Deposition of Multicomponent AlTiCrMoN Protective Coatings for Metal Cutting Applications

The high potential of the protective coatings for metal machining applications using the physical vapor deposition (PVD) processes is one to be valued, and will accelerate development of the multicomponent coating design and increase the cutting efficiency. In this study, nanostructured AlTiCrMoN coatings in a multilayered structure were fabricated using cathodic-arc deposition (CAD). Controlling the cathode current of both CrMo and AlTi alloy targets in a nitrogen environment, multilayered AlTiN/CrMoN coatings were deposited. The AlTiN and AlTiN/CrMoN multilayered coatings exhibit a face-centered cubic (fcc) structure with columnar morphologies. The highest hardness of 35.6 ± 1.5 GPa was obtained for the AlTiN coating; however, the H3/E*2 and H/E* values were the lowest (0.124 and 0.059, respectively). The multilayered AlTiN/CrMoN coatings possessed higher H3/E*2 and H/E* values of up to 0.157 and 0.071, respectively. The present study investigated the cutting performance of end mills in the milling of SUS316L stainless steel. The cutting performance was evaluated in terms of cutting length and tool wear. Because of high resistance to adhesive and abrasive wear, the end mills coated with multilayered AlTiN/CrMoN showed less flank wear than monolithic AlTiN. The introduction of CrMoN sublayers improved the cutting tool life of AlTiN.


Introduction
In order to improve the tribological and mechanical performance, many tool materials are deposited with hard coatings on the basis of nitrides. Hard coatings, such as TiN, AlTiN and CrN, have been used extensively as protective coatings, which can improve the surface properties of the mechanical tools and prolong the service life [1][2][3][4]. These coatings enhance the resistance to wear and therefore increase the tool lifetime. Especially in the case of metal cutting, using advanced hard coatings has been a regular technique to meet the wide range of machining materials, especially for difficult-to-cut mold steel, stainless steel as well as composite materials [5]. In previous studies, AlTiN and AlCrN have been widely used as substitutes for TiN and CrN coatings for industrial applications. However, the high friction coefficient and poor resistance to brittle fracture of AlTiN coating is still a challenge for modern manufacturing. Compared to AlTiN coatings, AlCrN coatings exhibit excellent thermal stability and have a potential to improve oxidation resistance [6,7]. Therefore, many studies have focused on AlCrN coatings, which are a promising candidate for protective coating applications [8][9][10][11][12]. The introduction of Al atoms into the CrN phase showed a single-phase face-centered cubic (fcc) structure when the Al content was less than~65 at.%; at higher Al contents, phase transformation from the fcc structure to the hexagonal closed packed (hcp) structure could be observed. Hence, adopting another transition element is an important technique to enhance mechanical properties and wear behavior of CrN and AlTiN. Because of the lower hardness of CrN coating (13∼20 GPa), it limits its Scientific Inc., Waltham, MA, USA) equipped with an EDS system. The milling process was interrupted for each experiment after completion and then value of the flank wear was measured.

Microstructure Characterization of the Deposited Coatings
The XRD patterns obtained for the as-deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings are shown in Figure 1. All AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings consist of cubic B1-NaCl structure and orientations of (111) (200), (220) and (311) planes were observed. For the AlTiN coating, the diffraction peak corresponding to the (111) reflection (2θ = 37.1°) indicated the existence of an fcc AlTiN solid solution [33][34][35]. The lattice parameter of the AlTiN was 0.419 nm, which was between the values of cubic TiN (0.424 nm, JCPDF file No.: #650565) and cubic AlN (0.405 nm, JCPDF file No.: #461200). The incorporation of Al to form fcc AlTiN solid solution was caused by the substitution of Ti atoms by smaller Al atoms.
The AlTiCrMoN-1 and AlTiCrMoN-2 coatings deposited at different cathode current ratios of AlTi/CrMo (I[AlTi]/I[CrMo]) also exhibited fcc crystal structure. The diffraction peaks of AlTiCrMoN-1 and AlTiCrMoN-2 coatings are also located between those of monolithic AlN and TiN. The diffraction peaks were similar to monolithic AlTiN except that the diffraction peak were shifted to higher angles, and a smaller lattice parameter of 0.418 nm was found. Similar results were also found in previous studies [36][37][38]. The peak shift was generally attributed to the lattice distortion due to the different atomic radius between Al, Ti Cr and Mo. Besides, no diffraction peaks of hexagonal AlN phase could be identified from the XRD patterns. AlN under thermodynamic equilibrium conditions forms a Wurtzite-type structure (hexagonal), whereas TiN has an fcc structure. In the multicomponent AlTiCrMoN, different growth directions of nitride grains appeared, showing that the cubic structure was formed by continuous growth of AlTiN and CrMoN when AlTi and CrMo alloy targets were evaporated in nitrogen environment. Stabilization of the cubic phase was observed.

