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Article

Performance Evaluation of PVD and CVD Multilayer-Coated Tools in Machining High-Strength Steel

1
Department of Mechanical and Manufacturing Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia
2
Department of Industrial and Manufacturing Engineering, NED University of Engineering and Technology, University Road, Karachi 75270, Pakistan
3
Mechanical Engineering Department, College of Engineering, Prince Sattam Bin Abdulaziz University, Alkharj 11942, Saudi Arabia
4
Laboratoire de Mécanique, Matériaux et Procédés (LR99ES05), École Nationale Supérieure d’Ingénieurs de Tunis, Université de Tunis, Tunis 1008, Tunisia
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(7), 865; https://doi.org/10.3390/coatings14070865
Submission received: 12 June 2024 / Revised: 2 July 2024 / Accepted: 4 July 2024 / Published: 10 July 2024
(This article belongs to the Special Issue Surface Engineering Processes for Reducing Friction and Wear)

Abstract

:
To curtail the negative effects of traditional flood machining, dry cutting using carbide tools has emerged as a prominent alternative for manufacturers, owing to its low cost and phenomenal surface qualities. In line with this view, high-speed machining of high-strength AISI 4340 alloy steel was carried out using multilayer Al2O3/TiCN-CVD and TiAlN/AlCrN-PVD carbide tools in a dry environment. The experimental scheme was adopted, as per Taguchi’s L18 orthogonal array, to investigate the two most crucial machinability aspects, namely tool life and surface roughness. An analysis of variance (ANOVA) was performed on the obtained data, and it was inferred that the feed rate exhibited the strongest effects on both the tool life and surface roughness, with corresponding percentage contributions of 46.22% and 68.96%, respectively. The longest tool lives of 14.75 min and 10.08 min were obtained at a low cutting speed and feed rate for CVD and PVD tools, respectively. However, the lowest surface roughness of 0.276 µm and 0.307 µm was achieved at a high cutting speed and low feed rate for PVD and CVD tools, respectively. The evolution of tool wear, studied through the microscopic images of the worn tools, revealed that a high cutting speed and feed rate accelerated the flank wear for both types of tools. Nevertheless, the CVD tool exhibited better results due to the thick and effective Al2O3/TiCN coating layer, which protected the carbide substrate against thermal–mechanical loads. Moreover, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) performed on the worn tools revealed that adhesion, oxidation, diffusion, and abrasion were the main wear mechanisms for both types of tools.

