⇦Chinese Physics Letters, 2018, Vol. 35, No. 6, Article code 066202⇨ Effect of Bias Voltage on Microstructure and Mechanical Properties of Nanocomposite ZrCN Films Deposited by Filtered Cathodic Vacuum Arc * Han Zhou(周晗), Fu-Zeng Zhou(周福增), Yong-Qing Shen(沈永青), Bin Liao(廖斌), Jing-Jing Yu(于晶晶), Xu Zhang(张旭)** Affiliations College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875 Received 18 December 2017, online 19 May 2018 ^*Supported by the National Natural Science Foundation of China under Grant No 51171028.
^**Corresponding author. Email: zhangxu@bnu.edu.cn Citation Text: Zhou H, Zhou F Z, Shen Y Q, Liao B and Yu J J et al 2018 Chin. Phys. Lett. 35 066202 Abstract Nanocomposite ZrCN films consisting of nanocrystalline ZrCN grains embedded in nitrogen-doped amorphous carbon film are deposited by filtered cathodic vacuum arc technology under different bias voltages ranging from 50 to 400 V. The influence of bias voltage on the characterization and the mechanical properties of the ZrCN films are investigated by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and nano-indentation. The bias voltage has a subtle effect on the ZrCN grain size, which is around 9.5 nm and keeps almost constant. A slight increase of the bias voltage induces a relatively high $sp^{3}$ fraction about 40% in N-doped amorphous C films but leads to the graphitization of the films under a higher voltage. The best mechanical property of the ZrCN film with the hardness of 41 GPa is obtained under the bias voltage of 200 V, indicating the positive effect of slight increase of ion bombardment on the hardness of the films. DOI:10.1088/0256-307X/35/6/066202 PACS:62.25.-g, 52.77.Dq © 2018 Chinese Physics Society Article Text Carbon-based nanocomposite films, including transition metal carbides (Me/a-C, nc-MeC/a-C) and carbonitrides, have attracted significant interest in recent years because of their unique structure comprising of nanocrystalline embedded in amorphous mixture. They are widely used for supercapacitors, biocompatible materials, protective coatings for anti-corrosion and anti-abrasion, etc.[1-4] Filtered cathodic vacuum arc (FCVA) is one of the most promising methods to fabricate carbon-based nanocomposite films because of its high ionization rate and the effect of ion bombardment during the deposition.[5,6] Nanocomposite ZrCN films have become attractive as candidates in scientific and industrial applications owing to their remarkable mechanical, decorative, tribological and chemical properties.[7,8] The versatile properties of ZrCN films are mainly determined by the microstructural characteristics (e.g. the nanoparticles size, the average thickness and the type of amorphous film), which are closely related to the deposition parameters. During the deposition, the sub-implantation, collision, etching and re-sputtering induced by the ion bombardment under different bias voltages have a significant influence on the microstructure and the properties of the films.[9] Anders[10] focused on the energetic deposition by filtered cathodic arcs and high power impulse magnetron sputtering, and an extended structure zone diagram based on the effect of ion energy was proposed. In this work, the ZrCN films are prepared under different bias voltages by FCVA technology and the effects of the bias voltage on the microstructure and the mechanical properties of the ZrCN films are investigated. The nanocomposite ZrCN films were deposited on silicon (100) and NaCl (100) substrates by the FCVA technology (the sketch of the deposition system is shown in Ref. [11]). To be brief, a pure Zr cathode (99.99%) was triggered to generate zirconium plasma. The inlet gas flows of C$_{2}$H$_{2}$ and N$_{2}$ were fixed at 25 sccm and 25 sccm, respectively. The cathodic arc supply was operated at a directed current of 120 A. The filter coil current was set at 1.5 A. The applied substrate bias voltage ranged from 50 to 400 V. The base pressure in the vacuum chamber was at least 1.0$\times$10$^{-3}$ Pa. Prior to the deposition of ZrCN films, Zr and ZrN interlayers were deposited for improving the adhesion between the substrate and ZrCN film. The fracture cross-sections of deposited ZrCN films were examined on a Merlin Vp compact scanning electron microscope (SEM), and the microstructure was examined by a high-resolution transmission electron microscope (HRTEM, Jem-2100). X-ray diffraction (XRD) was conducted to assess the phase structure of ZrCN films, using Cu $K_{\alpha 1}$ radiation, setting the incident angle at 1$^{\circ}$. The grain size was calculated by Scherrer's formula $d=k\lambda/(B\cos\theta)$, where $k$, $B$, $\lambda$ and $\theta$ are the constant of 0.89, the full width at half maximum (FWHM), the wavelength and the distraction angle, respectively. The relative content, chemical composition and bonding state of the ZrCN films were investigated by an x-ray photoelectron spectroscopy (XPS) measurements, using Al $K\alpha$ radiation at a constant power of 320 W. Raman spectra were obtained with a Jobin–Yvon HR800 monochromator, using 532 nm line of an Ar-Kr laser and setting the scanning range to be 100–1900 cm$^{-1}$. Surface morphology instrument was used to measure the film thickness. The hardness and reduced modulus measurements of the films were performed using a Wrexham Micro Materials Ltd nanotest system equipped with a Berkovich indenter. Nine indentations were measured at different places on the film surface for each sample.

