Wear Behavior of TiAlVN-Coated Tools in Milling Operations of INCONEL^® 718

Abstract

The use of coatings on cutting tools offers several advantages from the point of view of wear resistance. A recent technique with great coating deposition potential is PVD HiPIMS. TiAlN-based coatings have good resistance to oxidation due to the oxide layer that is formed on their surface. However, by adding doping elements such as Vanadium, it is expected that the wear resistance will be improved, as well as its adhesion to the substrate surface. INCONEL^® 718 is a nickel superalloy with superior mechanical properties, which makes it a difficult-to-machine material. Milling, due to its flexibility, is the most suitable technique for machining this alloy. Based on this, in this work, the influence of milling parameters, such as cutting speed (V_c), feed per tooth (f_z), and cutting length (L_cut), on the surface integrity and wear resistance of TiAlVN-coated tools in the milling of INCONEL^® 718 was evaluated. The cutting length has a great influence on the process, with the main wear mechanisms being material adhesion, abrasion, and coating delamination. Furthermore, it was noted that delamination occurred due to low adhesion of the film to the substrate, as well as low resistance to crack propagation. It was also observed that using a higher cutting speed resulted in increased wear. Moreover, in general, by increasing the milling parameters, machined surface roughness also increased.

1. Introduction

It is known that engineering materials are selected and designed into functional components to efficiently perform required purposes [1]. For this, the material’s surface is extremely important and is directly related to its performance, especially in applications where contact with the work surface has a great influence [2]. At this point, thin film deposition stands out, which aims to improve both resistances to wear [3] and corrosion [4] as well as more specific properties [5].

Surface engineering comprises the process of overlaying, modification techniques, and coating deposition [6,7]. In the first technique, a material with physical and chemical properties superior to its substrate is deposited on it, forming a thick and solid film [8]. Examples include laser cladding, laser additive manufacturing, and welding [9,10,11,12,13]. In the case of modification techniques, there are heat treatments, in which the chemical properties of the surface are changed [14]. Lastly, which is addressed in this work, there is the coating deposition technique, which can be deposited by chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes [15].

The CVD process, as the name suggests, occurs through a series of chemical reactions under specific conditions such as temperature, pressure, reaction rates, and others [16]. Therefore, the quality of the film obtained is very dependent on the parameters involved in the process, as well as the chemical reactions between the reagent and the substrate. The process occurs through the chemical reaction of the target in the vapor phase on the surface of the substrate [1]. Thus, it is essential that monitoring occurs throughout the entire process, as the chemical composition generated from the reaction and the physical structure (grain size, for example) can be adapted by changing parameters, such as temperature, pressure, gas flow, and reactor geometry [17]. The process can reach 900 °C [18], and the films produced have good hardness, a wide range of thicknesses, which are uniform across the substrate surface, and low porosity even on substrates with complex geometry [19].

Some authors have carried out depositions using the CVD process. For example, Schalk et al. [20] deposited TiB₂ coatings on carbide substrate with and without TiN-based layers, aiming to evaluate the presence of residual stresses. The authors verified that the TiN base layer presented tensile stresses and in the TiB₂ layer compressive stresses. In turn, Kainz et al. [21] deposited several types of coating using the PVD process, such as TiN, TiBN, TiCN, and TiBCN, because according to the authors, when elements are added to the TiN coating, its properties are improved. Thus, confirming what was predicted, it was found that TiBCN has the highest hardness (32.2 ± 1 GPa), Young’s modulus (587 ± 29 GPa), and level of residual stresses (8.5 ± 0.4 GPa).

On the other hand, in the PVD process, the material to be deposited is transformed into atomic particles through a physical thermal collision process [22]. This material is ejected from the target by ion bombardment [23]. As a result, it is condensed on the surface of the substrate [24]. Compared to the CVD process, the energy expenditure is much lower [25], and it does not produce toxic gases [26]. However, PVD coatings deposition on complex geometries is considerably more difficult to conduct when compared to the CVD process [27] due to its physical nature. The advantages of this process include deposition on substrates with lower melting points, variation in coating characteristics, and the possibility of multilayer deposition, as well as greater energy efficiency [28].

Basically, there are two ways in which particles can be extracted from the target: sputtering and evaporation [29]. Within the sputtering process, a recent technique with great potential is High Power Impulse Magnetron Sputtering (HiPIMS) [30]. This technique allows the direction and ionization fraction of the sputtered species to be tuned, which has the advantage of producing films with differentiated properties [15]. As it is a recent technique, much needs to be studied and developed, and there is still a clear gap in information in the literature. One paper using this technique was published by Wang et al. [31], in which a TiN coating with different N₂ flow rates was deposited, aiming to verify the importance of this parameter. According to the authors’ results, N₂ flow rates had an important impact on the surface microstructure of the coatings.