Microstructure Characterization of the Deposited Coatings
The XRD patterns obtained for the as-deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings are shown in Figure 1. All AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings consist of cubic B1-NaCl structure and orientations of (111) (200), (220) and (311) planes were observed. For the AlTiN coating, the diffraction peak corresponding to the (111) reflection (2θ = 37.1 • ) indicated the existence of an fcc AlTiN solid solution [33][34][35]. The lattice parameter of the AlTiN was 0.419 nm, which was between the values of cubic TiN (0.424 nm, JCPDF file No.: #650565) and cubic AlN (0.405 nm, JCPDF file No.: #461200). The incorporation of Al to form fcc AlTiN solid solution was caused by the substitution of Ti atoms by smaller Al atoms.
The AlTiCrMoN-1 and AlTiCrMoN-2 coatings deposited at different cathode current ratios of AlTi/CrMo (I [AlTi] /I [CrMo] ) also exhibited fcc crystal structure. The diffraction peaks of AlTiCrMoN-1 and AlTiCrMoN-2 coatings are also located between those of monolithic AlN and TiN. The diffraction peaks were similar to monolithic AlTiN except that the diffraction peak were shifted to higher angles, and a smaller lattice parameter of 0.418 nm was found. Similar results were also found in previous studies [36][37][38]. The peak shift was generally attributed to the lattice distortion due to the different atomic radius between Al, Ti Cr and Mo. Besides, no diffraction peaks of hexagonal AlN phase could be identified from the XRD patterns. AlN under thermodynamic equilibrium conditions forms a Wurtzite-type structure (hexagonal), whereas TiN has an fcc structure. In the multicomponent AlTiCrMoN, different growth directions of nitride grains appeared, showing that the cubic structure was formed by continuous growth of AlTiN and CrMoN when AlTi and CrMo alloy targets were evaporated in nitrogen environment. Stabilization of the cubic phase was observed. The surfaces of all deposited coatings were similar in appearance to those investigated by using a laser scanning microscope, and all deposited coatings showed a higher Ra value of 0.25 ± 0.1 μm than that of the polished substrates (Ra = 0.1 ± 0.03 μm). The presence of micro-droplets was the main reason for the higher surface roughness observed on the cathodic arc evaporated coatings. Crosssectional SEM micrographs, including SEI and BEI images, of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings are provided in Figure 2. As observed in the SEI images, the coatings exhibited columnar growth morphologies expanding from the initial growth on the substrate to the top layer of the whole coating. Especially for the AlTiN, the columns were slightly V-shaped with open column boundaries. Distinct columnar growth can be seen for conventional single-phase and cubic structured coatings, typical for the metallic and nitride thin films deposited at low temperature compared to melting point [39][40][41][42][43]. As shown in the SEI and BEI images of Figure 2, all deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings had TiN and TiN/AlTiN interlayers. The top layer of AlTiN/CrMoN and the interlayer of TiN/AlTiN exhibited multilayered structures. A multilayered structure could be formed by alternately depositing with different cathodic arc sources. The AlTiN/CrMoN top layer was deposited by alternate co-evaporation of CrMo and AlTi targets in a nitrogen environment. For all three coatings, the thicknesses of TiN and TiN/AlTiN interlayers were 0.28 and 0.23 μm, respectively. The TiN/AlTiN interlayer, which was deposited by Ti and AlTi targets in nitrogen, was used as a transition layer from TiN to AlTiN and AlTiN/CrMoN top layers in order to obtain good adhesion strength. The total thicknesses of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings were 1.43, 1.48, and 1.4 μm, respectively. Table 2 shows the EDS results, which revealed the chemical composition of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings. The ratio N/(Al + Ti + Cr + Mo) was 0.97~1.09 to be stoichiometric. The highest N/(Al + Ti) ratio was obtained for the AlTiN with atomic stoichiometry of Al0.63Ti0.37N. The Al/(Ti + Al) atomic ratio in the Al0.63Ti0.37N coating decreased to 0.63 compared with the Al67Ti33 cathode material, which was attributed to the different average ionization of Al and Ti and the lower atomic mass of Al that led to a lower Al density in the evaporated vapor [35,44]. For the AlTiCrMoN-1 and AlTiCrMoN-2 coatings, the results showed stoichiometric Al0.29Ti0.17Cr0.46Mo0.08N and Al0.38Ti0.27Cr0.30Mo0.05N, respectively. At the AlTi/CrMo cathode current (I[AlTi]/I[CrMo]) of 1:1, which had the same AlTi and CrMo currents, the higher atomic ratio of Cr and Mo as compared to Al and Ti exhibited higher evaporation rate of the CrMo alloy target. The surfaces of all deposited coatings were similar in appearance to those investigated by using a laser scanning microscope, and all deposited coatings