Investigation of Tribological Properties of Inconel 601 under Environmentally Friendly MQL and Nano-Fluid MQL with Pack Boronizing

Abstract

Friction and high temperatures greatly affect the hardness and processing efficiency of superalloys. Therefore, it is important to provide a coating on their surfaces with a hard layer. In this study, pack boronizing was applied on Inconel 601 to improve its microstructure and tribological properties. In this regard, tribological tests were performed under MQL, nano-MQL1 (MQL + CuO), and nano-MQL2 (MQL + TiO₂) environments. The research results showed that the lowest wear depth, friction force, coefficient of friction (CoF), and volume loss values were obtained in pack-boronized Inconel 601 in a nano-MQL2 environment. In the nano-MQL2 environment, the wear depth decreased by 17.81% (from 57.922 µm to 47.605 µm) with package-boronized Inconel 601 compared to as-received Inconel 601 at a 45 N load. Pack-boronized Inconel 601 experienced an average reduction of 30.23%, 41.60%, and 52.32% in friction force when switching from dry to MQL, nano-MQL1, and nano-MQL2 environments, respectively. It was also observed that the coefficient of friction (CoF) and volume loss values decreased with pack boronizing in an MQL/nano-MQL environment. In a nano-MQL2 environment at 15 N load, volume losses for as-received and boron-coated Inconel 601 were determined as 0.288 mm³ and 0.249 mm³, respectively (13.54% decrease). The findings of this study demonstrate that pack boronizing and MQL and nano-MQL techniques enhance the tribological characteristics of Inconel 601 alloys.

1. Introduction

Superalloys are extensively used in high-tech industrial sectors, including petrochemical, aviation, and marine industries, as well as in advanced engineering applications, like chemical processing, combustion chambers, turbine blades, and marine oil extraction. They are recognized for their remarkable tensile strength, high-temperature yield strength, resistance to creep, and corrosion resistance [1]. Superalloys are classified as nickel-based, iron-based, or cobalt-based superalloys, depending on the base metal [2]. Among these alloys, nickel-based superalloys are the most commonly used group, making up almost half of the materials used in aircraft engines [3], while Ni-based superalloys are an essential material for oxidizing and high-temperature environments because they can withstand corrosion and maintain their mechanical properties at high temperatures. Therefore, it is crucial to apply a hard layer to their surfaces when using them in corrosive environments, because of their low hardness. By applying surface treatments, the hardness of the material surface can be increased and wear can be reduced. The application of surface coatings is considered an effective method to minimize the effect of corrosion on metallic materials.

A well-used surface modification method for enhancing the resistance to wear and corrosion in steels and superalloys is boronizing. By impregnating the material’s surface with boron, a thermochemical process known as “boronizing” creates hard borides at elevated temperatures [4]. There are various methods for boronizing, including electrolytic, plasma, package, liquid, and gas methods [5]. The package boronizing approach is particularly noteworthy among these techniques since it is less expensive and easier to use than other boronizing techniques. The boron coating method has been indicated by many researchers as a successful and promising method due to the high hardness of the surfaces [6]. Even though wear resistance is improved by the coating method, Inconel materials are difficult to cut and have limited thermal conductivity, which creates problems related to their machinability.