One area of application in which the use of coatings can be widely used is in machining tools, especially when it comes to materials that are difficult to machine. These coatings, when deposited on cutting tools, tend to improve their wear resistance [32], thus improving process performance [33] and the quality of the machined surface [34]. In addition, coatings are not only applied to cutting tools but also to injection molds [35,36,37] and in the stamping process [38,39], given the range of benefits they bring [40,41]. INCONEL^® 718 is a material that has superior properties compared to other types of alloys [42] and weldability [43]. It is a nickel superalloy, with high hardness, a tendency to undergo work hardening, and low thermal conductivity, which makes it widely used in aeronautical, aerospace, biomedical, and petrochemical industries [44,45,46]. Moreover, because of its superior properties, it is considered a difficult-to-machine material [47]. The milling process is the most used in the machining of this alloy, as it is a more flexible process and produces parts with a good surface finish [48]. Furthermore, PVD coatings tend to perform better in finishing operations, as they are usually thinner [49]. There is still a gap in the literature regarding the milling operation of INCONEL^® 718 and variation in its parameters and process optimization, especially when it comes to finishing operations.

The first coating system applied to cutting tools was TiN (titanium nitride), which has excellent corrosion resistance and high hardness [50]. Aiming to improve the performance of the machining process, aluminum was added to it, forming TiAlN (titanium aluminum nitride), which resulted in a coating with good oxidation resistance due to the oxide layer formed on its surface [51]. Some authors, such as Yi et al. [52], Çomaklı [53], Li et al. [54], and Wang et al. [55], analyzed this type of base coating (TiAlN) as an object of study. Furthermore, some authors added doping elements to it with the aim of improving wear resistance and generating better corrosion resistance, hardness, adhesion, and toughness 18. For example, Yang et al. [56] analyzed the influence of Mo additions on the properties of TiAlN coatings, verifying better wear resistance for 8.3 at.% Mo due to the formation of molybdenum trioxide. In turn, Liu et al. [57] evaluated the influence of different percentages of Ru additions. The authors noted an improvement by ~32% through 5 at.% Ru addition in tool life. Furthermore, Aninat et al. [58] carried out a comparative study between TiAlN, TiAlYN, TiAlTaN, and TiAlTaYN coatings, observing that with the addition of Y the hardness was increased, and with the addition of Ta the compressive stresses increased.

Vanadium is another element with great potential to contribute to this type of coating; however, in the literature, studies are focused on characterizing the physical and chemical properties of this coating, causing it to have a gap in terms of its performance and wear resistance in milling operations. Added to this is the fact that there is a gap in the literature on milling of INCONEL^® 718 aimed at finishing operations, wear behavior, and the performance of the process, with the majority concentrated on the turning process [59].

The analysis of the surface roughness of the machined part, as well as the resulting wear on the cutting tool, indicates the process performance and, consequently, its productivity [40,41]. Thus, it is possible to verify the influence of the parameters used and to make adjustments to improve this performance, including the analysis and suitability of the coating used [60]. Another important aspect regarding the machining of INCONEL^® 718 is the cutting forces developed in the process [61]. Therefore, one of the methods used to analyze and verify the wear suffered by cutting tools is Scanning Electron Microscopy (SEM) [48]. This technique allows observation with high resolution at different magnifications [62], which is very beneficial for analyzing the performance of the machining process. The microscope has two imaging modes, Secondary Electrons (SEs) and Backscattered Secondary Electrons (BSEs), as well as an integrated Energy Dispersive Spectrometer (EDS) [63].

Thus, to analyze the wear of cutting tools in the machining process, authors such as Cabibbo et al. [64], Kayaba et al. [65], Lindvall et al. [66], and Olsson et al. [67] used the SEM technique, as it offers important and relevant information in terms of morphology and chemical composition. Thus, many conclusions can be drawn from this technique, which can assist in discussions about improving wear resistance and process performance, through the wear mechanisms identified there. Based on this, SEM analysis was crucial in the development of this work, which addressed the influence of machining parameters, such as cutting speed, feed per tooth, and cutting length, on the surface integrity and wear behavior of TiAlVN-coated end-milling tools in the milling operation of INCONEL^® 718. The coating was deposited using the PVD HiPIMS process. Therefore, this work intends to fill a gap in the literature regarding the wear behavior of tools coated with TiAlVN when milling INCONEL^® 718.

2. Materials and Methods

2.1. Materials

2.1.1. Machining Tools

The substrate of the end-mill cutting tools was composed of tungsten carbide (WC-Co) grade 6110, with 6% Co used as a binder. Furthermore, it had an average grain size of 0.3 μm. The geometry of the tools is shown in

Table 1. Geometry of the end-mill WC-Co cutting tools used in the machining tests.

Tool Geometry Dimensions	Value
Cutting diameter	6 mm
Maximum cutting depth	13 mm
Total length	57 mm
Chamfer	45°; 0.20 mm
Helix angle	35°/38°
Rake angle	12°
Clearance angle	10°
Number of flutes	4

. These tools were provided by INOVATOOLS, S.A. (Leiria, Portugal).

The cutting tools were coated with TiAlVN by PVD HiPIMS process using CemeCom CC800/HiPIMS equipment (CemeCon, AG, Wuersele, Germany), with four targets. The deposition parameters are presented in

Table 2. Deposition parameters of TiAlVN coating.

