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Structural evolution of TaN-alloyed Cr–Al–Y–N coatings
Surface & Coatings Technology 288 (2016) 203–210
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Structural evolution of TaN-alloyed Cr–Al–Y–N coatings Marián Mikula a,b,⁎, Dušan Plašienka a, Tomáš Roch a, Kamila Štyráková a, Leonid Satrapinskyy a, Marián Drienovský c, Vladimír Girman d, Branislav Grančič a, Andrej Pleceník a, Peter Kúš a a
Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, Bratislava, Slovakia Institute of Materials and Machine Mechanics, SAS Račianska 75, Bratislava, Slovakia Institute of Materials Science, Faculty of Materials Science and Technology STU, Bottova 25, Trnava, Slovakia d Department of Condensed Matter Physics, Institute of Physics, P.J. Šafárik University, Park Angelinum 9, Košice, Slovakia b c
a r t i c l e
i n f o
Article history: Received 8 December 2015 Revised 15 January 2016 Accepted in revised form 20 January 2016 Available online 22 January 2016 Keywords: Hard coatings Cr–Al–Y–Ta–N Magnetron sputtering Ab initio calculations Thermal stability
a b s t r a c t Substitutions of metallic elements in Al-containing ternary transition metal nitrides are a promising method for improving the alloy thermal stability and oxidation resistance. In this work, combined experimental and ab initio analysis of thermal stability and structural evolution of CrAlYN alloyed to tantalum nitride is presented. Asdeposited reactively-sputtered Cr1 − x − y − zAlxYyTazN coatings (z = 0 ÷ 0.21) exhibit single phase cubic sodium chloride (B1) structure identified as fcc-CrAlY(Ta)N solid solution. The presence of Ta in the solid solution shifts the decomposition process to higher temperatures (N1000 °C for Ta content of z = 0.21) compared to CrAlYN (~900 °C), thus enhancing the alloy thermal stability. Improved thermal stability of tantalum-containing solid solutions may be attributed to their higher cohesive energies, as revealed from ab initio calculations. X-ray diffraction and transmission electron microscopy investigation of the N2-depleted structure after exposure at high temperature (1200 °C) in Ar + H2 atmosphere revealed the presence of wurtzite AlN (w-AlN), cubic, hexagonal and tetragonal Cr- and Ta-containing binary or ternary nitrides (h-Cr2N, h-TaN, h-Ta2N, t-Cr0.8Ta1.2N) and metallic phases (bcc-Cr(AlTa), fcc-Cr2Ta). First-principle calculations show negative values of mixing free energies for fcc-Cr1 − xTaxN over the whole composition range at 1600 K indicating its enhanced thermodynamic stability compared to cubic Al1 − xTaxN and Al1 − xCrxN. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Development of materials with characteristics optimized to the requirements of modern applications demands great efforts for understanding the intrinsic material properties and their suitable combinations. Improvement of hard coatings leads to achievement of new excellent properties such as high hardness combined with good oxidation resistance above 1000 °C [1], extremely low friction with high wear resistance and enhanced toughness, especially combined with high resistance to cracking [2,3]. A promising way to produce hard coatings with these desired properties is through the use of non-equilibrium thin film growth deposition techniques like sputtering, that allows synthesizing nanostructured (nanocomposite) metastable systems that often surpass the physical and chemical properties of their stable components [4,5]. Crucial factor that limits the unique properties of the nanostructure formed in the films is its temperature stability, which is strongly affected by the annihilation of structural defects, rearrangement of atoms leading to reduction of internal stresses, interdiffusion,
⁎ Corresponding author at: Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, Bratislava, Slovakia. E-mail address: [email protected] (M. Mikula).
http://dx.doi.org/10.1016/j.surfcoat.2016.01.031 0257-8972/© 2016 Elsevier B.V. All rights reserved.
recrystallization, decomposition mechanism and phase transformation during high temperature exposure and oxidation processes [6–8]. Typical representatives of nanostructured hard coatings are ternary and quaternary transition metal nitrides (TMNs — Transition Metal Nitrides, Ti, Cr-, Zr, Nb-, V-, Ta-, Y- and HfN), where their attractive mechanical and chemical properties arise mainly from strong interatomic bonds, which can generally be present as mixtures of metallic, ionic and covalent contributions [9]. The structures of common Alcontaining ternary coatings, Ti1 − xAlxN and Cr1 − xAlxN, consist of metastable face-centered cubic NaCl-type (B1) fcc-TiAlN and fcc-CrAlN solid solutions. These supersaturated cubic structures remain stable due to large activation energies required for phase separation. However, at elevated temperatures, diffusion processes are sufficiently frequent to initiate decomposition and subsequent formation of nanostructured material. Spinodal decomposition is characteristic for Ti1 − xAlxN. In its initial stage (~ 900 °C), spinodal decomposition is accompanied by an increase in hardness due to the formation of nanocomposites consisting of fcc-TiN and fcc-AlN-enriched coherent phases [4]. In the case of Cr1 − xAlxN precipitation of incoherent hexagonal ZnS-type (wurtzite) w-AlN nanograins occur within the solid solution at the temperature of about 700 °C. [4] The formation of this fine-grained nanocomposite is also accompanied by a slight increase in hardness. The continuing process of decomposition leads to the dual-phase structure
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containing stable w-AlN and fcc-TiN or fcc-CrN phases (stable up to ~1000 °C, then transformation to h-Cr2N and Cr occurs), grain coarsening and degradation of mechanical properties. Nowadays, research activity is focused on improvement of thermal stability and oxidation resistance of ternary nitrides via a concept of alloying TMNs with nitrides or other elements [10]. Many articles [11–19] report mainly on quaternary systems based on Ti1 − xAlxN and alloyed with nitrides of elements from group III–VI (Y, Zr, Hf, V, Nb, Ta, Mo, W), where alloying is realized by substitution of Ti and Al atoms in the metallic sublattice. This allows for a huge variety of size and bonding types, due to different electronic configurations of constituting atoms (e.g., additional d- and f-states) [4]. Investigations of several authors [20–22] showed that Ta alloying (a pentavalent element) results in a pronounced increase in thermal stability of Ti1 − x − yAlxTayN by shifting the onset of the nitride phase decomposition by ~200 °C. Additionally, alloying Ta into Ti1 − xAlxN is also very beneficial for oxidation resistance [20]. Unlike for Ti1 − xAlxN, effects of alloying TM elements on the improvement of Cr1 − xAlxN coatings are much less investigated. Rovere [23–25] reported enhanced oxidation resistance of Cr0.30Al0.68Y0.02N with very small content of yttrium (~1 at.%.). In such coatings, Y promotes the formation of a dense and adherent mixed Al2O3 + Cr2O3 scale which results in a very promising oxidation resistance up to temperatures exceeding 1000 °C [25]. In this study, we investigate the influence of tantalum addition on the phase stability and structural evolution of Cr1 − x − y − zAlxYyTazN coatings prepared by reactive magnetron co-sputtering. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA) were used to analyze thickness, morphology, chemical composition and mass changes of the coatings, respectively. Thermal stability and decomposition route after annealing at temperatures up to 1200 °C were examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ab initio calculations were performed to support experimental data and to predict thermodynamical phase stabilities of quaternary solid solutions and products of their decompositions.
