Thermal stability of arc evaporated Al-Cr-O-N coatings

Accepted Manuscript Thermal stability of arc evaporated Al-Cr-O-N coatings

R. Raab, C.M. Koller, S. Kolozsvári, J. Ramm, P.H. Mayrhofer PII: DOI: Reference:

S0257-8972(18)31019-3 doi:10.1016/j.surfcoat.2018.09.036 SCT 23809

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

2 August 2018 14 September 2018 15 September 2018

Please cite this article as: R. Raab, C.M. Koller, S. Kolozsvári, J. Ramm, P.H. Mayrhofer , Thermal stability of arc evaporated Al-Cr-O-N coatings. Sct (2018), doi:10.1016/ j.surfcoat.2018.09.036

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ACCEPTED MANUSCRIPT Thermal Stability of Arc Evaporated Al-Cr-O-N Coatings

R. Raaba, C.M. Kollera,b,*, S. Kolozsváric, J. Rammd, P.H. Mayrhofera,b

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Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria b Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria c Plansee Composite Materials GmbH, Siebenbürgerstrasse 23, 86983 Lechbruck am See, Germany d Oerlikon Balzers, Oerlikon Surface Solutions AG, Iramali 18, 9496 Balzers, Liechtenstein

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Keywords: Al-Cr-O-N; oxynitride; thermal stability; cathodic arc evaporation

* Corresponding author:

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Christian M. Koller e-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract (AlxCr1-x)δ(O1-yNy)ξ coatings were arc evaporated using Al0.70Cr0.30 cathodes and different nitrogen-tooxygen gas flow ratios and investigated with respect to their phase evolution and thermal stability. The utilisation of specific nitrogen-to-oxygen ratios underlies elaborated cross-sectional transmission electron microscopy and selected area electron diffraction studies carried out on (Al,Cr)N–(Al,Cr)2O3– gradient

coatings.

Based on

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differential scanning calorimetry

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(Al,Cr)N

thermogravimetric analysis up to 1500 °C, in combination with subsequent X-ray powder diffraction,

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we can conclude, that the thermal stability of (AlxCr1-x)δ(O1-yNy)ξ oxynitrides increase with increasing O

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content, up to ~81 at.% of the non-metallic sublattice. Especially, the high O-containing coatings are

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prone for the formation of volatile Cr-oxides. However, the typically observed two-step dissociation process of Cr–N bonds towards Cr (via the formation of Cr2N and N2-release), is delayed to a one-step

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process (up to 1500 °C) for N-contents below 10 at.% of the non-metallic sublattice. Consequently, by knowledge-driven fine-tuning of the nitrogen-to-oxygen ratio, the thermal stability

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of (AlxCr1-x)δ(O1-yNy)ξ oxynitrides can be designed for their application-based specific needs.

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ACCEPTED MANUSCRIPT 1. Introduction Metal oxynitride coatings in general have recently attracted considerable attention due to their special physical and chemical properties, which make them promising candidates for new applications [1–12]. Here, especially the development of Al-Cr-based oxynitrides, has gained enormous interest as protective hard coatings in the last decades [13–17]. Their specific properties

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arise from their special mixture of ionic and covalent bonding contributions. When nitrogen

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substitutes for oxygen (or vice versa) at the non-metallic sublattice, maintaining a global charge

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neutrality (trivalent N3- replacing bivalent O2-) promotes the formation of vacancies, and/or interstitials or forces the metal ions to change their valance state, which directly influences the

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bonding states. For example Najafi et al. [16] reported that the substitution of nitrogen by oxygen leads to a decrease of the cubic lattice parameter a, due to the formation of an AlxCr1-x(OyN1-y) solid

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solutions with vacancies on the metallic sublattice. The maximum oxygen content of the non-metallic sublattice to still prepare single-phased face-centred cubic (fcc)-structured solid solutions can be as

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high as 97 at.%, depending on the deposition conditions. But for more than 60 at.% oxygen the

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hardness decrease from approximately 31.5±1.5 GPa (AlxCr1-xN) to 25.5±1 GPa, due to changed primary bonding contributions [16]. The high affinity of oxygen to the metallic species requires a

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rather high nitrogen content (of the reactive atmosphere during deposition) prepare oxynitride coatings with a high nitrogen content. Oxynitrides easily form with relatively low O2 contents of the

