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TaAlN coatings
Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings C.M. Koller, R. Hollerweger, C. Sabitzer, R. Rachbauer, S. Kolozsv´ari, J. Paulitsch, P.H. Mayrhofer PII: DOI: Reference:
S0257-8972(14)00923-2 doi: 10.1016/j.surfcoat.2014.10.024 SCT 19816
To appear in:
Surface & Coatings Technology
Received date: Accepted date:
16 August 2014 8 October 2014
Please cite this article as: C.M. Koller, R. Hollerweger, C. Sabitzer, R. Rachbauer, S. Kolozsv´ ari, J. Paulitsch, P.H. Mayrhofer, Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.10.024
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ACCEPTED MANUSCRIPT Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings C. M. Kollera,b,*, R. Hollerwegera,b, C. Sabitzera,b, R. Rachbauerc, S. Kolozsvárid, J. Paulitsche,
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P. H. Mayrhofera,e
Christian Doppler Laboratory for Application Oriented Coating Development at the Institute
of Materials Science and Technology, Vienna University of Technology, Vienna, Austria Christian Doppler Laboratory for Application Oriented Coating Development at the
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Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben,
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Leoben, Austria
Oerlikon Balzers Coating AG, Balzers, Liechtenstein
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Plansee Composite Materials GmbH, Lechbruck am See, Germany
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Institute of Materials Science and Technology, Vienna University of Technology, Vienna,
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Austria
Keywords: Titanium nitride; Tantalum nitride; Arc evaporation; Multilayer, Thermal stability
* Corresponding author: Christian Martin Koller e-mail: [email protected]
ACCEPTED MANUSCRIPT Abstract Phase stability and oxidation resistance are main objectives when synthesising hard and protective coatings for applications requiring also high thermal stability. Even though TiAlN
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is a well-studied and nowadays widely used high performance coating, the demand for further
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optimisation is omnipresent. Recent investigations on quaternary compounds demonstrate that
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the alloying of Ta to TiAlN films not only results in enhanced phase stability, but also in a significantly increased oxidation resistance. In this study we address thermal investigations of
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reactive cathodic arc evaporated coatings and elucidate the diverse performance of monolithically grown TiAlN, TaAlN, TiAlTaN, and a multilayered architecture of TiAlN and
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TaAlN layers. Subtle variations of the bilayer period between 30 and 38 nm were realised by varying the arc current at the TaAl cathode.
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Our research demonstrates that the quaternary Ti0.45Al0.36Ta0.19N and the multilayered
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TiAlN/TaAlN coatings feature enhanced mechanical properties and thermal stability as
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compared with their monolithically grown constituents Ti0.54Al0.46N and Ta0.89Al0.11N. All coatings synthesised exhibit as-deposited hardness values of 32±1 GPa, but only the
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quaternary Ti0.45Al0.36Ta0.19N and the multilayered TiAlN/TaAlN coatings demonstrate pronounced age-hardening with peak hardness values of 37±2 and 35±2 GPa for annealing temperatures of ~1000 and 1100 °C, respectively. Complementary X-ray diffraction and differential scanning calorimetry confirm their enhanced phase stability. Even though, also the multilayered design shifts the formation of wurtzite-structured AlN to higher temperatures, only quaternary Ti0.45Al0.36Ta0.19N could withstand ambient air oxidation at 850 °C for 20 h. This is based on the ability of forming a nearly single-phased dense protective mixed oxide scale, having an outermost Al-rich composition. Approximately 70 % of this quaternary nitride remained unaffected from oxidation. With the present study we conclusively demonstrate that thermomechanical properties of cathodic arc deposited TiAlN coatings can
ACCEPTED MANUSCRIPT significantly be enhanced by either forming a quaternary compound or sophisticated
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architectural design with tantalum, both allow for wide-ranged industrial applications.
ACCEPTED MANUSCRIPT 1. Introduction The development of protective hard coatings for forming and machining application is still in focus of extensive research activities to increase the performance and service lifetime of
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thereby protected tools [1–6]. On this account several approaches, including alloying concepts
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as well as sophisticated architectural design on the nanoscale, are conducted [7,8]. Ternary transition metal nitrides such as Ti1-xAlxN [9,10] or Cr1-xAlxN [11,12] exceed their binary counterparts in numerous properties like hardness and thermal stability [13]. Nonetheless,
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there is still an industrially driven demand for further improvements in terms of thermomechanical properties and oxidation resistance. Both are crucial requirements for high
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performance coatings utilised in forming or machining application [14–16].
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Ti1-xAlxN (for reasons of simplicity we will use TiAlN and similar terms to name our
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coatings) as one of the most prominent and widely used material system for protective coating applications, is still intensively studied in order to further improve the properties and hence
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the performance [8,17,18]. On this account, the formation of quaternary Ti1-x-yAlxMyN compounds (M being a 4 or 5d transition metal element) turns out to be a promising approach.
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Already small quantities of Ta within the supersaturated Ti1-xAlxN matrix successfully improve the hardness and retard the onset temperature for decomposition towards the stable constituents face-centred cubic TiN (B1-structured, NaCl prototype, abbreviated with c-TiN) and hexagonal close-packed AlN (B4-structured, wurtzite-ZnS prototype, therefore abbreviated with w-AlN) [19–23]. Another crucial requirement for hard protective coatings is the resistance against oxidation, especially at high temperatures and in aggressive environment. For instance, Reddy et al. [20] described the positive effect of Ta which can specifically be used to adapt the coatings performance by promoting the formation of a dense stable Al2O3 outer layer, reducing the in- and outward diffusion processes during high temperature oxidation of a Ti3Al-Ta alloy.
ACCEPTED MANUSCRIPT Sophisticated film architecture is another topic of great interest for wear protective coatings. Multilayers, superlattices, and nanocomposites are already successfully deployed to optimise and exceed the performance of conventional coatings in industry. Here, for example increased
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hardness or enhanced thermal stability can be realised by atomistic mechanisms at the very
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interface between two different material systems or crystallographic domains [24–31]. The focus of the present study is set on the mechanical and thermal properties as well as oxidation resistance of architectural designed TiAlN/TaAlN multilayer coatings which are
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systematically compared to their monolithically grown constituents TiAlN and TaAlN, and a
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cathodic arc evaporation (CAE).
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monolithically grown quaternary TiAlTaN films. All coatings were synthesised by reactive
ACCEPTED MANUSCRIPT 2. Experimental TiAlN, TaAlN, TiAlTaN, and multilayered TiAlN/TaAlN coatings were grown in an industrial-scaled batch-type Oerlikon Balzers Innova CAE facility, using powder-
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metallurgical (PM) produced Ti0.5Al0.5, Ta0.75Al0.25, and Ti0.45Al0.45Ta0.10 targets (PLANSEE
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CM GmbH). The substrates – low-alloyed steel foil, silicon and alumina stripes (Si (100), 20 × 7 × 0.38 mm3; Al2O3 (polycrystalline), 20 × 7 × 0.4 mm3) – were ultrasonically cleaned in acetone and ethanol for five minutes prior to mounting on a two-fold rotating carousel with a
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minimum substrate-to-target distance of ~25 cm. The deposition temperature of 500 °C was achieved by a combined plasma and radiator heating process. Substrate etching was
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conducted for 25 minutes in pure Ar atmosphere using the Oerlikon Balzers Central Beam Etching technology. A cathode current of 140 A was set for the cathodic arc evaporation of
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the monolithically grown TiAlN and TiAlTaN coatings, and 120 A for the TaAlN cathode.
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Variations in the bilayer period of our multilayer coatings were realised by powering the
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Ta0.75Al0.25 target at ITaAl = 100, 120, and 140A while keeping the current of the Ti0.5Al0.5 cathode constant at ITiAl = 140 A for all depositions. The substrate bias voltage was set to -40 V. An Ar and N2 flow rate of 400 and 1000 sccm, respectively, was used resulting in a
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process pressure of approximately 4.7 × 10−2 mbar (4.7 Pa). Vacuum annealing of coated Al2O3 stripes and powdered coating material (by chemical removal from the coated low-alloyed steel foil in diluted nitric acid) was conducted in a HTM Reetz vacuum furnace up to annealing temperatures Ta of 1400 °C in steps of 100 °C. A heating rate of 20 °C·min−1, a holding time of one minute and a cooling rate of 50 °C·min−1 was used. Additionally, we performed differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of powdered sample material in a Setaram Labsys EVO. The process included two equal heating steps, which were separated by a 60 minutes resting time at a base temperature of 50 °C. The samples were heated up in He atmosphere to 1500 °C with a heating rate of 20 °C min−1, held for one minute and cooled to room temperature
ACCEPTED MANUSCRIPT with approximately 50 °C min−1 was used. Additionally, investigations on the oxidation resistance were conducted on coated Al2O3 stripes and powdered coating material, which were annealed in ambient air at 850 °C for 20 h using a Nabertherm N11/HR furnace.
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Structural analyses were carried out by X-ray diffraction (XRD) in Bragg-Brentano
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arrangement using a Bruker-AXS D8 Advance equipped with a CuKα radiation source as well
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as a PANalytical XPert Pro MPD diffractometer. Hardness measurements on as-deposited and vacuum annealed samples were performed by the use of an Ultra-Micro-Indentation II system
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equipped with a Berkovich diamond tip. The applied normal loads were decreased from 20 to 2 mN with steps of 1 mN. Attention was paid to assure indentation depths below 10 % of the
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coating thickness. Hardness values were calculated from the load-displacement curves according to Oliver and Pharr [32]. Scanning electron microscopy (SEM) as well as
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investigations on the chemical composition by energy-dispersive X-ray spectroscopy (EDS)
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were obtained by a Zeiss EVO 50 SEM with an attached Oxford Instruments INCA EDS
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detector (acceleration voltage of 20 keV). Cross-sectional SEM micrographs and EDS investigations of the elemental distributions across the oxidised samples were carried out on embedded and mechanically polished samples. In order to protect the oxidised layers and the
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oxide scale formed during the polishing procedure a CrN layer was sputter-deposited on top of the samples. Detailed investigation on the morphology and microstructure was obtained by transmission electron microscopy (TEM) in a Philips CM12 operating with an acceleration voltage of 120 kV, and a FEI TECNAI F20 operated with 200 kV.
