Mechanical Behavior and Thermal Stability of (AlCrTiZrMo)N/ZrO 2 Nano-Multilayered High-Entropy Alloy Film Prepared by Magnetron Sputtering

: A new type of high-entropy alloy, a nitride-based (AlCrTiZrMo)N/ZrO 2 nano-multilayered ﬁlm, was designed to investigate the effect of ZrO 2 layer thickness on the microstructure, mechanical properties, and thermal stability. The results show that when the thickness of the ZrO 2 layer is less than 0.6 nm, it can be transformed into cubic-phase growth under the template effect of the (AlCrTiZrMo)N layer, resulting in an increased hardness. The (AlCrTiZrMo)N/ZrO 2 ﬁlm with a ZrO 2 layer thickness of 0.6 nm has the highest hardness and elastic modulus of 35.1 GPa and 376.4 GPa, respectively. As the thickness of the ZrO 2 layer further increases, ZrO 2 cannot maintain the cubic structure, and the epitaxial growth interface is destroyed, resulting in a decrease in hardness. High-temperature annealing treatments indicate that the mechanical properties of the ﬁlm decrease slightly after annealing at less than 900 ◦ C for 30 min, while the mechanical properties decrease signiﬁcantly after annealing for 30 min at 1000–1100 ◦ C. The hardness and elastic modulus after annealing at 900 ◦ C are still 24.5 GPa and 262.3 GPa, showing excellent thermal stability. This conclusion veriﬁes the “template” effect of the nano-multilayered ﬁlm, which improves the hardness and thermal stability of the high-entropy alloy.

Due to the exhibited outstanding properties of HEAs, HEA films have also received increasing attention in the application of protective coatings [30]. The attractive performance 99.99% (weight percent). The (AlCrTiZrMo)N layers were deposited from an AlCrTiZrMo HEA target by DC mode, and the power was set at 100 W. The ZrO 2 layers were sputtered from the ZrO 2 target by RF mode, and the power was set at 150 W. The film was deposited on a single-crystal silicon wafer with the size of 30 mm × 20 mm × 1 mm. The silicon wafer was cleaned before the deposition. Primarily, the silicon wafer was immersed in ethanol and then placed in an ultrasonic cleaning chamber for 15 min. After drying, the silicon wafer was placed in a vacuum chamber for reverse sputtering cleaning. The N 2 and Ar flow rates were 15 sccm and 20 sccm, respectively, with a working pressure of 0.6 Pa. The sputtering rate of ZrO 2 under the experimental condition was 0.1 nm/s based on a cross-sectional TEM observation. By controlling the deposition time of the ZrO 2 layer, (AlCrTiZrMo)N/ZrO 2 nano-multilayered films with ZrO 2 thicknesses of 0.2 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 1 nm, and 1.2 nm were obtained. The film thickness was about 2 µm. Under the same condition, a monolithic (AlCrTiZrMo)N film with a thickness of about 2 µm was prepared for comparison. Figure 1 is a schematic illustration of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film, in which Λ is the modulation period.
prepared, which is expected to improve the mechanical properties and high-temperature resistance of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film.

