Investigation of Properties of the Zr,Hf-(Zr,Hf)N-(Zr,Hf,Me,Al)N Coatings, Where Me Means Cr, Ti, or Mo

: The article describes the results of the investigation focused on the properties of the Zr,Hf-(Zr,Hf)N-(Zr,Hf,Me,Al)N coatings, where Me means chromium (Cr), titanium (Ti), or molybdenum (Mo). These coatings have three-layer architecture, including adhesion, transition, and wear-resistant layers, while the latter, in turn, has a nanolayer structure. Despite the fact that the coatings under study have close values of hardness and critical fracture load L C2 , there are noticeable differences in wear resistance during the turning of steel. The tools with the coatings under study demonstrated better wear resistance compared to an uncoated tool and the tool with the commercial ZrN coating. The best wear resistance was detected for a tool with the Zr,Hf-(Zr,Hf)N-(Zr,Hf,Ti,Al)N coating. The study of the pattern of cracking in the structure of the coatings has found that, during the cutting process, active cracking occurs in the coating with Cr, which leads to the fracture of the coating, while the process of cracking is noticeably less active in the coatings with Ti or Mo.


Introduction
Wear-resistant coatings for cutting tools have been widely and successfully used since 1980s and manufactures continue to improve them. While the intensification of cutting modes and, first of all, cutting speeds requires an increased heat resistance, the resistance of tools to brittle fracture and cracking also remains relevant [1][2][3][4][5][6][7]. Thus, if we formulate the basic requirements for wear-resistant coatings for cutting tools, the list is as follows: • Maximum hardness to resist abrasive wear; • Maximum strength to resist adhesive-fatigue wear and brittle fracture, including resistance to cracking; • Maximum heat resistance to maintain basic performance properties at high temperatures; • Ability to provide optimal tribological parameters in the cutting zone, including at high temperatures; • Ability to effectively perform barrier functions in relation to the diffusion of elements of the material being machined and oxygen.
To meet the listed requirements, the coatings should have an optimized composition and structure. In terms of structure, mutilayer coatings with functional layers for various purposes and with nanocomposite and nanolayer structures of the very functional layers are being actively investigated and introduced in the industry [8][9][10][11][12]. In terms of composition, multicomponent coatings are being developed, the combination of elements in which provide new properties and possibilities [13][14][15][16].
decrease with a further increase in temperature [37]. Additional experiments on annealing at temperatures of 850 and 950 • C during 20 h indicate a significant improvement in the oxidation resistance, with an increase in the content of Hf in the coating composition [37].
The (Ti,Hf,Cr)N coating was considered, which demonstrated good abrasion resistance while subjected to a pin-on-disc test [38]. The (Ti,Hf)N-CrN nanolaminated coating was considered, which had a high hardness combined with a low Young's modulus and good resistance to cracking [39]. The (Hf,Mo,Si)N coating was also studied as characterized by high hardness (up to 38.6 GPa) and increased impact resistance (up to 3.6 MPa × m 1/2 ). Research has explained the above by the synergic effect of solid solution hardening, the fine grain sizes, and the effect of grain boundaries in a nanocomposite system [40].
A coating combining in its composition the nitrides of Hf and Mo was also considered [41]. The investigation of the (Hf,Mo)N coating detected the presence of a single phase of the c-(Hf,Mo)N solid solution. When heated to 600 • C, the coating forms an outer oxide layer, due to which the coefficient of friction reduces significantly [41].
We should separately mention the studies of multicomponent coatings containing nitrides of Ti, Hf, and Zr in combination with nitrides of such metals as vanadium (V), niobium (Nb), and tantalum (Ta) [42][43][44][45][46]. In particular, the studies were focused on the multicomponent coatings of (Ti,Hf,Zr,V,Nb,Ta)N [42,46] and (Ti,Hf,Zr,V,Nb)N [43][44][45]. The studies detected high heat resistance of the above coatings. Furthermore, it was found that an increase in the temperature of tribological tests in the air from 20 to 460 • C improves the wear resistance of the coating. Such an increase in wear resistance and tribological properties is associated with the formation of oxide phases on the coating surface. The coatings of (Ti,Hf,Zr,V,Nb,Ta)N and (Ti,Hf,Zr,V,Nb)N have an extremely high hardness of up to 51 GPa [46].
