*Article* **Microwave-Assisted Synthesis, Characterization and Tribological Properties of a g-C3N4/MoS2 Nanocomposite for Low Friction Coatings**

**Mukul Saxena 1, Anuj Kumar Sharma 1,\*, Ashish Kumar Srivastava 2, Rabesh Kumar Singh 1, Amit Rai Dixit 3, Akash Nag <sup>4</sup> and Sergej Hloch <sup>5</sup>**


**Abstract:** This study explores the tribological performance of microwave-assisted synthesized *g-C3N4/*MoS2 coatings. The two-dimensional transition metal dichalcogenide (TMD) nanosheet is getting prominence in the study of tribology due to its layered structure. The graphitic carbon nitride (g-C3N4) nanosheet was made using the calcination method and its nanocomposite with molybdenum disulfide (MoS2) was produced using a microwave-assisted method. The structure and morphology of the samples were characterized by some well-known methods, and tribological properties were studied by a pin-on-disc (POD) apparatus. Morphological analysis revealed that graphitic carbon nitride and molybdenum disulfide coexisted, and the layer structured MoS2 was well dispersed on graphitic carbon nitride nanosheets. BET analysis was used to determine the pore volume and specific surface area of the synthesized materials. The inclusion of MoS2 nanoparticles caused the composite's pore volume and specific surface area to decrease. The reduction in g-C3N4 pore volume and specific surface area confirmed that the pores of calcinated graphitic carbon nitride were filled with MoS2 nanoparticles. The tribological property of g-C3N4/MoS2 nanocomposite was systematically investigated under different factors such as applied loads (5N to 15N), sliding speed (500 to 1000 mm/s) and material composition (uncoated, MoS2-coated, 9 wt.% of g-C3N4 and 20 wt.% of g-C3N4 in the composite). The optimal composite material ratio was taken 9%, by weight of g-C3N4 in the g-C3N4/MoS2 composite for a variety of levels of loads and sliding speeds. The results indicates that the incorporation of g-C3N4 in nanocomposites could reduce friction and improve wear life, which were better than the results with single MoS2. This study demonstrates a solution to broaden the possible uses of g-C3N4 and MoS2-based materials in the field of tribology.

**Keywords:** microwave synthesis; transition metal dichalcogenide; graphitic carbon nitride; coatings; characterization; tribology; wear; steel substrate

#### **1. Introduction**

Friction and wear are the primary causes of failure in many mechanical engineering components used in design. Much energy is used to overcome the resistance to motion caused by friction. Friction causes heat and wear, which in turn can cause material fatigue, noise emissions, surface deterioration, mechanical energy losses, and shortened service life of the components [1]. The cost of fitting, machinery and maintenance because of wear and tear and frictional faults affects the economy of a company. In passenger vehicles, around one third of the gasoline is utilized to overcome friction and wear [2].

**Citation:** Saxena, M.; Sharma, A.K.; Srivastava, A.K.; Singh, R.K.; Dixit, A.R.; Nag, A.; Hloch, S. Microwave-Assisted Synthesis, Characterization and Tribological Properties of a g-C3N4/MoS2 Nanocomposite for Low Friction Coatings. *Coatings* **2022**, *12*, 1840. https://doi.org/10.3390/ coatings12121840

Academic Editors: Cecilia Bartuli and Ajay Vikram Singh

Received: 27 October 2022 Accepted: 25 November 2022 Published: 28 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

One major way to reduce energy consumption is by improving the tribological characteristics of the mating surfaces. The requirements for enhanced lubricants are becoming more and more demanding as a result of their qualities, which include the ability to be used across a wider temperature range, larger loads, faster speeds, and increased endurance and service life [3]. To satisfy the needs of heavy machinery in severe working environments, it is essential to create novel and efficient friction-resistant plus high-bearing lubricant additives. Producing novel lubricants that meet changing requirements in many key domains, including as transportation, industry, and defense, is one of the major scientific problems of the twenty-first century [4].

