Composition driven phase evolution and mechanical properties of Mo – Cr – N hard coatings

Although many research activities concentrate on transition metal nitrides, due to their excellent properties, only little is known about Mo-N based materials. We investigate in detail the influenc ...


I. INTRODUCTION
Many research activities are conducted on TiN, ZrN, and CrN based coatings, [1][2][3][4][5][6][7][8] but only little is known about Mo-N based materials.0][11][12][13] Structure and phase development of Mo-N coatings are extremely sensitive to the N 2 -partial pressure used during depositions.With increasing nitrogen partial pressure, the formation of the hexagonal close packed hcp d-MoN (P6 3 /mmc) 14,15 is promoted next to face centered cubic (fcc) c-Mo 2 N (Fm-3 m). 16For extremely high nitrogen pressures, even the metastable n-MoN (Fm-3 m) is accessible. 17owever, the formation of interatomic bonds in Mo-N, by filling of antibonding electron states, results in low chemical and thermal stability. 18Therefore, we use the concept of alloying Mo-N with Cr, to increase the chemical and thermal stability.Especially, the oxidation resistance will significantly be improved due to the formation of a dense Cr-rich oxide, such as shown for Cr-N coatings. 19Zhou et al. 20 highlighted that single-phase cubic structured Mo-Cr-N phases exhibit a high driving force towards an isostructural decomposition, providing a high potential for spinodal decomposition similar to single-phase cubic structured Ti-Al-N. 21With increasing Mo content, the metallic bonding character within single-phase cubic Mo-Cr-N increases, leading to an improved ductility. 22Besides the computational studies, 20,23 there are further experimental investigations within the Mo-Cr-N system, 5,[24][25][26][27][28][29] but these often yield contradictory results.For example, the addition of Mo reduces 5,29 or increases 25,27 the mechanical properties of Cr-N.Furthermore, all experimental studies [25][26][27]30 focussed on the Cr-rich side with Cr/(MoþCr) atomic ratios above 0.5.
Therefore, we concentrate our investigations on the Mo-rich side of Mo-Cr-N coatings with Cr/(MoþCr) atomic ratios below 0.5.Varying the N 2 -to-total pressure ratio, p N 2 /p T , during deposition, allows the development of ternary coatings along the quasi-binary Mo 2 N-CrN or MoN-CrN tie line with predominant face centered cubic structure.

II. EXPERIMENTAL DETAILS
Various Mo-Cr-N coatings are synthesized, using a modified Leybold Heraeus Z400 magnetron sputtering system, in mixed Ar and N 2 (both gases with purity above 99.999%)glow discharges.Small cubes of Cr (99.99% purity, 3 Â 3 Â 3 mm 3 ) were uniformly arranged (4, 8, 12, 16,  20, and 36 pieces) on the race track of a molybdenum target (99.99%purity, Ø75 mm) to vary the Cr/(MoþCr) ratio within the films prepared.The substrates are centered parallel above the target at a distance of 55 mm.All depositions were prepared with 0.4 A DC target current and floating potential of the substrates ($À15 V), a total pressure, p T , of a) fedor.klimashin@tuwien.ac.at 0021-8979/2015/118(2)/025305/7 V C Author(s) 2015 118, 025305-1 0.35 Pa, and a substrate temperature of 450 6 20 C. The latter one corresponds to a homologous temperature T/T m of about 0.3 of these nitrides 14 and based on