Thus, the coatings based on (Zr,Hf)N and containing such elements as Al and Cr or Ti or Mo can be characterized by high wear resistance in combination with high heat resistance due to the stabilization of the cubic phase and the formation of protective oxide films. This investigation considered three (Zr,Hf)N-based coatings, containing about 10 at.% of aluminum, as well as Cr, Ti, or Mo. Theses coatings are hereinafter referred to as follows: Zr,Hf-(Zr,Hf)N-(Zr,Hf,Cr,Al)N-Coating H1; Zr,Hf-(Zr,Hf)N-(Zr,Hf,Ti,Al)N-Coating H2; and Zr,Hf-(Zr,Hf)N-(Zr,Hf,Mo,Al)N-Coating H3.
The nanolayer structure of the wear-resistant layer of the coatings under study was formed under the controlled planetary rotation of the specimens in the chamber, due to the combination of plasma flows of the FCVAD and CAA-PVD evaporators (IDTI RAS-MSTU STANKIN, Moscow, Russia) [47][48][49][50][51][52], at the turntable rotation speed n = 1.5 rpm. The coatings were deposited on commercial carbide inserts of SNUNISO 1832:2012 (WC + 15% TiC + 6% Co). Prior to the coating deposition process, the specimens were subjected to the usual preparation procedures for this technique, including washing in a solution of chemically active substances with ultrasonic stimulation and subsequent drying in a stream of hot purified air.
After the specimens had been placed in the chamber and vacuum-processed, they were subjected to ion cleaning in gas (argon) plasma. After the cleaning of the specimens, the coating deposition process was directly launched under the parameters as follows: arc current 160 A for the Al cathode, 80 A for the Zr,Hf cathode, 75 A for the Ti cathode, 125 A for the Mo cathode, and 73 A for the Cr cathode. During the coating deposition process, the pressure of the nitrogen in the chamber was 0.42 Pa. The substrate bias voltage was −50 DC. During the process of coating deposition, sources of infrared radiation were used to heat the surfaces of the specimens to a temperature of 600-650 • C.
The hardness (HV) of the coatings was determined by measuring the indentation at low loads according to the method proposed by Oliver and Pharr [63], which was conducted on a micro-indentometer hardness tester (CSM Instruments, Needham, MA, USA) at a fixed load of 10 mN. Each specimen was subjected to 15 measurements, after which the average value was calculated.
The measurement of the strength of the adhesion bond to the substrate was measured in accordance with the ASTMC1624-05 methods [64].
A scanning electron microscope (SEM, Oberkochen, Germany) Carl Zeiss EVO 50 was used to study the structure of the coating and the pattern of cracking. Backscatter electron imaging was used, at 20 kV, 750 pA.
A CU 500 MRD lathe (ZMM-BULGARIA HOLDING, Sofia, Bulgaria) with a ZMM CU 500 MRD variable-speed drive was used to turn steel 1045 with inserts with the coatings under study under dry cutting conditions. The geometric parameters of the cutting process were as follows: γ = −7 • , α = 7 • , λ = 0, r = 0.4 mm; under the cutting mode: f = 0.1 rpm, a p = 0.5 mm, v c = 400 m/min. Five experiments were carried out for each type of the coating, after which the results were processed statistically. The preliminary tests found the maximum tool life (with VB max = 0.4 mm as the criterion of the maximum flank face wear). Under the specified cutting conditions, the maximum tool life was 10 min.
Uncoated %tools and tools with the commercial coating of ZrN were used as objects of comparison while studying their cutting properties. The thickness of the ZrN coating (about 4 µm) was identical to the total thickness of the investigated coatings H1-H3.

