Trans. Nonferrous Met. Soc. China 24(2014) 2856-2863

Effect of bias voltage on compositional, mechanical and corrosion property of ZrNbAlN multilayer films by unbalanced magnetron sputtering

Yong-jing SHI^1,2, Fu-sheng PAN¹, Ming-dong BAO³, Hu-cheng PAN¹, Muhammad RASHAD¹

1. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China;

2. Department of Materials Science and Engineering, Chongqing University of Science and Technology, Chongqing 401331, China;

3. Institute of Materials Engineering, Ningbo University of Technology, Ningbo 315016, China

Received 6 August 2013; accepted 13 January 2014

Abstract: Nano-scaled ZrNbAlN films with different negative bias voltages (V_b) were deposited on bronze substrate and Si (100) wafers by a reactive unbalanced magnetron sputtering technique. Composition and structure properties were characterized by X-ray photoelectron spectroscopy and X-ray diffraction. It is found that mole concentrations of Zr and Nb are affected by V_b, which leads to the increase of binding energy of N 1s and Al 2p and decrease of binding energy of Zr 3d_5/2 and Nb 3d_5/2. Surface morphologies evolution controlled by V_b could be observed. Furthermore, X-ray diffraction patterns reveal that these films show a (111) preferred orientation. Moreover, mechanical property and corrosion behavior of ZrNbAlN films were characterized by nanoindentation test and corrosion test, respectively. A maximum value of 21.85 GPa at -70 V occurs in the ZrNbAlN- bronze system, which outperforms uncoated bronze. Corrosion experiments in 0.5 mol/L NaCl and 0.5 mol/L HCl solution show that corrosion potential and corrosion current are dependent on V_b, and better anti-corrosion property could be obtained at -90 V.

Key words: ZrNbAlN multilayer film; magnetron sputtering; composition; corrosion

1 Introduction

Aluminum-bronze alloy as an important structure material was widely used in machinery industry including bearing and parts. In order to enhance surface mechanical property and corrosion resistance of bronze substrates, some surface treatment techniques were developed, such as laser surface modification and thermal spray [1,2]. Moreover, physical vapor deposition (PVD) techniques were also promised and have obtained interesting application, such as cathode arc and magnetron sputtering, which could be used to synthesize films of nitride, oxide and carbide. Transition metal nitride films attracted more scientific interest than carbide and oxide films due to simple synthesis technology and high-speed deposition rate. With the development in magnetron sputtering technique, some novel films materials consisted of multilayer and multiphase, which were synthesized through a combination of multilayer concept and new materials with extremely nanometer-scaled structural ordering [3,4], such as TiN/NbN, Zr-Nb-N and Zr-Al-N [5-9]. Recently, CrTiAlN and TiZrAlN films were synthesized to enhance their physical and chemical stability [10,11]. Several theoretical models describing the effect of hardness enhancement were proposed. It has been proven that the performance of these films was dependent on their microstructure and chemical composition, which could be controlled by basic deposition parameters, such as sputtering power, N₂ flow rate and negative bias voltage (V_b) applied to a substrate during sputtering [12,13]. Especially, the effect of V_b during depositing was reported [14]. In previous research, the properties of ZrNbAlN film controlled by N₂ flow rate were researched [15]. In this work, composition, mechanical and corrosion property of ZrNbAlN with different negative bias voltages synthesized by unbalanced magnetron sputtering were reported.

2 Experimental

Nano-scaled ZrNbAlN multilayer films were synthesized on Al-bronze and silicon (100) substrates using metallic Zr, Nb and Al targets with 99.95% purity by a reactive DC unbalanced magnetron sputtering system. Sputtering sources were from four element targets arranged at 90° intervals around a stainless steel chamber, and a schematic diagram of film preparation system is shown in Fig. 1. Si (100) substrates were cleaned in acetone using an ultrasonic agitator for 30 min. Before depositing, base pressure was pumped to 7.0×10^-4 Pa. During film depositing, working pressure was 0.10 Pa, inlet flux rate of Ar gas was fixed at 25 mL/min (standard-state cubic centimeter per minute). Firstly, samples were bombarded by Ar⁺ ion for 30 min to eliminate surface adsorption and thin oxide layer. Secondly, the Zr layer and ZrN_x layer were deposited. At last, N₂ gas was put into vacuum chamber to form nitrides, and N₂ flow rate was controlled by a plasma optical emission monitor (OEM) with a feedback control. Sputtering powers of Zr, Al and Nb targets were 8, 7 and 7 mA/cm², respectively. Bias voltages of substrate samples were set at -40, -50, -60, -70, -80 and -90 V, respectively. Film thickness was measured using a ball-crater and SEM scale.

