Characterization and Evaluation of Engineered Coating Techniques for Different Cutting Tools—Review

Coatings are now frequently used on cutting tool inserts in the metal production sector due to their better wear resistance and heat barrier effect. Protective hard coatings with a thickness of a few micrometers are created on cutting tools using physical or chemical vapor deposition (PVD, CVD) to increase their application performance. Different coating materials are utilized for a wide range of cutting applications, generally in bi-or multilayer stacks, and typically belong to the material classes of nitrides, carbides, carbonitrides, borides, boronitrides, or oxides. The current study examines typical hard coatings deposited by PVD and CVD in the corresponding material classes. The present state of research is reviewed, and pioneering work on this subject as well as recent results leading to the construction of complete “synthesis–structure–property–application performance” correlations of the different coatings are examined. When compared to uncoated tools, tool coatings prevent direct contact between the workpiece and the tool substrate, altering cutting temperature and machining performance. The purpose of this paper is to examine the effect of cutting-zone temperatures on multilayer coating characteristics during the metal-cutting process. Simplified summary and comparisons of various coating types on cutting tools based on distinct deposition procedures. Furthermore, existing and prospective issues for the hard coating community are discussed.


Background
Coatings are typically described as thin layers that are deposited on the surface of the substrate to protect it during the application and, therefore, extend its lifetime. There are well-established studies covering different kinds of coatings with their respective applications [1,2]. One of the most critical coatings applications are in the cutting and machining industry, wherein the tool must be protected from extreme conditions such as high temperature, oxidation, corrosion, friction, and wear [3]. The tool material can be hard, coated, or uncoated, and cutting inserts can be found along the contact area, which is typically coated. In the cutting process, the workpiece remains motionless while the tool rotates throughout the machining operations. The rotating tool with several cutting blades moves over the workpiece to create a flat or straight surface in this machining operation. These coating processes are vital to the majority of machining operations. Coated cutters are also required for cutting materials with limited machinability. Along with the development of new and improved cutting tools, there has also been a breakthrough in high-speed steel (HSS) cutting tools. It can be extended from HSS tools with complex geometry to more contemporary coated cutting tools with a lubrication system that provides a smother cutting process [4]. For example, Martha et al. [5] investigated the durability and performance

Physical Vapor Deposition (PVD)
PVD refers to a class of non-equilibrium techniques in which the compositional diversity of the formed films is not restricted by thermodynamics, allowing for the deposition of metastable solid solutions. PVD techniques are categorized into sputtering and evaporation based on the physical approach used to transfer the solid precursor material (the so-called target) into the vapor phase. PVD methods are line-of-sight processes, which implies that they only coat the region of the substrate that is immediately exposed to the target. The ability to employ temperature-sensitive substrates at low substrate temperatures near to room temperature is a significant benefit of PVD [29]. Reactive deposition methods are often used, in which a reactive gas (e.g., N 2 or O 2 ) is supplied into the deposition chamber in addition to the working gas (typically Ar) [30]. The substrate holder can be grounded, or a negative substrate bias voltage can be supplied, resulting in intense ion bombardment of the growing film, which enhances activation of film growth due to atomic scale heating. PVD films often have high defect densities, tiny grain sizes, and compressive stresses because of the extra kinetic stimulation of film formation [31]. Since they are so adaptable to changing market conditions, a wide variety of techniques and procedures, some of which are shown in Figure 1, have been developed and improved. Two of the most prevalent PVD processes for the deposition of thin films are sputtering and evaporation, as shown in Figure 2.  ing, using ionized Argon (Ar+) gas [32]. Reproduced with permission from Baptista, A, Coatings; Published by MDPI, 2018.

Chemical Vapor Deposition (CVD)
CVD refers to any process in which gaseous precursors are injected into a reaction chamber to create a coating [33]. The activation of dissociation and chemical surface reactions of the precursors is mostly accomplished thermally, which can be accomplished, for example, using a hot-wall reactor in which the substrates are heated indirectly. Hot-wall reactors are commonly used in the hard coating industry. Moreover, the substrates can be warmed up inductively or resistively in a cold-wall reactor or with laser aid. Another option is plasma-assisted CVD, which relies on kinetic activation [34]. In addition to the substrate temperature, which is in the range of 800 to 1150 °C in thermally activated CVD, the  ing, using ionized Argon (Ar+) gas [32]. Reproduced with permission from Baptista, A, Coatings; Published by MDPI, 2018.

Chemical Vapor Deposition (CVD)
CVD refers to any process in which gaseous precursors are injected into a reaction chamber to create a coating [33]. The activation of dissociation and chemical surface reactions of the precursors is mostly accomplished thermally, which can be accomplished, for example, using a hot-wall reactor in which the substrates are heated indirectly. Hot-wall reactors are commonly used in the hard coating industry. Moreover, the substrates can be warmed up inductively or resistively in a cold-wall reactor or with laser aid. Another option is plasma-assisted CVD, which relies on kinetic activation [34]. In addition to the substrate temperature, which is in the range of 800 to 1150 °C in thermally activated CVD, the Schematic drawing of two conventional PVD processes, (a) sputtering and (b) evaporating, using ionized Argon (Ar+) gas [32]. Reproduced with permission from Baptista, A, Coatings; Published by MDPI, 2018.

Chemical Vapor Deposition (CVD)
CVD refers to any process in which gaseous precursors are injected into a reaction chamber to create a coating [33]. The activation of dissociation and chemical surface reactions of the precursors is mostly accomplished thermally, which can be accomplished, for example, using a hot-wall reactor in which the substrates are heated indirectly. Hot-wall reactors are commonly used in the hard coating industry. Moreover, the substrates can be warmed up inductively or resistively in a cold-wall reactor or with laser aid. Another option is plasma-assisted CVD, which relies on kinetic activation [34]. In addition to the substrate temperature, which is in the range of 800 to 1150 • C in thermally activated CVD, the deposition pressure is a critical process feature [35,36]. Because the inner gas stream velocities, which determine the retention time and the thickness of the boundary layer, and thus the reactant diffusion to the substrate surface are pressure-dependent, the deposition pressure influences the homogeneity of the deposition process within the reactor. The intended reaction to generate a coating is a heterogeneous reaction on the surface. Although CVD is far more driven by thermodynamics than PVD, it is not an equilibrium process and also has kinetic limits, which define the deposition rate and have a substantial influence on the coating microstructure. There are two rate restrictions to consider: (I) surface response limitation and (II) mass transport limitation. At low temperatures or pressures, reaction happens slowly, the boundary layer is thin, diffusion is rapid, and reactants reach the surface quickly. When mass transport is the limiting factor, the diffusion rate of the reactants across the boundary layer is the decisive parameter. At high temperatures and pressures, mass movement is frequently limited, resulting in low gas velocity and thicker boundary layers. Deposition inside the surface reaction kinetics regulated regime is commonly preferred because it results in more uniform deposition within the reactor [34]. Homogeneous gas phase reactions often occur at temperatures higher than the decomposition temperatures of the species inside the reactor, resulting in powder formation, which is undesirable in a deposition process. The tremendous throwing power of CVD and hence the ability to coat complicated shapes is a significant benefit. Additional advantages include large batch sizes and high coating thicknesses. Aside from the high deposition temperatures, which limit the substrate materials available, another disadvantage of CVD coatings is that they frequently display tensile residual stress and a high surface roughness, both of which are undesirable in application and demand post-treatment [37].

Electrodeposition Coating
Material electrodeposition is a method of protection that involves the deposition of metallic ions on a substrate. A potential difference between the anode and cathode poles induces an ion transfer in the unit cell in this process. By accepting ions from the other electrode, a coating layer develops on the submerged sample after a while. Extensive research has been conducted on popular electrodeposition materials. The most often researched metals include but are not limited to Ni-P, Ni-P/Sn, Ni-P-W, Ag/Pd, Cu/Ag, Cu/Ni, Co/Ag, and Co/Pt [38,39]. These studies show that electrodeposited coatings greatly improve the corrosion characteristics of the substrate. Furthermore, this approach has shown promise for enhancing the adhesion of hard coatings [40]. In general, electrodeposition is divided into two processes: electrolytic deposition (ELD) and electrophoretic deposition (EPD), which are further explored in the sections that follow.

Electrolytic Deposition (ELD) Coating
Electrolytic deposition (ELD) is an electrochemical method used on conductive surfaces to generate a dense metallic coating with a homogeneous thickness distribution. Substrate and deposition materials are selected as cathode and anode while placed inside an electrochemical unit cell. Metallic ions move toward the working electrolyte and then toward the substrate when a potential difference between the anode and cathode poles is applied. The deposition phase necessitates electrolyte supersaturation, which happens as a result of charging current in the circuit. During the coating process, the concentration of metallic ions in the electrolyte remains constant [41]. Although this technology is generally utilized for decorative and low-corrosion/wear applications, there have been reports of additional uses being developed, including optical, electronics, biomedical, high-temperature, and solid-oxide fuel cells [42]. Ceramic materials may be deposited on metallic substrates in a manner similar to the micro-arc oxidation (MAO) process by raising the potential difference in electrolytic unit cells. Tian et al. [43] deposited Ni-Co-Al 2 O 3 Materials 2022, 15, 5633 7 of 40 on steel pipes and found that it significantly improved corrosion of the substrate exposed to oil sand slurry. Yang et al. [44] showed considerable corrosion and erosion increased corrosion resistance after depositing Ni-Co-SiC on carbon steel pipes subjected to oil sand slurry. Fayomi et al. [45] obtained similar findings on a Zn-Ni-Al 2 O 3 -coated mild steel substrate. Furthermore, Redondo et al. [46] used a dihydrogen phosphate solution to deposit a corrosion-resistant polypyrrole (PPy) layer on a copper substrate.

