Structural and Tribological Analysis of Multilayer Carbon-Based Nanostructures Deposited via Modified Electron Cyclotron Resonance–Chemical Vapor Deposition

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Plasma-deposited high-adhesion carbon-based coating was confirmed to be good option for producing engineering steels with extended service lives, low wear rates, and low friction coefficients.

Abstract

The electron cyclotron resonance–chemical vapor deposition (ECR-CVD) plasma coating method was employed to bombard steel surfaces to achieve high-strength carbon-based structures. The surfaces to be coated were rotated using an Arduino-controlled rotation system at different orientations to ensure a homogeneous coating. The samples were fixed 10 mm away from the plasma gun (CH₄/N₂ plasma). The samples were characterized via XRD, EDX, Raman spectroscopy, SEM, and AFM. The coated surfaces were then subjected to tribological tests, including the wear rate, coefficient of friction, and surface hardness–roughness. Thermally reduced graphene oxide with an average nanocrystalline size of 5.19–24.58 nm and embedded carbon nanotube structures with sizes ranging from 150 to 600 nm were identified, as well as less-defective microcrystallines and nanodiamonds. The results demonstrated that carbon coating in the presence of N₂ gas led to a maximum reduction of 66% in the average wear rate, 14% improvement in the average surface hardness, 40% enhancement in the average coefficient of friction, and 48% enhancement in the average surface roughness. Consequently, a high-adhesion carbon-based coating deposited via plasma is likely to be a good candidate in the context of manufacturing engineering steels with a low friction coefficient, low wear rate, and long service life.

1. Introduction

Various characteristics of carbon are classified based on its smoothness, hardness, abrasion resistance, magnetic shielding, non-toxicity, environmental friendliness, resilience to harsh environments, and biocompatibility. These features make carbon-based structures appealing in various scientific fields, such as antimicrobial structures [1,2], semiconductor electronics [3], electromagnetic absorbance [4], tribological enhancement [5], etc. Self-assembling nanostructured compounds of carbon include the carbon nanotube (CNT), carbon nanowall (CNW), diamond-like carbon (DLC), graphene oxide (GO), reduced GO (rGO), and nanodiamond [6,7].

A technique to generate a wide range of carbon compounds is electron cyclotron resonance (ECR) [8]. Carbon nanostructures can be uniformly coated, even on surfaces with high roughness at low temperatures, both with and without a catalyst, thanks to the effective large-area and uniform plasma-generating method of electron cyclotron resonance–chemical vapor deposition (ECR-CVD) [9]. This approach uses an RF antenna in a magnetic field created by intense microwave radiation absorption to make plasma. The widely varying pressure values can result in a high-density plasma and augmented metal atom ionization in such a discharge [5,10,11,12]. An externally provided magnetic field and electron frequency allow the electrons to move in a cyclotron. The microwave power is effectively transferred to the plasma, which functions as a resonance cavity inside the plasma chamber [10,13,14,15,16]. Sirisantirut and Phetchakul [17] investigated the application of DLC films synthesized by ECR-CVD as anti-reflection coatings in heterojunction solar cells. They concluded that the integration of DLC films into heterojunction cells led to an efficiency increase from 0.74% to 0.78%, and nitrogen-doped DLC films further enhanced the efficiency to 0.79%. Akbari et al. [18] found that the vertical graphene nanosheet coatings formed by ECR-CVD significantly enhanced the corrosion resistance of Cu substrates, with a corrosion rate about two orders of magnitude lower than that of bare Cu.

