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Influence of bias voltage on the microstructure, mechanical and corrosion properties of AlSiN films deposited by HiPIMS technique
Journal of Alloys and Compounds 772 (2019) 112e121
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Influence of bias voltage on the microstructure, mechanical and corrosion properties of AlSiN films deposited by HiPIMS technique Ji Cheng Ding a, b, Qi Min Wang c, Zhe Ren Liu c, Seonhee Jeong b, Teng Fei Zhang c, *, Kwang Ho Kim a, b, ** a b c
School of Convergence Science, Pusan National University, Busan, 46241, South Korea Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan, 46241, South Korea School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2018 Received in revised form 3 September 2018 Accepted 6 September 2018 Available online 7 September 2018
AlSiN films were deposited with various bias voltages ranging from 0 V to - 150 V by high power impulse magnetron sputtering (HiPIMS) technique. The effects of bias voltage on the microstructure, mechanical and corrosion behavior were investigated. The AlSiN films exhibited an over-stoichiometric N content and both cubic and hexagonal AlN were observed in films. All deposited films showed a typical surface feature of granular structure and the cross-sectional images exhibited that all films possessed columnar structure, which was changed from coarse columnar to denser columnar structure with increasing the bias voltage. The AlSiN film deposited at - 150 V possessed the densest structure and exhibited the best mechanical properties, including hardness, toughness and nano-wear resistance. The corrosion test showed that all coated samples had better corrosion resistance compared to SUS304 in 3.5 wt.% NaCl solution and the AlSiN film deposited at bias voltage of - 150 V possessed the best corrosion resistance due to denser microstructure, which could act as a barrier layer for blocking the diffusion of corrosive substances. © 2018 Elsevier B.V. All rights reserved.
Keywords: AlSiN film HiPIMS Bias voltage Mechanical properties Corrosion
1. Introduction Transition metal nitride (TMN) films, such as CrN, TiN, and AlN, have been widely applied as protective films in machining industry due to their high hardness, good wear resistance and high thermal stability at elevated temperatures [1e4]. Among these TMN films, the AlN possesses a wide band gap of 6.2 eV, good optical transparence in the visible range of light, high melting temperature (2275 K), and also excellent corrosion/oxidation resistance [5e7]. These advantages make it a promising candidate as protective film for optically transparent substrates. Recently, many reports revealed that nano-composite AlSiN films can be synthesized by adding proper content of Si into the AlN films, where nanocrystalline AlN is surrounded by one to two monolayer of amorphous Si3N4 matrix, which can significantly improve the
* Corresponding author. ** Corresponding author. School of Convergence Science, Pusan National University, Busan, 46241, South Korea. E-mail addresses: [email protected] (T.F. Zhang), [email protected] (K.H. Kim). https://doi.org/10.1016/j.jallcom.2018.09.063 0925-8388/© 2018 Elsevier B.V. All rights reserved.
mechanical properties and thermal stability of the films [8e10]. In previous work by Pelisson et al. [6], the AlSiN films with various Si contents were deposited by magnetron sputtering (MS), which exhibited that the mean grain size decreased from 60 nm to 5 nm with increasing Si content and the maximum hardness exceeded 30 GPa was obtained at Si content of around 10 at. %. Musil et al. [11] reported that the AlSiN films synthesized by MS were polycrystalline when the Si content was lower than 10 at. %, and the films became amorphous structure with a relatively high Si content over than 20 at. % at a deposition temperature of 500 C. Furthermore, both the crystalline AlSiN films and the amorphous AlSiN films exhibited a high oxidation resistance up to 1000 C. Chang et al. [12] fabricated the AlSiN films by cathodic arc evaporation and showed that the bias voltage had great influence on the microstructure as well as the mechanical properties of the AlSiN films. As mentioned above, AlSiN films have been mainly fabricated by using MS and arc evaporation deposition methods. As an advanced physical vapor deposition (PVD) technology, high power impulse magnetron sputtering (HiPIMS) technique has been proven useful in depositing films as smooth as MS-PVD and as dense as arc-PVD [4,13]. By pulsing the sputtering target with high power (e.g.
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1e3 kW/cm2) and short pulse lengths, a high plasma density and high ionization of the sputtered species can be obtained. It has been widely reported that the HiPIMS was an excellent technique for synthesizing hard coatings with higher hardness, good adhesion between coating and substrate, and smooth surface, etc. [14,15]. However, so far, there have been very limited studies on AlSiN coatings by HiPIMS [16,17]. Meanwhile, the substrate bias determines the kinetic bombardment energies of the ions arriving on the substrate, which can significantly affect the microstructure and properties of the films. Since the HiPIMS plasma contains much higher ion fraction than those conventional sputtering, utilizing the bias voltage properly to attract the ions towards the substrate and control the kinetic energy of the ions will be critical. However, the investigations of the bias voltage effect on the microstructure and properties of the HiPIMS sputtered AlSiN coatings are limited. Therefore, the purpose of the present study is to synthesize the AlSiN films by HiPIMS and identify the influence of the bias voltage on the phase constituent, microstructure and mechanical properties of the films. Also, the relationship between evolution of microstructure and corrosion performance in simulated sea water environment was studied in detail. 2. Experimental details 2.1. Film deposition The AlSiN films were synthesized by a PVD equipment (Hybrid sputtering coater, KIMS, South Korea) with HiPIMS power (HIPIMS þ power, Hauzer Techno Coating BV) and pulsed direct current magnetron sputtering (pulsed DCMS) power supplies. The single crystalline Si wafers with (100) orientation and mirror polished SUS304 stainless steels were used as substrates. This stainless steel has the following chemical composition (in wt %): C (0.044), Si (0.43), Mn (1.12), P (0.032), S (0.004), Ni (8.03), Cr (18.13), N (0.04), and Fe in balance. The HiPIMS and pulsed DCMS powers were connected to the AlSi target and Cr target, respectively, which had a diameter of 80 mm and thickness of 8 mm. In order to guarantee the homogeneous feature of films along the surface plane the substrate holder was located at the center of vacuum chamber, and then continuously rotated at 10 r/min during whole deposition process. The center to center distance between the substrate holder and targets was approximately 120 mm. All the substrates were ultrasonically cleaned in acetone and ethanol for 15 min, sequentially, then fixed on the holder after being blown dry with nitrogen flow. The base pressure of chamber was below 3 103 Pa through using a rotary and turbo molecule pumps at temperature of 200 C. Before deposition, samples were cleaned by sputtering in argon plasma for 10 min using a bias voltage of - 800 V at 0.7 Pa. To enhance the adhesion strength of films with substrates, a thin Cr interlayer was deposited for 10 min by pulsed DCMS with fixed Cr target power of 0.7 kW at 0.5 Pa in argon atmosphere. Then, the Cr target was turn off and the Al70Si30 target was sputtered by HiPIMS for desired AlSiN films. The mixtures of gases were Ar and N2 with flow rates of 50 sccm and 20 sccm, respectively, controlled by separated mass flow controllers. The total working pressure and HiPIMS power were 0.6 Pa and 0.7 kW, respectively. The films were deposited for 2 h under different substrate bias voltages of 0 V, 50 V, - 100 V and - 150 V. The detailed pulsing information of HiPIMS during whole deposition process was listed in Table 1. 2.2. Film characterizations The element composition of the AlSiN films was conducted by an electron probe micro analysis (EPMA, CAMECA, SX 100) and corresponding result was presented in Fig. 1. The crystal structure