showed a higher Ra value of 0.25 ± 0.1 µm than that of the polished substrates (Ra = 0.1 ± 0.03 µm). The presence of micro-droplets was the main reason for the higher surface roughness observed on the cathodic arc evaporated coatings. Cross-sectional SEM micrographs, including SEI and BEI images, of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings are provided in Figure 2. As observed in the SEI images, the coatings exhibited columnar growth morphologies expanding from the initial growth on the substrate to the top layer of the whole coating. Especially for the AlTiN, the columns were slightly V-shaped with open column boundaries. Distinct columnar growth can be seen for conventional single-phase and cubic structured coatings, typical for the metallic and nitride thin films deposited at low temperature compared to melting point [39][40][41][42][43]. As shown in the SEI and BEI images of Figure 2, all deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings had TiN and TiN/AlTiN interlayers. The top layer of AlTiN/CrMoN and the interlayer of TiN/AlTiN exhibited multilayered structures. A multilayered structure could be formed by alternately depositing with different cathodic arc sources. The AlTiN/CrMoN top layer was deposited by alternate co-evaporation of CrMo and AlTi targets in a nitrogen environment. For all three coatings, the thicknesses of TiN and TiN/AlTiN interlayers were 0.28 and 0.23 µm, respectively. The TiN/AlTiN interlayer, which was deposited by Ti and AlTi targets in nitrogen, was used as a transition layer from TiN to AlTiN and AlTiN/CrMoN top layers in order to obtain good adhesion strength. The total thicknesses of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings were 1.43, 1.48, and 1.4 µm, respectively. Table 2 shows the EDS results, which revealed the chemical composition of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings. The ratio N/(Al + Ti + Cr + Mo) was 0.97~1.09 to be stoichiometric. The highest N/(Al + Ti) ratio was obtained for the AlTiN with atomic stoichiometry of Al 0.63 Ti 0.37 N. The Al/(Ti + Al) atomic ratio in the Al 0.63 Ti 0.37 N coating decreased to 0.63 compared with the Al 67 Ti 33 cathode material, which was attributed to the different average ionization of Al and Ti and the lower atomic mass of Al that led to a lower Al density in the evaporated vapor [35,44]. For the AlTiCrMoN-1 and AlTiCrMoN-2 coatings, the results showed stoichiometric Al 0.29 Ti 0.17 Cr 0. 46     In order to describe the coating constitution, microstructure and compositional distribution, especially the TiN/AlTiN interlayer and the AlTiN/CrMoN top layer, the deposited AlTiCrMoN-2 coating was investigated by TEM. The cross-sectional TEM bright field image of the AlTiCrMoN-2 multilayered coating are presented in Figure 3. The selected-area electron diffraction (SAED) pattern is shown for the top AlTiN/CrMoN layer. It exhibits columnar structure without the presence of voids. The cross-sectional bright field TEM image showed well-adherent layers within the multilayered architecture and the interlayers of TiN/AlTiN and TiN. Previous studies [26,45,46] also indicated that the TiN interlayer and composition-gradient layers increased the adhesion strength and enhanced the wear resistance of the AlTiN and multilayered TiAlSiN/TiN coatings. From the result of SAED pattern, the diffraction rings were identified as the reflections consistent with fcc NaCl type phases, and the pattern showed the characteristic diffraction rings of (111), (200) and (220). In agreement with the results obtained by XRD, the SAED patterns taken from the top multilayered  In order to describe the coating constitution, microstructure and compositional distribution, especially the TiN/AlTiN interlayer and the AlTiN/CrMoN top layer, the deposited AlTiCrMoN-2 coating was investigated by TEM. The cross-sectional TEM bright field image of the AlTiCrMoN-2 multilayered coating are presented in Figure 3. The selected-area electron diffraction (SAED) pattern is shown for the top AlTiN/CrMoN layer. It exhibits columnar structure without the presence of voids. The cross-sectional bright field TEM image showed well-adherent layers within the multilayered architecture and the interlayers of TiN/AlTiN and TiN. Previous studies [26,45,46] also indicated that the TiN interlayer and composition-gradient layers increased the adhesion strength and enhanced the wear resistance of the AlTiN and multilayered TiAlSiN/TiN coatings. From the result of SAED pattern, the diffraction rings were identified as the reflections consistent with fcc NaCl type phases, and the pattern showed the characteristic diffraction rings of (111), (200) and (220). In agreement with the results obtained by XRD, the SAED patterns taken from the top multilayered AlTiN/CrMoN exhibited only fcc phases. The diffraction rings corresponding to AlTiN and CrMoN could not be easily separated owing to the small lattice misfit between the AlTiN and CrMoN sublayers [47,48].

