Phase composition and mechanical properties of homostructure NbN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering

Journal Pre-proof Phase composition and mechanical properties of homostructure NbN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering

Y.G. Li, H. Yuan, Z.T. Jiang, N. Pan, M.K. Lei PII:

S0257-8972(20)30056-6

DOI:

https://doi.org/10.1016/j.surfcoat.2020.125387

Reference:

SCT 125387

To appear in:

Surface & Coatings Technology

Received date:

18 July 2019

Revised date:

15 January 2020

Accepted date:

19 January 2020

Please cite this article as: Y.G. Li, H. Yuan, Z.T. Jiang, et al., Phase composition and mechanical properties of homostructure NbN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering, Surface & Coatings Technology (2020), https://doi.org/10.1016/j.surfcoat.2020.125387

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Phase composition and mechanical properties of homostructure NbN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering Y.G. Li, H. Yuan, Z.T. Jiang, N. Pan, M.K. Lei* Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

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ABSTRACT

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Homostructure NbN nanocomposite coatings with cubic δ-NbN phase and hexagonal δ’-NbN phase were deposited by modulated pulsed power magnetron sputtering (MPPMS)

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under varied nitrogen flow rate fN2 from 15% to 30%. Low fN2 favored the formation of

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δ’-NbN phase with (100) preferred orientation and δ-NbN phase with (200) preferred orientation, while high fN2 favored the formation of δ-NbN phase with (200) preferred

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orientation. The NbN coatings were characterized as the nanocomposite coatings with

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δ’-NbN nanocrystallites embedded into δ-NbN matrix. The hardness and modulus of NbN coatings went up to 36 GPa from 30 GPa and 460 GPa from 366 GPa as fN2 increased to 20%

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with residual compressive stress from 0.47 GPa to 1.93 GPa, then decreased to 29 GPa and 389 GPa with residual compressive stress of 1.01 GPa as fN2 further increased. Meanwhile, the toughness of coatings increased with the content of δ’-NbN phase showing an abrupt change of NbN coating toughness under the fN2 of 30%. The homostructure NbN nanocomposite coatings with both enhanced hardness and toughness were achieved by forming nanocomposite structure at low fN2. The results showed that the essential conditions of the design criterion for hard and tough coatings should be arranged in the impact of weight as follows: 1) the designed microstructure, 2) the compressive residual stress, 3) high H/E and H3/E2, 4) high We.

Keywords: NbN nanocomposite coatings, modulated pulsed power magnetron sputtering, nitrogen flow rate, peak power, toughness, phase transition 1

Journal Pre-proof 1. Introduction Heterostructure coatings, nanomultilayer or nanocomposite, were proved to be a good route to attain high hardness [1-4]. Hard coatings with enhanced toughness can be specially tailored by proper control of coating microstructure [5-7].The presence of second phase on grain boundaries, in a nanocomposite coating which generally composed of two phases, helped to deflect and terminate nanocracks enhancing grain boundary sliding, thus improving the coating toughness. However, Friedel oscillation in heterostructure materials was believed

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to have an impact on the valence charge density limiting the achievable strength and hardness [8].

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NbNx coatings on a wide range of nitrogen content presented with several different

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phases, thereafter NbN was a promising material for depositing homostructure coatings

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which may avoid the potential drawbacks in heterostructure coatings. Given the recent investigations on nitrogen-rich NbNx phase, four typical structures were expected: NaCl-type

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δ-NbN, body-centered tetragonal γ-NbN, anti-WC-type ε-NbN, and anti-NiAs-type δ’-NbN. Wen et al. [9] deposited NbN coatings on Si(100) substrates by using dc reactive magnetron

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sputtering under varied nitrogen flow rate. Increasing the nitrogen flow rate from 7% to 25%, the NbN coatings only formed δ-NbN phase, then the NbN coatings were composed of