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of films was confirmed by an X-ray diffractometer (XRD, D8Discover Bruker, Cu Ka, 40 kV, 40 mA). An unlocked coupled q-2q mode with a 0.02 step size and a 0.2 s step time was performed. The diffraction angle was collected from 20 to 80 . The chemical bonding states of the AlSiN films were analyzed by X-ray photoelectron spectra (XPS, VG Scientifics, ES-CALAB 250) with monochromatic Al Ka (1486.6 eV) as the excitation source. To remove the surface contamination layer, an Arþ ion beam with energy of 4 keV was used to etch the sample surface for 180 s. The carbon C 1s peak at 284.5 eV was calibrated as reference. Surface and fractured crosssectional morphologies of all films were observed using fieldemission scanning electron microscopy (FE-SEM, S4800, 15 kV, Hitachi). Typically sample for transmission electron microscopy (TEM) analysis were prepared by a focused ion beam (FIB) technique. Relevant cross-sectional TEM analysis was carried out in a field emission analytical electron microscope (FEI, TALOS F200X) operated at 200 kV to further investigate the microstructure of film. The average roughness values of films were measured (1 1 mm2) by using an atomic force microscopy (AFM, MFM-3D, Asylum Research) digital image. The mechanical properties of films, i.e. hardness and elastic modulus were determined from load vs. displacement curves measured by a nano-indentation tester (Hysitron, TI950 TriboIndenter) at load of 4 mN. For all sputtered films, the indentation depth was kept below 10% of films thickness for avoiding influence of substrate. Fifteen indentations were performed for each sample and then an average value was proposed. Residual stress was obtained from the curvature of film/substrate composite with Stoney equation [18]. Vickers indentation tests were performed on the Vickers hardness testing system (Mitutoyo HM-220) with load of 10 N. The nano-wear tests were measured under a load condition of 90 mN across an area of 1 1 mm2 by three passes. The wear depth and wear volumes of films were measured by imaging mode of an in-situ scanning probe microscopy (SPM) (scan area was 3 3 mm2), the detailed calculation process can be found in previous report [19]. The corrosion properties of the samples (SUS304 specimens coated with and without films) were conducted in a 3.5 wt % NaCl solution (simulating the natural marine environment). A conventional three-electrode system was used which consist of the samples as the working electrode, platinum (Pt) mesh as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode [20]. The effective working area of samples was 1.0 cm2. The corrosion current density and corrosion potential were obtained by the intersection of the extrapolation of cathodic and anodic Tafel curves. All tests were performed in room temperature. Before testing, the samples were dipped in the solution for 30 min to reach a stable open-circuit potential. Then the electrode potential range of the measurement was carried out from 1 V to 1 V and the SCE with a scanning rate of 1 mV/s. 3. Results and discussion 3.1. Chemical composition and phase structure The elemental compositions of Al, N, Si and O in the obtained films as a function of bias voltage are shown in Fig. 1. It can be seen that the N and Al contents are around 60.0 at.% and 25.0 at.%, respectively, and slightly changed with increasing bias voltage from 0 to - 150 V. The Si content (10.0 at.%) is independent with the change of bias voltage. It was considered that the bias voltage showed limited influence on the elemental contents. It needs to be noted that the N was over-stoichiometric in all AlSiN films sputtered by HiPIMS, which is different from the previous results by using conventional MS or arc ion plating [12,21]. In their AlSiN films, the N content is commonly approximately/sub-
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Table 1 The HiPIMS pulsing parameters for the deposition of AlSiN films. Pulsing Parameters
Duty Cycle (%)
Repetition frequency (Hz)
Pa (kW)
Wt (w/cm2)
Id (A/cm2)
10.5
110.6
0.7
13.9
0.60
(i) Pa is the power of average target. (ii) Wt is the density of average target power. (iii) Id is the peak current density of target.
Fig. 1. The chemical compositions of the AlSiN films with various bias voltages.
stoichiometric. The over-stoichiometric N in all AlSiN films could be attributed to two possible reasons. First, large amount of N2 was ionized in HiPIMS plasma to promote the formation of the stoichiometric nitride during the deposition process due to the plasma density of HiPIMS was several orders of magnitude higher than that of conventional MS. It indicated that Al, Si and N atoms were completely reacted to transform into the nitride state and a part of superfluous N atoms could squash into the interstitial positions of AlN structure [4]. Second, the substrates did not face to the AlSi target for most of the rotation time during deposition. At a low duty cycle, the pulse-on time was much shorter than the pulse-off time in HiPIMS process, therefore, the films suffered excessed N and was independent of bias voltage, which could be described as nitriding. The oxygen existence in the films was attributed to the trace oxygen contamination from the target and sputtering chamber during deposition. Fig. 2 shows the X-ray diffraction patterns of AlSiN films at different bias voltages. Crystalline AlN phase were detected in all films and the c-AlN (100), c-AlN (200) and h-AlN (110) diffraction peaks were identified. As the bias voltage increasing, intensity of all peaks was gradually strengthened, indicating the crystallinity enhancement of AlSiN films. The additional energy, delivered to films by the ions bombardment through applying bias voltage, lead to increase of the surface adatom mobility, which in turn facilitated higher crystallinity of the films [16,22]. No obvious peaks associated with Si3N4 or any other aluminum silicide phases were observed, implying the Si3N4 was amorphous phase existed in films and Al, Si elements were bonded to N entirely. It is well known that the Si content has great effect on the structure of nitride films. After Si addition, the film structure firstly converted from solid solution to the nano-composite structure when the Si content overpassed its solution limitation in the base material of the films, and then
Fig. 2. The XRD patterns of the AlSiN films with various bias voltages.
formed amorphous structure as Si content further increased. The solid solution limitation of Si was approximately 6 at.% for the AlSiN film, whereas, the Si content was around at 10 at.% under various bias conditions in this work. According to previous report, such Si content did not result in amorphous structure in AlSiN film since it was confirmed that the Si content surpassed 15 at.% and 20 at.% would lead to the formation of amorphous structure in AlSiN films [9,11]. Therefore, it was considered that these films consisted of nano-crystalline AlN embedded in the amorphous Si3N4 matrix in the study. Fig. 3 shows the XPS spectroscopy for AlSiN films deposited at various bias voltages. From Fig. 3a, the Al2p spectra could be deconvoluted into two peaks (73.1 eV and 74.4 eV), which was related to the AlN and Al2O3 [23]. No any information from metallic AleAl (72.9 eV) bonds was detected. Similar results were reported by Chang et al. [12] and Erik et al. [16]. A small amount of Al2O3 existed in films probably came from oxygen contamination from the target or sputtering chamber during deposition. Fig. 3b shows two bonding energies in the N1s spectra, which could be assigned to NeAl bonds (AlN) at 396.3 eV and NeSi bonds (Si3N4) at 397.4 eV [12,24]. Additionally, the spectral peak shifted slightly to lower binding energy with increase of the bias voltage in the deposition process. From Fig. 3c, it was easy to distinguish the sole Si3N4 at binding energy of 101.1 eV. Combined with the XPS and XRD results, it could be speculated that Si3N4 was in amorphous structure and only one crystalline phase, AlN, formed in the films. 3.2. Microstructure and morphology of AlSiN films The SEM surface images of AlSiN films with different bias voltages are shown in Fig. 4. It could be clearly seen that all the films showed a typical feature of the granular structure. In Fig. 4a,
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Fig. 3. The XPS spectra of (a) Al 2p, (b) N1s, and (c) O1s of the AlSiN films with various bias voltages.
Fig. 4. The SEM surface morphology of AlSiN films with various bias voltages.