Electrophoretic Deposition (EPD) Coating
Electrophoretic deposition (EPD) is another type of electrodeposition that produces thicker colloidal coating layers. Thin films produced on surfaces by coagulation of colloidal particles using an electric field in a unit cell similar to that used in ELD. EPD is a multiphase approach that includes the following steps: (I) Electrophoresis is the process by which an external electric field pushes suspended particles in an electrolyte toward one electrode. (II) Moving particles congregate in one electrode and combine to produce a bigger coagulated particle. (III) The bigger particles settle on the electrode's surface, which is a to-be-coated substrate. Eventually, a thick coating layer with a powder-shaped structure will be formed on the substrate. Densification methods (e.g., furnace curing, light curing, sintering, etc.) are advised to improve the protective layer's quality. EPD has been used for a variety of applications, including coating, selective deposition, graded material deposition, porous structure deposition, and biological applications [47]. Borides, carbides, oxides, phosphates, and metals are widely utilized in EPD [48]. Castro et al. [49] used sol-gel and EPD to create corrosion-resistant coatings on stainless steel AISI 304 and observed two-and four-times improvements in corrosion resistance for each technique, respectively. Gebhart et al. [50] used chitosan to cover an AISI 316L stainless steel for biological purposes in another investigation. They found that this coating had a good influence on the corrosion behavior of the substrate. They also claimed that the applied electric field in EPD is the most important aspect in determining coating properties such hydrophobicity, thickness, and structure. Chen et al. [51] used graphene to cover TC4 Ti-alloy orthopedic implants. They discovered that graphene-coated artificial joint implants had a much longer life expectancy. They discovered that micro-cracks in coating surfaces were the cause of any corrosion on substrates. Fei et al. [52] investigated the wear resistance of EPD coatings and effectively deposited SiC particles on paper-based friction materials, achieving good wear enhancement.

Thermal Spray Coating
Thermal spray coating is a collective term of methods that use a plasma, electric, or chemical combustion heat source to melt a specific set of materials and spray the molten substance onto a surface to form a protective layer. These are dependable corrosionand wear-resistant coatings. In this method, a heat source, which is often produced by chemical combustion or plasma discharge, warms the materials to a molten or semi-solid state before spraying them on the substrate with a high-speed jet. Thermal spray coating methods may produce thicknesses ranging from 20 m to several millimeters, which is much more than electroplating, CVD, or PVD procedures [53]. Furthermore, the materials that may be utilized as feedstock for thermal spray coatings range from refractory metals and metallic alloys to ceramics, polymers, and composites and have the ability to cover a reasonably large surface area of a substrate. Thermal spray coatings are classified according to their features and process criteria (Table 1). Plasma spray coating, high-velocity oxyfuel (HVOF), cold spray coating, warm spray coating, and arc wire spray coating are the most prevalent types [54,55].

A Comparison of Coatings on Cutting Tools
For cutting tool coatings, the most popular and latest options will be discussed below as well as the deposition process. In addition, novel tool geometries will be demonstrated, including those applied to the coated insert substrate to enhance chip removal and breakage. The cutting inserts on the cutting edges of modern cutting tools are commonly made of stainless steel. There are several coatings available, ranging from monolayers to nanocomposite coating materials, which generate a two-phase composition during the deposition process [75,76].
Hard coatings, or nitrides, carbides, borides, and oxides of transition metals, are commonly employed in tool coatings. TiN, TiAlN, CrN, ZrN, TiSiN, TiAlSiN, CrAlN, TiAlCrN, and cBN are some of the nitride coatings used on cutting tools. Carbide coatings include TiC, CrC, and WC. With its chemical inertness and excellent hardness, TiB 2 is ideal for boride coatings. Because they have strong adhesion properties, they may be applied on tool steel [77][78][79]. The borocarbide coatings MoBC, WBC, and TaBC can also exhibit good fracture resistance and high hardness and extend the tool lifetime. In high-friction machining operations such as micro milling, boride coatings are used [10,80]. Al 2 O 3 is a commonly used oxide coating. Diamond-like carbon (DLC), MoS 2 , and WC-C are also prevalent coatings for cutting tools. As a consequence of several lines of research that continue to compare coated tools to uncoated tools, it is known that coatings applied to milling inserts increase milling process efficiency. It all comes down to the material being machined and the desired results, such as a higher MRR or a higher-grade surface finish. To further understand the cutting capabilities of Nimonic alloy 75, uncoated and TiAlNcoated tungsten carbide micro end mills are compared. The authors determined cutting performance by considering tool wear, groove geometry, bulge formation, and surface quality under the similar wear conditions. The TiAlN-coated tools often outperformed the uncoated ones. As a result, the surface polish quality and slot geometry improved [81]. Masooth et al. [82] analyzed the surface roughness of an Al6061-T6 alloy end mill using uncoated and TiAlN coated carbide tools. Figure 3 shows the comparison of these two workpieces. The Taguchi technique was employed to optimize the process, and an analysis of variance (ANOVA) was utilized to establish that spindle speed was the most important factor in surface roughness. The employment of both uncoated and coated tools helped to improve the efficiency of the process. Results show that the coated carbide end mill produced a superior surface polish. techniques. Tools deposited with VN coating exhibited an improved tribological and mechanical properties at high temperatures. Therefore, the VN-coated tools might be a proper choice when high temperature is inevitable. The effects of carbon and boron codoping vs. doping alone on the microstructure and performance of as-deposited AlTiN, AlTiCN, AlTiBN, and AlTiBCN coatings was discussed by Mei et al. [86]. The results indicated that typical columnar crystal structures were observed with dopped coating.  [82]. Reproduced with permission from Elsevier.

Diamond Coatings
In recent studies on coated cutting tools, diamond-like carbon (DLC) coatings have been a good solution for dealing with abrasion issues. These coatings are smooth with a typical coefficient of friction (COF), which often leads to a low wear rate. Therefore, DLC coatings can reduce the friction in the contact area and hence avoid early heat generation during the machining operation. DLC can also be combined with tough layers to increase the toughness of the coating. For instance, the characteristics of a three-layer CrN/CrCN/DLC have been evaluated [87]. Furthermore, these coatings were compared to a TiAlSiN coating and an untreated surface. It was 58 times more resistant to wear than an untreated surface. Ucun et al. [88] investigated tool performance during machining in terms of tool wear, surface roughness, burr formation, and cutting forces, as illustrated in Figures 4 and 5; because of the decreased friction of the DLC coating, the cutting forces for the coated tool test were lower than those for the uncoated tool. When milling with an untreated tool, burr creation increases as the cut length grows. At low cutting lengths, Martinho et al. [83] investigated the performance of comparable carbide insert coated tools with varying coatings. PVD (monolayer) AlTiN and CVD (multilayer) TiN/TiCN/Al 2 O 3 coatings were employed. During the cutting process, the tool was monitored for vibration, wear, and the ultimate condition of the machined surface. Compared to CVD-coated inserts, PVD-coated inserts showed higher failures and were found to be brittle. Surprisingly, wear of the CVD synthesized coating did not change the machined surface. It was reported that PVD coatings exhibited a better adhesion to the substrate than the CVD coatings, which was conducted by the same researchers utilizing a GX2CrNiMoN266-7-4 super duplex stainless steel alloy to manufacture inserts and coatings [84].
There were no significant differences in surface quality between the PVD-deposited monolayer coating and the CVD multilayer coating; however, a severe wear and abrasion was observed in the former coating. Furthermore, it was found that the substrate was more severely damaged by wear once the coatings had worn away. Hosokawa et al. [85] evaluated the cutting properties of PVD-coated tools synthesized by filtered arc deposition (FAD). This deposition approach was used to create two novel kinds of films in this investigation. A high-speed milling procedure was used to apply these coatings on a pre-hardened stainless steel. In the study, TiCN and VN coatings were investigated. The TiCN coatings exhibited excellent hardness and adherence to the substrate. In the machining application, TiCN-coated tools were found to be more effective than coated tools available commercially, obtained by arc ion plating (AIP) or hollow cathode discharge (HCD) techniques. Tools deposited with VN coating exhibited an improved tribological and mechanical properties at high temperatures. Therefore, the VN-coated tools might be a proper choice when high temperature is inevitable. The effects of carbon and boron co-doping vs. doping alone on the microstructure and performance of as-deposited AlTiN, AlTiCN, AlTiBN, and AlTiBCN coatings was discussed by Mei et al. [86]. The results indicated that typical columnar crystal structures were observed with dopped coating.