Deposition Parameters	TiAlVN Layer
Reactor gases	Ar+ + Kr + N2
Target amount/composition	4/TiAlV
Pressure [mPa]	580
Bias voltage [V]	−60
Holder rotational speed [rpm]	1

.

The adopted deposition parameters were selected from successful experiments performed before [15], and a rotational speed of 1 rpm was applied to the substrate holder to ensure that the coating was homogeneous throughout its entire useful surface.

2.1.2. Machined Material

The material selected to be machined is a difficult-to-machine nickel superalloy, INCONEL^® 718. It was supplied as a heat-treated round bar with a diameter of 158 mm. The heat treatment conditions pointed out by the supplier are as follows: (1) solution annealing at 970 °C, followed by rapid cooling by water quenching; (2) precipitation hardening at 718 °C for 8 h; (3) cooling in a furnace until 621 °C for 8 h; and (4) air cooling. According to the material supplier, Paris Saint-Denis Aéro (Grândola, Évora), its mechanical properties are shown in

Table 3. INCONEL^® 718 mechanical properties, according to the supplier.

Material Property	Value
Yield strength [MPa]	1200
Ultimate tensile strength [MPa]	1427
Hardness [HBW]	441

, and its chemical composition (%wt) is shown in

Table 4. Machined material chemical composition (%wt) [44].

Elements (%wt)
Ni	Cr	Fe	Nb	Mo	Ti	Al	Co
53.89	18.05	17.78	5.350	2.900	0.960	0.510	0.200
Cu	Si	Mg	B	C	P	N	Mg
0.100	0.080	0.078	0.039	0.023	0.010	0.007	0.0017

.

Moreover, some residual elements, such as Ag, Ca, Pb, Bi, S, and O, are also part of its composition.

2.2. Methods

2.2.1. Machining Tests

The tests were performed using an HAAS VF-2 CNC machining center (HAAS Automation, Oxnard, CA, USA). The maximum speed the equipment reaches is 10,000 rpm, and its maximum power is 20 kW. As INCONEL^® 718 was supplied as a cylindrical bar, the best strategy to use was spiral milling, which starts in the center of the workpiece towards its periphery, avoiding a previous partial lateral contact of the tool with the external skin of the round bar. Figure 1 shows the machined workpiece and the spiral marks on its surface.

The machining parameters were initially determined based on the tool’s substrate provided. However, some parameters had to be adjusted so that the tool could have a better performance. For example, the initial values for the axial depth of cut (a_p) were 0.2 and 0.1 mm; however, in initial tests it was found that the cutting tools failed after the initial plunge. Therefore, the value of 0.08 mm was selected for this parameter. Furthermore, the radial depth of cut (a_e) was kept constant in all tests, preventing unwanted wear phenomena. In addition, the lubrication regime with cutting fluid provided with 5% oil in water was used, as it is a material that is difficult to machine, highly hard, and tends to increase the temperature in the process. All parameters and test conditions are shown in

Table 5. Parameters used in machining tests.

Reference	Vc [m/min]	fz [mm/tooth]	Lcut [m]	ap [mm]	ae [mm]
S75F75L5	75	0.0525	5	0.08	4.5
S75F75L15	75	0.0525	15	0.08	4.5
S75F100L5	75	0.0700	5	0.08	4.5
S75F100L15	75	0.0700	15	0.08	4.5
S75F150L5	75	0.1050	5	0.08	4.5
S75F150L15	75	0.1050	15	0.08	4.5
S100F75L5	100	0.0525	5	0.08	4.5
S100F75L15	100	0.0525	15	0.08	4.5
S100F100L5	100	0.0700	5	0.08	4.5
S100F100L15	100	0.0700	15	0.08	4.5
S100F150L5	100	0.1050	5	0.08	4.5
S100F150L15	100	0.1050	15	0.08	4.5
S125F75L5	125	0.0525	5	0.08	4.5
S125F75L15	125	0.0525	15	0.08	4.5
S125F100L5	125	0.0700	5	0.08	4.5
S125F100L15	125	0.0700	15	0.08	4.5
S125F150L5	125	0.1050	5	0.08	4.5
S125F150L15	125	0.1050	15	0.08	4.5

.

As can be seen in Table 5, the radial cutting depth value (a_e) was 4.5 mm, as this represents 75% of the diameter of the cutting tool (6 mm). The parameters that varied in the tests were V_c, F_z, and L_cut. The central cutting speed value was 100 m/min, and it varied by 25% higher and 25% lower. In the case of feed per tooth, the central value was 0.07 mm/tooth, varied 25% for less, and 50% for more. As for the cutting length, 5 m and 15 m were used. It is important to highlight that three tools were used for each test condition.