1. Introduction

In this era of rapid development, manufacturing industries remain the main source of environmental pollution. The precarious effluents produced by the industrial sector, when released into the atmosphere, pose serious environmental and health hazards. Metal cutting is a critical manufacturing process that relies heavily on lubricants. However, conventional metal-cutting fluids, while effective at suppressing heat, pose severe medical and ecological risks. Studies have linked exposure to conventional metal-cutting fluids to an increased risk of various types of cancer, skin allergies, and even physical disabilities [1]. Moreover, their chemical dissolution after disposal is a potential cause of soil contamination and water pollution, emphasizing the need for expensive, yet necessary, pre-disposal treatment [2]. According to statistics presented by a leading German automobile company, the procurement and disposal expenses associated with cutting fluids make up approximately 7% to 17% of the direct labor cost [3]. Due to these ecological and economic challenges, dry cutting has emerged as a growing field of research. Dry cutting, being carried out without coolants or lubricants, offers significant environmental advantages over flood machining. However, the involvement of high thermal–mechanical loads, particularly in the machining of difficult-to-cut materials, such as titanium, nickel, and high-strength steel, compromises surface quality and the dimensional accuracy of machined components in a dry environment [4]. To address these challenges, advanced tool materials and coatings possessing excellent tribological properties and wear resistance have been introduced. Although the application of advanced materials such as ceramics, cubic boron nitride (CBN), and polycrystalline cubic boron nitride (PCBN) have substantially improved the chemical, mechanical, and cutting capabilities of cutting tools, cemented carbide tools, due to their cost-effectiveness, still capture a major share (~53%) of the market [5]. Among these, 80%–90% are categorized with single- or multilayer-coated carbide tools [6]. Thin films covering the surface of a carbide substrate function as hard coatings and serve various purposes. For instance, coating a substrate with a highly thermally resistant material can protect it from temperature-driven damage in applications involving elevated temperatures [7]. Grigoriev et al. [2] reported that Ti-based coatings can withstand high machining temperatures and considerably inhibit the diffusion and oxidation wear mechanism.
The most common types of hard coatings that are applied to cutting tools are TiCN, HSN2, Al2O3, TiN, TiAlN, TiB2, TiBN, and AlTiCrN [8]. The synthesis of coatings can generally be achieved through physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes. Both techniques differ in their method of application and depend on the type and nature of the coating and substrate material. PVD refers to a vacuum-coating process that allows for the deposition of solid material as a thin film layer on the substrate body through vaporization and condensation [9]. The coating temperature in PVD remains lower than 500 °C, and a thin layer of coated material is deposited on the base material, making it mostly suitable for finishing applications [7]. On the other hand, CVD is a heat-activated process in which gaseous precursors are injected into a reaction chamber for chemical surface reaction and dissociation, resulting in the formation of a potential coating. The temperature of the substrate is maintained between 800 °C and 1150 °C [10]. Both deposition methods can form single-layer and multilayer coatings. The effectiveness of single- and multilayer coating has also been investigated by many researchers; multilayer coatings have been claimed to be more effective when machining difficult-to-cut materials. Chinchanikar and Choudhury [11] reported that coated tools can substantially reduce tool wear mechanisms. They noticed a significant reduction in tool wear when a single-layer TiAlN-coated tool was compared with an uncoated tool while machining hardened steel, demonstrating satisfactory performance in increasing cutting from 62 to 200 m/min. However, the cutting efficiency of multilayer TiCN/Al2O3/TiN tools was remarkably improved up to a cutting speed of 350 m/min. It was concluded that single-layer coatings could not sustain the harsh machining environment and were, thus, severely affected by accelerated crater wear, weakening the cutting edge.
High-strength steel has widespread engineering applications (e.g., transmission shafts, gears, bearings, rolling dies, building structures, pipeline transportation, etc.) due to its superior qualities, such as high hardness and torsional strength, enhanced fatigue and abrasion resistance, and excellent shock-absorbing abilities [12,13]. Moreover, its high abrasiveness, low thermal conductivity, and high plastic nature are associated with poor machining capabilities [14]. The high cutting resistance exhibited by heat-treated steel, particularly at high machining regimes, is the leading cause of intense heat generation, high power consumption, unfavorable chip formation, shorter tool life, and increased machining time and cost [15]. With advancements in coating technologies, these machinability challenges can be addressed. The literature has reported that abrasion, adhesion, and temperature-driven diffusion are the main tool wear mechanisms in the machining of AISI 4340 high-strength alloy steel [16,17]. Therefore, Al2O3 coatings, being thermally and chemically resistant [18], can be paired with TiCN [19] or TiAlN coatings, which are categorized by high adhesion and corrosion resistance [20], to provide a better machinability performance. According to Li et al. [21], multilayer PVD-coated carbide tools can improve the machinability of heat-treated alloy steel during a high-speed machining process. They employed three types of PVD coatings, namely AlTiN single-layer, TiAlN/TiN bi-layer, and TiN/TiCN/TiAIN multilayer coatings, on the carbide substrate and a PVD bi-layer-TiCN/NbC composite coating on the ceramic substrate. It was reported that TiN/TiCN/TiAlN tools considerably reduced the tearing and collapse of the coatings and resulted in less plastic deformation, crack nucleation, edge chipping, and abrasive wear.