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3. Results and discussion
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3.1 Morphology and chemical composition
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All monolithically grown coatings, TiAlN, TaAlN, and TiAlTaN, exhibit a fine fibrous structure, as can be seen in SEM cross-sections provided in Figs. 1a-c, respectively. However, TiAlN indicates a slightly increased columnar character, whereas the other films show a
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smoother cross-section. Surface irregularities arise from the presence of macroparticles which
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are usually observed in non-filtered arc evaporation depositions [33,34]. Based on the overall film thickness and deposition time we obtain growth rates of 33, 27, and 31 nm min-1. Cross-
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sections of our TiAlN/TaAlN multilayers, processed with ITaAl = 100, 120, and 140 A are
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shown in Figs. 1d to f, respectively, indicate a fine and dense film morphology, comparable to
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their monolithically grown constituents. Bright field cross-section TEM studies of multilayered TiAlN/TaAlN coatings prepared with constant arc currents at the TiAl cathode (ITiAl = 140 A) combined with variations in the arc
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current at the TaAl cathode (ITaAl = 100, 120, and 140 A, see Figs. 1g-i, respectively), show a well-defined layered structure. The individual Ti1-xAlxN and Ta1-yAlyN layers can clearly be distinguished by their brighter and darker contrast, respectively. Evidence for epitaxial growth between TiAlN and TaAlN is given by the high resolution (HR) TEM inset in Fig. 1h. Estimated bilayer periods based on the growth rate obtained from monolithically grown coatings are in excellent agreement with the TEM investigations. The bilayer period (TiAlNlayer + TaAlN-layer thickness) of our multilayer coatings processed with ITaAl = 100, 120, and 140 A are 32±3, 35±3, and 36±3 nm, respectively. The TEM images also indicate thin horizontal lines within the TiAlN-layers, which stem from the two-fold-rotation of the substrate holder representing localised variations in chemical composition and microstructure.
ACCEPTED MANUSCRIPT This finding is most likely based on variations in particle flux and plasma density as well as resputtering effects [35]. Chemical analyses obtained by SEM-EDS are summarised in Table I. For the monolithically
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grown coatings the Al/Ti ratio (1.17) of our Ti0.54Al0.46N coating is comparable with the
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Ti0.5Al0.5 targets (Al/Ti = 1.00) used. Contrary the Ta0.89Al0.11N coatings with an Al/Ta ratio of 0.12 and the Ti0.45Ta0.36Ta0.19N coatings with Ti/Ta ratios of 1.89 and Al/Ta ratios of 2.36 are lower than their respective targets with Al/Ta = 0.33 for the Ta0.75Al0.25 and Ti/Ta = Al/Ta
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= 4.5 for the Ti0.45Al0.45Ta0.10 targets. The differences between coatings and targets stem from gas scattering and resputtering effects during film formation and the hugely different atomic
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masses of Ti, Al, and Ta. Therefore, the coatings contain more Ta than the respective target, for example the Ti1-x-yAlxTayN films contain even 19 at% Ta on the metallic sublattice
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although the target contained only 10 at%. Changes in cathode current ITaAl mainly influences
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the growth rate of our TaAlN coatings and not their morphology or chemical composition.
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The multilayered TiAlN/TaAlN coatings exhibit an almost even distribution of Al, Ti, and Ta on the metallic sublattice. Here, we have to emphasise that the values obtained for our
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multilayered coatings represent an overall composition measured through several bilayers. Therefore, the increase in current at the Ta-rich TaAl cathode (ITaAl = 100, 120, and 140 A)— while keeping the TiAl target powering constant (ITiAl = 140 A)—is mainly visible by the change in Ti to Ta ratio. These results suggest that the chemistry of the individual TiAlN and TaAlN layers of the TiAlN/TaAlN multilayers is similar to the monolithically grownTi1xAlxN
and Ta1-yAlyN films, respectively.
ACCEPTED MANUSCRIPT 3.2 Thermal stability
The X-ray diffraction pattern of as-deposited Ti0.54Al0.46N, see Fig. 2a, demonstrates a single-
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phased face-centred cubic (c, B1, NaCl-type) solid solution structure with well-defined 111
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(2θ ~37.5 deg.) and 200 (2θ ~43.5 deg.) peaks. Vacuum annealing of these samples up to Ta = 900 °C shows no significant changes in the XRD patterns, see Fig. 2a. First evidence for a decomposition of the supersaturated cubic solid solution towards the stable constituents c-TiN
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and hexagonal (B4, ZnS-wurtzite-type) w-AlN is given by the formation of left-hand shoulders of the 111 and 200 peaks. These peaks indicate the formation cubic Ti-rich nitride
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domains, having a larger lattice parameter than the coating matrix. Based on previous investigations by Rachbauer et al. using atom probe tomography [8], the formation of such Ti-
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rich domains also results in the formation of Al-rich domains within the TiAlN matrix. A
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further increase in annealing temperature to Ta ≥ 1300 °C clearly results in the formation of
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w-AlN, indicated by emerging XRD peaks at 2θ ~33.6 and 38 deg., respectively. The supplementary pattern of the powdered Ti0.54Al0.46N coating material obtained after DSC measurements (presented later) to 1500 °C finally demonstrates the complete decomposition
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of the material into its stable constituents c-TiN and w-AlN. Figure 2b shows the XRD patterns of as-deposited and vacuum annealed Ta0.89Al0.11N in which we can also identify a single-phase cubic structure with pronounced 111 and 200 peaks. Upon annealing to 1100 °C neither significant peak shifting nor the formation of additional peaks can be detected. However, for Ta = 1200 °C a pronounced phase transformation to mainly hexagonal ε-TaN occurred (2θ ~34.5 and 37 deg.) with small fractions of hexagonal Ta2N (2θ ~39 deg.). Further annealing results in continuous phase transformation due to nitrogen loss and precipitation of hexagonal Ta2N on the expanse of TaN. After annealing at Ta = 1400 °C only hexagonal Ta2N can be detected, which is confirmed by the XRD pattern
ACCEPTED MANUSCRIPT of the sample after DSC measurements to 1500 °C. The formation of stable w-AlN phase, due to its small phase fraction is hardly detectable especially next to Ta-N phases. Ti0.45Al0.36Ta0.19N (Fig. 2c) was grown in a single-phased cubic structure with pronounced
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111 and 200 orientations. Thermal annealing at Ta ≥ 800 °C causes a collective shift of the
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XRD peaks towards lower diffraction angles. This peak shift proceeds until 1400 °C, where tiny XRD peaks at the position for w-AlN (at 2θ ~33 and 38 deg.) can be detected. After the DSC measurement to 1500 °C the formation of w-AlN can clearly be identified by XRD. The
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collective shift of the major XRD peaks of the matrix (TiAlTaN) towards smaller diffraction angles—between c-TiN and δ-TaN—combined with the formation of w-AlN phases suggests
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for strain reduction and decomposition processes. Strain reduction is obtained by the thermally activated rearrangement and annihilation of structural built-in defects to lower-
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energy sites. The formation of AlN is the result of a decomposition of the supersaturated
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Ti0.45Al0.36Ta0.19N matrix towards c-Ti1-zTazN and w-AlN. The formation of a solid solution
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type c-Ti1-zTazN—z corresponds to the chemical composition of the coating with z = y/(1-x) with y being the Ta content and x being the Al content of our coating—is additionally
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responsible for the shift of the major XRD peaks towards the position between c-TiN and δTaN. Ab initio calculations suggest that the cubic-structured Ti1-zTazN is stable for chemical compositions of approximately 0.2 ≤ z ≤ 0.8 even at 0 K [36]. The formation of a hexagonal Ta2N phase is not detectable within the annealing range up to 1500 °C. Comparing the structural evolution after heat-treatment of our monolithically arc-evaporated Ti0.54Al0.46N and Ti0.45Al0.36Ta0.19N coatings, we can confirm that this 5d element tantalum increases the thermal stability as suggested earlier for reactive cathodic arc evaporated [21] as well as for sputter deposited coatings [22]. XRD patterns of vacuum annealed TiAlN/TaAlN multilayers—prepared with constant ITiAl = 140 A combined with ITaAl = 100, 120, and 140 A, Figs. 3a, b, and c, respectively—indicate a single-phased cubic structure—comparable to the monolithically grown films. The initially
ACCEPTED MANUSCRIPT broader 111 and 200 XRD peaks originate from overlapping c-TiAlN and c-TaAlN reflections of the individual layers as well as from additional lattice-mismatch-induced coherency strains. Annealing of the substrate-free and powdered coating samples leads to progressive sharpening
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of the XRD peaks. This is the consequence of reduced microstrains due to recovery of
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structural defects, grain growth, and inter-diffusion processes at the interfaces between the
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TiAlN and TaAlN layers, which reduces coherency strains. For annealing temperatures above Ta ~1200 °C the XRD peaks are nearly symmetric and much narrower as in the as-deposited
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state, indicative for an intermixing of the individual layers. Corresponding to the results obtained for the monolithically grown TiAlTaN coatings (see above and Fig. 2c), the major
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matrix XRD peaks shift to lower diffraction angles—towards values between c-TiN and δTaN. The concomitant formation of w-AlN, see the small XRD peak at 2θ ~33 deg. for Ta ≥
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1300 °C indicates decomposition of the supersaturated phases towards their stable
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constituents w-AlN and c-Ti1-zTazN. For Ta ≥ 1500 °C a N2-loss (as obtained by DSC-TGA
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measurements, see next paragraph) leads to the formation of hex-Ta2N, as indicated by the emerging XRD peaks at 2θ ~33.9 and 38.8 deg. The XRD patterns of the coatings after DSC measurements to 1500 °C, at the top of each column in Fig. 3, clearly show that the
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predominant phase of the coatings is c-Ti1-zTazN with small fractions of wurtzite AlN and hexagonal Ta2N. The three different multilayers prepared with ITaAl = 100, 120, and 140 A, Figs. 3a, b, and c, exhibit a similar structural evolution with annealing temperature. Only the phase fraction of hexagonal Ta2N—formed during annealing—increases with increasing power at the TaAl cathode. The DSC spectrum for monolithically grown single-phased cubic Ti0.54Al0.46N (Fig. 4a, black curve) shows pronounced exothermic features up to ~1200 °C. There is no clear separation of the individual reactions on-going, other than obtained for sputter deposited coatings [8]. The decrease in mass between Ta = 900–1000 °C is based on nitrogen loss. A review of published data, especially DSC-TGA data of Ti1-xAlxN coatings [2,37,38], shows that this is regularly
ACCEPTED MANUSCRIPT accompanied by the formation of w-AlN. Hence, we suggest that this type of N2-loss is a strong indicator for the formation and growth of w-AlN phases. The DSC curve of monolithically grown Ta0.89Al0.11N coatings, orange line in Fig. 4a,
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suggests only for minor exothermic contributions in the temperature range 600–800 °C
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followed by a more pronounced endothermic feature for Ta ≥ 1200 °C. The latter is due to nitrogen loss as confirmed by the TGA obtained mass reduction. The detected ―two-step‖ process for N2-release agrees with the XRD observations made for Ta0.89Al0.11N. Between
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1100 and 1200 °C the structure of face-centred cubic TaAlN changes to form ε-TaN, see Fig. 2b. Based on the results available, we conclude that this is triggered by the out-diffusion of
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AlN form the supersaturated TaAlN phase. As shown by previous studies, small additions of Al to TaN stabilise the δ-TaN phase over the ε-TaN phase [39]. Consequently, if Al diffuses
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out, this stabilisation effect is reversed and hence ε-TaN becomes favourable. Thus, nitrogen
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is released from a δ-TaN-based phase at the beginning to be followed by N2-release from an
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ε-TaN-based phase, resulting in the observed two-step N2-release. Increasing N2-release leads to the further formation of hex-Ta2N at the expanse of TaN.
