ICMCTF 2024 Session PP4-2-WeA: Deposition Technologies for Carbon-based Coatings II

PVD based carbon coatings show superior electrochemical resistance enabling durable coatings for metallic bipolar plates in PEM fuel cells etc. High durability and high conductivity (low interfacial contact resistance – ICR) of bipolar plates are essential, nevertheless only a cost-efficient coating method will have the chance for industrialization. Therefore, we only investigated PVD methods which allow scaling of the bipolar plate coating volume to MW and GW powers per year, which are electron-beam based PVD, as the method for highest productivity and deposition rate, and plasma based PVD which is high power magnetron sputtering and (pulsed) cathodic laser arc in our case. The electrical conductivity or ICR and corrosion performance of all those carbon coatings have been optimized to compete with gold coatings at fuel cell operation voltage. Anyway, the limits of the electrochemical corrosion performance of different carbon coatings at high electrochemical potentials occurring for split seconds on the cathode side of the bipolar plate during start/stop of a fuel cell can be crucial for the overall layer stack performance and durability. Depending on the deposition method and parameters, the properties of the carbon layer stack can highly differ from each other which leads to a different corrosion performance at such a high electrochemical potential.

Negatively charged nitrogen vacancy centers (NV^-) in diamond have unique properties making them excellent candidates for nanoscale magnetic and electric field sensors, as quantum bits as well as other applications. As quantum sensors they promise comparable sensitivity at room temperature to commonly used magnetic field sensors that must be cooled to liquid helium temperature, for example. NV centers may occur as neutral NV⁰ or negatively charged NV^-, however, it is only NV^- that exhibits this magnetic sensitivity. With growing interest in the use of quantum sensor it is necessary to ensure predictable and reliable performance which requires uniform NV^- formation. However, the stability of NV^- centers is strongly influenced by the surface functionalization of diamond particles. Both nitridation and fluorination have been shown to help stabilize NV^- centers especially for shallow NV centers which provide greater sensitivity. Uniform functionalization of particles in batches is necessary for cost effective production. This has been investigated using an atmospheric pressure plasma system. Results of treatment in fluorine and nitrogen-based plasmas will be reviewed. Fluorescence spectroscopy was used as a means to determine the presence of NV⁰ and NV^- following treatment and the impact of the treatment will be discussed.

The advent of High-Power Impulse Magnetron Sputtering (HiPIMS) in the last two decades opened a new route for magnetron-sputtered coating. In HiPIMS, a large fraction of the sputtered atoms is ionized due to the several orders of magnitude higher plasma densities than in DCMS. Although HiPIMS has been successfully implemented for many metals, it is much less effective for DLC coatings deposition since C has significantly higher ionization energy and lower ionization cross-section than typical metals.

In previous works, the authors have shown that adding Ne to the plasma significantly improves the properties of DLC films deposited by Deep Oscillation Magnetron Sputtering (DOMS), a variant of HiPIMS. Replacing half of the Ar process gas by Ne allowed for the deposition of denser films, with hardness up to 25 GPa, while still retaining a low CoF. Furthermore, the specific wear rate (SWR) of the DLC films decreased by close to 50 %, both in linearand reciprocating slidingagainst steel counterparts, being comparable with state-of-the-art hard DLC deposited by CAD and PLD.

The main objectives of the present work were to quantitively evaluate the sp³ content in DLC films deposited by DOMS, to identify the relevant film formation mechanisms and to better understand process-properties relationships resulting from the addition of Ne to the plasma. DLC films were deposited by DOMS both in pure Ar and in mixed Ar + Ne plasmas. Quantitative evaluation of the sp³ content in the films was performed by Electron Energy Loss Spectroscopy (EELS) and Near Edge X-ray Absorption Fine Structure (NEXAFS). Additionally, for comparison purposes, Raman spectroscopy was also used for qualitative assessments of the film's sp³ content. Although hydrogen was not purposefully incorporated in the DLC films deposited in this work, the hydrogen content was measured by Elastic Recoil Detection Analysis (ERDA) in combination with Rutherford Backscattering Analysis (RBS). T. The surface morphology of the DLC films was characterized by Scanning Electron Microscopy (SEM) while their microstructure was investigated by High-Resolution Transmission Electron Microscopy (HRTEM).