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Atmospheric pressure plasma enhanced chemical vapor deposition of SiOx films for improved corrosion resistant properties of AZ31 magnesium alloys
Surface & Coatings Technology 283 (2015) 194–200
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Atmospheric pressure plasma enhanced chemical vapor deposition of SiOx films for improved corrosion resistant properties of AZ31 magnesium alloys Yu-Lin Kuo ⁎, Kuang-Hui Chang Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 14 July 2015 Revised 5 October 2015 Accepted in revised form 2 November 2015 Available online 9 November 2015 Keywords: Atmospheric pressure plasma enhanced chemical vapor deposition Magnesium alloys Corrosion
a b s t r a c t This study investigated the feasibility of using atmospheric pressure plasma enhanced chemical vapor deposition (APPECVD) system using tetraethoxysilane (TEOS)/O2 plasma to deposit SiOx films as anti-corrosion layer on the AZ31 magnesium alloys. SiOx films were characterized by a scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) to study the surface morphology, and composition. In addition, the corrosion protection afforded by such thin films was investigated by electrochemical techniques, and relationships among composition, structure, and electrochemical properties of these films are reported. The results showed that deposition of silica-like dense film at an O2 carrier gas flow rate of 1800 sccm represented a near stoichiometric composition of O/Si ratio (2.0) and a lower degree of porosity, while silicone-like film with an O/Si ratio of 3.7 and higher degree of porosity was obtained at 600 sccm. The potentiodynamic polarization tests show that both SiOx films coated on AZ31 alloys have more positive corrosion potential and lower corrosion current density than AZ31 substrates, indicating the corrosion resistance of AZ31 can be improved by depositing SiOx film on its surface. In particular, as the surface is more compact and crosslinked, the silica-like film has a better corrosion resistance than the silicone-like film. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are used in a broad range of applications including automobile, aerospace, communications and computer components owing to their excellent physical and mechanical properties, such as low density, high specific strength, good cast and weld ability, excellent electrical and thermal conductivity, high dimensional stability, and good electromagnetic shielding characteristics [1–3]. Unfortunately, magnesium alloys possess lower corrosion resistance, especially in acidic environments and salt-water conditions. The ability to increase the corrosion resistance of magnesium and its alloys is crucial to increase the range of applications [4–6]. To improve the fault, it is important to impart a high corrosion resistance without losing the superior physical and mechanical properties. Several coating techniques were proposed to protect the surfaces of magnesium alloys, including chemical conversion [7], sol–gel process [8,11,13,17], plasma electrolytic oxidation (microarc oxidation) [9], chemical vapor deposition [10], physical vapor deposition [12,20], anodization [14,16], electrochemical plating [18], and low pressure plasma enhanced chemical vapor deposition (LP-PECVD) [15,19]. More recently, it was reported that the deposition of functional films was feasibly ⁎ Corresponding author at: 43 Keelung Road Section 4, Taipei 106, Taiwan. E-mail address: [email protected] (Y.-L. Kuo).
http://dx.doi.org/10.1016/j.surfcoat.2015.11.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
achieved through the use of atmospheric pressure plasma techniques [21–24]. Compared to plasma deposition under vacuum conditions, atmospheric pressure plasma deposition is a more versatile technology and enables the deposition of coatings on large and/or complex geometry substrates [25,26]. The versatility stems from the fact that vacuum equipment is not necessary, decreasing the initial capital investment and theoretically allowing for the deposition on substrates of any size and shape when integrated with other tools. The coating film with an excellent uniformity and low levels of defects were shown to act as an efficient anti-corrosion layer. Several researchers have extensively investigated that SiOx films improve corrosion resistance by acting as a protective barrier on the titanium alloy [27], galvanized steel [28], aluminum and alloys [29], and carbon steel [30]. This study focused on the development of atmospheric pressure plasma enhanced chemical vapor deposition (APPECVD) system using tetraethoxysilane (TEOS)/O2 plasma under atmospheric pressure to deposit SiOx films on the surface of magnesium alloys in order to improve the resistance to corrosion. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were employed to understand the surface characteristics and chemical composition of SiOx films, respectively. The electrochemical analysis and immersion of the surface with APPECVD-deposited SiOx films and bare Mg alloy in 3.5% NaCl solution were evaluated.