Coatings Al(at %) Ti(at %) Cr(at %) Mo(at %) N(at %)
between the AlTiN and CrMoN when the deposition temperature was as low as 280-320 °C. According to the previous studies [49,50], the epitaxial growth of a multilayered film resulted from the lower interfacial energy during the deposition process. The stabilization of epitaxial growth occurs at small layer thicknesses in non-isostructural multilayers. The alternating AlTiN and CrMoN layers are perpendicular to the growth direction, shown as the bright and dark interval fringes with a bilayer thickness of 27.31 nm. The AlTiN/CrMoN multilayers of the AlTiCrMoN-2 coating was deposited by alternate evaporation of CrMo and AlTi targets in nitrogen atmosphere under AlTi/CrMo cathode current ratios of 2:1. The higher the cathode current, the higher the evaporation rate of the target. Larger thickness of AlTiN was obtained, and the layer thickness ratio of AlTiN to CrMoN was 1.86. Assisted by the fast Fourier transformation (FFT) of square region in each AlTiN and CrMoN layer outlined in Figure 4, the d value measured from the selected AlTiN layer was 0.242 nm matches well with AlTiN (111), while the d value measured from the selected CrMoN was 0.238 nm for CrMoN (111).   The interfaces between AlTiN to CrMoN were coherent. B. Gao et al. [36] had shown that the ion energy would be influenced by the temperature during deposition, which affected the movement and diffusion of atoms in the multilayered films. In this study, we observed a distinct interface between the AlTiN and CrMoN when the deposition temperature was as low as 280-320 • C. According to the previous studies [49,50], the epitaxial growth of a multilayered film resulted from the lower interfacial energy during the deposition process. The stabilization of epitaxial growth occurs at small layer thicknesses in non-isostructural multilayers. The alternating AlTiN and CrMoN layers are perpendicular to the growth direction, shown as the bright and dark interval fringes with a bilayer thickness of 27.31 nm. The AlTiN/CrMoN multilayers of the AlTiCrMoN-2 coating was deposited by alternate evaporation of CrMo and AlTi targets in nitrogen atmosphere under AlTi/CrMo cathode current ratios of 2:1. The higher the cathode current, the higher the evaporation rate of the target. Larger thickness of AlTiN was obtained, and the layer thickness ratio of AlTiN to CrMoN was 1.86. Assisted by the fast Fourier transformation (FFT) of square region in each AlTiN and CrMoN layer outlined in Figure 4, the d value measured from the selected AlTiN layer was 0.242 nm matches well with AlTiN (111), while the d value measured from the selected CrMoN was 0.238 nm for CrMoN (111).