Fig. 1. The composition of ZrCN films under different bias voltages.

Fig. 2. (a) The x-ray diffraction patterns and (b) average grain size of ZrCN films deposited under different bias voltages.

Figure 1 shows the relative content of ZrCN films under different bias voltages. The bias voltage has a slight effect on the relative content in ZrCN films and the contents of C, Zr and N atoms in the films are around 44–51 at.%, 7–10 at.% and 42–46 at.%, respectively. The x-ray diffraction patterns of ZrCN films deposited under different bias voltages are presented in Fig. 2(a). The peaks located at 33$^{\circ}$, 55.5$^{\circ}$ and 66.5$^{\circ}$ correspond with the (111), (220) and (311) diffraction peaks of ZrCN phase, respectively. The appearance of these peaks demonstrates the well-defined face-centered cubic (fcc) structure of ZrCN crystalline phase. The increase of the bias voltage induces the continuous increase of the ZrCN (311) peak and the increase of the ZrCN (111) peak at first and then the decrease as the bias voltage increases to 400 V. The peak around 38.6$^{\circ}$ can be labeled as ZrN (200)[12] and decreases with increasing the bias voltage from 50 V to 200 V and disappears as further increasing the bias voltage, which is associated with the drastic ion bombardment under the higher voltage. The energy of the C atoms might be accelerated by the higher voltage and thus more C atoms could insert into ZrN clusters as interstitial atoms and form a ZrCN phase. The average grain size of the ZrCN films deposited under different bias voltages calculated from the ZrCN (111) peak is shown in Fig. 2(b). The average grain size is around 9.5 nm and keeps almost consistent with increasing the bias voltage, indicating the subtle influence of the bias voltage on the grain size.

Fig. 3. SEM images of fracture cross-sections of ZrCN films deposited under the bias voltages of 100 V (a), 200 V (b), 300 V (c) and 400 V (d), and the bar scale is 1 μm.

Fig. 4. Typical HRTEM image of the nanocomposite ZrCN film prepared under bias voltage of 200 V and the corresponding SADE pattern in the inset.