Many studies have been carried out regarding the machining of high-strength steel, with different coatings and tool materials. Das et al. [22] utilized thin multilayer Ti (C N O) PVD-coated cermet tools in machining heat-treated AISI 4340 alloy steel and achieved the lowest surface roughness of 1.065 µm in cutting speed, V = 250 m/min, feed rate, F = 0.09 mm/rev, and depth of cut, DOC = 0.4 mm. Interestingly, they found that the cutting depth was the most influential factor affecting the surface roughness, followed by the feed rate and cutting speed. However, Zheng et al. [23], in the milling of AISI 4340 (43 HRC) steel using an Al2O3/TiCN CVD-coated carbide tool, found considerably less surface roughness, 0.40 µm, at an identical machining speed and feed rate (V = 250 m/min, F = 0.09 mm/rev) but with a low depth of cut, DOC = 0.1 mm. The performance of a TiAlN PVD-coated tool in machining Ni-based super-alloy was evaluated in reference [24]. The results showed that the TiAlN coating initially suppressed the adherence of the work material, but, with subsequent cutting, crack initiation led to the occurrence of flaking and exposure of the substrate. The literature has also highlighted the comparative studies of different types of coatings. For instance, Sonawane and Sargade [25] compared uncoated AlTiCrN and AlTiN tungsten carbide inserts in tuning duplex stainless-steel 2205 under a dry environment. It was reported that AlTiCrN exhibited the lowest amount of tool wear, Vb = 0.16 mm, after completing 2450 mm of machining length for each tool. Likewise, Boing et al. [6] conducted a comparative assessment of TiCN/ Al2O3/TiN CVD and TiAlN PVD multilayer-coated tools, during which they reported the sequence of events of tool wear progression. It was observed that CVD tools were initially affected by abrasion, followed by crack nucleation and propagation, coating delamination, and, lastly, complete spalling. In contrast, the sequence of tool wear events for PVD tools included abrasion, high plastic deformation, crack nucleation, leading to weakening of the substrate and coating bonding, and, eventually, the detachment of the entire coating. In a recent study, Bag et al. [26] reported the best surface quality of machined surfaces (Ra = 0.40 µm) with a a TiN/TiCN/ Al2O3 CVD-coated tool at a cutting speed of 260 m/min. An ANOVA performed on surface roughness data revealed that the feed rate had the highest percentage contribution rate (PCR) of 71.93%, making it the most influential parameter. This was followed by machining speed, with a PCR of 14.8%, and depth of cut, with a PCR of 3.4%. Moreover, they analyzed worn tools and reported abrasion and adhesion as the main wear mechanisms. An ANOVA on tool wear data demonstrated the dominating effect of cutting speed, exhibiting the highest PCR of 66.06%, followed by the feed rate at 13.91% and depth of cut at 3.83%. Alok et al. [27] also acknowledged the highest statistical influence of the cutting speed on flank wear while machining AISI 4340 steel (52 HRC) using a HSN2-coated carbide insert. Furthermore, Kumar et al. [28] investigated the influence of different lubrication conditions, namely water-soluble, nano-cutting fluid, and compressed air, on the cutting performance of uncoated cermet tools. It was determined that nano-cutting fluid effectively reduced the cutting force, produced curlier chips, and inhibited the tool flank wear compared to the other two conditions. These results were in line with the findings of Singh et al. [29], who utilized uncoated textured carbide inserts under nano-fluid-based MQL lubrication. In another study, Zenjanab et al. [30] conducted high-speed machining in a range between V = 300 and 400 m/min, F = 0.1–0.2 mm/rev, using ceramic tools under dry conditions, CuO-based cutting fluids, and soluble cutting fluids. The results revealed that CuO-based cutting fluids exhibited better machinability under all cutting conditions due to favorable anti-friction and anti-wear properties.
Shalaby and Veldhuis [31] suggested that pure alumina + ZrO2-based ceramic tools produced better tool life at a low machining speed, V = 150 and 250 m/min, while mixed alumina+ Al2O3 + TiCN-based ceramic outperformed, with a high cutting speed at V = 700 and 1000 m/min. Furthermore, Suyama and Diniz [32] performed high-speed turning of AISI 4340 using a CBN+ Al2O3/TiCN ceramic phase insert and obtained the longest tool life of 15 min at a low cutting speed of 300 m/min. Additionally, they studied the effect of vibration on tool wear by varying the tool overhang distance and reported that diffusion was the main wear mechanism at high vibration, while abrasion predominated at low vibration. Da Silva et al. [33] compared the results of hard turning with the traditional grinding process by using ceramic and polycrystalline cubic boron nitride (PCBN) tools. It was claimed that hard turning in dry cutting conditions with both tools produced the lowest Ra of 0.18 µm at a high cutting speed, V = 300 m/min. On the contrary, there was a comparatively high Ra in grinding (0.20 µm), suggesting that hard turning could be a potential replacement for the traditional grinding process.
After reviewing the literature, it was determined that tools made up of advanced materials such as CBN and ceramics were primarily evaluated for the high-speed hard machining of AISI 4340 alloy steel. However, the utilization of multilayer-coated carbide tools has not been investigated in detail under high machining parameters. Therefore, this study aimed to provide an in-depth analysis on the performance of multilayer CVD and PVD-coated carbide tools in a dry environment, considering two crucial machinability aspects: surface roughness and tool life. Moreover, the evolution of tool wear and the behavior of different coatings in inhibiting the wear progression, followed by a comprehensive analysis of wear mechanisms, were also included in the scope of this study. It was anticipated that this research would be of potential interest to tool manufacturers and industry professionals who are striving to achieve better product quality and machinability performance by utilizing sustainable methods of manufacturing.