Furthermore, dislocation drag stress can be subjected to viscous drag effects resulting from dislocation motion at high strain rates. Increased temperatures can reach levels that can have detrimental effects on both the work material and the work tool. In this case, the life of the work tool decreases as a result of various negative factors, such as thermal cracks and wear [7]. Accordingly, the work tool loses its ability to perform its operation effectively. In order to prevent damage to both the abrasive and the workpiece material and to prevent an increase in the amount of energy consumed, different lubrication environments have recently been preferred by researchers and manufacturers. Not only surface treatments and coatings but also the creation of suitable lubrication environments is stated as methods used to reduce wear in metallic materials. The use of a lubricating fluid is an effective method to reduce friction and thus wear at the abrasive ball–workpiece material interface by effectively removing heat [8]. One such method is minimum quantity lubrication (MQL), which entails sprinkling a tiny quantity of lubricant into the cutting zone using pressurized air [9]. In the MQL method, the volume of the lubricant used is small, and most of it evaporates, thus minimizing the negative effects on the environment and reducing processing costs. Moreover, different types of fluids can be used as lubricants, but conventional fluids are not sustainable due to their high toxicity, costly transportation and disposal, and negative effects on human health and the environment [10]. In the MQL method, negative environmental and health problems can be minimized by using water-/plant-based emulsion coolants [11]. For this purpose, studies have been carried out on the use of MQL in the processing of Inconel materials [12]. In addition, to obtain more successful results in the MQL method, the addition of nanoparticles has recently become popular among researchers [13]. It is aimed to improve lubrication and cooling properties by adding nanoparticles to the fluid in varying concentrations. Compared to a traditional flood system, the MQL and nano-MQL method lowers cooling costs, the environmental impact, and disposal issues [14]. In general, nano-MQL is one of the cooling techniques that show promise and need further investigation. There are studies using nanoparticle-reinforced MQL systems to facilitate and make the wear characteristics of Inconel materials more efficient. Vardhaman et al. [15] carried out experiments by adding MWCNTs as lubricant additives to 10W40 engine oil, adding ZnO nanoparticles, and adding these two substances together in certain amounts and investigated tribological behaviors. As a result, they experienced that ZnO/MWCNT hybrid nanomaterials mixed with 10W40 engine oil leads to better friction-reducing and superior wear resistance effects compared to adding ZnO nano-additive and MWCNTs to the same engine oil individually. The mixture obtained by adding ZnO/MWCNT hybrid nanomaterials to engine oil at a concentration of 0.25% by weight and in a ratio of 3:1 reduced the friction coefficient by 32.30% and the wear volume by 74.48% compared to pure engine oil. Abdullah et al. [16] conducted an experimental study to investigate the effect of using hexagonal boron nitride (h-BN) nanoparticles in engine oil on friction and wear. They used SAE 15W-40 oil in their study. The h-BN nanoparticles they used were 70 nm. They carried out the process of dispersion of h-BN nanoparticles in oil using the sonication technique. Yılmaz [17] conducted a series of experiments to investigate the tribological changes that will occur as a result of adding copper oxide (CuO), copper iron oxide (CuFe₂O₄), and copper zinc iron oxide (CuZnFe₂O₄) nano-additives to engine oil. Wear significantly reduced as a result of adding nanoparticles to pure oil. The smoothest surface was obtained in the sample with CuZnFe₂O₄ nano-additive, while the roughest surface was obtained in the sample with CuO nano-additive.

The MQL system has recently become popular among researchers and manufacturers both because of the advantage of using environmentally friendly fluids and because it provides more successful results. Moreover, studies are ongoing to improve the MQL system with methods such as adding nanoparticles. As mentioned in the aforementioned literature, some studies have been carried out on the machinability of different Inconel materials with MQL and nano-MQL systems, but there are limited studies related to Inconel 601. Moreover, limited studies were found in the literature related to boron-coated Inconel 601. Based on these conditions, in this study, the change in wear, friction force, and friction coefficient values of boron-coated Inconel 601 was compared to its as-received state by using the MQL system reinforced with CuO and TiO₂ nanoparticles.

2. Materials and Methods

In the experiments, Inconel 601 material and abrasive tungsten carbide (WC) balls were used for wear testing of Inconel 601 samples. The chemical configurations of Inconel 601 and the abrasive balls are tabulated in

Table 1. Inconel 601’s and the WC abrasive ball’s chemical composition.

Inconel 601	Ni (%)	Cr (%)	Cu (%)	Si (%)	Fe (%)
	~58–63	~21–25	~1.00	~0.50	Balance
WC Ball	Tungsten Carbide (%)			Co (%)
	94			6

. Additionally, the experimental setup and equipment are shown in Figure 1. The tests were performed at 60 m/s speed for 33 min at three different forces: 15, 30, and 45 N.