electron microscope Tescan Lyra (SEM). The chemical composition of as-deposited and annealed samples was determined during the electron microscopy studies by means of the energy-dispersive X-ray analysis (EDX, INCA Wave, Oxford Instruments Analytical). Mass changes associated with changes in the chemical composition of the coatings during annealing were monitored by thermogravimetric analysis (TGA) in Thermal Analysis Apparatus Netzsch STA 409 CD using a heating rate of 10 K min−1, 10 sccm flow of high pure Ar (99.9999% purity), and a maximum temperature of 1400 °C. Prior to TGA the coatings were chemically removed, cleaned in acetone and isopropyl alcohol, rinsed with distilled water, grounded and then inserted into the Al2O3 crucible. Structural investigations of the as-deposited and annealed samples were performed by Bragg–Brentano X-ray diffraction (XRD) measurements using PANalytical X'Pert PRO MRD diffractometer with Cu Kα radiation (λ = 0.15418 nm). Patterns of as-deposited and annealed samples were fitted by Rietveld refinement in order to determine more detailed information, such as grain size and lattice parameters. Detailed studies of the nanostructure of selected samples were conducted with transmission electron microscopy (TEM) using JEOL 2100 operated at 200 keV. Elemental distribution in the observed structure was determined with EDX during investigations in scanning mode (STEM). Specimen preparation was performed using a Tescan Lyra focused ion beam (FIB) work station. 2.2. Computational details Ab initio density functional theory (DFT) calculations were performed with the VASP 5.3 code [27,28] employing projector
2. Methods 2.1. Experimental details Cr–Al–Y–Ta–N coatings were reactively deposited using unbalanced magnetron co-sputtering from powder metallurgically prepared CrAlY target (49.5/49.5/1 at.%, 100 mm dia., 99.5% purity) and Ta target (100 mm dia., 99.99% purity) in Ar + N2 discharge. The magnetrons were tilted with the angle of ~50° between their axes; see Ref. [26] for more details. The substrates were mounted on the sample holder at different positions with respect to the targets, yielding variable tantalum content in Cr–Al–Y–Ta–N. The magnetron power densities were fixed at 6.5 W cm− 2 and 1.4 W cm−2 for the CrAlY and the Ta targets, respectively. The base pressure in the chamber was below 1 × 10−3 Pa and the total working gas pressure during depositions was kept at 0.53 Pa with a N2-partial pressure at ~0.17 Pa. All depositions were carried out at a negative substrate bias of Us = −50 V. No additional heating of samples was used during deposition. The as-deposited Cr–Al–Y–Ta–N coatings were annealed in Ar + 5% H2 (99.999% purity) atmosphere (10− 1 Pa) at the temperatures of 900 °C, 1000 °C, 1100 °C and 1200 °C for 3 min using a heating rate of 50 K min−1 and self-limiting cooling rate. Heating chamber base pressure was 1 × 10− 4 Pa before each heat treatment. Mirror-polished Al2O3 plates (sapphire, c-cut, 8 × 8 × 0.5 mm3) were chosen to avoid interdiffusion of substrate and coating material at the high annealing temperatures. Prior to the deposition, all substrates were ultrasonically cleaned in acetone and isopropyl alcohol. Morphological and thickness characterization of the coatings was obtained from imaging of their cross section by a dual-beam scanning
Fig. 1. TGA and DTGA analysis of annealed Cr–Al–Y–Ta–N coatings in Ar atmosphere.
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augmented-wave (PAW) pseudopotentials and the Perdew–Burke– Ernzerhof (PBE) parameterization of exchange-correlation functional. Since CrN as well as Cr1 − xTixN and Cr1 − xAlxN are known to be paramagnetic at room temperatures [29–32], we employed fully spin-polarized calculations of paramagnetic configurations of all Cr-containing ternary and quaternary phases, as far as we are interested in properties of coatings in the temperature interval, where paramagnetism takes place (room-temperature and heated samples). Paramagnetism was modeled with disordered local moments (DLM), where local magnetic moments on Cr sites were initiated randomly with spin up and spin down orientations in the same amount. Ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (Al:Cr)1 − xTaxN systems were modeled as bulk materials with 216 explicitly treated atoms. For the starting configurations in structural optimization runs were taken 3 × 3 × 3 supercells of fcc-Al–Cr–Ta–N with NaCl structure, where one sublattice is fully occupied by N atoms and the other one randomly with Al and TM = Cr, Ta atoms. We used 2 × 2 × 2 k-point grid for the Brillouin-zone sampling of 216-atomic cubic samples (864 k-points ∗ atoms), while total energy and force convergence criteria during structural optimizations were set to 10−6 eV/atom and 10−3 eV/Å, respectively.
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3. Results and discussion Elemental analysis by EDX reveals that our as-deposited coatings are stoichiometric (the nitrogen content ≈ 50 ± 2 at.%) with N / (Cr + Al + Y + Ta) ratios of 1 ± 0.01. Content of metals is normalized to 50 at.% nitrogen leading to the notation Cr1 − x − y − zAlxYyTazN and compositions of Cr0.49Al0.50Y0.01N, Cr0.48Al0.48Y0.01Ta0.03N, Cr0.45Al0.42Y0.01Ta0.12N and Cr0.46Al0.46Y0.01Ta0.07N, Cr0.42Al0.36Y0.01Ta0.21N. Small amount of impurities of carbon and oxygen not exceeding 3 at.% in total was detected as well. TGA results indicated chemical transitions during exposure to high temperatures in an inert Ar + H2 atmosphere. Fig. 1a shows the mass loss of the Cr–Al– Y–Ta–N coatings as a function of annealing temperature. The weight of the samples remains almost stable up to 900 °C. Very stable behavior is most apparent for highest Ta content, whereas sample without Ta shows more than 2% initial mass loss. Above 1300 °C the sample without Ta shows about 4% higher final mass loss than Ta containing samples. According to derivative TGA curves (DTGA: dm / dt = f(T)) (Fig. 1b) the fastest mass loss occurs for tantalum-free Cr0.49Al0.50Y0.01N, where a DTGA peak minimum is observed at ~ 1115 °C. In the case of Cr0.45Al0.42Y0.01Ta0.12N and Cr0.42Al0.36Y0.01Ta0.21N a DTGA peak minimum is shifted to higher temperatures by ~ 50 °C. Further increase in
Fig. 2. Typical SEM cross-sectional micrographs of Cr1 − x − y − zAlxYyTazN coatings with Ta content of (a) z = 0; (b) z = 0.21; (c) TEM micrograph of as dep. Cr0.42Al0.36Y0.01Ta0.21N with inset showing high-angle boundary between two grains; (d) BF-TEM micrograph of Cr0.42Al0.36Y0.01Ta0.21N annealed at 1200 °C in Ar + H2 atmosphere for 3 min.
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annealing temperature above 1150 °C is accompanied by continuing mass loss of the samples. The obtained TGA and DTGA curves indicate that the presence of tantalum in the coatings shifts onset of processes associated with the mass loss to higher temperatures and also lowers their intensity. EDX analysis of all annealed Cr–Al–Y–Ta–N coatings revealed that exposure of the coatings to high temperatures leads primarily to the loss of nitrogen eventually resulting in N2-depleted structure after annealing to the temperature of 1200 °C. Typical SEM cross-sectional images of several micrometers thick Cr1 − x − y − zAlxYyTazN coatings are displayed in Fig. 2. The asdeposited coatings with Ta content of z = 0 (Fig. 2a) and z = 0.21 (Fig. 2b) exhibited the morphology with densely packed columnar grains. The inserted bright-field TEM micrograph obtained from the as-deposited Cr0.42Al0.36Y0.01Ta0.21N in Fig. 2c shows strong image contrast within columnar grains, caused by the formation of highly strained areas throughout the volume of the coatings. TEM-EDX map (not shown) depicts a homogeneous distribution of all elements across coatings. Changes in the chemical composition after annealing were also reflected in the morphological changes of the coatings when disappearance of columnar grains, formation of a dense featureless structure and reduced thickness due to N2 loss was observed (Fig. 2d). The inset in Fig. 2d showing bright field-TEM micrograph demonstrates the structure formed of equiaxed grains in Cr0.42Al0.36Y0.01Ta0.21N coating annealed at 1200 °C in Ar + 5%H2 atmosphere for 3 min. The difference in the structure of as-deposited Cr1 − x − y − zAlxYyTazN coatings with various compositions is illustrated in Fig. 3a by XRD patterns in the 2θ range between 34° and 84°. XRD patterns suggest that all as-deposited coatings crystallize in a single phase cubic (B1) structure identified as fcc-CrAlY(Ta)N solid solution. Increasing of Ta content from z = 0 to z = 0.21 results in a shift of the diffraction peaks to lower angles, corresponding to an increase in lattice parameter from 4.129 Å to 4.205 Å, as determined by the Rietveld analysis. Comparison of ab initio (calculated for (Cr0.5Al0.5)1 − xTaxN model) and experimentally obtained lattice parameters as functions of the tantalum fraction x is plotted in Fig. 3b. It is clearly seen, that the fits are not linear as suggested by Vegard's rule, but must be approximated with an additional bowing parameter δ describing the positive deviation from the linear interpolation. Therefore, the plotted data are fitted with the following expression: a(x) = ao(1 − x) + a1x + δx(x − 1) where a0 and a1 are the lattice parameters of the boundary Cr0.5Al0.5N and TaN systems. Lattice parameters of as-deposited coatings are higher in comparison with the calculated data. In as-deposited coatings, there are large amounts of defects which cause the structures to be strained. Also, small amount of Y atoms (that larger than Cr, Al) may also contribute to lattice parameter increase. The fit gives lower δ = 0.111 for ab initio simulation considering relaxed material without the presence of lattice defects. On the other hand, higher δ = 1.753 obtained for experimental data is caused by sputtering processes that generate defects leading to macro-strains. From the Rietveld fit the micro-strain values are close to zero for all Ta contents. However, the crystallite size of cubic phase does not show any clear steady change with Ta, but fluctuates within 30 and 100 nm. In comparison to as-deposited samples, the patterns of coatings annealed at 900 °C show slight positive shift of reflection positions (thinner lines, Fig. 3a). Corresponding values of lattice parameter for as-deposited samples and after annealing at 900 °C are shown in Fig. 3b. After annealing the fcc-CrAlYTaN phase is preserved in all samples. However, its lattice parameter is decreased probably due to structure recovery, stress release and some partial leakage of nitrogen. The lattice parameter decrease is smallest (0.5%) for highest Ta content of z = 0.21 and largest (1.4%) for z = 0.07. Overally the dependence of lattice parameter is almost linear and the bowing parameter is δ = 0.1. The crystallite size obtained from the peak width after 900 °C annealing is reduced roughly in half compared to as-deposited values. This means the coherent crystallite size in normal direction is decreasing which is consistent with the tendency towards disappearing of long columnar
features in SEM, TEM micrographs (Fig. 2a-c) and observation of fine grains after annealing at higher temperatures (Fig. 2d). In XRD patterns of all as-deposited coatings (Fig. 3a) relatively intense peaks corresponding to the fcc-CrAlY(Ta)N 111 and 222 reflections were observed around 2θ ~ 37° and 2θ ~ 80° respectively. This implies the preferential orientation of (111) planes parallel to the ccut Al2O3 surface corresponding to columnar character of grains observed in cross-sectional TEM micrograph (Fig. 2c) of single phased as-deposited coatings. Such texture is typical feature for fcc transition metal nitride coatings deposited with magnetron sputtering on biased substrates causing energetic ion bombardment [33]. By comparing the normalized integral XRD peak intensities Ih/∑ Ih of fcc-CrAlY(Ta)N measured in the 2θ range of 32–115° we have not observed any clear dependence of (111) texture on Ta content. After annealing at 900 °C the preferential orientation slightly decreased only for the highest Ta content z = 0.21. We suppose that upon annealing the decomposition of original single fcc phase starts with splitting of columns into finer and more equiaxed grains probably due to some recrystallization processes induced by nitrogen release. The thermally induced structure development of Cr1 − x − y − zAlx YyTazN coatings is depicted in a series of XRD patterns in Fig. 4. Measurements in Bragg–Brentano setup were performed at room temperature after annealing. Vertical lines indicate 2θ nominal positions for
Fig. 3. (a) The difference in the structure of as-deposited Cr1 − x − y − zAlxYyTazN coatings with various compositions; (b) Comparison of ab initio (calculated for (Cr0.5Al0.5)1 − xTaxN model) and experimentally obtained lattice parameters.