2.2.2. Machined Surface Roughness Analysis

The roughness of the machined surface was measured using a Mahr Perthometer M2 profilometer (Mahr, Gottingen, Germany), in accordance with DIN EN ISO 4288:1996 [68], and it was evaluated after each test under all conditions. Five measurements were taken in different regions of the workpiece surface, both in the radial and tangential directions, and the arithmetic average roughness value (R_a) was assessed. Therefore, it may be possible to observe differences in measurements at the center and periphery of the workpiece, since the latter is the end of the spiral trajectory; thus, the roughness is expected to be greater due to the sustained wear of the cutting tool. As for the roughness measurement parameters, a cut-off of 0.8 mm and a measuring length of 5.6 mm were used. Thus, it is possible to observe the stability of the milling process, as well as the performance of the cutting tools, and correlate them with the resulting sustained wear.

2.2.3. Tool Wear Analysis

The tool wear analysis was evaluated using SEM equipment, according to the ISO 8688-2:1986 [69]. For this, an FEI QUANTA 400 FEG scanning electron microscope was used (FEI, Hillsboro, OR, USA), equipped with an EDAX Genesys Energy Dispersive X-Ray Spectroscopy (EDAX Inc., Mahwah, NJ, USA) microanalysis system. For the analysis, BSE (Backscattered Secondary Electron) mode was used at magnifications between 40× and 2000×. For EDS, a beam potential of 15 kV was used, which was sporadically reduced to 10 kV, avoiding interference of the substrate in the analysis.

According to ISO 8688-2:1986 [69], the wear phenomenon with the greatest influence must be considered as a life criterion. Therefore, VB3 was selected to analyze and check the wear of the cutting tools, as it was located close to the chamfer. In addition, wear measurements were performed in “Position 1”. The top view, rake face, and clearance face of the cutting tools were analyzed. Furthermore, with the EDS analysis, it was possible to confirm that material from the workpiece adhered to the tool. To facilitate identification, the tool teeth were numbered from 1 to 4, as shown in Figure 2, which also illustrates how the VB3 measurement was carried out.

3. Results and Discussion

3.1. Machined Surface Roughness Analysis

As previously stated, measurements for surface roughness were carried out in the radial and tangential directions; however, no significant differences were observed between them. The results obtained for machined surface roughness in each condition are shown in

Table 6. Surface roughness (R_a) for all conditions tested.

Reference	Average Ra Value [µm]
S75F75L5	0.809 ± 0.064
S75F75L15	0.694 ± 0.087
S75F100L5	0.800 ± 0.054
S75F100L15	0.902 ± 0.011
S75F150L5	0.813 ± 0.032
S75F150L15	1.009 ± 0.074
S100F75L5	0.932 ± 0.192
S100F75L15	0.719 ± 0.137
S100F100L5	0.678 ± 0.109
S100F100L15	1.217 ± 0.394
S100F150L5	0.815 ± 0.167
S100F150L15	2.819 ± 0.474
S125F75L5	0.563 ± 0.068
S125F75L15	0.795 ± 0.102
S125F100L5	0.899 ± 0.060
S125F100L15	1.307 ± 0.419
S125F150L5	0.728 ± 0.083
S125F150L15	2.142 ± 0.303

. For comparison and analysis purposes, Figure 3 illustrates the values for each condition tested. The graph in Figure 3 shows the test conditions in the X-axis, and three series of roughness results corresponding to the values of feed per tooth used in this work in the Y-axis. The number after the “S” indicates the cutting speed, the number after “F” points out the feed per tooth, and the number after the “L” indicates the cutting length.

As can be seen, when increasing the cutting length from 5 m to 15 m, the roughness tends to increase. This can be observed in almost all conditions, except when comparing condition S75F75L5 with S75F75L15 and condition S100F75L5 with S100F75L15. The increase in surface roughness with increasing cutting length was already expected, since INCONEL^® 718 is a difficult-to-machine material that can generate high levels of wear and consequent loss of surface quality [44]. In conditions where roughness decreased, it can be pointed out that the stability of the process was not in compliance with high levels of vibration, considering tests under 5 m of cutting length, which generates higher surface roughness of the machined part.

Regarding the feed per tooth, under condition S75L5, no significant influence was observed since the values were similar when changing this parameter. In turn, under condition S100L5, roughness decreased when increasing from 75% to 100%, followed by a slight increase when increasing feed per tooth to 150%. For condition S125L5, the opposite of the previous conditions occurred. In this case, there was an increase in roughness when increasing feed per tooth from 75% to 100%, followed by a slight decrease when increasing this parameter to 150%. On the other hand, in all conditions with 15 m cutting length, the trend was clearer, with an increase in surface roughness when increasing the feed per tooth.

As other have authors stated [70], it is expected that the feed per tooth influences the roughness of the machined surface. In general, as referred to in [71], when this parameter is increased, roughness tends to increase. In situations where this did not occur, as reported by other authors [72], it can be said that the chip generation mechanism may have influenced the process.