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reactive N2+O2 gas atmosphere [18]. Although, considerable research activities were carried out with the focus on quaternary Al-Cr-O-N hard coatings (driven by the possibility to stabilise the fcc structure even for high Al contents), only little is known about their thermal stability and phase evolution during annealing at elevated temperatures. Therefore, we developed (AlxCr1-x)δ(O1-yNy)ξ coatings with different N-to-O compositions y and studied their thermal stability (during annealing in inert atmosphere up to 1500 °C) using a combination of differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) with subsequent X-ray diffraction (XRD). 3

ACCEPTED MANUSCRIPT 2. Experimental All coatings investigated were synthesised in an Oerlikon Balzers Innova batch-type cathodic arc evaporation system equipped with powder metallurgically (PM) prepared Al0.70Cr0.30 cathodes (Plansee Composite Materials GmbH). The substrates, low alloyed iron steel foil, polycrystalline Al2O3 and Si (100) stripes (20x7x0.4 mm³), were ultrasonically cleaned in acetone and ethanol for 10

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minutes and then mounted on a two-fold rotation carousel with a minimum cathode-to-substrate

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distance of approximately 25 cm. The deposition chamber was heated to 500 °C by a combined

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radiation and plasma heating process for approximately 30 minutes. Ar ion etching, using the Balzers Central Beam Etching Technology, was performed for another 30 minutes, in order to remove any

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residuals and oxides from the substrate surfaces, and in further consequence to enhance adhesion. One (AlxCr1-x)N–(AlxCr1-x)2O3–(AlxCr1-x)N gradient-coating (transition from a nitride to an oxide and

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back to a nitride), five different (AlxCr1-x)δ(O1-yNy)ξ oxynitride coatings, as well as the corresponding (AlxCr1-x)N and (AlxCr1-x)2O3 coatings were deposited with four Al0.70Cr0.30 cathodes operated for 60

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minutes with an arc current of 180 A. The oxynitride coatings were prepared with N2/O2 reactive gas

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mixtures (gas flow ratios) of either 90/10, 85/15, 80/20, 75/25 or 70/30 at a pressure of 2.6±0.5 Pa. The gradient-coating was prepared by linear decreasing the nitrogen flow rate from 1000 to 0 sccm,

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while increasing the oxygen flow rate from 0 to 1000 sccm (within 30 min) and decreasing the oxygen flow back from 1000 to 0 sccm, while increasing the nitrogen flow rate from 0 to 1000 sccm (within

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another 30 min). In order to enhance growth kinetics, a negative substrate bias of -40 V DC was used for all coatings prepared. Chemical and morphological investigations of our coatings were carried out using a FEI Quanta 200 field emission gun scanning electron microscope (FEG SEM) with an energy dispersive X-Ray spectroscopy (EDS) detector attached. The morphology, chemistry, and microstructure od the gradient-coating was investigated in detail by transmission electron microscopy (TEM) including high-resolution TEM (HR-TEM), scanning TEM 4

ACCEPTED MANUSCRIPT (STEM), selected-area electron diffraction (SAED, aperture diameter ~160 nm), and EDS, using a TECNAI F20 FEG TEM operated with an acceleration voltage of 200 keV with an EDAX Apollo XLT2 EDS detector attached. The thermal stability of our coatings was investigated by a combination of DSC and TGA using a DSCSTA 449 F1 Jupiter®, calibrated with 7 elements (In, Sn, Bi, Zn, Al, Ag, Au). The dynamical

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measurements were carried out with a heating rate of 20 K/min up to 1500 °C in flowing He

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atmosphere (protective=50 ml/min; purge=50 ml/min). In order to remove volatile contaminations,

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an isothermal annealing for 20 min at 150 °C was conducted. The crucibles for the powder samples are made of Pt-Ir alloy with an α-Al2O3 inlay. An empty crucible (inlay included) was utilized as

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reference. Powdered coating material was used in order to avoid substrate interference during the DSC–TGA measurements. Therefore, the coatings were chemically removed from the iron foil

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substrate by dissolving it in 35 % hydrochloride acid at 70 °C for approximately 30 min. The remaining film material was rinsed with acetone and manually ground to powder.

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Coated polycrystalline α-Al2O3 substrates were annealed in a Centorr LF22-2000 vacuum furnace at

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annealing temperatures Ta ranging from 800 to 1500 °C in steps of 100 °C using a heating rate of 20 K/min, a holding time of 1 h. Cooling was performed passively, by switching off the heater.