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cubic δ-TaN (ICDD:03–065-9404), hex ε-TaN (ICDD:01–071-0253), hex Ta2N (ICDD:00–026-0985), cubic AlN (ICDD: 00–046-1200), hex w-AlN (ICDD:01–076-0702), cubic CrN (ICDD:01–076-2494), hex β-Cr2N (ICDD:00–035-0803), cubic Cr (ICDD:01–074-7045), cubic Cr2Ta (ICDD:00–020-0317) and tetragonal Cr0.8Ta1.2N (ICDD:00–073-6154). Cubic fcc-CrAlYN solid solution forming tantalum-free Cr0.49Al0.50Y0.01N coatings begins to decompose when the annealing temperature exceeds 900 °C (Fig. 4a). Further increase in the annealing temperature is accompanied by nitrogen release from the structure (confirmed by EDX and TGA). XRD pattern of the coatings annealed at
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1000 °C shows a reduction in the fcc-CrAlYN peak intensity and appearance of additional reflections at 2θ ~33°, 36°, and 38° indicating formation of hexagonal h-Cr2N and w-AlN phases. This means that AlN precipitates begin to form in the structure and a simultaneous N2release from the coatings leads to the transformation of the remaining Cr-enriched matrix into metastable h-Cr2N phase. Increase in the annealing temperature above 1100 °C leads to further N2 loss and subsequently to transformation of Cr2N into Cr which is confirmed by the presence of strong reflection at 2θ ~ 44° belonging to bcc-Cr. The completely decomposed structure is formed by a dual-phase system
Fig. 4. Structural evolution of Cr1 − x − y − zAlxYyTazN coatings with Ta content of (a) z = 0; (b) z = 0.03; (c) z = 0.07; (d) z = 0.12; (e) z = 0.21.
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bcc-Cr–w-AlN. EDX analysis of this N2-depleted structure after annealing at 1200 °C in Ar + 5%H2 atmosphere revealed remaining nitrogen content of approximately 10 at.%. The Cr0.46Al0.46Y0.01Ta0.07N coating with Ta content of z = 0.03 and z = 0.07, respectively, annealed at 900 °C still retain the character of the fcc-CrAlYTaN solid solution (Fig. 4b,c). Onset of the decomposition, which is accompanied by the loss of nitrogen, occurs after the temperature exceeds 1000 °C. For the coating with Ta content of z = 0.12 the XRD pattern after annealing at 1100 °C (Fig. 4d) shows that decomposition products are not thermally stable (e.g. Cr2N), while in the pattern after 1200 °C annealing is dominant reflection from w-AlN and probably Cr-rich bcc-Cr(AlTa) phase. The series of XRD patterns for the highest Ta containing samples (z = 0.21) after 1000 °C annealing (Fig. 4e) show an increase of the right-hand shoulder of the strongest fcc peak, broadening of peaks and a weak scattering around 2θ ~ 32°, probably due to starting w-AlN formation. The structural evolution with increasing annealing temperature was similar in all measured samples. From 900 to 1000 °C peak intensities of initial fcc-CrAlY(Ta)N phase is reduced and additional reflections appeared indicating the formation of hexagonal h-Cr2N and w-AlN phases. Upon further release of N2 by annealing above 1000 °C the metastable h-Cr2N phase transforms into bcc-Cr(AlTa) metallic phase. Eventually from 1200 °C predominantly w-AlN and bcc-Cr(AlTa) phases remain. Higher content of Ta causes in addition formation of h-Ta2N and hTaN phases. In summary, the higher the Ta content, the higher decomposition temperature of the cubic solid solution. For better overview of the extent of decomposition processes, Fig. 5 shows XRD diffractograms of samples with various Ta content annealed at 1000 °C. For Ta-free coatings the decomposition of the fcc-CrAlYN phase is fully accomplished, whereas for the tantalum containing samples (z = 0.12 and z = 0.21, respectively), the fcc-CrAlYTaN phase just starts to decompose. Broadening of peaks indicates the decrease of crystallite size and formation of broad maximum around 2θ ~32° implies appearance of another phase. Probably formation of t-Cr0.8Ta1.2N, h-Cr2N or w-AlN is initiated. This is in accordance with TGA data (Fig. 1), where we observe about 50–100 °C shift of the beginning of mass reduction. The samples with smaller Ta content, in addition, show clearer presence of h-Cr2N and for the smallest Ta amount wAlN is already visible, together with some cubic CrTa metallic phase. For samples annealed at higher temperatures, there is certain ambiguity in assigning diffraction peaks to a specific diffraction pattern due to multiphase mixture of overlapping patterns and relatively broad peaks. In addition, it is notable that for pure Ta–N system more than 10 (meta)stable phases has been reported. For these reasons and for possibility of Cr–Ta–N and Ta–Al–N ternaries formation it is difficult to identify, distinguish and index peaks of all crystal phases in patterns from high annealing temperature, especially above 1100 °C. Positive influence of tantalum alloying on thermal stability of Cr1 − x − y − zAlxYyTazN coatings can be explained by increased cohesive energy Ec, which is defined as energy required to dissociate crystalline material into individual atoms. The calculated values of Ec per atom are 5.054 eV, 5.302 eV, 5.541 eV and 7.57 eV for fcc CrN, (CrAl)N, (CrAl)0.79Ta0.21N and TaN, respectively (for Al:Cr = 1:1). These results show that by adding the highest content of Ta (z = 0.21) into (CrAl)N solid solution increases the cohesive energy per atom by about 4.5%. Higher value of Ec for Ta-containing system demonstrates the strengthening effect of alloying elements that can be understood as a pure electronic effect when dopant atoms increase valence electron density compared to the original structure [36]. These additional electrons take part in bonding and hence cause stronger bonds between atoms in the doped solid solution. DFT calculations also allow determining mixing free energies ΔF(x) of solid solutions, where negative values represent thermodynamical stability of solutions with respect to considered decomposition products, while positive values reflect their tendency to decompose. Structural optimization runs were carried out to obtain energies of
Fig. 5. Comparison of XRD patterns of the coatings with various Ta content annealed at 1000 °C in Ar + H2 atmosphere.
relaxed configurations at zero pressure and temperature, which were then used to calculate mixing free energies shown in Fig. 6. These mixing free energies were calculated as ΔF(x) = ΔE(x) − TΔSc(x), where ΔE(x) is the standard mixing enthalpy term (energy, at pressure P = 0) and TΔSc(x) represents the mean-field estimation of configurational entropy Sc(x) of a solid solution weighted by temperature (electronic and vibrational entropy were not considered). Configurational entropy of a substitutional alloy comes from the irregular presence of different types of atoms occupying individual lattice sites. If occupancy of these sites is strictly random, configurational entropy can be estimated (for two types of substituting elements in a system defined solely by x) from the mixing entropy formula [34] Sc(x) = − kB [xln(x) + (1 − x)ln(1 − x)] per atom. In ternary (AlTM)N solid solutions, Al and TM atoms occupy one out of two sublattices in NaCl-type fcc structure and hence the prefactor one half has to be included into the entropy formula. The values of ΔF(x) given in Fig. 6 then represent values of free-energy differences per atom. In Fig. 6, mixing free energies of ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (CrAl)1 − xTaxN systems for T = 1600 K are shown. The free-energy differences were calculated for fcc ternaries (and quaternary) with respect to w-AlN and fcc-TaN and CrN binaries. The configurational entropy promotes lattice disorder for increasing temperatures by lowering the alloy free energy. For the negative (though relatively small in absolute value) mixing free energy for fcc-(CrTa)N solid solution, we may conclude that the calculations indicate its stability with respect to decomposition into
Fig. 6. Mixing free energies ΔF of ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (CrAl)1 − xTaxN systems for T = 1600 K. The presented data suggests table or unstable (negative/positive ΔF) solid solutions as a function of x.