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δ-NbN and δ’-NbN phases under fN2 ranging from 33% to 38%. Further increasing fN2 to 42%, the NbN coating turned to form δ’-NbN phase only. Qi et al. [10] reported that when fN2 increased to 40%, the NbN coatings were only with δ-NbN phase without observing any δ’-NbN phase, and higher fN2 tended to sacrifice the mechanical properties of NbN coatings. Ding et al. [11] deposited NbN coatings under fN2 ranging from 28% to 50% by using HiPIMS with cubic δ-NbN phase only observed in the coatings. The reported works gave a controversial condition to deposit NbN coatings with nanocomposite structure, nitrogen flow rate fN2 should be carefully selected to grain required two phases. Compared with those deposition techniques, MPPMS was a promising technique to deposited coatings with both enhanced hardness and toughness. The shape and magnitude of the long pulses in MPPMS, also known as the macropulses, was usually controlled by 2

Journal Pre-proof modulating the duty cycle of the micropulses, thus presenting more flexible control. Lin et al. [12] gave a comparative study of the CrN coating deposited by dcMS, pulsed dcMS, and MPPMS. The coatings deposited by MPPMS showed higher hardness and crack resistance to that of dc and pulsed dc coatings. In this paper, homostructure NbN nanocomposite coatings were deposited by MPPMS containing showing both δ-NbN and δ’-NbN phases. The microstructure and mechanical properties of homostructure NbN coatings were studied, and the essential requirements of the

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design criterion were discussed for hard NbN nanocomposite coatings with enhanced

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toughness.

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2. Experimental details

All NbN coatings were deposited in a closed field unbalance magnetron sputtering

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system. Detailed descriptions of the deposition system were detailed in our previous work [3].

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The sputtering target is niobium plate (99.95% purity) with the size of 440 mm×140 mm×8 mm, which is powered at a fixed macropulse frequency of 75 Hz, generated through a

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Zpulser AXISTM with a charging voltage of 380 V. The background pressure was vacuumed superior to 4.5×10-4 Pa. The sputtering pressure and mass flow rate were kept at 0.3 Pa and

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80 sccm. The nitrogen to argon flow ratio, was ranging from 15% to 30% with their purity of 99.99%. The substrate holder was paralleled to the target surface, and the mean substrate-target distance was kept constant at 100 mm without applying additional heat or bias. AISI 304 stainless steel and Si(100) with of size 20-20 mm were used as substrates, the substrates were ultrasonically cleaned in alcohol and acetone for at least 15 minutes before mounted into the deposition chamber. The substrate surface was sputtering etched by using the Ar ion bombardment at 2.5 Pa with a pulsed dc bias of -350 V, under a frequency of 100 kHz with 90% duty cycle for 20 minutes, and the target surface was presputtered for 5 minutes to clean the target surface. The discharge waveform was recorded by Tektronix TDS2014C oscilloscope as shown in Fig. 1, and each discharge parameters were at least an average of ten recorded data. These parameters were peak voltage Vp, peak current Ip, peak power Pp and average power Pa which 3

Journal Pre-proof were listed in Table 1. Phase structure and microstructure of the coatings were characterized by using a Shimadzu LabX XRD-6000 diffractometer (XRD), a ZEISS SUPRA 55-32-76 scanning electron microscope (SEM) and Tecnai 220S-TWIM high resolution transmission electron microscope (HRTEM). The samples for HRTEM observation were ground mechanically to a foil with a thickness of about 30 μm. The samples with the supporting ring were further thinned down to electron transparency using a Gatan Model 691 precision ion polishing with a 4.5 keV Ar+ beam and a 3–7° angle of beam-to-sample surface. The hardness

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(H) and elastic modulus (E) of the coatings were measured by MTS Nanoindenter XPTM equipped with a Berkovich diamond indenter. The indentation depth was selected below 10%

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of the film thickness to minimize the substrate effect. For each sample, at least nine effective

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measurements separated by a distance of 200 μm were made to obtain the statistical results.