2. Materials and Methods

Round cylindrical bars of AISI 4340 alloy steel having dimensions Ø60 × 120 were employed for experimentation. The work samples were heat-treated to achieve the desired hardness of 50 HRC through a series of repetitive operations, including 3 h of curing at 840 °C, followed by 1 h of quenching at 830 °C and 4 h of annealing at 400 °C. The experiments were conducted on a CNC turning machine with high cutting speed in a range between 300 and 350 m/min, feed rate 0.05–0.15 mm/rev, and depth of cut 0.1–0.3 mm according to Taguchi L18 orthogonal array, as presented in Table 1. The machining parameters were selected as per Ref. [30], utilizing ceramic tools in high-speed (250–400 m/min) machining of heat-treated AISI 4340 alloy steel (52 HRC), using soluble cutting fluid-based nanofluids (CuO, CuO + boric acid), which have a detrimental impact on the environment. This study was conducted to realize sustainable machining using a dry approach and aimed to evaluate the effectiveness of multilayer-coated carbide tools, which are less costly than ceramic tools.
Two multilayer-coated carbide inserts TiAlN/AlCrN with physical vapor deposition (PVD) and Al2O3/TiCN chemical vapor deposition (CVD) were employed for the turning experiments. The selection of tool coating was based on reviewed literature, as hard turning generates intense machining heat, which promotes temperature-driven wear mechanisms. Therefore, effective coatings were selected to provide thermal stability and chemical resistance. For instance, Al2O3 coating forms a thermal barrier and offers greater oxidation and wear resistance at elevated temperatures [34], TiCN exhibits resistance to shock loading [21], TiAlN coating possesses hardness and self-lubricating properties [35], and TiCrN offers good wear resistance and chemical stability [36]. The characteristics of both tools are listed in Table 2. Tool performance was evaluated by measuring tool wear and surface roughness under each experimental condition. The progression of tool wear was tracked by capturing the wear images at different intervals using a Zeiss Stemi 2000-C optical microscope (Zeiss, Oberkochen, Germany), where an average tool life criterion of Vb ≥ 300 µm was used as per ISO 3685 [37] for tool life analysis. Next, the worn inserts were analyzed using a Zeiss SUPRA 55VP scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) to determine the associated wear mechanisms. Then, the surface quality of machined samples was measured by using a Mitutotyo Surfest SJ-301 surface roughness tester (Mitutotyo, Kawasaki, Japan). Three surface roughness readings were recorded at different positions on the cylindrical workpiece, and the average value was computed. Figure 1 illustrates the experimental scheme adopted for this study.

3. Results and Discussion

This section presents a comprehensive discussion on the crucial machinability aspects when high-strength steel was machined with Al2O3/TiCN-CVD and TiAlN/AlCrN-PVD carbide tools: tool life, tool wear progression, tool wear mechanisms, and surface roughness.

3.1. Statistical Analysis of Tool Life and Surface Roughness

The output variables tool life and surface roughness for each experimental run are tabulated in Table 3. Analysis of variance (ANOVA) was performed to infer meaningful information from the obtained data, and the most significant factors with their percentage contributions were determined by using Minitab 17 statistical software.

3.1.1. Tool Life

Based on the ANOVA results (Table 4), it was revealed that the feed rate, cutting speed, and tool type were the most influential parameters for tool life, with corresponding percentage contributions of 46.22%, 28.50%, and 12.92%, respectively. However, the depth of cut with p = 0.828 was found to be insignificant, and its contribution was only 0.46%. Similar results could also be deduced from the main effect plot of the means in Figure 2a, where the depth of cut showed a relatively shorter spread in comparison to other parameters. The result of the present study was unexpected based on the previous literature, where, generally, the cutting speed was the governing factor for tool life. The appreciable variation in tool life with changing feed rates can be justified by the fact that raising the feed rate leads to an increased tool–workpiece contact area, resulting in a high material removal rate. This exerts more force on the tool, in addition to a colossal increase in friction between the contacting surfaces. These factors substantially contributed in hastening the tool wear, particularly in the hard turning operation, which unequivocally involves high thermal stresses. Therefore, the range of variation in tool life was sufficient to contribute greater weight to the feed rate, and this nullified the presumed dominating influence of machining speed. The results obtained in this research were similar to López-Luiz et al. [39], who performed hard turning of AISI D2 steel with carbide inserts and found the feed rate to be the most controlling factor affecting the tool wear/life. Moreover, a recent study carried out by Fedai [40] in turning AISI 4340 alloy steel also found the feed rate to be a statistically significant exerting dominating effect on tool wear rather than cutting speed. Overall, the upward trend demonstrated by the CVD-coated tools in the main effect plot of tool type in Figure 2a showed that they outperformed in extending tool life within the conducted experimental scheme.

3.1.2. Surface Roughness

The results presented in Table 4 indicate that the feed rate continued to be the most significant factor affecting the surface roughness, exhibiting a percentage contribution of 68.98%. This was followed by cutting speed and tool types, with respective contributions of 8.75 % and 8.38%. However, depth of cut, with p-value = 0.059 and contributing 6%, remained the least significant factor in this study. The main effect plot of the mean in Figure 2b also supports these findings, showing that the graph of feed rate, cutting speed, and tool type exhibited a relatively broader spread compared to the depth of cut. The apparent reason for the increasing surface roughness with increasing feed rate is associated with the high chip volume and thrust force, which can lead to increased vibration and, consequently, decreased surface quality. This result is similar to the observation of Mia et al. [41] while machining heat-treated AISI 1060 steel. According to them, the feed rate and then the cutting speed were the most significant governing variables for surface roughness. The lowest mean value exhibited by CVD tools in Figure 2b indicated their superior performance within the conducted experimental scheme.