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The monolithically grown Ti0.45Al0.36Ta0.19N coating exhibits only small exothermic contributions to the DSC curve spanning over a broad temperature range, violet line in Fig. 5a. The corresponding XRD investigations suggest stress relaxation and grain growth (indicated by the small shift in position and sharpening of the XRD peaks) as the major origin for these exothermic features up to Ta = 1100 °C. For higher temperatures, the supersaturated phases decompose to form AlN and cubic Ti1-zTazN. Our multilayered coatings clearly exhibit at least two exothermic features during DSC measurements up to 1500 °C in He, see Fig. 4b. Between 600 and 800 °C thermally induced recovery of structural built-in defects takes place, which is comparable to the results obtained for our monolithically grown coatings. The exothermic feature between 1100 and 1200 °C can
ACCEPTED MANUSCRIPT be associated with inter-diffusion of the adjacent TiAlN and TaAlN layers—based on the XRD studies. The subsequent pronounced endothermic feature for Ta ≥ 1300 °C is due to N2release—please compare the corresponding TGA signals showing a mass loss for these
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temperatures. The N2-release is mainly caused by the transformation of TaAlN layers to form
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Ta2N, see the corresponding XRD patterns in Fig. 3. In this context an increasing TaAlN layer
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thickness results in a stronger and faster endothermic reaction, as observed for the multilayers prepared with ITaAl = 100, 120, and 140 A, see the green, blue, and red DSC and TGA curves
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in Fig. 4b, respectively. However, the onset temperature of approximately 1300 °C for this endothermic reaction shows no pronounced dependence on the layer thickness. The multilayer
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arrangement of TiAlN and TaAlN layers with bilayer periods between 30 and 38 nm exhibit an improved phase stability and resistance against N2-loss compared with their monolithically
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grown components Ti0.54Al0.46N and Ta0.89Al0.11N.
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The hardness of monolithically grown single-phased cubic Ti0.54Al0.46N is around 31 GP from
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the as-deposited state up to annealing temperatures Ta of 600 °C (black squares in Fig. 5a). The hardness decrease for higher temperatures to ~22 GPa at 1000 °C ≤ Ta ≤ 1200 °C is due
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to the decomposition of the supersaturated Ti0.54Al0.46N phase towards its stable constituents c-TiN and w-AlN. We prepared all coatings with relatively low bias potential of -40 V, therefore, our Ti0.54Al0.46N coatings exhibits slightly open and under-dense column boundaries. These are preferred sites for decomposition of the supersaturated matrix [40], as they provide higher diffusion as well as reduced retarding forces against changes in specific volume due to decomposition and precipitation [41]. Consequently, the precipitation of wurtzite-structured AlN—with its huge volume increase by ~26 %—is accelerated for coatings with open and under-dense column boundaries. This in turn leads to the observed hardness decrease between 900 and 1000 °C. The formation of c-TiN can already be detected in this temperature range by XRD, see Fig. 2a, which implies that also AlN-rich areas—often difficult to detect by XRD—need to develop [42].
ACCEPTED MANUSCRIPT Monolithically grown single-phased cubic Ta0.89Al0.11N coatings (orange circles in Fig. 5a) exhibit an as-deposited hardness of 30±2 GPa, which remains even for annealing treatments up to ~1100 °C. This is in excellent agreement with our XRD investigations, see Fig. 2.
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Further increase in Ta results in a pronounced hardness decrease caused by the decomposition
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and transformation of the single-phased cubic matrix into ε-TaN, hex-Ta2N, and w-AlN, as
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discussed in the previous section.
Monolithically grown single-phased cubic Ti0.45Al0.36Ta0.19N (purple triangles in Fig. 5a)
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exhibits an as-deposited hardness value of 32±2 GPa, similar to Ti0.54Al0.46N. But here, the asdeposited hardness value is maintained upon annealing to ~800 °C. For Ta ≥ 800 °C we even
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observe an increase in hardness due to the first onset of decomposition processes into Ti-rich and Al-rich cubic domains resulting in a hardness peak value of ~37 GPa for Ta = 1000 °C.
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Such a pronounced age-hardening effect of our quaternary Ti0.45Al0.36Ta0.19N coating is in
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good agreement with previous reports by Rachbauer et al. for sputtered coatings [22]. Beyond
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the peak hardness, we observe a slight decrease to approximately 33±1 GPa at Ta = 1100 °C, which is still about 3 GPa above that of the monolithically grown Ta0.89Al0.11N coating and
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almost 10 GPa above the hardness of Ti0.54Al0.46N. The as-deposited hardness of all TiAlN/TaAlN multilayer variations is between 30 and 32 GPa and is maintained up to annealing temperatures of ~900 °C, see Fig. 5b. The first significant hardness increase starts at Ta ≥ 900 °C for the multilayers prepared with ITaAl = 100 and 120 A. Due to spallation no hardness measurements could be performed for the coatings deposited with ITaAl = 100 A after annealing at temperatures of 1100 and 1200 °C. Nevertheless, the increasing hardness values with increasing the annealing temperature from 800 to 1000 °C suggest that the annealing behaviour of this coating is similar to the multilayer coating deposited with ITaAl = 120 A. The latter exhibits a peak hardness of 35 GPa with Ta = 1100 °C. The multilayer prepared with 140 A at the Ta0.75Al0.25 cathode does not feature a pronounced age-hardening effect but behaves like the monolithically grown Ta0.89Al0.11N
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Ta = 1100 °C.
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3.3 Oxidation resistance
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The monolithically grown Ti0.54Al0.46N and Ta0.89Al0.11N coatings are fully oxidised after 20 h
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exposure to ambient air at 850 °C. Their fracture cross-sections, Figs. 6a and b, show the typical morphology of a porous oxide scale with reduced adhesion to the substrate. Also their
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multilayered TiAlN/TaAlN arrangements prepared with 100, 120, or 140 A at the Ta0.75Al0.25 cathode are completely oxidised after 20 h exposure to ambient air at 850 °C. Only the
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monolithically grown quaternary Ti0.45Al0.36Ta0.19N coating was still intact after this treatment. Therefore, the latter is studied in more detail by cross-sectional EDS line-scan
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analysis and compared to TiAlN/TaAlN multilayered coatings prepared with ITaAl = 120 A,
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Figs. 7a and b, respectively. To prevent spallation of the oxide scale during preparation—
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mechanically grinding and polishing—for the SEM-EDS analysis a supporting CrN layer was deposited onto the oxidation-treated samples. Consequently, a Cr signal can be detected
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between embedding material and oxide scale. The monolithically grown quaternary Ti0.45Al0.36Ta0.19N coating exhibits an ~1 µm thick dense oxide scale with an inhomogeneous chemical composition, Fig. 7a. The outermost oxide region (~0.25 µm thickness) is higher in Al and lower in Ti and Ta content as the ~0.75 µm thick oxide region underneath. These dense oxide layers, and especially the dense Al 2O3based outermost region, effectively protect the nitride layer underneath against further oxidation. Consequently, almost 70 % of the monolithically grown Ti0.45Al0.36Ta0.19N coating remains intact after 20 h exposure to ambient air at 850 °C. The oxygen content of this remaining nitride coating—with an as-deposited-like chemical composition—is below the detection limit of the EDS system used. These results for cathodic arc evaporated
ACCEPTED MANUSCRIPT Ti0.45Al0.36Ta0.19N are in excellent agreement with reactively sputter deposited Ta-alloyed Ti1xAlxN
coatings [21,22].
EDS line-scans of our multilayered TiAlN/TaAlN coatings, Fig. 7b, exhibit and evenly
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distributed Al, Ti, Ta, and high oxygen content throughout the entire coating thickness.
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Nitrogen can only be detected in the region of the CrN supporting layer, hence, the initial multilayered TiAlN/TaAlN nitride coating is completely oxidised.
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Ti0.54Al0.46N coatings clearly exhibit a multi-phased composition of rutile TiO2, corundumtype Al2O3, and possibly small phase fractions of anatase TiO2 after oxidation, see Fig. 8.
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Ta0.89Al0.11N mainly transforms into Ta2O5 with a minor phase fraction of α-Al2O3. For the monolithically grown Ti0.45Al0.36Ta0.19N coating the major oxide phase is rutile TiO2-based.
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Also here, only small indications for a crystalline α-Al2O3 structure can be detected, see the
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small XRD peaks at 2θ ~25.5 and 43 deg. The cross-sectional EDS analyses also suggest the formation of an Al2O3-based phase, as the outermost region of the oxide scale is Al-rich. It is
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important to mention, that in contrast to coated Al2O3 substrates, no evidence for a remaining nitride phase is given here. For the freestanding powdered coating material, used for these
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XRD analyses, the oxidation condition is way more demanding, but helps identifying the oxide phases formed. The XRD patterns of our multilayered coatings, exhibiting a homogeneous chemical composition—of Al, Ti, Ta, and O—across the entire oxide scale, indicate only the formation of a rutile TiO2-based phase with minor phase fractions of anatase. Based on the homogeneous chemical composition and the absence of an XRD peak at 2θ ~43 deg. we assign the XRD peak at 2θ ~ 25.5 deg. to an anatase TiO2 rather than an α-Al2O3based phase. This is similar for all three different multilayered coatings prepared with ITaAl = 100, 120, and 140 A and in agreement with recent studies by Hollerweger et al. showing, also referring to ab initio data, that for specific chemical compositions between Ti, Ta, and Al, single-phased, rutile-based oxides can form [43].