Many researchers have been utilizing the aforementioned features of carbon-based structures in a variety of applications. However, there have been some drawbacks of carbon-based coatings on steel substrates. Kapsiz et al. [19] investigated the performance of different hydrogenated DLC coatings used in hydraulic systems. According to the results of the study, DLC coatings have been effective in improving the wear resistance and friction properties in hydraulic applications. They also concluded that DLC coatings can have difficulty adhering to certain metal surfaces. Without proper surface treatments and interlayers, the risk of the coating peeling or delamination increases. Wang et al. [20] deposited coatings on metal substrates consisting of multiple phases, including amorphous carbon, fullerene-like carbon, nanographite, nanodiamond, and C₆₀. The results showed that when the coated sample was fretted against Ø3 mm 316 stainless steel balls in fresh 5W-30 oil under a load of 10 N, the coefficient of friction (COF) was approximately 0.13. In degraded oil, it varied between 0.11 and 0.20. The coatings showed excellent wear resistance, with no visible damage observed on the worn tracks. On the other hand, as the performance of the coatings is highly dependent on the substrate material, variations in the substrate properties could affect the adhesion strength and overall effectiveness of the coating. Yet, in general, it is still observed that the advantages of using carbon-based structures as coatings outweigh the disadvantages. Yin et al. [21] claimed that, when the surface is treated with boronized functionalization and lubricated with a 1 wt% GO nanosheet water-based additive, an ultralow friction state with a coefficient of friction of 0.03 could be achieved. Liu and Lai [22] observed the electromagnetic interference shielding effectiveness of nitrogen-doped carbon (CN) thin films deposited on an aluminum/polyethylene terephthalate substrate. They achieved an electromagnetic shielding efficiency of up to 74.1 dB at 10.11 GHz when the CN film with 3 sccm of N₂ was deposited on the Ti/Al/polyethylene terephthalate substrate. In their experimental investigation, Yilmaz and Esen [5] sought to ascertain the effect of N₂ incorporation in DLC coatings on the formation of carbon bonds and to gain a comprehensive understanding of the tribological characteristics of the coating by conducting thorough research on the coated substrates. CH₄ gas was used as a carbon source and N₂ was utilized to crystallize the carbon bonds in DLC/NCD form by adjusting the flow rates (CH₄/N₂ plasma) through the ECR-CVD process at a bias voltage of −600 V. The ideal flow rates for wear and friction resistance were determined to be 8 sccm of N₂ and 6.05 sccm of CH₄, providing a very low average coefficient of friction of 0.025, a hardness increment of up to 42% and a roughness reduction of 30%.

In general, the adhesion and uniform distribution of carbon-based coatings on rough surfaces are issues that need attention. Based on this, this study presents a variety of carbon-based structures (rGO, GO, CNT, nanodiamond) successfully deposited on metal surface via the ECR-CVD plasma technique and a specially designed microcontroller substrate rotating mechanism that allows variable plasma angle and duration. To the authors’ knowledge, the aforementioned studies and other related research in the literature include the production of only one type of carbon-based structures generally deposited on very smooth surfaces (very low roughness), its impacts on various scientific fields and its possible applications. Another issue in previous studies is that the coated surface has a regular geometry, such as a square or rectangle. For surfaces with high curvature (such as the ring sample in this study), an Arduino-controlled mechanism has been developed to ensure that all the surfaces that will come into physical contact within the mechanism could be uniformly coated. In this study, the novel methods used during the coating process via the specially designed ECR-CVD plasma set-up provided the simultaneous deposition of several carbon-based structures with superior features, even on curved and very rough steel substrates. Moreover, a comprehensive friction-wear analysis was conducted on the “Uncoated Sample (US)”, “Sample coated with only Carbon (SC)”, and “Sample coated with Carbon in the presence of Nitrogen (SCN)” to determine the mechanical strength improvement to the carbon coating under dry friction conditions. Therefore, it is believed that this study will be beneficial in terms of filling a gap in the literature and will be a good guide for many studies on carbon-based structures and industrial applications.

2. Materials and Methods

2.1. Coating Process

The AISI 1018 steel substrates (0.60–0.90% Mn, ≤0.04%, ≤0.05% S, 0.15–0.20% C, 98.05% Fe) with a size of 25 mm × 8 mm × 2.5 mm (L × D × 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 N₂ 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. N₂ and CH₄ 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 × 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°/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 °C without any external heating. The film thickness data were derived from profilometer tests, and the coating thicknesses were found to be 2.72 ± 0.2 µm and 3.55 ± 0.2 µm for SCN and SC, respectively. The reactive gas with the presence of N₂ 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. Etching and coating process parameters.

Sample ID	Process	Process Pressure (Torr)	N2 Flow Rate (sccm)	CH4 Flow Rate (sccm)	Plasma Current (mA)	Bias Voltage (V)	Rotation Rate (°/min)	Process Duration (min)	Coating Thickness (µm)
US	Etching	7.0 ± 0.5 × 10−3	11.28 ± 1.0	-	26	−580 ± 30	5	60 ± 2	-
SCN	Coating	5.1 ± 0.2 × 10−3	11.00 ± 1.0	6.82	26	−580 ± 20	5	60 ± 2	2.72 ± 0.2
SC	Coating	5.2 ± 0.2 × 10−3	-	6.74	26	−580 ± 20	5	60 ± 2	3.55 ± 0.2

US: uncoated sample; SC: sample coated with only carbon; SCN: sample coated with carbon in the presence of nitrogen.