3.2. Analysis of Tool Life/Wear for PVD- and CVD-Coated Carbide Tool

3.2.1. Tool Life and Tool Wear Progression

From the results illustrated in Figure 3, it was determined that the CVD-coated tool demonstrated the longest tool life of 14.75 min in experiment 10. However, at an identical machining speed (V = 300 m/min) and feed rate (F = 0.05 mm/rev), the useful life of the PVD-coated tool was 10.08 min, which was 31.3% less than the CVD tool. The trend of decreasing tool life was noticed when increasing the speed and feed for both types of tools. The higher order of both parameters produced significant heat generation and plastic deformation [42]. Overall, CVD-coated tools outperformed PVD tools in all experimental runs, as deduced from Figure 2. The obtained results of tool life can be comprehensively explained by analyzing the tool wear progression and mechanism with different parametric settings. Thorough analysis of tool wear progression can provide valuable insights into potential tool failures. The effects of significant input variables for a comparative assessment of tool wear were evaluated from the Taguchi L18 orthogonal array. As in Taguchi’s L18 design of experiment, only one factor varied, while others remained the same. Since, the depth of cut had been identified as the least influential variable, as per the discussion in Section 3.1.1, its variation was not expected to significantly influence the accuracy of the comparative assessment. Hence, the assessments of tool wear for both CVD and PVD tools were performed by examining the microscopic images of the tool with increasing machining time.
It is evident from Figure 4 that variation in machining parameters can substantially influence the evaluation of tool wear. Most particularly, as the level of cutting speed and feed rate increases, the tool wear accelerates and rapidly progresses towards the severe wear stage for both types of tools. The corresponding wear graphs in Figure 4a–c were, therefore, more skewed towards the left side as the cutting speed increased from 300 to 400 m/min and feed rate increased from 0.05 to 0.15 mm/rev. The most critical reason for this behavior was the excess heat generation with increasing cutting speed, resulting in plastic deformation and thermal softening of the tool material. Even a small rise in the feed rate in this scenario resulted in a proportional increase in the tool’s transverse speed across the rotating workpiece, causing friction between the tool and the workpiece, ultimately leading to degradation of the tool [43]. It was noticed that the progression of wear in the case of CVD tools remained nearly steady in all cutting speed and feed rate combinations. However, it notably accelerated for PVD tools when the feed value was raised to 0.1 mm/rev and 0.15 mm/rev, as evident from Figure 4b,c. In order to determine why CVD tools outperformed PVD tools, microscopic and SEM images of worn tools were analyzed with respect to machining time for low and high parametric settings, as shown in Figure 5. It was discernible that PVD tools in both cutting conditions (experiment 1 and 9) were highly affected by excessive chipping and abrasion, even during the initial wear stage (Vb ≥ 66.21 µm), indicating that the thin layer of the TiAlN/AlCrN coating (3 µm) could not protect the tool against thermal–mechanical loads. On the contrary, the double thick layer of the Al2O3/TiCN coating (18 µm) showed better resistance to chipping during the initial and gradual wear stages. It could also be observed from the tool cutting geometry in Figure 5 that it was relatively more stable at high wear width, Vb ≤ 194.14 µm @ 8.2 min in experiment 10 and at Vb ≤ 167.33 µm @ 1.83 min in experiment 18. According to Sonawane and Sargade [25], thicker coatings have low thermal conductivity, which plays a crucial role in protecting the carbide substrate from exposure to temperature variation. However, soon after the deterioration of the coating, the carbide substrate of both tools could not withstand the thermal–mechanical variations imposed by harsh machining conditions. Thus, they experienced faster progression towards the failure region. It is worth noting that PVD tools exhibited less flank wear width than CVD tools in experiment 1 (Vb = 339.70 µm < Vb = 366.18 µm) and experiment 9 (Vb = 415.2 µm < Vb = 841.12 µm). This behavior could be attributed to the following reasons:
  • Comparatively lower depth of cut of 0.1 mm for the PVD tool, which exerted less mechanical load on the tool edge, eventually causing less damage.
  • The higher hardness possessed by the PVD tool (93.2 HRA) compared to the CVD tool (90.5 HRA), which provided stability against thermal–mechanical loads.
  • The high thermal expansion coefficient of the inner coating in case of the CVD tool (TiCN~7.8 × 10−6 K−1 [44] > AlCrN ~6 × 10−6 K−1 [45]) compared to the carbide substrate (5.7–6.9 × 10−6 K−1 [46]), causing the coating to expand more than the carbide substrate. Consequently, this exerted compressive stresses and damaged the tool.
  • The high thermal conductivity of the TiCN coating (30 W/mk [47]) compared to AlCrN (4.63 W/mk [48]), causing excess heat to transfer to the carbide substrate, thereby weakening the base material of the tool and resulting in extensive flank wear.