Regarding cutting speed and as reported by other authors [73,74], it was expected that increasing this parameter would result in better surface quality. However, this only occurred under the conditions when increasing from S100F75L5 to S125F75L5, S75F100L5 to S100F100L5, and S100F150L5 to S125F150L5, i.e., only under the 5 m cutting length condition. This may be due to the high tool wear at 15 m cutting length, as referred to in [75,76], since INCONEL^® 718 even at small distances already generates excessive wear, and also due to the high abrasive wear that leads to tool chipping. Therefore, under these conditions, in general, when the cutting speed is increased, the roughness also increases.

Regarding the differences in measurements at the center and in the periphery of the part, it can be said that in the latter, due to the end of the spiral path, the roughness tends to increase due to the accumulation of wear, as previously reported in [77]. This can be observed based on the standard deviation resulting from the measurements, in which the conditions S100F100L15, S100F150L15, and S125F100L15 had high values, i.e., the roughness values in the center were lower than those at the periphery of the part. This illustrates the progress of tool wear over the cutting length.

Generally, as referred to in [78,79], due to the HiPIMS deposition process, the coating obtained has a significant level of residual compressive stress, and this is related to the surface integrity of the machined part. However, when observing the results, it appears that the roughness is above what was expected, which can result in poor performance of the milling process.

Aiming for a more accurate analysis and statistical validation of the results with proof of the influence of machining parameters, t-tests were carried out comparing two samples with different variances using Microsoft^® Excel™ software (2019 version). A p of 0.05 was used. Statistical tests were performed by varying and verifying the influence of L_cut, f, and s.

Table 7. Conclusions obtained from the t-test.

Condition	Comments
Lcut influence	As s increases, the influence of Lcut becomes greater. In the case of conditions S75F100L5 vs. S75F100L15, S100F75L5 vs. S100F75L15, and S125F100L5 vs. S125F100L15, surface quality is not affected by Lcut, as it is not statistically significant.
f influence	The influence of f is not clear, but it appears that by increasing this parameter the surface quality is impaired, especially regarding the existence of abrasive wear.
s influence	s is the most sensitive parameter, which has great variation and impact on the machined surface quality. In conditions considered more severe, it is a parameter that has a great influence on the machined surface quality.

summarizes the results of the statistical tests, with the results set out in Appendix A.

3.2. Wear Measurements and Characterization

The results obtained for VB3 in each condition tested are shown in

Table 8. Average VB3 values for each tested condition.

Reference	Average VB3 Value [µm]
S75F75L5	470.87 ± 95.54
S75F75L15	547.29 ± 73.92
S75F100L5	448.66 ± 85.82
S75F100L15	486.66 ± 27.80
S75F150L5	361.81 ± 86.81
S75F150L15	359.66 ± 92.08
S100F75L5	393.86 ± 90.26
S100F75L15	524.31 ± 57.41
S100F100L5	395.50 ± 89.94
S100F100L15	583.44 ± 31.10
S100F150L5	447.06 ± 53.43
S100F150L15	667.49 ± 59.56
S125F75L5	371.27 ± 109.8
S125F75L15	616.22 ± 136.2
S125F100L5	309.99 ± 45.16
S125F100L15	497.61 ± 117.6
S125F150L5	399.85 ± 32.37
S125F150L15	661.20 ± 133.3

and Figure 4, following the same nomenclature previously pointed out.

Figure 4 indicates that when the cutting length is increased from 5 m to 15 m, the resulting flank wear (VB3) increases. This is consistent, given that when this parameter is increased, the sustained wear tends to increase, and the quality of the machined surface tends to decrease [48]. Furthermore, even at a cutting length of 5 m, VB3 already had significant values, which confirms the fact that INCONEL^® 718 is a hard-to-machine material [80]. Figure 5 illustrates the conditions S100F150L5 and S100F150L15 with the top view of the cutting tools, which can be seen to verify the influence of the cutting length on the flank wear.

Regarding the feed per tooth, a single trend was not observed for all tools, and its influence was more difficult to detect. For example, for conditions S75L5 and S75L15, flank wear decreased with the increase in feed per tooth, whereas in conditions S100L5 and S100L15, flank wear increased with the increase in feed per tooth. On the other hand, in conditions S125L5 and S125L15, flank wear was reduced when increasing from 75% to 100% feed per tooth, followed by an increase when increasing to 150%, resulting in higher wear than in the situation of 75% feed per tooth. It is interesting to note that the behavior was the same for each analysis group regarding cutting speed.

The influence of feed per tooth is related to the chip formation mechanism [81], with the formation of fine chips being the most common cause of poor process performance, which causes greater abrasive wear [82]. Thus, it is more advantageous for the chip section to be thicker, which results in greater integrity and extraction flow, with care that it is not too high, as this can lead to failure and consequent breakage of the cutting tool’s edges [18].