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without bias voltage, the film exhibited relative large sizes of granules and less compact feature on the surface. As the bias voltage increased from - 50 V to - 150 V (Fig. 4bed), the AlSiN films showed smaller granules and a more compact structure, similar result was also observed in previous work [4]. It was contributed to the increased energy delivered to the surface adatom, which could effectively increase the surface adatom mobility after applying bias voltage. Fig. 5 shows the typical three-dimensional topographic images of the AlSiN film deposited under bias voltage of 0 V and 150 V, respectively. The average surface roughness was listed in Table 2. In Fig. 5a, the AlSiN film was composed of many column hillocks and islands with different sizes distributed randomly on the surface, which was consistent with the shapes of the granular structure shown in SEM surface. The film deposited without bias voltage showed the maximum roughness value of 8.2 nm and the roughness decreased to less than 6.0 nm under bias voltage of 150 V. The probable reasons are as follows: (1) The larger quantities of Arþ ion bombardment at higher bias voltage resulted in the further etching of the asperities and thus smoothing the surface of the films. (2) As shown in Fig. 5b, the surface hillocks seemed to be merged into bigger agglomerates appearing larger islands or bumps, which also leaded to the decrease of surface roughness. The cross-sectional SEM images of deposited films with various bias voltages are shown in Fig. 6. A typical loose columnar structure with visible gap was observed in Fig. 6a, which grew outward from the film-interlayer interface and then throughout the whole film thickness. The Cr interlayer with a thickness of about 130 nm was used to strength the adhesion between substrate and film. With increase of bias voltage, these films still exhibited pronounced columnar structure but three different features need to be stated. First, the columnar structure transformed into denser and more compact structure, which was ascribed to the higher energy came from bias voltage forcing atoms to move into voids between the grains. Second, the thickness of the films decreased from about 1.80 mm to 1.57 mm as the bias voltage increasing from 0 V to - 150 V. The enhanced bias voltage resulted in large augment energy of the ions impinging on the growing coating, leading to the re-sputtering phenomena and decrease of film thickness [25]. The change of microstructure evolved into denser structure was also conducive to the diminish thickness. The last feature was shown in Fig. 6d, the film exhibited duplex layers consisting of a stacking, dense glassy layer grown onto Cr interlayer, and then followed by a compact and dense columnar structure layer. It was speculated that the special structure was beneficial to enhance corrosion resistance, thus, more investigations were needed to understand the underlying causes for this duplex structure. Fig. 7 shows the cross-sectional TEM images and diffraction patterns of the AlSiN film deposited at a bias voltage of - 150 V. In Fig. 7a, it could be seen that the growth direction of the film was along with its thickness directions. The Cr interlayer clearly showed
Table 2 The RMS roughness values of AlSiN films with various bias voltages. Bias voltage (V) RMS roughness (nm)
0 8.2
50 7.9
100 7.8
150 6.0
a typical columnar structure with a thickness of 130 nm, which was grown along the perpendicular direction of the Si substrate. Fig. 7b and c showed the bright and dark-field TEM images of AlSiN film, respectively. The columnar crystals dominated the film microstructure, with column width ranging from 10 to 25 nm. The SAED pattern in Fig. 7d showed a polycrystalline structure, which were in good agreement with those diffraction peaks as shown in the XRD pattern (Fig. 2). The SAED pattern in TEM analysis did not reveal any crystalline silicon nitride. Combined with XRD results, it was confirmed that a silicon compound was formed as amorphous phase in the film. Fig. 7e showed the HR-TEM image and the corresponding Fast Fourier Transformation (FFT) pattern image (inset). It could be observed that the film was nano-composite structure consist of AlN nano-crystals (marked as curve) with a grain size below 10 nm embedded in the thick Si3N4 amorphous matrix. 3.3. Mechanical properties The hardness (H) and elastic modulus (E) of AlSiN films with different bias voltages are shown in Fig. 8a. With increasing of the bias voltage, the H values continuously increased to the maximum of 13 GPa at bias voltage of - 150 V, whereas E increased to maximum of 146.4 GPa as bias voltage increased from 0 V to - 100 V and then slightly decreased at - 150 V. Such tendency was consistent with the results reported by Chang et al. [12], where an increase of bias voltage lead to increased H and E in AlSiN films. The hardness enhancement in this study could be inferred as follows: First, the smaller Si atoms substitute larger Al atoms entered into the AlN host lattice structure and then caused the lattice distortion and hinder the movement of dislocation, which was defined as solid solution hardening. Second, it was attributed to the structure densification. The microstructure changed from porous to denser structure with increase of the bias voltage and the defects in films were effectively reduced. The hardness could be improved by the structure densification process, similar results were also reported by Wang et al. [26] and Ding et al. [27]. Third, the moderate compressive stress in films was partly in favor of hardness hardening, as shown in Fig. 8b, the stress value increased with increase of bias voltage, which was consistent with the evolution of hardness value. Finally, the formation of nano-composite structure, comprising of AlN crystalline encapsulated in an amorphous Si3N4 tissue, also improved the hardness. However, the effect of the improved hardness in this work was rather limited, comparing to the previous reports [10]. Although, the absence of pronounced
Fig. 5. The typical AFM topography of AlSiN films with various bias voltages of (a) 0 V and (b) - 150 V.
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Fig. 6. The cross-sectional morphology of AlSiN films with various bias voltages.
Fig. 7. The TEM results of AlSiN film deposited at bias voltage of - 150 V: (a) cross-sectional image, (b) bright-field image, (c) dark-field image, (d) SAED pattern and (e) HRTEM image and corresponding FFT pattern.
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Fig. 8. Hardness and elastic modulus (a), compressive stress (b), H3/E2 ratios (c) and wear volumes (d) of AlSiN films with various bias voltages.
hardness enhancement could be ascribed to the different deposition methods and parameters, other reasons were more worthwhile to be considered. As for the hardness properties of nanocomposite structured films, it is commonly agreed that hard nano-crystalline grains covered by an amorphous phase of only one or two monolayers could achieve a superhard effect [28,29]. In the present case, the Si content in films was up to 10 at.%, which resulted in a thickening of amorphous phase between nano-grains. The high content of soft amorphous matrix would enlarge the distance between nano-grains, which reduced the effect of nanocomposite strengthening. It could be verified by TEM observation. On the other hand, both AlN and Si3N4 were covalent nitrides with the same metalloid coordination numbers and similar mechanical properties. These similarities probably resulted in weak diffusion interfaces between the crystalline and amorphous phases. Such weak interface could not effectively hinder the movement of dislocations compared with the sharp interface, which deteriorated the hardness [6]. The mixed crystalline structure in films, as shown in Fig. 2, was also a reason for decrease of hardness. In addition, many reports have shown that oxygen in hard films can significantly decrease the film hardness because of its high electronegativity, which causes weakening of the neighbor bonds thus forms fairly large defects to limit the hardness defects [30,31], the oxygen contamination in films could be another reason for limited hardening effect limited in this work. It has been reported that the H3/E2 ratio could be used to predict the resistance against plastic deformation and fracture toughness of hard coatings, which has been also used to predict potential results of nano-wear for thin films. Fig. 8c presented the H3/E2 ratios for various bias voltages. As the bias voltage increased from 0 V to 150 V, the H3/E2 ratio, increased from 0.06 to 0.12, indicating that the AlSiN film had better plastic deformation and fracture toughness compare to the film depositing without bias voltage. Fig. 8d showed the nano-scale wear resistance of films at different bias
voltages. The nano-wear test was performed by using nanoindenter. To avoid the influence of the underlying substrates and environment parameters, small load was used, so that the nanowear properties were mainly determined by the intrinsic properties of film materials. The nano-wear could be represented by the wear out volume under identical wear conditions with different films, which was defined as the material volume of the AlSiN films being removed by diamond probe. The single crystalline Si substrate was as a standard and its nano-wear volume was 17.2 103 mm3 in previous report [32]. Under the same test condition, the nano-wear volume decreased from 9.7 103 mm3 at 0 V to a minimum value of 7.6 103 mm3 at bias voltage increased of 150 V. It was reported that the higher fracture toughness (KIC) and hardness (H) lead to higher wear resistance [33,34], which was in agreement with the variations of hardness and H3/E2 values in this work. The obtained result indicated that the nano-wear resistance of AlSiN film could be enhanced by applying bias voltage. Fig. 9 showed the typical Vickers indentation SEM images over a high load (10 N) on the AlSiN films with different bias voltages. As shown in Fig. 9a, serious cracks were observed at the circumambience of indenter tip center on the surface of film deposited with no bias voltage, indicating a brittle failure. While the AlSiN film deposited under bias voltage of - 150 V exhibited a relative smooth surface morphology, as shown in Fig. 9b, indicating a positive effect of bias voltage on the enhancement of the film toughness. This result was in accordance with the H3/E2 ratio values as mentioned above. In addition, the denser structure, higher hardness and lower surface roughness might be also conducive to toughness enhancement of film. 3.4. Corrosion properties The potentiodynamic polarization curves of the SUS304 steel substrate as well as the typically AlSiN films deposited under
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Fig. 9. The typical Vickers indentation images of AlSiN films with various bias voltages of (a) 0 V and (b) - 150 V.