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X-ray diffraction (XRD) was carried out in Bragg-Brentano arrangement (BB: 2θ=15–90°) for as-

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deposited and annealed powder samples, and in grazing-incidence arrangement (GI: angle of incidence Ω=2°, 2θ=15–90°) for as-deposited and annealed coatings on polycrystalline Al2O3 substrates using an Empyrean PANalytical θ-θ diffractometer with a Cu Kα radiation source and a scintillation-detector.

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Growth morphology, chemistry, and microstructure

The growth morphology of our gradient-coating, Fig. 1a shows a SEM cross section overview image, which contains dark and bright contrasted regions representing (Al,Cr)N and (Al,Cr)2O3, respectivley.

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Their intermediate transition zones thus correspond to (AlxCr1-x)δ(O1-yNy)ξ oxynitrides with different N-

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to-O compositions, y. The substrate near region of our coating (N-rich) is more compact (with a lower

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amount of droplets) than the columnar structured zone in the middle of the cross section (which is O-rich). This is confirmed by corresponding transmission electron microscopy cross-section studies,

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Fig. 1b. Selected area electron diffraction patterns of the transition zone from (Al,Cr)N to (Al,Cr)2O3 were obtained using a 160 nm aperture, placed at six positions (marked by numbered white circles)

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in the substrate near region directly next to each other, Fig. 1c. These diffraction patterns (presented inf Figs. 1d, e, f, g, h, and I with corresponding TEM cross sections) indicate a single-phase face-

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centred cubic structure for thes regions, with a change from random-like towards pronounced (200)

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and (220) orientation with increasing distance from the substrate.

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Fig. 1: Cross-sectional overview images of our (Al,Cr)N–(Al,Cr)2O3–(Al,Cr)N gradient coating by (a) SEM and (b,c) TEM. TEM details of the regions indicated in (c) are given in (d, e, f, g, h, and i) with corresponding SAED insets.

STEM-EDS measurements, Fig. 2, demonstrate that the nitride-to-oxide transition zone of the ~4.4 µm thick gradient-coating is already completed after 1 µm distance from the substrate. Within this 1

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µm, the nitrogen content decreases from ~30 to ~4 at.%, while the oxygen content increases from

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~10 to ~56 at.%. The Al and Cr content decrease from ~42 to ~28 at.% and ~20 to 13 at.%, respectively. After the first 1 µm, the chemical composition is nearly unchanged up to ~3.5 µm. This clearly highlights the much higher reactivity of oxygen to metal species than nitrogen.

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Fig. 2: (a) Cross-sectional STEM image with (b) EDS-linescan of the (Al,Cr)N–(Al,Cr)2O3–(Al,Cr)N gradient coating (total coating thickness of ~4.4 µm).

The (SEM-EDS obtained) chemical composition of our (AlxCr1-x)δ(O1-yNy)ξ oxynitride coatings, prepared

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with different N2/O2 gas flow rations, is presented in Table 1. Although, sample A was prepared with a very high N2/O2 gas flow ratio of 90/10 (%-ratio), their oxygen content is already ~16.0 at.%. This

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again (in addition to the EDS-linescan of our nitride-oxide-nitride gradient coating, Fig. 2) exhibits the much higher reactivity of O2 than N2 with the metallic species (Al and Cr). With further increasing the

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O2 content of the reactive atmosphere, hence decreasing the N2/O2-ratio to 70/30 (sample E), the

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oxygen content within our oxynitride increases to 54.5 at% and the nitrogen content decreases to 5.0 at%. Simultaneously, the cation-to-anion ratio [represented by (Al+Cr)/(O+N) in Table 1] decreases

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from 0.9 to 0.68. But, although the O-content and N-content of our (AlxCr1-x)δ(O1-yNy)ξ oxynitrides continuously increases and decreases, respectively, with decreasing N2/O2-ratio, their Al/(Al+Cr)

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ratio, x, is nearly unchanged at ~0.6, see Table 1 and Fig. 3b. Only the oxynitride prepared with the lowest N2/O2-ratio (70/30) has a slightly higher Al/(Al+Cr)-ratio of x ~0.63. This is also the coating, which shows already a significant formation of fcc-(Al,Cr)2O3 like phases and α-(Al,Cr)2O3, see Fig. 3a.