The SEM images of the cross-sectional structure of the ZrCN films deposited under different bias voltages are shown in Fig. 3. All the ZrCN films present columnar-free and smooth structures and change slightly under different bias voltages. Figure 4 shows the HRTEM image in both real and reciprocal space of the ZrCN film deposited under the bias voltage of 200 V. The ZrCN film presents a typical nanocomposite structure comprising of the ZrCN grains with the grain size around 9 nm embedded in the amorphous film. The ZrCN (111), ZrCN (220) and ZrCN (311) lattice fringes and corresponding interplanar spacings of ZrCN nanocrystallites are marked in Fig. 4. The selected area electron diffraction (SADE) image in the inset exhibits three visible and continuous diffraction rings, corresponding to ZrCN (111), ZrCN (220) and ZrCN (311), respectively. To better understand the structure evolution, the C (1$s$), Zr (3$d$) and N (1$s$) core-level spectra of ZrCN films deposited under different bias voltages are analyzed in Figs. 5(a)–5(c). In the C 1$s$ spectra, the peaks located at 282 eV, 283 eV, 284.5 eV, 285.3 eV and 288.2 eV represent C–Zr bonds, C–Zr–O bond, $sp^2$ C, $sp^{3}$ C and C–N bonds, respectively.[8,13] The intensity of N-doped amorphous C peak increases slightly with increasing the bias voltage, indicating a relatively higher content of N-doped amorphous C phase in the films. In the Zr $3d$ spectra, the Zr $3d$ core-level spectra are characterized by the Zr 3$d_{3/2}$ and Zr 3$d_{5/2}$ double peaks due to the spin-orbit coupling, with the splitting energy of 2.4 eV. For simplicity, the Zr $3d$ spectra analyzed here only refer to the Zr 3$d_{5/2}$ peaks. Zr 3$d_{5/2}$ XPS spectra are fitted with three deconvolution peaks, corresponding to Zr–C–N (179.5 eV), Zr–O–N (180.6 eV) and Zr–O (183.2 eV) bonds, respectively.[8,13] The relatively higher Zr–O peak is attributed to the high affinity between Zr and O atoms. The N 1$s$ XPS spectra can be fitted into two peaks: the peak located at 396.5 eV is assigned to the N–Zr bond and the peak located at 398.3 eV is assigned to the N–C bond.[13] The intensity of the N–Zr bond decreases with increasing the bias voltage, which is in agreement with the decrease of the intensity of the ZrN peak (200) obtained in the XRD result. Meanwhile, the intensities of C–C, C=C and C–N (N–C) bonds keep increasing, while the ones of C–Zr and N–Zr bonds have the opposite trends.

Fig. 5. The C 1$s$ (a), Zr $3d$ (b), N 1$s$ (c) and core-level spectra of ZrCN films deposited under different bias voltages. Every element is displayed at different scales to clearly identify the peaks (C=1 X, Zr=4.7 X and N=2.4 X).

Fig. 6. Raman spectroscopy of ZrCN films prepared under different bias voltages.

Fig. 7. The $sp^{3}$ fraction in the N-doped amorphous C film and $I({\rm D})/I({\rm G})$ values for ZrCN films prepared under different bias voltages.

The Raman spectroscopy of ZrCN films prepared under different bias voltages is given in Fig. 6. Three visible asymmetrical broad peaks in the region of 1000–1700 cm$^{-1}$ belong to the N-doped amorphous C phase, including D (1350 cm$^{-1}$), G (1550 cm$^{-1}$) peaks and CN peak (1100 cm$^{-1}$), respectively.[14] The relative intensities of D and G peaks increase slightly with the bias voltage, which is in agreement with the variation trend of N-doped amorphous C peak in the XPS results. The $I({\rm D})/I({\rm G})$ ratio and the content of $sp^{3}$ fraction in the N-doped amorphous C film deposited under different bias voltages are shown in Fig. 7. The $sp^{3}$ content in the N-doped amorphous C film increases from 35% to about 40% with increasing the bias voltage from 50 to 200 V and then decreases at the higher bias voltages and down to 27% at 400 V, while the $I({\rm D})/I({\rm G})$ ratio presents an opposite variation trend compared with the $sp^{3}$ fraction. The result obtained here indicates the graphitization of N-doped amorphous C phase under the higher bias voltages, which could be associated with the higher temperature induced by the severe ion bombardment under the higher bias voltages.

Fig. 8. The hardness and reduced modulus of the nanocomposite ZrCN film under different bias voltages.