ACCEPTED MANUSCRIPT The observation of an enrichment of Al, indicative for a protective outermost oxide scale, for the TiAlTaN coating and a homogeneous distribution of Al for the multilayered coatings (Fig. 7a, and b) may be explained by the different constitution of the oxides as well as the film
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architecture. The monolithically grown TiAlTaN coatings exhibit a denser morphology than
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the TiAlN coatings, suggesting for fewer high diffusion pathways along (underdense) grain
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and column boundaries. The results reported in Ref. [43] furthermore show that due to the addition of Ta the formation of the stable phase rutile during oxidation is promoted. Without
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tantalum, anatase forms initially on TiAlN, which then transforms upon further oxidation to the rutile phase. The associated volume change very often leads to the formation of a large
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crack network, which can clearly be reduced or even avoided by the addition of tantalum. For the multilayered TiAlN/TaAlN coatings, we have to consider the oxidation behaviour of the
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individual monolithically grown TiAlN and TaAlN coatings. The distinct peak shift to smaller
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2θ positions of the Ta2O5 phase formed on TaAlN in combination to the missing indications
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for an Al2O3 phase, Fig. 8, suggests that Al is incorporated to the Ta2O5 structure, hence not available to form a stable oxide. The large volume increase by the formation of Ta2O5 based oxides leads to the formation of underdense open porous oxides. The TiAlN layers will then
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also not be able to form dense oxides. This can explain the homogenous chemical composition (within the detection limit of the SEM-EDS system used) of the oxide formed.
ACCEPTED MANUSCRIPT Conclusions We have developed cathodic arc evaporated quaternary Ti0.45Al0.36Ta0.19N and multilayered TiAlN/TaAlN coatings and compared their mechanical properties, thermal stabilities, and
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oxidation resistance with their monolithically grown constituents Ti0.54Al0.46N and
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Ta0.89Al0.11N coatings.
All coatings exhibit a single-phased cubic solid solution in the as-deposited state, which is
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stable up to 900 and 1100 °C for the monolithically grown coatings Ti0.54Al0.11N and Ta0.89Al0.11N, respectively. Their multilayered combination to TiAlN/TaAlN coatings (with
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bilayer periods of 30 to 38 nm) exhibit an even higher thermal stability during vacuum annealing, as first indications for a w-AlN formation can be detected for Ta above 1300–1400
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°C, independent of the TaAlN layer thicknesses. Corresponding results are also obtained for
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the monolithically grown quaternary Ti0.45Al0.36Ta0.19N coatings. Furthermore, this quaternary coating does not show any Ta2N formation even when annealed at 1500 °C. Contrary, the
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Ta0.89Al0.11N and the multilayered TiAlN/TaAlN coatings, especially with thicker TaAlN layers, show a Ta2N formation due to N2-release when annealed to Ta ≥ 1200 and 1400 °C,
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respectively.
All coatings investigated exhibit as-deposited hardness values of around 32±2 GPa. However, only the quaternary Ti0.45Al0.46Ta0.19N coating and the TiAlN/TaAlN multilayers (prepared with ITaAl = 100 and 120 A) feature pronounced age-hardening for Ta = 1000–1100 °C with peak hardness values of 35 to 37 GPa. This is a clear improvement when compared to their monolithically grown counterpart coatings Ti0.45Al0.46N and Ta0.89Al0.11N, which exhibit hardnesses below 30 GPa for Ta ≥ 1000 °C. Based on long-term oxidation tests at 850 °C in ambient air, we can conclude that tantalum within the monolithically grown Ti0.45Al0.36Ta0.19N coatings is responsible for the formation of a dense and protective single-phased oxide scale with an Al-enrichment at the outermost
ACCEPTED MANUSCRIPT region. This is not the case for the other coatings, hence they are fully oxidised after the 20 h exposure to air at 850 °C. According to our results we conclude that quaternary Ti0.45Al0.36Ta0.19N coatings and
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TiAlN/TaAlN multilayers feature enhanced mechanical properties and thermal stability with
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respect to their monolithically grown counterpart coatings Ti0.54Al0.46N and Ta0.89Al0.11N. But a significant increase in oxidation resistance could only be achieved for the quaternary
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Ti0.45Al0.36Ta0.19N coatings.
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
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acknowledged. We also thank for the financial support of Plansee Composite Materials
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GmbH and Oerlikon Balzers Coating AG. We also acknowledge the support by the X-Ray Center of the Vienna University of Technology Austria during as well as by the University Service Centre for Transmission Electron Microscopy, Vienna University of Technology
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Austria.
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[31] X. Chu, S.A. Barnett, Model of superlattice yield stress and hardness enhancements, J. Appl. Phys. 77 (1995) 4403.
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[35] A.O. Eriksson, J.Q. Zhu, N. Ghafoor, M.P. Johansson, J. Sjölen, J. Jensen, et al., Layer formation by resputtering in Ti–Si–C hard coatings during large scale cathodic arc deposition, Surf. Coatings Technol. 205 (2011) 3923–3930.
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[38] I.A. Abrikosov, A. Knutsson, B. Alling, F. Tasnádi, H. Lind, L. Hultman, et al., Phase Stability and Elasticity of TiAlN, Materials (Basel). 4 (2011) 1599–1618. [39] Z. Zhang, Impact of Al on structure and mechanical properties of NbN and TaN, Montanuniversität Leoben, 2011. [40] R. Rachbauer, S. Massl, E. Stergar, P. Felfer, P.H. Mayrhofer, Atom probe specimen preparation and 3D interfacial study of Ti–Al–N thin films, Surf. Coatings Technol. 204 (2010) 1811–1816. [41] P.H. Mayrhofer, F.D. Fischer, H.J. Böhm, C. Mitterer, J.M. Schneider, Energetic balance and kinetics for the decomposition of supersaturated Ti1−xAlxN, Acta ater. 55 (2007) 1441–1446. [42] R. Rachbauer, E. Stergar, S. Massl, M. Moser, P.H. Mayrhofer, Three-dimensional atom probe investigations of Ti–Al–N thin films, Scr. Mater. 61 (2009) 725–728. [43] R. Hollerweger, H. Riedl, J. Paulitsch, M. Arndt, R. Rachbauer, P. Polcik, et al., Origin of High Temperature Oxidation Resistance of Ti-Al-Ta-N Coatings, (2014).
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[44] Q. Li, I.W. Kim, S.A. Barnett, L.D. Marks, I. Introduction, Structures of AlN/VN Superlattices with Different AlN Layer Thicknesses, J. Mater. Res. 17 (2011) 1224– 1231.
ACCEPTED MANUSCRIPT Tables
Table I: Metallic sublattice population of the monolithically (TiAlN, TaAlN, and TiAlTaN)
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and multilayered (TiAlN/TaAlN) grown coatings obtained by energy-dispersive X-ray
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spectroscopy.
Cathode powering [A]
Al [at%]
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Monolithic coatings1 ITiAl = 140 ITiAlTa = 140 TiAlN/TaAlN multilayers²
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ITaAl = 100
Ta [at%]
54
46
-
11
-
89
45
36
19
36
38
26
34
34
32
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32
35
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ITaAl = 120 ITaAl = 140
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ITaAl = 120
Ti [at%]
The Ti0.5Al0.5 and Ti0.45Al0.45Ta0.10 cathodes were powered with 140 A, and the
Ta0.75Al0.25 cathode was powered with 120 A. Constant powering of the Ti0.5Al0.5 cathode with ITiAl = 140 A and varying
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powering of the Ta0.75Al0.25 cathode with ITaAl = 100, 120, and 140 A.
ACCEPTED MANUSCRIPT List of figure captions
Fig. 1: Cross–section SEM images of monolithically grown (a) TiAlN (ITiAl = 140 A), (b)
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TaAlN (ITaAl = 120 A), and (c) TiAlTaN (ITiAlTa = 140 A) thin films. (d), (e), and (f) are cross-
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section SEM images of multilayered TiAlN/TaAlN coatings prepared with constant ITiAl =
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140 A combined with ITaAl = 100, 120, and 140 A, respectively. The corresponding TEM images of these multilayers with ITaAl = 100, 120, and 140 A are shown in (g), (h), and (i),
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respectively.
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Fig. 2: XRD patterns of arc evaporated monolithically grown (a) Ti0.54Al0.46N, (b) Ta0.89Al0.11N, and (c) Ti0.45Al0.36Ta0.19N coatings in the as-deposited state and after vacuum
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annealing at Ta. The patterns are obtained from powdered coating materials—removed from
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their low-alloyed steel substrates—see Experimental. The 2θ standard positions for δ-TaN
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(lattice parameter a = 4.339 Å, ICDD: 03-065-9404), c-TiN (a = 4.247 Å, ICDD: 00-0381420), c-AlN (calculated for a = 4.06 Å [44]), wurtzite AlN (lattice parameters a = 3.11 Å and c = 4.98 Å, ICDD: 01-076-0702), ε-TaN (a = 5.196 Å, c = 2.911 Å, ICDD: 01-071-0253), and
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hex-Ta2N (a = 3.044 Å, c = 4.914 Å, ICDD: 00-026-0985) are indicated. Fig. 3: XRD patterns of vacuum annealed multilayer TiAlN/TaAlN coatings prepared with constant ITiAl = 140 A combined with ITaAl = (a) 100, (b) 120, and (c) 140 A. The XRD pattern of the multilayered coatings after DSC measurements to 1500 °C are added (top-most pattern). The 2θ positions for δ-TaN (a = 4.339 Å, ICDD: 03-065-9404), c-TiN (a = 4.247 Å, ICDD: 00-038-1420), c-AlN (calculated for a = 4.06 Å [44]), wurtzite w-AlN (a = 3.11 Å, c = 4.98 Å, ICDD: 01-076-0702), and hex-Ta2N (a = 3.044 Å, c = 4.914 Å, ICDD: 00-026-0985) are indicated.
ACCEPTED MANUSCRIPT Fig. 4: Heat flow (baseline subtracted) and relative mass as a function of the annealing temperature, Ta, during DSC/TGA investigations of (a) monolithically grown Ti0.54Al0.46N, Ta0.89Al0.11N, and Ti0.45Al0.36Ta0.19N coatings, and (b) multilayered TiAlN/TaAlN coatings
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prepared with constant ITiAl = 140 A combined with ITaAl = 100, 120, and 140 A.
Fig. 5: Hardness evolution as a function of the vacuum annealing treatments at T a of (a)
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monolithically grown Ti0.54Al0.46N, Ta0.89Al0.11N, and Ti0.45Al0.36Ta0.19N coatings, and (b) multilayered TiAlN/TaAlN coatings prepared with constant ITiAl = 140 A combined with ITaAl
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= 100, 120, and 140 A.
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Fig. 6: Monolithically grown (a) Ti0.54Al0.46N and (b) Ta0.89Al0.11N coatings after oxidation in
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ambient air at 850 °C for 20 h.
