Fabricated Using DC Magnetron Sputtering at Various Nitrogen Flow Rates and Deposition Temperatures

Ti-Al-N coatings were deposited by direct current magnetron sputtering (DCMS) onto IN 718 at different nitrogen flow rates and deposition temperatures. The coatings’ properties were characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) as well as nanoindentation. It was found that higher deposition temperature leads to higher surface roughness and nitrogen flux influences the shape of grains. According to XPS, the bonding structure of all coatings exhibited the (Ti,Al)N phase. Mechanical properties depend on the Al content within the films. The coating with the best mechanical properties (deposited at 500 ◦C and 20 standard cubic centimeters per minute (sccm)) was further deposited onto tungsten carbide (WC) cutting tools for cylindrical turning experiments. A quasi-constant flank wear was observed until a machining volume of 23,500 mm3.


Introduction
Ti-Al-N coatings have been successfully developed since the 1980s because they could significantly increase lifetime of high speed machining tools in comparison with conventional TiN coatings [1,2].Ti-Al-N films with cubic NaCl (c) structure, where Al substitutes Ti in the TiN based structure, are widely used in the tool industry due to their very good oxidation resistance [3].Furthermore, these films exhibit high hardness, good wear resistance and age-hardening abilities, along with lower thermal expansion, lower thermal conductivity and enhanced erosion resistance compared to TiN [4][5][6].
Properties of Ti-Al-N are strongly dependent on their chemical composition [7,8].The Al content in Ti-Al-N coatings overcomes the oxidation problems, but a superficial Al 2 O 3 layer formed at high temperature [9][10][11].Al 2 O 3 curbs oxygen diffusion and oxidative wear.Depending on the elemental and phase composition, hardness of these coatings can range from 20 to 35 GPa [12][13][14].
A number of different coating techniques, such as arc ion plating [15], arc evaporation [12,16], radio frequency magnetron sputtering [17], and chemical vapor deposition [4] have been used.However, Metals 2017, 7, 52 2 of 10 magnetron sputtering [18,19] is a widely used method for depositing Ti-Al-N coatings due to control of the film thickness, structure and properties.However, the influence of nitrogen flow rate at different deposition temperatures on the chemical bonding structure and surface morphology of the coatings is hardly discussed.
In the present investigation, Ti-Al-N films were deposited at 200 • C and 500 • C with different nitrogen flow rates on Inconel 718 via the reactive sputter deposition process.The aim of this work is to analyze the influence of deposition temperature and nitrogen flow rate on chemical composition, surface morphology and mechanical properties by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and nanoindentation.Furthermore, the coating with the best properties was deposited onto a tungsten carbide (WC) cutting tool for an application oriented turning test.

Experimental Section
Ti-Al-N films were deposited in an industrial coater CemeCon 800/9 from CemeCon AG (Würselen, Germany) onto Inconel 718 substrates and WC cutting tools for a cylindrical turning experiment.Prior to coating, the substrates were mirror polished, ultrasonic cleaned and dried.A composite TiAl target (Ti-50%, Al-50%) with a size of 88 mm × 500 mm from PLANSEE AG (Reutte, Austria) was used for the experiments in a reactive sputtering mode.The initial coating process consists of an argon ion etching process, which was performed for 30 min at a substrate MF bias of 650 V and a pressure of 350 MPa.Later, a cathode power of 4 kW with an argon pressure of 600 MPa at a substrate bias of 60 V was used for the deposition process.Moreover, a deposition time of 30 min in a static mode was opted to ensure high deposition rates.Further information about deposition temperature and nitrogen flow rates in standard cubic centimeters per minute (sccm) are given in Table 1.A scanning electron microscope (SEM) and wavelength dispersive spectroscopy (WDS) from TESCAN MIRA II (Brno, Czech Republic) were used for characterizing coating growth and elemental analysis.
An ES300 KRATOS spectrometer (Kratos Analytical, Manchester, UK) was used for XPS studies.The surface of the samples was cleaned by washing several times in pure ethanol, dried and placed into a spectrometer.Photoelectron spectra were acquired after evacuation to ultra-high vacuum (UHV, 5 × 10 −9 mbar).The electron analyzer was calibrated against Au4f7/2 (84 eV) and Cu2p2/3 (932.7 eV) photoelectron line positions obtained from cleaned gold and copper metallic surfaces.Non-monochromatic Mg Kα radiation was used for analysis without calibration because of the conductivity of the samples.Additionally, all samples were characterized after evacuation to UHV and an Ar + ion etching beam at 1 keV and 20 mA was used for 30 min to remove the topmost contaminated layers.Mathematical spectra processing was performed using XPS-Calc software (Boreskov Institute of catalysis, Novosibirsk, Russia), described in [20].
Surface images and roughness of the films were obtained by AFM ("Smena" microscope by NT-MDT, Zelenograd, Russia) operating in the contact mode.Silicon cantilever NSG 11 Pt (NT-MDT, Zelenograd, Russia) with a tip radius of 10 nm was used in this work.The hardness of the coatings was measured by UNAT nanoindenter with a Berkovich indenter (ASMEC GmbH, Radeberg, Germany).Hardness and elastic modulus of the coatings were calculated by analyzing the load-displacement curves using the Oliver Pharr method [21] at a load of 10 mN.The penetration depth did not reach more than one-tenth of the coating thickness to avoid effects from the substrate [22].An average value of 15-20 measurements was calculated for each film.A cylindrical turning experiment was Metals 2017, 7, 52 3 of 10 carried out under wet cutting conditions on the computer numeric control (CNC) slant bed lathe type VDF 180 C U (Oerlikon-Boehringer, Göppingen, Germany).The turning test of the WC cutting tool with the geometry of CNMN 120408 according to ISO 1832 was carried out with a cutting speed of v c = 100 m/min, a feed of f = 0.1 mm/rev and a depth of cut of a p = 0.5 mm.A cutting edge radius of approximately r β = 35 µm was determined for the cutting tool.In this work, cold work tool steel, type X210CrW12, was used for the turning test.For the examination of the wear form, the flank wear VB was measured until reaching the defined tool life criterion VB max = 0.5 mm.The clearance angle was α eff = 10 • , and the rake angle was γ eff = −10 • [23,24].