Thornton's zone model 31 suggests for a zone T structure.The chamber was always evacuated to a base pressure of p base 5 Â 10 À4 Pa.Pre-studies showed that single-phase face centered cubic Mo 2 N coatings can be prepared with N 2 -to-total pressure ratios, p N 2 /p T , between 0.32 and 0.44.Higher ratios result in an increasing phase fraction of hcp d-MoN.Contrary, single phase face centered cubic c-CrN coatings can be synthesized for ratios p N 2 /p T !0.44, and lower ratios promote the crystallization of hexagonal close packed h-Cr 2 N. Figure 1 shows a scheme of these phase evolutions.Based on these prestudies, the Mo-Cr-N coatings are prepared with three different N 2 -to-total pressure ratios, p N 2 /p T ¼ 0.32, 0.44, or 0.69, to allow for the development of single-phase ternary coatings.
Phase analyses were performed with an X-ray diffractometer Philips X'Pert using monochromized Cu K a radiation.Lattice parameters and dimensions of coherently scattering domains, d, were obtained by Williamson Hall plots and pseudo-Voigt approximations, respectively.
Fracture cross-sections of coated Si-substrates are investigated by scanning electron microscopy (FEI Quanta 200 FEGSEM with a spatial resolution of about 2 nm) for evaluating the film growth morphology.An integrated EDAX Genesis system allows analysing the elemental composition by means of energy dispersive X-ray spectroscopy (EDS).Several samples were investigated by Time-of-flight Elastic Recoil Detection (TOF-ERDA) to calibrate the EDS using 36 MeV 127 I primary ions at the tandem accelerator at Uppsala University, Sweden.The recoil detection angle in the ERDA experiments was 45 .More details on TOF-ERDA can be found elsewhere. 32Further investigation of the growth morphology and film structure is conducted by transmission electron microscopy (TEM) using a FEI TECNAI F20 operated at 200 kV, with lattice resolution of about 0.14 nm.
Indentation hardness, H, and modulus, E, of the coatings on austenitic substrates (due to the better adhesion) are characterized with a UMIS unit equipped with a Berkovich diamond tip and applying loads within the range of 3-30 mN.The film-only indentation hardness, H, and modulus, E, are obtained by evaluating the load-displacement curve after Oliver and Pharr. 33Subsequently, in order to guarantee for minimized substrate interference, only indentations from a fully developed plastic zone are used, i.e., a region of nearly constant values of indentation hardness and modulus over the penetration depth ("plateau" method).In the case of partially developed plastic zone, i.e., insufficient coating thickness, the film-only indentation modulus, E, resulted from extrapolation of the measured values back to zero indentation depth. 34All data are analysed with a Poisson's ratio, , of 0.25 for our Mo-Cr-N thin films.The indenter geometry correction factor was obtained through indentation tests with varying maximum loads of fused silica with known E and values of 72.5 GPa and 0.17, respectively. 35Verification of H and E is carried out with a series of well-characterized single phase and polycrystalline reference samples: fused silica, silicon, and sapphire.The coatings are also characterized for their as-deposited biaxial stresses using the cantilever beam method and applying the Stoney equation. 36