Fig. 7: EDS line-scan and back-scattered SEM cross-section of monolithically grown (a)
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Ti0.45Al0.36Ta0.19N and (b) multilayered TiAlN/TaAlN (ITaAl = 120 A) coatings after oxidation in ambient air at 850 °C for 20 h. The cross-sections are polished.
Fig. 8: XRD pattern of oxidised (20 h at 850 °C in ambient air) powdered coating material. ICCD standard positions for rutile TiO2 (green diamonds, ICDD: 00-001-1292), anatase TiO2 (green squares, ICDD: 00-021-1272), corundum-type Al2O3 (red hexagons, ICDD: 01-0760702), and Ta2O5 (blue circles, ICDD: 00-019-1298) are indicated. The top three XRD patterns are oxidised TiAlN/TaAlN multilayer coatings prepared with 100, 120, and 140 A at the TaAl cathode.
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ACCEPTED MANUSCRIPT Highlights:
Comparison of monolithically grown and multilayered CAE coatings
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Ti0.45Al0.36Ta0.19N and TiAlN/TaAlN coatings exhibit enhanced thermal stability
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Pronounced age-hardening of 37±2 and 35±2 GPa between 1000 and 1100 °C
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Only monolithic Ti0.45Al0.36Ta0.19N withstands 20 h oxidation tests at 850 °C
S0257-8972(14)00923-2 doi: 10.1016/j.surfcoat.2014.10.024 SCT 19816
To appear in:
Surface & Coatings Technology
Received date: Accepted date:
16 August 2014 8 October 2014
Please cite this article as: C.M. Koller, R. Hollerweger, C. Sabitzer, R. Rachbauer, S. Kolozsv´ ari, J. Paulitsch, P.H. Mayrhofer, Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.10.024
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ACCEPTED MANUSCRIPT Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings C. M. Kollera,b,*, R. Hollerwegera,b, C. Sabitzera,b, R. Rachbauerc, S. Kolozsvárid, J. Paulitsche,
a
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P. H. Mayrhofera,e
Christian Doppler Laboratory for Application Oriented Coating Development at the Institute
of Materials Science and Technology, Vienna University of Technology, Vienna, Austria Christian Doppler Laboratory for Application Oriented Coating Development at the
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b
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben,
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Leoben, Austria
Oerlikon Balzers Coating AG, Balzers, Liechtenstein
d
Plansee Composite Materials GmbH, Lechbruck am See, Germany
e
Institute of Materials Science and Technology, Vienna University of Technology, Vienna,
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Austria
Keywords: Titanium nitride; Tantalum nitride; Arc evaporation; Multilayer, Thermal stability
* Corresponding author: Christian Martin Koller e-mail: [email protected]
ACCEPTED MANUSCRIPT Abstract Phase stability and oxidation resistance are main objectives when synthesising hard and protective coatings for applications requiring also high thermal stability. Even though TiAlN
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is a well-studied and nowadays widely used high performance coating, the demand for further
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optimisation is omnipresent. Recent investigations on quaternary compounds demonstrate that
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the alloying of Ta to TiAlN films not only results in enhanced phase stability, but also in a significantly increased oxidation resistance. In this study we address thermal investigations of
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reactive cathodic arc evaporated coatings and elucidate the diverse performance of monolithically grown TiAlN, TaAlN, TiAlTaN, and a multilayered architecture of TiAlN and
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TaAlN layers. Subtle variations of the bilayer period between 30 and 38 nm were realised by varying the arc current at the TaAl cathode.
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Our research demonstrates that the quaternary Ti0.45Al0.36Ta0.19N and the multilayered
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TiAlN/TaAlN coatings feature enhanced mechanical properties and thermal stability as
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compared with their monolithically grown constituents Ti0.54Al0.46N and Ta0.89Al0.11N. All coatings synthesised exhibit as-deposited hardness values of 32±1 GPa, but only the
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quaternary Ti0.45Al0.36Ta0.19N and the multilayered TiAlN/TaAlN coatings demonstrate pronounced age-hardening with peak hardness values of 37±2 and 35±2 GPa for annealing temperatures of ~1000 and 1100 °C, respectively. Complementary X-ray diffraction and differential scanning calorimetry confirm their enhanced phase stability. Even though, also the multilayered design shifts the formation of wurtzite-structured AlN to higher temperatures, only quaternary Ti0.45Al0.36Ta0.19N could withstand ambient air oxidation at 850 °C for 20 h. This is based on the ability of forming a nearly single-phased dense protective mixed oxide scale, having an outermost Al-rich composition. Approximately 70 % of this quaternary nitride remained unaffected from oxidation. With the present study we conclusively demonstrate that thermomechanical properties of cathodic arc deposited TiAlN coatings can
ACCEPTED MANUSCRIPT significantly be enhanced by either forming a quaternary compound or sophisticated
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architectural design with tantalum, both allow for wide-ranged industrial applications.
ACCEPTED MANUSCRIPT 1. Introduction The development of protective hard coatings for forming and machining application is still in focus of extensive research activities to increase the performance and service lifetime of
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thereby protected tools [1–6]. On this account several approaches, including alloying concepts
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as well as sophisticated architectural design on the nanoscale, are conducted [7,8]. Ternary transition metal nitrides such as Ti1-xAlxN [9,10] or Cr1-xAlxN [11,12] exceed their binary counterparts in numerous properties like hardness and thermal stability [13]. Nonetheless,
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there is still an industrially driven demand for further improvements in terms of thermomechanical properties and oxidation resistance. Both are crucial requirements for high
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performance coatings utilised in forming or machining application [14–16].
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Ti1-xAlxN (for reasons of simplicity we will use TiAlN and similar terms to name our
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coatings) as one of the most prominent and widely used material system for protective coating applications, is still intensively studied in order to further improve the properties and hence
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the performance [8,17,18]. On this account, the formation of quaternary Ti1-x-yAlxMyN compounds (M being a 4 or 5d transition metal element) turns out to be a promising approach.
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Already small quantities of Ta within the supersaturated Ti1-xAlxN matrix successfully improve the hardness and retard the onset temperature for decomposition towards the stable constituents face-centred cubic TiN (B1-structured, NaCl prototype, abbreviated with c-TiN) and hexagonal close-packed AlN (B4-structured, wurtzite-ZnS prototype, therefore abbreviated with w-AlN) [19–23]. Another crucial requirement for hard protective coatings is the resistance against oxidation, especially at high temperatures and in aggressive environment. For instance, Reddy et al. [20] described the positive effect of Ta which can specifically be used to adapt the coatings performance by promoting the formation of a dense stable Al2O3 outer layer, reducing the in- and outward diffusion processes during high temperature oxidation of a Ti3Al-Ta alloy.
ACCEPTED MANUSCRIPT Sophisticated film architecture is another topic of great interest for wear protective coatings. Multilayers, superlattices, and nanocomposites are already successfully deployed to optimise and exceed the performance of conventional coatings in industry. Here, for example increased
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hardness or enhanced thermal stability can be realised by atomistic mechanisms at the very
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interface between two different material systems or crystallographic domains [24–31]. The focus of the present study is set on the mechanical and thermal properties as well as oxidation resistance of architectural designed TiAlN/TaAlN multilayer coatings which are
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systematically compared to their monolithically grown constituents TiAlN and TaAlN, and a
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cathodic arc evaporation (CAE).
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monolithically grown quaternary TiAlTaN films. All coatings were synthesised by reactive
ACCEPTED MANUSCRIPT 2. Experimental TiAlN, TaAlN, TiAlTaN, and multilayered TiAlN/TaAlN coatings were grown in an industrial-scaled batch-type Oerlikon Balzers Innova CAE facility, using powder-
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metallurgical (PM) produced Ti0.5Al0.5, Ta0.75Al0.25, and Ti0.45Al0.45Ta0.10 targets (PLANSEE
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CM GmbH). The substrates – low-alloyed steel foil, silicon and alumina stripes (Si (100), 20 × 7 × 0.38 mm3; Al2O3 (polycrystalline), 20 × 7 × 0.4 mm3) – were ultrasonically cleaned in acetone and ethanol for five minutes prior to mounting on a two-fold rotating carousel with a
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minimum substrate-to-target distance of ~25 cm. The deposition temperature of 500 °C was achieved by a combined plasma and radiator heating process. Substrate etching was
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conducted for 25 minutes in pure Ar atmosphere using the Oerlikon Balzers Central Beam Etching technology. A cathode current of 140 A was set for the cathodic arc evaporation of
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the monolithically grown TiAlN and TiAlTaN coatings, and 120 A for the TaAlN cathode.
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Variations in the bilayer period of our multilayer coatings were realised by powering the
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Ta0.75Al0.25 target at ITaAl = 100, 120, and 140A while keeping the current of the Ti0.5Al0.5 cathode constant at ITiAl = 140 A for all depositions. The substrate bias voltage was set to -40 V. An Ar and N2 flow rate of 400 and 1000 sccm, respectively, was used resulting in a
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process pressure of approximately 4.7 × 10−2 mbar (4.7 Pa). Vacuum annealing of coated Al2O3 stripes and powdered coating material (by chemical removal from the coated low-alloyed steel foil in diluted nitric acid) was conducted in a HTM Reetz vacuum furnace up to annealing temperatures Ta of 1400 °C in steps of 100 °C. A heating rate of 20 °C·min−1, a holding time of one minute and a cooling rate of 50 °C·min−1 was used. Additionally, we performed differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of powdered sample material in a Setaram Labsys EVO. The process included two equal heating steps, which were separated by a 60 minutes resting time at a base temperature of 50 °C. The samples were heated up in He atmosphere to 1500 °C with a heating rate of 20 °C min−1, held for one minute and cooled to room temperature
ACCEPTED MANUSCRIPT with approximately 50 °C min−1 was used. Additionally, investigations on the oxidation resistance were conducted on coated Al2O3 stripes and powdered coating material, which were annealed in ambient air at 850 °C for 20 h using a Nabertherm N11/HR furnace.
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Structural analyses were carried out by X-ray diffraction (XRD) in Bragg-Brentano
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arrangement using a Bruker-AXS D8 Advance equipped with a CuKα radiation source as well
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as a PANalytical XPert Pro MPD diffractometer. Hardness measurements on as-deposited and vacuum annealed samples were performed by the use of an Ultra-Micro-Indentation II system
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equipped with a Berkovich diamond tip. The applied normal loads were decreased from 20 to 2 mN with steps of 1 mN. Attention was paid to assure indentation depths below 10 % of the
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coating thickness. Hardness values were calculated from the load-displacement curves according to Oliver and Pharr [32]. Scanning electron microscopy (SEM) as well as
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investigations on the chemical composition by energy-dispersive X-ray spectroscopy (EDS)
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were obtained by a Zeiss EVO 50 SEM with an attached Oxford Instruments INCA EDS
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detector (acceleration voltage of 20 keV). Cross-sectional SEM micrographs and EDS investigations of the elemental distributions across the oxidised samples were carried out on embedded and mechanically polished samples. In order to protect the oxidised layers and the
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oxide scale formed during the polishing procedure a CrN layer was sputter-deposited on top of the samples. Detailed investigation on the morphology and microstructure was obtained by transmission electron microscopy (TEM) in a Philips CM12 operating with an acceleration voltage of 120 kV, and a FEI TECNAI F20 operated with 200 kV.