Materials and Methods
The equiatomic AlCrTiZrMo HEA target was prepared by vacuum arc melting under a pure Ar atmosphere, with each component having a purity of at least 99.99% (weight percent). The ZrO2 target was prepared by powder metallurgy with a purity of at least 99.99% (weight percent). The (AlCrTiZrMo)N layers were deposited from an AlCrTiZrMo HEA target by DC mode, and the power was set at 100 W. The ZrO2 layers were sputtered from the ZrO2 target by RF mode, and the power was set at 150 W. The film was deposited on a single-crystal silicon wafer with the size of 30 mm × 20 mm × 1 mm. The silicon wafer was cleaned before the deposition. Primarily, the silicon wafer was immersed in ethanol and then placed in an ultrasonic cleaning chamber for 15 minutes. After drying, the silicon wafer was placed in a vacuum chamber for reverse sputtering cleaning. The N2 and Ar flow rates were 15 sccm and 20 sccm, respectively, with a working pressure of 0.6 Pa. The sputtering rate of ZrO2 under the experimental condition was 0.1 nm/s based on a crosssectional TEM observation. By controlling the deposition time of the ZrO2 layer, (AlCrTiZrMo)N/ZrO2 nano-multilayered films with ZrO2 thicknesses of 0.2 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 1 nm, and 1.2 nm were obtained. The film thickness was about 2 μm. Under the same condition, a monolithic (AlCrTiZrMo)N film with a thickness of about 2 μm was prepared for comparison. Figure 1 is a schematic illustration of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film, in which Λ is the modulation period. The microstructures of (AlCrTiZrMo)N/ZrO2 nano-multilayered films and monolithic (AlCrTiZrMo)N films were characterized by the D8 Advance X-ray diffractometer (XRD, Bruker, Mannheim, Germany) using CuKα radiation (λ = 0.15406 nm), the Quanta FEG450 field-emission environmental electron microscope (SEM, FEI, Oregon, USA), and Tecnai G220 high-resolution field-emission transmission electron microscope (HRTEM, FEI, Oregon, USA). The XRD patterns were measured using the Bragg-Brentano (θ/2θ) scan mode with parameters of 30 kV and 20 mA. The diffraction angle of the XRD pattern ranges from 25° to 90°. The elemental distributions were analyzed by an electron probe microanalyzer (EPMA, JXA-8530F PLUS, Tokyo, Japan). The hardness and elastic modulus of the films were measured by a G200 nano-indentation system (Agilent, California, USA). A Berkovich diamond indenter was used with a loading rate of 0.05/s, a load of 5 mN, and a press-in depth of 100 nm, which was less than 1/10 of the film thickness so that the influence of the substrate on the mechanical properties of the film could be eliminated. At least 16 points were selected for the hardness measurement of each film, and the data The microstructures of (AlCrTiZrMo)N/ZrO 2 nano-multilayered films and monolithic (AlCrTiZrMo)N films were characterized by the D8 Advance X-ray diffractometer (XRD, Bruker, Mannheim, Germany) using CuK α radiation (λ = 0.15406 nm), the Quanta FEG450 field-emission environmental electron microscope (SEM, FEI, Hillsboro, OR, USA), and Tecnai G220 high-resolution field-emission transmission electron microscope (HRTEM, FEI, Hillsboro, OR, USA). The XRD patterns were measured using the Bragg-Brentano (θ/2θ) scan mode with parameters of 30 kV and 20 mA. The diffraction angle of the XRD pattern ranges from 25 • to 90 • . The elemental distributions were analyzed by an electron probe microanalyzer (EPMA, JXA-8530F PLUS, Tokyo, Japan). The hardness and elastic modulus of the films were measured by a G200 nano-indentation system (Agilent, Santa Clara, CA, USA). A Berkovich diamond indenter was used with a loading rate of 0.05/s, a load of 5 mN, and a press-in depth of 100 nm, which was less than 1/10 of the film thickness so that the influence of the substrate on the mechanical properties of the film could be eliminated. At least 16 points were selected for the hardness measurement of each film, and the data with a coefficient of variation of less than 10% were selected to calculate the average hardness value of the film. In order to evaluate the thermal stability of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film, the sample was placed in a furnace and annealed in air for 30 min at 700 • C, 800 • C, 900 • C, 1000 • C, and 1100 • C. The sample was then removed, placed in air, and cooled to room temperature at a cooling rate of about 100 • C/min. The structural changes and mechanical properties of the annealed samples were measured by the XRD and nanoindentation techniques. Table 1 shows the deposition parameters and the element contents of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films.