Fig. 1 Schematic diagram of coating preparation system for ZrNbAlN multilayer films

Chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB250) with Al K_α source (1486.84 eV) radiation and large area XL lens mode, energy step size of 0.1 eV and pass energy of 50 eV. Typical XPS spectra were fitted using Lorentzian-Gaussian function. Binding energy (BE) of surface C 1s spectrum was 285.4 eV for all samples. Surface morphologies were observed with a high resolution field emission scanning electron microscope (FESEM). Cross-sectional morphologies were observed with a high resolution transmission electron microscope (HRTEM, H9000NAR), and fast Fourier transformation spectra (FFT) were obtained from selective area electron diffraction (SAD). Crystallographic microstructure patterns were obtained by grazing incidence X-ray diffraction (GIXRD, Rigaku D/Max 2500), and X-ray source was a Cu K_α radiation at θ=2°. Microhardness and elastic modulus were checked by a Vickers ultra microhardness tester (Fischer scope H100), which were carried out by an indenter at a load of 5 mN. At this load level, indentation depth was much less than one tenth of the film thickness to eliminate the effect of substrate.

Electrochemical corrosion behavior of these films was tested in a solution of 0.5 mol/L NaCl and 0.5 mol/L HCl by an Autolab setup (III+VA663) consisting of three-electrode system. Film samples served as working electrode, and a Pt electrode served as counter electrode kept parallel to working electrode, while a saturated calomel electrode (SCE) served as reference electrode. This reference electrode was kept near surface of working electrode and used to measure the potential of working electrode. Corrosion samples were firstly coated with epoxy resin, leaving only 1.5386 cm² surface area exposed, and then coated samples were immersed in corrosion solution for approximately 1 h so that a steady state equilibrium potential, known as open circuit potential (OCP), was obtained. Corrosion experiment was carried out, first cathodically and then anodically, and scan rate and step potential were 0.01 V/s and 0.00213 V, respectively. Scan range was approximately ±0.450 V above OCP to cover Tafel region (±0.060 to ±0.120 V). The polarization curve was recorded simultaneously. Corrosion current density (J_corr) and corrosion potential relative to SCE (φ_corr) were determined by extrapolating straight-line section of cathodic and anodic Tafel lines of polarization curve.

3 Results and discussion

ZrAlNbN multilayer films were deposited at N₂ flow rate f_N=14 mL/min. Film thickness was 2.0-2.5 μm. To fabricate ZrAlNbN film with multilayer structure, a Zr layer of 50 nm was deposited as the adhesion layer, and then a ZrN_x layer of about 500 nm was deposited, as shown in Fig. 2(a), which shows a TEM cross-sectional morphology of as-deposited film. Figure 2(b) shows interface of film-substrate corresponding to Region 1, yielding a mixing interface layer (several nanometers in thickness) due to ion flux impingement. Figure 2(c) shows interface between ZrN_x and nitride multilayer corresponding to Region 2, indicating the growth direction. Nitride phase clearly presents a periodical modulation structure composed of four layers materials of ZrN, NbN, ZrN, and AlN (Fig. 2(d)). The inset shows micro-area structure and crystallization behavior of nitride multilayer.

Figure 3 shows the relationship between surface morphology evolution and V_b. Granular surface morphologies of ZrNbAlN with different V_b can be seen, and voids in films decrease with the increase in V_b. Surface morphology derived from nucleation and coalescence of deposition flux during film thickening under low mobility conditions, was dependent on atomic mobility, which could be controlled by impingement energy and ion flux density. Some literatures reported growth model for popular film’s deposition was controlled by ion flux [16,17]. Increased V_b caused increase of ion flux to thickening interface, therefore surface roughness decreases. However, surface morphology also could be controlled by thickening rate and temperature, yielding a typical crystallographic microstructure.

Fig. 2 TEM image of ZrAlNbN films (a) (Regions A, B, C, and D represent substrate, adhesion, ZrN_x layer and nitride multilayer, respectively) and HRTEM images of Region 1 (b), Region 2 (c) and Region 3 (d) (Inset is corresponding FFT spectrum)