Mechanical Properties and Cutting Performances of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 Coatings
The adhesion strength between the hard-ceramic coating and the substrate material is one of the most important properties of PVD hard coatings as it influences the employment for mechanical applications such as metal cutting and forming, particularly for tribologically stressed cutting tools. The adhesion strength of all AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coated samples were investigated using the Rockwell indentation test. The coating damage at the indentation imprint is assigned to four adhesion classes. Class 0 reveals acceptable adhesion. Class 1 shows no adhesive delamination; adhesion is acceptable. In the cases of class 2 and class 3, delaminations are present around the indentation imprint and adhesion is unacceptable. Figure 5 shows optical images to reveal the crack behavior of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coated samples. Radial cracks, as indicated by the arrows in the figures, around the cone indentation were found on all coatings. No delaminations were present around the indentation imprint. The results showed strong adhesive strength (class 1). The radial cracks without chipping and delaminations exhibited good adhesion originating from the proper TiN and TiN/AlTiN interlayer configurations. Our previous study [26] had shown that the improvement of adhesion strength and mechanical properties of multilayered AlTiCrN/TiSiN coatings were obtained by the interlayer design of CrN and composition-gradient

Mechanical Properties and Cutting Performances of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 Coatings
The adhesion strength between the hard-ceramic coating and the substrate material is one of the most important properties of PVD hard coatings as it influences the employment for mechanical applications such as metal cutting and forming, particularly for tribologically stressed cutting tools. The adhesion strength of all AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coated samples were investigated using the Rockwell indentation test. The coating damage at the indentation imprint is assigned to four adhesion classes. Class 0 reveals acceptable adhesion. Class 1 shows no adhesive delamination; adhesion is acceptable. In the cases of class 2 and class 3, delaminations are present around the indentation imprint and adhesion is unacceptable. Figure 5 shows optical images to reveal the crack behavior of the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coated samples. Radial cracks, as indicated by the arrows in the figures, around the cone indentation were found on all coatings. No delaminations were present around the indentation imprint. The results showed strong adhesive strength (class 1). The radial cracks without chipping and delaminations exhibited good adhesion originating from the proper TiN and TiN/AlTiN interlayer configurations. Our previous study [26] had shown that the improvement of adhesion strength and mechanical properties of multilayered AlTiCrN/TiSiN coatings were obtained by the interlayer design of CrN and composition-gradient transition layers. Y.W. Lin et. al. [51] had also shown that the TiZrN possessed improved adhesion strength and wear resistance by properly introducing a Ti interlayer. Compared to the single-layer coating, the design of adding composition-gradient interlayers and multilayers is proved to improve the mechanical properties and wear resistance.
Coatings 2020, 10, 605 9 of 18 coating, the design of adding composition-gradient interlayers and multilayers is proved to improve the mechanical properties and wear resistance. The hardness and Young's modulus of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings were measured using the nanoindentation test. Table 3 shows the measured hardness, Young's modulus, H/E* and H 3 /E* 2 value of the coatings. The hardness of the AlTiN coating was 35.6 ± 1.5 GPa, which was higher than that of the AlTiN (32.47 GPa) coating synthesized by a filtered cathodic vacuum arc deposition method and the TiAlN (33.1 GPa) coating deposited by magnetron The hardness and Young's modulus of the deposited AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings were measured using the nanoindentation test. Table 3 shows the measured hardness, Young's modulus, H/E* and H 3 /E* 2 value of the coatings. The hardness of the AlTiN coating was 35.6 ± 1.5 GPa, which was higher than that of the AlTiN (32.47 GPa) coating synthesized by a filtered cathodic vacuum arc deposition method and the TiAlN (33.1 GPa) coating deposited by magnetron sputtering [3,5,52]. The AlTiCrMoN-1 and AlTiCrMoN-2 coatings displayed lower hardness of 29.4 ± 0.2 and 32.0 ± 1.5, respectively. Higher values for both hardness and Young's modulus were found in the monolayer AlTiN coatings when compared with the multilayer AlTiCrMoN systems. Compared to the AlTiCrMoN-1 coating deposited under AlTi/CrMo cathode current ratios of 1:1, AlTiCrMoN-2 deposited under AlTi/CrMo cathode current ratios of 2:1 had higher hardness due to the higher Al and Ti contents in the film. The H/E* and H 3 /E* 2 values of the deposited AlTiN were 0.059 and 0.124, respectively. Although the AlTiN possesses higher hardness than that of AlTiCrMoN-1 and AlTiCrMoN-2, higher values of H/E* (0.068~0.071), which indicates a higher resistance to cracking [53][54][55], and H 3 /E* 2 (0.137~0.157), which indicates a higher resistance to plastic deformation, a major factor of coating's wear resistance property [54,56], were obtained for AlTiCrMoN-1 and AlTiCrMoN-2 coatings. E* is expressed as E/(1-ν 2 ), where E and ν are the Young's modulus and the Poisson ratio, respectively. In this study, the AlTiCrMoN-2 coating with AlTiN/CrMoN multilayers had the highest H/E* (0.071) and H 3 /E* 2 (0.157) among the deposited coatings. A recent study [57] also showed that the CrN/Si 3 N 4 multilayered coatings with a high H/E* and H 3 /E* 2 values exhibited high cracking resistance to improve friction and wear. The multilayer interfaces could reduce crack propagation and adhesion failure. Compared to the AlTiN coating, the AlTiCrMoN-1 and AlTiCrMoN-2 coatings with nanolayered structure of the AlTiN/CrMoN top layer had high H/E* and H 3 /E* 2 that could be beneficial to enhance the tribological and cutting performances.