Diamond Coatings
In recent studies on coated cutting tools, diamond-like carbon (DLC) coatings have been a good solution for dealing with abrasion issues. These coatings are smooth with a typical coefficient of friction (COF), which often leads to a low wear rate. Therefore, DLC coatings can reduce the friction in the contact area and hence avoid early heat generation during the machining operation. DLC can also be combined with tough layers to increase the toughness of the coating. For instance, the characteristics of a three-layer CrN/CrCN/DLC have been evaluated [87]. Furthermore, these coatings were compared to a TiAlSiN coating and an untreated surface. It was 58 times more resistant to wear than an untreated surface. Ucun et al. [88] investigated tool performance during machining in terms of tool wear, surface roughness, burr formation, and cutting forces, as illustrated in Figures 4 and 5; because of the decreased friction of the DLC coating, the cutting forces for the coated tool test were lower than those for the uncoated tool. When milling with an untreated tool, burr creation increases as the cut length grows. At low cutting lengths, there was no discernible difference in burr development between uncoated and coated tools. To conclude, the DLC-coated tools give rise to a reduced COF and wear, hence providing a smoother polished surface, as shown in Figure 6.
It has recently been reported that CVD diamond cutting tools have been developed for use in the micro milling of oxygen-free copper. Because of their high hardness, diamond coatings are a viable micro-milling tool. Using laser-induced graphitization and precision grinding, Zhao et al. [89] suggested a unique approach for the fabrication of these micro-cutting tools. These micro-cutting tools claimed to show better performance than commercially available coated cemented carbide micro-cutting tools used for oxygen-free copper machining. Additionally, the CVD diamond tools exhibit a superior surface polish and reduced cutting forces throughout the milling operation.
Several studies have recently documented the milling process and diamond coatings. Wang et al. [90] examined the cutting performance of several high-speed machining of graphite molds using diamond-coated tools. The microcrystalline diamond (MCD), sub microcrystalline diamond (SMCD), nanocrystalline diamond (NCD), and micro/nanocrystalline composite diamond (MCD/NCD) coatings were tested to assess their ability against scratching. While NCD was found to have the lowest friction coefficient, MCD exhibited a better adhesion to the substrate, as shown in Figure 7. Furthermore, as compared to MCD and NCD, the SMCD coating's performance was subpar utilizing these parameters. However, the MCD/NCD composite showed strong adhesion to WC-Co. The friction was decreased because the top layer of the coating was NCD, which has a lower surface roughness and finer diamond particles.        different feed rates (ϕ1 > ϕ2 > ϕ3 > ϕ4 and L1 > L2 > L3 > L4) [88]. Reproduced with permission from Elsevier.
Several studies have recently documented the milling process and diamond coatings. Wang et al. [90] examined the cutting performance of several high-speed machining of graphite molds using diamond-coated tools. The microcrystalline diamond (MCD), sub microcrystalline diamond (SMCD), nanocrystalline diamond (NCD), and micro/nanocrystalline composite diamond (MCD/NCD) coatings were tested to assess their ability against scratching. While NCD was found to have the lowest friction coefficient, MCD exhibited a better adhesion to the substrate, as shown in Figure 7. Furthermore, as compared to MCD and NCD, the SMCD coating's performance was subpar utilizing these parameters. However, the MCD/NCD composite showed strong adhesion to WC-Co. The friction was decreased because the top layer of the coating was NCD, which has a lower surface roughness and finer diamond particles.
The use of MCD/NCD composite coated tools in the milling of hot bending graphite molds and related materials establish a compelling case for their use. Wang et al. [91] reported the same occurrence in their investigation. This coating has a reduced friction coefficient when compared to the monolayer NCD coating owing to a lack of adhesion to the substrate. It was found, however, that the multilayer diamond film with surface coating of NCD layer (MNMN-CD)-coated tools had 7.5 times longer lifetime than the monolayer-coated tools.  . Schematic drawing of chip formation process depending on angular position of tool for different feed rates (φ1 > φ2 > φ3 > φ4 and L1 > L2 > L3 > L4) [88]. Reproduced with permission from Elsevier. Figure 6. Schematic drawing of chip formation process depending on angular position of tool for different feed rates (ϕ1 > ϕ2 > ϕ3 > ϕ4 and L1 > L2 > L3 > L4) [88]. Reproduced with permission from Elsevier.
Several studies have recently documented the milling process and diamond coatings. Wang et al. [90] examined the cutting performance of several high-speed machining of graphite molds using diamond-coated tools. The microcrystalline diamond (MCD), sub microcrystalline diamond (SMCD), nanocrystalline diamond (NCD), and micro/nanocrystalline composite diamond (MCD/NCD) coatings were tested to assess their ability against scratching. While NCD was found to have the lowest friction coefficient, MCD exhibited a better adhesion to the substrate, as shown in Figure 7. Furthermore, as compared to MCD and NCD, the SMCD coating's performance was subpar utilizing these parameters. However, the MCD/NCD composite showed strong adhesion to WC-Co. The friction was decreased because the top layer of the coating was NCD, which has a lower surface roughness and finer diamond particles.
The use of MCD/NCD composite coated tools in the milling of hot bending graphite molds and related materials establish a compelling case for their use. Wang et al. [91] reported the same occurrence in their investigation. This coating has a reduced friction coefficient when compared to the monolayer NCD coating owing to a lack of adhesion to the substrate. It was found, however, that the multilayer diamond film with surface coating of NCD layer (MNMN-CD)-coated tools had 7.5 times longer lifetime than the monolayer-coated tools.  The use of MCD/NCD composite coated tools in the milling of hot bending graphite molds and related materials establish a compelling case for their use. Wang et al. [91] reported the same occurrence in their investigation. This coating has a reduced friction coefficient when compared to the monolayer NCD coating owing to a lack of adhesion to the substrate. It was found, however, that the multilayer diamond film with surface coating of NCD layer (MNMN-CD)-coated tools had 7.5 times longer lifetime than the monolayer-coated tools.

Carbon Nano Tubes (CNTs)-Coated Tool
Currently, carbon nanotubes (CNTs) have attracted significant attention in PVD hard coatings due to their exceptional tensile and shear strength. Additionally, a faster heat dissipation during machining is aided by their high thermal conductivity (4000-6000 W/mK) [92]. The wear characteristics and machinability features of CNT-coated inserts were studied by Pazhanivel et al. [93]. These inserts exhibited CoF, which can point out higher machinability and better surface polish.
Hirata et al. [94] studied the sliding friction characteristics of CNTs deposited on silicon, cemented carbide, and silicon nitride substrates, and the lubrication and adhesion properties of CNT-coated substrates were shown to be superior on substrates with surface porosity. Borkar et al. [95] found that pulsed electrode deposition of CNT coatings resulted in a considerable increase in wear resistance compared to pure nickel coatings.
At various coating conditions, Chandru et al. [96] deposited carbon nanotubes (CNT) over the HSS tool and evaluated the machining performance of the coated tool with respect to the important machinability aspects such as cutting tip temperature, cutting forces, surface roughness, and tool wear and life. Due to the excellent mechanical and thermal properties of CNTs, drastic reductions in cutting tip temperature and cutting forces of 70 to 80% were observed for coated tools. However, there was no significant effect of coating on surface roughness at lower levels of cutting conditions, but significant improvement was observed at higher levels of cutting conditions. The tool wear and life analysis explored the steady improvement to a remarkable extent. Very specifically, under elevated machining conditions, tool life investigations concluded the fact that the CNT-coated tool has a tool life of more than 60 min with negligible failure.
The study achieved by Chenrayan et al. [97] investigated the deposition of carbon nanotubes (CNT) over the HSS tool through PECVD. It also evaluated the coated tool's machining capability in terms of mechanical performance, surface quality, cutting parameters, temperature, cutting forces, friction coefficient, and service life. The analysis of experimental outcomes for three different cutting environments shows that the CNT-coated tool represent a viable candidate for machining of harder materials. Furthermore, dramatic reduction of the cutting tool tip temperature and cutting forces due to the excellent mechanical and thermal properties of CNTs was recorded. This demonstrates that the CNT-coated tools outperform the DLC-coated tools in terms of applicability, as shown in  Reproduced with permission from Elsevier.

AlTiN and AlTiSiN Coatings.
A series of full studies on the surface morphology, hardness, surface section morphology, and wear resistance of AlTiN/AlTiSiN coatings were carried out [99,100], and the effects of different modulation periods on the microstructure, mechanical properties, and tribological properties of the coatings were systematic in order to improve  Reproduced with permission from Elsevier.

AlTiN and AlTiSiN Coatings.
A series of full studies on the surface morphology, hardness, surface section morphology, and wear resistance of AlTiN/AlTiSiN coatings were carried out [99,100], and the effects of different modulation periods on the microstructure, mechanical The most critical finding of other studies can be summarized as follows: CNTs deposited using the PECVD technique have exceptional adhesive strength. The scratch test findings show that the adhesive stress value is greater than 75% of the yield strength of the substrate material. CNT coating reduced heat generation by 70-80% compared to the uncoated HSS tool and improved the metal cutting performance at elevated temperatures. In diverse cutting circumstances, coated tools showed a 70-80% reduction in cutting forces. In all machining settings, the tangential cutting force is larger than the feed and radial forces. As a result, the observations validate the theoretical reality that cutting parameters increase cutting forces. However, the coating had a minor effect on surface roughness at the lower cutting level.
Coated tools can be used to manufacture challenging materials at higher cutting rates since they minimize surface roughness by 30-40%. The amplitude of surface texture peaks and valleys rises with cutting conditions for both coated and uncoated tools. According to rigorous AFM and optical microscope measurements, the CNT-coated tool has a tool lifetime of >60 min even in the severe cutting conditions. In all cutting circumstances, the wear of coated tool flank was decreased by 60-70%. It was also observed that the rate of flank wear increased gradually for coated tools in the same conditions. Uncoated tools failed catastrophically in poor, moderate, and extreme cutting conditions at 24, 18, and 7 min, respectively, when the coated tool survived even for >60 min. However, the homogeneity in the CNT dispersion of the CNT-coated tool resulted in a dramatically shortened tool life [98].

AlTiN and AlTiSiN Coatings
A series of full studies on the surface morphology, hardness, surface section morphology, and wear resistance of AlTiN/AlTiSiN coatings were carried out [99,100], and the effects of different modulation periods on the microstructure, mechanical properties, and tribological properties of the coatings were systematic in order to improve the mechanical and tribological properties and expand the microstructure and properties of AlTiN/AlTiSiN coatings by arc ion plating. The influence of the modulation period on the microstructure and characteristics of TiN/TiAlN multilayer coatings was studied by Yongqiang et al. [101]. In all TiN/TiAlN multilayer coatings, XRD examination revealed a significantly preferred orientation (111) plane. It explains why this result occurred, which might be due to an increase in the inner tension as the coating grows. The rapid atom movement was aided by the powerful ion bombardment, which allowed them to easily align along the densely packed (111) direction. The nano hardness of TiN/TiAlN multilayer coatings, on the other hand, achieved a maximum of 38.9 GPa at a modulation period of 164 nm. The TiN/TiAlN multilayered coatings had greater hardness values, between 28.9 and 38.9 GPa, than the TiN and TiAlN monolithic coatings. The nanoscale multilayer design is responsible for this improvement. Because the individual layers of TiN and TiAlN had identical shear moduli, the coherent interface generated by epitaxial growth of TiAlN and TiN had an alternating strain field in the multilayers due to lattice misfit, which may prevent dislocation motion and promote multilayer strengthening. According to this study, the TiN/TiAlN multilayer coatings had the best adhesion strength at a modulation period of 54 nm. Tuffy et al. [102] investigated the influence of TiN coating thickness applied by the PVD method on tungsten carbide insert machining performance in dry hard turning of AISI 1040 steel. The TiN layer with a thickness of 3.5 mm had the best cutting efficiency, with a tool life of roughly 40 times. Sargade et al. [92] investigated the effect of TiN coating thickness on cemented carbide tools deposited between 1.8 and 6.7 mm during dry turning of hardened C40 steel. The critical load for scratch testing grew until the thickness reached 4 mm and then decreased. They discovered that a 4 mm thick covering increased the tool life of untreated cemented carbide by 9.5 times. The characteristics and structure of TiAlN/CrAlN-and TiAlN-coated cermets were investigated by Yang et al. [103]; during the machining of 9CrSi 2 Mn steel, SEM, EDS, and scratch tests were used to investigate cutting performance, wear mechanism, and adhesive strength. The TiAlN coating has a hybrid columnar structure that resists adhesive failure better than the TiAlN/CrAlN coating. The flank wear was more severe in TiAlN/CrAlN coating than in TiAlN coating. Kumar et al. [104] investigated the effect of different AlTiN (2-4 mm) thicknesses on Al 2 O 3 -TiCN mixed ceramic inserts coated by cathodic arc evaporation while hard twisting AISI 52,100 steel (62 HRC). The application of coating resulted in a considerable increase in machinability, with a coating thickness of 4 mm exhibiting higher machining performance.
Colombo-Pulgarin et al. [105] used laser powder bed fusion (LPBF) to manufacture Inconel 718 (IN718) specimens, which were then PVD-coated with TiN and AlTiSiN to physically and chemically characterize their surface. Microhardness tests and chemical analysis using glow discharge optical emission spectroscopy (GDOES) were carried out in this regard. The mechanical performance of both coatings as well as their surface and morphological properties were assessed using roughness and wear behavior studies. The experimental findings allowed for a comparison of the two coatings tested with the LPBFprocessed substrate ( Table 2). The microhardness of AlTiSiN and TiN was found to be greater than that of the substrate materials. Between the two, the former has a greater Vickers microhardness than the latter. In a wear process, AlTiSiN outperformed TiN in terms of wear resistance. When the identical test-time values of both coatings were compared with the rubber wheel, it was discovered that AlTiSiN had a lower relative weight loss when the weight loss of the substrate material was taken into account. The greater the mechanical adherence of the coating to the substrate material, the lesser the comparable weight loss.