The wear marks from the tests were measured to calculate volume loss values. Equation (1) was used to compute the volume loss.

$$Volume Loss={\displaystyle \frac{2}{3}}\times Wear width\times Wear depth\times Trace length$$

(1)

As shown in Figure 2, four distinct conditions were chosen as lubrication conditions in experimental studies: dry, MQL, nano-MQL1 (MQL + CuO), and nano-MQL2 (MQL + TiO₂). The Werte brand 15 model (İzmir, Turkey) MQL cooling system, appropriate for a range of bench and experimental conditions, was used in the research. Nanoparticles of copper oxide (CuO) and titanium dioxide (TiO₂) were introduced to the coolant to generate nano-MQL1 and nano-MQL2 environments, which improved the MQL system’s performance. For this purpose, the coolant was supplemented with CuO and TiO₂ nanoparticles (the physical properties are shown in

Table 2. Thermophysical properties of nanoparticles.

Property	CuO	TiO2
Density (kg/m3)	6000	4230
Thermal conductivity (W/m-K)	33	8.4
Specific heat (J/kg-K)	551	692
Diameter of nanoparticles (nm)	25–50	25–50

) that included 0.2% nanoparticles by volume [18], resulting in the creation of nano-MQL1 and nano-MQL2, respectively. SEM images of CuO and TiO₂ are shared in Figure 3. In Figure 4, the preparation procedure is shown.

2.1. Boriding Process

In this investigation, the boriding combination used was a powder mixture with a particle size of less than 25 µm that contained 90% by weight of B4C and 10% by weight of NaBF₄. Using SiC abrasive sheets of 800, 1000, and 1200 mesh diameters, the samples were ground prior to boriding. To acquire a clean and uniform surface, they were then rinsed in ethanol, cleaned in acetone using ultrasonic waves for 15 min, and then cleaned again with distilled water. The samples were then placed into a stainless-steel crucible so that each sample was completely covered from all sides by a 10 mm thick layer of boriding powder. The crucible’s top was coated with a layer of pure Al₂O₃ powder, 10 mm thick, to guard against oxidation at high temperatures. For 5 h, temperatures of 950 °C were maintained during the boriding process (Figure 5). Following the boriding procedure, nano-hardness was tested at 2500 Hv.

2.2. Material Characterization

Specimens for metallography and XRD were prepared from the boronized samples (60 × 30 × 10 mm³) using a precision cutter. The cross sections were hot-mounted, ground with SiC papers ranging from 320 to 2500 grit, and polished with diamond paste (1 µm and 0.25 µm) to achieve a mirror-like surface. Scanning electron microscopy (SEM) analyses were performed with a TESCAN S8000 BrightBeam™ (Brno, Czech Republic) field emission scanning electron microscope operating at an accelerating voltage of 25 kV and equipped with EDS capabilities. X-ray examinations were conducted on the sample surfaces to determine the phase structure of the boride layers formed on the surface of Inconel 601. XRD phase identification was performed using a computer-controlled Empyrean analytical instrument, with parameters set to a 2θ range of 20 to 100°, a scanning step size of 0.0525211, and Cu Kα radiation (1.5418 Å). The cross sections of the generated layers were subjected to microhardness tests with a Future-Tech FM-700 hardness tester (Kawasaki, Japan). A Vickers pyramid indenter with 25 g applied tension and a 15 s dwell time was fitted to the tester. The average of the five readings was used to calculate the hardness value for each depth.

3. Results and Discussion

3.1. Characterization of Boride Layer

Figure 6 shows the XRD patterns of the untreated and boronized Inconel 601 superalloy samples, taken from the surface.

In Figure 6a, the XRD analysis of the untreated Inconel 601 sample shows dominant peaks at 43.82°, 50.98°, 91.12°, and 96.46°, corresponding to the γ-austenite phase (DB card number 01-077-9326), which consists of Ni, Cr, and Fe, with no evidence of carbide or precipitate phases. In the boronized sample, dominant peaks were observed at 25.12°, 35.81°, 42.45°, 45.71°, 51.51°, 58.11°, 79.94°, 81.27°, and 82.41°, corresponding to Ni2B (DB card number 01-089-1995) and Fe2B (DB card number 03-065-2693). Additionally, minor peaks for the Cr₂Ni₃B₆ phase (DB card number 00-027-0125) were detected at 44.84°, 45.71°, and 81.27°. The formation of Ni2B and Fe2B phases during the boronizing of Ni-based superalloys has been widely reported in studies [19]. However, the detection of the Cr₂Ni₃B₆ ternary phase is less common. This phase likely forms due to partial substitution of Fe atoms by Cr atoms during the boride formation process.