Comparison of the Cutting Performance of the End Mills Coated with the AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 Coatings
AlTiN, AlTiCrMoN-1 and AlTiCrMoN-2 coatings were deposited on the end mills for the cutting test. The experiments were conducted at a high cutting speed (Vc) of 226.19 m/min with a constant spindle speed of 9000 rpm on austenitic SUS316L stainless steel. The influence of AlTiN and multilayered AlTiCrMoN hard coatings on the wear behavior and cutting performance of end mills was investigated. When machining the difficult-to-cut austenitic stainless steel, in addition to abrasion wear, adhesion failure of the tungsten carbide tools was usually found. The challenge in cutting the austenitic stainless steel materials is the serious adhesion of the work piece material to the cutting edge, which causes early chipping of the end mills. Figure 6 presents the lifetime curves of average flank wear obtained with different tools at different cutting lengths. In this study, the flank wear of 60 µm was used as a criterion to evaluate the primary lifetime of the end mills. The results showed that the flank wear increased rapidly before the cutting length of 10 m. As shown in Figure 6, the cutting distance of the uncoated end mill is approximately 6 m due to severe wear, whereas the monolithic AlTiN coated end mill had longer lifetime, i.e.,~21.9 m. All AlTiN and multilayered AlTiCrMoN coated end mills had lower flank wear than the uncoated tools. By adding CrMoN to form a multilayered coating, the AlTiCrMoN-1 coated end mill outperformed the AlTiN. The AlTiCrMoN-2 with a multilayered AlTiN/CrMoN top layer possessed the longest tool life of 79.1 m. Table 4 shows a comparison of cutting performance of different coatings for machining austenitic stainless steels [58][59][60][61][62]. AlTiN, AlTiCrN and AlTiSiN are usually used as the protective hard coatings for cutting stainless steels and resist thermal shock and severe abrasive wear at the tool-chip interface. Selective laser melting (SLM) and laser texturing before applying the PVD coatings were also used to improve the cutting performance. In this study, the multilayered AlTiCrMoN-2 coated tool outperformed the AlTiN by having a tool life 3.6 times longer than the AlTiN, and it possessed the best cutting performance.
AlTiN/CrMoN multilayers (bilayer thickness = 27.31 nm). Coating delamination and longitudinal cracking due to brittle failure are the major failure mechanisms of the cutting tool with hard coatings, especially in the case of AlTiN. The coating failure, wear and delamination on tool surfaces are based on the compositions and architectures of the coatings [24]. The coalesced top coating layer and gradient interlayers provided good adhesion strength of the hard coating to cutting tools and improved machining behavior [65,66]. In this study, the nanolayered AlTiCrMoN-2 coating had high H/E* and H 3 /E* 2 values, which improved the resistance to plastic deformation and provided inhibition of crack formation and propagation, and outperformed monolithic AlTiN at high cutting speed. Therefore, the nanolayered AlTiCrMoN-2 coated tools possessed the best cutting performance among the investigated coatings and had the longest tool lifetime of the end mills.  Decrease of 10-20% in cutting forces, a decrease of 10-15% in cutting temperature and a high surface quality of the machined workpieces at the high cutting speed. [54]

AlTiN
Cathodic arc vapor deposition technique The nano-crystalline AlTiN coating outperformed the traditional AlTiN coating that almost doubled the tool life due to the reduction of the sticking of the stainless steel to the tool's surface. [55]