Boron Nitride Coatings
In mechanical cutting and grinding, cubic boron nitride (cBN) is a synthetic wearresistant substance with a hardness comparable to diamond [139]. Furthermore, cubic boron nitride particles have superior thermal stability to diamond. To further increase the wear resistance of the alloy, laser cladding (LC) technology was utilized to create (cBN)/Ti6Al4V and Ni-plated (cBN)/Ti6Al4V composite coatings on Ti6Al4V substrates. Cubic boron nitride particles lose their raw characteristics when exposed to laser light in a molten pool. Composite coatings' wear resistance, worn track shape, and microstructures were studied with scanning electron microscopy (SEM), as was the interface bonding between cubic boron nitride nanoparticles and the Ti6Al4V matrix (WTM-2E). By comparing the two coatings, it was discovered that the Ni-plated cubic boron nitride/Ti6Al4V composite coating had many fewer thermal flaws. The composite coating's cubic boron nitride particles were spared heat break thanks to a Ni plating applied to their surfaces. A TiN reaction layer is produced between cubic boron and the Ti6Al4V matrix, thus preventing further disintegration of cubic boron nitride particles. Between the cubic boron nitride particles and Ti6Al4V, a TiN reaction layer formed. The composite coating's wear resistance was improved by nickel plating the cubic boron nitride particles.
Due to a lower cutting zone temperature, a protective coating's physical and mechanical characteristics are evaluated to see how this affects cutting tool life in turning hardened steel [140]. In the amorphous state, boron nitride is believed to be a solid lubricant in the tool-cutting chip contact that reduces the contact length and cutting force, resulting in a gain in tool life and dependability, particularly during the run-in period. According to physico-mechanical and thermal studies, the hardness and Young's modulus, coefficient of thermal conductivity, thermal capacity, and friction of the coating on the ShKh15 steel were found to be 15 GPa, 200-220 GPa, 70 W/mK, 800 J/kg K and 0.3, correspondingly. When cutting rate and feed are increased while using a tool with a protective coating on the working surfaces, the total contact length decreases as the friction coefficient decreases. It is feasible to extend the tool life and dependability, especially during the run-in stage, by applying a coating of amorphous boron nitride to the area where a chip meets the cutting pressures. Tool wear is reduced by 22-25% with the boron nitride coating while turning hardened steels due to a difference in the thermobaric loading compared to tools without the coating. Boron nitride nanotube (BNNT) has emerged as an outstanding mechanical, chemical, and thermal material for ceramic and metal matrix reinforcement. In this study, the atmospheric plasma spraying approach was used to add 5% BNNT to the alumina (Al 2 O 3 ) matrix in this work [141].
BNNTs have been able to withstand the plasma plume's tremendous temperatures. It has been discovered that BNNTs can increase the relative density of an Al 2 O 3 coating from 90% to 94% while also increasing the hardness, elastic modulus, and fracture toughness of the BNNT-reinforced coatings by 117%, 35%, and 51%, respectively. With the addition of BNNT, the critical energy release rate rose from 3.90 1.18 J/m 2 to 6.50 1.22 J/m 2 . The increased attributes are ascribed to improvements in density, homogeneous distribution of BNNTs, and toughening mechanisms such as BNNT truss, nanotube pull-out, and fracture bridging. This BNNT-enhanced Al 2 O 3 coating might be a suitable material for automobiles, aircraft, high-speed cutting tools, and bioimplant applications. Sugihara et al. [142] investigated high-speed milling of Inconel 718 with CBN cutting tools to understand the tool wear behavior better and extend tool life. The following results were gathered: Inconel 718 is a difficult material for machining at high speeds because of its high melting point, which causes a diffusion wear phase at the beginning of cutting and a following cracking step due to the creation of a thick adhesion layer on the worn flank face and its flaking, which accelerates CBN tool retreat. The anchoring effect of the textured surface was used to design new CBN cutting tools with textured flank faces, utilizing a femtosecond laser in order to stabilize the adhesion layer on the flank face and prevent it from flaking. A series of cutting experiments revealed that the CBN-ORE cutting tool significantly reduced tool retreat compared to a conventional CBN cutting tool without surface texture, indicating that surface roughening on a flank face is a promising approach for enhancing CBN durability during high-speed milling of Inconel 718, as demonstrated by a series of cutting experiments shown in Figures 10 and 11.  Azinee et al. [143] coated the instrument with cubic boron utilizing electroless nickel co-deposition. The Ni-CBN surface coating was applied electrolessly using nickel, using chemical methods to integrate the ceramic CBN particles. Electroless nickel can minimize coating costs and time. Corrosion-, wear-, and abrasion-resistant coatings have been widely employed in the industry. This study compared the surface tolerance of Ni-CBN HSS tool substrates to uncoated HSS tools for cutting aluminum alloy 7075. Using Taguchi L9 experiments, the authors chose to examine cutting speed, feed rate, and depth of cut. The lowest and most significant flank wear measures differ significantly for coated surfaces, as shown in Figure

Tungsten Carbide-Cobalt (WC/Co) Coatings
It is a superb anti-friction and wear substance used in the preparation of hard coatings and in industrial applications. The University of Connecticut used high-temperature flame spraying to generate nanostructured WC/10Co coatings with excellent hardness and bonding strength in 1994 [144]. Since then, nanostructured WC/Co coatings have become a significant issue of discussion. For the most part, traditional WC/Co coatings are only melted on the surface, while nano-WC/Co coatings are melted in the complete volume. Nano-WC cermet powder has a denser coating, granular strengthening, improved toughness and hardness, better adhesion, more strength, greater strength and crushing Cutting Time [143]. Reproduced under CC BY license.

Tungsten Carbide-Cobalt (WC/Co) Coatings
It is a superb anti-friction and wear substance used in the preparation of hard coatings and in industrial applications. The University of Connecticut used high-temperature flame spraying to generate nanostructured WC/10Co coatings with excellent hardness and bonding strength in 1994 [144]. Since then, nanostructured WC/Co coatings have become a significant issue of discussion. For the most part, traditional WC/Co coatings are only melted on the surface, while nano-WC/Co coatings are melted in the complete volume. Nano-WC cermet powder has a denser coating, granular strengthening, improved toughness and hardness, better adhesion, more strength, greater strength and crushing resistance, and improved wear resistance. Together with sophisticated surface modification technology and 3D intelligent programming technology, they shield the cutting tools from abrasion and extend the tool's service life [145]. The WC phase in the coating is more equally distributed in the Co and Cr phases at a single high temperature. There was a considerable improvement in the microhardness value as well as a more uniform distribution of microhardness.
The coating's wear and sediment erosion resistance have been substantially enhanced. Myalska et al. [146] used mechanical blending and ultrasonic-aided mixing to combine WC/17Co raw material powder with various quantities of TiC nanoparticles and thermally spray them onto a carbon steel surface using the high-velocity air-fuel (HVAF) process. In the Co matrix, because the lower temperature of the HVAF spraying process slowly reduces powder oxidation during spraying, and consequently, phase composition and microstructure of powder are preserved, TiC was slowly oxidized but did not breakdown or dissolve. Nanoscale TiC also slowed the dissolution and decarburization of WC, making it more resistant to corrosion. When the coating was cured and slid at 400 • C, clusters of TiC nanoparticles protruded from the coated surface and were responsible for the majority of the contact stresses. Because of this, the wear rate decreased. Friction oxidation debris clusters (based on CoWO 4 and WO 3 -2H 2 O) developed on the worn track and assisted in lowering the friction coefficient by eliminating direct contact between the WC/Co surface and the matching body. The structure, mechanical characteristics, and wear parameters of nano-WC/Co coatings generated by the high-velocity oxygen-fuel (HVOF) technique were examined at high temperatures. Studies showed that the nano-WC/Co coating had a dense structure, and the friction coefficient fell progressively with increasing temperature. The rate of wear dropped first but subsequently began to rise. 450 • C has the lowest wear rate. The friction coefficient and wear rate might be reduced by a continuous oxide coating of WO 3 /WO 3 -CoWO 4 on the worn surface. Utilizing detonation spray coating (DSC) on mild steel (MS) substrates, Babu et al. [147] investigated the microstructure and phase composition of nano-WC/12Co raw materials and coatings.
Using SiC as a grinding media, an abrasive wear test was conducted, and the coating wear rate was estimated. Hardness and Weibull modulus was discovered to be altered by variations in indentation load and coating microstructure. The number of notches necessary to obtain the specified hardness value was established based on the results, and the structure-performance connection was investigated. To obtain the best performance and performance, Weibull parameter-based strategies were proposed.

