Structural changes in Ti1-xAlxN coatings during turning: A XANES and EXAFS study of worn tools

Structural changes in Ti1-xAlxN coated tool inserts used for turning in 316L stainless steel were investigated by XANES, EXAFS, EDS, and STEM. For coarse-grained fcc-structured Ti1-xAlxN coatings, with 0 \leq x \leq 0.62, the XANES spectrum changes with Al-content. XANES Ti 1s line-scans across the rake face of the worn samples reveals that TiN-enriched domains have formed during turning in Ti0.47Al0.53N and Ti0.38Al0.62N samples as a result of spinodal decomposition. The XANES spectra reveal the locations on the tool in which the most TiN-rich domains have formed, indicating which part of the tool-chip contact area that experienced the highest temperature during turning. Changes in the pre-edge features in the XANES spectra reveal that structural changes occur also in the w-TiAlN phase in fine-grained Ti0.38Al0.62N during turning. EDS shows that Cr and Fe from the steel adhere to the tool rake face during machining. Cr 1s and Fe 1s XANES show that Cr is oxidized in the end of the contact length while the adhered Fe retains in the same fcc-structure as that of the 316L stainless steel.


Introduction
Titanium aluminum nitride (TiAlN) is one of the most important coating materials for increasing the service life and performance of tool inserts in metal machining. It is well suited for high-speed cutting operations due to its favorable high temperature behavior in terms of phase stability and mechanical properties. The spinodal decomposition of the face centered cubic (fcc) TiAlN phase and subsequent formation of wurtzite (w) AlN that occurs when the material is exposed to high temperatures is well characterized, see for example Refs. [1][2][3][4][5]. The decomposition of the metastable cubic TiAlN phase influences the mechanical properties and thus the wear behavior of the coating [6,7]. Decomposition of the coating takes place during machining [8,9] and it has been observed that interdiffusion of species from the workpiece into the coating is enhanced in the decomposed structure [10,11]. The small contact area between the tool and the workpiece material, and the large temperature and stress gradients [8], limit the understanding on how the decomposition affects the wear behavior as the characterization of the nanostructure formation requires advanced sample preparation by focused ion beam (FIB) in combination with scanning transmission electron spectroscopy (STEM). This restricts the possibilities of mapping out the material behavior across the contact zone between the tool and the workpiece material.
Non-destructive techniques such as x-ray diffraction (XRD) [2][3][4]12] and x-ray absorption spectroscopy (XAS) techniques [13][14][15][16][17][18] have successfully been used to separate between single phase and two-phase materials for as-deposited or post-annealed coatings. We have previously observed that XRD is not sensitive enough to detect the initial stage of decomposition because of the small difference in lattice parameter between the as formed cubic phase AlN and TiN-rich domains [2]. X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) are, on the other hand, more sensitive to the short-range order compared to diffraction as they probe the local environment, which changes character at early stages of decomposition. Using a combination of XANES, EXAFS and XRD, Tuilier et al. [13] studied magnetron sputtered TiAlN coatings and observed that Ti presents a hexagonal-like local order for high Alcontents (Ti0.14Al0.86N) while for lower Al-contents (Ti0.50Al0.50N) there is a cubic-like local order.
XANES of the Al 1s-and Ti 1s-edges revealed that there is a discrepancy between the structure obtained by XRD and EXAFS, understood as while XRD only probes well crystallized domains, also distorted or amorphous-like domains, e.g., grain boundaries, contribute to the EXAFS signal [14]. The sensitivity of EXAFS to non-crystalline domains was used to confirm an increased disorder within the hexagonal TiAlN domains during nanoindentation of TiAlN coatings [15]. This could not be observed by selected area electron diffraction (SAED) in a combined TEM/SAED study since only crystalline domains are probed by SAED. Gago et al. could identify the presence of a wurtzite phase by recording spectra at the N 1s, Al 1s and Ti 2p edges by XAS on magnetron sputtered TiAlN films [16,18]. The segregation of TiAlN upon annealing could also be identified by XANES [17].