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ACCEPTED MANUSCRIPT Table 1. Elemental composition of monolithically grown (Al xCr1-x)δ(O1-yNy)ξ obtained by SEM-EDS. sample

chemical composition [at.%] Al 27.9 26.8 25.6 24.2 25.7

Cr 19.3 17.6 17.5 16.2 14.8

O 16.0 25.5 35.0 48.3 54.5

N 36.7 30.0 21.9 11.3 5.0

𝐀𝐥 (𝐀𝐥 + 𝐂𝐫)

𝐂𝐫 (𝐀𝐥 + 𝐂𝐫)

𝐎 (𝐎 + 𝐍)

𝐍 (𝐎 + 𝐍)

[%] 59.1 60.3 59.4 60.0 63.4

[%] 40.9 39.7 40.6 40.0 36.6

[%] 30.3 45.9 61.6 81.0 91.6

[%] 70.7 54.1 38.4 19.0 8.4

(𝐀𝐥 + 𝐂𝐫) (𝐎 + 𝐍) 0.90 0.80 0.76 0.68 0.68

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A B C D E

N2/O2 in gas flow [%] 90/10 85/15 80/20 75/25 70/30

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The XRD patterns in Fig. 3a show that up to a N2/O2-ratio of 80/20 the (AlxCr1-x)δ(O1-yNy)ξ oxynitrides

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basically crystallise in the fcc-(Al,Cr)N–like structure, please see the (111), (200), and (220) peaks at 2θ ~38, 45, and 65°, respectively (indicated by red solid squares in Fig. 3a). For N2/O2-ratios of 75/25

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and 70/30, especially the (111) XRD peaks are hardly detectable. The decreasing intensity of the (111) XRD peak intensity with increasing O-content of our oxynitrides, might be related to the

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formation of X-ray amorphous phases or/and changed surface energies. Castaldi et al. [19]

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suggested, that for fcc-structured CrNyO1-y, the surface energy of (111) planes increases with the oxygen content, because substituting N with O would increases the polarization due to the higher electronegativity. Consequently, (111) planes are less preferable for higher O contents and thus the

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(111) XRD peaks decrease in intensity.

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With increasing O-content, the XRD peak positions at 2θ ~38, 45 and 65° continuously shift to higher diffraction angles, from fcc-(Al,Cr)N towards fcc-(Al,Cr)2O3. Consequently, this indicates decreasing lattice parameters (presented in Fig. 4) and a change from fcc-(Al,Cr)N–like to fcc-(Al,Cr)2O3 like phases, which also suggest for the formation of metal vacancies (for the B1-like NaCl-type structure). This is in agreement with the concomitant reduction of the cation-to anion ratio, Table 1, and recent results by Stüber et al. [13] and Khatibi et al. [14] reporting the formation of fcc-structured Al–Cr–O– N and Al–Cr–O thin films, respectively. The comparison with the (Al,Cr)N coating, prepared with comparable deposition conditions (shown at the bottom of Fig. 3a, same deposition system, 9

ACCEPTED MANUSCRIPT Al0.70Cr0.30 cathodes, bias potential, deposition pressure, substrate temperature) in N2 atmosphere (hence, N2/O2 flow ratio of 100/0) clearly highlights that already small additions of oxygen promote the crystallisation in fcc structure. Without oxygen, a significant fraction of wurtzite structured wAlN–based phases are present. This behaviour is in agreement with other density functional theory calculations, highlighting that the fcc structure (contrary to hexagonal structured counterparts) is

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relatively insensitive for vacancies. These are preferred, when oxygen substitutes for nitrogen to keep the charge balance (N3- vs. O2-), which is also indicated by the above-mentioned decrease in

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cation-to anion ratio in parallel.

When evaluating the lattice parameters from the SAED patterns taken during the cross sectional

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studies of our (Al,Cr)N–(Al,Cr)2O3–(Al,Cr)N gradient coating, and correlating these regions with the N2/O2 flow rate present during deposition, we obtain an excellent correlation with the lattice

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parameters of our (AlxCr1-x)δ(O1-yNy)ξ oxynitrides, see Fig. 4.

Fig. 3: (a) XRD patterns and (b) chemical composition of powdered (AlxCr1-x)δ(O1-yNy)ξ oxynitride coatings. The XRD patterns are labelled with the N2/O2 gas flow ratio (in %) used during their deposition. 10

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Fig. 4: Lattice parameter of the fcc structure of our (AlxCr1-x)δ(O1-yNy)ξ coatings and of the transition zone in our nitride-oxide-nitride gradient coating obtained from the XRD and SAED patterns, respectively.