Hafnium and Vanadium Nitride Coatings
Hafnium and vanadium nitride multilayer coatings [HfN/VN] n coatings were investigated as a replacement for TiN coatings for gear cutting tools in the 1980s due to their higher thermal barrier and hot hardness, but their production costs and times were so high due to TiN deposition processes at the time that new PVD coatings on a base of transition metal nitride compounds were required [148]. HfN monolayers, VN monolayers, and [HfN/VN] n coatings were deposited onto a variety of substrates using cutting-edge equipment, and their microstructure, chemical composition, morphological topography, and layer thickness were studied to identify properties such as high hardness, high modulus of elasticity, and a critical load increase; these properties result in outstanding mechanical behavior.
According to Duran et al. [149], deposition of the HfN monolayer coating enhances the surface smoothness of the machined component, exhibits homogeneity of the manufactured piece, and decreases surface microcracks, thus preventing probable early failures in the manufacturing process [60]. An HfN monolayer-coated cutting tool has several advantages at the industrial level, including boosting the quality of made products, lowering production time and costs, as well as minimizing the adhesive wear caused in the tribological pair [150]. Using a computer numerically controlled (CNC) machine, the temperature of the steel bar and the tool was monitored; the results showed that the Monolayer HfN coating reduces tool wear and improves the surface smoothness of the machined item, which was reflected in the process temperature change, adding HfN coating to a tool can extend the tool's life, improve quality, and save manufacturing costs.
The use of [HfN/VN] n multilayer coatings on the flank and rake faces of HSS cutting tools improved tribological compatibility, reduced friction coefficient, and increased tool life, according to Navarro-Devia et al. [151]. Using a cutting tool with [HfN/VN] n multilayer coatings enhance workpiece surface integrity by reducing friction and increasing thermal stability proportional to bilayer number. Because the coating affects the cutting process, it improves corrosion resistance. Using [HfN/VN] n multilayer thin films as protective coatings on an HSS cutting tool can increase tribological compatibility in low carbon steel turning. The coatings' outstanding tribological properties not only increase cutting tool wear resistance but also reduce cutting forces by reducing friction and acting as a thermal barrier. A thinner chip reduces the chip compression ratio by decreasing stresses and temperatures at the interface, and softer surfaces in milled steel improve workpiece quality by increasing resistance to corrosion. With a bilayer number (n), [HfN/VN] n multilayer coatings promote tribo-pair interactions, enhancing workpiece surface integrity and tool life.
Escobar et al. [152] deposited HfN and VN films using magnetron sputtering. The elastic modulus of HfN (224 GPa) is larger than that of VN (205 GPa), which is evident in variations in plastic deformation resistance values. The tribological pair (steel pin and nitride film) had low friction coefficients for both films, 0.44 for HfN and 0.62 for VN, and critical loads for HfN and VN films were about 41 and 34 N, respectively. SEM identified the wear processes in both films: adhesive wear in HfN and abrasive wear in VN. Finally, the HfN films had 9% better mechanical characteristics than the VN films and 18% better tribological properties (friction coefficients) than the VN materials. Escobar et al. [153] investigated the wear and tribological behavior in great detail. As bilayer periods in the coatings were reduced, an increase in hardness and elastic modulus of up to 37 GPa and 351 GPa was observed. In terms of friction coefficient (0.15) and critical load (72 N), the sample with a bilayer period of 15 nm and a bilayer number of n = 80 had the lowest friction coefficient and the greatest critical load, respectively. WC inserts were utilized as substrates to enhance the mechanical and tribological characteristics of [HfN/VN] n coatings as a function of increasing contact numbers and to achieve improved efficiency in various industrial applications, including machining and extrusion.
Cutting experiments with AISI 1020 steel (workpiece) and bilayer number and bilayer period were carried out to evaluate wear as a function of the bilayer period and number of bilayers. Compared to uncoated tungsten carbide inserts, WC inserts coated with [HfN/VN] 80 showed a decrease in flank wear of around 24%. New coatings for tool machining with high industrial performance may be achieved by employing [HfN/VN] multilayers.
With bilayer periods ranging from 1200 to 15 nm, Escobar et al. [154] succeeded in depositing multilayered hafnium/vanadium nitride systems onto AISI 4140 steel substrates using the multi-target magnetron sputtering approach. Images from the SEM ( Figure 13) and TEM (Figure 14) revealed layers of evidence with a well-defined and consistent periodic pattern. Over uncoated AISI 4140 industrial steel, multilayered coatings significantly improved corrosion characteristics, strengthened the material's defenses against rust, and lowered the corrosion rate. Compared to untreated steel, the multilayered coatings demonstrated improved resistance to corrosion due to an interface effect that created a protective layer over the steel substrate. Because of the decreased porosity and shorter bilayer period (15 nm) of the [HfN/VN] 80 multilayered coatings, these reactive species were unable to diffuse as easily into the substrate. Therefore, when the bilayer period of the multilayer coating was reduced, the best conductivity was discovered. layered coatings demonstrated improved resistance to corrosion due to an interface effect that created a protective layer over the steel substrate. Because of the decreased porosity and shorter bilayer period (15 nm) of the [HfN/VN]80 multilayered coatings, these reactive species were unable to diffuse as easily into the substrate. Therefore, when the bilayer period of the multilayer coating was reduced, the best conductivity was discovered.  In the study developed by Caicedo et al. [155], they used carbon nitride coatings (V-C-N) to study the mechanical and tribological properties of V-C-N-coated industrial steel (AISI 8620). Using a magnetron sputtering method, the coatings were applied on silicon (100) and steel substrates by changing the applied bias voltage. Nanoindentation, pin-on-disk, and scratch test curves were used to evaluate the hardness, friction coefficient, and critical load of V-C-N surface material. As a function of bias voltage deposition, mechanical and tribological behavior in the VCN/steel system showed an increase in hardness of 58% and a reduction in friction coefficient of 39%. This shows that VCN coatings might be a potential material for industrial applications such as high-speed cutting (HSC) covered with a VCN layer evaluated in machining operations against heat-treated steels (AISI 8620). In the study developed by Caicedo et al. [155], they used carbon nitride coatings (V-C-N) to study the mechanical and tribological properties of V-C-N-coated industrial steel (AISI 8620). Using a magnetron sputtering method, the coatings were applied on silicon (100) and steel substrates by changing the applied bias voltage. Nanoindentation, pin-on- Staia et al. [156] studied the tribological behavior of milling tools covered with HfN. The early wear behavior of multilayer HfN coatings is documented in laboratory friction and wear experiments against WC as a tribological pair. Vickers micro indentation experiments were also performed to characterize the coatings' mechanical characteristics, evaluating the coating-substrate composite hardness. A model recently constructed by one of the authors provided information on the absolute hardness of HfN and TiCN monolayer films. These data are used to determine the HfN/TiCN multilayer coating characteristics. For the same number of cycles, the wear volume of single-layer HfN coatings is roughly 15 times that of HfN/TiCN coatings.

ZrO 2 Nanostructured Coatings
Nano-zirconia coatings are often used as thermal barrier coatings because to their low temperature coefficient, high thermal expansion coefficient, and strong stability at high temperatures. The tie layer and zirconia coating make up the thermal barrier coating. Wang et al. [157] created nanostructured ZrO 2 -8 wt. % Y 2 O 3 (8YSZ) thermal barrier coatings (TBCs) on steel substrates using atmospheric plasma spraying. When it comes to elastic modulus, nanostructured 8YSZ coatings were shown to have a lower value than conventional 8YSZ coatings, but their elastic recovery rate was greater. The uneven and chaotic distribution of fractures in typical TBC may be clearly seen in Figure 15. Cracks appear to be very tiny and rare, with no clear development direction in nanostructured TBC. This may be due to the increased fine grain and durability of nanostructured coatings. Yin et al. [158] employed FeAl powder, ZrO 2 nanoparticles, and CeO 2 additives to spray dry innovative multi-component raw materials. Plasma spraying was utilized to cover 1Cr18Ni9Ti stainless steel with FeAl/CeO 2 /ZrO 2 nanocomposite coatings and1Cr18Ni9Ti stainless steel treated with FeAl/CeO 2 /ZrO 2 nanocomposite coatings through plasma spraying. Both a Vickers micro indentation tester and a ball disc sliding wear friction mill were employed. In this study, the mechanical, friction, and wear characteristics of nano-composite coatings and pure FeAl coatings were studied and assessed. The microstructures and mechanical characteristics of the two coatings were examined in relation to their wear mechanisms. The strengthening effect of ZrO 2 nanoparticles may explain the increased hardness, fracture toughness, and wear resistance of the nanocomposite coatings compared to pure FeAl coatings.
Materials 2022, 15, 5633 25 conventional 8YSZ coatings, but their elastic recovery rate was greater. The uneven chaotic distribution of fractures in typical TBC may be clearly seen in Figure 15. C appear to be very tiny and rare, with no clear development direction in nanostruct TBC. This may be due to the increased fine grain and durability of nanostructured ings. Yin et al. [158] employed FeAl powder, ZrO2 nanoparticles, and CeO2 additiv spray dry innovative multi-component raw materials. Plasma spraying was utiliz cover 1Cr18Ni9Ti stainless steel with FeAl/CeO2/ZrO2 nanocomposite coa and1Cr18Ni9Ti stainless steel treated with FeAl/CeO2/ZrO2 nanocomposite coa through plasma spraying. Both a Vickers micro indentation tester and a ball disc sl wear friction mill were employed. In this study, the mechanical, friction, and wear acteristics of nano-composite coatings and pure FeAl coatings were studied and asse The microstructures and mechanical characteristics of the two coatings were examin relation to their wear mechanisms. The strengthening effect of ZrO2 nanoparticles explain the increased hardness, fracture toughness, and wear resistance of the nano posite coatings compared to pure FeAl coatings. Using a plasma spray ZrO2/B2O3/Al system, Zhang et al. [159] examined the im of ZrO2 particle size on the structure and characteristics of ZrB2-containing comp coatings. Plasma spraying resulted in the formation of ZrB2 from the reaction of Z B2O3, and Al. When plasma spraying with nano ZrO2 particles, more ZrB2 was gene than when using micro ZrO2 particles, indicating that the reaction degree betwee ZrO2/B2O3/Al composite powder and the nano-ZrO2 particles was greater. There w layered structure to the plasma-sprayed ZrO2/B2O3/Al composite powder and nanocoatings. There was an excellent distribution of ZrB2 particles, and the microstructure dense. Compared to micro-ZrO2, nano-ZrO2 coatings have substantially greater hard and toughness. Figure 15. The section image the as-sprayed (a) conventional 8YSZ TBC and (b) nanostruc 8YSZ TBC [157]. Reproduced with permission from Elsevier.