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fcc-CrN and fcc-TaN. In the cases of Al1 − xTaxN, Al1 − xCrxN and (CrAl)1 − xTaxN systems, ΔF(x) is mostly positive, indicating a tendency for decomposition. It should be noted that according to the paper [36], the addition of yttrium leads to increase of the driving force for decomposition of the cubic solid solution Ti1 − x − yAlxYyN with y = 0.1 due to the large difference lattice parameter between c-YN and c-AlN. However, the Y content in our Cr1 − x − y − zAlxYyTazN coatings was negligible (y = 0.02) and therefore yttrium was not included in ab initio calculations. We note that our calculated mixing free energies (excluding the entropy term) are in good agreement with previous calculations of mixing enthalpies ΔE(x) for Al1 − xTaxN [22], Al1 − xCrxN [29] and also Cr1 − xTaxN [35]. Annealed Cr0.45Al0.42Y0.01Ta0.12N was chosen for further structure study by means of TEM in order to provide an additional insight into the nanostructure. STEM micrograph (Fig. 7) of the coating illustrates the structural evolution taking place during the annealing at 1200 °C. The structure was composed of equiaxed grains with diameter in the range of approx. 20–50 nm. It can also be seen that the elemental distribution in the structure is not homogeneous (lighter and darker regions). Fig. 7b–d depict overlaid color-coded EDX maps of Al, Cr and Ta after the sample annealing at 1200 °C. Aluminium and chrome create almost inverse maps (Fig. 7d), indicating that they do not have a tendency to coexist in the grains. However, tantalum and chromium maps are highly overlapped (Fig. 7b), indicating the formation of a mixed Cr1 − xTaxN phase, in agreement with ab initio calculations presented above. On the map of tantalum with aluminium (Fig. 7c) several dark regions
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can be seen which contain only chromium atoms, but there are also places that are overlapping, which may indicate mixture of w-AlN and Ta(Cr)N grains. 4. Conclusions We have grown Cr1 − x − y − zAlxYyTazN coatings with Ta contents in the range of z = 0 to z = 0.21 and N / (Cr + Al + Y + Ta) = 1. The obtained experimental results and analysis have been supported by ab initio calculations. All prepared Cr1 − x − y − zAlxYyTazN coatings in the as-deposited state exhibit a single phase cubic structure identified as fcc-CrAlY(Ta)N solid solution. The structural investigation indicates that tantalum-free Cr0.49Al0.50Y0.01N coatings start to decompose when the annealing temperature exceeds 900 °C and results in the formation of w-AlN precipitates in the remaining Cr-rich matrix. Unstable Cr–N bonds at higher temperatures and N-loss lead to dual-phase structure consisting of w-AlN and bcc-Cr. The presence of Ta in the solid solution shifts the start of the decomposition process to higher temperatures (N 1000 °C for Cr0.42Al0.36Y0.01Ta0.21N with Ta content of z = 0.21), leading to improvement of thermal stability compared to CrAlYN. This improved thermal stability can be attributed to higher cohesive energies of the Tacontaining solid solutions, according to ab initio calculations. Decomposition of solid solutions during gradual increase of temperature to 1200 °C accompanied by a loss of nitrogen continues with the formation of cubic and hexagonal Cr- and Ta-containing binaries or ternaries. DFT
Fig. 7. (a) STEM micrograph of the Cr0.42Al0.36Y0.01Ta0.21N coating illustrating the structural evolution after annealing at 1200 °C; (b–d) EDX maps of Al, Cr and Ta from STEM micrograph.
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calculations found negative values of mixing free energies for cubic Cr1 − xTaxN at 1600 K indicating its stability over the whole composition range with respect to the cubic binaries. This prediction was confirmed by the STEM investigation, where the presence of CrTa-containing grains after annealing at 1200 °C was clearly visible. Acknowledgments This work has been supported by the Slovak Research and Development Agency under the contract numbers APVV-14-0173, APVV-010811 and Operational Program Research and Development (projects ITMS:26240220002 and ITMS:26220220004). References [1] D. McIntyre, J.E. Greene, G. Håkansson, J.-E. Sundgren, W.-D. Münz, Oxidation of metastable single-phase polycrystalline Ti0.5Al0.5N films: kinetics and mechanisms, J. Appl. Phys. 67 (1990) 1542. [2] H. Kindlund, D.G. Sangiovanni, L. Martínez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J.E. Greene, V. Chirita, L. Hultman, Toughness enhancement in hard ceramic thin films by alloy design, App. Phys. Lett. Mater. 1 (2013) 042104. [3] H. Kindlund, D.G. Sangiovanni, J. Lu, J. Jensen, Vacancy-induced toughening in hard single-crystal V0.5Mo0.5Nx/MgO(001) thin films, Acta Mater. 77 (2014) 394. [4] P.H. Mayrhofer, F. Rachbauer, D. Holec, F. Rovere, J.M. Schneider, Protective Transition Metal Nitride Coatings, in: M.S.J. Hashmi (Ed.) Comprehensive Materials Processing, 4, Elsevier, Amsterdam 2014, pp. 355–388. [5] O. Knotek, M. Bohmer, T. Leyendecker, On structure and properties of sputtered Ti and Al based hard compound films, J. Vac. Sci. Technol. A 4 (1986) 2695. [6] L. Hultman, Thermal stability of nitride thin films, Vacuum 57 (2000) 1–30. [7] C. Mitterer, P.H. Mayrhofer, J. Musil, Thermal stability of PVD hard coatings, Vacuum 71 (2003) 279–284. [8] P.H. Mayrhofer, C. Mitterer, L. Hultman, H. Clemens, Microstructural design of hard coatings, Prog. Mater. Sci. 51 (2006) 1032–1114. [9] D.G. Sangiovanni, L. Hultman, V. Chirita, Supertoughening in B1 transition metal nitride alloys by increased valence electron concentration, Acta Mater. 59 (2011) 2121. [10] H. Lind, R. Forsén, B. Alling, N. Ghafoor, F. Tasnádi, M.P. Johansson, I.A. Abrikosov, M. Odén, Improving thermal stability of hard coating films via a concept of multicomponent alloying, Appl. Phys. Lett. 99 (2011) 091903. [11] R. Forsén, M.P. Johansson, M. Odén, N. Ghafoor, Effects of Ti alloying of AlCrN coatings on thermal stability and oxidation resistance, Thin Solid Films 534 (2013) 392–402. [12] R. Forsén, N. Ghafoor, M. Odén, Coherency strain engineered decomposition of unstable multilayer alloys for improved thermal stability, J. Appl. Phys. 114 (2013) 244303. [13] R. Forsén, M. Johansson, M. Odén, N. Ghafoor, Decomposition and phase transformation in TiCrAlN thin coatings, J. Vac. Sci. Technol. A 30 (2012) 061506. [14] R. Rachbauer, A. Blutmager, D. Holec, P.H. Mayrhofer, Effect of Hf on structure and age hardening of Ti–Al–N thin films, Surf. Coat. Technol. 206 (2012) 2667–2672. [15] R. Rachbauer, D. Holec, M. Lattemann, L. Hultman, P.H. Mayrhofer, Electronic origin of structure and mechanical properties in Y and Nb alloyed Ti–Al–N thin films, Int. J. Mater. Res. 102 (2011) 6.