Fig. 10. The typical potentiodynamic polarization curves of the SUS304 steel and AlSiN films (from 0 V to - 150 V) in the 3.5 wt.% NaCl aqueous solution.
Table 3 Corrosion results by potentiodynamic polarization curves obtained in 3.5 wt.% NaCl solution. Sample
Ecorr (mV vs. SCE)
Icorr (A/cm2)
SUS304 AlSiN (0 V) AlSiN (- 50 V) AlSiN (- 100 V) AlSiN (- 150 V)
220 198 197 184 178
1.06 106 8.10 107 3.41 107 2.52 108 5.94 1010
various bias voltages were tested in 3.5 wt.% NaCl solution as shown in Fig. 10. The corrosion potential (Ecorr) and the corrosion current density (Icorr) were determined by means of linear fitting of Tafel extrapolation of the curves. Table 3 lists the respective Icorr and the Ecorr. It was clearly seen that the SUS304 substrates coated with AlSiN films displayed lower corrosion current densities than that of the uncoated SUS304 substrates, indicating better corrosion resistance. The Icorr of SUS304 substrate was 1.06 106 A/cm2, which was higher than that of the deposited AlSiN film at bias voltage of 0 V (8.10 107 A/cm2). With further increase of bias voltage to 150 V, the Icorr of AlSiN film reached to 5.94 1010 A/cm2, which was one and four orders of magnitude lower than that of uncoated
SUS304 substrate and coated film without bias voltage, respectively. This demonstrated that the corrosion rate of AlSiN films was lower than that of SUS304 substrate during polarization test [35]. Also, the Ecorr shows a slight shift to positive side with increasing bias voltage, which implies better corrosion resistance of films. The corresponding SEM surface morphologies of the SUS304 substrate as well as AlSiN films synthesized at bias voltages of 0 and - 150 V after corrosion test are shown in Fig. 11. It is obvious that numerous deep pitting corrosion with different dimensions in length appeared on the surface of the bare SUS304 (Fig. 11a, and d), while as to the AlSiN film at bias voltage of 0 V (Fig. 11b and e), few tiny shallow pits with smaller dimension were distributed on the surface of the film. No apparent pits on the surface of AlSiN film synthesized at bias voltage of - 150 V were seen at the same observation scale with the other two samples. All of these SEM results were consistent with Icorr values as shown in Fig. 10. The improvement of corrosion resistance could be attributed to the dense structure of the film, especially the duplex denser layers at high bias voltage as shown in Fig. 6d. The film possessed dense structure with low defects could act as protective passive film and effectively block the paths of corrosion medium to the substrate [20]. The result demonstrates that AlSiN film can work as a promising protective barrier layer against corrosion of substrates in corrosive environment.
4. Conclusions The AlSiN films were deposited at different bias voltages by a high power impulse magnetron sputtering. The composition, microstructure, mechanical and corrosion properties of films were investigated. The obtained conclusions were as follows: (1) All AlSiN films deposited in this work presented overstoichiometric N and the result was independent of bias voltage, which was ascribed to the higher ionization rate in HiPIMS. h-AlN and c-AlN crystalline phase co-existed in the films with nano-crystalline AlN embedded in Si3N4 amorphous phase, forming a nano-composite structure. (2) With increasing bias voltage, the granular on the surface of film became smaller and the cross-section morphology changed from porous columnar structure with visible gaps to denser and more compact columnar structure. Especially, at bias voltage of - 150 V, the AlSiN film exhibited a stacking and dense-glassy layer at the bottom and then followed by a dense columnar structure. The average roughness value also exhibited a descending trend with increase of bias voltage.
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Fig. 11. SEM images of the (a,d) SUS304 steel, (b,e) AlSiN film (0 V) and (c,f) AlSiN film (- 150 V) after corrosion test in the 3.5 wt.% NaCl aqueous solution.
(3) The mechanical properties including hardness and toughness, and nano-wear resistance of films, were improved with increasing bias voltage, which could be explained by the higher energy bombardment, denser structure and higher H3/E2 ratio. In addition, potentiodynamic polarization tests showed that the AlSiN film deposited at - 150 V bias voltage exhibited the best corrosion resistance in 3.5 wt.% NaCl solutions. Acknowledgments The work was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013M3A6B1078874). References [1] S.K. Pradhan, C. Nouveau, A. Vasin, M.A. Djouadi, Deposition of CrN coatings by PVD methods for mechanical application, Surf. Coating. Technol. 200 (2005) 142e145. [2] S. Khamseh, H. Araghi, A study of the oxidation behavior of CrN and CrZrN ceramic thin films prepared in a magnetron sputtering system, Ceram. Int. 42 (2016) 9988e9994. [3] G. Dorcioman, G. Socol, D. Craciun, N. Argibay, E. Lambers, M. Hanna, C.R. Taylor, V. Craciun, Wear tests of ZrC and ZrN thin films grown by pulsed laser deposition, Appl. Surf. Sci. 306 (2014) 33e36. [4] J.C. Ding, T.F. Zhang, H.J. Mei, J.M. Yun, S.H. Jeong, Q.M. Wang, K.H. Kim, Effects of negative bias voltage and ratio of nitrogen and argon on the structure and properties of NbN coatings deposited by HiPIMS deposition system, Coatings 8 (2018) 1e12. [5] F. Ansart, H. Ganda, R. Saporte, J.P. Traverse, Study of the oxidation of aluminium nitride coatings at high temperature, Thin Solid Films 260 (1995) 38e46. lisson, M. Parlinska-Wojtan, H.J. Hug, J. Patscheider, Microstructure and [6] A. Pe mechanical properties of Al-Si-N transparent hard coatings deposited by magnetron sputtering, Surf. Coating. Technol. 202 (2007) 884e889. [7] A. Khanna, D.G. Bhat, Effects of deposition parameters on the structure of AlN coatings grown by reactive magnetron sputtering, J. Vac. Sci. Technol. A 25 (2007) 557e565. [8] A. Bendavid, P.J. Martin, H. Takikawa, The properties of nanocomposite aluminium-silicon based thin films deposited by filtered arc deposition, Thin Solid Films 420 (2002) 83e88. lisson, H.J. Hug, J. Patscheider, Morphology, microstructure evolution and [9] A. Pe optical properties of Al-Si-N nanocomposite coating, Surf. Coating. Technol. 257 (2014) 114e120. [10] H. Liu, W. Tang, D. Hui, L. Hei, F. Lu, Characterization of (Al,Si)N films deposited by balanced magnetron sputtering, Thin Solid Films 517 (2009)