Al2O3 and Al2O3/TiO2 Nanostructured Coatings
There are several military and industrial uses for nanostructured Al2O3 and its posite coatings. Several countries' naval fleets are now using Al2O3/TiO2 nanocoatin an anti-friction and wear material. Using plasma sprayed multi-layer coatings, Dar parvar et al. [160] were able to effectively coat magnesium alloys. In the experiment, ings of NiCrAlY, nano-Al2O3-13% TiO2, and nano-TiO2 were applied to the magne alloy matrix in the experiment and generated three plasma spraying coatings. An inv gation of the corrosion behavior of these three distinct plasmas spraying coatings conducted. According to the findings of the study, the novel coating (NiCrAlY/n Al2O3/13% TiO2/nano-TiO2) reduced corrosion and increased the impedance value of nesium alloy, according to the novel coating. Certain micro-holes in the coating ma filled by a layer of titanium dioxide, which slows down the corrosion rate of Al2O3 Using a plasma spray ZrO 2 /B 2 O 3 /Al system, Zhang et al. [159] examined the impact of ZrO 2 particle size on the structure and characteristics of ZrB 2 -containing composite coatings. Plasma spraying resulted in the formation of ZrB 2 from the reaction of ZrO 2 , B 2 O 3 , and Al. When plasma spraying with nano ZrO 2 particles, more ZrB 2 was generated than when using micro ZrO 2 particles, indicating that the reaction degree between the ZrO 2 /B 2 O 3 /Al composite powder and the nano-ZrO 2 particles was greater. There was a layered structure to the plasma-sprayed ZrO 2 /B 2 O 3 /Al composite powder and nano-ZrO 2 coatings. There was an excellent distribution of ZrB 2 particles, and the microstructure was dense. Compared to micro-ZrO 2 , nano-ZrO 2 coatings have substantially greater hardness and toughness.

Al 2 O 3 and Al 2 O 3 /TiO 2 Nanostructured Coatings
There are several military and industrial uses for nanostructured Al 2 O 3 and its composite coatings. Several countries' naval fleets are now using Al 2 O 3 /TiO 2 nanocoatings as an anti-friction and wear material. Using plasma sprayed multi-layer coatings, Daroonparvar et al. [160] were able to effectively coat magnesium alloys. In the experiment, coatings of NiCrAlY, nano-Al 2 O 3 -13% TiO 2 , and nano-TiO 2 were applied to the magnesium alloy matrix in the experiment and generated three plasma spraying coatings. An investigation of the corrosion behavior of these three distinct plasmas spraying coatings was conducted. According to the findings of the study, the novel coating (NiCrAlY/nano-Al 2 O 3 /13% TiO 2 /nano-TiO 2 ) reduced corrosion and increased the impedance value of magnesium alloy, according to the novel coating. Certain micro-holes in the coating may be filled by a layer of titanium dioxide, which slows down the corrosion rate of Al 2 O 3 -13% TiO 2 . There are three layers of NiCrAlY/nano-Al 2 O 3 -13% Ti/nano-TiO 2 coating. Spray drying, heat treatment, and plasma treatment were all effective methods for producing nanocomposite powders, according to Yang et al. [161]. The microstructure and characteristics of Al 2 O 3 /TiO 2 nanocomposite powders were examined because of processing technique and nano additions. The Al 2 O 3 /TiO 2 nanostructured composite powder was made by spray drying and heat treatment, and it had nanoparticle sizes and was very spherical. An amorphous inter-crystalline network layer rich in Ti, Zr, and Ce encircled the alfa-Al 2 O 3 colony in the plasma-treated nanocomposite powder, resulting in a three-dimensional network structure. Plasma treatment resulted in spherical powders with a smooth surface and dense microstructures because of their quick melting and solidification. It was thus possible to increase the fluidity and density of plasma-treated powders.

Cr Nanostructured Coatings
Wear-resistant coatings based on the (Ti,Cr,Al)N system is commonly utilized to enhance the performance qualities of metal cutting tools. Incorporating silicate (Si) with the aforementioned coating composition further enhances its performance [162,163]. (Ti,Cr,Al,Si)N coatings with up to 60% Al and Si content were found to have a cubic structure of the NaCl type. The hexagonal (wurtzite) structure dominates when the content is high. Coatings with a maximum (Al,Si) concentration of 56% were found to have a maximum hardness of 33 GPA, which concurrently retains the cubic structure [164,165]. According to the research of Yamamoto et al. [166], (Ti,Al,Cr)N coatings can have both hexagonal and cubic phases. This coating's oxidation resistance reached 1000 • C (as opposed to 850 • C for the (Ti,Al)N coating). When heated to 1000 • C, the (Ti,Al,Cr)N coating's dominating crystal structure altered from cubic to hexagonal. There was a drop in lattice parameter in this situation because of the probable migration of Al from cubic to hexagonal grains. The coating's microhardness decreased as a result. At room temperature, the hardness of coatings containing varying amounts of different elements was almost identical (about 29 GPa). When the temperature was raised to 1000 • C, the (Ti 0.27 Cr 0.11 Al 0.62 ) N coating had the highest hardness (27 GPa). The hardness of the material was increased by 31 GPa by heating it to 800 • C. Grain sizes were decreased to 5-10 nm with the addition of Al to the coating mixture.
Introducing Si to the (Ti,Cr,Al)N coating's composition (2-3%) might alter the coating's characteristics, as Yamamoto et al. [167] discovered. Researchers found that adding Si to the coating formulation enhanced its hardness and resistance to thermal oxidation for temperatures over 1000 • C. As Al,Si surpassed 60%, the hexagonal phase content rose, and the cubic phase content dropped. During research of temperature oxidation of this layer, coatings with high Si content were found to have formed a thin oxide layer between the basic oxide and the coating. According to the scientists, the barrier layer played an important role in preventing oxygen and metal ions from entering the coating structure. The composition of the (Ti,Al,Cr)N coating had no influence on its tribological factors at room temperature, but at 650 • C, the coating with a high concentration exhibited excellent tribological properties leading to the incorporation of a chromium oxide film, which is preferable in anti-friction features to films of aluminum and titanium oxides. Using a coating with a high Cr content resulted in a lower tool life at low cutting speeds (and, accordingly, at relatively low temperatures). Nonetheless, the coating with a Cr concentration of roughly 50% demonstrated greater wear resistance when cutting speed exceeded 100 m/min [168]. Ti 0.22 Cr 0.22 Al 0.44 Si 0.12 N's cubic structure was maintained until the temperature reached 1000 • C, while Ti 0.20 Cr 0.20 Al 0.55 Si 0.05 N's cubic and hexagonal phases were already disintegrated at 900 • C in thermal stability investigations. Si atoms were included in the architecture of the phases to slow down their disintegration. The hardness of the Ti 0.20 Cr 0.20 Al 0.55 Si 0.05 N composition rose from 32 to 34 GPa when the temperature was raised to 1000 • C, due to the coherence deformation between the matrix and the newly created cubic domains. The hardness of the Ti 0.22 Cr 0.22 Al 0.44 Si 0.12 N composition decreased from 34 GPa to 32 GPa when the temperature was raised to 1000 • C [162]. When Ti and Al were added to the CrN structure, the B1-NaCl-type cubic structure with the desirable orientation was formed, according to Tam et al. [169]. In the (Cr 0.40 , Ti 0.1 , Al 0.1 )N coating, the CrN, TiN, and AlN nitride phases were found, but no Cr2N phase was found. The (Ti,Al, and Cr)N coating was also shown to have higher wear resistance than the CrN, (Cr,Ti)N, and (Cr,Al)N coatings. At room temperature, the friction coefficient of the chosen coating was between 0.4 and 0.5. A comparison of the characteristics of the CrN, (Cr,Ti), (Cr,Al), and (CrTi,Al)N coatings was carried out by Wang et al. [170]. In the investigation, it was observed that the (Cr,Ti,Al)N coating had smaller grains and a much greater hardness and fracture resistance than other coatings. It was found that the adhesion strength of the provided coating to the carbide substrate was much decreased, and considerable residual compressive stresses were also found, resulting in the production of annular fissures. The characteristics of (Ti,Al) N-CrN-type nanolayer coatings were also examined. Al x Ti 1 − x N and CrN have been widely used as a protective coating material in many types of tools and mechanical components because of high wear performance and high temperature resistance. Chang et al. [171] investigated the monolayer of Al 0.63 Ti 0.37 N and the nanolayer of Al 0.63 Ti 0.37 N/CrN. Thermal stability and resistance to oxidation at temperatures between 700 • C and 1000 • C were examined. An oxide film was generated on the Al 0.63 Ti 0.37 N and CrN layers that was significantly thinner than the one on the Al 0.63 Ti 0.37 N coating, even at temperatures of up to 1000 • C, whereas the Al 0.63 Ti 0.37 N coating had a thicker oxide film. The nanolayer (Al,Ti,Si)N and (Cr,Si)N coating was explored by Yang et al. [172]. The B1-NaCl type structure was also present in this coating. With a rise in silicate percentage, coating hardness (up to 35 GPa) increased, but with an increase in silicate content of over 8%, coating hardness and adhesive binding strength decreased.
Fukumoto et al. [173] investigated the nanolayer (Ti,Cr,Al)N/(Al,Si)N coating. When it came to layer structure, it was discovered that the (Ti,Cr,Al)N layer had a hexagonal structure, whereas the (Al,Si)N layer was cubic. When the nanolayer thickness was reduced, the coating's hardness rose. Furthermore, it was discovered that the coating was able to withstand temperatures of up to 1100 • C. Chang et al. [174] investigated nanocrystalline CrAlSiN and CrTiAlSiN coatings, which were deposited by using a cathodic-arc deposition system with lateral rotating arc cathodes. Titanium, chromium, and Al 89 Si 11 cathodes also were used for the deposition of CrAlSiN and CrTiAlSiN coatings. Zhang et al. [175] looked at the nanolayer Cr,Al,Si,N coating as a possible alternative. Cr,Al,Si hexagonal and cubic nanolayers were alternated in the coating's structure, with a nanolayer thickness of roughly 7 nanometers. The provided coating was discovered to have an incredibly high hardness rating (up to 52 GPa). The provided coating, on the other hand, demonstrated great plastic deformation resistance. In the ball-on-disk test, the coating had an extremely low friction coefficient of 0.1-0.2. These numbers were explained by the high cohesive energy of interfacial bonds and an epitaxial rise that formed a field of alternating stresses that inhibited dislocation movement and improved the hardness of a given coating. The low value of elastic recovery revealed that the coating had appropriate strength and ductility, as demonstrated by the low value of elastic recovery.