Microstructures
In Figure 2, the values of the ZrO 2 thicknesses are marked on their corresponding XRD patterns. The monolithic (AlCrTiZrMo)N film and the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films present an fcc structure. The diffraction peaks of the monolithic (AlCrTiZrMo)N film are weak. As the thickness of the ZrO 2 layer increases, the (111) and (200) peaks of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films first become large and then small. When the ZrO 2 layer thickness is 0.6 nm, the peak intensity reaches the maximum value. According to the previous reports of nano-multilayered films [41], it can be inferred that when the ZrO 2 layer thickness is less than 0.6 nm, the ZrO 2 layer changes from a monoclinic structure to an fcc metastable structure under the template effect of the (AlCrTiZrMo)N layer, and it forms co-epitaxial growth with the (AlCrTiZrMo)N layers. Therefore, the crystallinity is improved, and, correspondingly, the intensity of the diffraction peak becomes strong. However, when the ZrO 2 layer thickness is more than 0.6 nm, the effect of the improved crystallinity of the nano-multilayered film gradually disappears. Therefore, the crystallinity becomes generally weak, and the intensity of the diffraction peak tends to decrease. It can also be seen in Figure 2 that the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film shows a shift toward a lower angle in the (111) diffraction peak when the ZrO 2 layer thickness is 0.6 nm.  The crystal structure transition of ZrO2 occurs due to the "template effect" of the nano-multilayered film. The (AlCrTiZrMo)N layer has a stable fcc crystal structure. Since the thickness of the modulation layer generally does not exceed a few nanometers, the nucleation and growth of the ZrO2 layer are greatly affected by the structure of the template layer. When the ZrO2 layer is nucleated, the total energy per unit area of the ZrO2 layer, ET, can be expressed as [42] = + (1) where EB is the volume free energy of ZrO2, t is the thickness of the ZrO2 layer, and EI is the interfacial energy. When ZrO2 starts to deposit, t → 0, EI is the main part of ET. Compared with the ZrO2 of the monoclinic structure, the interface energy of the cubic-structured ZrO2 and (AlCrTiZrMo)N is smaller. Therefore, ZrO2 tends to grow epitaxially in The crystal structure transition of ZrO 2 occurs due to the "template effect" of the nano-multilayered film. The (AlCrTiZrMo)N layer has a stable fcc crystal structure. Since the thickness of the modulation layer generally does not exceed a few nanometers, the nucleation and growth of the ZrO 2 layer are greatly affected by the structure of the template layer. When the ZrO 2 layer is nucleated, the total energy per unit area of the ZrO 2 layer, E T , can be expressed as [42] E T = E B t + E I (1) where E B is the volume free energy of ZrO 2 , t is the thickness of the ZrO 2 layer, and E I is the interfacial energy. When ZrO 2 starts to deposit, t → 0, E I is the main part of E T . Compared with the ZrO 2 of the monoclinic structure, the interface energy of the cubic-structured ZrO 2 and (AlCrTiZrMo)N is smaller. Therefore, ZrO 2 tends to grow epitaxially in the cubic structure. As a result, the crystal growth of the (AlCrTiZrMo)N layer is promoted, and the intensity of the cubic-phase diffraction peak is enhanced as can be seen in Figure 2. However, as the thickness of the ZrO 2 layer increases, E B becomes larger, and E I is no longer the main part of E T . ZrO 2 nucleates and grows on the surface of the (AlCrTiZrMo)N layer, and the effect of the volume free energy becomes increasingly evident. When the critical thickness reaches 0.6 nm, the total energy of the cubic structure of ZrO 2 is greater than that of the monoclinic structure of ZrO 2 . As a result, ZrO 2 is converted to monoclinic phase growth, and the crystallinity of the (AlCrTiZrMo)N layer is lowered, and the intensity of the cubic-phase diffraction peak is weakened when the thickness of ZrO 2 is more than 0.6 nm.