[16] H. Riedl, D. Holec, R. Rachbauer, P. Polcik, R. Hollerweger, J. Paulitsch, P.H. Mayrhofer, Phase stability, mechanical properties and thermal stability of Y alloyed Ti–Al–N coatings, Surf. Coat. Technol. 235 (2013) 174–180. [17] D.G. Sangiovanni, V. Chirita, L. Hultman, Toughness enhancement in TiAlN-based quarternary alloys, Thin Solid Films 520 (2012) 4080–4088. [18] D.G. Sangiovanni, V. Chirita, L. Hultman, Electronic mechanism for toughness enhancement in TixM1 − xN (M = Mo and W), Phys. Rev. B 81 (2010) 104107. [19] T. Reeswinkel, D.G. Sangiovanni, V. Chirita, L. Hultman, J.M. Schneider, Structure and mechanical properties of TiAlN-WNx thin films, Surf. Coat. Technol. 205 (2011) 4821–4827. [20] R. Hollerweger, H. Riedl, J. Paulitsch, M. Arndt, R. Rachbauer, P. Polcik, S. Primig, P.H. Mayrhofer, Origin of high temperature oxidation resistance of Ti–Al–Ta–N coatings, Surf. Coat. Technol. 257 (2014) 78–86. [21] R. Rachbauer, D. Holec, P.H. Mayrhofer, Increased thermal stability of Ti–Al–N thin films by Ta alloying, Surf. Coat. Technol. 211 (2012) 98–103. [22] R. Rachbauer, D. Holec, P.H. Mayrhofer, Phase stability and decomposition products of Ti–Al–Ta–N thin films, Appl. Phys. Lett. 97 (2010) 151901. [23] F. Rovere, P.H. Mayrhofer, Thermal stability and thermo-mechanical properties of magnetron sputtered Cr–Al–Y–N coatings, J. Vac. Sci. Technol. A A26 (2008) 29–36. [24] F. Rovere, P.H. Mayrhofer, A. Reinholdt, J. Mayer, J.M. Schneider, The effect of yttrium incorporation on the oxidation resistance of Cr–Al–N coatings, Surf. Coat. Technol. 202 (2008) 5870–5875. [25] F. Rovere, D. Music, J.M. Schneider, P.H. Mayrhofer, Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings, Acta Mater. 58 (2010) 2708–2715. [26] M. Mikula, T. Roch, D. Plašienka, L. Satrapinskyy, P. Švec sr, D. Vlčková, M. Dvoranová, B. Grančič, M. Gregor, A. Plecenik, P. Kúš, Thermal stability and structural evolution of quaternary Ti–Ta–B–N coatings, Surf. Coat. Technol. 259 (2014) 698–706. [27] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (1996) 11169. [28] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set Comput, Mater. Sci. 6 (1996) 15. [29] B. Alling, T. Marten, I.A. Abrikosov, A. Karimi, Comparison of thermodynamic properties of cubic Cr1 − xAlxN and Ti1 − xAlxN from first-principles calculations, J. Appl. Phys. 102 (2007) 044314. [30] B. Alling, T. Marten, I.A. Abrikosov, Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN, Phys. Rev. B 82 (2010) 184430. [31] B. Alling, Theory of the ferromagnetism in Ti1 − xCrxN solid solutions, Phys. Rev. B 82 (2010) 054408. [32] P. Steneteg, B. Alling, I. Abrikosov, Equation of state of paramagnetic CrN from ab initio molecular dynamics, Phys. Rev. B 85 (2012) 144404. [33] R. Daniel, K.J. Martinschnitz, J. Keckes, C. Mitterer, Texture development in polycrystalline CrN coatings: the role of growth conditions and a Cr interlayer, J. Phys. D. Appl. Phys. 42 (2009) 075401. [34] B. Alling, A.V. Ruban, A. Karimi, O.E. Peil, S.I. Simak, L. Hultman, I.A. Abrikosov, Mixing and decomposition thermodynamics of c-Ti1 − xAlxN from firstprinciples calculations, Phys. Rev. B 75 (2007) 045123. [35] L. Zhou, D. Holec, P.H. Mayrhofer, Ab initio study of the alloying effect of transition metals on structure, stability and ductility of CrN, J. Phys. D. Appl. Phys. 46 (2013) 365301. [36] D. Holec, L. Zhou, R. Rachbauer, P.H. Mayrhofer, Alloying-related trends from first principles: an application to the Ti–Al–X–N system, J. Appl. Phys. 113 (2013) 113510.
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Structural evolution of TaN-alloyed Cr–Al–Y–N coatings Marián Mikula a,b,⁎, Dušan Plašienka a, Tomáš Roch a, Kamila Štyráková a, Leonid Satrapinskyy a, Marián Drienovský c, Vladimír Girman d, Branislav Grančič a, Andrej Pleceník a, Peter Kúš a a
Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, Bratislava, Slovakia Institute of Materials and Machine Mechanics, SAS Račianska 75, Bratislava, Slovakia Institute of Materials Science, Faculty of Materials Science and Technology STU, Bottova 25, Trnava, Slovakia d Department of Condensed Matter Physics, Institute of Physics, P.J. Šafárik University, Park Angelinum 9, Košice, Slovakia b c
a r t i c l e
i n f o
Article history: Received 8 December 2015 Revised 15 January 2016 Accepted in revised form 20 January 2016 Available online 22 January 2016 Keywords: Hard coatings Cr–Al–Y–Ta–N Magnetron sputtering Ab initio calculations Thermal stability
a b s t r a c t Substitutions of metallic elements in Al-containing ternary transition metal nitrides are a promising method for improving the alloy thermal stability and oxidation resistance. In this work, combined experimental and ab initio analysis of thermal stability and structural evolution of CrAlYN alloyed to tantalum nitride is presented. Asdeposited reactively-sputtered Cr1 − x − y − zAlxYyTazN coatings (z = 0 ÷ 0.21) exhibit single phase cubic sodium chloride (B1) structure identified as fcc-CrAlY(Ta)N solid solution. The presence of Ta in the solid solution shifts the decomposition process to higher temperatures (N1000 °C for Ta content of z = 0.21) compared to CrAlYN (~900 °C), thus enhancing the alloy thermal stability. Improved thermal stability of tantalum-containing solid solutions may be attributed to their higher cohesive energies, as revealed from ab initio calculations. X-ray diffraction and transmission electron microscopy investigation of the N2-depleted structure after exposure at high temperature (1200 °C) in Ar + H2 atmosphere revealed the presence of wurtzite AlN (w-AlN), cubic, hexagonal and tetragonal Cr- and Ta-containing binary or ternary nitrides (h-Cr2N, h-TaN, h-Ta2N, t-Cr0.8Ta1.2N) and metallic phases (bcc-Cr(AlTa), fcc-Cr2Ta). First-principle calculations show negative values of mixing free energies for fcc-Cr1 − xTaxN over the whole composition range at 1600 K indicating its enhanced thermodynamic stability compared to cubic Al1 − xTaxN and Al1 − xCrxN. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Development of materials with characteristics optimized to the requirements of modern applications demands great efforts for understanding the intrinsic material properties and their suitable combinations. Improvement of hard coatings leads to achievement of new excellent properties such as high hardness combined with good oxidation resistance above 1000 °C [1], extremely low friction with high wear resistance and enhanced toughness, especially combined with high resistance to cracking [2,3]. A promising way to produce hard coatings with these desired properties is through the use of non-equilibrium thin film growth deposition techniques like sputtering, that allows synthesizing nanostructured (nanocomposite) metastable systems that often surpass the physical and chemical properties of their stable components [4,5]. Crucial factor that limits the unique properties of the nanostructure formed in the films is its temperature stability, which is strongly affected by the annihilation of structural defects, rearrangement of atoms leading to reduction of internal stresses, interdiffusion,
⁎ Corresponding author at: Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, Bratislava, Slovakia. E-mail address: [email protected] (M. Mikula).
http://dx.doi.org/10.1016/j.surfcoat.2016.01.031 0257-8972/© 2016 Elsevier B.V. All rights reserved.