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2. Experiment 2.1. Material preparation This study used commercial grade Mg alloy AZ31 with the following chemical composition (in mass %): 3% Al, 1% Zn, 0.3% Mn and Mg (balance) obtained in the form of cast ingots. Samples were cut into test strips 2 cm × 2 cm squares with a thickness of 0.2 cm. The test strips were ground using 4000 grit sandpaper and polished to produce a uniformly flat surface with mirror-like reflectivity. 2.2. Atmospheric pressure plasma deposition system The AP-PECVD systems depicted in Fig. 1 was performed using a plasma head, a DC power supply, and a bubbling and gas delivery system. The plasma field is generated using a 24-kHz pulsed-DC power supply and compressed dry air is used as the working gas, which is supplied at a constant flow rate of 40 standard liters per minute (slm). In AP-PECVD, the deposition of SiOx films on AZ31 substrates was obtained in a plasma head with a glass bubbler that contained TEOS monomer as precursor maintained at 160 °C. The TEOS vapor was transferred by O2 as the carrier gas with a flow rate of 600 and 1800 standard cubic centimeters per minute (sccm) through a stainless delivery pipe maintained at 160 °C by a heating tape for the prevention of condensation. The nozzle-to-sample distance is fixed at 20 mm, while the applied plasma power, stage movement velocity, and deposition time are set at 500 W, 10 mm/s, and 2 min, respectively. 2.3. Characterization and measurement of corrosion resistance in SiOx films Surface morphologies of AP-PECVD deposited SiOx films were observed by field emission scanning electron microscopy (FESEM, JSM6500F, JOEL, Japan) and atomic force microscopy (AFM, ICON2-SYS, Bruker, USA). The chemical structures of SiOx films were characterized by the Fourier transform infrared spectrometer (FTIR, Digilab FTS3500, Bio-Rad, USA). Each spectrum was obtained from an average of 32 scans in the range of 500–4000 cm−1 with a resolution of 4 cm−1. The surface compositions by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, PHI-1600, USA) measurements were taken to identify the elemental composition of each SiOx film. The XPS system with an Al Kα X-ray beam (energy = 1486.6 eV and power = 250 W) was operated at 15 kV. The detailed microstructures of SiOX films were studied by cross sectional transmission electron microscopy (TEM, TecnaiG2-F20, UK).
Fig. 1. Schematic diagram of the atmospheric pressure plasma enhanced chemical vapor deposition system.
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To evaluate the corrosion resistance of samples, potentiodynamic polarization curves were performed. The corrosion cell consisted of a saturated calomel electrode (SCE) as a reference with platinum foil as a counter electrode, while samples were used as working electrodes with 1 cm2 exposed to the solution during the electrochemical measurements. Polarization curves were plotted in triplicate using five discrete specimens immersed in fresh 3.5% NaCl solution with a pH value of 6.5 at room temperature under atmospheric pressure. Potentiodynamic polarization scans of each specimen were obtained at a scan rate of 1 mV s−1 from an initial potential of - 1.7 V/SCE relative to the open circuit potential to a final potential of −0.7 V/SCE. The potentials for all the electrochemical analysis are referred to Ag/AgCl electrode (0.207 V vs SCE). 3. Results and discussion The images of SEM and AFM shown in Fig. 2 illustrated the high quality, smoothness and uniform appearance of the resulting films. In fact, the film was smoother (Rrms = 3–5 nm) than untreated AZ31 (Rrms = 9–10 nm) in terms of surface appearance, demonstrating the efficacy of atmospheric-pressure plasma-enhanced chemical vapor deposition with regard to the evenness of the resulting films. The detailed microstructure of as-deposited SiOx film was further characterized using cross-sectional TEM. As can be seen in Fig. 3, SiOx films in contact with the AZ31 substrate were in a highly compact amorphous state and no inner layer or precipitated phase was observed. The results referred that the reactions in the plasma deposition process might be contributed to electron collisions, ion bombardment, and free radical reaction which caused active-species from the decomposition of TEOS and the formation of organic species (i.e., hydrocarbon organic content (SiRx) in tetraethoxysilane (TEOS)/O2 plasma) under atmospheric pressure to produce SiOx nanoparticles deposited on the substrates. The thin film had a good scratch resistance with a pencil hardness of 8H and good adhesion with a rating of 5B. The photograph of the SiOx films surface after Cross hatch cutter test are given in Fig. S1. As can be seen, even after removal of the adhesive tape, the corners and the sides of the lattice pattern remained almost unchanged. The result is in accordance with the ASTM (D3359-93) classification of adhesion test results 5B and indicates that there is a good bonding between the substrate and the coatings. Fig. 4(a) presents an FTIR spectrograph of SiOX film. Strong bands associated with the asymmetric