As for cutting speed, usually an increase in this parameter results in a smoother cutting behavior by the cutting tools [73]. However, under the conditions analyzed, no clear trend was observed. In conditions F75L5 and F100L5, wear decreased when cutting speed increased. In the case of conditions F150L5, F100L15, and F150L15, wear increased when increasing the cutting speed from 75 m/min to 100 m/min, followed by a decrease in wear when increasing to 125 m/min. In turn, in condition F75L5, flank wear decreased with increasing cutting speed from 75 m/min to 100 m/min and increased when increasing the speed to 125 m/min. This is not commonly observed and may be related to the adhesion of material to the surface of the coating, creating more abrasion and leading to its delamination [83]. Figure 6 illustrates the tools used under condition F100L5, in which with increasing cutting speed, flank wear decreased. Based on this, condition S125F100L5 proved to be the optimal parameter with regard to maximum wear VB3, as this had the lowest value in addition to being at higher speed and intermediate feed per tooth, which makes them great parameters for minimizing wear and maintaining high performance and productivity.

3.3. Analysis of Wear Mechanisms

3.3.1. Cutting Speed of 75 m/min

Regarding the type of flank wear identified for tools tested at 75 m/min,

Table 9. Wear mechanisms for conditions tested at 75 m/min.

Condition	Wear Mechanisms Observed
S75F75L5	Abrasion
	Adhesion
	Delamination
S75F75L15	Abrasion
	Adhesion
	Delamination
	Chipping
	Cracking
	Bubbles (coating)
S75F100L5	Abrasion
	Adhesion
	Delamination
	Bubbles (coating)
S75F100L15	Abrasion
	Adhesion
	Delamination
	Bubbles
	Cracking
S75F150L5	Abrasion
	Adhesion
	Delamination
	Bubbles
S75F150L15	Abrasion
	Adhesion
	Delamination

shows the main wear mechanisms identified for each tested condition.

As can be seen, in all conditions, abrasive and adhesive wear and coating delamination were observed, varying according to intensity and determined by SEM analysis. For example, in conditions with a cutting length of 15 m, abrasive wear was of significant intensity, whereas at 5 m this same wear was moderate. Abrasive and adhesive wear are very common and predominant when it comes to wear on cutting tools in the machining process [84]. Furthermore, it was seen that INCONEL^® 718 commonly adheres to cutting tools [85]. Therefore, due to material adhesion, the tendency is to generate severe wear [86] and, consequently, higher abrasive wear, which leads to coating delamination [87]. The sequence observed for these wear mechanisms led to the understanding that, initially, the main wear mechanism is abrasion, because the coating is hard enough to make contact with the workpiece material and sustain its hardness effect. However, any surface defect or the abrasion effect of the workpiece surface material over the tool surface leads to the initial wear. When the wear progresses and an initial failure of the coating occurs, the coating starts detaching in small fragments, because the workpiece material flows over the coating and, finding a failure, tends to concentrate the main wear effect in this area. Thus, the wear tends to progress in a more accentuated way in these areas, progressing in a normal way (abrasion) in the remaining area where the contact between the chip and the tool edge is more intense. The heat generated in this area will be even higher as the coating is removed because the tool surface is rougher than it was initially, and the insulation effect of the coating no longer exists. Thus, the tool substrate starts degrading in a more accentuated way, and phenomena like chipping and breaking start appearing. This is the sequence observed in general in the set of machining tests performed, independent of the set of parameters used. It is evident that the evolution speed of these phenomena is higher as the machining parameters selected are more severe.

Figure 7 illustrates the wear present in condition S75F75L5, and Figure 8 shows the predominant wear in condition S75F75L15. Therefore, it can be seen that the intensity of abrasive wear in the second condition is more intense, as well as the appearance of chipping, cracking, and bubbles in the coating. Because this condition has a lower speed, finding other mechanisms was expected, such as a built-up edge, since INCONEL^® 718 is a material with high heat resistance and has high reactivity with tool materials [48]. Furthermore, milling performance is impaired by the presence of chips due to the change in the geometry of the workpiece that this wear promotes, causing the chip generation mechanism to also be modified, impacting the respective surface quality [81].

In the case of conditions S75F100L5 and S75F100L15, abrasive wear and coating delamination were of high intensity in the second. Figure 9 illustrates these two conditions, with Figure 9a referring to abrasive wear, bubbles, and adhered material in condition S75F100L5, and Figure 9b presenting the delamination, adhered material, bubbles, and cracking in condition S75F100L15. The presence of cracks can be justified by the fact that the coating does not exhibit crack propagation resistance enough to avoid this phenomenon, as it is a monolayered coating [88]. Typically, multilayer or nanostructured coatings have greater resistance to crack propagation due to the adhesion layer in contact with the tool substrate and the successive interfaces between layers, which tend to block crack propagation [18].

In turn, the conditions with 150% feed per tooth exhibited adhered material, abrasive wear, and delamination. However, condition S75F150L5 also showed some bubbles on the coating surface, which in some situations led to delamination of this coating. Figure 10a illustrates adhered material, bubbles, and delamination in condition S75F150L5, and Figure 10b illustrates adhered material and delamination in condition S75F150L15.

3.3.2. Cutting Speed of 100 m/min

Regarding the type of flank wear identified for tools tested at 100 m/min,

Table 10. Wear mechanisms for conditions tested at 100 m/min.