recrystallization, decomposition mechanism and phase transformation during high temperature exposure and oxidation processes [6–8]. Typical representatives of nanostructured hard coatings are ternary and quaternary transition metal nitrides (TMNs — Transition Metal Nitrides, Ti, Cr-, Zr, Nb-, V-, Ta-, Y- and HfN), where their attractive mechanical and chemical properties arise mainly from strong interatomic bonds, which can generally be present as mixtures of metallic, ionic and covalent contributions [9]. The structures of common Alcontaining ternary coatings, Ti1 − xAlxN and Cr1 − xAlxN, consist of metastable face-centered cubic NaCl-type (B1) fcc-TiAlN and fcc-CrAlN solid solutions. These supersaturated cubic structures remain stable due to large activation energies required for phase separation. However, at elevated temperatures, diffusion processes are sufficiently frequent to initiate decomposition and subsequent formation of nanostructured material. Spinodal decomposition is characteristic for Ti1 − xAlxN. In its initial stage (~ 900 °C), spinodal decomposition is accompanied by an increase in hardness due to the formation of nanocomposites consisting of fcc-TiN and fcc-AlN-enriched coherent phases [4]. In the case of Cr1 − xAlxN precipitation of incoherent hexagonal ZnS-type (wurtzite) w-AlN nanograins occur within the solid solution at the temperature of about 700 °C. [4] The formation of this fine-grained nanocomposite is also accompanied by a slight increase in hardness. The continuing process of decomposition leads to the dual-phase structure
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containing stable w-AlN and fcc-TiN or fcc-CrN phases (stable up to ~1000 °C, then transformation to h-Cr2N and Cr occurs), grain coarsening and degradation of mechanical properties. Nowadays, research activity is focused on improvement of thermal stability and oxidation resistance of ternary nitrides via a concept of alloying TMNs with nitrides or other elements [10]. Many articles [11–19] report mainly on quaternary systems based on Ti1 − xAlxN and alloyed with nitrides of elements from group III–VI (Y, Zr, Hf, V, Nb, Ta, Mo, W), where alloying is realized by substitution of Ti and Al atoms in the metallic sublattice. This allows for a huge variety of size and bonding types, due to different electronic configurations of constituting atoms (e.g., additional d- and f-states) [4]. Investigations of several authors [20–22] showed that Ta alloying (a pentavalent element) results in a pronounced increase in thermal stability of Ti1 − x − yAlxTayN by shifting the onset of the nitride phase decomposition by ~200 °C. Additionally, alloying Ta into Ti1 − xAlxN is also very beneficial for oxidation resistance [20]. Unlike for Ti1 − xAlxN, effects of alloying TM elements on the improvement of Cr1 − xAlxN coatings are much less investigated. Rovere [23–25] reported enhanced oxidation resistance of Cr0.30Al0.68Y0.02N with very small content of yttrium (~1 at.%.). In such coatings, Y promotes the formation of a dense and adherent mixed Al2O3 + Cr2O3 scale which results in a very promising oxidation resistance up to temperatures exceeding 1000 °C [25]. In this study, we investigate the influence of tantalum addition on the phase stability and structural evolution of Cr1 − x − y − zAlxYyTazN coatings prepared by reactive magnetron co-sputtering. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA) were used to analyze thickness, morphology, chemical composition and mass changes of the coatings, respectively. Thermal stability and decomposition route after annealing at temperatures up to 1200 °C were examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ab initio calculations were performed to support experimental data and to predict thermodynamical phase stabilities of quaternary solid solutions and products of their decompositions.
electron microscope Tescan Lyra (SEM). The chemical composition of as-deposited and annealed samples was determined during the electron microscopy studies by means of the energy-dispersive X-ray analysis (EDX, INCA Wave, Oxford Instruments Analytical). Mass changes associated with changes in the chemical composition of the coatings during annealing were monitored by thermogravimetric analysis (TGA) in Thermal Analysis Apparatus Netzsch STA 409 CD using a heating rate of 10 K min−1, 10 sccm flow of high pure Ar (99.9999% purity), and a maximum temperature of 1400 °C. Prior to TGA the coatings were chemically removed, cleaned in acetone and isopropyl alcohol, rinsed with distilled water, grounded and then inserted into the Al2O3 crucible. Structural investigations of the as-deposited and annealed samples were performed by Bragg–Brentano X-ray diffraction (XRD) measurements using PANalytical X'Pert PRO MRD diffractometer with Cu Kα radiation (λ = 0.15418 nm). Patterns of as-deposited and annealed samples were fitted by Rietveld refinement in order to determine more detailed information, such as grain size and lattice parameters. Detailed studies of the nanostructure of selected samples were conducted with transmission electron microscopy (TEM) using JEOL 2100 operated at 200 keV. Elemental distribution in the observed structure was determined with EDX during investigations in scanning mode (STEM). Specimen preparation was performed using a Tescan Lyra focused ion beam (FIB) work station. 2.2. Computational details Ab initio density functional theory (DFT) calculations were performed with the VASP 5.3 code [27,28] employing projector
2. Methods 2.1. Experimental details Cr–Al–Y–Ta–N coatings were reactively deposited using unbalanced magnetron co-sputtering from powder metallurgically prepared CrAlY target (49.5/49.5/1 at.%, 100 mm dia., 99.5% purity) and Ta target (100 mm dia., 99.99% purity) in Ar + N2 discharge. The magnetrons were tilted with the angle of ~50° between their axes; see Ref. [26] for more details. The substrates were mounted on the sample holder at different positions with respect to the targets, yielding variable tantalum content in Cr–Al–Y–Ta–N. The magnetron power densities were fixed at 6.5 W cm− 2 and 1.4 W cm−2 for the CrAlY and the Ta targets, respectively. The base pressure in the chamber was below 1 × 10−3 Pa and the total working gas pressure during depositions was kept at 0.53 Pa with a N2-partial pressure at ~0.17 Pa. All depositions were carried out at a negative substrate bias of Us = −50 V. No additional heating of samples was used during deposition. The as-deposited Cr–Al–Y–Ta–N coatings were annealed in Ar + 5% H2 (99.999% purity) atmosphere (10− 1 Pa) at the temperatures of 900 °C, 1000 °C, 1100 °C and 1200 °C for 3 min using a heating rate of 50 K min−1 and self-limiting cooling rate. Heating chamber base pressure was 1 × 10− 4 Pa before each heat treatment. Mirror-polished Al2O3 plates (sapphire, c-cut, 8 × 8 × 0.5 mm3) were chosen to avoid interdiffusion of substrate and coating material at the high annealing temperatures. Prior to the deposition, all substrates were ultrasonically cleaned in acetone and isopropyl alcohol. Morphological and thickness characterization of the coatings was obtained from imaging of their cross section by a dual-beam scanning
Fig. 1. TGA and DTGA analysis of annealed Cr–Al–Y–Ta–N coatings in Ar atmosphere.
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augmented-wave (PAW) pseudopotentials and the Perdew–Burke– Ernzerhof (PBE) parameterization of exchange-correlation functional. Since CrN as well as Cr1 − xTixN and Cr1 − xAlxN are known to be paramagnetic at room temperatures [29–32], we employed fully spin-polarized calculations of paramagnetic configurations of all Cr-containing ternary and quaternary phases, as far as we are interested in properties of coatings in the temperature interval, where paramagnetism takes place (room-temperature and heated samples). Paramagnetism was modeled with disordered local moments (DLM), where local magnetic moments on Cr sites were initiated randomly with spin up and spin down orientations in the same amount. Ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (Al:Cr)1 − xTaxN systems were modeled as bulk materials with 216 explicitly treated atoms. For the starting configurations in structural optimization runs were taken 3 × 3 × 3 supercells of fcc-Al–Cr–Ta–N with NaCl structure, where one sublattice is fully occupied by N atoms and the other one randomly with Al and TM = Cr, Ta atoms. We used 2 × 2 × 2 k-point grid for the Brillouin-zone sampling of 216-atomic cubic samples (864 k-points ∗ atoms), while total energy and force convergence criteria during structural optimizations were set to 10−6 eV/atom and 10−3 eV/Å, respectively.
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3. Results and discussion Elemental analysis by EDX reveals that our as-deposited coatings are stoichiometric (the nitrogen content ≈ 50 ± 2 at.%) with N / (Cr + Al + Y + Ta) ratios of 1 ± 0.01. Content of metals is normalized to 50 at.% nitrogen leading to the notation Cr1 − x − y − zAlxYyTazN and compositions of Cr0.49Al0.50Y0.01N, Cr0.48Al0.48Y0.01Ta0.03N, Cr0.45Al0.42Y0.01Ta0.12N and Cr0.46Al0.46Y0.01Ta0.07N, Cr0.42Al0.36Y0.01Ta0.21N. Small amount of impurities of carbon and oxygen not exceeding 3 at.% in total was detected as well. TGA results indicated chemical transitions during exposure to high temperatures in an inert Ar + H2 atmosphere. Fig. 1a shows the mass loss of the Cr–Al– Y–Ta–N coatings as a function of annealing temperature. The weight of the samples remains almost stable up to 900 °C. Very stable behavior is most apparent for highest Ta content, whereas sample without Ta shows more than 2% initial mass loss. Above 1300 °C the sample without Ta shows about 4% higher final mass loss than Ta containing samples. According to derivative TGA curves (DTGA: dm / dt = f(T)) (Fig. 1b) the fastest mass loss occurs for tantalum-free Cr0.49Al0.50Y0.01N, where a DTGA peak minimum is observed at ~ 1115 °C. In the case of Cr0.45Al0.42Y0.01Ta0.12N and Cr0.42Al0.36Y0.01Ta0.21N a DTGA peak minimum is shifted to higher temperatures by ~ 50 °C. Further increase in
Fig. 2. Typical SEM cross-sectional micrographs of Cr1 − x − y − zAlxYyTazN coatings with Ta content of (a) z = 0; (b) z = 0.21; (c) TEM micrograph of as dep. Cr0.42Al0.36Y0.01Ta0.21N with inset showing high-angle boundary between two grains; (d) BF-TEM micrograph of Cr0.42Al0.36Y0.01Ta0.21N annealed at 1200 °C in Ar + H2 atmosphere for 3 min.