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ACCEPTED MANUSCRIPT 3.2 Thermal stability and phase evolution

The base-line subtracted DSC (heat flow) and TGA (mass loss) signals of the five different (AlxCr1x)δ(O1-yNy)ξ

oxynitrides during annealing in He atmosphere up to 1500 °C, show reduced endothermic

contributions and significantly reduced mass-loss, especially for the coatings prepared with N2/O2-

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ratios of 75/25 and 70/30 (hence, O-rich), see Fig. 5. Based on previous studies on the thermal stability of (Al,Cr)N [20–22] and (Al,Cr)2O3 [23][24], we can

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conclude that the exothermic DSC features up to ~1000 °C, basically stem from recovery processes of

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deposition-induced structural defects. The superimposed endothermic features, for example for the

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highest N-containing oxynitride (prepared with N2/O2 = 90/10), with peak temperatures of ~1150 and 1180 °C are due to dissociation of Cr–N bonds to form hexagonal close packed (hcp) Cr2N and body

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centred cubic (bcc) Cr, respectively, via N2-release [21,25]. The connected two-step N2-release (from CrN to Cr2N to Cr) can be seen by the mass-loss curve having two different slopes, Fig. 5, due to the

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different bonding states of nitrogen in fcc-CrN and hcp-Cr2N. The rather strong exothermic DSC

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features between 1300 and 1400 °C are due to grain growth, recrystallization, and/or sintering processes. (The comparison between DSC and TGA shows that the TGA signal is more suitable to rate the thermal stability of our (AlxCr1-x)δ(O1-yNy)ξ oxynitrides with respect to their dissociation of Cr–N

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bonds and the connected N2-release).

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The comparison of the TGA curves of the five different (AlxCr1-x)δ(O1-yNy)ξ oxynitrides suggest, that the coatings prepared with N2/O2-ratios of 90/10, 85/15, and 80/20, end up with virtually the same massloss after annealing up to 1500 °C, Fig 5. This is somehow surprising as their N-content is different. The coating prepared with N2/O2-ratios of 75/25 and 70/30 exhibit significantly lower mass-loss of ~4.7 and < 1 %, respectively, according to their lower N-content. These coatings also show the highest onset temperature of ~1150 °C for the mass-loss.

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Fig. 5: DSC and TGA signals of powdered (AlxCr1-x)δ(O1-yNy)ξ oxynitride coatings during annealing in He atmosphere up to 1500 °C.

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The structural investigations by XRD of these five different (AlxCr1-x)δ(O1-yNy)ξ oxynitrides prepared with N2/O2-ratios of 90/10, 85/15, 80/20, 75/25, and 70/30, Figs. 6a, b, c, d, and e, respectively, show

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their phase transformations annealing at selected temperatures. The three N-rich oxynitrides show a

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comparable transformation of their fcc structure with increasing Ta. With increasing oxygen content, the formation of w-AlN phases is postponed to higher temperatures, see Figs. 6a, b, and c. In the intermediate temperature ranges (1100–1300 °C) the formation of hcp-Cr2N phases can be detected,

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which upon further annealing transform to bcc-Cr, in agreement with their significant mass-loss

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curves. After annealing at 1500 °C, these coatings exhibit a significant fraction of metallic bcc-Cr, wAlN, and α-Al2O3. The formation of the latter can be detected after annealing at Ta = 1500 °C, their α(Al,Cr)2O3 phases are rather Al-rich, as indicated by their XRD peak position being close to α-Al2O3. Contrary to these observations, the two O-rich coatings prepared with N2/O2-ratios of 75/25 and 70/30, Figs. 6d and e, respectively, exhibit significant smaller fractions of metallic bcc-Cr after annealing at Ta = 1500 °C (indicated by their rather low XRD peak intensities). For the oxynitride with highest oxygen content (prepared with N2/O2 = 70/30), almost no bcc-Cr but still some hcp-Cr2N can be detected for Ta = 1500 °C, see the small XRD peak at 2θ ~ 40° in Fig. 6e. Furthermore, these two 13

ACCEPTED MANUSCRIPT oxynitrides exhibit a significant formation of crystalline α-(Al,Cr)2O3 phases for Ta ≥ 1120 °C. This temperature corresponds to the exothermic feature during their DSC investigations (compare Fig. 5). Due to their lower fraction of metallic bcc-Cr present, the XRD peak positions for these α-(Al,Cr)2O3 phases, are rather close to the calculated position for a α-(Al0.70Cr0.30)2O3 solid solution. Hence, there is a higher Cr content incorporated in the α-(Al,Cr)2O3 phases than for the other three oxynitride

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coatings (Figs. 6a, b, and c).