AlTiN AlTiCrN AlTiSiN
Cathodic arc vapor deposition technique Effectively resists thermal shock and severe abrasive wear at the tool-chip interface due to the high H 3 /E 2 ratio. [56]

TiAlN
Cathodic arc vapor deposition with laser irradiation technique The anti-adhesive wear properties of laser textured TiAlN coated tool were significantly improved over that of the conventional TiAlN. [57]

TiN/MoN (Multilayer)
Magnetron sputtering The numbers of drilling holes the coated drills were 4.2 times larger than that of uncoated high speed steel (HSS) drills. Adhesive friction and the formation of the build-up edge was avoided. [58]

AlTiN/CrMoN (Multilayer)
Cathodic arc vapor deposition technique The nanolayered AlTiCrMoN-2 coated tools outperformed the AlTiN that had a tool life 3.6 times longer than that of AlTiN. And it possessed the best cutting performance.
This study Figure 7 shows the optical microscopic observations of worn faces of the uncoated and coated end mills after machining of SUS316L stainless steel tested during cutting lengths of 6 m, 34 m and 82 m.
Distinct abrasion was observed on the flank faces of the uncoated tools at the early cutting length of 6 m. The flank wear at the cutting edge resulted from the increasing cutting heat and friction wear during milling. The end mill blunted during the high-speed cutting process due to its poor heat resistance, which facilitated serious abrasion. After cutting a length of 34 m, the AlTiN and AlTiCrMoN-1 coated end mill had an average flank wear of~66 µm and~64 µm, respectively, and showed obvious abrasion wear on the cutting edge, which caused the chipping and delamination of the coating, while smaller flank wear was obtained for the AlTiCrMoN-2 coated end mill. Previous studies [63,64] revealed that the failure mechanism of the AlTiN coated end mills included abrasion, chipping due to the thermal fatigue, adhesion of workpiece material with formation of build-up edge (BUE). After the protective coating had worn away, the tool substrate was exposed, leading to the adhesive BUE formation on the coated tools at high temperature. The AlTiCrMoN-2 with multilayered AlTiN/CrMoN top layer (layer thickness ratio of AlTiN to CrMoN = 1.86) had average flank wear of~61 µm after cutting length of 82 m, and possessed the lowest progression of tool flank wear. In addition, the AlTiCrMoN-2 coating possessed high H/E* and H 3 /E* 2 values, which improved the wear resistance.    Figure 8 shows the SEM image and EDS mapping results of the AlTiMoN-2 coated end mills after cutting length of 82 m when the flank wear exceeded 60 µm. As displayed in the figure, the compositional EDS mapping images revealed the exposure of the tool substrate (W mapping) and the adhered material (Fe mapping). The machined material mainly adhered to coated end mills on the cutting edge of the flank wear zone. A small exposure area of the tool substrate (W mapping) was found at the cutting edge, indicating the excellent wear resistance of the AlTiCrMoN-2 with AlTiN/CrMoN multilayers (bilayer thickness = 27.31 nm). Coating delamination and longitudinal cracking due to brittle failure are the major failure mechanisms of the cutting tool with hard coatings, especially in the case of AlTiN. The coating failure, wear and delamination on tool surfaces are based on the compositions and architectures of the coatings [24]. The coalesced top coating layer and gradient interlayers provided good adhesion strength of the hard coating to cutting tools and improved machining behavior [65,66]. In this study, the nanolayered AlTiCrMoN-2 coating had high H/E* and H 3 /E* 2 values, which improved the resistance to plastic deformation and provided inhibition of crack formation and propagation, and outperformed monolithic AlTiN at high cutting speed. Therefore, the nanolayered AlTiCrMoN-2 coated tools possessed the best cutting performance among the investigated coatings and had the longest tool lifetime of the end mills.

Conclusion
In this study, Al 0. 63 30 Mo 0.05 N, which was synthesized in a multilayered structure, possessed the longest tool life. Due to its high H/E* and H 3 /E* 2 , which exhibits high cracking resistance to improve wear performance, the Al 0.38 Ti 0.27 Cr 0.30 Mo 0.05 N coating with AlTiN to CrMoN layer thickness ratio of 1.86 is suggested to be suitable for in machining of SUS316L stainless steels. The architecture and composition of the multilayered hard coating are the key parameters for the cutting applications of coated tools. Further works, including carbon addition and controlling the periodic thickness of the multilayered coatings, will be conducted.