In the present work, we investigate high-resolution Ti 1s XANES and EXAFS data from different positions across the contact area of the rake face of worn Ti1-xAlxN coated tools, utilizing the excellent spatial resolution available at the Balder beamline at MAX IV [19]. The XANES spectra changes with the Al-content of the coating which enables us to identify small variations in the chemical composition of the fcc-TiAlN phase. We use this to show that decomposition occurs in the crater region of the worn Ti1-xAlxN coatings with highest Al-content.

Sample deposition and cutting tests
Cobalt-toughened tungsten carbide (WC-Co) inserts (TPUN 160308E30-K) were coated with Ti1-xAlxN in a Metaplas MZR323 arc deposition system. Four separate depositions using TiAl-alloy cathodes with different Ti:Al ratio were performed for the four different coatings. The depositions were performed in nitrogen gas (N2) atmosphere at the pressure of 3.5 Pa, a substrate temperature of 500 °C, and using a negative substrate bias between 25 and 40 V. Two depositions were performed with 33:67 Ti:Al cathodes of different sizes (diameter of 63 and 100 mm, respectively) to obtain coarsegrained and fine-grained Ti0.38Al0.62N. More details on the deposition can be found in Ref. [11].
The coated inserts were used for longitudinal turning of 316L stainless steel using a cutting speed of 220 m/min, depth of cut of 2 mm, and feed of 0.2 mm/rev. Three edges were used for each sample, and they were run for 0.5, 1, and 3 min, respectively. After initial analysis by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS), the edges used for 1 min of turning were selected for further experiments.
XANES data was also collected for magnetron sputtered (MS), single crystal reference TiN and fcc-Ti0.35Al0.65N coatings. The reference coatings were grown on MgO substrates ((111) orientation for TiN and (001) orientation for Ti0.35Al0.65N) from a Ti or Ti0.35Al0.65 target (99.9% purity) in an Ar/N2 gas mixture. For more details on the deposition conditions, the reader is referred to Refs. [20] and [21].

XANES and EXAFS
The Ti 1s, Fe 1s, and Cr 1s XANES and Ti 1s EXAFS spectra were measured using a Si(111) crystal monochromator at the wiggler beamline Balder on the 3 GeV electron storage ring at MAX IV in Lund, Sweden. The data was collected in fluorescence yield mode using a 7-element SDD detector (X-PIPS, Mirion Technologies) and an 80° incidence angle on the sample. The beam size was defined by slits to 50 µm in the vertical direction. The horizontal width of the beam was approximately 200 µm. A 6 µm thick Ti metal foil was used for energy calibration where the first inflection point in the first derivative of the Ti 1s absorption spectrum was set to 4.9660 keV. The measurements were performed using scans with 0.2 eV step size and 0.1 s dwell time per step. The total intensity was accumulated over three scans for Cr 1s and Fe 1s and five scans for Ti 1s. The lower statistics for the Ti 1s XAS was a result of de-tuning the monochromator to reduce the contribution of unwanted undulator-derived harmonics. The XANES data from the seven channels was summed, background corrected by fitting a linear function to the pre-edge data, and normalized by setting the highest intensity to one. The procedure was repeated ten times at each position at the sample and a summation provided the final XAS spectrum. A metal bulk sample was measured for each of the three elements. For Fe and Cr, a 316L steel reference was measured with the same parameters. For each worn insert, a line scan was performed across the rake face as illustrated in Figure 3.
EXAFS data was measured at the Ti 1s absorption edge with a step size of 0.25 eV for 0.1 s per step.
The EXAFS analysis was made by using the Visual Processing in EXAFS Researches (VIPER) software package [22]. The Ti-N, Ti-Ti, and Ti-Al scattering paths obtained from the Effective Scattering Amplitudes (FEFF) [23,24] were included in the EXAFS fitting. The k 2 -weighted χ EXAFS oscillations were extracted from the raw absorption data, the average of 10 absorption spectra, after removing known monochromator-induced glitches and subsequent atomic background subtraction and normalization. The bond distances (R), number of neighbors (N), Debye-Waller factors (σ 2 , representing the amount of disorder) and the reduced χr 2 as the squared area of the residual, were determined by fitting the back-Fourier-transform signal between k=0-12 Å −1 originally obtained from the forward Fourier-transform within R=0-3.2 Å of the first coordination shell using a Hanning window function [23,24] with a many body factor of S0 2 =0.8.