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annealing temperature above 1150 °C is accompanied by continuing mass loss of the samples. The obtained TGA and DTGA curves indicate that the presence of tantalum in the coatings shifts onset of processes associated with the mass loss to higher temperatures and also lowers their intensity. EDX analysis of all annealed Cr–Al–Y–Ta–N coatings revealed that exposure of the coatings to high temperatures leads primarily to the loss of nitrogen eventually resulting in N2-depleted structure after annealing to the temperature of 1200 °C. Typical SEM cross-sectional images of several micrometers thick Cr1 − x − y − zAlxYyTazN coatings are displayed in Fig. 2. The asdeposited coatings with Ta content of z = 0 (Fig. 2a) and z = 0.21 (Fig. 2b) exhibited the morphology with densely packed columnar grains. The inserted bright-field TEM micrograph obtained from the as-deposited Cr0.42Al0.36Y0.01Ta0.21N in Fig. 2c shows strong image contrast within columnar grains, caused by the formation of highly strained areas throughout the volume of the coatings. TEM-EDX map (not shown) depicts a homogeneous distribution of all elements across coatings. Changes in the chemical composition after annealing were also reflected in the morphological changes of the coatings when disappearance of columnar grains, formation of a dense featureless structure and reduced thickness due to N2 loss was observed (Fig. 2d). The inset in Fig. 2d showing bright field-TEM micrograph demonstrates the structure formed of equiaxed grains in Cr0.42Al0.36Y0.01Ta0.21N coating annealed at 1200 °C in Ar + 5%H2 atmosphere for 3 min. The difference in the structure of as-deposited Cr1 − x − y − zAlxYyTazN coatings with various compositions is illustrated in Fig. 3a by XRD patterns in the 2θ range between 34° and 84°. XRD patterns suggest that all as-deposited coatings crystallize in a single phase cubic (B1) structure identified as fcc-CrAlY(Ta)N solid solution. Increasing of Ta content from z = 0 to z = 0.21 results in a shift of the diffraction peaks to lower angles, corresponding to an increase in lattice parameter from 4.129 Å to 4.205 Å, as determined by the Rietveld analysis. Comparison of ab initio (calculated for (Cr0.5Al0.5)1 − xTaxN model) and experimentally obtained lattice parameters as functions of the tantalum fraction x is plotted in Fig. 3b. It is clearly seen, that the fits are not linear as suggested by Vegard's rule, but must be approximated with an additional bowing parameter δ describing the positive deviation from the linear interpolation. Therefore, the plotted data are fitted with the following expression: a(x) = ao(1 − x) + a1x + δx(x − 1) where a0 and a1 are the lattice parameters of the boundary Cr0.5Al0.5N and TaN systems. Lattice parameters of as-deposited coatings are higher in comparison with the calculated data. In as-deposited coatings, there are large amounts of defects which cause the structures to be strained. Also, small amount of Y atoms (that larger than Cr, Al) may also contribute to lattice parameter increase. The fit gives lower δ = 0.111 for ab initio simulation considering relaxed material without the presence of lattice defects. On the other hand, higher δ = 1.753 obtained for experimental data is caused by sputtering processes that generate defects leading to macro-strains. From the Rietveld fit the micro-strain values are close to zero for all Ta contents. However, the crystallite size of cubic phase does not show any clear steady change with Ta, but fluctuates within 30 and 100 nm. In comparison to as-deposited samples, the patterns of coatings annealed at 900 °C show slight positive shift of reflection positions (thinner lines, Fig. 3a). Corresponding values of lattice parameter for as-deposited samples and after annealing at 900 °C are shown in Fig. 3b. After annealing the fcc-CrAlYTaN phase is preserved in all samples. However, its lattice parameter is decreased probably due to structure recovery, stress release and some partial leakage of nitrogen. The lattice parameter decrease is smallest (0.5%) for highest Ta content of z = 0.21 and largest (1.4%) for z = 0.07. Overally the dependence of lattice parameter is almost linear and the bowing parameter is δ = 0.1. The crystallite size obtained from the peak width after 900 °C annealing is reduced roughly in half compared to as-deposited values. This means the coherent crystallite size in normal direction is decreasing which is consistent with the tendency towards disappearing of long columnar
features in SEM, TEM micrographs (Fig. 2a-c) and observation of fine grains after annealing at higher temperatures (Fig. 2d). In XRD patterns of all as-deposited coatings (Fig. 3a) relatively intense peaks corresponding to the fcc-CrAlY(Ta)N 111 and 222 reflections were observed around 2θ ~ 37° and 2θ ~ 80° respectively. This implies the preferential orientation of (111) planes parallel to the ccut Al2O3 surface corresponding to columnar character of grains observed in cross-sectional TEM micrograph (Fig. 2c) of single phased as-deposited coatings. Such texture is typical feature for fcc transition metal nitride coatings deposited with magnetron sputtering on biased substrates causing energetic ion bombardment [33]. By comparing the normalized integral XRD peak intensities Ih/∑ Ih of fcc-CrAlY(Ta)N measured in the 2θ range of 32–115° we have not observed any clear dependence of (111) texture on Ta content. After annealing at 900 °C the preferential orientation slightly decreased only for the highest Ta content z = 0.21. We suppose that upon annealing the decomposition of original single fcc phase starts with splitting of columns into finer and more equiaxed grains probably due to some recrystallization processes induced by nitrogen release. The thermally induced structure development of Cr1 − x − y − zAlx YyTazN coatings is depicted in a series of XRD patterns in Fig. 4. Measurements in Bragg–Brentano setup were performed at room temperature after annealing. Vertical lines indicate 2θ nominal positions for
Fig. 3. (a) The difference in the structure of as-deposited Cr1 − x − y − zAlxYyTazN coatings with various compositions; (b) Comparison of ab initio (calculated for (Cr0.5Al0.5)1 − xTaxN model) and experimentally obtained lattice parameters.
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cubic δ-TaN (ICDD:03–065-9404), hex ε-TaN (ICDD:01–071-0253), hex Ta2N (ICDD:00–026-0985), cubic AlN (ICDD: 00–046-1200), hex w-AlN (ICDD:01–076-0702), cubic CrN (ICDD:01–076-2494), hex β-Cr2N (ICDD:00–035-0803), cubic Cr (ICDD:01–074-7045), cubic Cr2Ta (ICDD:00–020-0317) and tetragonal Cr0.8Ta1.2N (ICDD:00–073-6154). Cubic fcc-CrAlYN solid solution forming tantalum-free Cr0.49Al0.50Y0.01N coatings begins to decompose when the annealing temperature exceeds 900 °C (Fig. 4a). Further increase in the annealing temperature is accompanied by nitrogen release from the structure (confirmed by EDX and TGA). XRD pattern of the coatings annealed at
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1000 °C shows a reduction in the fcc-CrAlYN peak intensity and appearance of additional reflections at 2θ ~33°, 36°, and 38° indicating formation of hexagonal h-Cr2N and w-AlN phases. This means that AlN precipitates begin to form in the structure and a simultaneous N2release from the coatings leads to the transformation of the remaining Cr-enriched matrix into metastable h-Cr2N phase. Increase in the annealing temperature above 1100 °C leads to further N2 loss and subsequently to transformation of Cr2N into Cr which is confirmed by the presence of strong reflection at 2θ ~ 44° belonging to bcc-Cr. The completely decomposed structure is formed by a dual-phase system
Fig. 4. Structural evolution of Cr1 − x − y − zAlxYyTazN coatings with Ta content of (a) z = 0; (b) z = 0.03; (c) z = 0.07; (d) z = 0.12; (e) z = 0.21.