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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 °C oven to be annealed for 1 h in order to remove impurities from the carbon coating structure.

2.2. Surface Characterization

SEM is an efficient technique for the investigation of organic and inorganic materials from the scale of a nanometer (nm) to a micrometer (μm). EDX produces qualitative and semi-quantitative data in conjunction with SEM [28]. Raman spectroscopy is an effective technique for determining the characteristics of carbonaceous structures, such as the phases, functions, and defects [29]. A potent nondestructive method for describing crystalline materials is XRD, which offers details on the phases, preferred crystal orientations (texture), and other structural characteristics, such as the average grain size, strain, crystallinity, and crystal defects [30]. AFM offers high-resolution images with surface height data that can be used to obtain three-dimensional sample morphology information. Thus, the carbon-based structures deposited on the substrates were analyzed with reference to the related analyses. The morphology of the coated sample surfaces was analyzed via SEM, and XRD was utilized to procure comprehensive details about the coated surface’s chemical composition, physical characteristics, and crystallographic structure. Raman spectroscopy was used to analyze the intramolecular and chemical bonding of the coating. In order to determine the elemental and chemical composition of the coated surface, EDX analysis was carried out. The surface topography was analyzed via AFM and the profilometer. The technical specifications of the equipment for the aforementioned analyses are tabulated in

Table 2. Technical specifications of the analysis devices.

Analysis	Device	Technical Specs
Laser-cutting	Senfeng (Jinan City, China)	Laser power: 1.5–3 kW Positioning accuracy: ±0.05 mm Max. speed: 80 m/min
SEM and EDX	FEI Quanta 650 (Hillsboro, OR, USA)	Resolution: 1.2 nm @ 30 kV Magnification: 6–106×
XRD	Rigaku Miniflex II (Tokyo, Japan)	X-ray source: Cu anode (λ Cu Kα = 1.5418 Å), Measurement range (2θ) = 2–145°
Raman	Renishaw In Via Qontor (Wotton-under-Edge, UK)	Wavelength range: 200–2200 nm Lasers: 229–1064 nm Stability: <±0.01 cm−1
AFM	Park Systems NX10 (Suwon, Republic of Korea)	Field of view: 480 × 360 µm (10× objective lens) CCD: 5 MP
Profilometer	Filmetrics Profilm 3D (Quezon City, Philippines)	Thickness range: 50–10 mm Camera: 2592 × 1944 (5 MP)

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2.3. Mechanical Tests

The coated piston ring samples underwent hardness, roughness and abrasion tests to determine the wear resistance of the coating and possible usage area in industry. A reciprocating tribometer abrasion test device was used to scratch the surface using a 100Cr6 steel ball (Ø5) moving across the coated surfaces under dry-sliding circumstances, as shown in Figure 2. With a standard load of 20 N and a stroke of 4.5 mm, the scratcher moved at a linear sliding speed of 2400 mm/min, subtending to a distance of 280 m. The temperature and relative humidity ranges for the tribological analyses of the steel specimens were 20–25 °C and 55–60%, respectively.

The electronic balance (Shimadzu ATX224) was used to correctly weigh each sample before and after the friction test. The coefficient of friction (COF) was automatically calculated by the system considering the normal load and friction force between the pairs. To determine the wear resistance limits of the surface coatings, a constant high load was applied under dry-sliding conditions (20 N, Hertzian contact stress of approximately 1.7 GPa) [31]. The mass loss values of the samples obtained from the abrasion tests were transformed into wear rates using Equation (1):

$$\varphi ={\displaystyle \frac{A*\mathcal{l}}{F*s}}$$

(1)

• Ø: wear rate (mm³/Nm)
• A: trace area (mm²)
• ℓ: amount of reciprocation (mm)
• F: normal load (N)
• s: stroke (m)

The tribological tests were performed on the region where the main plasma bombardment (perpendicular plasma from 10 mm distance) was applied. A portable roughness measuring device (MarSurf M300) with a capacity of up to 350 µm was utilized to capture the macro roughness data, and AFM imaging was employed to assess the R_a of the samples within a scanning area of 5 μm × 5 μm. The surface hardness of all the specimens were measured using a Bruker Universal Mechanical Tester (UMT) with a Vickers indenter tip, applying a 100 g load for a duration of 15 s.