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The elastic recovery We was calculated by the load-displacement curves by using mean value of the nine effective measurements in nanoindentation. Surfcorder ET4000AK surface

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profilometry was used to measure curvature change of the Si substrate with size of 30×3×0.36 mm to calculate the residual stress by using Stoney equation [13], and the coating

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residual stress for each case was an average of two sample measurements. The toughness resistance of NbN nanocomposite coatings to crack was assessed by

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another Vickers indention test with a diamond indenter at a load of 1000 mN. Failures of the NbN nanocomposite coatings were expressed by the radical crack formation originated from indent impression. The morphologies of Vickers diamond indenter imprints were analyzed by a ZEISS SUPRA 55-32-76 scanning electron microscope (SEM).

3. Results and discussions 3.1. Microstructure Fig. 2 shows the peak discharge current waveforms of MPPMS at varied nitrogen flow rate fN2. As nitrogen flow rate fN2 increased from 0% to 30%, the average power decreased linearly from 1.7 kW to 0.5 kW. When fN2 ranged from 15% to 20%, the discharge peak current waveform oscillated in a small amplitude before reaching the maximum. When fN2 4

Journal Pre-proof was above 20%, the peak current fluctuated in a much smaller oscillation amplitude, which were still an order higher than the average current. The decreased oscillation amplitude of current waveform was less significant compared to the low fN2 conditions suggesting the decrease of plasma ionization degree. When introducing the nitrogen flow rate fN2 from 15% to 30%, the peak voltage didn’t show a significant change as peak current. The peak power decreased from 54 kW to 16 kW according with the energy delivered in each macropulse from 23.2 J to 9.8 J, as illustrated in Table 1.

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Fig. 3 shows the XRD patterns of NbN nanocomposite coatings deposited by MPPMS under varied nitrogen flow rate fN2. Almost all NbN coatings were composed of cubic δ-NbN

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and hexagonal δ’-NbN phases. When nitrogen flow rate fN2 was at 15%, NbN coatings were

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mainly composed of δ’-NbN phase textured along (100) and (102) preferred orientation and δ-NbN textured along (111) and (200) preferred orientation. As nitrogen flow rate fN2

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increased, the content of δ’-NbN phase in NbN coatings gradually decreased with preferred

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orientation turned to (100), while δ-NbN phase was mainly textured along (200) preferred orientation. When fN2 reached 30%, the NbN coatings were mainly composed of δ-NbN

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phase showing (200) preferred orientation.

Fig. 4 shows the cross-sectional SEM images of NbN coatings under the varied nitrogen

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flow rate fN2 from 15% to 30%. At relative low nitrogen flow rate fN2 of 15%, the coatings exhibited dense microstructure with short columnar grains. While short columnar grains in the coatings gradually turned into long penetrated columnar grains with the increase of fN2. The changed microstructure should be caused by the decrease of discharge peak power owing to the introduction of nitrogen. High peak power could promote efficiency of momentum transfer between the growing film and the incoming metal ions, leading to a less pronounced columnar microstructure [14]. Fig. 5 shows the cross-sectional HRTEM images of NbN coatings. Fig.5a-f were for the NbN coatings deposited at nitrogen flow rate fN2 of 25%, Fig. 5b was fast Fourier transformation of Fig. 5a showing that the coatings were composed of cubic δ-NbN nanocrystalline phase and hexagonal δ’-NbN phase. Figs. 5d and 5e showed the typical 5

Journal Pre-proof electron diffraction of cubic and hexagonal NbN phases, the results further confirmed that the coatings were formed nanocomposite structure with hexagonal δ’-NbN phase embedded into cubic δ-NbN matrix. Fig. 5f gave an illustration of HRTEM image with δ and δ’- NbN phases well distributed. Fig. 5g-5l for nitrogen flow rate fN2 of 30%, Fig. 5h was fast Fourier transformation of Fig. 5g suggesting the coatings were mainly composed of cubic δ-NbN nanocrystalline phase. The content of δ’-NbN phase further decreased with the nitrogen flow rate reaching 30%, and no obvious δ’-NbN phase was observed which may exist as interface