Fig. 6: XRD patterns of our (AlxCr1-x)δ(O1-yNy)ξ oxynitrides prepared with N2/O2-ratios of (a) 90/10, (b) 85/15, (c) 82/20, (d) 75/25, and (e) 70/30. The individual patterns are labelled with the annealing temperatures Ta.

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70/30), respectively.

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Fig. 7: Estimated phase composition as a function of the annealing temperature Ta of the (AlxCr1x)δ(O1-yNy)ξ oxynitrides prepared with N2/O2-ratios of (a) 90/10 and (b) 70/30.

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To further proof the high thermal stability of the O-rich (AlxCr1-x)δ(O1-yNy)ξ oxynitrides, we have vacuum-annealed coated polycrystalline Al2O3 substrates at Ta ≤ 1500 °C (for 1 h), and subsequently measured the structural evolution, see Fig. 8b-c and Fig. 9a-c (Fig. 8a shows the XRD patterns of

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annealed monolithically grown (Al,Cr)N). For the reader’s convenience the measurement of an empty

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substrate is added at the bottom. The (Al,Cr)N coating in as-deposited state crystallise in a single phase wurtzite-type structure, as evident from XRD peaks at 33.2, 36.2, 38.1, and 66.1° 2θ. Generally, with increasing the annealing temperature to 1500 °C these peaks narrowing and increase in intensity due to recovery processes of built-in defects as well as grain growth [21]. At Ta=800 °C small fractions of h-Cr2N as well as metallic Cr (or a Cr(Al) solid solution) appear, indicated by peaks at 40.1 and 42.5° 2θ as well as 44.3 and 64.4° 2θ, respectively. The formation of h-Cr2N exhibit a maximum at Ta=1000 °C before it gradually decreases to 1200 °C, as a result of further dissociation of h-Cr2N to Cr. At elevated annealing 15

ACCEPTED MANUSCRIPT temperatures between 1100 and 1500 °C a slight shift of the w-AlN peaks to lower diffraction angles can be detected, accompanied by a simultaneous decrease of Cr peaks and once again an increase in h-Cr2N at 1500 °C. This behaviour might be explained by the temperature enhanced diffusion rate and high driving forces. We envision that Cr diffuses to the growing grain boundaries of w-AlN, leading to a temporary formation of a w-(Al,Cr)N solid solution, which in turn dissociates again to h-

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Cr2N, as it is indicated by small fraction of h-Cr2N without Cr phases present at 1500 °C.

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Fig. 8: XRD evolution of monolithically grown (a) (Al,Cr)N, (b) (Al,Cr)(O,N)90/10 (sample A), and (c) (Al,Cr)(O,N)85/15 (sample B), annealed at Ta=800–1500 °C in vacuum for 1 h, respectively.

XRD patterns of our (Al,Cr)(O,N) coating with the highest nitrogen and consequently lowest oxygen

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content (sample A) are shown in Fig. 8b. In the as-deposited state the (Al,Cr)(O,N) peaks can be assigned to (111), (200), and (220) at position 38.2, 44.5, and 64.6° 2θ, respectively. At Ta=900 °C (Al,Cr)(O,N) dissociates into α-Al2O3, indicated by increasing substrate peaks, w-AlN (33.3, 36.2, 38.0, and 66.0° 2θ, solid black pentagons), h-Cr2N (37.5 and 42.8° 2θ, half-filled blue hexagons), and Cr (44.2 and 64.4° 2θ, solid grey diamonds). At Ta=1300 °C there is no Cr detectable, indicating the formation of volatile chromium-oxides (with excess O in the coatings [23] ) as we have seen in our previous study for monolithically grown (Al,Cr)2O3 [24]. Increasing the annealing temperature to 1500 °C α-Al2O3 as well as w-AlN peaks narrow and increase in intensity, indicating grain growth. XRD 16

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bonds with Al, to form α-Al2O3, due to the higher affinity of Al to O than Cr has to O [18]. The formation of Cr-oxides requires further O. The formation of an (Al,Cr)2O3 solid solution is more

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pronounced for vacuum annealed sample D and E, see Fig. 9b and Fig. 9c, respectively.