5988e5993. [11] J. Musil, G. Remnev, V. Uglov, A. Lebedynskiy, A. Lauk, J. Prochazka, S. Haviar, E. Smolyanskiy, Flexible hard Al-Si-N films for high temperature operation, Surf. Coating. Technol. 307 (2016) 1112e1118. [12] C.L. Chang, C.S. Huang, Effect of bias voltage on microstructure, mechanical and wear properties of Al-Si-N coatings deposited by cathodic arc evaporation, Thin Solid Films 519 (2011) 4923e4927. [13] J.C. Ding, T.F. Zhang, J.M. Yun, K.H. Kim, Q.M. Wang, Effect of Cu addition on the microstructure and properties of TiB2 films deposited by a hybrid system combining high power impulse magnetron sputtering and pulsed dc magnetron sputtering, Surf. Coating. Technol. 344 (2018) 441e448. [14] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: a review on scientific and engineering state of the art, Surf. Coating. Technol. 204 (2010) 1661e1684. [15] V. Kouznetsov, K. Macak, J.M. Schneider, U. Helmersson, I. Petrov, A novel pulsed magnetron sputter technique utilizing very high target power densities, Surf. Coating. Technol. 122 (1999) 290e293. [16] E. Lewin, D. Loch, A. Montagne, A.P. Ehiasarian, J. Patscheider, Comparison of Al-Si-N nanocomposite coatings deposited by HiPIMS and DC magnetron sputtering, Surf. Coating. Technol. 232 (2013) 680e689. [17] X. Jiang, F.C. Yang, W.C. Chen, J.W. Lee, C.L. Chang, Effect of nitrogen-argon flow ratio on the microstructural and mechanical properties of AlSiN thin films prepared by high power impulse magnetron sputtering, Surf. Coating. Technol. 320 (2017) 138e145. [18] S.S. Zhao, H. Du, W.G. Hua, J. Gong, The depth distribution of residual stresses in (Ti,Al)N films: measurement and analysis, J. Mater. Res. 22 (2007) 2659e2662. [19] T.F. Zhang, B. Gan, S.M. Park, Q.M. Wang, K.H. Kim, Influence of negative bias voltage and deposition temperature on microstructure and properties of superhard TiB2 coatings deposited by high power impulse magnetron sputtering, Surf. Coating. Technol. 253 (2014) 115e122. [20] D. Wang, Y. Fu, M. Hu, D. Jiang, X.M. Gao, Q. Wang, J. Yang, J. Sun, L. Weng, Effect of Nb content on the microstructure and corrosion resistance of the sputtered Cr-Nb-N coatings, J. Alloys Compd. 740 (2018) 510e518. [21] J. Musil, M. Sasek, P. Zeman, R. Cerstvy, D. Herman, J.G. Han, V. Satava, Properties of magnetron sputtered Al-Si-N thin films with a low and high Si content, Surf. Coating. Technol. 202 (2008) 3485e3493. [22] G.F. Iriarte, F. Engelmark, I.V. Katardjiev, Reactive sputter deposition of highly oriented AlN films at room temperature, J. Mater. Res. 17 (2002) 1469e1475. [23] D. Manovaa, V. Dimitrovaa, W. Fukarekb, D. Karpuzovc, Investigation of d.c.reactive magnetron-sputtered AlN thin films by electron microprobe analysis, X-ray photoelectron spectroscopy and polarised infra-red reflection, Surf. Coating. Technol. 106 (1998) 205e208. [24] M. Witthaut, R. Cremer, D. Neuschuetz, Electron spectroscopy of single-phase (Al,B)N films, Sur. Interface Anal. 30 (2000) 580e584. [25] P. Wang, T. Takeno, J. Fontaine, M. Aono, K. Adachi, H. Miki, T. Takagi, Effect of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings, Mater. Chem. Phys. 145 (2014) 434e440. [26] T.G. Wang, D. Jeong, Y. Liu, Q.M. Wang, S. Iyengar, S. Melin, K.H. Kim, Study on nanocrystalline Cr2O3 films deposited by arc ion plating: II. Mechanical and tribological properties, Surf. Coating. Technol. 206 (2012) 2638e2644. [27] J.C. Ding, T.F. Zhang, J.M. Yun, M.C. Kang, Q.M. Wang, K.H. Kim, Microstructure,
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Influence of bias voltage on the microstructure, mechanical and corrosion properties of AlSiN films deposited by HiPIMS technique Ji Cheng Ding a, b, Qi Min Wang c, Zhe Ren Liu c, Seonhee Jeong b, Teng Fei Zhang c, *, Kwang Ho Kim a, b, ** a b c
School of Convergence Science, Pusan National University, Busan, 46241, South Korea Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan, 46241, South Korea School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2018 Received in revised form 3 September 2018 Accepted 6 September 2018 Available online 7 September 2018
AlSiN films were deposited with various bias voltages ranging from 0 V to - 150 V by high power impulse magnetron sputtering (HiPIMS) technique. The effects of bias voltage on the microstructure, mechanical and corrosion behavior were investigated. The AlSiN films exhibited an over-stoichiometric N content and both cubic and hexagonal AlN were observed in films. All deposited films showed a typical surface feature of granular structure and the cross-sectional images exhibited that all films possessed columnar structure, which was changed from coarse columnar to denser columnar structure with increasing the bias voltage. The AlSiN film deposited at - 150 V possessed the densest structure and exhibited the best mechanical properties, including hardness, toughness and nano-wear resistance. The corrosion test showed that all coated samples had better corrosion resistance compared to SUS304 in 3.5 wt.% NaCl solution and the AlSiN film deposited at bias voltage of - 150 V possessed the best corrosion resistance due to denser microstructure, which could act as a barrier layer for blocking the diffusion of corrosive substances. © 2018 Elsevier B.V. All rights reserved.
Keywords: AlSiN film HiPIMS Bias voltage Mechanical properties Corrosion
1. Introduction Transition metal nitride (TMN) films, such as CrN, TiN, and AlN, have been widely applied as protective films in machining industry due to their high hardness, good wear resistance and high thermal stability at elevated temperatures [1e4]. Among these TMN films, the AlN possesses a wide band gap of 6.2 eV, good optical transparence in the visible range of light, high melting temperature (2275 K), and also excellent corrosion/oxidation resistance [5e7]. These advantages make it a promising candidate as protective film for optically transparent substrates. Recently, many reports revealed that nano-composite AlSiN films can be synthesized by adding proper content of Si into the AlN films, where nanocrystalline AlN is surrounded by one to two monolayer of amorphous Si3N4 matrix, which can significantly improve the
* Corresponding author. ** Corresponding author. School of Convergence Science, Pusan National University, Busan, 46241, South Korea. E-mail addresses: [email protected] (T.F. Zhang), [email protected] (K.H. Kim). https://doi.org/10.1016/j.jallcom.2018.09.063 0925-8388/© 2018 Elsevier B.V. All rights reserved.
mechanical properties and thermal stability of the films [8e10]. In previous work by Pelisson et al. [6], the AlSiN films with various Si contents were deposited by magnetron sputtering (MS), which exhibited that the mean grain size decreased from 60 nm to 5 nm with increasing Si content and the maximum hardness exceeded 30 GPa was obtained at Si content of around 10 at. %. Musil et al. [11] reported that the AlSiN films synthesized by MS were polycrystalline when the Si content was lower than 10 at. %, and the films became amorphous structure with a relatively high Si content over than 20 at. % at a deposition temperature of 500 C. Furthermore, both the crystalline AlSiN films and the amorphous AlSiN films exhibited a high oxidation resistance up to 1000 C. Chang et al. [12] fabricated the AlSiN films by cathodic arc evaporation and showed that the bias voltage had great influence on the microstructure as well as the mechanical properties of the AlSiN films. As mentioned above, AlSiN films have been mainly fabricated by using MS and arc evaporation deposition methods. As an advanced physical vapor deposition (PVD) technology, high power impulse magnetron sputtering (HiPIMS) technique has been proven useful in depositing films as smooth as MS-PVD and as dense as arc-PVD [4,13]. By pulsing the sputtering target with high power (e.g.