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phase as illustrated in Fig. 5l. The discharge peak power changes of MPPMS was usually related to nitrogen flow rate

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fN2. Lin et al. [15] deposited CrN coatings by using MPPMS, the discharge peak current went

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up linearly from 90 A to 197 A when the nitrogen flow rate increased from 0% to 100%

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showing a linear change. Kimura et al. [16] deposited carbon nitride films by using HiPIMS with low sputtering yield carbon target employed, the peak current still got an obvious

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increase as fN2 increased from 2.5% to 50%. In this work, the discharge peak power decreased with the increase of nitrogen flow rate fN2, and the results were inverse with most reported

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works. Generally, two factors should be responsible for this tendency, the increase of nitrogen flow rate led to the change of target poisoning process with sputtering yield of the N2 gas

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lower with respect to Ar, as well as electron temperature Te [17], and the other factor should be attributed to the ionization potential difference between Nb and Ar resulting in more Nb2+. The second ionization potential of Nb is 14.0 eV which is lower than Ar first ionization potential of 15.76 eV. With the introduction of reactive nitrogen gas flow rate from 15% to 30%, the maximum of electron density ne should decrease, and electron temperature Te should increase [17]. Higher Te also promoted the generation of secondly charged Nb ions. For conventional dcMS, higher fN2 assisted the formation δ’-NbN phase. In this case, low nitrogen flow rate tended to assist the formation of δ’-NbN phase. When the nitrogen flow rate decreased from 30% to 15%, the energy delivered in each macropulse increased from 9.8 J to 23.2 J with the increase of discharge peak power. The high energy input in each macropulse should tend to assist the formation of δ’-NbN phase, since δ-NbN was 6

Journal Pre-proof unfavorable judging from the view point of system energy [18]. In cubic systems, the elastic strain energy density was usually lowest for the (100) orientation and highest for the (111) orientation [19]. The preferred orientation of NbN coatings were observed favoring the minimization of strain energy either.

3.2. Mechanical Properties Fig. 6 shows the hardness and elastic modulus of NbN coatings as a function of nitrogen flow rate fN2. The hardness and modulus of NbN coatings didn’t show a linear change with

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the nitrogen flow rate fN2. As fN2 increased from 15% to 20%, the hardness and modulus

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increased from 30.9 GPa to 36.3 GPa and 366.3 GPa to 460.4 GPa. Further increasing the fN2

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to 30%, the hardness and modulus then decreased to 29.4 GPa and 389.6 GPa. δ’-NbN phase was generally harder than δ-NbN phase, however the hardness of the NbN coatings didn’t

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follow the rule of mixture (ROM) values showing an indirect correlation with the

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composition of δ’-NbN phase. The nanocomposite structure should be one essential reason responsible for the anomalous increase in hardness [20-21].

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Fig. 7 shows the residual stress of NbN coatings as a function of nitrogen flow rate fN2. All NbN coatings showed compressive residual stress. When the nitrogen was introduced into

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the chamber of which rate varied from 15% to 30%, the compressive residual stress first increased almost a factor of 4 from 0.47 GPa to 1.93 GPa, then decreased to 1.01 GPa. Note that, the residual stress was composed of by intrinsic, extrinsic and thermal stress [22]. High peak current tended to assist the formation of compressive residual stress owing to the intrinsic stress, and thermal stress was excluded for the temperature showing no significant fluctuation. The residual stress of the NbN coatings deposited under fN2 of 15% was 0.47 GPa which was the lowest in all the NbN coatings, the coatings were deposited under the highest peak power indicating that the intrinsic stress should not play the major role. δ’-NbN phase was expected under high intrinsic stress which should be controlled by strain energy [23], however, the δ’-NbN phase in this study should not be controlled by the high stress. The increase of the residual stress should be generated by phase transition of the NbNx phases which could be classified into extrinsic stress. 7