Results
The study of the composition of the coatings revealed the following content of elements (see Figure 1 and Table 1). The result table exhibits the content of metals only: the total metal content is assumed as 100%, with the nitrogen content of 49 at.%. The study of the structure of the coatings (see Figure 1) shows the presence of three functional layers, the thickness of which corresponds to the specified one (see Section 2). The coatings H1-H3 have close values of hardness and critical fracture load L C2 ( Table 1).
The study of the wear resistance of coated and uncoated tools during the turning of steel 1045 has revealed that all coated tools have better wear resistance compared to uncoated tools ( Figure 2). After 5 min of cutting, the average flank wear VB detected for the uncoated tool was 1.8 times higher than for the tools with the coatings H1, H2, and H3 and 1.5 times higher than for the ZrN-coated tool. On average, the tools with Coatings H1, H2, and H3 demonstrated the rate of wear 1.2 times lower after 5 min of cutting compared to that of the ZrN-coated tool. The ZrN-coated tool and the tool with Coating H1 had a tool life 1.5 times longer and the tools with Coating H2 and H3-2 times longer compared to the uncoated tools. The tool with Coating H1 demonstrated the highest wear resistance after 5 min of cutting; however, after 7.5 min of cutting, its wear resistance noticeably decreased to the level of the ZrN-coated tool. Since a catastrophic wear was detected on two out of five specimens after 5 min of cutting, it can be assumed it is not advisable to use uncoated tools under the given cutting conditions. At the same time, all the coated tools under consideration demonstrated fairly balanced patterns of wear with no signs of catastrophic wear or failure.

H3
(e) (f)   to the uncoated tools. The tool with Coating H1 demonstrated the highest wear resistance after 5 min of cutting; however, after 7.5 min of cutting, its wear resistance noticeably decreased to the level of the ZrN-coated tool. Since a catastrophic wear was detected on two out of five specimens after 5 min of cutting, it can be assumed it is not advisable to use uncoated tools under the given cutting conditions. At the same time, all the coated tools under consideration demonstrated fairly balanced patterns of wear with no signs of catastrophic wear or failure. We consider in more detail the patterns of rake wear for the tools with considered Coatings H1, H2, and H3 after 10 min of cutting ( Figure 3). An obvious notch wear is typical for the rake face, while no formation of noticeable wear crater is detected. Such a pattern of wear is typical for high cutting speeds and, respectively, active diffusion and oxidation processes at elevated temperatures [65,66]. The specimens with Coatings H1, H2, and H3 demonstrate fairly close patterns of wear. However, for the specimen with Coating H3, the wear rate is slightly higher (KB = 520 μm) than for the specimens with Coatings H1 and H2 (KB = 450 μm). In terms of notchwear, the more active wear was detected on the specimens with Coatings H1 and H2, while the specimen with Coating H3 demonstrated less active wear. We consider in more detail the patterns of rake wear for the tools with considered Coatings H1, H2, and H3 after 10 min of cutting ( Figure 3). An obvious notch wear is typical for the rake face, while no formation of noticeable wear crater is detected. Such a pattern of wear is typical for high cutting speeds and, respectively, active diffusion and oxidation processes at elevated temperatures [65,66]. The specimens with Coatings H1, H2, and H3 demonstrate fairly close patterns of wear. However, for the specimen with Coating H3, the wear rate is slightly higher (KB = 520 µm) than for the specimens with Coatings H1 and H2 (KB = 450 µm). In terms of notchwear, the more active wear was detected on the specimens with Coatings H1 and H2, while the specimen with Coating H3 demonstrated less active wear.  For a better understanding of the specifics of wear and fracture of the coated specimen during the turning, we consider cross-sections passing through the center of the wear area on the rake surface, perpendicular to the cutting edge. In general, all three coatings under consideration have fairly close patterns of cracking and delamination formation, but certain differences are also obvious.