Microstructural characterization
The phase content was determined by XRD in the Bragg-Brentano as well as grazing incidence (GI) configuration using Cu Kα radiation (Bruker D8 Advance). The lattice parameters were extracted from the fcc-111 diffraction peak by fit of a pseudo-Voigt function to the θ-2θ diffractogram to extract the peak position. The corresponding Ti-N and Ti-Ti/Ti-Al interatomic distances were calculated assuming a NaCl structure. SEM and EDS were performed in a Leo 1550 Gemini (Zeiss) instrument equipped with an Oxford X-MAX detector. The SEM was operated at 10 kV for imaging and EDS. Lamellas for STEM were prepared by FIB in a Zeiss Neon 40 instrument using the lift-out technique. They were coated by a thin Pt layer to protect the samples and were coarse milled using a 30 kV/2 nA Ga + ion beam followed by final polishing using 200 pA and 50 pA currents. The TEM lamellas were viewed in a FEI Tecnai G2 TF 20 UT instrument operating at 200 kV. STEM images were recorded using the HAADF detector and a camera length of 140 mm. Figure 1 shows GIXRD diffractograms of the as-deposited coatings. The coarse-grained coatings have a cubic structure while the fine-grained (FG) coating reveals a mixture of fcc and wurtzite phases. All coatings have a columnar structure and a thickness from 2.5-2.7 µm [11]. The chemical composition specified in Figure 1 was determined by EDS. The nitrogen-to-metal ratio as determined by EDS was close to one and similar between the samples and is not considered further here. For more details on the coating microstructure and properties, the reader is referred to Ref. [11].  presence of grain boundaries does, however, not seem to influence the XANES spectra. Table 1 lists the energy corresponding to the intensity maximum of the peaks A, C, D, and F-H and the features appear at the same energy for both the arc-evaporated and MS reference TiN coatings. The mainedge region shows insensitivity toward the Al content. The large signal changes, observed when the Al-content increases in the arc-deposited coatings, are visual at the post-edge region. For the coarsegrained samples, increasing the Al-content above x=0.23 causes the features G and H to disappear.

Results and discussion
Endrino et al. observed that the features at ~5060 eV and ~5090 eV (G and H) disappear when the Al-content of the fcc-TiAlN phase decreased during annealing [17]. There is also a broadening and shift to higher energies of F, similar to what was observed by Endrino et al. with the change of Alcontent in the fcc-TiAlN phase [17]. For the highest Al-content, x=0.62, a new feature E appears. The reference MS-Ti0.35Al0.65N sample also show a broadening of feature F compared to TiN and a loss of feature G (Fig. 2b), thus similar to the arc deposited coatings. There is, however, a larger shift of feature F for the arc deposited films when x increases from x=0 to x=0.62, compared to when x increases from 0 to 0.65 for the MS reference coatings. Further, the feature F is broader for the arc deposited coating than for the MS reference coating. One reason for this can be the higher defect density and the presence of grain boundaries in the arc deposited coatings, resulting in a locally more disordered structure compared to the MS reference coating. However, the origin of feature F would need to be identified for a better interpretation of the exact position and broadening of this peak.
The pre-edge peak A, which actually is a double feature assigned to the Ti 1s transitions to the empty Ti-N t2g-eg orbitals [25,26], is located at 4971 eV for all coarse-grained samples and only small changes of this feature is observed for the coarse-grained samples in the inset of Fig. 2a, indicating that the strength of the Ti-N bonds is rather insensitive toward the Al content. For Ti0.47Al0.53N and Ti0.38Al0.62N, the intensity of the pre-edge feature has decreased and it consists of a peak at 4971 eV and a clear shoulder at 4968 eV. Thus, the t2g-eg crystal-field splitting of the Ti 3d orbitals [25,26] is best observed for these samples and the two shoulders suggest a t2g-eg crystal-field splitting of about 3 eV.