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Please note that the nitrogen-rich phase is face centered cubic for Cr-N, but hexagonal for Mo-N.The packing density is significantly higher for the face centered cubic phases than that for the hexagonal phases.Consequently, the deposition rate increases with increasing fraction of the hexagonal phases (for Cr-N as well as for Mo-N).
The further reduction in deposition rate for CrN y coatings to 0.8 nm/s upon increasing p N 2 /p T to 0.69 is mainly based on poisoning effects, 37,38 as the coating is still singlephase cubic structured (see Fig. 2(b)).The chromium content within our Mo-Cr-N films increases from 0 to $32 at.% with increasing number of Cr-cubes (from 0 to 36) at the Mo-target.Our elemental analysis suggests that with increasing Cr content also the incorporated nitrogen increases, for example, from 35 to 38 at.% when using p N 2 /p T ¼ 0.32 (see Fig. 3).Corresponding results are also obtained for p N 2 / p T ¼ 0.44 and 0.69, where the Cr-content also increases from 0 to $30 at.%, but the nitrogen increases approximately from 39 to 45 at.% and from 40 to 51 at.%, respectively.These data indicate that the nitrogen content within our Mo-Cr-N films is closely related to the metal sublattice population, as highlighted in Fig.

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the lowest Cr content of the ternary Mo-Cr-N coatings, small XRD peaks of the hexagonal d-MoN-based phase can be detected (see Fig. 7(c)).However, these coatings are also single-phase face centered cubic structured, if their Cr content is above 15 at.% (or the Cr/(MoþCr)-ratio, x, is above 0.29).
The dimensions of coherently scattering domains, obtained by Warren Averbach analysis of the XRD patterns, of all ternary Mo-Cr-N coatings investigated are between 10 and 30 nm.The pronounced (200)-peaks are used to derive the corresponding lattice parameters of the cubic Mo 1-x Cr x N y solid solution with respect to their Cr content and p N 2 /p T ratios used during deposition (see Fig. 8).The lattice parameters of the coatings prepared with p N 2 / p T ¼ 0.32 and 0.44 are in excellent agreement with ab initio calculated values for face centered cubic Mo 1Àx Cr x N 0.5(1þx) solid solutions.Consequently, in addition to the chemical composition (Fig. 4), also the lattice parameters suggest that these coatings are composed of fcc solid solutions along the Mo 2 N-CrN quasi-binary tie line.
The lattice parameters of the coatings prepared with the highest p N 2 /p T -ratio of 0.69 meets the ab initio obtained lattice parameters of fcc solid solution Mo 1Àx Cr x N 20 along the quasi-binary MoN-CrN tie line for Cr contents x above 0.4 (see Fig. 8).The lattice parameter variations are actually in excellent agreement with the chemical variation, especially with respect to the nitrogen content.For example, with increasing nitrogen content of the binary MoN y coating from 35 to 41 at.%, the lattice parameter increases from 4.199 to 4.242 A ˚, suggesting that the fcc c-Mo 2 N structure (half occupied N sublattice, hence actually, fcc MoN 0.5 ) approaches the metastable fcc n-MoN structure (with fully occupied N sublattice).This significant increase in lattice parameter of the fcc structure within our Mo-N coatings is not represented in their compressive biaxial stresses, which are at around À1.2 GPa for p N 2 /p T ¼ 0.32, 0.44, and 0.69 (see Fig. 9).We envision that the formation of a small fraction of d-MoN phases next to fcc-Mo 2 N for p N 2 /p T ¼ 0.44 and 0.69-as suggested by the XRD studies, see Fig. 2-counteracts for the expected increasing compressive stresses when filling the Nsublattice.The formation of d-MoN phases, when increasing p N 2 /p T from 0.32 to 0.44 or 0.69, is also represented by the decreasing hardness from 33 to 28 GPa (see Fig. 10).The indentation modulus only slightly decreases from 425 to 412 GPa.
The binary CrN exhibits an even more pronounced change in hardness from H ¼ 17 to 26 GPa upon increasing the N 2 -to-total pressure ratio from 0.32 to 0.69, because here the structure significantly changes from mixed h-Cr 2 N þ c-CrN to single phase c-CrN (see Fig. 2(b)).Thereby, also the tensile stresses are reduced from þ0.3 to þ0.1 GPa (Fig. 9).Corresponding results (increasing hardness and decreasing tensile stresses of magnetron sputtered CrN coatings with increasing N 2 content of the working gas) are already reported earlier. 39he majority of the ternary Mo 1Àx Cr x N y coatings exhibit hardness between 28 and 31 (62) GPa (almost within the error of measurement) with no significant dependence on the Cr content within the investigated region of 0.1 x 0.6 (see Fig. 10).However, the data clearly suggest that the coatings prepared with the lowest p N 2 /p T exhibit hardness at the upper limit, whereas those prepared with the highest p N 2 /p T exhibit hardness at the lower limit of the 28-31 GPa range.Consequently, the two ternary Mo-Cr-N coatings with hardness above this range (H ¼ 33 6 1 GPa for Mo 0.89 Cr 0.11 N 0.54 and H ¼ 34 6 2 GPa for Mo 0.81 Cr 0.19 N 0.52 ) are prepared with p N 2 /p T ¼ 0.32, and the only ternary Mo-Cr-N coating with a hardness below this range (H ¼ 27 6 1 GPa for Mo 0.71 Cr 0.29 N 0.92 ) is prepared with p N 2 /p T ¼ 0.69.This coating exhibits also the lowest indentation modulus of 353 6 5 GPa among all ternary Mo-Cr-N coatings studied.Similar to the hardness, most Mo-Cr-N coatings have similar indentation moduli (between 390 6 10 and 410 6 10 GPa) with no significant dependence on the Cr content.
The slightly decreasing hardness with increasing Cr content of our ternary Mo 1Àx Cr x N y coatings-especially when prepared with p N 2 /p T ¼ 0.32-is well represented by their decreasing compressive stresses.