2.4. Uncertainty Analysis

The independent and dependent parameters’ relative error levels were determined using the Gaussian distribution and the root mean square (RMS) method. The uncertainty of the independent and dependent parameters was determined using Equations (2) and (3).

Table 3. Error data.

Variable	Error (%)
COF	±1.6
ϕ	±2.2
Ra	±2.5
Hardness	±3.7

shows the average uncertainty of a few measured metrics. Considering the standard variation (σ), the average of the standard variable (X_m), the uncertainty of the independent variable (∆X), and the uncertain error (ΔR) can be derived from the measured variables X₁, X₂, X₃, …, and X_n [32]:

$$\Delta X=\mp {\displaystyle \frac{2\sigma }{X_m}}\times 100$$

(2)

$$\Delta R={\left[{\left({\displaystyle \frac{\mathsf{\partial }R}{\mathsf{\partial }{X}_1}}\Delta X_1\right)}^2+{\left({\displaystyle \frac{\mathsf{\partial }R}{\mathsf{\partial }{X}_2}}\Delta X_2\right)}^2+{\left({\displaystyle \frac{\mathsf{\partial }R}{\mathsf{\partial }{X}_3}}\Delta X_3\right)}^2+\dots +{\left({\displaystyle \frac{\mathsf{\partial }R}{\mathsf{\partial }{X}_n}}\Delta X_n\right)}^2\right]}^{1/2}$$

(3)

3. Results and Discussion

Figure 3 presents the coated samples’ Raman spectra that were produced by fitting the baseline corrected normalized Lorentz curves (FWHM), allowing the intensities of the D and G peaks—which represent diamond-like sp³ bonds, and graphite-like sp² bonds, respectively. A 2D peak, centered at around 2640 cm⁻¹, was also observed, which is also a characteristic peak of GO [33,34,35]. The I_D/I_G ratio likewise tended to diminish in parallel with a decrease in the nitrogen flow rate. It was observed that, for the coated surfaces (SCN and SC), a typical spectrum of GO and its other forms, such as CNT and nanodiamond, were grown. The presence of nanocrystalline diamond is confirmed by the peaks at around 1620 cm⁻¹ and 1330 cm⁻¹, which indicate that the structure contains graphitized materials [36,37,38,39]. The I_D/I_G ratios of 0.95 and 1.02 (FWHM) show the D peaks centered at around 1320 cm⁻¹ and 1314 cm⁻¹, and the G peaks centered at around 1595 cm⁻¹ and 1598 cm⁻¹, for each N₂-incorporated plasma-coated surface [40,41,42]. The formation of less-defective graphite microcrystallines (smooth film surface) can also be attributed to these I_D/I_G values. The disorder (D) peaks (1320 cm⁻¹), which also match the typical nanodiamond properties, are the strongest peaks [43,44,45].

The GO peaks in the XRD analysis (Gaussian fit) of SCN (Figure 4a) indicate that the dominant crystalline structure was formed, and GO was composed on the surface in multiple forms [46]. Nanodiamond (111) was observed on the surface, and the average nanocrystalline size (L) was computed (

Table 4. XRD parameters and calculated grain size information for SCN.

2Ɵ	FWMH (°)	Grain Size (nm)
10.44	4.47	1.86
14.48	0.30	27.87
17.33	0.55	15.25
26.26	0.76	11.21
43.98	0.02	447.27
75.5	0.02	524.59

and

Table 5. XRD parameters and calculated grain size information for SC.

2Ɵ	FWMH (°)	Grain Size (nm)
11.88	0.18	46.33
14.52	1.06	7.89
16.76	0.96	8.73
26.02	0.1	85.13
45.78	0.38	23.69
73.02	0.04	258.03

) using the Scherrer formula, as shown in Equation (4). The shape factor, $\kappa$, was taken as 0.94, the XRD radiation wavelength, λ, as 0.15406 nm and the FWHM as β.

$$L={\displaystyle \frac{\kappa \lambda }{\beta \cos 2\theta }}$$

(4)

For SC, there was a decrease in the rGO (002) peak, while the C (002) seemed to become the dominant peak, showing that the structure would tend to shift to an amorphous rather than a crystalline form. GO revealed a multi-orientation for both samples. The peaks around 45° are indications of carbon nanocomposite structures like nanocrystalline diamond (111) [47], which is also supported by the Raman analysis.