SEM and AFM
A typical cross-sectional SEM image of the Ti-Al-N coating on IN 718 is shown in Figure 1.The dense columnar microstructure of the coating can be attributed to ion bombardment during deposition.
Metals 2017, 7, 52 3 of 10 experiment was carried out under wet cutting conditions on the computer numeric control (CNC) slant bed lathe type VDF 180 C U (Oerlikon-Boehringer, Göppingen, Germany).The turning test of the WC cutting tool with the geometry of CNMN 120408 according to ISO 1832 was carried out with a cutting speed of vc = 100 m/min, a feed of f = 0.1 mm/rev and a depth of cut of ap = 0.5 mm.A cutting edge radius of approximately rβ = 35 µ m was determined for the cutting tool.In this work, cold work tool steel, type X210CrW12, was used for the turning test.For the examination of the wear form, the flank wear VB was measured until reaching the defined tool life criterion VBmax = 0.5 mm.The clearance angle was αeff = 10°, and the rake angle was γeff = −10° [23,24].

SEM and AFM
A typical cross-sectional SEM image of the Ti-Al-N coating on IN 718 is shown in Figure 1.The dense columnar microstructure of the coating can be attributed to ion bombardment during deposition.WDS analysis of the deposited Ti-Al-N coatings can be seen in Table 2. Al content first increases and then decreases to 44.78 at.% with rising nitrogen flow rate at 200 °C.In contrast, a decrease of Al content from 43.76 to 36.08 at.% takes place with rising nitrogen flow rate at 500 °C.The surface morphology of the coated IN 718 samples examined by AFM is presented in Figure 2. Topography of the coatings produced at different conditions shows a significant difference in surface roughness and typical surface features.The main characteristics of the films, such as thickness and surface roughness (root-mean-square, RMS) are listed in Table 3.
Coatings fabricated at 200 °C exhibit a densely packed structure consisting of well-separated grains with apparently spherical form (Figure 2a-c).With an increasing nitrogen flow rate, a decrease in roughness (root-mean-square values) was measured.This decrease is mainly attributed WDS analysis of the deposited Ti-Al-N coatings can be seen in Table 2. Al content first increases and then decreases to 44.78 at.% with rising nitrogen flow rate at 200 • C. In contrast, a decrease of Al content from 43.76 to 36.08 at.% takes place with rising nitrogen flow rate at 500 • C. The surface morphology of the coated IN 718 samples examined by AFM is presented in Figure 2. Topography of the coatings produced at different conditions shows a significant difference in surface roughness and typical surface features.The main characteristics of the films, such as thickness and surface roughness (root-mean-square, RMS) are listed in Table 3.
Coatings fabricated at 200 • C exhibit a densely packed structure consisting of well-separated grains with apparently spherical form (Figure 2a-c).With an increasing nitrogen flow rate, a decrease in roughness (root-mean-square values) was measured.This decrease is mainly attributed to a denser packing of surface features.According to Shum et al. [25], the Al content plays an important role in smoothing of the film, especially in the range between 20 and 50 at.