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1e3 kW/cm2) and short pulse lengths, a high plasma density and high ionization of the sputtered species can be obtained. It has been widely reported that the HiPIMS was an excellent technique for synthesizing hard coatings with higher hardness, good adhesion between coating and substrate, and smooth surface, etc. [14,15]. However, so far, there have been very limited studies on AlSiN coatings by HiPIMS [16,17]. Meanwhile, the substrate bias determines the kinetic bombardment energies of the ions arriving on the substrate, which can significantly affect the microstructure and properties of the films. Since the HiPIMS plasma contains much higher ion fraction than those conventional sputtering, utilizing the bias voltage properly to attract the ions towards the substrate and control the kinetic energy of the ions will be critical. However, the investigations of the bias voltage effect on the microstructure and properties of the HiPIMS sputtered AlSiN coatings are limited. Therefore, the purpose of the present study is to synthesize the AlSiN films by HiPIMS and identify the influence of the bias voltage on the phase constituent, microstructure and mechanical properties of the films. Also, the relationship between evolution of microstructure and corrosion performance in simulated sea water environment was studied in detail. 2. Experimental details 2.1. Film deposition The AlSiN films were synthesized by a PVD equipment (Hybrid sputtering coater, KIMS, South Korea) with HiPIMS power (HIPIMS þ power, Hauzer Techno Coating BV) and pulsed direct current magnetron sputtering (pulsed DCMS) power supplies. The single crystalline Si wafers with (100) orientation and mirror polished SUS304 stainless steels were used as substrates. This stainless steel has the following chemical composition (in wt %): C (0.044), Si (0.43), Mn (1.12), P (0.032), S (0.004), Ni (8.03), Cr (18.13), N (0.04), and Fe in balance. The HiPIMS and pulsed DCMS powers were connected to the AlSi target and Cr target, respectively, which had a diameter of 80 mm and thickness of 8 mm. In order to guarantee the homogeneous feature of films along the surface plane the substrate holder was located at the center of vacuum chamber, and then continuously rotated at 10 r/min during whole deposition process. The center to center distance between the substrate holder and targets was approximately 120 mm. All the substrates were ultrasonically cleaned in acetone and ethanol for 15 min, sequentially, then fixed on the holder after being blown dry with nitrogen flow. The base pressure of chamber was below 3 103 Pa through using a rotary and turbo molecule pumps at temperature of 200 C. Before deposition, samples were cleaned by sputtering in argon plasma for 10 min using a bias voltage of - 800 V at 0.7 Pa. To enhance the adhesion strength of films with substrates, a thin Cr interlayer was deposited for 10 min by pulsed DCMS with fixed Cr target power of 0.7 kW at 0.5 Pa in argon atmosphere. Then, the Cr target was turn off and the Al70Si30 target was sputtered by HiPIMS for desired AlSiN films. The mixtures of gases were Ar and N2 with flow rates of 50 sccm and 20 sccm, respectively, controlled by separated mass flow controllers. The total working pressure and HiPIMS power were 0.6 Pa and 0.7 kW, respectively. The films were deposited for 2 h under different substrate bias voltages of 0 V, 50 V, - 100 V and - 150 V. The detailed pulsing information of HiPIMS during whole deposition process was listed in Table 1. 2.2. Film characterizations The element composition of the AlSiN films was conducted by an electron probe micro analysis (EPMA, CAMECA, SX 100) and corresponding result was presented in Fig. 1. The crystal structure
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of films was confirmed by an X-ray diffractometer (XRD, D8Discover Bruker, Cu Ka, 40 kV, 40 mA). An unlocked coupled q-2q mode with a 0.02 step size and a 0.2 s step time was performed. The diffraction angle was collected from 20 to 80 . The chemical bonding states of the AlSiN films were analyzed by X-ray photoelectron spectra (XPS, VG Scientifics, ES-CALAB 250) with monochromatic Al Ka (1486.6 eV) as the excitation source. To remove the surface contamination layer, an Arþ ion beam with energy of 4 keV was used to etch the sample surface for 180 s. The carbon C 1s peak at 284.5 eV was calibrated as reference. Surface and fractured crosssectional morphologies of all films were observed using fieldemission scanning electron microscopy (FE-SEM, S4800, 15 kV, Hitachi). Typically sample for transmission electron microscopy (TEM) analysis were prepared by a focused ion beam (FIB) technique. Relevant cross-sectional TEM analysis was carried out in a field emission analytical electron microscope (FEI, TALOS F200X) operated at 200 kV to further investigate the microstructure of film. The average roughness values of films were measured (1 1 mm2) by using an atomic force microscopy (AFM, MFM-3D, Asylum Research) digital image. The mechanical properties of films, i.e. hardness and elastic modulus were determined from load vs. displacement curves measured by a nano-indentation tester (Hysitron, TI950 TriboIndenter) at load of 4 mN. For all sputtered films, the indentation depth was kept below 10% of films thickness for avoiding influence of substrate. Fifteen indentations were performed for each sample and then an average value was proposed. Residual stress was obtained from the curvature of film/substrate composite with Stoney equation [18]. Vickers indentation tests were performed on the Vickers hardness testing system (Mitutoyo HM-220) with load of 10 N. The nano-wear tests were measured under a load condition of 90 mN across an area of 1 1 mm2 by three passes. The wear depth and wear volumes of films were measured by imaging mode of an in-situ scanning probe microscopy (SPM) (scan area was 3 3 mm2), the detailed calculation process can be found in previous report [19]. The corrosion properties of the samples (SUS304 specimens coated with and without films) were conducted in a 3.5 wt % NaCl solution (simulating the natural marine environment). A conventional three-electrode system was used which consist of the samples as the working electrode, platinum (Pt) mesh as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode [20]. The effective working area of samples was 1.0 cm2. The corrosion current density and corrosion potential were obtained by the intersection of the extrapolation of cathodic and anodic Tafel curves. All tests were performed in room temperature. Before testing, the samples were dipped in the solution for 30 min to reach a stable open-circuit potential. Then the electrode potential range of the measurement was carried out from 1 V to 1 V and the SCE with a scanning rate of 1 mV/s. 3. Results and discussion 3.1. Chemical composition and phase structure The elemental compositions of Al, N, Si and O in the obtained films as a function of bias voltage are shown in Fig. 1. It can be seen that the N and Al contents are around 60.0 at.% and 25.0 at.%, respectively, and slightly changed with increasing bias voltage from 0 to - 150 V. The Si content (10.0 at.%) is independent with the change of bias voltage. It was considered that the bias voltage showed limited influence on the elemental contents. It needs to be noted that the N was over-stoichiometric in all AlSiN films sputtered by HiPIMS, which is different from the previous results by using conventional MS or arc ion plating [12,21]. In their AlSiN films, the N content is commonly approximately/sub-
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Table 1 The HiPIMS pulsing parameters for the deposition of AlSiN films. Pulsing Parameters
Duty Cycle (%)
Repetition frequency (Hz)
Pa (kW)
Wt (w/cm2)
Id (A/cm2)
10.5
110.6
0.7
13.9
0.60
(i) Pa is the power of average target. (ii) Wt is the density of average target power. (iii) Id is the peak current density of target.