Figure 5 depicts the SEM images of the surface of SCN. The visual analyses demonstrate that the CNT and rGO are the dominant structures with wrinkles and crumbles [48,49]. Moreover, when the examination area is expanded (20,000×), carbon heaps can be clearly observed on the coated substrate. CNT structures embedded in the GO sheets were spotted, and the mean size of the GO sheets on SCN and SC was determined to be 223 nm, with a standard deviation of 106 nm, and 336 nm, with a standard deviation of 189 nm, respectively. Thanks to the high-energy plasma, the effective penetration of carbon atoms was achieved, leading to the filling of the grooves in the very rough steel substrate (polishing effect) and the formation of nanocomposite rGO/CNT.

In Figure 6, similar structures are seen on the surface of SC with wrinkles and crumbles [50,51]. Larger GO clusters were observed due to the absence of N₂ during the coating process entailing a decrease in the sp³/sp² ratio and the clustering of carbon in the various regions of the surface [52].

AFM imaging was utilized to investigate the surface morphology of the substrates, using a scan size of 5 µm × 5 µm, as seen in Figure 7. The images clearly depict that the grooves and asperities of the surfaces were filled with carbon composites, i.e., CNT and GO. Referring to the images of both samples, the surfaces were generally dominated by GO structures, and CNT structures were also observed throughout the sample surfaces. The SEM images also confirm this phenomenon. Considering the fact that the coating was made from a very close distance to the plasma gun (10 mm), and given the variability of the incidence angle due to the rotating mechanism, the sample surface enabled the uniform formation of carbon structures even on a very rough metal surface, as shown in Figure 5, Figure 6 and Figure 7. The plasma flux with variable angles provides enhancement in terms of the continuity and atomic order of carbon structures throughout the surface.

The surface topography and coating thickness data of the SCN are shown in Figure 8a. It was observed that the mean coating thickness was found to be 5 ± 1 μm, and the amount of tight carbon clusters was higher than those of the SC. For SC, the average coating thickness was found to be around 10 ± 1 μm. The lower number of carbon islands, in general, increased the size of the carbon heaps, which is consistent with the other analyses.

The elemental compositions of the coated substrates are demonstrated in

Table 6. Elemental composition of the samples (EDX).

	US (Before N2 Etching)		SC		SCN
Element	Weight %	Atomic %	Weight %	Atomic %	Weight %	Atomic %
C	16.86	44.67	29.35	59.41	27.06	55.74
N	-	-	2.42	4.24	3.64	6.37
O	4.87	9.68	7.3	10.61	7.42	12.54
Si	1.89	2.14	1.52	1.44	1.41	1.16
Fe	76.37	43.5	59.41	24.3	60.45	24.2

. Carbon deposition on both substrates is clearly seen from the EDX analyses. Referring to a previous study [5], the C–C bonds were prone to becoming stronger as the nitrogen amount in the plasma was adjusted at about 8–12 sccm. Due to the higher amount of N₂ during the deposition process, the carbon amount throughout the surface of the SCN was observed to be lower when compared to that of the SC, as shown in Figure 9.

The hardness values of carbon-based materials are predominantly determined by the strength and brevity of the carbon bonds; therefore, an increased sp³/sp² ratio corresponds to enhanced hardness [53]. The AFM and optical microscopy images (Figure 10) indicate that the coating on the surface of SCN maintained its uniform structure better than that of SC despite the severe abrasion test conditions. An optimal blend of N₂ and CH₄ yields enhanced hardness and a more polished surface, resulting in superior durability and better resistance to abrasion. Deposition factors exert a substantial influence on the mechanical and tribological properties of the structure. SCN has a notably higher abundance of preserved carbon clusters compared to SC, suggesting that its surface structure exhibited higher endurance during the abrasion tests. Figure 10a,b depict the worn surface morphology of SCN, and it is clearly visualized that the abrasion ball was not even able to scratch the region where the diamond structure (only one of many diamond regions was depicted) was formed due to its higher hardness than that of the scratcher. The absence of a scratcher trace is also confirmed by the AFM image of the diamond region (Figure 10b). The incorporation of N₂ gas into the plasma enhanced the adhesion of the carbon to the substrate, which is also consistent with the other tribological analyses.