The SEM microstructure image of the boronized Inconel 601 superalloy revealed the presence of three distinct regions based on color concentration. These regions were (i) the boride layer region, where the coating layer appeared nearly homogeneous across the area; (ii) the transition region, where a darker color concentration was observed at the grain boundaries due to boron diffusion, while the grains themselves exhibited a relatively lighter microstructure; and (iii) the matrix region, where only the grain boundaries were black, and no significant color variation was present within the coating layer. As a result of boronizing at 950 °C for 3 h, a dense boride layer of 50–60 µm thickness formed on the surface, in addition to a transition layer of 60–70 µm where the boride layer was occasionally discontinuous (Figure 7). This was confirmed by EDS analysis. In the dense region, the boron content was 56.87 at.%, while it was 27.36 at.% in the transition region and 0% in the matrix. Microhardness measurements across the coating cross section revealed values of 2584 ± 78 HV0.1 for the boride layer, 815 ± 42 HV0.1 for the transition layer, and 509 ± 25 HV0.1 for the matrix. The SEM images, EDS findings, and hardness values are consistent with the literature [19].

3.2. Assessment of Wear Rate

Figure 8 displays the wear profiles of as received and pack boronizing Inconel 601 under different loads and lubrication conditions. The figures show that under all environmental conditions (including a dry environment), the load increase (from 15 N to 30 N and 45 N) caused an increase in the wear depth. However, with the use of MQL, nano-MQL1, and nano-MQL2, a significant decrease was achieved in the wear depth values (both in as-received and boron-coated material) compared to the dry environment. These decreases were realized at much higher rates, especially with MQL systems with nanoparticle additions. In addition, the wear values in boron-coated material under all conditions were lower than in as-received material. Based on these results, it can be concluded that MQL, nanoparticle reinforcement to MQL, and boron coating all are beneficial in reducing the wear depth. According to the results obtained at a 15 N load in a dry environment, the wear depth obtained in as-received Inconel 601 was determined as 141.554 µm, while it was determined as 120.518 µm in boron-coated material. Accordingly, it is understood that the wear depth is reduced by 14.86% with boron coating at a 15 N load in a dry environment. The reduction rates at other loads (30 N and 45 N) were 28.22% (from 175.319 µm to 125.836 µm) and 28.40% (from 181.097 µm to 129.674 µm), respectively. However, in the nano-MQL2 environment, where the lowest wear depth values occur, the wear depth values with boron coating compared to the as-received state decreased by 9.96% (from 43.811 µm to 39.45 µm), 11.63% (from 48.931 µm to 43.242 µm), and 17.81% (from 57.922 µm to 47.605 µm) at loads of 15 N, 30 N, and 45 N, respectively. When the average wear depths are considered according to the comparison of all environments, the order of wear depth values for both boron-coated and as-received materials is dry > MQL > nano-MQL1 > nano-MQL2.

On the one hand, the decrease in the viscosity of the lubricant fluid due to the increase in temperature as the load increases causes a decrease in the protective effect of the material surface. The increase in wear depth with increasing load can be explained in this way. On the other hand, the wear-depth-reducing effect of the boron coating can be explained by the fact that the hard and flexible surfaces formed on the material surface by boronizing become resistant to wear [20]. Moreover, the effect of using MQL in reducing the wear zone temperatures and facilitating the process provides a significant reduction in wear levels [21]. Furthermore, the wear depth values decrease much more with the addition of CuO and TiO₂ nanoparticles. It can be said that this is due to the increase in heat transfer by increasing the surface area of nanoparticles [22].