Film composition is an important factor affecting film property and structure. Full spectra of these films show that six elements, including O 1s, C 1s, N 1s, Zr 3d, Nb 3d and Al 2p, occur on the surface of these films before etching (Fig. 4). However, the spectra of C 1s and O 1s disappear after being etched. The C 1s derives from an adsorption contaminant from air, while the O 1s derives from residual atmosphere in vacuum chamber during depositing. Mole concentration of O of these films is lower than 3%. Mole ratio of N to metal is about 1:1.1-1:1.2 for all these films. N atomic concentration is seriously affected by V_b, as described in Ref. [17]. Typical XPS spectra were fitted using Lorentzian- Gaussian function, and all the calculated peaks can be assigned to corresponding species (Fig. 5) according to the reference BE from literatures [18-20]. It was found that BE values of N 1s, Zr 3d_5/2, Nb 3d_5/2 and Al 2p at -40 V are 396.78, 179.68, 203.54 and 73.26 eV, respectively, which can be assigned to ZrN (Figs. 5(a, b)), NbN (Figs. 5 (a, c)) and AlN (Figs. 5(a, d)). Moreover, also a few oxynitrides (ZrO_xN_y, NbO_xN_y) and Al₂O₃ formed besides metal Al. These oxynitride species derived from nitrides with dissolved oxygen [21]. Through comparison of several fitting XPS data from ZrNbAlN films with different V_b, it was found that BE is affected by V_b (Figs. 5(e, f)). BE of N 1s and Al 2p increases with the increase in V_b, while BE of Zr 3d_5/2 and Nb 3d_5/2 decreases. Area ratio of nitride increases with the increase in V_b and that of oxynitride according to the calculated data, while that of Al₂O₃ and Al decreases. In order to depict variation of chemical composition at different V_b, relative mole concentration of metal is obtained (Fig. 6). Mole concentration of Zr decreases with the increase in V_b, while mole concentration of Nb increases. However, mole concentration of Al is hardly affected. The phenomenon of relative concentration of metal affected by V_b could be explained by the variation of atomic adsorption rate and different resputtering yields due to ion bombardment during film thickening [22].

Fig. 3 SEM images of ZrAlNbN films on bronze with different V_b

Fig. 4 XPS full spectra of ZrAlNbN film

Fig. 5 N 1s spectrum (a), Zr 3d_5/2 spectrum of nitride (b), Nb 3d_5/2 spectrum of nitride (c), Al 2p spectrum of nitride (d) (These XPS spectra were fitted by Lorentzian-Gaussian function, and ∑χ2 value is lower than 2), BE of N 1s and Zr 3d_5/2 (e) and BE of Nb 3d_5/2 and Al 2p (f)

Fig. 6 Relative mole concentration of metal in ZrAlNbN films

Glancing incidence XRD patterns of these films show that there is a relationship between structure of ZrNbAlN films and V_b (Fig. 7). ZrN film presents a crystallographic microstructure of multiphase containing a face-centered cubic (FCC) structure (ZrN, PDF741217) and orthorhombic structure at f_N=12 mL/min and V_b= -40 V (Zr₂N, PDF461204) according to powder diffraction data (JCPDS). ZrNbAlN films show a broadened peak which ranged from 30° to 45° in 2θ value, which could be assigned to FCC structure of (111) out-plane preferred crystallographic orientation according to JCPDS data of ZrN, NbN (PDF741218) and AlN (PDF800010). NbN film showed a phase transition from FCC structure of (200) preferred orientation at V_b=-40 V to mixture phase containing FCC and hexagonal close-packed (HCP) structure at V_b =-80 V [23], and AlN film showed a hexagonal wurtzite-type structure [24]. Obviously, HCP phase could not be observed in ZrNbAlN films. Scherrer’s equation revealed that θ value is inversely proportional to grain size.

Fig. 7 GIXRD patterns of ZrAlNbN films on bronze (ZrN film was prepared at f_N=12 mL/min)

So, small grain size and low crystallinity could contribute to broadened reflection peak.

Vickers hardness of these films with different V_b on bronze substrates was checked (Fig. 8). Notably, a maximum hardness with 21.85 GPa and elastic modulus with 340 GPa could be obtained at about V_b=-70 V. The inset shows the corresponding bias current (I_b) at different V_b, from which a maximum I_b of 2.14A at V_b=-90 V and a minimum I_b of 1.65 A at V_b=-40 V could be obtained, and I_b shows an exponential increase with the increase in V_b. There is a fact that hardness is dependent on V_b [25,26]. Moreover, Vickers hardness of these films on bronze outperforms that of bronze substrate (3.8 GPa). TUNG et al [27] revealed that the single phase ZrN indicated the maximum micro-hardness at V_b=-80 V. Hardness enhancement can be explained in terms of some following reasons: 1) increased film density [17], 2) higher residual stress [27], 3) strain hardening and high defect density effect.