Journal Pre-proof Generally, the hardness, modulus, and residual stress showed a direct correlation with peak power. Ferreira et al. [24] reported that the hardness of Cr coatings increased linearly from 9.8 GPa to 17.7 GPa with the peak power from 39 kW to 129 kW. Lin et al. [25] studied the peak power can control the preferred orientation and mechanical properties of AlN coatings, and the hardness of AlN coatings increased from 8.5 GPa to 13.4 GPa with the peak power increased from 35.5 kW to 62.2 kW. The anomalous change of mechanical properties in this study should be caused by the forming nanocomposite structure.

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Fig. 8 shows the surface view of the indentation imprints at a load of 1000 mN under varied nitrogen flow rate fN2. The 1000 mN load was intentionally chosen to generate the

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radical cracks which were used to evaluate the coating toughness, since no cracks was

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observed under the load of 500 mN. The imprints on the NbN coatings showed similar shapes

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and sizes after indentation. When the nitrogen flow rate fN2 increased from 15% to 25%, the indentation imprints were relative intact with several cracks in the imprints paralleled to the

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edge. These cracks caused the cohesive failure of NbN coatings owing to the large applying load. When fN2 increased to 30%, radical cracks could be observed indicating that the sudden

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decrease of coating toughness. The toughness evaluated by the radical crack was about 5.4 MPa∙m-1/2, however the exact toughness calculated by the radical cracks showed limited

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meanings owing to high applying load. Assuming the applying load down to 500 mN with the radical crack showing the same size, the toughness of the coating was still about 2.7 MPa∙m-1/2 showing obvious better toughness. Fig. 9 shows the H/E and the H3/E2 of NbN coatings as a function of nitrogen flow rate fN2. Generally, H/E stands for the elastic limit of strain, and the H3/E2 is the plastic resistance parameter. The H/E and H3/E2 showed similar decreasing trends with two obvious decreasing steps observed as the increase of fN2 from 15% to 30%. The H/E and H3/E2 got the first reduction at fN2 of 17.5%, then went a second decrease at fN2 from 25% to 30% attaining the minimum suggesting the poor response of elastic and plastic behavior of NbN coating deposited under fN2 of 30%. Fig. 10 shows the elastic recovery We of NbN coatings as a function of nitrogen flow rate fN2. As the increase of fN2 from 15% to 30%, the elastic recovery 8

Journal Pre-proof We first decreased to 40.6% from 43.9% at fN2 of 17.5%, then increased to the maximum of 51.9% at fN2 of 25%. Further increasing fN2 to 30%, the elastic recovery We showed a sudden decrease to the minimum of 40.2%. All elastic recovery We of NbN coatings was lower than 60%. When increasing fN2 to 30%, H/E, H3/E2 and We all hit the minimum suggesting the decrease of coating toughness in accordance with the result showed in Fig. 9. However, the H/E, H3/E2 showed quite different results when the fN2 was at 17.5 %. The difference lied between the two coatings toughness should be owing to the nanocomposite microstructure.

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3.3. Design Criterion Discussion

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For homostructure NbN nanocomposite coatings deposited by MPPMS, both enhanced

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hardness and toughness could be achieved by proper control of cubic δ-NbN phase and hexagonal δ’-NbN phase in the nanocomposite structure, as well as coating residual stress,

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hardness, modulus and elastic recovery. Musil [26-27] proposed a design criterion for hard

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and tough coatings demanding four essential conditions, a high ratio H/E ≥ 0.1; a high elastic recovery We ≥ 60%; the compressive macrostress  < 0; dense, void-free microstructure. In

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this case, the NbN coatings deposited by MPPMS were all with compressive residual stress and dense structure, however H/E* and We didn’t attain the required values as proposed in

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the design criterion. Meanwhile, the NbN coatings with similar We exhibited distinct toughness, the results suggested that the four essential conditions of the design criterion didn’t show the same importance. The four essential conditions should be arranged in the impact of weight.