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3. Results and discussion
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3.1 Morphology and chemical composition
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All monolithically grown coatings, TiAlN, TaAlN, and TiAlTaN, exhibit a fine fibrous structure, as can be seen in SEM cross-sections provided in Figs. 1a-c, respectively. However, TiAlN indicates a slightly increased columnar character, whereas the other films show a
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smoother cross-section. Surface irregularities arise from the presence of macroparticles which
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are usually observed in non-filtered arc evaporation depositions [33,34]. Based on the overall film thickness and deposition time we obtain growth rates of 33, 27, and 31 nm min-1. Cross-
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sections of our TiAlN/TaAlN multilayers, processed with ITaAl = 100, 120, and 140 A are
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shown in Figs. 1d to f, respectively, indicate a fine and dense film morphology, comparable to
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their monolithically grown constituents. Bright field cross-section TEM studies of multilayered TiAlN/TaAlN coatings prepared with constant arc currents at the TiAl cathode (ITiAl = 140 A) combined with variations in the arc
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current at the TaAl cathode (ITaAl = 100, 120, and 140 A, see Figs. 1g-i, respectively), show a well-defined layered structure. The individual Ti1-xAlxN and Ta1-yAlyN layers can clearly be distinguished by their brighter and darker contrast, respectively. Evidence for epitaxial growth between TiAlN and TaAlN is given by the high resolution (HR) TEM inset in Fig. 1h. Estimated bilayer periods based on the growth rate obtained from monolithically grown coatings are in excellent agreement with the TEM investigations. The bilayer period (TiAlNlayer + TaAlN-layer thickness) of our multilayer coatings processed with ITaAl = 100, 120, and 140 A are 32±3, 35±3, and 36±3 nm, respectively. The TEM images also indicate thin horizontal lines within the TiAlN-layers, which stem from the two-fold-rotation of the substrate holder representing localised variations in chemical composition and microstructure.
ACCEPTED MANUSCRIPT This finding is most likely based on variations in particle flux and plasma density as well as resputtering effects [35]. Chemical analyses obtained by SEM-EDS are summarised in Table I. For the monolithically
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grown coatings the Al/Ti ratio (1.17) of our Ti0.54Al0.46N coating is comparable with the
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Ti0.5Al0.5 targets (Al/Ti = 1.00) used. Contrary the Ta0.89Al0.11N coatings with an Al/Ta ratio of 0.12 and the Ti0.45Ta0.36Ta0.19N coatings with Ti/Ta ratios of 1.89 and Al/Ta ratios of 2.36 are lower than their respective targets with Al/Ta = 0.33 for the Ta0.75Al0.25 and Ti/Ta = Al/Ta
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= 4.5 for the Ti0.45Al0.45Ta0.10 targets. The differences between coatings and targets stem from gas scattering and resputtering effects during film formation and the hugely different atomic
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masses of Ti, Al, and Ta. Therefore, the coatings contain more Ta than the respective target, for example the Ti1-x-yAlxTayN films contain even 19 at% Ta on the metallic sublattice
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although the target contained only 10 at%. Changes in cathode current ITaAl mainly influences
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the growth rate of our TaAlN coatings and not their morphology or chemical composition.
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The multilayered TiAlN/TaAlN coatings exhibit an almost even distribution of Al, Ti, and Ta on the metallic sublattice. Here, we have to emphasise that the values obtained for our
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multilayered coatings represent an overall composition measured through several bilayers. Therefore, the increase in current at the Ta-rich TaAl cathode (ITaAl = 100, 120, and 140 A)— while keeping the TiAl target powering constant (ITiAl = 140 A)—is mainly visible by the change in Ti to Ta ratio. These results suggest that the chemistry of the individual TiAlN and TaAlN layers of the TiAlN/TaAlN multilayers is similar to the monolithically grownTi1xAlxN
and Ta1-yAlyN films, respectively.
ACCEPTED MANUSCRIPT 3.2 Thermal stability
The X-ray diffraction pattern of as-deposited Ti0.54Al0.46N, see Fig. 2a, demonstrates a single-
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phased face-centred cubic (c, B1, NaCl-type) solid solution structure with well-defined 111
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(2θ ~37.5 deg.) and 200 (2θ ~43.5 deg.) peaks. Vacuum annealing of these samples up to Ta = 900 °C shows no significant changes in the XRD patterns, see Fig. 2a. First evidence for a decomposition of the supersaturated cubic solid solution towards the stable constituents c-TiN
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and hexagonal (B4, ZnS-wurtzite-type) w-AlN is given by the formation of left-hand shoulders of the 111 and 200 peaks. These peaks indicate the formation cubic Ti-rich nitride
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domains, having a larger lattice parameter than the coating matrix. Based on previous investigations by Rachbauer et al. using atom probe tomography [8], the formation of such Ti-
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rich domains also results in the formation of Al-rich domains within the TiAlN matrix. A
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further increase in annealing temperature to Ta ≥ 1300 °C clearly results in the formation of
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w-AlN, indicated by emerging XRD peaks at 2θ ~33.6 and 38 deg., respectively. The supplementary pattern of the powdered Ti0.54Al0.46N coating material obtained after DSC measurements (presented later) to 1500 °C finally demonstrates the complete decomposition
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of the material into its stable constituents c-TiN and w-AlN. Figure 2b shows the XRD patterns of as-deposited and vacuum annealed Ta0.89Al0.11N in which we can also identify a single-phase cubic structure with pronounced 111 and 200 peaks. Upon annealing to 1100 °C neither significant peak shifting nor the formation of additional peaks can be detected. However, for Ta = 1200 °C a pronounced phase transformation to mainly hexagonal ε-TaN occurred (2θ ~34.5 and 37 deg.) with small fractions of hexagonal Ta2N (2θ ~39 deg.). Further annealing results in continuous phase transformation due to nitrogen loss and precipitation of hexagonal Ta2N on the expanse of TaN. After annealing at Ta = 1400 °C only hexagonal Ta2N can be detected, which is confirmed by the XRD pattern
ACCEPTED MANUSCRIPT of the sample after DSC measurements to 1500 °C. The formation of stable w-AlN phase, due to its small phase fraction is hardly detectable especially next to Ta-N phases. Ti0.45Al0.36Ta0.19N (Fig. 2c) was grown in a single-phased cubic structure with pronounced
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111 and 200 orientations. Thermal annealing at Ta ≥ 800 °C causes a collective shift of the
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XRD peaks towards lower diffraction angles. This peak shift proceeds until 1400 °C, where tiny XRD peaks at the position for w-AlN (at 2θ ~33 and 38 deg.) can be detected. After the DSC measurement to 1500 °C the formation of w-AlN can clearly be identified by XRD. The
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collective shift of the major XRD peaks of the matrix (TiAlTaN) towards smaller diffraction angles—between c-TiN and δ-TaN—combined with the formation of w-AlN phases suggests
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for strain reduction and decomposition processes. Strain reduction is obtained by the thermally activated rearrangement and annihilation of structural built-in defects to lower-
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energy sites. The formation of AlN is the result of a decomposition of the supersaturated
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Ti0.45Al0.36Ta0.19N matrix towards c-Ti1-zTazN and w-AlN. The formation of a solid solution
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type c-Ti1-zTazN—z corresponds to the chemical composition of the coating with z = y/(1-x) with y being the Ta content and x being the Al content of our coating—is additionally
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responsible for the shift of the major XRD peaks towards the position between c-TiN and δTaN. Ab initio calculations suggest that the cubic-structured Ti1-zTazN is stable for chemical compositions of approximately 0.2 ≤ z ≤ 0.8 even at 0 K [36]. The formation of a hexagonal Ta2N phase is not detectable within the annealing range up to 1500 °C. Comparing the structural evolution after heat-treatment of our monolithically arc-evaporated Ti0.54Al0.46N and Ti0.45Al0.36Ta0.19N coatings, we can confirm that this 5d element tantalum increases the thermal stability as suggested earlier for reactive cathodic arc evaporated [21] as well as for sputter deposited coatings [22]. XRD patterns of vacuum annealed TiAlN/TaAlN multilayers—prepared with constant ITiAl = 140 A combined with ITaAl = 100, 120, and 140 A, Figs. 3a, b, and c, respectively—indicate a single-phased cubic structure—comparable to the monolithically grown films. The initially
ACCEPTED MANUSCRIPT broader 111 and 200 XRD peaks originate from overlapping c-TiAlN and c-TaAlN reflections of the individual layers as well as from additional lattice-mismatch-induced coherency strains. Annealing of the substrate-free and powdered coating samples leads to progressive sharpening
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of the XRD peaks. This is the consequence of reduced microstrains due to recovery of
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structural defects, grain growth, and inter-diffusion processes at the interfaces between the
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TiAlN and TaAlN layers, which reduces coherency strains. For annealing temperatures above Ta ~1200 °C the XRD peaks are nearly symmetric and much narrower as in the as-deposited
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state, indicative for an intermixing of the individual layers. Corresponding to the results obtained for the monolithically grown TiAlTaN coatings (see above and Fig. 2c), the major
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matrix XRD peaks shift to lower diffraction angles—towards values between c-TiN and δTaN. The concomitant formation of w-AlN, see the small XRD peak at 2θ ~33 deg. for Ta ≥
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1300 °C indicates decomposition of the supersaturated phases towards their stable
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constituents w-AlN and c-Ti1-zTazN. For Ta ≥ 1500 °C a N2-loss (as obtained by DSC-TGA
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measurements, see next paragraph) leads to the formation of hex-Ta2N, as indicated by the emerging XRD peaks at 2θ ~33.9 and 38.8 deg. The XRD patterns of the coatings after DSC measurements to 1500 °C, at the top of each column in Fig. 3, clearly show that the
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predominant phase of the coatings is c-Ti1-zTazN with small fractions of wurtzite AlN and hexagonal Ta2N. The three different multilayers prepared with ITaAl = 100, 120, and 140 A, Figs. 3a, b, and c, exhibit a similar structural evolution with annealing temperature. Only the phase fraction of hexagonal Ta2N—formed during annealing—increases with increasing power at the TaAl cathode. The DSC spectrum for monolithically grown single-phased cubic Ti0.54Al0.46N (Fig. 4a, black curve) shows pronounced exothermic features up to ~1200 °C. There is no clear separation of the individual reactions on-going, other than obtained for sputter deposited coatings [8]. The decrease in mass between Ta = 900–1000 °C is based on nitrogen loss. A review of published data, especially DSC-TGA data of Ti1-xAlxN coatings [2,37,38], shows that this is regularly
ACCEPTED MANUSCRIPT accompanied by the formation of w-AlN. Hence, we suggest that this type of N2-loss is a strong indicator for the formation and growth of w-AlN phases. The DSC curve of monolithically grown Ta0.89Al0.11N coatings, orange line in Fig. 4a,
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suggests only for minor exothermic contributions in the temperature range 600–800 °C
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followed by a more pronounced endothermic feature for Ta ≥ 1200 °C. The latter is due to nitrogen loss as confirmed by the TGA obtained mass reduction. The detected ―two-step‖ process for N2-release agrees with the XRD observations made for Ta0.89Al0.11N. Between
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1100 and 1200 °C the structure of face-centred cubic TaAlN changes to form ε-TaN, see Fig. 2b. Based on the results available, we conclude that this is triggered by the out-diffusion of
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AlN form the supersaturated TaAlN phase. As shown by previous studies, small additions of Al to TaN stabilise the δ-TaN phase over the ε-TaN phase [39]. Consequently, if Al diffuses
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out, this stabilisation effect is reversed and hence ε-TaN becomes favourable. Thus, nitrogen
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is released from a δ-TaN-based phase at the beginning to be followed by N2-release from an
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ε-TaN-based phase, resulting in the observed two-step N2-release. Increasing N2-release leads to the further formation of hex-Ta2N at the expanse of TaN.