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bcc-Cr–w-AlN. EDX analysis of this N2-depleted structure after annealing at 1200 °C in Ar + 5%H2 atmosphere revealed remaining nitrogen content of approximately 10 at.%. The Cr0.46Al0.46Y0.01Ta0.07N coating with Ta content of z = 0.03 and z = 0.07, respectively, annealed at 900 °C still retain the character of the fcc-CrAlYTaN solid solution (Fig. 4b,c). Onset of the decomposition, which is accompanied by the loss of nitrogen, occurs after the temperature exceeds 1000 °C. For the coating with Ta content of z = 0.12 the XRD pattern after annealing at 1100 °C (Fig. 4d) shows that decomposition products are not thermally stable (e.g. Cr2N), while in the pattern after 1200 °C annealing is dominant reflection from w-AlN and probably Cr-rich bcc-Cr(AlTa) phase. The series of XRD patterns for the highest Ta containing samples (z = 0.21) after 1000 °C annealing (Fig. 4e) show an increase of the right-hand shoulder of the strongest fcc peak, broadening of peaks and a weak scattering around 2θ ~ 32°, probably due to starting w-AlN formation. The structural evolution with increasing annealing temperature was similar in all measured samples. From 900 to 1000 °C peak intensities of initial fcc-CrAlY(Ta)N phase is reduced and additional reflections appeared indicating the formation of hexagonal h-Cr2N and w-AlN phases. Upon further release of N2 by annealing above 1000 °C the metastable h-Cr2N phase transforms into bcc-Cr(AlTa) metallic phase. Eventually from 1200 °C predominantly w-AlN and bcc-Cr(AlTa) phases remain. Higher content of Ta causes in addition formation of h-Ta2N and hTaN phases. In summary, the higher the Ta content, the higher decomposition temperature of the cubic solid solution. For better overview of the extent of decomposition processes, Fig. 5 shows XRD diffractograms of samples with various Ta content annealed at 1000 °C. For Ta-free coatings the decomposition of the fcc-CrAlYN phase is fully accomplished, whereas for the tantalum containing samples (z = 0.12 and z = 0.21, respectively), the fcc-CrAlYTaN phase just starts to decompose. Broadening of peaks indicates the decrease of crystallite size and formation of broad maximum around 2θ ~32° implies appearance of another phase. Probably formation of t-Cr0.8Ta1.2N, h-Cr2N or w-AlN is initiated. This is in accordance with TGA data (Fig. 1), where we observe about 50–100 °C shift of the beginning of mass reduction. The samples with smaller Ta content, in addition, show clearer presence of h-Cr2N and for the smallest Ta amount wAlN is already visible, together with some cubic CrTa metallic phase. For samples annealed at higher temperatures, there is certain ambiguity in assigning diffraction peaks to a specific diffraction pattern due to multiphase mixture of overlapping patterns and relatively broad peaks. In addition, it is notable that for pure Ta–N system more than 10 (meta)stable phases has been reported. For these reasons and for possibility of Cr–Ta–N and Ta–Al–N ternaries formation it is difficult to identify, distinguish and index peaks of all crystal phases in patterns from high annealing temperature, especially above 1100 °C. Positive influence of tantalum alloying on thermal stability of Cr1 − x − y − zAlxYyTazN coatings can be explained by increased cohesive energy Ec, which is defined as energy required to dissociate crystalline material into individual atoms. The calculated values of Ec per atom are 5.054 eV, 5.302 eV, 5.541 eV and 7.57 eV for fcc CrN, (CrAl)N, (CrAl)0.79Ta0.21N and TaN, respectively (for Al:Cr = 1:1). These results show that by adding the highest content of Ta (z = 0.21) into (CrAl)N solid solution increases the cohesive energy per atom by about 4.5%. Higher value of Ec for Ta-containing system demonstrates the strengthening effect of alloying elements that can be understood as a pure electronic effect when dopant atoms increase valence electron density compared to the original structure [36]. These additional electrons take part in bonding and hence cause stronger bonds between atoms in the doped solid solution. DFT calculations also allow determining mixing free energies ΔF(x) of solid solutions, where negative values represent thermodynamical stability of solutions with respect to considered decomposition products, while positive values reflect their tendency to decompose. Structural optimization runs were carried out to obtain energies of
Fig. 5. Comparison of XRD patterns of the coatings with various Ta content annealed at 1000 °C in Ar + H2 atmosphere.
relaxed configurations at zero pressure and temperature, which were then used to calculate mixing free energies shown in Fig. 6. These mixing free energies were calculated as ΔF(x) = ΔE(x) − TΔSc(x), where ΔE(x) is the standard mixing enthalpy term (energy, at pressure P = 0) and TΔSc(x) represents the mean-field estimation of configurational entropy Sc(x) of a solid solution weighted by temperature (electronic and vibrational entropy were not considered). Configurational entropy of a substitutional alloy comes from the irregular presence of different types of atoms occupying individual lattice sites. If occupancy of these sites is strictly random, configurational entropy can be estimated (for two types of substituting elements in a system defined solely by x) from the mixing entropy formula [34] Sc(x) = − kB [xln(x) + (1 − x)ln(1 − x)] per atom. In ternary (AlTM)N solid solutions, Al and TM atoms occupy one out of two sublattices in NaCl-type fcc structure and hence the prefactor one half has to be included into the entropy formula. The values of ΔF(x) given in Fig. 6 then represent values of free-energy differences per atom. In Fig. 6, mixing free energies of ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (CrAl)1 − xTaxN systems for T = 1600 K are shown. The free-energy differences were calculated for fcc ternaries (and quaternary) with respect to w-AlN and fcc-TaN and CrN binaries. The configurational entropy promotes lattice disorder for increasing temperatures by lowering the alloy free energy. For the negative (though relatively small in absolute value) mixing free energy for fcc-(CrTa)N solid solution, we may conclude that the calculations indicate its stability with respect to decomposition into
Fig. 6. Mixing free energies ΔF of ternary Al1 − xTaxN, Cr1 − xTaxN, Al1 − xCrxN and quaternary (CrAl)1 − xTaxN systems for T = 1600 K. The presented data suggests table or unstable (negative/positive ΔF) solid solutions as a function of x.
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fcc-CrN and fcc-TaN. In the cases of Al1 − xTaxN, Al1 − xCrxN and (CrAl)1 − xTaxN systems, ΔF(x) is mostly positive, indicating a tendency for decomposition. It should be noted that according to the paper [36], the addition of yttrium leads to increase of the driving force for decomposition of the cubic solid solution Ti1 − x − yAlxYyN with y = 0.1 due to the large difference lattice parameter between c-YN and c-AlN. However, the Y content in our Cr1 − x − y − zAlxYyTazN coatings was negligible (y = 0.02) and therefore yttrium was not included in ab initio calculations. We note that our calculated mixing free energies (excluding the entropy term) are in good agreement with previous calculations of mixing enthalpies ΔE(x) for Al1 − xTaxN [22], Al1 − xCrxN [29] and also Cr1 − xTaxN [35]. Annealed Cr0.45Al0.42Y0.01Ta0.12N was chosen for further structure study by means of TEM in order to provide an additional insight into the nanostructure. STEM micrograph (Fig. 7) of the coating illustrates the structural evolution taking place during the annealing at 1200 °C. The structure was composed of equiaxed grains with diameter in the range of approx. 20–50 nm. It can also be seen that the elemental distribution in the structure is not homogeneous (lighter and darker regions). Fig. 7b–d depict overlaid color-coded EDX maps of Al, Cr and Ta after the sample annealing at 1200 °C. Aluminium and chrome create almost inverse maps (Fig. 7d), indicating that they do not have a tendency to coexist in the grains. However, tantalum and chromium maps are highly overlapped (Fig. 7b), indicating the formation of a mixed Cr1 − xTaxN phase, in agreement with ab initio calculations presented above. On the map of tantalum with aluminium (Fig. 7c) several dark regions
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can be seen which contain only chromium atoms, but there are also places that are overlapping, which may indicate mixture of w-AlN and Ta(Cr)N grains. 4. Conclusions We have grown Cr1 − x − y − zAlxYyTazN coatings with Ta contents in the range of z = 0 to z = 0.21 and N / (Cr + Al + Y + Ta) = 1. The obtained experimental results and analysis have been supported by ab initio calculations. All prepared Cr1 − x − y − zAlxYyTazN coatings in the as-deposited state exhibit a single phase cubic structure identified as fcc-CrAlY(Ta)N solid solution. The structural investigation indicates that tantalum-free Cr0.49Al0.50Y0.01N coatings start to decompose when the annealing temperature exceeds 900 °C and results in the formation of w-AlN precipitates in the remaining Cr-rich matrix. Unstable Cr–N bonds at higher temperatures and N-loss lead to dual-phase structure consisting of w-AlN and bcc-Cr. The presence of Ta in the solid solution shifts the start of the decomposition process to higher temperatures (N 1000 °C for Cr0.42Al0.36Y0.01Ta0.21N with Ta content of z = 0.21), leading to improvement of thermal stability compared to CrAlYN. This improved thermal stability can be attributed to higher cohesive energies of the Tacontaining solid solutions, according to ab initio calculations. Decomposition of solid solutions during gradual increase of temperature to 1200 °C accompanied by a loss of nitrogen continues with the formation of cubic and hexagonal Cr- and Ta-containing binaries or ternaries. DFT
Fig. 7. (a) STEM micrograph of the Cr0.42Al0.36Y0.01Ta0.21N coating illustrating the structural evolution after annealing at 1200 °C; (b–d) EDX maps of Al, Cr and Ta from STEM micrograph.
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