The hardness and reduced modulus of the ZrCN films as a function of the substrate bias are demonstrated in Fig. 8. The hardness (and reduced modulus) of the ZrCN films increases gently from 36 GPa (and 275 GPa) to 41 GPa (and 310 GPa) with increasing the bias voltage from 50 to 200 V, and a drastic reduction to 26 GPa (and 215 GPa) as the substrate bias rises further up to 400 V. According to the proposed mechanism for hardness enhancement of the carbon-based nanocomposite films, three conditions should be fulfilled for the possible formation of the superhard film: (i) the fine grain size should be within the nanoscale (not exceed 10 nm) to prevent grain deformation; (ii) the suitable average grain separation should not exceed 0.5 nm to limit crack initiation in the surrounding phase; (iii) there should be a sufficient dense and rigid network of amorphous film evaluated by the relative content of $sp^{3}$ fraction.[15-17] Considering the results obtained above, the fine grain size around 9.5 nm and the formation of dense and rigid amorphous film with a relatively high $sp^{3}$ fraction about 40% are responsible for the formation of the superhard ZrCN film deposited under the bias voltages of 100 V and 200 V. Our results indicate that the mild ion bombardment helps to improve the hardness of the film by enhancing the $sp^{3}$ content in the films. In summary, the ZrCN films exhibit a typical nanocomposite structure comprising of the nanoscale ZrCN grains embedded in the N-doped amorphous C film. The increase of the bias voltage has a slight effect on the grain size and induces the graphitization of the films, which is associated with the structure modification induced by the ion bombardment. The best mechanical property with the hardness of 41 GPa is obtained under the bias voltage of 200 V, indicating that the slight ion bombardment helps to enhance the hardness of ZrCN films. References 60 years of DLC coatings: Historical highlights and technical review of cathodic arc processes to synthesize various DLC types, and their evolution for industrial applicationsNanomechanical and nanotribological properties of carbon-based thin films: A reviewHistory of diamond-like carbon films — From first experiments to worldwide applicationsEffect of Chromium on Structure and Tribological Properties of Hydrogenated Cr/a-C:H Films Prepared via a Reactive Magnetron Sputtering SystemSynthesis of Ti-doped DLC film on SS304 steels by Filtered Cathodic Vacuum Arc (FCVA) technique for tribological improvementA review of investigations on biocompatibility of diamond-like carbon and carbon nitride filmsStructure and properties of Zr/ZrCN coatings deposited by cathodic arc methodInvestigation of the corrosion resistance of ZrCN hard coatings fabricated by advanced controlled arc plasma depositionEffect of the bias voltage on the structure of nc-CrC/a-C:H coatings with high carbon contentA structure zone diagram including plasma-based deposition and ion etchingEffect of carbon content on nanostructural, mechanical and electrochemical characteristics of self-organized nc-ZrCN/a-CNx nanocomposite filmsWear tests of ZrC and ZrN thin films grown by pulsed laser depositionCorrosion resistance, mechanical properties and biocompatibility of Hf-containing ZrCN coatingsInterpretation of infrared and Raman spectra of amorphous carbon nitridesA concept for the design of novel superhard coatingsRecent advances on understanding the origin of superhardness in nanocomposite coatings: A critical reviewThe grain refining effect of energy competition and the amorphous phase in nanocomposite materials

[1]	Vetter J 2014 Surf. Coat. Technol. 257 213
[2]	Charitidis C A 2010 Int. J. Refract. Met. Hard Mater. 28 51
[3]	Bewilogua K and Hofmann D 2014 Surf. Coat. Technol. 242 214
[4]	Liu L, Zhou S G, Liu Z B, Wang Y C and Ma L Q 2016 Chin. Phys. Lett. 33 026801
[5]	Bookul D, Saenphinit N, Supsermpol B, Aramwit C and Intarasiri S 2014 Appl. Surf. Sci. 310 293
[6]	Cui F Z and Li D J 2000 Surf. Coat. Technol. 131 481
[7]	Braic M, Braic V, Balaceanu M, Zoita C N, Kiss A, Vladescu A, Popescu A and Ripeanu R 2011 Mater. Chem. Phys. 126 818
[8]	Gu J D and Chen P L 2006 Surf. Coat. Technol. 200 3341
[9]	Yate L, Martínez-de-Olcoz L, Esteve J and Lousa A 2012 Surf. Coat. Technol. 206 2877
[10]	Anders A 2010 Thin Solid Films 518 4087
[11]	Zhou F Z, Fu K H, Liao B, Yu J J, Yang C L and Zhang X 2015 Appl. Surf. Sci. 327 350
[12]	Dorcioman G, Socol G, Craciun D, Argibay N, Lambers E, Hanna M, Taylor C R and Craciun V 2014 Appl. Surf. Sci. 306 33
[13]	Cotrut C M, Braic V, Balaceanu M, Titorencu I, Brai M and Parau A C 2013 Thin Solid Films 538 48
[14]	Ferrari A C, Rodil S E and Robertson J 2003 Phys. Rev. B 67 155306
[15]	Vepřek S and Reiprich S 1995 Thin Solid Films 268 64
[16]	Lu C S, Mai Y W and Shen Y G 2006 J. Mater. Sci. 41 937
[17]	Guo J, Liu Z J, Wang S W and Shen Y G 2013 Scr. Mater. 69 662