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The monolithically grown Ti0.45Al0.36Ta0.19N coating exhibits only small exothermic contributions to the DSC curve spanning over a broad temperature range, violet line in Fig. 5a. The corresponding XRD investigations suggest stress relaxation and grain growth (indicated by the small shift in position and sharpening of the XRD peaks) as the major origin for these exothermic features up to Ta = 1100 °C. For higher temperatures, the supersaturated phases decompose to form AlN and cubic Ti1-zTazN. Our multilayered coatings clearly exhibit at least two exothermic features during DSC measurements up to 1500 °C in He, see Fig. 4b. Between 600 and 800 °C thermally induced recovery of structural built-in defects takes place, which is comparable to the results obtained for our monolithically grown coatings. The exothermic feature between 1100 and 1200 °C can
ACCEPTED MANUSCRIPT be associated with inter-diffusion of the adjacent TiAlN and TaAlN layers—based on the XRD studies. The subsequent pronounced endothermic feature for Ta ≥ 1300 °C is due to N2release—please compare the corresponding TGA signals showing a mass loss for these
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temperatures. The N2-release is mainly caused by the transformation of TaAlN layers to form
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Ta2N, see the corresponding XRD patterns in Fig. 3. In this context an increasing TaAlN layer
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thickness results in a stronger and faster endothermic reaction, as observed for the multilayers prepared with ITaAl = 100, 120, and 140 A, see the green, blue, and red DSC and TGA curves
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in Fig. 4b, respectively. However, the onset temperature of approximately 1300 °C for this endothermic reaction shows no pronounced dependence on the layer thickness. The multilayer
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arrangement of TiAlN and TaAlN layers with bilayer periods between 30 and 38 nm exhibit an improved phase stability and resistance against N2-loss compared with their monolithically
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grown components Ti0.54Al0.46N and Ta0.89Al0.11N.
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The hardness of monolithically grown single-phased cubic Ti0.54Al0.46N is around 31 GP from
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the as-deposited state up to annealing temperatures Ta of 600 °C (black squares in Fig. 5a). The hardness decrease for higher temperatures to ~22 GPa at 1000 °C ≤ Ta ≤ 1200 °C is due
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to the decomposition of the supersaturated Ti0.54Al0.46N phase towards its stable constituents c-TiN and w-AlN. We prepared all coatings with relatively low bias potential of -40 V, therefore, our Ti0.54Al0.46N coatings exhibits slightly open and under-dense column boundaries. These are preferred sites for decomposition of the supersaturated matrix [40], as they provide higher diffusion as well as reduced retarding forces against changes in specific volume due to decomposition and precipitation [41]. Consequently, the precipitation of wurtzite-structured AlN—with its huge volume increase by ~26 %—is accelerated for coatings with open and under-dense column boundaries. This in turn leads to the observed hardness decrease between 900 and 1000 °C. The formation of c-TiN can already be detected in this temperature range by XRD, see Fig. 2a, which implies that also AlN-rich areas—often difficult to detect by XRD—need to develop [42].
ACCEPTED MANUSCRIPT Monolithically grown single-phased cubic Ta0.89Al0.11N coatings (orange circles in Fig. 5a) exhibit an as-deposited hardness of 30±2 GPa, which remains even for annealing treatments up to ~1100 °C. This is in excellent agreement with our XRD investigations, see Fig. 2.
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Monolithically grown single-phased cubic Ti0.45Al0.36Ta0.19N (purple triangles in Fig. 5a)
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exhibits an as-deposited hardness value of 32±2 GPa, similar to Ti0.54Al0.46N. But here, the asdeposited hardness value is maintained upon annealing to ~800 °C. For Ta ≥ 800 °C we even
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observe an increase in hardness due to the first onset of decomposition processes into Ti-rich and Al-rich cubic domains resulting in a hardness peak value of ~37 GPa for Ta = 1000 °C.
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Such a pronounced age-hardening effect of our quaternary Ti0.45Al0.36Ta0.19N coating is in
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good agreement with previous reports by Rachbauer et al. for sputtered coatings [22]. Beyond
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the peak hardness, we observe a slight decrease to approximately 33±1 GPa at Ta = 1100 °C, which is still about 3 GPa above that of the monolithically grown Ta0.89Al0.11N coating and
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almost 10 GPa above the hardness of Ti0.54Al0.46N. The as-deposited hardness of all TiAlN/TaAlN multilayer variations is between 30 and 32 GPa and is maintained up to annealing temperatures of ~900 °C, see Fig. 5b. The first significant hardness increase starts at Ta ≥ 900 °C for the multilayers prepared with ITaAl = 100 and 120 A. Due to spallation no hardness measurements could be performed for the coatings deposited with ITaAl = 100 A after annealing at temperatures of 1100 and 1200 °C. Nevertheless, the increasing hardness values with increasing the annealing temperature from 800 to 1000 °C suggest that the annealing behaviour of this coating is similar to the multilayer coating deposited with ITaAl = 120 A. The latter exhibits a peak hardness of 35 GPa with Ta = 1100 °C. The multilayer prepared with 140 A at the Ta0.75Al0.25 cathode does not feature a pronounced age-hardening effect but behaves like the monolithically grown Ta0.89Al0.11N
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3.3 Oxidation resistance
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The monolithically grown Ti0.54Al0.46N and Ta0.89Al0.11N coatings are fully oxidised after 20 h
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exposure to ambient air at 850 °C. Their fracture cross-sections, Figs. 6a and b, show the typical morphology of a porous oxide scale with reduced adhesion to the substrate. Also their
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multilayered TiAlN/TaAlN arrangements prepared with 100, 120, or 140 A at the Ta0.75Al0.25 cathode are completely oxidised after 20 h exposure to ambient air at 850 °C. Only the
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monolithically grown quaternary Ti0.45Al0.36Ta0.19N coating was still intact after this treatment. Therefore, the latter is studied in more detail by cross-sectional EDS line-scan
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analysis and compared to TiAlN/TaAlN multilayered coatings prepared with ITaAl = 120 A,
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Figs. 7a and b, respectively. To prevent spallation of the oxide scale during preparation—
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mechanically grinding and polishing—for the SEM-EDS analysis a supporting CrN layer was deposited onto the oxidation-treated samples. Consequently, a Cr signal can be detected
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between embedding material and oxide scale. The monolithically grown quaternary Ti0.45Al0.36Ta0.19N coating exhibits an ~1 µm thick dense oxide scale with an inhomogeneous chemical composition, Fig. 7a. The outermost oxide region (~0.25 µm thickness) is higher in Al and lower in Ti and Ta content as the ~0.75 µm thick oxide region underneath. These dense oxide layers, and especially the dense Al 2O3based outermost region, effectively protect the nitride layer underneath against further oxidation. Consequently, almost 70 % of the monolithically grown Ti0.45Al0.36Ta0.19N coating remains intact after 20 h exposure to ambient air at 850 °C. The oxygen content of this remaining nitride coating—with an as-deposited-like chemical composition—is below the detection limit of the EDS system used. These results for cathodic arc evaporated
ACCEPTED MANUSCRIPT Ti0.45Al0.36Ta0.19N are in excellent agreement with reactively sputter deposited Ta-alloyed Ti1xAlxN
coatings [21,22].
EDS line-scans of our multilayered TiAlN/TaAlN coatings, Fig. 7b, exhibit and evenly
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distributed Al, Ti, Ta, and high oxygen content throughout the entire coating thickness.
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Nitrogen can only be detected in the region of the CrN supporting layer, hence, the initial multilayered TiAlN/TaAlN nitride coating is completely oxidised.
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Ti0.54Al0.46N coatings clearly exhibit a multi-phased composition of rutile TiO2, corundumtype Al2O3, and possibly small phase fractions of anatase TiO2 after oxidation, see Fig. 8.
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Ta0.89Al0.11N mainly transforms into Ta2O5 with a minor phase fraction of α-Al2O3. For the monolithically grown Ti0.45Al0.36Ta0.19N coating the major oxide phase is rutile TiO2-based.
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Also here, only small indications for a crystalline α-Al2O3 structure can be detected, see the
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small XRD peaks at 2θ ~25.5 and 43 deg. The cross-sectional EDS analyses also suggest the formation of an Al2O3-based phase, as the outermost region of the oxide scale is Al-rich. It is
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important to mention, that in contrast to coated Al2O3 substrates, no evidence for a remaining nitride phase is given here. For the freestanding powdered coating material, used for these
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XRD analyses, the oxidation condition is way more demanding, but helps identifying the oxide phases formed. The XRD patterns of our multilayered coatings, exhibiting a homogeneous chemical composition—of Al, Ti, Ta, and O—across the entire oxide scale, indicate only the formation of a rutile TiO2-based phase with minor phase fractions of anatase. Based on the homogeneous chemical composition and the absence of an XRD peak at 2θ ~43 deg. we assign the XRD peak at 2θ ~ 25.5 deg. to an anatase TiO2 rather than an α-Al2O3based phase. This is similar for all three different multilayered coatings prepared with ITaAl = 100, 120, and 140 A and in agreement with recent studies by Hollerweger et al. showing, also referring to ab initio data, that for specific chemical compositions between Ti, Ta, and Al, single-phased, rutile-based oxides can form [43].