Rosentsveig et al. [5] suggested that the use of nanoparticles in lubricants might improve their tribological characteristics, significantly lowering the coefficient of kinetic friction in working equipment. Numerous nanoparticle classes have been experimentally explored as potential lubricants [6]. Several nanoparticles are made up of two-dimensional structures that are adjacent to one another and are held together by weak Van der Waal forces. These structures are responsible for lowering the shear strength of the system as well as generating lubricating or sliding effects on the adjoining layer structure [7,8].

When compared to other nanomaterials, 2D nanomaterials have a greater specific surface area for absorbing onto the surface of a substrate, hence reducing, or eliminating, the amount of friction that occurs between the contact surfaces [9]. Because of its chemical and physical stability in a coating, molybdenum-disulfide is currently found as a promising two dimensional transition metal chalcogenide. The materials are resistant to most acids, chemically balanced and irradiation-resistant. The rate at which the lubricant works is regulated by the crystalline lamella structure of the lubricant, in which the sulfur lamella is connected by a weak Van der Waals affinity, resulting in lower fiction [10]. The layers of molybdenum disulfide efficiently slide against each other and align parallel to the relative motion during sliding, resulting in the lubricating effect. The strong ionic connection between sulfur and molybdenum atoms, on the other hand, makes the lamellar very resistant to asperity penetration [11]. However, pure molybdenum disulfide quickly absorbs the moisture in any surrounding environment that is humid and can be oxidized in an environment that contains either atomic or molecular oxygen [12]. This results in a rapid increase in the coefficient of friction and a reduction in the lifespan of the frictional surface. Most of the time, the coefficient of friction increases to a value greater than 0.2 because of the oxidation of molybdenum disulfide [13,14]. Because of these factors, its practical application is restricted and limited. The super-lubricity of molybdenum disulfide can be enhanced at the molecular or atomic scale under appropriate ordering of structure or orientations in a humid free dry setting, or can be used in vacuum environment [15].

MoS2 is often mixed with other materials, such as polyurethane and graphene, as well as metals such as titanium [16], aluminum [17], copper [18], and chromium [19,20] to circumvent these restrictions and acquire its superior frictional resistance, mechanical, and thermal qualities. It is presently recognized as a major issue to improve the wear life and coefficient of friction of molybdenum sulfide for its use as a solid lubricant in different fields of application. It is now recognized as a substantial problem to extend the wear life of MoS2 for its application as a solid lubricant in different field while keeping the coefficient of friction at a low level [21]. However, the expensive cost of the metal additions in MoS2 coatings restricts their utilization. Theoretically, materials that are not metals, such as layered graphitic carbon nitride, which has the characteristics of being affordable, easily accessible, adequately stable, and environmentally acceptable, can replace metal components [22,23].

Currently, a variety of disciplines make use of polymer semiconductor graphitic carbon nitride (CN), which exhibits weak Van Der Waals forces between its layers and tris-triazine units [22,24]. CN is frequently added to lubricating oil to improve friction performance. For example, the bonding of CN with octadecylamine resulted in the creation of an efficient boundary coating on the friction surface, which increased the material's resistance to wear [25]. Duan et al. [26] adopted CN as a base oil additive because it considerably improves the wear resistance of thermoset polyimide. Zhu et al., created g-C3N4/poly vinylidene difluoride (PVDF) composites and discovered that the g-C3N4 filler was advantageous in reducing composite wear loss [27].