3.2.2. Tool Wear Mechanisms

To comprehend the wear mechanisms for both types of tools, SEM analysis was performed on the worn tool from experiments 1 and 10. Three regions of the worn tools were also examined in detail for SEM and EDX analysis: (a) crater face, (b) cutting edge, and (c) flank face. Figure 6a represented the high crater formation of the PVD tool, indicating a severe tribological condition between the flowing chips and the tool rake face. Reis et al. [49] stated that the combined effect of diffusion and adhesion caused the evolution of crater wear. Traces of elemental composition in spectra 1 and 2 showed the strong peaks of Fe and an appreciable amount of oxygen. This confirmed the adherence of the workpiece material, as well as elemental diffusion and oxidation, which were likely caused by the formation of iron oxide and a high-stress field at elevated temperature. Since the machining was performed in a high-speed regime, the generation of intense heat was inevitable. The rough surface on the crater surface also indicated diffusion. Moreover, chipping and abrasion were also noticed near the cutting edge. The presence of a few traces of Ti and Al elements in all spectra suggested that the tool coating had deteriorated. However, a noticeable percentage of W in spectra 1 and 2 represented exposure to the carbide substrate. It was expected that the periodic entry of the tool during the cutting cycle may have smashed the adhered material onto the tool, thereby leading to carbide breakage in the form of chipping. Das et al. [50] stated that stress concentration and uneven force distribution near the cutting edge increase the possibility of chipping followed by tool fracture. Parallel friction marks in the direction of material flow indicated abrasion, which was identifiable on the tool flank face near the main cutting edge. The hard carbide micro-inclusion in the workpiece material rubbed against the tool, which caused abrasive wear. Furthermore, a significant percentage of workpiece material in spectra 1 and 2 indicated serious adhesion and was a possible cause of accelerated tool flank wear in the case of the PVD tool.
In comparison, the CVD tool exhibited a shallower crater depth, as shown in Figure 6b. This was due to the effective multilayer coating, which substantially minimized the wear and friction in comparison to the PVD tools. However, the EDX analysis of spectrum 4 showed high percentages of W, Co, and O, with a few traces of Al, suggesting the deterioration of the Al2O3 coating and the occurrence of oxidation wear. Interestingly, the presence of the inner TiCN coating on the rake face benefitted the carbide substrate against the varied thermo-mechanical loads and caused less damage, as compared to the PVD tool. Therefore, the profile of the main edge was relatively more stable. Previous studies have also reported that the TiCN coating exhibits excellent shock resistance capabilities [51]. The EDX analysis on the cutting edge (spectrum 5) revealed the existence of the Al2O3/TiCN coating, which effectively prevented material adhesion and chipping, thereby maintaining a stable cutting-edge geometry. Bjerke et al. [52] stated that the outer layer of the Al2O3 coating can form a protective layer of aluminum oxide during the machining process, thus providing thermal stability and abrasion resistance to the cutting edge. The wear morphology at the flank face represents abrasion and adhesion to the tool due to constant rubbing of hard micro-inclusions from the workpiece and carbide substrate. Meanwhile, the adherence on the flank face (as evident from spectrum 6) was an extrusion of flowing chips between the tool and workpiece. Due to the strong chemical affinity of the work material with the tool substrate, further adherence led to the weakening of the binder in the carbide matrix, while the adhered material, being unstable in nature, was continuously dislodged from the tool surface, promoting a traction effect, causing interlayer coating delamination [53]. Considering this fact, it was expected that the high depth of cut in this scenario exerted an increased cutting force, which was sufficiently high to cause tool flaking and ultimately led to the complete tool life criteria.
Based on SEM and EDX analysis, it is summarized that adhesion, oxidation, and abrasion constituted the primary wear mechanisms for both Al2O3/TiCN-CVD- and TiAlN/AlCrN-PVD-coated tools. However, excessive chipping on the main cutting edge and severe diffusion on the crater face were the contributing factors that accelerated wear mechanisms in the case of the PVD-coated tools.