It should be noted that Figure 2 does not detect the diffraction peak of the monoclinic phase of ZrO 2 , probably because the thickness of ZrO 2 is too small to be detected. Figure 3 presents the HRTEM images of the cross-sectional morphology of the (AlCrTiZ rMo)N/ZrO 2 nano-multilayered film with the ZrO 2 layer thickness of 0.6 nm. In the lowmagnified image of Figure 3a, the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film has a good columnar structure along the direction of the film growth. The width of the columnar crystal is about 20-50 nm, the length runs through several modulation periods of the nano-multilayered film, and the insertion of the ZrO 2 layer does not damage the columnar crystal growth. Figure 3b presents the medium-magnified cross-sectional morphology, which indicates a well-grown nano-multilayered film structure. The layers with the gray contrast are (AlCrTiZrMo)N, and the ones with the bright contrast refer to ZrO 2 . The (AlCrTiZrMo)N and the ZrO 2 layers are alternately deposited to form a continuous nanomultilayered film structure. Figure 3c presents a high-magnified HRTEM image from the selected area of Figure 3b. The modulation period of the nano-multilayered film is measured as 8 nm, and the nanocrystal distribution is very dense and uniform. Chu et al. [43] calculated the hardness behavior of multilayered film, and the hardness was significantly improved when Λ = 5~10 nm. The lattice fringes can continuously move across several modulation periods, indicating that the ZrO 2 layer transforms into an fcc structure under the "template effect" of the (AlCrTiZrMo)N layers and grows epitaxially with them. The (111), (200), and (220) fcc-structured diffraction rings are presented in the selected area electron diffraction (SAED) patterns of Figure 3d, which are consistent with the results in the XRD patterns.
Crystals 2021, 11, x FOR PEER REVIEW 6 of 13 uous nano-multilayered film structure. Figure 3c presents a high-magnified HRTEM image from the selected area of Figure 3b. The modulation period of the nano-multilayered film is measured as 8 nm, and the nanocrystal distribution is very dense and uniform. Chu et al. [43] calculated the hardness behavior of multilayered film, and the hardness was significantly improved when Λ = 5 ~ 10 nm. The lattice fringes can continuously move across several modulation periods, indicating that the ZrO2 layer transforms into an fcc structure under the "template effect" of the (AlCrTiZrMo)N layers and grows epitaxially with them. The (111), (200), and (220) fcc-structured diffraction rings are presented in the selected area electron diffraction (SAED) patterns of Figure 3d, which are consistent with the results in the XRD patterns. The cross-sectional SEM morphology of the nano-multilayered films with various ZrO2 layer thicknesses is presented in Figure 4. In Figure 4a, when the ZrO2 layer thickness is 0.2 nm, the columnar crystals cannot be clearly observed in the cross-sectional image. The thickness of the ZrO2 layer in Figure 4b is 0.4 nm, which shows a clear columnar crystal structure. As the ZrO2 layer thickness increases, the ZrO2 layer gradually transforms The cross-sectional SEM morphology of the nano-multilayered films with various ZrO 2 layer thicknesses is presented in Figure 4. In Figure 4a, when the ZrO 2 layer thickness is 0.2 nm, the columnar crystals cannot be clearly observed in the cross-sectional image. The thickness of the ZrO 2 layer in Figure 4b is 0.4 nm, which shows a clear columnar crystal structure. As the ZrO 2 layer thickness increases, the ZrO 2 layer gradually transforms into an fcc structure under the "template effect" of the (AlCrTiZrMo)N layer. Therefore, the crystallinity is improved. When the thickness of the ZrO 2 layer of the nano-multilayered film in Figure 4c is 0.6 nm, the columnar crystal growth of the nano-multilayered film is complete, suggesting that the insertion of the ZrO 2 layer does not destroy the columnar growth feature of the (AlCrTiZrMo)N film, which is consistent with the XRD patterns. However, as the thickness of the ZrO 2 layer continues to increase, the columnar crystal structure in Figure 4d,e begins to weaken, indicating that the crystallinity deteriorates, which is in accordance with the XRD results.   