Active cracking is detected in Coating H1 (Figure 4), with exclusively longitudinal cracks, the formation of which is usually related to the action of internal compressive stresses [67][68][69]. It is the active cracking resulted from the action of compressive stresses that can explain the noticeable decrease in the wear resistance of the tool with Coating H1 after 5 min of cutting. (d) (e) (f) For a better understanding of the specifics of wear and fracture of the coat men during the turning, we consider cross-sections passing through the center of area on the rake surface, perpendicular to the cutting edge. In general, all three under consideration have fairly close patterns of cracking and delamination fo but certain differences are also obvious.
Active cracking is detected in Coating H1 (Figure 4), with exclusively long cracks, the formation of which is usually related to the action of internal com stresses [67][68][69]. It is the active cracking resulted from the action of compressive that can explain the noticeable decrease in the wear resistance of the tool with Co after 5 min of cutting. The structure of the coating hardly contains any transverse cracks ( Figure 5) mation of which contributes to a larger degree to the fracture of the coating and f the cutting tool [70]. According to the analysis of the data exhibited in  The structure of the coating hardly contains any transverse cracks ( Figure 5), the formation of which contributes to a larger degree to the fracture of the coating and failure of the cutting tool [70]. According to the analysis of the data exhibited in Figures 4 and 5, the dominant direction of crack propagation in Coating H1 is the direction parallel to the surface of the substrate. The nanolayer structure of the coating does not have any noticeable effect on the pattern of crack propagation. Cracks propagate by cutting through the nanolayers of the coating. The Focused Ion Beam (FIB) cross-section analysis of the coating (Figure 5e,f) also detects a number of longitudinal cracks, which can indicate that the cracks are contained in the structure of the coatings, rather than being formed during the obtaining of the cross-sections for SEM.
The patterns of cracking slightly differ for Coatings H1 and H2. In particular, a slightly less active cracking takes place in Coating H2, with interlayer delaminations dominating, rather than longitudinal cracks (Figures 6 and 7).
Thus, it can be assumed that the interlayer (cohesive) bond between the nanolayers is weaker and that the internal compressive stresses are lower in Coating H2. Given that the tool with Coating H2 demonstrated noticeably better wear resistance during the turning compared to the tool with Coating H1, it can be concluded that the mentioned parameters have a significant influence on the cutting properties of the tool. Earlier, in [70], it was noted that the formation of delaminations between the nanolayers of the coating could, to a certain extent, favorably affect the wear resistance of the coating due to a decrease in the internal stresses. The patterns of cracking slightly differ for Coatings H1 and H2. In particular, a slightly less active cracking takes place in Coating H2, with interlayer delaminations dominating, rather than longitudinal cracks (Figures 6 and 7). In terms of the pattern of cracking, Coating H3 occupies an intermediate position between Coatings H1 and H2 (Figures 8 and 9). In particular, the structure of worn Coating H3 contains both delaminations between nanolayers and longitudinal cracks. At the same time, the process of cracking is less intensive compared to Coating H1 and is approximately equal to that of Coating H2. Thus, it can be assumed that the interlayer (cohesive) bond between the na is weaker and that the internal compressive stresses are lower in Coating H2. G the tool with Coating H2 demonstrated noticeably better wear resistance during ing compared to the tool with Coating H1, it can be concluded that the mentioned eters have a significant influence on the cutting properties of the tool. Earlier, i was noted that the formation of delaminations between the nanolayers of the could, to a certain extent, favorably affect the wear resistance of the coating due crease in the internal stresses. Thus, it can be assumed that the interlayer (cohesive) bond between the nanolayers is weaker and that the internal compressive stresses are lower in Coating H2. Given that the tool with Coating H2 demonstrated noticeably better wear resistance during the turning compared to the tool with Coating H1, it can be concluded that the mentioned parameters have a significant influence on the cutting properties of the tool. Earlier, in [70], it was noted that the formation of delaminations between the nanolayers of the coating could, to a certain extent, favorably affect the wear resistance of the coating due to a decrease in the internal stresses.  In terms of the pattern of cracking, Coating H3 occupies an intermediate between Coatings H1 and H2 (Figures 8 and 9). In particular, the structure of wo ing H3 contains both delaminations between nanolayers and longitudinal crack same time, the process of cracking is less intensive compared to Coating H1 an proximately equal to that of Coating H2. The described pattern and intensity of the cracking process in Coating H3 well with a considerably high wear resistance of the tool with Coating H3, whic slightly inferior to the wear resistance of the tool with Coating H2.  In terms of the pattern of cracking, Coating H3 occupies an intermediate position between Coatings H1 and H2 (Figures 8 and 9). In particular, the structure of worn Coating H3 contains both delaminations between nanolayers and longitudinal cracks. At the same time, the process of cracking is less intensive compared to Coating H1 and is approximately equal to that of Coating H2. The described pattern and intensity of the cracking process in Coating H3 correlate well with a considerably high wear resistance of the tool with Coating H3, which is only slightly inferior to the wear resistance of the tool with Coating H2.  The described pattern and intensity of the cracking process in Coating H3 correlate well with a considerably high wear resistance of the tool with Coating H3, which is only slightly inferior to the wear resistance of the tool with Coating H2.