3.3. Analysis of Surface Roughness for PVD- and CVD-Coated Carbide Tool

A pictorial representation of average surface roughness (Ra) for PVD and CVD tools is shown in Figure 7. It can be seen that the best surface roughness of 0.276 µm was obtained with the PVD tool in experiment 4 at V = 350 m/min, F = 0.05 mm/rev, and DOC = 0.1 mm. However, the minimum Ra of 0.307 µm in the case of the CVD tool was obtained at a cutting speed of 400 m/min, feed rate of 0.05 mm/rev, and depth of cut of 0.3 mm. Overall, CVD tools outperformed in delivering the best surface quality, showing 66.6% better results than PVD tools. These results were consistent with the tool wear/life findings discussed in Section 3.2, where the thick layer of the Al2O3/TiCN coating effectively decreased the coefficient of friction and resulted in lower surface roughness. On the contrary, the PVD tool was only 33.33% effective in delivering a better surface finish, as per the results of experiments 1, 4, and 7. This meant that the PVD tool with high hardness and a thin coating (TiAlN/AlCrN) was merely effective at low machining parameters because all these experiments were conducted at a low machining speed V = 300 m/min, low feed rate 0.05 mm/rev, and low depth of cut, DOC = 0.2–0.3 mm.
From Figure 7, it is also inferred that with an increase in the machining speed, there was a decrease in surface roughness. This is because increasing the speed minimizes the tool–chip contact area, thereby lowering the friction, and this produces better surface quality. Thermal softening of the work material by increasing the cutting speed is one of the governing factors, contributing to a decreasing cutting force as well as decreasing peaks and valleys present in the work surface, thereby allowing for a better surface finish [27]. It was also revealed that the surface roughness for both tools substantially increased with an increment in the feed rate. Most importantly, the value of Ra was augmented at a higher feed F = 0.15 mm/rev for both tools due to higher undeformed chip thickness, which exerted a load on the cutting edge, leading to tool wear and increased surface roughness. The highest Ra of 1.97 µm and 1.211 µm were observed at low machining speeds V = 300 m/min and the highest feed rate F = 0.15 mm/rev for the PVD and CVD tools, respectively, in experiments 3 and 12. Though the effect of the cutting depth was not statistically significant, as discussed in Section 3.1.2, its higher value with high feed rates (0.1 mm/rev and 0.15 mm/rev) considerably increased the surface roughness. A combination of higher feed rate and depth of cut resulted in an increase in the chip cross-sectional area, which is associated with a higher force requirement to shear the material, resulting in chatter and a subsequent deterioration in surface quality [54]. Hence, it is suggested that a high machining speed with a low feed rate and depth of cut is favorable for lower surface roughness.

4. Conclusions

In this study, the performance of PVD and CVD multilayer-coated tools was analyzed with respect to the tool life, tool wear, and surface roughness undergoing high-speed turning under a dry environment. The following conclusions can be drawn based on this analysis:
  • Based on the ANOVA results, it was found that the feed rate, cutting speed, and tool type significantly influence both the tool life and surface roughness. The feed rate exhibited the highest percentage contributions of 46.22% and 68.96% for tool life and surface roughness, respectively, thereby establishing it as the most influential parameter in this study. However, the cutting depth, with a high p-value (>0.05), was found to be insignificant for both response variables.
  • The highest tool lives of 14.75 min and 10.08 min were obtained at a low cutting speed (300 m/min) and low feed rate (0.05 mm/rev) for CVD- and PVD-coated tools, respectively. The wear progression graph for both tools skewed towards the left as the machining speed and feed rate increased. Additionally, optical microscopic images captured at different time intervals of the worn tool indicated that the CVD-coated tool with the Al2O3/TiCN coating protected the tool from thermal–mechanical loads, thus maintaining the cutting-edge geometry, despite high flank wear width at the end of the tool life. Conversely, the PVD tool with the TiAlN/AlCrN coating exhibited excessive chipping in the early phases of cutting, resulting in a comparatively shorter tool life.
  • SEM and EDX analysis revealed that adhesion, oxidation, and abrasion constituted the primary wear mechanisms for both types of tools. Furthermore, excessive chipping on the main cutting edge and severe diffusion on the crater face were also identified as contributing factors to the wear mechanisms in the case of PVD-coated tools.
  • The best surface roughness of 0.276 µm and 0.307 µm was achieved at a high cutting speed and low feed rate for both PVD and CVD tools, respectively. Remarkably, the CVD-coated tool exhibited the most favorable outcomes in terms of low surface roughness due to the reduced thermal conductivity of the Al2O3/TiCN coating, which notably reduced the coefficient of friction.
  • From the perspective of future research, a more comprehensive understanding of tool wear mechanisms can be achieved by analyzing the cutting force, chip formation, and cutting temperature. This deeper knowledge is crucial for optimizing the tool life and surface integrity in high-speed machining applications. Additionally, emerging trends in the field of tool coating, for instance, advanced multilayer composite coatings and nano-structural hard coatings, can offer outstanding tribological features and wear resistance to cutting tools. Thus, research in this domain can improve the cutting performances of the PVD- and CVD-coated tools during the machining of high-strength steel. Lastly, the cost quantification and sustainability assessment of coated carbide tools in machining high-strength steel could demonstrate significant economic and environmental benefits, motivating manufacturers to implement these tools in wide-scale industrial applications.

Author Contributions

S.Y.: Methodology, investigation, data curation, writing—original draft. J.A.G.: Methodology, investigation, data curation, writing—review and editing, supervision. N.J.: Writing—review and editing, funding acquisition. A.Z.J.: Writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