Figure 5 shows the hardness and elastic modulus of the (AlCrTiZrMo)N/ZrO2 nanomultilayered films with the different ZrO2 layer thicknesses. The hardness and elastic modulus of the (AlCrTiZrMo)N film are 27.5 GPa and 260.2 GPa, respectively. When the thickness of the ZrO2 layer is 0.2 nm, the hardness and elastic modulus of the nano-multilayered film are slightly lower than those of the monolithic (AlCrTiZrMo)N film, which is due to the fact that the integrity of the crystal growth of the (AlCrTiZrMo)N film is destroyed, which is a result of the insertion of thin ZrO2 layers. With an increase in ZrO2 layer thickness, the hardness and elastic modulus of the nano-multilayered film gradually increase, and they reach the maximum levels of 35.1 GPa and 376.4 GPa, respectively, when the ZrO2 layer thickness is 0.6 nm. The strengthening effect can be attributed to the fact that the ZrO2 layers transform into an fcc metastable structure under the "template effect" of the (AlCrTiZrMo)N layers, and they form co-epitaxial growth with the (AlCrTiZrMo)N layers, which promotes the film's integrity and blocks the dislocation movements. When the ZrO2 layer thickness exceeds 0.6 nm, the ZrO2 layers transform back into a stable monoclinic phase structure, and the co-epitaxial growth of the nanomultilayered film can no longer be maintained, leading to the disappearance of the strengthening effect. Accordingly, the hardness and elastic modulus of the nano-multilayered film decrease.  Figure 5 shows the hardness and elastic modulus of the (AlCrTiZrMo)N/ZrO 2 nanomultilayered films with the different ZrO 2 layer thicknesses. The hardness and elastic modulus of the (AlCrTiZrMo)N film are 27.5 GPa and 260.2 GPa, respectively. When the thickness of the ZrO 2 layer is 0.2 nm, the hardness and elastic modulus of the nanomultilayered film are slightly lower than those of the monolithic (AlCrTiZrMo)N film, which is due to the fact that the integrity of the crystal growth of the (AlCrTiZrMo)N film is destroyed, which is a result of the insertion of thin ZrO 2 layers. With an increase in ZrO 2 layer thickness, the hardness and elastic modulus of the nano-multilayered film gradually increase, and they reach the maximum levels of 35.1 GPa and 376.4 GPa, respectively, when the ZrO 2 layer thickness is 0.6 nm. The strengthening effect can be attributed to the fact that the ZrO 2 layers transform into an fcc metastable structure under the "template effect" of the (AlCrTiZrMo)N layers, and they form co-epitaxial growth with the (AlCrTiZrMo)N layers, which promotes the film's integrity and blocks the dislocation movements. When the ZrO 2 layer thickness exceeds 0.6 nm, the ZrO 2 layers transform back into a stable monoclinic phase structure, and the co-epitaxial growth of the nano-multilayered film can no longer be maintained, leading to the disappearance of the strengthening effect. Accordingly, the hardness and elastic modulus of the nano-multilayered film decrease. effect" of the (AlCrTiZrMo)N layers, and they form co-epitaxial growth with the (AlCrTiZrMo)N layers, which promotes the film's integrity and blocks the dislocation movements. When the ZrO2 layer thickness exceeds 0.6 nm, the ZrO2 layers transform back into a stable monoclinic phase structure, and the co-epitaxial growth of the nanomultilayered film can no longer be maintained, leading to the disappearance of the strengthening effect. Accordingly, the hardness and elastic modulus of the nano-multilayered film decrease.

Thermal Stability
To evaluate the thermal stability of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film, the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film with the ZrO 2 layer thickness of 0.6 nm was annealed in air at 700 • C, 800 • C, 900 • C, 1000 • C, and 1100 • C for 30 min. The XRD patterns of the annealed films can be seen in Figure 6. It can be observed that as the temperature increases, the number of XRD peaks of the oxides of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film increases, suggesting that the degree of oxidation rises with an increase in the annealed temperature. After annealing in the temperature range from 700 • C to 900 • C, based on the XRD patterns, only the ZrO 2 can be found in the annealed film, indicating that the nano-multilayered film is slightly oxidized, and the basic composition of the nano-multilayered film is not changed. After annealing at 1000 • C and 1100 • C, more oxides, such as SiO 2 and Al 2 O 3 , are found in the XRD patterns, indicating that the oxidation degree of the nano-multilayered film remarkably increases.

