is usually governed by mechanical factors, however sometimes chemical factors also strongly affect the wear. For example, a previous experimental study has shown that when water is present at the diamond-like carbon (DLC)/Fe sliding interface wear amount increases due to chemical reactions induced at the sliding interface [1]. Therefore, it is important to clarify the atomic-scale wear mechanism in which mechanical and chemical actions are involved.However, the observation of the atomic-scale wear phenomena at the sliding interface from experiments is difficult.In this study, we successfully revealed the tribochemical reactions and their effects on the atomic-scale wear mechanism at the sliding interface of DLC and Fe pair, which is typically used in automobile engines, in the presence of H₂O and O₂by using reactive MD simulation.

The sliding simulation model is shown in a supplementary document (SD) Fig.1. To investigate the effects of H₂O and O₂, we prepared the following three models: H₂O model, O₂ model and H₂O+O₂ model. H₂O molecules, O₂ molecules and both H₂O and O₂ molecules were placed at the DLC/Fe sliding interface of H₂O model, O₂ model and H₂O+O₂ model, respectively. In the sliding simulations, the DLC substrate on the Fe substrate was slid along the x-direction at a speed of 100 m/s with the normal load of 1 GPa.

In all sliding simulation models, when the Fe and DLC substrates came into contact with each other, the Fe atoms were scraped off and adhered to the DLC surface with a formation of Fe-C bonds. The amount of atomic-scale wear was smaller in the following order, H₂O+O₂ model < O₂ model < H₂O model. In H₂O model, H₂O molecules were adsorbed on the surface to prevent adhesion (SD Fig.2(a)). However, they were ejected from the contact surface when the high contact pressure was applied, allowing the direct contact. In contrast, in O₂ model O₂ molecules reacted with the Fe surface, forming chemically inert layer, thereby prevented atomic-scale adhesive wear (SD Fig.2(b)) [2]. Moreover, in H₂O+O₂ model, in addition to the above roles of H₂O and O₂, the reaction of H₂O with the oxide layer promotes the formation of Fe-O-H groups which passivate nascent Fe surface (SD Fig.2(c)). Thus, the results indicate that H₂O and O₂ play the different role to reduce atomic-scale wear. Moreover, when H₂O and O₂ coexist at the sliding interface they play a collaborative role to reduce atomic-scale wear.

[1] A. Alazizi, et al., Langmuir, 32, 1996 (2016).

[2] M. Yokoi, M. Kubo, et al., J. Comput. Chem. Jpn. in press

High entropy material provides an unlimited compositional space that allows an unparallel possibilities for tailoring the electronic structure favorable for catalysts reaction.We report herein two advanced high entropy electrocatalysts having earth-abundant metals for oxygen evolution reaction (OER) in water splitting.The first one is a high entropy perovskite oxide (HEPO) and the second one is high entropy sulfide (HES).The B-site lattice in the HEPO consists of 5 consecutive first-row transition metals, including Cr, Mn, Fe, Co, and Ni, which have quite similar ionic radii.Equimolar and five non-equimolar HEPO electrocatalysts have been studied for their OER electrocatalytic performances.Optimized HEPO having outstanding OER activity is demonstrated.The metals used in the HES are Fe, Ni, Co, Cr, and X where X = Mn, Cu, Zn, or Al.We show that the obtained sulfate-containing FeNiCoCrMnS₂ exhibits superior OER activity with exceptionally low overpotential of 199, 246, 285, and 308 mV at current densities of 10, 100, 500 and 1000 mA cm^-2, respectively.The electrocatalyst yields exceptional stability after 12000 cycles and 55 h of durability even at a high current density of 500 mA cm^-2.Various in-situ and ex-situ analyses were used to investigate the self-reconstruction of the sulfides during the oxygen evolution reaction (OER) for the first time.Density function theory calculation is in a good agreement with the experimental result.The result demonstrates a viable strategy that leads to the development of new catalyst materials with excellent OER performance.It also opens a new avenue to explore novel, outstanding high entropy materials for various applications.