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Fig. 9: XRD evolution of monolithically grown (a) (Al,Cr)(O,N)80/20 (sample C), (b) (Al,Cr)(O,N)75/25 (sample D), and (c) (Al,Cr)(O,N)70/30 (sample E), annealed at Ta=800–1500 °C in vacuum for 1 h, respectively. The XRD measurements of the annealed sample E, Fig. 9c, indicate the formation of a corundum-type α-(Al,Cr)2O3 solid solution between 900 and 1200 °C, by XRD peaks at 34.7, 37.2, 42.7, 65.5 and 67.2° 2θ and metallic Cr fractions (44.3 and 64.6° 2θ, solid grey diamonds). Increasing the annealing temperature to 1400 °C and beyond result in an XRD peak shift of α-(Al,Cr)2O3 to higher diffraction angles thus towards the α-Al2O3 reference, suggesting for an depletion of Cr of the (Al,Cr)2O3 solid solution.

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Fig. 10: Hardness and indentation modulus of the (AlxCr1-x)δ(O1-yNy)ξ oxynitrides prepared with N2/O2ratios of 75/25.

The hardness of the O-rich (AlxCr1-x)δ(O1-yNy)ξ oxynitride coating (on Sapphire substrates) prepared

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with N2/O2-ratios of 75/25 decreases with increasing annealing temperature from ~ 19 GPa (asdeposited value) to ~ 13 GPa (Ta ≥ 1000 °C), while the indentation modulus of ~275 GPa stays

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relatively constant up to even 1500 °C, see Fig. 10. The hardness decrease after annealing Ta = 1000

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°C can be assigned to the decomposition of Cr–N bonds and thus the dissociation of CrN to Cr2N and further to Cr via N2-release. The transformation of fcc-(Al,Cr)2O3 towards α-(Al,Cr)2O3 at this

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temperature might effectively help to keep the hardness relatively constant up to 1500 °C.

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ACCEPTED MANUSCRIPT Summary and conclusions Cathodic arc evaporated monolithically grown gradient-layer—with a continuous transition from (Al,Cr)N to (Al,Cr)2O3 and back to (Al,Cr)N—as well as (AlxCr1-x)δ(O1-yNy)ξ coatings were synthesised using powder metallurgically prepared Al0.70Cr0.30 cathodes. The transition zone from nitride to oxide was investigated in detail by TEM-SAED and STEM-EDS, to understand the structural and chemical

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evolution taking place through continuous transitions or blurred interfaces between individual

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(Al,Cr)N and (Al,Cr)2O3 layers.

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The (AlxCr1-x)δ(O1-yNy)ξ films could then be deposited with specifically tailored structural and chemical properties and studied in detail for their thermal stability.

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According to DSC/TGA results we can conclude that, in the case of (Al,Cr)(O,N), the thermal stability

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increases, with increasing O fraction. The decomposition of CrN into h-Cr2N and subsequently to bccCr, via N2 release, can be successfully delayed from ~1090 to 1150 °C.

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For the powder annealed (AlxCr1-x)δ(O1-yNy)ξ , the cubic (Al,Cr)(O,N) phase—within the film with the

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highest N and lowest O fractions (sample A)—decomposes into w-AlN, α-Al2O3 and metallic Cr fractions, while the oxynitride coating with the highest O fraction (sample E) dissociates into

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corundum-type (Al,Cr)2O3 solid solution and small metallic Cr fractions at Ta=1500 °C, respectively. A depletion of the corundum-type (Al,Cr)2O3 solid solution with Cr of the (AlxCr1-x)δ(O1-yNy)ξ films with

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high O fraction (sample D and sample E) deposited on polycrystalline Al2O3 substrates after annealing at 1300 °C and beyond for 1 h was found by XRD investigations.

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ACCEPTED MANUSCRIPT Acknowledgements The financial support by the Austrian Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged. We also thank for the financial support of Plansee Composite Materials GmbH and Oerlikon Balzers, Oerlikon Surface Solutions AG. We also acknowledge the support by the X-Ray Center of the TU Wien Austria

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during as well as by the University Service Centre for Transmission Electron Microscopy, TU Wien

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Austria.

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ACCEPTED MANUSCRIPT Highlights:

Monolithically grown (AlxCr1-x)δ(O1-yNy)ξ with different oxygen contents

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Formation of single-phase cubic structured (AlxCr1-x)δ(O1-yNy)ξ

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Increasing thermal stability with increasing oxygen content

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