Several preliminary approaches for the synthesis of MoS2 nanoparticles have been devised, including, high-temperature sulfurization [28], CVD (chemical vapor deposition) [29], laser ablation, hydrothermal process [30], and sometimes even thermal reduction [31]. MoS2 nanoparticles were synthesized with higher yields using hydrothermal and microwave (MW) synthesis processes. The hydrothermal approach is widely employed because to the ease of access to processing equipment, although it suffers from insufficient uniform heating. However, during MW-assisted synthesis, the compounds can be rapidly heated, which results in a more consistent temperature ramp in comparison to the conventional oven based hydrothermal process. Aside from that, the Teflon lined vessel that is used for the reaction is translucent to microwaves, which ensures that the heating is continuous and sustained throughout the reaction vessels. Furthermore, the microwave has advantages over traditional approaches in terms of speedy and accelerated heating, high-temperature uniformity, and selective heating compared to traditional methods [32]. The reactions are essentially dependent on the capacity of their precursors, particularly solvents, to effectively use microwave energy. Because of its homogeneous heating, low energy consumption, greater yield, and quicker synthesis time, the microwave synthesis process outperforms the hydrothermal technique.

In certain articles [33–35], the MoS2 nanosheets are synthesized using microwave technology, which takes less than 30 min, as opposed to conventional heating in the oven, which takes around 24 h. Both MoS2 and CN can be used as lubricants, however little research has examined the use of graphitic carbon nitride as an additive in MoS2 to create solid nanocomposite lubricants. Because MoS2 and CN have atomic layers, they may readily move from one to the next when combined, enhancing the friction capabilities. Even though sophisticated microwave-assisted synthesis of MoS2 and its composite with g-C3N4 haven't been reported in the literature much, and their composites made via microwave heating for the field of tribology haven't been published yet, this study shows that it is possible to make these materials.

In this study, a one-step microwave-assisted (MW) process was used to synthesize the g-C3N4/MoS2 nanocomposite, and its tribological properties as an anti-friction coating were examined. The details of synthesis, characterization, and tribological property of the g-C3N4/MoS2 nanocomposite are described. For tribological analysis, it is important to find the endurance of the coating, and for this purpose three factors at different levels were considered. These factors include the applied load, sliding speed, and composition. Due to the three factors, with three levels of sliding speed, applied load and four levels of composition, the design contained 36 different possible combinations to perform experimental work.

#### **2. Materials and Methods**

For the synthesis of the material, a microwave reactor (Model Monowave450, Anton-Paar, CAS-Lucknow, India) was used, and a sample drying oven (Model HTF-T-1600, Tempsens, CAS-Lucknow, India) was used for the drying process. XRD (Ultima IV with Copper Kα radiation, current 150 mA, and potential 40 kV (wavelength = 1.5418 Å), Rigaku, IIT ISM, Dhanbad, India) was used in examine the phase composition of the developed samples in the range 10–70 ◦C at 4 ◦C steps. A FTIR spectrometer (Alpha II, BRUKER, CAS-Lucknow, India) was used to analyse the FTIR spectra of the sample materials. The microstructures were observed using a FESEM (Gemini500, Zeiss equipped with EDS, CAS-Lucknow, India). BET analysis was performed on Belsorp (mini X) to determine the pore volume, mean pore diameter, and specific surface area of synthesized materials in the relative pressure range (*P* to *P*0) of 0 to 1. The friction pairs and worn surface of the coated steel substrate were characterized using Stereo microscope (Olympus, SZX10, CAS-Lucknow, India).

The analytical grade reagents used in this investigation were sodium molybdate dihydrate (Na2MoO4·H2O), thioacetamide (CH3CSNH2), absolute ethanol, and urea (NH2·CO·NH2). All of the reagents employed were of analytical quality and did not require further purification. First, graphitic carbon nitride powder was prepared by a solid reaction. Urea (20 g) taken into a 50 mL crucible and dried at about 80 ◦C. After the crucible had been allowed to dry, it was covered with a lid, then coated in several layers of aluminum foil, and finally put in a muffle furnace. After that, it underwent calcination at 550 ◦C for four hours at a rate of 3 ◦C per minute. Following the lowering of the temperature to room temperature, the g-C3N4 powder was prepared using the grinding technique. After being cooled to room temperature (~22 ◦C), the g-C3N4 powder was produced by grinding. The conversion of urea (NH2·CO·NH2) into g-C3N4 in the process was around 4.5 percent, as calculated from Equation (1) and presented in Table 1.

$$\begin{array}{c} \textbf{Yield\\_percent} = \left(\frac{m\_t - m\_c}{m\_u}\right) \textbf{\underline{\times}} \textbf{\underline{100}}\\ \cdot \quad . \qquad . \qquad . \qquad . \qquad . \qquad . \qquad . \qquad . \end{array} \tag{1}$$

where, *mt* = mass of obtained catalyst, *mc* = mass of crucible, *mu* = initial urea mass.