Since the coatings were composed of two phases, the toughness of each phase can be evaluated through documented classic criteria [28-30]. Pugh [28] analyzed plastic flow and brittle failure in polycrystalline metals, and proposed a criterion to estimate the toughness of materials using shear modulus G and bulk modulus B. When G/B < 0.5 trended to be tough, G/B > 0.5 tended to be brittle. Frantsevich et al. [29] distinguished the brittleness and ductility by the Poisson ratio υ following Pugh's criteria, materials with υ>0.26 tended to be ductile, υ<0.26 tended to be brittle. Pettifor [30] investigated cubic pseudobinary aluminide properties by using Cauchy pressures of c12-c44. When c12-c44>0 tended to be more ductile, 9

Journal Pre-proof c12-c44<0 tended to have more directional bonding and be more brittle. For cubic δ-NbN phase and hexagonal δ’-NbN phase, experimental and computational works have been carried to acquire the basic elastic coefficients cij, Young’s modulus E, shear modulus G, bulk modulus B and Possion’s ratio υ of NbN coatings [31-32]. Through evaluation of the cubic δ-NbN phase by the criteria listed above, the coatings with δ-NbN phases met all the requirements suggesting that δ-NbN phase should be tough and ductile. Meanwhile, δ’-NbN phase should be tough either, since the Cauchy pressure for hexagonal phase calculated by

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using c12-c66 was also positive. Cubic δ-NbN phase had more dislocation sliding systems which could be initiated than hexagonal δ’-NbN phase, thereafter cubic δ-NbN phase should

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be more ductile than hexagonal δ’-NbN phase. However, forming the NbN nanocomposite

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coatings, the indentation imprints showed that the coatings with more δ’-NbN phase showed

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better toughness, the enhanced toughness should attributed to the nanocomposite structure. Upon the four essential conditions, the nanocomposite structure should thereafter show the

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utmost important. The compressive residual stress in the coatings prohibit the cracks from propagation which should be listed as the second. When the NbN nanocomposite coatings

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with compressive residual stress were under indentation load, the dislocation would not easily go across the homostructure interface boundaries resulting in the enhancement of coating

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hardness and toughness [33]. When the content of δ’-NbN phase decreased to a certain level, the second phase was not enough to prohibit the crack from propagation leading to the decrease in the coating toughness. Meanwhile, unlike H/E*, We got two extremes inconsistence with the variation of coating toughness, as the increase of fN2 from 15% to 30%. Thereafter, We should be a less significant condition compared with H/E* in a nanocomposite coatings. In this work, a series of homostructure NbN coatings composed of cubic δ-NbN phase and hexagonal δ’-NbN phase were deposited by MPPMS under varied nitrogen flow rate fN2. Low fN2 led to high discharge peak current, and high discharge peak current promoted the formation of hexagonal δ’-NbN phase. NbN coatings with both enhanced hardness and toughness should be controlled by the nanocomposite structure. In a nanocomposite coatings, 10

Journal Pre-proof the four conditions in the design criterion should be reordered in the impact of weight: 1) the designed microstructure, 2) the compressive residual stress, 3) H/E and H3/E2, 4) We.

4. Conclusions 1) Homostructure NbN nanocomposite coatings were deposited by MPPMS under varied nitrogen flow rate fN2. Increasing fN2 from 15% to 30%, the discharge peak power decreased accordingly from 54 kW to 16 kW. The decrease in discharge peak power was attributed to two factors: target poisoning process resulting in the decrease of electron density

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ne and increase of electron temperature Te, and the second ionization potential difference

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between Nb and argon.

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2) The NbN coatings were composed of cubic δ-NbN and hexagonal δ’-NbN phases. Low fN2 favored the formation of δ’-NbN phase with (100) preferred orientation and δ-NbN

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phase with (200) preferred orientation, high fN2 favored the formation of δ-NbN phase with

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preferred orientation (200). Increasing fN2, the coatings gradually turned from dense microstructure with short columnar grains to long penetrated columnar grains.