The authors extend their appreciation to Prince Sattam bin Abdulaziz University for funding this research, with project number (PSAU/2024/01/29747).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Experimental setup adopted for machining.
Figure 1. Experimental setup adopted for machining.
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Figure 2. Main effect plot of (a) mean of tool life and (b) mean of surface roughness.
Figure 2. Main effect plot of (a) mean of tool life and (b) mean of surface roughness.
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Figure 3. Tool life results for CVD and PVD tools in different experimental runs.
Figure 3. Tool life results for CVD and PVD tools in different experimental runs.
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Figure 4. Wear progression for PVD and CVD tools at feed rates of (a) 0.05 mm/rev, (b) 0.1 mm/rev, and (c) 0.15 mm/rev.
Figure 4. Wear progression for PVD and CVD tools at feed rates of (a) 0.05 mm/rev, (b) 0.1 mm/rev, and (c) 0.15 mm/rev.
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Figure 5. Wear images of both tools with the machining time at low and high parametric settings.
Figure 5. Wear images of both tools with the machining time at low and high parametric settings.
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Figure 6. SEM and EDX analysis of the worn tools at V = 300 m/min and F = 0.05 mm/rev. (a) PVD tool; (b) CVD tool.
Figure 6. SEM and EDX analysis of the worn tools at V = 300 m/min and F = 0.05 mm/rev. (a) PVD tool; (b) CVD tool.
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Figure 7. Surface roughness results for PVD and CVD tools in different experimental runs.
Figure 7. Surface roughness results for PVD and CVD tools in different experimental runs.
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Table 1. Experimental scheme, as per Taguchi L18.
Table 1. Experimental scheme, as per Taguchi L18.
Exp. NoTool Type
(T)
Cutting Speed
V (m/min)
Feed Rate
F (mm/rev)
Depth of Cut DOC (mm)Exp. NoTool Type
(T)
Cutting Speed
V (m/min)
Feed Rate
F (mm/rev)
Depth of Cut DOC (mm)
E1PVD3000.050.1E10CVD3000.050.3
E2PVD3000.100.2E11CVD3000.100.1
E3PVD3000.150.3E12CVD3000.150.2
E4PVD3500.050.1E13CVD3500.050.2
E5PVD3500.100.2E14CVD3500.100.3
E6PVD3500.150.3E15CVD3500.150.1
E7PVD4000.050.2E16CVD4000.050.3
E8PVD4000.100.3E17CVD4000.100.1
E9PVD4000.150.1E18CVD4000.150.2
Table 2. Characteristics of PVD and CVD tools [38].
Table 2. Characteristics of PVD and CVD tools [38].
CharacteristicsPVD ToolCVD Tool
GradeH-gradeP-grade
CoatingTiAlN/AlCrNAl2O3/TiCN
Coating thickness3 µm18 µm
Hardness of substrate93.2 HRA90.5 HRA
Nose radius0.4 mm0.4 mm
Rake angle−6°−6°
Cutting edge angle95°95°
Table 3. Results obtained for each experimental run.
Table 3. Results obtained for each experimental run.
Exp. NoTool TypeSurface RoughnessTool LifeExp. NoTool TypeSurface RoughnessTool Life
E1PVD0.349010.080E10CVD0.491014.750
E21.08006.590E110.832010.570
E31.97232.980E121.21104.540
E40.27557.830E130.305014.110
E51.16603.290E140.71906.850
E61.74301.940E150.99202.840
E70.29404.530E160.30704.830
E80.72100.943E170.58204.230
E91.20600.493E180.79402.510
Table 4. ANOVA table for response variables.
Table 4. ANOVA table for response variables.
SourceDFSeq SSAdj SSAdj MSFp% Cont.Significance
Tool life (T) (R-Sq 86.62%)
T139.14439.14439.14410.850.008 *12.92%Significant
V286.38286.38243.19111.970.002 *28.50%Significant
F2140.08140.08070.04019.410.000 *46.22%Significant
DOC21.3861.3860.69280.190.8280.46%Insignificant
Residual Error1036.07536.0753.6075
Total17303.067
Surface Roughness (Ra) (R-Sq 88.09%)
T10.36800.36800.3680211.130.008 *8.75%Significant
V20.35260.35260.176315.330.027 *8.38%Significant
F22.89962.89961.4497843.830.000 *68.98%Significant
DOC20.25230.25230.126143.810.0596.002%Insignificant
Residual Error100.33080.33080.03308
Total174.2033
* Significant factor.
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Yaqoob, S.; Ghani, J.A.; Jouini, N.; Juri, A.Z. Performance Evaluation of PVD and CVD Multilayer-Coated Tools in Machining High-Strength Steel. Coatings 2024, 14, 865. https://doi.org/10.3390/coatings14070865

AMA Style

Yaqoob S, Ghani JA, Jouini N, Juri AZ. Performance Evaluation of PVD and CVD Multilayer-Coated Tools in Machining High-Strength Steel. Coatings. 2024; 14(7):865. https://doi.org/10.3390/coatings14070865

Chicago/Turabian Style

Yaqoob, Saima, Jaharah A. Ghani, Nabil Jouini, and Afifah Z. Juri. 2024. "Performance Evaluation of PVD and CVD Multilayer-Coated Tools in Machining High-Strength Steel" Coatings 14, no. 7: 865. https://doi.org/10.3390/coatings14070865

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