Thermal stability
To evaluate the thermal stability of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film, the (AlCrTiZrMo)N/ZrO2 nano-multilayered film with the ZrO2 layer thickness of 0.6 nm was annealed in air at 700 ℃, 800 ℃, 900 ℃, 1000 ℃, and 1100 ℃ for 30 min. The XRD patterns of the annealed films can be seen in Figure 6. It can be observed that as the temperature increases, the number of XRD peaks of the oxides of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film increases, suggesting that the degree of oxidation rises with an increase in the annealed temperature. After annealing in the temperature range from 700 ℃ to 900 ℃, based on the XRD patterns, only the ZrO2 can be found in the annealed film, indicating that the nano-multilayered film is slightly oxidized, and the basic composition of the nano-multilayered film is not changed. After annealing at 1000 ℃ and 1100 ℃, more oxides, such as SiO2 and Al2O3, are found in the XRD patterns, indicating that the oxidation degree of the nano-multilayered film remarkably increases. The hardness and elastic modulus of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film with the ZrO2 layer thickness of 0.6 nm after annealing at different temperatures for 30 min are shown in Figure 7. In the annealing temperature range of 700 ℃-900 ℃, the mechanical properties of the film decrease slightly. The hardness and elastic modulus of the (AlCrTiZrMo)N/ZrO2 film at 900 ℃ are 24.5 GPa and 262.3 GPa, respectively. The indistinctive drop in mechanical properties can be attributed to the fact that the nano-multilayered film is only slightly oxidized. The hardness and elastic modulus of the (AlCrTiZrMo)N/ZrO2 film decrease rapidly following annealing at 1000 ℃-1100 ℃. The hardness and elastic modulus of the films annealed at 1,100 ℃ decrease to 9.8 GPa and 158.6 GPa, respectively. After annealing at 1,000 ℃ or 1,100 ℃, more oxides are produced during the annealing process, suggesting that the (AlCrTiZrMo)N/ZrO2 film is severely (AlCrTiZrMo)N/ZrO 2 film decrease rapidly following annealing at 1000-1100 • C. The hardness and elastic modulus of the films annealed at 1100 • C decrease to 9.8 GPa and 158.6 GPa, respectively. After annealing at 1000 • C or 1100 • C, more oxides are produced during the annealing process, suggesting that the (AlCrTiZrMo)N/ZrO 2 film is severely oxidized. As a result, the mechanical properties of the film degrade rapidly. It is worth noting that the mechanical properties of the (AlCrTiZrMo)N/ZrO 2 film after annealing at 900 • C are nearly equivalent to those of the monolithic (AlCrTiZrMo)N film at room temperature, indicating that the (AlCrTiZrMo)N/ZrO 2 film is stable at high temperatures and has a good high-temperature resistance.

Effect of structures on mechanical properties of the (AlCrTiZrMo)N/ZrO2 nano-multilayered film
The modulus-difference strengthening theory [35] and the alternating stress field theory [44] can explain the strengthening phenomenon of the (AlCrTiZrMo)N/ZrO2 nanomultilayered films.
Koehler [35] proposed the modulus-difference strengthening theory and suggested that when dislocations move across the coherent interface within the nano-multilayered film, they become subjected to the mirror force of the coherent interface. This force could be caused by the fact that the dislocations have different line energies in the layers with different shear moduli. Additional stress is required when the dislocations cross from the layer with a low shear modulus to that with a high shear modulus. Therefore, the dislocation movements can be hindered, and the nano-multilayered film becomes strengthened. The greater the difference in the shear modulus between the two layers, the more remarkable the strengthening effect. The shear modulus, G, can be expressed as = 2(1 + ) (2) where E is the elastic modulus, and ν is the Poisson ratio. In this study, E(AlCrTiZrMo)N = 260.2 GPa, EZrO2 = 160 GPa, and the Poisson ratio is 0.25; the shear moduli of (AlCrTiZrMo)N and ZrO2 can be calculated as G(AlCrTiZrMo)N = 104.1 GPa and GZrO2 = 64 GPa, respectively. According to the modulus-difference strengthening theory [45], the hardness increment (ΔH1) relative to the layer with the lowest hardness can be calculated. In the (AlCrTiZrMo)N/ZrO2 nano-multilayered film, the layer with the lowest hardness refers to ZrO2, and its hardness is measured as 11.5 GPa. The hardness increment (ΔH1) can be expressed as [45] (1 − )( − ) sin

Effect of Structures on Mechanical Properties of the (AlCrTiZrMo)N/ZrO 2 Nano-Multilayered Film
The modulus-difference strengthening theory [35] and the alternating stress field theory [44] can explain the strengthening phenomenon of the (AlCrTiZrMo)N/ZrO 2 nanomultilayered films.