As a protective film, NaCl and HCl solution on the top of these films in a real work environment may easily cause corrosion degradation, leading to the decrease of service life. Therefore, a conventional potentiodynamic corrosion experiment was carried out in 0.5 mol/L NaCl and HCl solution for ZrNbAlN multilayer films with different V_b. As a reference, potentiodynamic polarization curves of bronze substrate in 0.5 mol/L NaCl and HCl solution are shown in Fig. 9, which shows corrosion potential (φ_corr) of -316 mV in 0.5 mol/L NaCl and φ_corr of -269 mV in 0.5 mol/L HCl, yielding a corresponding J_corr of 5.523 mA/cm² and 4.479 mA/cm², respectively. To characterize the bias effect on corrosion behavior, corrosion test of these films was carried out, as shown in Fig. 10. It can be seen that φ_corr and J_corr in 0.5 mol/L NaCl are higher than those in 0.5 mol/L HCl, leading to φ_corr of -200 to -300 mV in 0.5 mol/L NaCl and -230 to -330 mV in 0.5 mol/L HCl. Obviously, it is found that J_corr decreases with the increase in V_b, which results in occurrence of the lowest J_corr of 0.216 μA/cm² in 0.5 mol/L NaCl and 0.622 μA/cm² in 0.5 mol/L HCl at V_b=-90 V. φ_corr values of ZrN, NbN, AlN were about 210 mV, -420 mV and -566 mV respectively [28-30], which shows that φ_corr and J_corr firstly depend on material properties. Furthermore, corrosion behavior was seriously affected by V_b, leading to an increase of defect and stress due to the ion flux impingement [31]. So, an excellent anti-corrosion property could be obtained at higher V_b.

Fig. 8 Vickers hardness and elastic modulus of ZrAlNbN films (Inset shows corresponding V_b-I_b curve)

Fig. 9 Polarization curves of bronze substrate in 0.5 mol/L NaCl and 0.5 mol/L HCl solution

Fig. 10 φ_corr (a) and J_corr (b) from polarization curve of ZrAlNbN films in 0.5 mol/L HCl and 0.5 mol/L NaCl solutions

4 Conclusions

ZrNbAlN films were successfully deposited on bronze substrate. Composition, mechanical property and corrosion behavior of ZrNbAlN film were characterized. The results reveal that Zr content decreases and Nb content increases with the increase in V_b. XPS analysis shows that BE of N 1s and Al 2p increases with the increase in V_b, while that of Zr 3d_5/2 or Nb 3d_5/2 decreases. There is a notable enhanced mechanical property of surface in the ZrNbAlN film-bronze substrate system, yielding a maximum Vickers hardness of 21.85 GPa and elastic modulus of 340 GPa. Corrosion behavior of ZrNbAlN film was affected by V_b.

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偏压对非平衡磁控溅射ZrNbAlN薄膜的成分、力学性能及腐蚀特性的影响

石永敬^1,2，潘复生¹，鲍明东³，潘虎成¹，Muhammad RASHAD¹

1. 重庆大学 材料科学与工程学院，重庆 400044；

2. 重庆科技学院 冶金与材料学院，重庆 401331；

3. 宁波工程学院 材料工程研究所，宁波 315016

摘  要：采用反应非平衡磁控溅射技术在青铜及Si(100)衬底上沉积不同负偏压(V_b)的纳米ZrNbAlN薄膜。薄膜结构及成分采用X射线光电子能谱及X射线衍射进行表征。结果表明，Zr和Nb的原子浓度受负偏压影响，V_b导致N 1s谱和Al 2p谱的结合能增加及Zr 3d_5/2和Nb 3d_5/2谱的结合能降低，薄膜表面形貌的演化受控于V_b。X射线衍射谱显示这些薄膜具有(111)择优取向。此外，薄膜的力学特性及腐蚀行为分别通过纳米压痕测试及腐蚀测试表征。当负偏压为-70 V时，纳米压痕测试显示的最大显微硬度为21.85 GPa，ZrNbAlN膜在青铜衬底上的性能远优于未涂层处理的衬底。在0.5 mol/L NaCl和0.5 mol/L HCl溶液中的腐蚀实验表明，腐蚀势能及腐蚀电流依赖于衬底偏压，在-90 V时能够获得较高的抗腐蚀特性。

关键词：ZrNbAlN多层膜；磁控溅射；成分；腐蚀

(Edited by Xiang-qun LI)

Foundation item: Project (50725413) supported by the National Natural Science Foundation of China; Project (2010BB4290) supported by Natural Science Foundation Project of CQ CSTC, China

Corresponding author: Yong-jing SHI; Tel/Fax: +86-23-65112635; E-mail: yjshi_cqust@163.com

DOI: 10.1016/S1003-6326(14)63418-X