Discussion
We consider several possible causes for the formation of interlayer delaminations in the coatings under study and the differences in the patterns of those delaminations.
The formation of delaminations between the nanolayers could be caused by a difference in the coefficients of thermal expansion (CTEs) α between the materials of separate layers. The data available (Table 2) exhibit the greatest difference in the CTEs between ZrN (α = 8.1) and Mo 2 N (α = 6.4). A slightly less difference is detected between ZrN (α = 8.1) and TiN (α = 7.1). The closest CTEs are detected for ZrN (α = 8.1) and CrN (α = 7.5). That is, using the given approach, we can predict the highest tendency to delamination between the layers of ZrN and Mo 2 N, and the lowest tendency between the layers of ZrN and CrN. In fact, there is an almost opposite situation when the highest number of delaminations is observed for the coating with alternating layers of (Zr,Hf)N and CrN. It is clear that the coefficient of thermal expansion α for (Zr,Hf)N differs from that for ZrN, and aluminum contained in the coating composition also has a certain effect. The contents of (Zr,Hf)N and Al are identical for all the coatings under consideration, and, therefore, only the difference in the CETs α for CrN, TiN, and Mo 2 N has an effect on the different expansion of some separate layers. --* A dash means "not available for this temperature range" (authors found no appropriate data in references); ** Not available for hafnium nitride (authors found no appropriate data in references).
Another possible cause for the formation of interlayer delaminations could be an inconsistency in such parameters of the material as the atomic radius and electronegativity in terms of the Hume-Rothery rules [73,74] and the Pauling scale [75][76][77]. The analysis of the data contained in Table 3 exhibits that Zr and Hf have very close parameters and are almost ideally combined. In terms of the considered approach, Ti and Al have the parameters closest to Zr and Hf, while the parameters of Cr, and, in particular, Mo, greatly differ from those of Zr and Hf. It is Coating H2, containing titanium, in which the most active delaminations occur, while Coating H3 containing Mo has fewer delaminations and more transverse cracks, cutting through the structure of the coating. Table 3. Comparison of the atomic radii and electronegativity of the elements contained in the coatings under consideration (compiled on the basis of analysis of information presented in [78]).

Metal
Atomic Radius, pm Electronegativity (Pauling Scale) Thus, Coating H3, containing the layers of Mo 2 N, which differ significantly from the layers of (Zr,Hf)N, both, in terms of the coefficient of thermal expansion and in terms of the Pauling scale, did not demonstrate a high tendency to interlayer delamination. At the same time, the mentioned tendency was identified for Coating H2, containing the layers of TiN quite close in their properties to the layers of (Zr,Hf)N.