3.3. Assessment of Friction Force

Figure 9 displays the variations in friction forces recorded by sensors during wear testing based on various load levels, various environmental conditions, and boron coating. The increasing load had a negative effect on the friction force and the wear depth. The average friction force values of both as-received and boron-coated Inconel 601 increased with increasing load under all environmental conditions. The average friction force values of the as-received material in the dry environment were 14.36 N, 23.07 N, and 33.68 N, respectively, at loads of 15, 30, and 45 N. In as-received Inconel 601, a 30.87% reduction was achieved in the MQL environment compared to the dry environment based on the average of all loads. Similarly, these reductions with nano-MQL1 and nano-MQL2 were 53.07% and 59.31%, respectively. However, the values of 12.49 N, 16.50 N, and 21.12 N were obtained with the boron-coated material, respectively, at loads of 15, 30, and 45 N. With boron coating, a decrease of 13.05%, 28.49%, and 37.29% was obtained at loads of 15 N, 30 N, and 45 N in the dry environment, respectively. Furthermore, in boron-coated Inconel 601, a 30.23% reduction was achieved in the MQL environment compared to the dry environment, based on the average of all loads. Additionally, these reductions with nano-MQL1 and nano-MQL2 were 41.60% and 52.32%, respectively. As a result, the highest friction force was 33.675 N with as-received Inconel 601 at a 45 N load in the dry environment, while the lowest friction force was determined as 7.302 N with boron-coated Inconel 601 at a 15 N load in a nano-MQL2 environment. It can be clearly understood from all the graphs that the friction force values are reduced with MQL and nanoparticle-supported MQL environments. Compared to the dry environment, the decrease in the heat between the workpiece and the work tool due to the cooling and lubricating effect of MQL and nanoparticles and, at the same time, the formation of an oil film layer at the interface between the workpiece and the work tool by the MQL environment cause this situation [23].

3.4. Evaluation of CoF

The CoF changes in boron-coated and as-received Inconel 601, depending on the changing forces in different environments, are shown in Figure 10. It is clearly seen that contrary to the wear depth and friction force, the CoF values decreased in both boron-coated Inconel 601 and as-received Inconel 601 due to the increasing load. Moreover, not only the increase in the load but also the transition to the MQL environment provided a decrease in CoF. The lowest CoF value was found with boron-coated Inconel 601 in the nano-MQL2 environment at 45 N, while the highest CoF value was determined with as-received Inconel 601 in a dry environment at a 15 N load. These values were 0.267 and 0.958, respectively. However, it is clearly understood that boron coating is effective in reducing CoF. This situation can be explained by the fact that more surface damage occurs in as-received Inconel 601 due to the lower surface hardness. In addition, it can be said that the effect of nanoparticles in reducing CoF is due to their large surface areas and the formation of a lubricating film layer between the workpiece and the tool.

3.5. Evaluation of Volume Loss

The volume loss values of both as-received and boron-coated Inconel 601, depending on the varying loads, are presented in Figure 11. The highest volume losses in both materials occurred in the dry environment. In addition, increasing load values in each environment and material caused an increase in volume loss. Therefore, the highest volume loss occurred in the dry environment at a 45 N load. The lowest volume loss occurred in the nano-MQL2 environment at a 15 N load. In terms of environmental conditions, the volume loss values of both materials were as follows: dry > MQL > nano-MQL1 > nano-MQL2. On the one hand, in as-received Inconel 601, the volume loss under these conditions (dry environment and 45 N load) was 2.46 mm³, while this value was 1.85 mm³ in boron-coated Inconel 601 (24.8% decrease). On the other hand, volume losses in a nano-MQL2 environment and at a 15 N load were determined as 0.288 mm³ and 0.249 mm³, respectively (13.54% decrease). The results show that the box boriding process, MQL application, and nanoparticle reinforcement all are effective in reducing volume loss. It can be said that the volume loss is reduced due to the increase in surface hardness with boron coating and the decrease in friction in the wear environment with MQL and nano-MQL application [14,24].