IV. DISCUSSION
As mentioned in Sec.I, PVD allows to prepare metastable n-MoN (Fm-3 m), 17 if the ionization degree and/or the nitrogen partial pressure is very high.This metastable phase is based on the stable high-temperature phase fcc c-Mo 2 N (Fm-3 m), 14,40 but with (theoretically) fully occupied octahedral sites of the fcc structure formed by Mo.Within fcc c-Mo 2 N, these octahedral sites are only half-filled with randomly distributed nitrogen.Hence, our metal-sublattice-normalized notification (see Sec. I), MoN 0.5 , would be more correct.However, due to the possible formation of n-MoN or c-Mo 2 N, both quasi-binary tie lines (Mo 2 N-CrN and MoN-CrN) are important for the discussion of chemistry, structure, and phase evolution within ternary Mo-Cr-N coatings prepared at higher N 2 -to-total pressures.We do not consider the quasi-binary tie line Mo 2 N-Cr 2 N, which would be relevant for rather low N 2 -partial pressures, where also metallic Mo-phases develop (see Fig. 1).
Even for the highest N 2 -to-total pressure ratio used (p N 2 / p T ¼ 0.69), no n-MoN phase, with a (theoretically) fully occupied N-sublattice, was formed.Their nitrogen content is about 41 at.% (i.e., MoN 0.68 , see Fig. 4), but here already a small phase fraction of hcp d-MoN can be detected by XRD, compare Figs. 4 and 2. The maximum nitrogen solubility within fcc MoN y is y % 0.55 under thermodynamic equilibrium conditions. 14However, when investigating the lattice parameter of the face centered cubic structure, a pronounced increase with increasing p N 2 /p T is obtained (see Fig. 8).The comparison with the calculated values for c-Mo 2 N and n-MoN clearly suggests that the amount of nitrogen-vacancies, which is 50% of the fcc N-sublattice within c-Mo 2 N, decreases to nearly 38% with increasing p N 2 /p T .Hence, nearly single-phase c-MoN 0.62 (using the simplified linear interpolation between MoN 0.5 and MoN) can be synthesized by reactive magnetron sputtering.A corresponding result was already suggested by Anitha et al. 16 However, we need to mention that the accuracy for nitrogen detection within the EDS system used is by around 62 at.%, which we confirmed by ERDA measurements of MoN 0.

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CrN y would agree better with the XRD investigations, suggesting a small fraction of h-Cr 2 N next to the dominating c-CrN for the coating prepared with p N 2 /p T ¼ 0.32.
Nitrogen over-stoichiometric compositions can be explained by metal vacancies, nitrogen ions on anti-sites (N on metal places), and the formation of nitrogen interstitials. 41,42However, for CrN y , the latter is energetically unpreferred (by more than 0.1 eV/at) over the previous two mechanisms for nitrogen contents above 52 at.%, i.e., y !1.1. 42ased on the chemical (Fig. 4) and structural (Fig. 7) investigations of our ternary Mo 1Àx Cr x N y coatings, we can conclude that the Cr content and the N 2 -partial pressure used during deposition determine the population of the Nsublattice of the face centered cubic structure.If we use a high nitrogen partial pressure (p N 2 /p T ¼ 0.69), the nitrogen content in our ternary coatings rapidly increases to y ! 1 (i.e., N-content !50 at.%) with increasing Cr-content to x !0.4 (i.e., Cr-content !20 at.%), suggesting for a (theoretically) fully occupied N-sublattice.Consequently, the lattice parameter of the fcc phase approaches and follows the calculated values for fcc c-Mo 1Àx Cr x N. Our results (chemistry and structure) clearly suggest that the single-phase solid solution fcc c-Mo 1Àx Cr x N can be prepared by reactive magnetron sputtering for x !0.4 (i.e., up to 60 at.% of Mo at the metal sublattice); although, Mo is thermodynamically insoluble (at least for contents above 1 at.%) in c-CrN. 43or low nitrogen-partial pressures (p N 2 /p T ¼ 0.32 and 0.44), the nitrogen content closely follows the quasi-binary tie line Mo 2 N-CrN (see Fig. 4).Consequently, the chemistry of our ternary Mo 1Àx Cr x N y coatings can nearly be described with Mo 1Àx Cr x N 0.5(1þx) , meaning that half as many nitrogen ions are additionally added to the half-filled N-sublattice as Cr ions substitute for Mo, y ¼ 0.5(1þx).(In other words, for the substitution of two Mo-ions with two Cr-ions, one vacancy of the N-sublattice within a c-MoN 0.5 based structure is removed.)This is also represented by the lattice parameter variation of the fcc phase with the Crcontent x, which follows the ab initio calculated values for fcc c-Mo 1Àx Cr x N 0.5(1þx) (see Fig. 8).
Contrary to some previous studies, 5,30 which actually concentrate only on the Cr-rich side of Mo-Cr-N (with Cr/ (MoþCr) ratios above 0.4, hence x > 0.4), we obtain higher hardness and indentation moduli for all of our ternary Mo 1Àx Cr x N y coatings than for CrN (see Fig. 9).The highest hardness is obtained for the single-phase fcc c-Mo 0.81 Cr 0.19 N 0.52 coating, prepared with p N 2 /p T ¼ 0.32, which actually exhibits comparable indentation moduli to the other ternaries.Therefore, this coating also yields the highest resistance against plastic deformation (H 3 /E 2 (Ref.44)) of 0.2 GPa.