% due to more nuclei.For the samples prepared at 500 °C (Figure 2d-f), a completely different evolution was found.While for a low nitrogen flux, a network-like surface structure consisting of non-separable grains is present, at higher fluxes, larger, well-separated features appear.In contrast to the low temperature, the coatings have more surface defects, accompanied by increasing surface roughness.Thus, the process temperature has major influence on the growth and surface roughness (an average quantity) of the coating, whereas nitrogen flux determines the shape of single grains (a local quantity).These findings may be caused by different growth rates and nucleation properties during deposition.Furthermore, the thicknesses for both series decrease with higher nitrogen content, because the densities of gas molecules increase with rising nitrogen flow rate, which leads to more collisions and reduced thickness of the coatings [26].Poisoning of the target by nitrogen can be a reason for this change, too [27].For the samples prepared at 500 • C (Figure 2d-f), a completely different evolution was found.While for a low nitrogen flux, a network-like surface structure consisting of non-separable grains is present, at higher fluxes, larger, well-separated features appear.In contrast to the low temperature, the coatings have more surface defects, accompanied by increasing surface roughness.Thus, the process temperature has major influence on the growth and surface roughness (an average quantity) of the coating, whereas nitrogen flux determines the shape of single grains (a local quantity).These findings may be caused by different growth rates and nucleation properties during deposition.Furthermore, the thicknesses for both series decrease with higher nitrogen content, because the densities of gas molecules increase with rising nitrogen flow rate, which leads to more collisions and reduced thickness of the coatings [26].Poisoning of the target by nitrogen can be a reason for this change, too [27].

XPS
The surface of the coatings was analyzed by XPS before an ion bombardment cleaning process, in order to understand the chemical bonding state of the surface.The Ar + etching rate was about Metals 2017, 7, 52 5 of 10 2 Å/min.The initial surfaces were strongly contaminated and oxidized in all cases.After Ar + etching, a substantial decrease of carbon content and higher intensity of Al2p, N1s and Ti2p lines due to surface cleaning were noticed.This behavior was typical for all samples.
Thus, all samples were analyzed after 30 min argon etching to remove the top contaminant layer.Figure 3a-f shows typical high resolution XPS-spectra Ti2p core levels of coatings prepared at different nitrogen flow rates and temperatures, respectively.Titanium is represented by three main states, which correspond to (Ti,Al)N (455.6-455.7 eV, black), TiON (457.3-457.7 eV, blue) and TiO 2 (459.0-459.4eV, red) [28 -30].The titanium states do not correlate with the nitrogen content as well as the preparation temperature of the samples.No significant differences in binding energy between the analysed samples was found.
Aluminum was represented by two states with binding energies of 74.1-74.2eV and 74.9-75.4eV (Figure 4a-f), which correspond to oxynitride and nitride AlN (a standard binding energy of 73.5-73.8eV) [31,32] and oxide Al 2 O 3 [33,34].A correlation of aluminum oxide concentration with the nitrogen content was found in the 200 • C series.Increasing N content leads to an increase of the oxynitride (nitride) fraction and decrease of the Al 2 O 3 fraction.No correlation of the Al state with the nitrogen content in the 500 • C series was observed; the corresponding spectra are almost the same.This means that at high temperature treatment, the equilibrium surface composition is reached independently of the initial aluminum content.