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3) Increasing fN2, the hardness, modulus and residual stress first increased to 36 GPa from 30 GPa, 460 GPa from 366 GPa, and 0.47 GPa to 1.93 GPa, then decreased to 29 GPa

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and 389 GPa with residual compressive stress of 1.01 GPa showing a nonlinear response with fN2. Meanwhile, the toughness of coatings increased with the content of δ’-NbN phase showing an abrupt change of NbN coating toughness under the fN2 of 30%. 4) The essential conditions of the design criterion for hard and tough coatings were rearranged in the impact of weight as follows: 1) the designed microstructure, 2) the compressive residual stress, 3) high H/E and H3/E2, 4) high We.

Acknowledgements This work is supported by the National Natural Science Foundation of China under Grants No. 51601029, 51575077 and U1508218, the Foundation for Innovative Research Groups of the National Natural Science Foundation of China under Grants No. 51621064, and the Fundamental Research Funds for the Central Universities. 11

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[17] B.C Zheng, Z.L. Wu, B. Wu, Y.G. Li, and M.K. Lei, A global plasma model for reactive deposition of compound films by modulated pulsed power magnetron sputtering discharges, Journal of Applied Physics, 121 (2017) 171901. [18] V.I. Ivashchenko, P.E.A. Turchi, E.I. Olifan, Phase stability and mechanical properties of niobium nitrides, Physical Review B, 82 (2010) 054109. [19] H. Lee, S.S. Wong, Correlation of stress and texture evolution during self- and thermal annealing of electroplated Cu films, Journal of Applied Physics, 93 (2003) 3796-3804. [20] A.D. Pogrebnjak, O.V. Bondar, G. Abadias, V. Ivashchenko, O.V. Sobol, S. Jurga, E. Coy, Structural and mechanical properties of NbN and Nb-Si-N films: Experiment and molecular 13

Journal Pre-proof dynamics simulations, Ceramics International, 42(10) (2016) 11743-11756. [21]V.N. Zhitomirsky, Structure and properties of cathodic vacuum arc deposited NbN and NbN-based multi-component and multilayer coatings, Surface and Coatings Technology, 201(13) (2007) 6122-6130. [22] E. Chason, B.W. Sheldon, L.B. Freund, J.A. Floro, S.J. Hearne, Origin of compressive residual stress in polycrystalline thin films, Physical Review Letters, 88(15) (2002) 156103. [23] M. Wen, C.Q. Hu, C. Wang, T. An, Y.D. Su, Q.N. Meng, W.T. Zheng, Effects of substrate

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bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering, Journal of Applied Physics, 104 (2008)

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[24] F. Ferreira, R. Serra, J.C. Oliveira, A. Cavaleiro, Effect of peak target power on the

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[26] J. Musil, Advanced hard coatings with enhanced toughness and resistance to cracking, Chapter 7 in the Book, in: S. Zhang (Ed.), Thin Films and Coatings. Toughening and

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Toughness Characterization, CRC Press, USA, 2015, pp. 377–464. [27] J. Musil, Flexible hard nanocomposite coatings, RSC Advances, 5 (2015) 60482-60495. [28] S.F. Pugh, Relations between the elastic moduli and the plastic properties of polycrystalline pure metals, Philosophical Magazine, 45 (1954) 823-843. [29] I.N. Frantsevich, F.F. Voronov, S.A. Bokuta, Elastic constant and Elastic Moduli of Metals and Insulators Handbook, Kiev: Naukova Dumka, 1983, pp. 60-180. [30] D.G. Pettifor, Theoretical predictions of structure and related properties of intermetallics, Materials Science and Technology, 8 (1992) 345-349. [31] X.-J. Chen, V.V. Struzhkin, Z. Wu, M. Somayazulu, J. Qian, S. Kung, A.N. Christensen, Y. Zhao, R.E. Cohen, H.K. Mao, R.J. Hemley, Hard superconducting nitrides, National Academy of Science of USA, 102 (2005) 3198-3201. 14

