Authors: S. Frangini | Livia Della Seta | D. Stocchi | Alessandra Bellucci
Source: ECS Meeting Abstracts (article, 2025-11-24)
Field: Materials Science — Materials Chemistry

Si- and Si/N-Doped DLC Films as a New Coating Approach for Solid Oxide Interconnect Protection Interconnects (ICs) are key multifunctional components of Solid Oxide Cell (SOC) stacks, which serve primarily as physical separators between two adjacent single cells, but also serve as current collector, for reactant gas transport to anode and cathode compartments and for the mechanical stability of the entire stack of cells. ICs consist of stamped and welded thin metallic sheets characterized by high geometrical complexity and typically made of commercial or special-designed ferritic stainless steel grades with a Cr content varying between 18 and 22 weight percentage. Due to the high operating temperatures involved (600-750°C), chromia evaporation and oxide scale growth are two well-known degradation modes that critically limit IC performance and lifetime [1]. For this reason, it is nowadays a common industrial practice to use coated ICs protected by thin layers of conductive composite spinel- or perovskite structured ceramic oxides. Although effective, spinel and perovskite coatings are not totally impervious to outward Cr diffusion from steel substrate, which means that some reaction layers will form with the time at steel/coating interface leading to progressive loss of surface electrical conductivity and reduced coating mechanical stability [1,2]. Moreover, these ceramic coatings are usually prepared with hazardous or costly transition metals such as Ni, Co, Cu and rare earth elements, which may arise questions about the effective environmental sustainability of the current SOC interconnect technology. Diamond-Like Carbon (DLC) is a special kind of carbon-based material characterized by a metastable and amorphous structure [3]. DLCs show some unique mechanical and chemical properties that make them ideal materials in many demanding industrial applications. For instance, thanks to their high chemical inertness and corrosion resistance even in extremely aggressive water environments, very dense and smooth DLC coatings are nowadays commonly applied in cutting tools, biomedical, oil&amp;gas and marine sectors, among others [4]. However, there are also some limitations for a their wider industrial application. As DLC coatings are produced by energetic deposition techniques, this results in the formation of coatings with high residual internal stress that limit the deposition of DLCs to a few micrometer-thickness and often require an interlayer bonding approach. Thermal stability and oxidation behavior are other two important issues, since DLC coatings degrade rapidly in air above 250-300°C due to graphitization (transition from C-C sp 3 to sp 2 bonds) and oxidation reactions. Several studies have demonstrated that some of these issues can be alleviated by doping DLC coatings with non-metal or metal elements [5]. In particular, there are several reports indicating that doping with non-metal elements such as Si and/or N can significantly improve adhesion, thermal stability and also modify electrical properties [6,7]. Si-doping promotes the formation of Si-C sp 3 bonds inhibiting thus the graphitization process and resulting in higher thermal stability. According to some authors, co-doping Si and N into the DLC structure has been shown to further extend thermal stability at extremely high temperatures up to the 800-1100 °C range [7]. The effect of N doping on the electrical properties of DLC films has been also the object of several studies. For instance, high electrical performance has been recently reported for N-doped DLC coatings applied to low-temperature fuel cell bipolar plates [8,9]. Thermal expansion compatibility with the substrate is another important property to consider to skip delamination or flake off of coatings during high temperature applications. Thermal expansion coefficient of DLC coatings may vary in the range 2-9 ppm being its exact value highly dependent from the sp 2 and sp 3 C-C bond proportions existing in the carbon structure [4]. This suggests that a good thermal compatibility with ferritic stainless steel substrates such as those used for SOC IC fabrication is achievable by fine tuning of the sp 2 /sp 3 bond structure. Based on the above considerations, the purpose of this work was therefore to explore the possibility of using doped DLC coatings for protection of SOC interconnects. This study was motivated by two main reasons, namely i) DLC coatings make little or no use of critical raw materials and ii) plasma-assisted CVD methods often applied