The surface roughness has a direct impact on the wear rate and COF, making it an essential parameter to be evaluated in tribological analyses. When compared to other surfaces, SCN’s surface had the lowest R_a values, showing that the carbon atoms were successfully introduced by the high-energy plasma, which led to the deposition of the rGO/CNT nanocomposite and the filling of the voids in the steel substrates. As a result, there was a significant reduction in the roughness, as confirmed by the SEM and AFM images. Similarly, the addition of N₂ to the plasma improved the adherence and homogeneity of the carbon-based coating, resulting in a smoother surface. The addition of nitrogen to the process reduced the reaction rate; however, it increased the sp³/sp² ratio, leading to the formation of more crystalline structures on the surface. This, in turn, enhanced the hardness and wear resistance.

Figure 11 illustrates the relationship between the COF and the sliding distance during friction tests conducted on the US, SC, and SCN. Especially for US, the COF values displayed fluctuation until the sliding distance of 170 m. The average decrease in the COF was found to be roughly 40% for SCN compared to that of the US sample. Based on the absence of a sudden rise in the COF for SC and SCN, it may be inferred that none of the coatings were fully worn down to the substrate, even when subjected to dry-sliding circumstances. The decrease in the COF can be attributed to the reduction in surface roughness thanks to the polishing effect of the carbon-based deposition. The overall data related to the mechanical tests are tabulated in

Table 7. Summary of the tribological tests (σ: standard deviation).

	Ø × 10−9(mm3/Nm)	Hardness (HV)					Roughness (μm)
							Ra			Rz			Av.	Av.	σ	σ
		1	2	3	Av.	σ	No.1	No.2	No.3	No.1	No.2	No.3	Ra	Rz	Ra	Rz
US	37.2	264	254	250	256	5.89	0.83	0.64	0.74	5.32	4.53	4.73	0.74	4.86	0.09	0.41
SCN	12.5	293	287	292	291	2.63	0.26	0.31	0.37	2.08	2.69	2.79	0.31	2.52	0.06	0.38
SC	16.1	280	272	273	275	3.56	0.35	0.36	0.43	2.83	2.61	3.07	0.38	2.85	0.04	0.23

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4. Conclusions

CH₄ plasma produced in an ECR-CVD reactor provided a coating of highly crystallized and ordered carbon structures on the steel engine piston rings. The combination of high-density plasma, N₂ pre-treatment on the substrates, custom-made rotating mechanism, and thermal annealing of the sample surfaces yielded precise deposition of carbon-based nanostructures on very rough substrates, leading to the filling of the crevices with regular graphene oxide and carbon nano-clusters. By this means, the advantages of the ECR-CVD method were further enhanced. Carbon deposition along with N₂ gas facilitated a considerable improvement in vital parameters regarding the service life of piston rings, such as a reduction of roughly 66% in the average wear rate, an improvement of 14% in the average hardness, an enhancement of 40% in the COF, and a decrease of 48% in the average R_a. Considering the results procured from the modified ECR-CVD coating method, this study will be a good guide in terms of providing superior features to the moving parts of mechanical systems such as internal combustion engines, pneumatic-hydraulic systems, etc. Improved fuel efficiency and overall engine performance due to reduced friction between contact regions of piston ring–cylinder liner can be achieved. The increase in surface hardness suggests the improved wear resistance of piston rings, potentially leading to longer service intervals and reduced maintenance costs for internal combustion engines. Reduced friction may lead to diminished exhaust emissions aligned with stricter environmental regulations, making this technology attractive for manufacturers aiming to produce cleaner engines. Apart from piston rings, this crystalline carbon deposition technique could be applied to other engine components subject to high wear and friction, such as cylinder liners and valve trains, further enhancing engine durability and performance. On the other hand, comprehensive evaluations encompassing long-term durability, manufacturing feasibility, broader environmental impacts, and compatibility with other engine components are essential and could be future work for assessing the practicality of implementing such coatings in commercial engines.