Condition	Wear Mechanisms Observed
S100F75L5	Abrasion
	Adhesion
	Delamination
	Cracking
	Bubbles
S100F75L15	Abrasion
	Adhesion
	Delamination
	Cracking
	Bubbles
S100F100L5	Abrasion
	Adhesion
	Delamination
	Cracking
S100F100L15	Abrasion
	Adhesion
	Delamination
	Fracture/break
S100F150L5	Abrasion
	Adhesion
	Delamination
S100F150L15	Abrasion
	Adhesion
	Delamination
	Bubbles
	Fracture/break

shows the main wear mechanisms identified for each tested condition.

In the same way as for the conditions tested at 75 m/min, in all conditions at 100 m/min abrasive wear, adhered material and delamination were observed. Furthermore, in some conditions it was possible to observe bubbles and cracks formed in the coating, which tend to lead to its delamination, as well as breaks or failures in the cutting tool substrate. It should also be noted that abrasive wear was more intense for conditions with 15 m of cutting length, and material adhesion was found both on the tool substrate and on the coating, being greater in the former.

Figure 11 illustrates the conditions tested at 75% feed per tooth. Both have the same associated wear mechanisms; however, the abrasion in the condition tested at 15 m cutting length is of greater intensity, which is consistent with the result obtained for VB3.

An important aspect is that the delamination of the coating, which may have occurred due to lack of adhesion of the coating to the substrate, can be followed by crack propagation due to the high thermal coefficient of expansion of the coating material compared to the tool substrate, as reported in [89]. This mismatch creates excessive stress levels in the interface between the coating and the substrate, leading to preferential conditions for developing cracks and their propagation, as well as further delamination of the coating. Furthermore, INCONEL^® 718 tends to generate high temperatures in the milling process [90], which can generate a thermal gradient between the tool substrate and the coating layer, causing rapid delamination; consequently, the tool substrate will be in direct contact with the workpiece to be machined, which may result, as observed in conditions S100F100L15 and S100F150L15, in the cutting edge’s breakage and failure [91]. Figure 12 illustrates condition S100F100L15, in which the cutting-edge fracture was observed.

Furthermore, an EDS analysis was carried out to confirm that the adhered material provides from the workpiece. For this, condition S100F100L5 was considered, as illustrated in Figure 13. Figure 14 shows the composition analysis of each zone identified in Figure 13. Additionally,

Table 11. Percentage of elements detected in zone 1—tool substrate.

Zone 1—Elements (%wt)
W	Co	C	Ti	Al	O	Cr
86.52	7.66	1.52	1.39	1.32	0.8	0.78

shows the percentage of each element in zone 1,

Table 12. Percentage of elements found in zone 2—INCONEL^® 718.

Zone 2—Elements (%wt)
Ni	Fe	Cr	Nb	Mo
51.49	16.97	16.91	4.95	3.0
C	Co	Ti	W	Al
2.04	1.85	1.11	1.05	0.62

in zone 2, and

Table 13. Percentage of elements noticed in zone 3—TiAlVN coating.

Zone 3—Elements (%wt)
Ti	Al	V	N
45.08	32.56	13.24	9.13

in zone 3.

According to the EDS analysis, zone 1, rich in tungsten, refers to the tool substrate, zone 2, with a large amount of nickel, confirms the presence of INCONEL^® 718 adhered to the tool substrate, and zone 3 concerns the coating, given the presence of Ti, Al, V, and N. Therefore, the EDS results obtained for the three selected zones are consistent with the chemical composition of the areas indicated in Figure 13.

3.3.3. Cutting Speed of 125 m/min

Regarding the type of flank wear identified for tools tested at 125 m/min,

Table 14. Wear mechanisms for conditions tested at 125 m/min.

Condition	Wear Mechanisms Observed
S125F75L5	Abrasion
	Adhesion
	Delamination
	Cracking
	Bubbles
S125F75L15	Abrasion
	Adhesion
	Delamination
	Cracking
	Bubbles
	Fracture/break
S125F100L5	Abrasion
	Adhesion
	Delamination
	Cracking
S125F100L15	Abrasion
	Adhesion
	Delamination
	Fracture/break
	Cracking
	Bubbles
S125F150L5	Abrasion
	Adhesion
	Delamination
	Chipping
S125F150L15	Abrasion
	Adhesion
	Delamination
	Bubbles
	Fracture/break
	Cracking

shows the main wear mechanisms identified for each tested condition.

In the same way as in the previous conditions, at 75 m/min and 100 m/min, in the conditions tested at 125 m/min, abrasive, adhesive wear, and delamination of the coating were observed, the first being at greater intensity in 15 m cutting length conditions. In addition, bubbles, cracking, and chipping were also observed in some specific conditions, and breakage of the cutting edge was observed under the conditions tested at 15 m cutting length. It was also verified that abrasive wear was more intense on the tools’ clearance face, and the adhesion of material was registered on the tools’ flank, edges, and rake face.