Journal Pre-proof [32] M. Wen, C.Q. Hu, C. Wang, T. An, Y.D. Su, Q.N. Meng, W.T. Zheng, Effect of substrate bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering, Journal of Applied Physics, 104 (2008) 023527. [33] X. Chu, M.S. Wong, W.D. Sproul, S.L. Rohde, S.A. Barnett, Deposition and properties of polycrystalline TiN/NbN superlattice coatings. Journal of Vacuum Science and Technology

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A, 10(4) (1992) 1604-1609.

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Figure Captions Fig. 1 The typical pulse waveform of MPPMS discharge curve at nitrogen flow rate fN2 = 20%, including a 250 μs weak ionization stage and a 500 μs strong ionization stage constituted by adjusting the different micro-pulse switching time (τon/τoff) Fig. 2 The peak discharge current waveforms of MPPMS at varied nitrogen flow rate fN2 Fig. 3 The XRD patterns of NbN nanocomposite coatings deposited by MPPMS under varied nitrogen flow rate fN2

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Fig. 4 The cross-sectional SEM images of NbN coatings under the varied nitrogen flow rate

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fN2 from 15% to 30%

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Fig. 5 The cross-sectional HRTEM images of NbN coatings at nitrogen flow rate fN2 of 25% and 30%

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Fig. 6 The hardness and elastic modulus of NbN coatings as a function of nitrogen flow rate

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fN2

Fig. 7 The residual stress of NbN coatings as a function of nitrogen flow rate fN2

flow rate fN2

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Fig. 8 The surface view of the indentation imprints at a load of 10 N under varied nitrogen

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Fig. 9 The H/E and the H3/E2 of NbN coatings as a function of nitrogen flow rate fN2 Fig. 10 The elastic recovery We of NbN coatings as a function of nitrogen flow rate fN2 Table 1 The discharge parameters of MPPMS under varied nitrogen flow rate fN2

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Journal Pre-proof Table 1 The discharge parameters of MPPMS Charging No

voltage (V)

Depositi on time (s)

Vp

Ip

Pp

Pa

(V)

(A)

(kW)

(kW)

Weak

Strong

Macropulse

ionization

ionization

Energy per

frequency

region

region

macropulse

(Hz)

toff/ton

toff/ton

(J)

(μs)

(μs)

380

7200

427.8±4.0

37.7±2.1

16.1±1.0

0.5±0.1

75

34/6

10/9

9.8+0.5

2

380

7200

421.1±2.6

51.2±2.7

21.6±1.2

0.6±0.1

75

34/6

10/9

12.7±0.9

3

380

4800

419.8±5.6

69.3±3.4

29.1±1.7

0.8±0.1

75

34/6

10/9

16.5±0.5

4

380

3000

416.3±6.2

97.2±9.8

40.1±4.3

1.1±0.1

75

34/6

10/9

20.6±1.4

5

380

3000

408.4±5.1

132.3±8.5

54.0±3.7

1.3±0.1

75

34/6

10/9

23.2±1.2

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Credit author statement Y.G. Li: Conceptualization, Methodology, Writing- Reviewing and Editing. H. Yuan: Data curation, Writing- Original draft preparation. Z.T. Jiang: Investigation. N. Pan: Data curation.

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M.K. Lei: Supervision.

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Declaration of Interest Statement

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We declare that we have no conflict of interest.

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Highlights

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1 NbN nanocomposite coatings were deposited by MPPMS under varied nitrogen flow rate. 2 Hexagonal δ’-NbN phase was observed in coatings under low nitrogen flow rate. 3 The mechanical properties showed a nonlinear response with discharge peak power. 4 Homostructure in NbN nanocomposite coatings could alleviate the Friedel oscillations. 5 The conditions of design criterion for tough and hard coatings were rearranged.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10