3.6. SEM Assessment of Worn Surfaces

Figure 12 SEM images of the worn specimens under various conditions (dry, MQL, nano-MQL1, and nano-MQL2) for Inconel 601 alloys, as received and box boronized, amply demonstrate the wear marks and plastic deformation on the surface, both of which are brought on by the plowing wear process. After exposing the samples to a dry atmosphere, EDX analysis was used to determine the quantity of each component on the worn surfaces and SEM images were generated. Cracks and wear indicators grew in tandem with the amount of material lost from the workpiece surface in the dry environment. Crack formation and material accumulation were greatly decreased by altering the lubrication conditions. Material collection on the workpiece surface was found to have significantly decreased, particularly under nano-MQL lubrication settings. The graphs in MQL, nano-MQL1, and nano-MQL2 environments differed from the one depicting the friction force in the dry state for both as-received and boronized Inconel 601 alloys, which is not surprising. The sample surface in the dry environment showed an increase in plastic deformation patterns with increasing load, because of the friction force. The degree of cleanliness of the metal contact surfaces directly relates to the strength of the connection between the two metals. Numerous factors, including lubricants, moisture, absorbed gases, and oxides, all contribute to the weakening of the connection, which leads to wear. Consequently, the MQL and nano-MQL approaches produce surfaces that are noticeably smoother and prevent the creation of micro-welding zones.

4. Conclusions

In this paper, changes in the wear, friction, and volume loss characteristics of as-received and box-boronized Inconel 601 alloys in different processing environments, according to changing loads, were investigated. The research was carried out under dry, MQL, nano-MQL1 (MQL + CuO), and nano-MQL2 (MQL + TiO₂) lubrication conditions. The results obtained from this study are as follows:

• The research results revealed that MQL, nanoparticle reinforcement on MQL, and boron coating all are beneficial in reducing the wear depth. It turns out that the hard and flexible surfaces formed on the material surface by boriding provide resistance to abrasion, reducing the depth of wear. Under a 15 N load in a dry environment, the wear depth was determined as 141.554 µm for as-received Inconel 601, while it was determined as 120.518 µm for boron-coated Inconel 601 (14.86% reduction). In the nano-MQL2 environment, the wear depth value of boron-coated Inconel 601 was reduced by 9.96% (from 43.811 µm to 39.45 µm) at 15 N compared to as-received Inconel 601.
• With as-received Inconel 601, the average friction force in the MQL environment decreased by 30.87% compared to the dry environment. These reductions were 53.07% and 59.31% in nano-MQL1 and nano-MQL2 environments, respectively. In boron-coated Inconel 601, the average friction forces in MQL, nano-MQL1, and nano-MQL2 environments decreased by 30.23%, 41.60%, and 52.32%, respectively, compared to the dry environment. This situation is explained by the decrease in the heat between the workpiece and the work tool due to the cooling and lubricating effect of MQL and nanoparticles compared to the dry environment.
• The results showed that the maximum CoF value was 0.958 with as-received Inconel 601 in a dry environment at a 15 N load, while the lowest CoF value was 0.267 with boron-coated Inconel 601 in a nano-MQL2 environment at 45 N. It has been understood that the increase in surface hardness with boron coating and the formation of a lubricating film layer between the workpiece and the tool by nanoparticles support the reduction of CoF.
• In as-received Inconel 601, the volume loss in the dry environment and at a 45 N load was 2.46 mm³, while this value was 1.85 mm³ in boron-coated Inconel 601; thus, a 24.8% decrease was achieved. However, in the nano-MQL2 environment and at a 15 N load, the volume losses for as-received and boron-coated Inconel 601 were determined as 0.288 mm³ and 0.249 mm³, respectively (13.54% decrease). It can be claimed that the volume loss decreases as a result of the boron coating’s increased surface hardness and the MQL and nano-MQL application’s decreased wear environment friction.
• The higher frequency of cracks and wear indicators that show up under dry conditions is due to the increased removal of material from the surface. Cracks and the deposition of materials can be significantly decreased by changing the lubricating conditions. Furthermore, the application of nano-MQL lubrication reduces the build-up of material on the object’s surface.