**Table 1.** Yield percent calculation.


g-C3N4, sodium molybdate dehydrate and thioacetamide were mixed to a 100 mL beaker containing 70 mL deionized (DI) water in a molar ratio 1:0.89:1.91 and 1:2.33:5.01. This mixture was then subjected to a sonication treatment for three hours, during which time the reagents were mixed thoroughly in the distilled water, and then magnetic stirring for 1 h at room temperature. A schematic of the process is shown in Figure 1.

The solution was transferred into a 30 mL vial and put into microwave reactor for the synthesis. The heat ramp for the microwave-assisted synthesis is shown in Figure 2.

Following the completion of the reaction, the mixture was allowed to reach room temperature by means of a flow of compressed air. Next, filtration was carried out, and g-C3N4/MoS2 powder was collected. After that, the powder was dried in a hot air oven at a temperature of 80 degrees Celsius for two hours. For comparison, the same process was used to create MoS2 without the inclusion of g-C3N4. In order to prepare the surface of steel substrate for coating, acetone was used to clean it, and then it was ultrasonically stirred for one hour. Composition of the substrate was calculated from EDX analysis and listed in Table 2.

**Table 2.** Composition of steel substrate analyzed by EDX spectroscopy.


**Figure 1.** Synthesis of material.

**Figure 2.** Heat ramp for microwave synthesis.

The MoS2 powder and g-C3N4/MoS2 nanocomposite were dissolved separately and dispersed into a medium size beaker that contained 50 mL of ethanol. The solution was coated on the substrate by a spin coater after 30 min of ultra-sonication. Spin coating is an easy and efficient technique for coating nanomaterial on a flat surface. The coating solution spreads out over the substrate at low rotational speeds, and coated films are created at high rotational speeds. The substrate was first mounted on the spin chuck. The substrate was held in place by turning on the vacuum line. To coat the sample material, a predetermined volume of substrate was dispensed onto it with a disposable pipette. The substrate was spun after the coating solution was applied, and the lid was then put on the spin coated. After the solution was entirely suspended, the MoS2 and g-C3N4/MoS2 nanomaterial suspension were deposited on the substrate.

A pin-on-disc (POD) Tester (Model: TR-20LE PHM-400, DUCOM Instruments) was employed for the friction studies on the coating. Figure 3 is a schematic illustration of this POD tester. The counterpart pin of diameter 8 mm was made out of AISI304 stainless steel pin. All of the studies were conducted in an environment that had a relative humidity (RH) of around 30% ± 2, and the surrounding temperature was approximately 24 degrees Celsius.

The design of experiment (DOE) was created with the help of MINITAB-19, and for the purpose of conducting an analysis, three factors at their different levels were considered. These factors include the applied load, sliding speed, and composition. Table 3 presents the parameters used as inputs in the investigation.


**Table 3.** Parameters utilized as inputs for the investigation.

Due to three factors with three levels of sliding speed, applied load and four levels of composition, the design contained 36 different possible combinations to perform experimental work. The real time data of the coefficient of friction were continually recorded in software. To rule out the possibility of inadvertent mistakes, every experiment was carried out three times, and the average of the results was then determined.