Koehler [35] proposed the modulus-difference strengthening theory and suggested that when dislocations move across the coherent interface within the nano-multilayered film, they become subjected to the mirror force of the coherent interface. This force could be caused by the fact that the dislocations have different line energies in the layers with different shear moduli. Additional stress is required when the dislocations cross from the layer with a low shear modulus to that with a high shear modulus. Therefore, the dislocation movements can be hindered, and the nano-multilayered film becomes strengthened. The greater the difference in the shear modulus between the two layers, the more remarkable the strengthening effect. The shear modulus, G, can be expressed as where E is the elastic modulus, and ν is the Poisson ratio. In this study, E (AlCrTiZrMo)N = 260.2 GPa, E ZrO2 = 160 GPa, and the Poisson ratio is 0.25; the shear moduli of (AlCr-TiZrMo)N and ZrO 2 can be calculated as G (AlCrTiZrMo)N = 104.1 GPa and G ZrO2 = 64 GPa, respectively. According to the modulus-difference strengthening theory [45], the hardness increment (∆H 1 ) relative to the layer with the lowest hardness can be calculated. In the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film, the layer with the lowest hardness refers to ZrO 2 , and its hardness is measured as 11.5 GPa. The hardness increment (∆H 1 ) can be expressed as [45] ( For the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films, G (AlCrTiZrMo)N > G ZrO2 ; thus, G A is G (AlCrTiZrMo)N , and G B is G ZrO2 ; θ is the angle between the slip surface of the low modulus layer and the coherent interface, taking 45 • [45] in the investigation; m is the Taylor factor, and the nitride ceramic phase can take 0.3 [46]. According to the calculation, the hardness increment (∆H 1 ) of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films, relative to ZrO 2 , is 21.5-28.7 GPa.
Kato [44] introduced the theory of the alternating stress field to nano-multilayered films to explain the phenomenon of the hardness improvement. It is suggested that the stress field at the coherent interface of a nano-multilayered film could lead to the strengthening effect. Since (AlCrTiZrMo)N and ZrO 2 have different lattice constants, in order to achieve coherent growth at the interface, lattice mismatch occurs between the two layers of the materials, and the stress magnitude at the interface depends on the mismatch between the two layers. In a nano-multilayered film with a coherent structure, the layer with a larger lattice constant tends to become smaller and, thus, is subjected to compressive stress, while the layer with a smaller lattice constant is subjected to tensile stress. As shown in Figure 2, the nano-multilayered film with the ZrO 2 layer thickness of 0.6 nm shifted to a smaller angle, suggesting that the (AlCrTiZrMo)N layers are subject to tensile stress. Therefore, the alternating stress field forms along the growth direction in the nano-multilayered films. Kato [44] stated that hardening in an fcc structure mainly occurs in the positive region of the internal stress. The corresponding increase in the film, relative to the hardness of ZrO 2 (∆H 2 ), can be expressed as [47] where ε is the lattice mismatch between the two layers of the material. Due to the transformation of the structure of ZrO 2 , it is difficult to calculate the lattice mismatch between (AlCrTiZrMo)N and ZrO 2 , which takes 3~4% in this study [47]. A is the modulation amplitude of the nano-multilayered films, which is related to the modulation period and the interfacial mixing degree of the film; A takes 0.5 [48]. E is the average of the elastic moduli of the two layers of the materials and is calculated as 210.1 GPa. According to calculation (4), the hardness increment (∆H 2 ) is 3.9~5.1 GPa. Based on the theories of the elastic modulus difference and the alternating stress field, the total hardness increase (∆H 1 + ∆H 2 ) in the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film is 25.4~33.8 GPa; that is, the theoretical hardness of the film is 36.9~45.3 GPa. However, the actual maximum hardness of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film is 35.1 GPa, which is slightly lower than the theoretical value. This trend may be due to the structural transformation of the ZrO 2 layers under the coherent structure in the film, which results in a change in the modulus. The existing techniques cannot determine the modulus of a single-layer film within a nano-multilayered film, which leads to the deviation of the actual hardness from the theoretical value.

