## 2 Materials and Methods  
### Coating Process  
The AISI 1018 steel substrates (0.60-0.90% Mn, \(\leq\)0.04%, \(\leq\)0.05% S, 0.15-0.20% C, 98.05% Fe) with a size of 25 mm \(\times\) 8 mm \(\times\) 2.5 mm (L \(\times\) D \(\times\) W) were laser-cut from the piston ring of a fired engine. The samples' surfaces were cleaned for 15 min in an ultrasonic and diluted HCl (5 wt%) solution before each deposition procedure in order to remove the contaminants on the substrate surface. Methanol and filtered water were used to clean the surfaces before they were blown by nitrogen gas.  
In the first step, the samples were ion-etched with N\({}_{2}\) gas using the ECR-CVD method to increase the surface reactivity and eliminate surface contaminants. The ECR-CVD coating technique was utilized to bombard the sample surface via high-density plasma and augmented atom ionization. The extra components of the ECR-CVD plasma ion source (Tectra GmbH, Frankfurt, Germany), including the substrate changer, accelerator bias systems, high-strength steel chamber, plasma shutter, and substrate holder were specially designed and produced. This technique uses a 2.45 GHz microwave source to create microwaves that are then delivered into the plasma. The required magnetic field (875 Gs) for ECR was produced by a set of excitation coils. N\({}_{2}\) and CH\({}_{4}\) gases, both of which were 99.99% pure, were fed into the chamber as precursors for the plasma production using precise flowmeters. A turbomolecular pump was used to vacuum the chamber to a low pressure of about 2.0 \(\times\) 10\({}^{-5}\) Torr prior to the deposition process.  
The coating procedure was carried out using a microwave power of 70 W. The substrate was fixed to a specially designed rotating mechanism to provide uniform deposition on the surface with various plasma orientation angles. The sample was placed at a distance of 10 mm from the plasma gun and coated for a total deposition period of 60 min with an orientation rate of 5\({}^{\circ}\)/min to guarantee the adhesion to the substrate, as shown in Figure 1. This approach minimizes the risk of uncoated areas, even on highly rough surfaces like steel. The surface temperature was kept at roughly 25 \({}^{\circ}\)C without any external heating. The film thickness data were derived from profilometer tests, and the coating thicknesseswere found to be 2.72 \(\pm\) 0.2 \(\upmu\)m and 3.55 \(\pm\) 0.2 \(\upmu\)m for SCN and SC, respectively. The reactive gas with the presence of N\({}_{2}\) gas interacts with the target, forming a compound that lowers the sputtering yield, thereby decreasing the deposition rate and reducing the thin film thickness [23]. These parameters were obtained as a result of long-term experiments and fall within the range where the system operates most stably. Applying a bias voltage between 500 and 600 V ensures good adhesion and penetration of carbon structures to the surface. The reason for placing the sample very close to the plasma, at a distance of about 10 mm, is to create a more suitable reactive environment for atoms to settle into surface voids. The parameters of the process are tabulated in Table 1.  
Thermal annealing has proven to be an effective technique for increasing the graphitization of GO structures and eliminating impurities in GO, amorphous carbon, metal, and metal oxides [24; 25; 26; 27]. The samples coated at room temperature were placed in a 200 \({}^{\circ}\)C oven to be annealed for 1 h in order to remove impurities from the carbon coating structure.