The sharp pre-edge peak for the fine-grained Ti1-xAlxN coating, located at 4969 eV, can be interpreted as Ti 1s transitions to unoccupied Ti 3d-4p hybridized orbitals [27,28] that is superposed on the t2g-eg features. The Ti 3d-4p hybridized orbitals feature suggests Ti-Ti or Ti-Al interaction in the fine-grained film that is absent in the coarse-grained film. One hypothesis could be that the appearance of the Ti-Ti or Ti-Al interaction is related to the increased number of grain boundaries and thus larger total interface area in the fine-grained material compared to the coarse-grained materials. However, there is no pronounced difference in the intensity of the pre-edge feature Applied Surface Science 612, 155907, (2022) between the single-crystal MS reference samples (Fig. 2b) and the coarse-grained arc deposited samples (Fig. 2a). Thus, the grain boundaries are not contributing to the appeared Ti-Ti or Ti-Al interaction in the fine-grained coating. Furthermore, the fine-grained coating consists of a mixture of cubic and wurtzite phases that have been suggested causing dipole allowed electronic transitions into 3d-4p mixing orbitals for four-fold coordinated atoms in a tetrahedral coordination [13,14].
Also our results presented here suggest that 3d-4p hybridized orbitals appear in the XANES spectrum because of a w-TiAlN phase while being absent for an fcc-TiAlN phase. This observation agrees with that of Endrino et al. [17], that also observed the sharp pre-edge peak when a wurtzite phase was present in the coating. They assigned the pre-edge peak to Ti-d states hybridized with Al-p states. It is however not possible to determine from the current data whether the 3d contribution originates from neighboring Al or Ti atoms.   (Fig. 2), and the middle of the crater region of the worn samples (orange spectra in Fig. 5). The absorption energies are expressed in electron volt (eV).   Figure 3b shows the corresponding EDS maps of selected elements. It can be observed that across the contact length of the tool, Fe and Cr species from the steel has adhered to the surface. At the end of the contact where the coating is in exposed to air while the temperature still is relatively high, oxide phases commonly form [11] and also in this case we observe an enrichment of O and Cr at this position. In addition to Cr, Fe and O, Ni has also adhered at the tool surface together with smaller amounts of Mg, Ca, and P (not shown).  suppressed intensity as an effect of destructive interference between Ti-Ti and Ti-Al scattering paths of similar length due to the difference in their phase shift close to 180° [14]. It is also observed that the Ti-N distance becomes shorter when Al is incorporated in the fcc structure (see Table 2), as also shown by Tuilier et al. [14].

References
There is some discrepancy between the interatomic distances extracted from EXAFS and XRD. This is likely an effect of that the two methods probe short-vs long-range order, respectively, together with presence of strain in the coating. XRD in the Bragg-Brentano configuration probes only lattice planes parallel to the surface while EXAFS averages over all directions. In the presence of residual stress in the coating the lattice parameter will be altered in different directions. For the as-deposited coatings, there is a compressive in-plane strain [11], as commonly observed for arc deposited coatings. Thus, the lattice spacing is largest in the growth direction which is the direction probed by XRD here. This explains the larger interatomic distances obtained by this method. Tuilier et al. also reported a deviation between the lattice parameter determines by EXAFS and XRD [14].   Ti0.38Al0.62N (Fig. 5d-e), the feature F shifts to lower energies in the middle of the contact length (orange) compared to the region closest to the edge (green) and the end of the contact (dark blue).