XPS
The surface of the coatings was analyzed by XPS before an ion bombardment cleaning process, in order to understand the chemical bonding state of the surface.The Ar + etching rate was about 2 Å /min.The initial surfaces were strongly contaminated and oxidized in all cases.After Ar + etching, a substantial decrease of carbon content and higher intensity of Al2p, N1s and Ti2p lines due to surface cleaning were noticed.This behavior was typical for all samples.
Thus, all samples were analyzed after 30 min argon etching to remove the top contaminant layer.Figure 3a-f shows typical high resolution XPS-spectra Ti2p core levels of coatings prepared at different nitrogen flow rates and temperatures, respectively.Titanium is represented by three main states, which correspond to (Ti,Al)N (455.6-455.7 eV, black), TiON (457.3-457.7 eV, blue) and TiO2 (459.0-459.4eV, red) [28][29][30].The titanium states do not correlate with the nitrogen content as well as the preparation temperature of the samples.No significant differences in binding energy between the analysed samples was found.
Aluminum was represented by two states with binding energies of 74.1-74.2eV and 74.9-75.4eV (Figure 4a-f), which correspond to oxynitride and nitride AlN (a standard binding energy of 73.5-73.8eV) [31,32] and oxide Al2O3 [33,34].A correlation of aluminum oxide concentration with the nitrogen content was found in the 200 °C series.Increasing N content leads to an increase of the oxynitride (nitride) fraction and decrease of the Al2O3 fraction.No correlation of the Al state with the nitrogen content in the 500 °C series was observed; the corresponding spectra are almost the same.This means that at high temperature treatment, the equilibrium surface composition is reached independently of the initial aluminum content.The nitrogen valence state represented mainly by the peak at 396.8-396.9eV is corresponding to nitride as well as oxynitride (396.6 eV) (Figure 5a-f) due to the close position of N1s for pure TiN (396.8 eV) and AlN (397.3 eV), respectively [29,31,35]; these states are not resolved on the binding energy.The peak centered at approximately 398.5 eV may be attributed to the shake-up satellite [29].An increase in N1s intensity with increasing nitrogen amount can be observed for coatings The nitrogen valence state represented mainly by the peak at 396.8-396.9eV is corresponding to nitride as well as oxynitride (396.6 eV) (Figure 5a-f) due to the close position of N1s for pure TiN (396.8 eV) and AlN (397.3 eV), respectively [29,31,35]; these states are not resolved on the binding energy.The peak centered at approximately 398.5 eV may be attributed to the shake-up satellite [29].An increase in N1s intensity with increasing nitrogen amount can be observed for coatings deposited at 200 • C, whereas no correlation was observed for coatings at 500 • C.This is similar to the aluminum behavior and could be related with reaching the equilibrium surface composition, regardless of initial elements' concentrations.
Metals 2017, 7, 52 6 of 10 deposited at 200 °C, whereas no correlation was observed for coatings at 500 °C.This is similar to the aluminum behavior and could be related with reaching the equilibrium surface composition, regardless of initial elements' concentrations.Metals 2017, 7, 52 6 of 10 deposited at 200 °C, whereas no correlation was observed for coatings at 500 °C.This is similar to the aluminum behavior and could be related with reaching the equilibrium surface composition, regardless of initial elements' concentrations.Metals 2017, 7, 52 7 of 10

Nanoindentation
Figure 6 shows the relationship between hardness and elastic modulus as a function of nitrogen content at two deposition temperatures.At 500 • C, the coating deposited with 20 sccm has the highest hardness and lowest modulus (30.5 GPa and 215 GPa, respectively).In comparison to this, at 200 • C, hardness first decreases and then rises with increasing nitrogen flow rate.Nanoindentation results correlate with the WDS analysis, because mechanical properties increase with Al content and reach a maximum value at 43-44 at.% of Al, close to the results reported by other groups [25,36].Moreover, the deterioration of mechanical properties with the increase in nitrogen content can be related to the achievement of saturation level due to higher nitrogen pressure leading to excess of nitrogen in the interstitial sites [37].This excess of nitrogen could lead to an increase in grain size, a change in crystallographic orientations, and affect film density, lattice parameter and the stoichiometry of the coatings [38,39].A reason for high standard deviations is the high surface roughness of the coating, which influences the indentation depth during the nanoindentation measurements.

Nanoindentation
Figure 6 shows the relationship between hardness and elastic modulus as a function of nitrogen content at two deposition temperatures.At 500 °C, the coating deposited with 20 sccm has the highest hardness and lowest modulus (30.5 GPa and 215 GPa, respectively).In comparison to this, at 200 °С, hardness first decreases and then rises with increasing nitrogen flow rate.Nanoindentation results correlate with the WDS analysis, because mechanical properties increase with Al content and reach a maximum value at 43-44 at.% of Al, close to the results reported by other groups [25,36].Moreover, the deterioration of mechanical properties with the increase in nitrogen content can be related to the achievement of saturation level due to higher nitrogen pressure leading to excess of nitrogen in the interstitial sites [37].This excess of nitrogen could lead to an increase in grain size, a change in crystallographic orientations, and affect film density, lattice parameter and the stoichiometry of the coatings [38,39].A reason for high standard deviations is the high surface roughness of the coating, which influences the indentation depth during the nanoindentation measurements.