ACCEPTED MANUSCRIPT The observation of an enrichment of Al, indicative for a protective outermost oxide scale, for the TiAlTaN coating and a homogeneous distribution of Al for the multilayered coatings (Fig. 7a, and b) may be explained by the different constitution of the oxides as well as the film
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architecture. The monolithically grown TiAlTaN coatings exhibit a denser morphology than
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the TiAlN coatings, suggesting for fewer high diffusion pathways along (underdense) grain
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and column boundaries. The results reported in Ref. [43] furthermore show that due to the addition of Ta the formation of the stable phase rutile during oxidation is promoted. Without
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tantalum, anatase forms initially on TiAlN, which then transforms upon further oxidation to the rutile phase. The associated volume change very often leads to the formation of a large
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crack network, which can clearly be reduced or even avoided by the addition of tantalum. For the multilayered TiAlN/TaAlN coatings, we have to consider the oxidation behaviour of the
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individual monolithically grown TiAlN and TaAlN coatings. The distinct peak shift to smaller
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2θ positions of the Ta2O5 phase formed on TaAlN in combination to the missing indications
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for an Al2O3 phase, Fig. 8, suggests that Al is incorporated to the Ta2O5 structure, hence not available to form a stable oxide. The large volume increase by the formation of Ta2O5 based oxides leads to the formation of underdense open porous oxides. The TiAlN layers will then
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also not be able to form dense oxides. This can explain the homogenous chemical composition (within the detection limit of the SEM-EDS system used) of the oxide formed.
ACCEPTED MANUSCRIPT Conclusions We have developed cathodic arc evaporated quaternary Ti0.45Al0.36Ta0.19N and multilayered TiAlN/TaAlN coatings and compared their mechanical properties, thermal stabilities, and
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oxidation resistance with their monolithically grown constituents Ti0.54Al0.46N and
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Ta0.89Al0.11N coatings.
All coatings exhibit a single-phased cubic solid solution in the as-deposited state, which is
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stable up to 900 and 1100 °C for the monolithically grown coatings Ti0.54Al0.11N and Ta0.89Al0.11N, respectively. Their multilayered combination to TiAlN/TaAlN coatings (with
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bilayer periods of 30 to 38 nm) exhibit an even higher thermal stability during vacuum annealing, as first indications for a w-AlN formation can be detected for Ta above 1300–1400
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°C, independent of the TaAlN layer thicknesses. Corresponding results are also obtained for
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the monolithically grown quaternary Ti0.45Al0.36Ta0.19N coatings. Furthermore, this quaternary coating does not show any Ta2N formation even when annealed at 1500 °C. Contrary, the
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Ta0.89Al0.11N and the multilayered TiAlN/TaAlN coatings, especially with thicker TaAlN layers, show a Ta2N formation due to N2-release when annealed to Ta ≥ 1200 and 1400 °C,
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respectively.
All coatings investigated exhibit as-deposited hardness values of around 32±2 GPa. However, only the quaternary Ti0.45Al0.46Ta0.19N coating and the TiAlN/TaAlN multilayers (prepared with ITaAl = 100 and 120 A) feature pronounced age-hardening for Ta = 1000–1100 °C with peak hardness values of 35 to 37 GPa. This is a clear improvement when compared to their monolithically grown counterpart coatings Ti0.45Al0.46N and Ta0.89Al0.11N, which exhibit hardnesses below 30 GPa for Ta ≥ 1000 °C. Based on long-term oxidation tests at 850 °C in ambient air, we can conclude that tantalum within the monolithically grown Ti0.45Al0.36Ta0.19N coatings is responsible for the formation of a dense and protective single-phased oxide scale with an Al-enrichment at the outermost
ACCEPTED MANUSCRIPT region. This is not the case for the other coatings, hence they are fully oxidised after the 20 h exposure to air at 850 °C. According to our results we conclude that quaternary Ti0.45Al0.36Ta0.19N coatings and
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TiAlN/TaAlN multilayers feature enhanced mechanical properties and thermal stability with
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respect to their monolithically grown counterpart coatings Ti0.54Al0.46N and Ta0.89Al0.11N. But a significant increase in oxidation resistance could only be achieved for the quaternary
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Ti0.45Al0.36Ta0.19N coatings.
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
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acknowledged. We also thank for the financial support of Plansee Composite Materials
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GmbH and Oerlikon Balzers Coating AG. We also acknowledge the support by the X-Ray Center of the Vienna University of Technology Austria during as well as by the University Service Centre for Transmission Electron Microscopy, Vienna University of Technology
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Austria.
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[44] Q. Li, I.W. Kim, S.A. Barnett, L.D. Marks, I. Introduction, Structures of AlN/VN Superlattices with Different AlN Layer Thicknesses, J. Mater. Res. 17 (2011) 1224– 1231.
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Table I: Metallic sublattice population of the monolithically (TiAlN, TaAlN, and TiAlTaN)
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and multilayered (TiAlN/TaAlN) grown coatings obtained by energy-dispersive X-ray
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spectroscopy.
Cathode powering [A]
Al [at%]
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Monolithic coatings1 ITiAl = 140 ITiAlTa = 140 TiAlN/TaAlN multilayers²
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ITaAl = 100
Ta [at%]
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46
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11
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89
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36
19
36
38
26
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32
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32
35
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ITaAl = 120 ITaAl = 140
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ITaAl = 120
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The Ti0.5Al0.5 and Ti0.45Al0.45Ta0.10 cathodes were powered with 140 A, and the
Ta0.75Al0.25 cathode was powered with 120 A. Constant powering of the Ti0.5Al0.5 cathode with ITiAl = 140 A and varying
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powering of the Ta0.75Al0.25 cathode with ITaAl = 100, 120, and 140 A.
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Fig. 1: Cross–section SEM images of monolithically grown (a) TiAlN (ITiAl = 140 A), (b)
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TaAlN (ITaAl = 120 A), and (c) TiAlTaN (ITiAlTa = 140 A) thin films. (d), (e), and (f) are cross-
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section SEM images of multilayered TiAlN/TaAlN coatings prepared with constant ITiAl =
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140 A combined with ITaAl = 100, 120, and 140 A, respectively. The corresponding TEM images of these multilayers with ITaAl = 100, 120, and 140 A are shown in (g), (h), and (i),
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Fig. 2: XRD patterns of arc evaporated monolithically grown (a) Ti0.54Al0.46N, (b) Ta0.89Al0.11N, and (c) Ti0.45Al0.36Ta0.19N coatings in the as-deposited state and after vacuum
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annealing at Ta. The patterns are obtained from powdered coating materials—removed from
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their low-alloyed steel substrates—see Experimental. The 2θ standard positions for δ-TaN
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(lattice parameter a = 4.339 Å, ICDD: 03-065-9404), c-TiN (a = 4.247 Å, ICDD: 00-0381420), c-AlN (calculated for a = 4.06 Å [44]), wurtzite AlN (lattice parameters a = 3.11 Å and c = 4.98 Å, ICDD: 01-076-0702), ε-TaN (a = 5.196 Å, c = 2.911 Å, ICDD: 01-071-0253), and
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hex-Ta2N (a = 3.044 Å, c = 4.914 Å, ICDD: 00-026-0985) are indicated. Fig. 3: XRD patterns of vacuum annealed multilayer TiAlN/TaAlN coatings prepared with constant ITiAl = 140 A combined with ITaAl = (a) 100, (b) 120, and (c) 140 A. The XRD pattern of the multilayered coatings after DSC measurements to 1500 °C are added (top-most pattern). The 2θ positions for δ-TaN (a = 4.339 Å, ICDD: 03-065-9404), c-TiN (a = 4.247 Å, ICDD: 00-038-1420), c-AlN (calculated for a = 4.06 Å [44]), wurtzite w-AlN (a = 3.11 Å, c = 4.98 Å, ICDD: 01-076-0702), and hex-Ta2N (a = 3.044 Å, c = 4.914 Å, ICDD: 00-026-0985) are indicated.
ACCEPTED MANUSCRIPT Fig. 4: Heat flow (baseline subtracted) and relative mass as a function of the annealing temperature, Ta, during DSC/TGA investigations of (a) monolithically grown Ti0.54Al0.46N, Ta0.89Al0.11N, and Ti0.45Al0.36Ta0.19N coatings, and (b) multilayered TiAlN/TaAlN coatings
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prepared with constant ITiAl = 140 A combined with ITaAl = 100, 120, and 140 A.
Fig. 5: Hardness evolution as a function of the vacuum annealing treatments at T a of (a)
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monolithically grown Ti0.54Al0.46N, Ta0.89Al0.11N, and Ti0.45Al0.36Ta0.19N coatings, and (b) multilayered TiAlN/TaAlN coatings prepared with constant ITiAl = 140 A combined with ITaAl
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= 100, 120, and 140 A.
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Fig. 6: Monolithically grown (a) Ti0.54Al0.46N and (b) Ta0.89Al0.11N coatings after oxidation in
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ambient air at 850 °C for 20 h.
Fig. 7: EDS line-scan and back-scattered SEM cross-section of monolithically grown (a)
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Ti0.45Al0.36Ta0.19N and (b) multilayered TiAlN/TaAlN (ITaAl = 120 A) coatings after oxidation in ambient air at 850 °C for 20 h. The cross-sections are polished.
Fig. 8: XRD pattern of oxidised (20 h at 850 °C in ambient air) powdered coating material. ICCD standard positions for rutile TiO2 (green diamonds, ICDD: 00-001-1292), anatase TiO2 (green squares, ICDD: 00-021-1272), corundum-type Al2O3 (red hexagons, ICDD: 01-0760702), and Ta2O5 (blue circles, ICDD: 00-019-1298) are indicated. The top three XRD patterns are oxidised TiAlN/TaAlN multilayer coatings prepared with 100, 120, and 140 A at the TaAl cathode.
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Comparison of monolithically grown and multilayered CAE coatings
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Ti0.45Al0.36Ta0.19N and TiAlN/TaAlN coatings exhibit enhanced thermal stability
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Pronounced age-hardening of 37±2 and 35±2 GPa between 1000 and 1100 °C
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Only monolithic Ti0.45Al0.36Ta0.19N withstands 20 h oxidation tests at 850 °C