Fig. 1. The chemical compositions of the AlSiN films with various bias voltages.
stoichiometric. The over-stoichiometric N in all AlSiN films could be attributed to two possible reasons. First, large amount of N2 was ionized in HiPIMS plasma to promote the formation of the stoichiometric nitride during the deposition process due to the plasma density of HiPIMS was several orders of magnitude higher than that of conventional MS. It indicated that Al, Si and N atoms were completely reacted to transform into the nitride state and a part of superfluous N atoms could squash into the interstitial positions of AlN structure [4]. Second, the substrates did not face to the AlSi target for most of the rotation time during deposition. At a low duty cycle, the pulse-on time was much shorter than the pulse-off time in HiPIMS process, therefore, the films suffered excessed N and was independent of bias voltage, which could be described as nitriding. The oxygen existence in the films was attributed to the trace oxygen contamination from the target and sputtering chamber during deposition. Fig. 2 shows the X-ray diffraction patterns of AlSiN films at different bias voltages. Crystalline AlN phase were detected in all films and the c-AlN (100), c-AlN (200) and h-AlN (110) diffraction peaks were identified. As the bias voltage increasing, intensity of all peaks was gradually strengthened, indicating the crystallinity enhancement of AlSiN films. The additional energy, delivered to films by the ions bombardment through applying bias voltage, lead to increase of the surface adatom mobility, which in turn facilitated higher crystallinity of the films [16,22]. No obvious peaks associated with Si3N4 or any other aluminum silicide phases were observed, implying the Si3N4 was amorphous phase existed in films and Al, Si elements were bonded to N entirely. It is well known that the Si content has great effect on the structure of nitride films. After Si addition, the film structure firstly converted from solid solution to the nano-composite structure when the Si content overpassed its solution limitation in the base material of the films, and then
Fig. 2. The XRD patterns of the AlSiN films with various bias voltages.
formed amorphous structure as Si content further increased. The solid solution limitation of Si was approximately 6 at.% for the AlSiN film, whereas, the Si content was around at 10 at.% under various bias conditions in this work. According to previous report, such Si content did not result in amorphous structure in AlSiN film since it was confirmed that the Si content surpassed 15 at.% and 20 at.% would lead to the formation of amorphous structure in AlSiN films [9,11]. Therefore, it was considered that these films consisted of nano-crystalline AlN embedded in the amorphous Si3N4 matrix in the study. Fig. 3 shows the XPS spectroscopy for AlSiN films deposited at various bias voltages. From Fig. 3a, the Al2p spectra could be deconvoluted into two peaks (73.1 eV and 74.4 eV), which was related to the AlN and Al2O3 [23]. No any information from metallic AleAl (72.9 eV) bonds was detected. Similar results were reported by Chang et al. [12] and Erik et al. [16]. A small amount of Al2O3 existed in films probably came from oxygen contamination from the target or sputtering chamber during deposition. Fig. 3b shows two bonding energies in the N1s spectra, which could be assigned to NeAl bonds (AlN) at 396.3 eV and NeSi bonds (Si3N4) at 397.4 eV [12,24]. Additionally, the spectral peak shifted slightly to lower binding energy with increase of the bias voltage in the deposition process. From Fig. 3c, it was easy to distinguish the sole Si3N4 at binding energy of 101.1 eV. Combined with the XPS and XRD results, it could be speculated that Si3N4 was in amorphous structure and only one crystalline phase, AlN, formed in the films. 3.2. Microstructure and morphology of AlSiN films The SEM surface images of AlSiN films with different bias voltages are shown in Fig. 4. It could be clearly seen that all the films showed a typical feature of the granular structure. In Fig. 4a,
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Fig. 3. The XPS spectra of (a) Al 2p, (b) N1s, and (c) O1s of the AlSiN films with various bias voltages.
Fig. 4. The SEM surface morphology of AlSiN films with various bias voltages.
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without bias voltage, the film exhibited relative large sizes of granules and less compact feature on the surface. As the bias voltage increased from - 50 V to - 150 V (Fig. 4bed), the AlSiN films showed smaller granules and a more compact structure, similar result was also observed in previous work [4]. It was contributed to the increased energy delivered to the surface adatom, which could effectively increase the surface adatom mobility after applying bias voltage. Fig. 5 shows the typical three-dimensional topographic images of the AlSiN film deposited under bias voltage of 0 V and 150 V, respectively. The average surface roughness was listed in Table 2. In Fig. 5a, the AlSiN film was composed of many column hillocks and islands with different sizes distributed randomly on the surface, which was consistent with the shapes of the granular structure shown in SEM surface. The film deposited without bias voltage showed the maximum roughness value of 8.2 nm and the roughness decreased to less than 6.0 nm under bias voltage of 150 V. The probable reasons are as follows: (1) The larger quantities of Arþ ion bombardment at higher bias voltage resulted in the further etching of the asperities and thus smoothing the surface of the films. (2) As shown in Fig. 5b, the surface hillocks seemed to be merged into bigger agglomerates appearing larger islands or bumps, which also leaded to the decrease of surface roughness. The cross-sectional SEM images of deposited films with various bias voltages are shown in Fig. 6. A typical loose columnar structure with visible gap was observed in Fig. 6a, which grew outward from the film-interlayer interface and then throughout the whole film thickness. The Cr interlayer with a thickness of about 130 nm was used to strength the adhesion between substrate and film. With increase of bias voltage, these films still exhibited pronounced columnar structure but three different features need to be stated. First, the columnar structure transformed into denser and more compact structure, which was ascribed to the higher energy came from bias voltage forcing atoms to move into voids between the grains. Second, the thickness of the films decreased from about 1.80 mm to 1.57 mm as the bias voltage increasing from 0 V to - 150 V. The enhanced bias voltage resulted in large augment energy of the ions impinging on the growing coating, leading to the re-sputtering phenomena and decrease of film thickness [25]. The change of microstructure evolved into denser structure was also conducive to the diminish thickness. The last feature was shown in Fig. 6d, the film exhibited duplex layers consisting of a stacking, dense glassy layer grown onto Cr interlayer, and then followed by a compact and dense columnar structure layer. It was speculated that the special structure was beneficial to enhance corrosion resistance, thus, more investigations were needed to understand the underlying causes for this duplex structure. Fig. 7 shows the cross-sectional TEM images and diffraction patterns of the AlSiN film deposited at a bias voltage of - 150 V. In Fig. 7a, it could be seen that the growth direction of the film was along with its thickness directions. The Cr interlayer clearly showed
Table 2 The RMS roughness values of AlSiN films with various bias voltages. Bias voltage (V) RMS roughness (nm)
0 8.2
50 7.9
100 7.8
150 6.0
a typical columnar structure with a thickness of 130 nm, which was grown along the perpendicular direction of the Si substrate. Fig. 7b and c showed the bright and dark-field TEM images of AlSiN film, respectively. The columnar crystals dominated the film microstructure, with column width ranging from 10 to 25 nm. The SAED pattern in Fig. 7d showed a polycrystalline structure, which were in good agreement with those diffraction peaks as shown in the XRD pattern (Fig. 2). The SAED pattern in TEM analysis did not reveal any crystalline silicon nitride. Combined with XRD results, it was confirmed that a silicon compound was formed as amorphous phase in the film. Fig. 7e showed the HR-TEM image and the corresponding Fast Fourier Transformation (FFT) pattern image (inset). It could be observed that the film was nano-composite structure consist of AlN nano-crystals (marked as curve) with a grain size below 10 nm embedded in the thick Si3N4 amorphous matrix. 3.3. Mechanical properties The hardness (H) and elastic modulus (E) of AlSiN films with different bias voltages are shown in Fig. 8a. With increasing of the bias voltage, the H values continuously increased to the maximum of 13 GPa at bias voltage of - 150 V, whereas E increased to maximum of 146.4 GPa as bias voltage increased from 0 V to - 100 V and then slightly decreased at - 150 V. Such tendency was consistent with the results reported by Chang et al. [12], where an increase of bias voltage lead to increased H and E in AlSiN films. The hardness enhancement in this study could be inferred as follows: First, the smaller Si atoms substitute larger Al atoms entered into the AlN host lattice structure and then caused the lattice distortion and hinder the movement of dislocation, which was defined as solid solution hardening. Second, it was attributed to the structure densification. The microstructure changed from porous to denser structure with increase of the bias voltage and the defects in films were effectively reduced. The hardness could be improved by the structure densification process, similar results were also reported by Wang et al. [26] and Ding et al. [27]. Third, the moderate compressive stress in films was partly in favor of hardness hardening, as shown in Fig. 8b, the stress value increased with increase of bias voltage, which was consistent with the evolution of hardness value. Finally, the formation of nano-composite structure, comprising of AlN crystalline encapsulated in an amorphous Si3N4 tissue, also improved the hardness. However, the effect of the improved hardness in this work was rather limited, comparing to the previous reports [10]. Although, the absence of pronounced
Fig. 5. The typical AFM topography of AlSiN films with various bias voltages of (a) 0 V and (b) - 150 V.