The Impact of Cutting-Zone Temperatures on Multilayer Coating Properties
During the metal cutting process, about 90% of the mechanical energy is transferred to thermal heat flux, resulting in a severe cutting temperature rise in the cutting zone, as shown in Figure 16 [176,177]. Increased cutting temperature causes excessive wear on the tool rake and flank faces, reducing tool life [178]. Furthermore, induced thermal softening of the tool and workpiece impairs chemical element diffusion and has an impact on surface quality, dimensional accuracy, and functioning of the machined item [179]. As a result, cutting temperature is an important metric for predicting tool wear and cutting performance [180].
According to Nguyen et al. [181], a coating made of (Ti,Al,Cr,Si)N may be thermally oxidized at 1000 • C. Cr 2 O 3 , "Al 2 O 3 ", and "Rutile" (a type of titanium dioxide) oxide phases developed on the coating's surface under these circumstances. Al 2 O 3 and Cr 2 O 3 were largely found in the inner section of the oxide layer, whereas TiO 2 occupied the outside part. In the oxide layer, the amount of SiO 2 was negligible because of its limited mobility. The researchers identified a four-sublayer oxide layer structure. The outer layer was created via the incorporation of Si ions into the TiO 2 phase (the layer was formed due to the diffusion of Ti, Si, and to a lesser extent). tool rake and flank faces, reducing tool life [178]. Furthermore, induced thermal softening of the tool and workpiece impairs chemical element diffusion and has an impact on surface quality, dimensional accuracy, and functioning of the machined item [179]. As a result, cutting temperature is an important metric for predicting tool wear and cutting performance [180]. According to Nguyen et al. [181], a coating made of (Ti,Al,Cr,Si)N may be thermally oxidized at 1000 °C. Cr2O3, "Al2O3", and "Rutile" (a type of titanium dioxide) oxide phases developed on the coating's surface under these circumstances. Al2O3 and Cr2O3 were largely found in the inner section of the oxide layer, whereas TiO2 occupied the outside part. In the oxide layer, the amount of SiO2 was negligible because of its limited mobility. The researchers identified a four-sublayer oxide layer structure. The outer layer was created via the incorporation of Si ions into the TiO2 phase (the layer was formed due to the diffusion of Ti, Si, and to a lesser extent).
The Cr2O3/Al2O3 with a minor number of Si ions was the next layer, followed by the TiO2 phase, which had a low inclusion of Cr, Al, and Si ions in the inner sublayer. Oxygen diffusion caused the formation of the inner layers. The (Ti,Cr,Al)N coating's thermal stability, oxidation resistance, and tribological characteristics were studied by Xu et al. [182]. This coating was found to have a hardness of between 32 and 33.9 GPa at room temperature. A Cr content of less than 38% prevailed over annealing in an inert environment, Figure 16. Microscale modeling of cutting temperature distribution at the tool-chip contact in the cutting zone [177].
The Cr 2 O 3 /Al 2 O 3 with a minor number of Si ions was the next layer, followed by the TiO 2 phase, which had a low inclusion of Cr, Al, and Si ions in the inner sublayer. Oxygen diffusion caused the formation of the inner layers. The (Ti,Cr,Al)N coating's thermal stability, oxidation resistance, and tribological characteristics were studied by Xu et al. [182]. This coating was found to have a hardness of between 32 and 33.9 GPa at room temperature. A Cr content of less than 38% prevailed over annealing in an inert environment, resulting in a hardness increase of up to 35 GPa when heated to 700 • C. High Cr concentration in the coatings led to the formation of Cr2N, which resulted in metallic Cr and the spinodal breakdown of the supersaturated c-(Ti,Cr,Al)N supersaturated solid solution when heated. There was also a sign of dispersion hardening. Solid solution (Cr,Ti) 2 N phase was produced due to the high Ti concentration. A considerable drop in hardness was seen when the Cr concentration in AlN approached 41%, which was followed by the quick formation of the hemodynamically stable wurtzite phase during annealing (associated with the dissolution of Cr-N bonds). Since these metals generate stable oxide coatings that inhibit oxidation, the oxidation resistance steadily rose as the Cr and Al amount increased. Thus, Ti 0.18 Al 0.35 Cr 0.47 N coating had the best oxidation resistance when heated to 900 • C, as shown in Figure 17.
In contrast, the low-Cr Ti 0.26 Al 0.48 Cr 0.26 N coating proved to be an excellent choice for cutting tools because of its oxidation resistance and strong hardness at high temperatures. The (Ti,Al)N/CrN coatings were examined for the influence of temperature and found that at 700 • C, the coating displayed an interface-directed spinodal breakdown, and Al atoms went to the closest interface, as shown in Figure 18. An anisotropic interface-directed decomposition mechanism developed as the temperature rose, accompanied by transboundary interdiffusion as a result (with intensive mixing of Cr and Ti). Columnar grains had clearly defined composition gradients at this stage of disintegration, oriented perpendicular to the layers' paths. At a temperature of 1000 • C, the underlying coating layer structure totally dissolved but was preserved in places that were far from the surface [183,184]. As a result of this, grain production also increased. Interlayer interdiffusion was seen in the (Ti,Al)N/CrN system at 500 • C, whereas hardness decreased at 600 • C, and active oxidative processes were activated at 900 • C on the coating's surface. At 800-1100 • C, an AlCrTiSiN nano multilayered coating was isothermally annealed by Chen et al. [185] principally consisting of fcc-AlCrTiSiN solid solution and some hcp-clusters at fcc-AlCrTiSiN nanotwin borders. The findings showed that annealing at 1100 • C promotes layered structure disintegration by pinching off hcp sublayers; the transition of fcc AlCrTiSiN solid solution to Cr, Cr 2 N, and hcp-AlN particles; and the coarsening of hcp-AlN particles, leading to a significant loss in hardness. In contrast, the low-Cr Ti0.26Al0.48Cr0.26N coating proved to be an excellent choice for cutting tools because of its oxidation resistance and strong hardness at high temperatures. The (Ti,Al)N/CrN coatings were examined for the influence of temperature and found that at 700 °C, the coating displayed an interface-directed spinodal breakdown, and Al atoms went to the closest interface, as shown in Figure 18. An anisotropic interface-directed decomposition mechanism developed as the temperature rose, accompanied by transboundary interdiffusion as a result (with intensive mixing of Cr and Ti). Columnar

Failure Behavior of Coated Tools
The coated tool performance is greatly influenced by residual stresses formed in the film structure [186]. Compressive stresses are thus regarded beneficial for loaded components and tools, as they prevent fracture initiation and propagation in the coating. Higher compressive stresses are linked to longer coated tool life until a particular stress limit is reached, which worsens coating brittleness and hence cutting efficiency [187]. PVD coatings on cutting tools have received widespread acceptance and have emerged as a key means of increasing tool cutting quality. Due to thermoelastic forces and grown-in flaws caused by particles with high kinetic energy during deposition, PVD coatings often have substantial residual stresses in their structure [188].
The amplitude of produced compressive residual strains during film deposition is influenced by process factors such bias voltage [189]. The goal of the research was to figure out how the degree of residual stresses in the PVD film structure affects the wear behavior of coated tools. The coating residual stresses are predicted to have a major impact on attributes such as fatigue, toughness, residual stresses, and adhesion, which are essential for cutting with coated tools. Novel experimental approaches combining with FEM-supported calculations were used to establish these features. Residual stresses are a critical aspect in component longevity and are a component of surface integrity.

Failure Behavior of Coated Tools
The coated tool performance is greatly influenced by residual stresses formed in the film structure [186]. Compressive stresses are thus regarded beneficial for loaded components and tools, as they prevent fracture initiation and propagation in the coating. Higher compressive stresses are linked to longer coated tool life until a particular stress limit is reached, which worsens coating brittleness and hence cutting efficiency [187]. PVD coatings on cutting tools have received widespread acceptance and have emerged as a key means of increasing tool cutting quality. Due to thermoelastic forces and grown-in flaws caused by particles with high kinetic energy during deposition, PVD coatings often have substantial residual stresses in their structure [188].
The amplitude of produced compressive residual strains during film deposition is influenced by process factors such bias voltage [189]. The goal of the research was to figure out how the degree of residual stresses in the PVD film structure affects the wear behavior of coated tools. The coating residual stresses are predicted to have a major impact on at- Compressive residual stresses are beneficial for tool life because cutting tools have alternating mechanical and high thermal demands. As a result, during the production of cutting tools, it must be guaranteed that the ready-made tools have appropriate compressive residual stress in their subsurface. Some special properties of PVD-coated cutting tools must be examined in this context. The base must be made up of at least two phases: one or more hard materials and a binder substance. Residual stresses are usually solely taken into account for the dominant hard material, while there may be interdependencies between coating and binder residual stresses. The fabricated tool's ultimate residual stress state is the outcome of a sequence of machining stages, each of which impacts or even produces a new residual stress state. Because the depth of susceptible to influence coating subsurface is restricted to around 10 mm, it is believed that the process step not only alters the current residual stress state but also creates a new subsurface condition.
The facts that each machining step removes material from the surface and the new subsurface only partially overlaps the old one support this. It is important to remember that not only thermal component loads cause a shift in the residual stress state in the direction of tensile stress while making PVD-coated cutting tools. During etching, for example, more binder material is removed than coating grains, resulting in a relaxation of the coating's compressive stress and a shift towards the tensile stress direction due to mechanical effect. Furthermore, the presence of a coating with a high compressive residual stress causes the substrate stress to shift in the direction of tensile stress. Mechanical sources in addition to thermal ones are responsible for this.
A compensation of the high compressive coating stress occurs in the substrate subsurface due to mechanical causes, resulting in the above-mentioned displacement. High compressive coating stress is necessary to extend tool life duration. There is an upper limit beyond which tool life begins to dwindle anew. However, significant compressive stress in the substrate subsurface is also necessary to prevent cohesive tool damage, which is only possible within specific limitations since coating and substrate residual stresses interact. In this case, finding a balance that combines a lengthy life duration with a reduced chance of cohesive tool damage is critical. These considerations must take into account the effects of surface condition on coating adhesion. The damage behavior of milling inserts is affected by thermal and mechanical loadings in milling operations. Majid Abdoos et al. [190] reported research into the cutting performance and associated coating features of multilayer-thick TiAlN coatings with various residual stress designs. During the deposition process, residual stress was controlled by adjusting the substrate bias voltage. Nano-indentation and scratch tests were used to investigate the influence of residual stress on attributes such as hardness, yield strength, and adhesion. Furthermore, a scanning electron microscope was used to examine the prevalent wear pattern, particularly on the rake face and cutting edge (SEM).
With larger substrate bias voltages and hence higher residual stresses, the findings showed enhanced mechanical characteristics such as hardness and yield strength. Due to a combination of good adherence to the substrate and minimal as-deposited deficiencies, the coating with the lowest compressive residual stress outperformed the other coatings during machining, thereby delaying cutting-edge exposure. Nemetz et al. [191] used the finite element approach to mimic industrial milling operations and obtain information about the underlying damage mechanisms. Two recently published experimental milling settings are used to validate the results. The creation and propagation of so-called comb fractures in planes perpendicular to the cutting edge is aided by these tensile residual stresses, which are damaging to the tool's efficiency. The insert is constructed of WC-Co hard metal with an average WC grain size of 1 µm and an 8 wt.% co-binder. As a thermal shield, it is covered with a 7 µm thick TiAlN layer. A Johnson-Cook constitutive material model describes the workpiece material, 42CrMO4. A 2D Arbitrary Lagrangian-Eulerian (ALE) technique is used to simulate the milling process. The present study's key findings are as follows: For 50 cycles of milling, the time-dependent development of fields of temperature and stress in a milling tool was computed in a 3D FE model as a function of milling process parameters. The computed thermal and mechanical stresses were projected to cause localized plastic deformation of the hard metal substrate and, as a result, the accumulation of tensile residual stress in a location near the tool's rake face's cutting edge. The suggested modeling approach may be used to forecast the lifetime of metal cutting tools that have failed owing to comb cracking. The fabrication and characterization of superposed structures made of Cr, CrN, and CrAlN layers are described by Tlili et al. [192]. Physical vapor deposition (PVD) was used to deposit CrN/CrAlN and Cr/CrN/CrAlN nano-multilayers onto Si (100) and AISI4140 steel substrates. The Cr, CrN, and CrAlN monolayers were created using a novel PVD coatings technology that corresponded to deposition with various residual stresses.
Tracks of composition and wear scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and a 3D-surface analyzer were used to describe the coating morphologies. Nanoindentation, interferometry, and microtribometry were used to study the mechanical characteristics (hardness, residual stresses, and wear) (fretting-wear tests). Multilayer coatings appear to be largely made up of nanocrystalline, according to observations. The number of residual stresses in the films has had a significant impact on all physicochemical and mechanical parameters as well as wear behavior. As a result, the coating with moderate residual stresses has a better wear behavior than the coating with larger residual stresses. Friction interaction between coated samples and alumina balls reveals a wide range of wear processes as well. Plastic deformation, tiny microcracking, and micro spallation were all involved in the abrasive wear of the coatings.