Cylindrical Turning Experiments
Ti-Al-N coating deposited at 500 °C with N2 flow of 20 sccm was chosen for the deposition on the WC cutting tool due to its high hardness and low modulus.The result of machining the hardened X210CrW12 steel is presented in Figure 7.A quasi-constant flank wear can be seen until a machining volume of 23,500 mm 3 , followed by rapid material removal until the criterion was fulfilled.In comparison to the results reported by Uhlmann et al. [24] (Figure 7), the Ti-Al-N coating in the current study shows virtually twice higher machining volume.An alternative method to the direct current magnetron sputtering (DCMS) is the high pulsed power magnetron sputtering (HPPMS), which is believed to provide better mechanical properties and high adhesion between the coating and the substrate [23].Industry has been using HPPMS as a coating method during the last years, but not much information can be found in the literature regarding the wear properties of these coatings.Hence, investigations on the wear behaviour of HPPMS deposited Ti-Al-N coatings will be made in the future.

Cylindrical Turning Experiments
Ti-Al-N coating deposited at 500 • C with N 2 flow of 20 sccm was chosen for the deposition on the WC cutting tool due to its high hardness and low modulus.The result of machining the hardened X210CrW12 steel is presented in Figure 7.A quasi-constant flank wear can be seen until a machining volume of 23,500 mm 3 , followed by rapid material removal until the criterion was fulfilled.In comparison to the results reported by Uhlmann et al. [24] (Figure 7), the Ti-Al-N coating in the current study shows virtually twice higher machining volume.An alternative method to the direct current magnetron sputtering (DCMS) is the high pulsed power magnetron sputtering (HPPMS), which is believed to provide better mechanical properties and high adhesion between the coating and the substrate [23].Industry has been using HPPMS as a coating method during the last years, but not much information can be found in the literature regarding the wear properties of these coatings.Hence, investigations on the wear behaviour of HPPMS deposited Ti-Al-N coatings will be made in the future.

Conclusions
Ti-Al-N coatings were prepared at different temperatures and nitrogen flow rates using a reactive sputter deposition process.The process parameters were optimized to achieve good mechanical properties of the films.AFM results reveal that the process temperature has major influence on growth and surface roughness of the films, whereas the nitrogen flow rate affects the shape of the grains.XPS results reveal that the bonding structure of all coatings exhibited the (Ti,Al)N phase.The nanoindentation data indicated that the coating deposited at 500 °C and 20 sccm exhibited the best mechanical properties.Moreover, mechanical properties depend on Al content within the coating.A coating with these parameters was used for deposition onto WC cutting tools, which showed low material removal rate during cylindrical turning experiments.

Conclusions
Ti-Al-N coatings were prepared at different temperatures and nitrogen flow rates using a reactive sputter deposition process.The process parameters were optimized to achieve good mechanical properties of the films.AFM results reveal that the process temperature has major influence on growth and surface roughness of the films, whereas the nitrogen flow rate affects the shape of the grains.XPS results reveal that the bonding structure of all coatings exhibited the (Ti,Al)N phase.The nanoindentation data indicated that the coating deposited at 500 • C and 20 sccm exhibited the best mechanical properties.Moreover, mechanical properties depend on Al content within the coating.A coating with these parameters was used for deposition onto WC cutting tools, which showed low material removal rate during cylindrical turning experiments.

Figure 1 .
Figure 1.Cross-sectional SEM of the Ti-Al-N film.

Figure 1 .
Figure 1.Cross-sectional SEM of the Ti-Al-N film.
Metals 2017, 7, 52 4 of 10 to a denser packing of surface features.According to Shum et al.[25], the Al content plays an important role in smoothing of the film, especially in the range between 20 and 50 at.% due to more nuclei.

Figure 6 .
Figure 6.Influence of the nitrogen flow rate on mechanical properties of the Ti-Al-N coatings at: (a) 200 °C and (b) 500 °C.

Figure 6 .
Figure 6.Influence of the nitrogen flow rate on mechanical properties of the Ti-Al-N coatings at: (a) 200 • C and (b) 500 • C.

Figure 7 .
Figure 7. Development of wear for the turning test of WC with the Ti-Al-N coating.

Figure 7 .
Figure 7. Development of wear for the turning test of WC with the Ti-Al-N coating.

Table 1 .
Deposition parameters of Ti-Al-N coatings at various process parameters.

Table 2 .
Chemical composition of the Ti-Al-N coatings by means of WDS analysis.

Table 2 .
Chemical composition of the Ti-Al-N coatings by means of WDS analysis.

Table 3 .
Influence of temperature and nitrogen flow rate on morphological properties of the Ti-Al-N films.Temperature (°C) Nitrogen Flow Rate (sccm) Thickness (µm) RMS (nm)