Thermal Stability of the (AlCrTiZrMo)N/ZrO 2 Nano-Multilayered Films
The mechanical properties of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered films are stable after annealing below 900 • C. The hardness and elastic modulus of the (AlCrTiZrMo) N/ZrO 2 film with the ZrO 2 layer thickness of 0.6 nm after annealing at 900 • C for 30 min are nearly equivalent to those of the (AlCrTiZrMo)N film at room temperature. The optimized thermal stability of the (AlCrTiZrMo)N/ZrO 2 nano-multilayered film is mainly related to the following factors: the favorable thermal stability of ZrO 2 , the lattice distortion of (AlCrTiZrMo)N, and the nano-multilayered film structure.
Firstly, ZrO 2 is a high-temperature-resistant material with low thermal conductivity. ZrO 2 layers in a nano-multilayered film can effectively prevent the diffusion of oxygen atoms, reduce the oxidation degree of the film, and improve the thermal stability of the nano-multilayered film. Secondly, high-entropy alloys have a hysteresis diffusion effect, and the diffusion is slower than that of conventional alloys [31], so the (AlCrTiZrMo)N layer can hinder the diffusion movement of oxygen atoms. Finally, there is an obvious lattice distortion at the epitaxial interface between the (AlCrTiZrMo)N and ZrO 2 layers. When oxygen atoms diffuse from one layer to another, they can be hindered by the epitaxial interface within the nano-multilayered film, reducing the diffusion rate of the oxygen atoms. Therefore, the structure of the multilayered film can effectively improve the thermal stability of the film. Furthermore, the alternating stress field along the film growth direction can also hinder the oxygen atom diffusion from the tensile stress layer to the compressive stress layer.
Hence, in future designs of nano-multilayered films, a modulation layer with good thermal stability can be selected, and HEA nitride film can be included to improve the thermal stability of the nano-multilayered film. Moreover, the thermal stability of the nano-multilayered film can be enhanced by optimizing a suitable modulation period and the formation of the epitaxial interface. Nano-multilayered films composed of HEA nitride and oxide layers are potential coating materials with good high-temperature resistance.

Conclusions
(1) The (AlCrTiZrMo)N/ZrO 2 nano-multilayered film presents an fcc structure. When the ZrO 2 layer thickness increases, the peak intensity first increases and then decreases. When the thickness of the ZrO 2 layer is less than 0.6 nm, the ZrO 2 layers convert into a cubic structure and form a coherent interface with the (AlCrTiZrMo)N layer. When the thickness of the ZrO 2 layer exceeds 0.6 nm, the fcc-structured ZrO 2 layers transform back into a monoclinic structure, leading to the deterioration of the crystallization integrity. (2) When the film thickness of ZrO 2 increases, the mechanical properties of the nanomultilayered film first increase and then decrease. When the ZrO 2 film thickness is 0.6 nm, the maximum values of the hardness and elastic modulus are obtained, which are 35.1 GPa and 376.4 GPa, respectively. The strengthening effect can be attributed to the modulus difference between ZrO 2 and (AlCrTiZrMo)N and the alternating stress field theory. The actual hardness is slightly lower than the theoretical hardness, which may be due to the structural transformation of the ZrO 2 layers under the coherent structure in the film, resulting in a change in the modulus. (3) The (AlCrTiZrMo)N/ZrO 2 nano-multilayered film presents high thermal stability.
The mechanical properties of the (AlCrTiZrMo)N/ZrO 2 film with the ZrO 2 layer thickness of 0.6 nm after annealing at 900 • C for 30 min are nearly equivalent to those of the (AlCrTiZrMo)N film at room temperature. The high thermal stability can be attributed to the favorable thermal stability of the ZrO 2 layer, the lattice distortion of (AlCrTiZrMo)N, and the nano-multilayered film structure.

Conflicts of Interest:
The authors declare no conflict of interest.