Residual Stresses Induced in Coating Process
Residual stresses present after the coating process depend heavily on the method and properties of the coating process, on the coating material, and lastly on the substrate. Three different processes have been identified as the source of majority of residual stress: ceramic phase transformation during cooling process, cooling of molten spray, and cooling of the whole coating system [193]. These processes are a factor in methods relying on a difference in temperatures (spray coating or vapor depositions, for example) and are not present in methods using electrolytic or electrophoretic processes to deposit the coating even though stresses produced by mismatch of a lattice constant will be present no matter the coating method. The severity of the residual stresses depends on the temperature during the coating process, cooling speed, and the thickness of the generated layer as well [194]. To reduce the residual stress, several methods have been developed, mostly focusing on temperature control and strain compatibility at layer interfaces [195,196]. Generally, a better matching coating material to the substrate needs to be used, but where that is not possible, there are several solutions. One of the approaches is a multi-layered coating. This can allow for a deposition of a more heat-resistant material first, reducing the stress caused by temperature change in the final layer. This combination can take the form of, for example, heat-resistant ceramic and a metallic coat [197]. While this method drastically reduces the thermal residual stress, it is important to take into account the mechanical properties of the different coatings as well because heat-resistant material might not have the best properties required of the final coating, such as vibration resistance or thermal/electric conductivity or isolation. A slightly different approach during layer deposition is to reduce the thermal expansion mismatch by using coatings with graded properties [193,198]. In this process, the physical properties change in a stepwise manner due to change in composition or morphology [199]. In this way, it is possible to transition from properties of the coating that will provide better adhesion with the substrate but do not possess the qualities of the desired top layer to more ideal top layer of the coating without the need to trade adhesion compatibility for desired hardness or resistance and vice versa. The third option is the combination of the previous two, where a multi-layered coating is improved by inserting a graded interlayer. This technique has the potential to provide the best possible result, but it has drawbacks, such as the technological requirements on the whole process, precision of material selection, and the final thickness of the whole coating, which can increase dramatically when including more and more layers. This large number of different methods combined into one increases the number of failure states, which increases the difficulty of the whole process [200].

Future Challenges for Hard Coatings
Since the introduction of hard coatings in the late 1960s, industry aspirations to further enhance these coatings have been fueled by the push to enhance the productivity and competitiveness of manufacturing processes and, later, by the desire to reduce the use of coolants and lubricants and to save resources by extending the life span of tools. This rapid and ongoing success of hard coatings contributes significantly to the sustainable development goals (SDGs), for example, by lowering the energy consumption of cutting procedures. While this motivation will undoubtedly continue, the already existing basic scientific foundation is being exploited to provide more and more functionality to these coatings. However, in the future, the hard coatings community, i.e., both academic and industry, will have to greatly strengthen their efforts to meet the SDGs, as will any other production-related group, pushed by the appropriate government laws. Some of these prospective difficulties will be addressed in the following paragraphs.

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Multi-functionality coatings There have previously been significant attempts to create multifunctional hard coatings in order to achieve a long tool lifespan, but only a handful of these have found their way into industrial applications. Self-hardening (for example, age hardening of Ti 1-x Al x Nbased coatings) has been a focus of study for over two decades [201]. In recent decades, complex coating topologies for damage-tolerant coatings capable of stopping fractures have been proposed. The obvious next step is the development of self-healing coatings, which is currently in its early stages. Surface self-organization processes during tribological sliding contacts are seen as a potential technique for not only self-healing of surface fractures but also for adding self-lubrication functions, making the coated product more environmentally friendly [202]. Furthermore, thermal management abilities have grown in popularity in recent years, as the conventionally required low thermal conductivity of hard coatings may result in high peak temperatures in the contact with the chip in cutting and distinct thermal gradients, implying that a tailored thermal conductivity allowing heat isolation along the coating thickness and heat distribution within the coating plane may be more advantageous [203]. Autonomously self-healing hard coatings are a novel technique in which microstructural changes are tracked by in situ measurements of relevant characteristics, such as electrical sheet resistance, to follow oxidation progress. All these characteristics should be combined in an original design for a future hard coating for metal-cutting applications [204].

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Simulated coatings and deposition processes Simulation-guided or at least -helped design of hard coatings with improved performance is already a well-established approach that aids in resource conservation. The existing technique, however, has to be encouraged and expanded further. There is now a significant gap between turn-key solutions for typical applications, which can be handled by semi-skilled individuals, and unique coating concepts, which require specially qualified academics. To bridge this gap and enable the rapid and easy construction, correction, and optimization of deposition processes without the immediate requirement for highly educated experts, the development of related technologies [205] and, as a result, expert systems for the individual deposition processes will be required. Adding multifunctional features to more complicated hard coatings will require the digitalization of our traditional coating development processes to profit from the ever-increasing stream of accessible data. Modern data management systems, for example, based on the FAIR Guiding Principles [206] (where FAIRness of data implies that it is findable, accessible, interoperable, and reusable), could enable machine learning in the future from a wealth of theoretical and experimental data that humans would never be able to overlook.

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Energy-efficient deposition techniques Coating deposition machines use a huge amount of energy to manufacture a relatively little amount of coating material. Normalization of the collected data in terms of coating thickness and number of tools per coating batch enables comparison of the efficiency of various deposition procedures (energy consumption and material fluxes in PVD and CVD processes). In addition to visualizing energy consumption and material fluxes, the developers suggest considering ways to improve the energy efficiency of deposition processes with high energy consumption (e.g., heating substrates or running power supplies for evaporation or sputtering) in next-generation coating facilities. For example, the utilization of heat-exchange modules for waste heat recovery offers previously untapped opportunities to improve the energy efficiency of deposition systems and so contribute to environmental sustainability.

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Coating thermal and mechanical properties.
Future research should focus on studying the impact of tool coating wear and failure progression on cutting temperature and tool life across the whole metal-cutting process with coated tools. Based on genuine experimental observations, the assumption of boundary conditions in prediction models of cutting temperature should be updated to be compatible with the actual cutting process. Though certain cutting temperature measuring techniques with uncoated tools are applicable to coated tools, future research should focus on developing and improving accurate cutting temperature measurement techniques with rapid reaction times. There have been few investigations into the temperature monitoring and control of coated smart cooling tools. Cutting temperature reductions are more noticeable with coated smart cutting tools than with uncoated smart cutting tools due to the coating's superior effect and internal cooling effects. Recent studies have focused mostly on the coating mechanical characteristics of micro-milling tools. There has been little research into the impact of coating thickness or thermal characteristics on micro-milling temperatures, including analytical models and trials. The tool edge radius increased as the coating layer grew, impacting the micro-milling temperature. However, the influencing mechanism remains unknown. The proper coated micro-milling tools must be built with both coating thermal and mechanical qualities in consideration.

Conclusions
This paper presents a review of the current state-of-the-art in hard coatings for metal cutting applications, focusing on widely used and well-established hard coatings used in the metal cutting industry and considering the similarities and differences between coatings grown using PVD and CVD techniques. Economic and environmental considerations in the machining industry result in an ongoing increase in cutting velocities and feed rates as well as a reduction in the use of coolants and lubricants, with the concurrent demand for longer tool lifetimes, necessitating continuous further development of the applied hard coatings.
Historically, strong wear resistance was frequently associated with great hardness and perhaps good thermal and oxidative stability. Additional qualities are now necessary to satisfy ever-increasing demands and withstand the severe circumstances that cutting instruments are subjected to. Modern hard coatings must be multifunctional and have qualities such as enhanced breaking resistance or advanced heat management, which are generally achieved through complex coating designs. Modern nanocoatings demonstrate greater adhesion and increased wear resistance, resulting in longer tool life. This demonstrates the coatings' tremendous promise in a variety of milling applications, particularly the milling of difficult-to-machine materials.
Recent studies have also concentrated on nanostructured and nanolayered coatings. These have been tested and found to have high tool-life values, rivalling those of more recent nanocoatings, with very satisfactory results.
In terms of tool wear processes and patterns, for milling tools, the most common wear pattern is the creation and spread of cracks, which is produced by thermal fatigue (high machining temperatures). Recent research is still heavily focused on the causes and patterns of tool wear that are experienced by coated tools because this directly affects the tool's lifespan. Studies on coating wear might benefit new coating development and process optimization. According to recent research, coatings can be used longer if they are applied using these procedures, providing excellent outcomes in terms of both surface finish and overall process efficiency.