#### **3. Results and Discussion**

X-ray diffraction and Fourier transform infrared spectroscopy were used, respectively, to investigate the crystal structures and functional groups of synthesized materials (pure MoS2, graphitic carbon nitride, wt.% 9 and 20 g-C3N4/MoS2 composite). Diffraction peaks for g-C3N4 were found at 2θ = 13.0◦ and 27.5◦, as shown in Figure 4 (JCPDS#87-1526). These values correspond to (100) and (002) crystallographic planes, respectively. These two peaks are indicative of conjugated aromatic systems with interlayer stacking [27]. The peaks at 2θ = 14◦, 32.7◦, 35.871◦, and 58.3◦, respectively, are attributable to the (002), (100), (102), and (110) planes of the hexagonal MoS2 (JCPDS#37-1492) [36,37].

**Figure 4.** XRD pattern of MoS2, g-C3N4 and their composites.

FTIR results of synthesized materials are shown in Figure 5. The peaks at 1237.4 and 808 cm−<sup>1</sup> in the FTIR spectrum are responsible for the aromatic ring structure and the breathing mode of a triazine ring of g-C3N4, respectively [22]. The extension and respiration of C=N and C-C heterocyclic structure in g-C3N4 results in the peaks at 1407.1 and 1638.9 cm−<sup>1</sup> [26,38]. Furthermore, the peak at wavenumber 3432 cm−<sup>1</sup> is attributable to stretching vibration due to -NH residual in the C-N ring as well as -OH vibration in the H2O that was absorbed on the surface [22,26,38].

The characteristic peaks of graphitic carbon nitride and molybdenum disulfide present in the FTIR pattern of g-C3N4/MoS2 suggests that MoS2 has no effect on the functional group for g-C3N4 in nanocomposite while microwave-assisted synthesis. The results of all of the characterizations point to the conclusion that the nano-composite of g-C3N4/MoS2 that was successfully synthesized by the microwave-assisted method consisted of two distinct structures of g-C3N4 and MoS2. FESEM was used to examine the surface morphology and microstructures of the samples. Figure 6 depicts FESEM images of the samples.

It can be seen from Figure 6 that the flower-like structure of molybdenum disulfide assembled in several nanosheets, while a smooth and layered structure of graphitic carbon nitride is seen in Figure 7.

Figure 8 shows that MoS2 and g-C3N4 in the developed composite are bundled together, showing that the components of composite were mixed successfully and may provide the lubrication property of the coating.

**Figure 5.** FTIR pattern of MoS2, g-C3N4 and their composites.

**Figure 6.** FESEM images of MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 7.** FESEM image for g-C3N4 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 8.** FESEM images 9 wt.% g-C3N4/MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

It can be seen from Figure 9 that on increasing the weight percentage of graphitic carbon nitride in the composite, MoS2 predominates.

**Figure 9.** FESEM images 20 wt.% g-C3N4/MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 10.** (**a**) SEM image; (**b**) live map; (**c**) element overlay; (**d**) elemental analysis; element (**e**) molybdenum, Mo; (**f**) sulfur, S; (**g**) carbon, C and (**h**) nitrogen, N of the g-C3N4/MoS2 composite.

An EDX detector was used to identify distributed elements in the composite. Carbon, nitrogen, molybdenum, and sulfur are among of the components that make up the

EDX elemental analysis results for graphitic carbon nitride, molybdenum disulfide and their composites are summarized in Table 4.

**Table 4.** Elemental composition of synthesized materials from EDX elemental analysis.


The nitrogen adsorption/desorption analysis determines the specific area of surface (*as*) of each sample (MoS2, g-C3N4, and their composite), as shown in Figure 11. The hysteresis curve loops of the samples were recorded and found to be the H3-IV type.

The specific surface areas of g-C3N4, MoS2 and g-C3N4/MoS2 were approximately 78.94, 1.46, and 25.71 m2g−1. The addition of stacked MoS2 nanoparticles may result in a reduction in the BET specific surface areas of g-C3N4/MoS2. The pore volumes of g-C3N4, MoS2 and g-C3N4/MoS2 were approximately 0.142, 0.072 and 0.021 cm3g−1. In this instance, the inclusion of MoS2 nanoparticles caused the composite's pore volume to decrease. The reduction in g-C3N4 pore volume confirms that the pores of calcinated graphitic carbon nitride were filled with MoS2 nanoparticles [39], so a range up to 15 wt.% was employed for the friction studies on the coating.