Furthermore, it was observed that the abrasive wear mark is wider for tools that were tested with 75% feed per tooth and deeper for tools tested with 150% of this same parameter. This must have occurred due to frictional wear that can modify the geometry of the tools; consequently, the wear is more severe. When comparing the wear resulting from the conditions tested at 125 m/min, it was noted that they are more developed than in the previous conditions, i.e., the cutting speed directly impacts the performance of the cutting tools in the milling operation.

Figure 15 illustrates the predominant wear mechanisms under conditions S125F75L5 and S125F75L15. In turn, Figure 16 illustrates conditions S125F100L5 and S125F100L15, and Figure 17 depicts conditions S125F150L5 and S125F150L15.

As observed, delamination of the coating was verified in all tested conditions, and this may be related to its low adhesion to the substrate, since the adhesion of Vanadium to TiAlN decreases resistance to oxidation, due to the formation of titanium oxides instead of aluminum oxide, which is more effective in protection.

4. Conclusions

This work presents a study and analysis regarding the performance of TiAlVN-coated tools in INCONEL^® 718 milling operations, under various machining conditions. The parameters that varied were cutting speed, feed per tooth, and cutting length. This study aims to fill a gap in the literature regarding TiAlN Vanadium-doped coatings. Therefore, the following conclusions can be drawn:

• The parameters had a direct influence on both the roughness of the machined surface and the resulting wear;
• In general, as for the roughness of the machined surface, the condition that allowed the best result was S125F75L5. However, when increasing the cutting length, the surface roughness values also increased;
• No clear trend was observed for the influence of feed per tooth and cutting speed regarding the roughness of the machined surface. However, in general, when these parameters were increased, the roughness also increased;
• As the selected milling strategy was a spiral, there were some measurement differences in the center and periphery of the part, causing the standard deviation to be higher in some situations. This happened due to the wear progress of the tool over the cutting length from the center to the periphery;
• Flank wear was higher for condition S100F150L15 and lower for condition S125F100L5, which show the influence of the cutting length and, maybe, a conjugated slight influence of the cutting speed;
• The longer the cutting length, the greater the abrasive wear, for example, when comparing conditions S75F100L5 and S75F100L15;
• The higher the cutting speed, the more developed was the wear, for example, when comparing the conditions tested at 100 m/min and 125 m/min; in the latter, the wear was more developed and of greater intensity;
• Abrasive wear, material adhesion, and delamination were the predominant wear mechanisms identified in all tested conditions. Furthermore, cracking, bubbles in the coating, chipping, and breakage of the cutting tool’s edge were also observed under different conditions;
• For condition S75F75L5, abrasive wear, delamination, and material adhesion were mainly observed, and for condition S75F75L15, the intensity of abrasive wear was more intense, as well as the appearance of chipping, cracking, and bubbles in the coating;
• In the case of conditions S75F100L5 and S75F100L15, the abrasive wear and coating delamination were of high intensity in the second, with the presence of delamination, adhered material, bubbles, and cracking;
• For conditions S100F75L5 and S100F75L15, the mechanisms were delamination, cracking, blisters, abrasive wear, and material adhesion;
• Conditions S100F100L15 and S100F150L15 had a failure and fracture due to the increase in process temperature, causing delamination and causing the tool substrate to be in direct contact with the workpiece to be machined;
• Conditions at 125 m/min cutting speed had abrasive wear, adhesion, delamination, cracking, blisters, and eventual fracture of the cutting edge;
• The factors that led to the delamination of the coating were the low resistance to crack propagation of the monolayer coating, as well as the presence of bubbles on its surface. These bubbles result from the PVD deposition process and made the chip flow over the tools’ surface difficult, promoting coating detachment in localized areas. These detachments leave the substrate visible, create favorable conditions to deposit adhered material from the workpiece, and work as anchors for the further development of cracks. Due to the high hardness of the coating, which implies lower toughness, the coating tends to break and detach in a more severe way from the substrate. This makes the wear drastically increase, leaving the substrate uncoated in the contact area. The remaining phenomena that originated from the severe cutting conditions promoted effects such as breakage of the tool’s cutting edges. Furthermore, the high heat generation in the process also had an influence on the phenomenon previously described.

As limitations of the work, it can be said that uncoated tools were not included in the analysis, the axial depth of cut had a limited value, and only one type of conventional coolant was used. Furthermore, due to the limited number of existing tools and equipment availability, the cutting speed was only varied by three values, feed per tooth by three values, and cutting length by two values.

As future work, it is suggested that other test conditions be analyzed, as well as compared with other TiAlN-based coatings with doping elements. Furthermore, it is proposed that this same coating be analyzed with the presence of an interlayer, which can be beneficial for resistance to crack propagation. Moreover, Taguchi analysis should be performed with regard to optimization of the machining parameters. The results obtained here are useful and can serve as a basis for future work given the difficulty of machining INCONEL^® 718 and regarding the optimization and selection of the best machining parameters for this alloy. Moreover, the literature about TiAlVN coatings is very scarce, mainly when used in machining tools. Thus, this work intends to act as a starting point to contribute to the development of this kind of coating in the near future.