V. CONCLUSIONS
Magnetron sputtered Mo-Cr-N coatings are studied in detail as a function of their chemical composition and N 2 -tototal pressure ratios, p N 2 /p T , used during deposition.The Mo-N and all ternary coatings studied exhibited a pronounced face centered cubic 200-growth orientation with small coherently scattering domains between 10 and 30 nm.
All ternary coatings prepared with p N 2 /p T ¼ 0.32 and 0.44 are single-phase fcc structured with a chemistry along the quasi-binary Mo 2 N-CrN tie line.This can be described by Mo 1Àx Cr x N 0.5(1þx) , indicating that with the addition of Cr, x, also the vacant sites at the N-sublattice decreases, by 0.5x.This is additionally confirmed by their lattice parameters, which are in excellent agreement with ab initio obtained values for fcc c-Mo 1Àx Cr x N 0.5(1þx) .
Preparing the Mo-N coatings with a high N 2 -to-total pressure ratio, p N 2 /p T ¼ 0.69, leads to the formation of fcc c-MoN 0.68 -where a part of the N-vacancies at the Nsublattice of c-MoN 0.5 is occupied-and a small fraction of hcp d-MoN.However, for Cr-contents of x !0.29, all coatings are again single-phase fcc structured and their chemistry as well as lattice parameters approach and follow the quasibinary MoN-CrN tie line, hence c-Mo 1Àx Cr x N.
The highest hardness values of 33-34 GPa are obtained for the coatings prepared with the lowest N 2 -to-total pressure ratio, p N 2 /p T ¼ 0.32, combined with low Cr-contents of x 0.19.All other ternary Mo 1Àx Cr x N y and MoN y coatings exhibit hardness within the range of 28-31 GPa, with a tendency for lower values when prepared with p N 2 /p T ¼ 0.69.These data indicate that filling the N-vacancies, of the Nsublattice, within the fcc c-MoN 0.5 based structure leads to a hardness reduction.The indentation moduli decrease from $440 GPa for fcc c-MoN 0.53 to $350 GPa for fcc c-Mo 0.38 Cr 0.62 N 1.06 with increasing Cr content, x.The binary CrN y coatings have indentation moduli of $310 GPa.Consequently, the coating c-Mo 0.81 Cr 0.19 N 0.52 , which exhibits the highest hardness of 34 6 2 GPa among all coatings studied, also exhibits the highest resistance against plastic deformation with H 3 /E 2 % 0.2 GPa.
Based on our results, we can conclude that fcc c-Mo 1Àx Cr x N y films-especially with a low Cr-content and a high density of N-vacancies-provide excellent mechanical properties to be beneficial for many industrial applications.
FIG. 1. Schematic illustration of the phase evolution with increasing N 2 -tototal pressure ratios, p N2 /p T , used during deposition of binary Mo-N and Cr-N coatings.The red dashed vertical lines indicate three p N2 /p T ratios, 0.32, 0.44, and 0.69, used for the current study.
FIG. 3. Elemental composition, obtained by EDS and verified for MoN 0.54 and Mo 0.62 Cr 0.38 N 1.04 by ERDA, of Mo-Cr-N coatings deposited with p N2 / p T ¼ 0.32 and different amount of Cr cubes placed on the Mo-target race track.The error bars for Cr-and Mo-concentrations are smaller than the symbol size.

FIG. 4 .
FIG. 4. Nitrogen concentration (in at.%) within our ternary Mo 1Àx Cr x N y coatings, prepared with p N2 /p T ¼ of 0.32, 0.44, and 0.69, as a function of their Cr/(MoþCr)-ratio, x.The two coatings, which are investigated by Elastic Recoil Detection Analysis, are labelled with ERDA.The error bars for Cr/(MoþCr)-ratios are smaller than the symbol size.

FIG. 6 .
FIG.6.Cross-section TEM image of Mo 0.70 Cr 0.30 N 0.61 prepared with p N2 / p T ¼ 0.32.The small inset is the SAED of a representative area within the coating.
FIG. 9. Biaxial residual stresses of our Mo-Cr-N coatings deposited with p N2 /p T ¼ 0.32.The stresses for binary Mo-N and Cr-N prepared with p N2 / p T ¼ 0.44 and 0.69 are added for comparison.The error bars for Cr/ (MoþCr)-ratios are smaller than the symbol size.