A pin-on-disc (POD) wear test was employed for the tribological investigation of the coating. The testing parameters for tribological investigations were selected as per the specifications of the machine available, and three loads of 5, 10 and 15 N with sliding speeds as 1000, 750 and 500 mm/s were used. The revolutions per minute required for the analysis were calculated from the equation *<sup>S</sup>* <sup>=</sup> *<sup>π</sup>*.*d*. *<sup>N</sup>* <sup>60</sup> , where *<sup>S</sup>* = sliding speed, *<sup>d</sup>* = track diameter and *N* = required revolutions per minute. The total track length covered by the pin that slid against disc was calculated from the equation *L* = (*π*.*d*).*N*.*T*, where *L* = sliding speed, *d* = track diameter, *N* = required revolutions per minute and *T* = test duration. Sliding speeds of 1000, 750 and 500 mm/s covered track lengths 600,000 mm, 450,000 mm and 300,000 mm, respectively which were required for effectively analyzing the effect of the coating and its tribological aspects for ultra-long wear life.

**Figure 11.** Nitrogen adsorption-desorption isotherm curves of (**A**) g-C3N4, (**B**) MoS2, (**C**) g-C3N4/MoS2 (9 wt.%) and (**D**) g-C3N4/MoS2 (20 wt.%).

Figure 12 depicts the wear of substrates coated with g-C3N4, MoS2, and g-C3N4/MoS2 coatings in various ratios under loads ranging from 5N to 15N and sliding speeds ranging from 500 to 1000 mm/s as per the design of experiment done with the help of MINITAB-19, as depicted in Table 3. Maximum wear depth was recorded approximately 900 μm for uncoated steel substrate disc for a test duration of 10 min, but was reduced to approximately 500 μm for the MoS2-coated disc (Figure 12B). The MoS2-coated disc restrict the friction and sliding forces for a short duration as compared to the g-C3N4/MoS2 coatings. The wear depth for coated disc dropped to approximately 66% and 50%, respectively at 9 wt.% (Figure 12C) and 20 wt.% (Figure 12D) of graphitic carbon nitride compared to the uncoated steel disc. Figure 12 shows that the optimal composite material ratio was proven to be weight percent 9 of g-C3N4 in the g-C3N4/MoS2 composite for a variety of levels of loads and sliding speeds. The figure also indicates that the friction lifetime might be increased while minimizing wear. Figure 12 also reveals a higher rate of wear for pure steel substrates, which may be due to the direct contact that occurs in between the respective pins and the steel substrates. Substrate coated with only MoS2 had lower wear than uncoated steel substrates but a shorter wear-life since MoS2 is heavily impacted by the environment. Due to the reciprocating type shear stress, the materials with g-C3N4 (9 wt.% and 20 wt.%) in MoS2 were compacted during the running-in phase, generating a dense layer on the coated surface, as shown in Figure 12c,d.

**Figure 12.** Wear depth vs. Time (**A**) for uncoated substrate; (**B**) for pure MoS2-coated substrate; (**C**) for 9 wt.% g-C3N4/MoS2 (**D**) 20 wt.% g-C3N4/MoS2, at different sliding speeds and loadings.

Furthermore, the friction process could exfoliate MoS2 nanosheets into lamella due to the reciprocating type shear stress that arises as a result of the sliding friction between the coating and counterpart pin. Instrument software recorded the mean coefficient of friction (COF) for different combinations of factors and levels as summarized in Table 5.