Another parameter influencing the formation of interlayer delaminations is the Gibbs energy, described by the well-known formula [79]: A negative value of ∆G • indicates a decrease in the action of adhesion as a result of the formation of interphase tension. As the coating is operated under elevated temperatures, then it is reasonable to consider ∆G T • , an isobaric reaction potential, at the temperature at which the cutting tool is operated. Therefore, the challenge is reduced to the determination of nitride phases, characterized by an optimal combination of properties, as too strong adhesion bonds will prevent the formation of separate delaminations able to reduce the level of internal stresses, and too weak adhesion bonds will provoke rapid fracture of the coating through its global delamination. The analytical solution of the described problem is being hampered by the lack of necessary coefficients for most nitride compounds, especially multicomponent ones [79,80]. However, during the determination of the mentioned coefficients, it is the calculation of isobaric reaction potential at the temperature at which the cutting tool is operated that can become an important element in modeling the fracture of multilayer coatings and developing the coatings with optimal properties. Similar problems in relation to the NiTi coating were considered in [81,82].
Based on the foregoing, it can be assumed that, to determine the optimal coating composition in terms of adhesive bonds between nanolayers, it is not sufficient to use a single method. Only the development of the methods to consider various approaches will be able to effectively solve the problem.
Given that tools with H1 and H2 coatings showed fairly similar wear rates, it is necessary to take into account the certain technological complexity of using molybdenum cathodes. Molybdenum is characterized by the active formation of microdroplets during the deposition of coatings; moreover, due to the high melting and boiling points, the use of molybdenum is associated with higher energy consumption [83,84].

Conclusions
The properties of three coatings based on the systems of Zr,Hf,Al were investigated, including Zr,Hf-(Zr,Hf)N-(Zr,Hf,Cr,Al)N, referred to as Coating H1, Zr,Hf-(Zr,Hf)N-(Zr,Hf,Ti,Al)N, referred to as Coating H2, and Zr,Hf-(Zr,Hf)N-(Zr,Hf,Mo,Al)N, referred to as Coating H3. Following the analysis of the studies carried out, we can make the following conclusions: 1.
The considered coatings have close values of hardness and critical fracture load L C2 ; 2.
After 5 min of cutting, the average flank wear VB detected for the uncoated tool was 1.8 times higher than that for the tools with Coatings H1, H2, and H3 and 1.5 times higher than for the ZrN-coated tool. For the tools with Coatings H1, H2, and H3, the rate of wear was in average 1.2 times lower after 5 min of cutting in comparison with that for the tool with the commercial ZrN coating. The ZrN-coated tool and the tool with Coating H1 demonstrated the tool life 1.5 times higher, and the tools with Coatings H2 and H3-2 times higher compared to that of the uncoated tool; 3.
A process of notchwear is typical for the rake faces of the tools with the coatings under study, with no formation of any noticeable wear crater. The patterns of rake wear were fairly close for all the tools with Coatings H1, H2, and H3. However, for the tool with Coating H3, the rate of wear was slightly higher (KB = 520 µm) than that for the tools with Coatings H1 and H2 (KB = 450 µm). In terms on notchwear, the more active wear was detected for the tools with Coatings H1 and H2, while less active wear was demonstrated by the tool with Coating H3; 4.
Coating H1 undergoes active wear during the cutting process due to the formation of a large number of transverse cracks, cutting through the nanolayer structure of the coating. A formation of interlayer delaminations is more typical for Coating H2, whereas a combination of interlayer delaminations and longitudinal cracks is more typical for Coating H3. In general, Coatings H2 and H3 demonstrate less active cracking than Coating H1; 5.
Thus, the highest wear resistance is detected for the tool with Coating H2, and Coating H2 demonstrates the most favorable pattern of cracking, i.e., an insignificant amount of interlayer delaminations. In typical cutting operations of steel 1045, tools with H2 coating would likely give the best overall performance and tool life. Despite the additional cost to apply this coating, we anticipate that cutting speed and the added tool life would make this choice economic.
Further research may be directed at varying such coating parameters as the value of the nanolayer period λ and the ratio of elements in the coating composition. The research focused on Coatings H2 and H3 is more advisable.

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