Applied Surface Science 612, 155907, (2022) Feature F is found at lower energies close to the edge compared to the end of the contact and compared to the as-deposited coating. For Ti0.38Al0.62N, feature G appears in the middle of the contact in the crater region. Figure 6 shows the position of the maximum intensity of feature F extracted from the spectra of the as-deposited and worn coatings. For the as-deposited coatings, the position of this feature is observed to shift to higher energies as more Al is introduced into the coating. This is observed both for arc-deposited and MS reference coatings (marked as stars). The exception is the fine-grained coating, for which part of the Ti is present in the w-TiAlN structure. Thus, in agreement with previous studies [17], the position of feature F is dependent on the Al content of the fcc phase. The position of feature F shifts to lower energies in the middle of the crater for all samples with Al content higher than x=0.23. Thus, this indicates that there is a Ti-rich fcc-Ti(Al)N phase forming in the crater region during turning. For the fine-grained sample, there is also a shift of the feature F to lower energies in the middle of the crater (Fig 5e). The inset in Figure 5e shows the pre-edge features for selected positions along the edge. In the middle to end of the contact length (orange to yellow), there is a change in shape of the pre-edge features. The intensity of the feature at 4968 eV decreases indicating less contributions from neighboring Ti or Al atoms. Based on the discussion above, where we assigned the Ti-Ti or Ti-Al interaction as specific for the w-TiAlN phase, this is interpreted as that there are changes in the w-TiAlN phase during turning. The w-TiAlN phase has been observed to have a better thermal stability compared to the fcc-TiAlN phase [29], however, there is still a lack of knowledge on the influence of temperature and pressure of the stability of this phase. For the coarse-grained samples, a wurtzite phase forming during turning is expected to be free of Ti and can thus not be detected here [29].     The XANES spectra suggest that there are TiN-enriched domains forming also in the fine-grained sample, thus spinodal decomposition occurs also in the fcc-phase of this coating. The shift of feature F is apparent across the entire contact (Fig. 3), thus spinodal decomposition occurs for all measured positions. A fine-grained microstructure has previously been observed to lower the temperature Applied Surface Science 612, 155907, (2022) required for spinodal decomposition, and this might be the case also here, since the Al-content of the fine-grained fcc-phase is lower compared to the coarse-grained sample with similar overall Alcontent [29]. Figure 8 shows the XANES spectra from the Cr 1s and Fe 1s absorption edges. The small SEM micrograph in Figure 8e illustrates the position where the spectra were recorded (compare with Figure 3). The white arrow illustrates the increasing distance from the tool edge and has a length of 500 µm. The EDS map of the worn inserts reveal that both Fe and Cr species from the stainless steel are found across the contact area (Fig. 3). For Cr, the spectra change appearance across the tool edge. For the first two positions, recorded in the crater region, the spectra are similar to that of the workpiece material. At the end of the crater region and the end of contact (yellow and blue), the spectra look different and are more similar to that of Cr2O3 [30,31]. At the end of the contact length, there is access to oxygen from the surrounding air that can react with the tool and workpiece material, thus it is likely that Cr-O compounds form in this region. This has also been observed in our previous study [11]. This kind of compound can act as a protective layer during turning operation, reducing friction between coating and chip, and delaying crater formation [32,33]. Even further away from the edge (purple and pink), the spectra are again similar to that of the steel workpiece.
For Cr, the structure of the adhered material is like that of stainless steel in the crater region. At this position of the tool the temperature is expected to be low, thus reactions with oxygen from the air does not occur here. For the TiN coating, the signal is weak for the last two spectra indicating that only small amounts of Cr are present on the surface. For Fe, the spectra do not change across the contact length of the tool and is similar for both TiN and Ti0.47Al0.53N coatings. The spectra are similar to the spectra recorded for the 316L stainless steel used here (dashed line) as well as that of a similar stainless steel (SS304) [34]. Thus, the Fe adhered to the tool has an fcc structure similar to that of the original stainless steel and no Fe-O phases form.

Conclusion
Changes in atomic arrangements in the crater region of Ti1-xAlxN coated metal cutting tools during a The high sensitivity of XANES to the chemical composition of the fcc-TiAlN phase and the small beam size provided by the Balder beamline, enabling high spatial resolution, makes it a promising technique for identification of local phase changes in hard coatings.