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Fig. 6. The cross-sectional morphology of AlSiN films with various bias voltages.
Fig. 7. The TEM results of AlSiN film deposited at bias voltage of - 150 V: (a) cross-sectional image, (b) bright-field image, (c) dark-field image, (d) SAED pattern and (e) HRTEM image and corresponding FFT pattern.
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Fig. 8. Hardness and elastic modulus (a), compressive stress (b), H3/E2 ratios (c) and wear volumes (d) of AlSiN films with various bias voltages.
hardness enhancement could be ascribed to the different deposition methods and parameters, other reasons were more worthwhile to be considered. As for the hardness properties of nanocomposite structured films, it is commonly agreed that hard nano-crystalline grains covered by an amorphous phase of only one or two monolayers could achieve a superhard effect [28,29]. In the present case, the Si content in films was up to 10 at.%, which resulted in a thickening of amorphous phase between nano-grains. The high content of soft amorphous matrix would enlarge the distance between nano-grains, which reduced the effect of nanocomposite strengthening. It could be verified by TEM observation. On the other hand, both AlN and Si3N4 were covalent nitrides with the same metalloid coordination numbers and similar mechanical properties. These similarities probably resulted in weak diffusion interfaces between the crystalline and amorphous phases. Such weak interface could not effectively hinder the movement of dislocations compared with the sharp interface, which deteriorated the hardness [6]. The mixed crystalline structure in films, as shown in Fig. 2, was also a reason for decrease of hardness. In addition, many reports have shown that oxygen in hard films can significantly decrease the film hardness because of its high electronegativity, which causes weakening of the neighbor bonds thus forms fairly large defects to limit the hardness defects [30,31], the oxygen contamination in films could be another reason for limited hardening effect limited in this work. It has been reported that the H3/E2 ratio could be used to predict the resistance against plastic deformation and fracture toughness of hard coatings, which has been also used to predict potential results of nano-wear for thin films. Fig. 8c presented the H3/E2 ratios for various bias voltages. As the bias voltage increased from 0 V to 150 V, the H3/E2 ratio, increased from 0.06 to 0.12, indicating that the AlSiN film had better plastic deformation and fracture toughness compare to the film depositing without bias voltage. Fig. 8d showed the nano-scale wear resistance of films at different bias
voltages. The nano-wear test was performed by using nanoindenter. To avoid the influence of the underlying substrates and environment parameters, small load was used, so that the nanowear properties were mainly determined by the intrinsic properties of film materials. The nano-wear could be represented by the wear out volume under identical wear conditions with different films, which was defined as the material volume of the AlSiN films being removed by diamond probe. The single crystalline Si substrate was as a standard and its nano-wear volume was 17.2 103 mm3 in previous report [32]. Under the same test condition, the nano-wear volume decreased from 9.7 103 mm3 at 0 V to a minimum value of 7.6 103 mm3 at bias voltage increased of 150 V. It was reported that the higher fracture toughness (KIC) and hardness (H) lead to higher wear resistance [33,34], which was in agreement with the variations of hardness and H3/E2 values in this work. The obtained result indicated that the nano-wear resistance of AlSiN film could be enhanced by applying bias voltage. Fig. 9 showed the typical Vickers indentation SEM images over a high load (10 N) on the AlSiN films with different bias voltages. As shown in Fig. 9a, serious cracks were observed at the circumambience of indenter tip center on the surface of film deposited with no bias voltage, indicating a brittle failure. While the AlSiN film deposited under bias voltage of - 150 V exhibited a relative smooth surface morphology, as shown in Fig. 9b, indicating a positive effect of bias voltage on the enhancement of the film toughness. This result was in accordance with the H3/E2 ratio values as mentioned above. In addition, the denser structure, higher hardness and lower surface roughness might be also conducive to toughness enhancement of film. 3.4. Corrosion properties The potentiodynamic polarization curves of the SUS304 steel substrate as well as the typically AlSiN films deposited under
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Fig. 9. The typical Vickers indentation images of AlSiN films with various bias voltages of (a) 0 V and (b) - 150 V.
Fig. 10. The typical potentiodynamic polarization curves of the SUS304 steel and AlSiN films (from 0 V to - 150 V) in the 3.5 wt.% NaCl aqueous solution.
Table 3 Corrosion results by potentiodynamic polarization curves obtained in 3.5 wt.% NaCl solution. Sample
Ecorr (mV vs. SCE)
Icorr (A/cm2)
SUS304 AlSiN (0 V) AlSiN (- 50 V) AlSiN (- 100 V) AlSiN (- 150 V)
220 198 197 184 178
1.06 106 8.10 107 3.41 107 2.52 108 5.94 1010
various bias voltages were tested in 3.5 wt.% NaCl solution as shown in Fig. 10. The corrosion potential (Ecorr) and the corrosion current density (Icorr) were determined by means of linear fitting of Tafel extrapolation of the curves. Table 3 lists the respective Icorr and the Ecorr. It was clearly seen that the SUS304 substrates coated with AlSiN films displayed lower corrosion current densities than that of the uncoated SUS304 substrates, indicating better corrosion resistance. The Icorr of SUS304 substrate was 1.06 106 A/cm2, which was higher than that of the deposited AlSiN film at bias voltage of 0 V (8.10 107 A/cm2). With further increase of bias voltage to 150 V, the Icorr of AlSiN film reached to 5.94 1010 A/cm2, which was one and four orders of magnitude lower than that of uncoated
SUS304 substrate and coated film without bias voltage, respectively. This demonstrated that the corrosion rate of AlSiN films was lower than that of SUS304 substrate during polarization test [35]. Also, the Ecorr shows a slight shift to positive side with increasing bias voltage, which implies better corrosion resistance of films. The corresponding SEM surface morphologies of the SUS304 substrate as well as AlSiN films synthesized at bias voltages of 0 and - 150 V after corrosion test are shown in Fig. 11. It is obvious that numerous deep pitting corrosion with different dimensions in length appeared on the surface of the bare SUS304 (Fig. 11a, and d), while as to the AlSiN film at bias voltage of 0 V (Fig. 11b and e), few tiny shallow pits with smaller dimension were distributed on the surface of the film. No apparent pits on the surface of AlSiN film synthesized at bias voltage of - 150 V were seen at the same observation scale with the other two samples. All of these SEM results were consistent with Icorr values as shown in Fig. 10. The improvement of corrosion resistance could be attributed to the dense structure of the film, especially the duplex denser layers at high bias voltage as shown in Fig. 6d. The film possessed dense structure with low defects could act as protective passive film and effectively block the paths of corrosion medium to the substrate [20]. The result demonstrates that AlSiN film can work as a promising protective barrier layer against corrosion of substrates in corrosive environment.
4. Conclusions The AlSiN films were deposited at different bias voltages by a high power impulse magnetron sputtering. The composition, microstructure, mechanical and corrosion properties of films were investigated. The obtained conclusions were as follows: (1) All AlSiN films deposited in this work presented overstoichiometric N and the result was independent of bias voltage, which was ascribed to the higher ionization rate in HiPIMS. h-AlN and c-AlN crystalline phase co-existed in the films with nano-crystalline AlN embedded in Si3N4 amorphous phase, forming a nano-composite structure. (2) With increasing bias voltage, the granular on the surface of film became smaller and the cross-section morphology changed from porous columnar structure with visible gaps to denser and more compact columnar structure. Especially, at bias voltage of - 150 V, the AlSiN film exhibited a stacking and dense-glassy layer at the bottom and then followed by a dense columnar structure. The average roughness value also exhibited a descending trend with increase of bias voltage.
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Fig. 11. SEM images of the (a,d) SUS304 steel, (b,e) AlSiN film (0 V) and (c,f) AlSiN film (- 150 V) after corrosion test in the 3.5 wt.% NaCl aqueous solution.
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