**Table 5.** Mean coefficient of friction for different combinations of factors.


**Table 5.** *Cont.*

The coating material doesn't peel off instantly, preventing substrate from exposure to the pin directly. Despite the fact that the mean coefficient of friction of g-C3N4/MoS2 is somewhat lower than that of pure MoS2, g-C3N4 exhibits outstanding resistance to wear and may greatly extend the wear life of MoS2. The coefficient of friction was determined to be at its lowest when the load being applied was approximately 15 Newton, which demonstrates that the greater the load, the lower the coefficient of friction. This is due to the fact that the rate of increases in load is higher than that for increase in contact area.

Figure 13 shows a microscopic view of the material wear loss for a nanocompositecoated steel substrate disc. It shows that the amount of wear loss leading to a small ploughing action. With different material composition in the coating, the chance of pinto-disc contact increased. While analyzing the worn surface, some small micro cracks and grooves could be seen in the MoS2-coated substrate (Figure 13a) which may be due to oxidation of molybdenum disulfide at high-speed sliding between pin and substrate disc. The increase in wear loss of the 20 wt.% g-C3N4/MoS2 composite at the applied load caused ploughing on the surface, and deep grooves with large amount of wear debris were found (Figure 13c). It can be seen from Figure 13b that the nanocomposite coating with 9 wt.% g-C3N4/MoS2 shows a lower value of wear loss when compared with MoS2, as well as 20 wt.% of g-C3N4 in the composite. An amount of coating remained on the substrate disc even after long-duration sliding, as can clearly be seen in Figure 13b. A few traces of g-C3N4/MoS2 material within the valleys would continue to provide lubrication, and affirms good adhesion of coating to substrate. Uncoated bare substrate was strongly affected by sliding with the pin due to shear fracture and a comparatively rough surface was found after the wear test. Some wear debris, deep groves, abrasive and ploughing lines can be clearly seen in Figure 13d. Worn surface analysis confirms the addition of g-C3N4 would reduce the wear loss of the MoS2 nanocomposite-coated substrate and increase the wear resistance.

The notable features of g-C3N4/MoS2 coating were excellent wear protection and lifetime of the composite coating. The aforementioned phenomena suggest that MoS2 (solid lubricant) and an additive of g-C3N4 have effective characteristics when used together. g-C3N4/MoS2 composites have a lower coefficient of friction (COF) and a longer wear-life because the proper quantity of addition of MoS2 can greatly reduce the coefficient of friction for g-C3N4, while g-C3N4 can enhance the wear-life of MoS2.

**Figure 13.** Micro-graphs of wear surfaces: (**a**) MoS2; (**b**) 9 wt.% g-C3N4/MoS2; (**c**) 20 wt.% g-C3N4/MoS2; (**d**) uncoated steel substrate.

#### **4. Conclusions**

Calcination of urea was used to make graphitic carbon nitride nanosheets, and a novel microwave-assisted synthesis process was used to make a g-C3N4/MoS2 nanocomposite. Physical and morphological characterizations were performed by XRD, FTIR, FESEM and EDX spectroscopy and showed the well-controlled morphology of MoS2 nanosheets and the structural features of nanocomposites. The tribological characteristics of coated steel substrate were studied using a pin-on-disc (POD) test for different loads and sliding speeds between AISI304 steel pin and substrate.

After analyzing and comparing the results, the following conclusions can be made:


• It was established that the coefficient of friction was at its lowest when the applied load was approximately 15 N, indicating that the higher the load, the lower the coefficient of friction. This is because the rate of increase in the applied load exceeded the rate of increase in contact area.

This study provides a solution to broaden the potential application of graphitic carbon nitride and molybdenum disulfide-based materials in the domain of tribology.

**Author Contributions:** M.S.: Conceptualization, Writing; A.K.S. (Anuj Kumar Sharma): Review and editing; A.K.S. (Ashish Kumar Srivastava): methodology, validation, Visualization, Writing—Review & Editing; R.K.S.: resources; A.R.D.: Supervison; Writing—Review & Editing, A.N.: software, data curation; S.H.: project administration, Writing—Review & Editing, Visualization. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Slovak Research and Development Agency under Contract No. APVV-17-0490.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