for DLC deposition may offer good environmental compatibility thanks to their low processing temperatures [10]. Thus, it is believed that DLC coatings, especially if produced with plasma-based CVD deposition methods could offer a more sustainable and environmental advantage over the more traditional oxide ceramic coatings. As an interconnect must have a good surface electrical conductivity, various combinations of Si and Si/N doping concentration were studied to optimize mechanical and functional properties of the doped DLC coatings. Due to the exploratory nature of this work, a commercial austenitic 316L stainless steel was used as substrate in place of a more obvious IC ferritic steel for its availability and easy machinability to produce parallelepiped shaped test samples (size: 50x10x1 mm). A Plasma-Assisted Chemical Vapor Deposition (PA-CVD) process was used for the deposition of hydrogenated amorphous DLC structures (a-C:H carbon phases). Thick Si- and Si/N-doped DLC coatings were deposited with thickness ranged from 4 to about 8µm. Doping level was in the range 10-30 at% for Si and in the range 0-10 at% for N. Properties were investigated on as-received and post-annealed samples. Thermal stability studies were conducted by exposing the DLC coated steel samples in air at various temperatures in the range 200-700°C for 24 hours and measuring the sample weight after each thermal exposition. Chemical composition and structure of the coatings were analyzed by SEM/EDX and ATR-FTIR spectroscopy analysis, respectively. Standard four-probe methods were used for electrical resistance measurements. Fig.1 shows the thermal stability of various as-received doped DLCs as compared to a standard undoped DLC coating. In general, Si and Si/N doping was seen to significantly improve the DLC thermal stability well above the 250-300°C, which is the maximum operating temperature of undoped DLC coatings. However, the effect of doping level on thermal stability showed not straightforward results. Thus, the behavior of the DLC samples containing the highest levels of Si doping (DLC/30Si and DLC/27Si+5N) was somewhat below expectations. A significant weight loss of the DLC/30Si sample was observed at 500°C as consequence of massive coating flake off probably due to residual internal stresses. On the other hand, the DLC/27Si+5N sample did not flake off, but its diamond-like characteristics were completely lost at around 500°C due to strong carbon evaporation. Results obtained with lower Si doping levels were easier to rationalize. Thus, raising the Si doping level from 10 to 16 at % was observed to dramatically increase the DLC thermal stability. The DLC/10Si started to degrade rapidly at 350°C by evaporation, whereas the DLC/16Si sample showed only a minimal weight loss starting at around 400°C keeping its distinct diamond-like characteristics up to 650°C. Some evident coating loss by evaporation was observed starting from 700°C. However, Fig.1 shows only minimal weight change of the sample at 700°C as compared to 650°C probably as carbon evaporation was compensated by oxygen adsorption from the air. Fig. 2 shows the air oxidation kinetics of the as-received DLC/16Si at 700°C in comparison to a bare 316L steel sample. It could be observed that in spite of some loss of diamond-like features the DLC/16Si sample promoted a dramatic improvement of the steel oxidation resistance. Further, no sign of chromium evaporation was observed from the surface of the DLC-coated steel sample. These preliminary results suggest that, under proper synthesis and design conditions, Si or Si/N doped DLC films could be a promising and eco-friendly approach for a very stable SOC interconnect protection. More detailed results including annealing effects on the DLC coating properties will be presented during the Conference. Bibliography [1] SJ Mc Phail et al, “Addressing planar solid oxide cell degradation mechanisms: A critical review of selected components”, Electrochem. Sci. Adv, 2022, 2(5), e2100024; [2] J. Mao et al, “Progress in metal corrosion mechanism and protective coating technology for interconnect and metal support of solid oxide cells”, Renew. Sust. Energ. Rev., 2023, 185, 113597; [3] J. Robertson, “Diamond-like amorphous carbon”, Mater. Sci. Eng. R, 2002, 37, 129-281; [4] D.K. Rajak et al. “Diamond-like carbon (DLC) coatings: classification, properties and applications”, Appl. Sci., 2021, 11(10), 4445; [5] Q. Zheng et al, “High-t Chromia ; Coating ; Oxide ; Ceramic ; Anode ; Solid oxide fuel cell ; Diffusion barrier ; Cathode ; Spinel ; Amorphous solid Diamond and Carbon-based Materials Research ; Nanotechnology research and applications ; Research, Science, and Academia