stretching and bending modes of the Si– O–Si bonds were observed at ∼1050 and 796 cm−1. The shape difference of the bands at ∼1150 cm−1 resulted from different molecular network structures of Si–O–Si as discussed below [31–33]. Peak characteristics of hydroxyl bonds were observed at 943 cm− 1 and 3400–3650 cm−1, which are hydrogen-bonded (Si–OH) and isolated hydroxyl groups [31–33]. This assumption is based on the fact that the proposed process was conducted at room temperature under nonvacuum conditions, which resulted in the reactions between the precursor and water vapor in the air. Other minor characteristic peaks in SiOX films included a CH2 bond at 1427 cm−1, a C_O bond at 1621 cm−1, and a CH2 bond at 2800–3000 cm−1 [34,35]. The occurrence of these bonds can be attributed to the dissociation of TEOS precursor in the plasma reaction, which entered the film resulting in the formation of SiOCHx and RCOSiR3. FTIR is a powerful method used to investigate the structural properties of silicon oxide. Previous reports [35,36] suggested that the shoulder peak at 1200 cm−1 and the main peak at 1090 cm−1 are in-phase (bond angle of less than 144°) and out-of-phase (bond angle of 150°) movement of Si–O–Si bonds, respectively (see Fig. 4(b)). In the previous study by Chou et al. [37], IR measurements of the refractive index demonstrated that materials with greater absorption at 1200 cm−1 possess a more porous structure, which suggests that the peak at 1200 cm−1 is to arise from porous oxide, i.e., Si–O–Si, in large voids. Based on these findings, Hicks et al. [38] used the ratio of the shoulder area to that of
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Fig. 2. SEM and AFM images of as-deposited SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
the primary peak to determine the degree of porosity of silicon oxide films, which could have a direct effect on chemical structure and mechanical properties. In accordance with the methods outlined in these studies, we used the ratio of the shoulder peak area associated with Si-O-Si stretching mode at ∼ 1150 cm−1 to the primary peak area at ∼1050 cm−1 to assess the correlation between FTIR results and the porosity of SiOX films. The calculate values of degree of porosity of these two SiOx films deposited by AP-PECVD were 1.43 (O2: 600 sccm) and 0.76 (O2: 1800 sccm). The normalized atomic concentrations obtained by XPS analysis and depth profile were presented in Figs. 5 and 6. The results showed that XPS surface analysis regarding the elemental composition of SiOx films shows the atomic percentages of Si, O, and C, while no peak associated with magnesium was observed in any samples of SiOx film. Therefore, we can further infer that SiOx films were formed merely on the surface. This typical depth profile reveals that the components (i.e., silicon (Si), oxygen (O), and carbon (C)) were uniformly distributed throughout the “bulk” of the film. The sharp interface between the SiOx films and Mg substrate indicates that no interfacial reaction was occurred, which was in good agreement with TEM observation displayed in Fig. 3. Besides, SiOx film deposited at an O2 carrier gas rate of 600 sccm with a thickness of 41 nm from bottom showed average chemical composition of 70.2 at.% O, 21.5 at.% Si, and 8.3 at.% C, while the chemical composition of 62.8 at.% O, 31.3 at.% Si, and 5.9 at.% C of SiOx film was obtained at 1800 sccm, with the thickness of 80 nm. These above results provide evidence supporting the effectiveness and high reproducibility of using AP-PECVD technology for the deposition of SiOX films on AZ31. FTIR and XPS results show that major bond of SiOx films is Si–O–Si with the characteristic peaks at 796 cm−1 and 1050–1150 cm− 1, and O/Si ratios of the SiOX film were 2.0 (O2: 1800 sccm) and 3.7 (O2: 600 sccm). Thus, we can infer a general formula for the atmospheric pressure plasma enhanced chemical vapor deposition of SiOX film via TEOS/O2 plasma, where X = 1, 2, 3. TEOS þ e− →ðCHX CHx−1 OÞx Si þ ðCHx CHx−1 OÞ þ e− : Fig. 3. Cross-section TEM image showing the structure of SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
ð3 1Þ
This reaction results in the production of chemical compounds, such as SiOCHx and RCOSiR3, according to the values selected for plasma
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Fig. 4. (a) FTIR spectra of as-deposited SiOX films prepared at different oxygen carrier gas flow rates of 600 and 1800 sccm. (b) Si–O–Si stretching mode: In-phase for atom pairs of Si–O–Si bond angle of less than 144°; out-of-phase for atom pairs of Si–O–Si bond angle of 150°.
power, processing time, and carrier gas flow rate. This study varied the flow rates for oxygen carrier gas in order to elucidate the mechanism underlying the deposition of SiOX film on AZ31 magnesium alloys (see Fig. 7). The AP-PECVD processes can be generally identified by three reactions inducing gas reaction, gas-surface process, and surface reaction. In the initial stage, the reaction process of precursors was inelastic collision free radicals, ions and electrons, referring that TEOS became a fragmented species due to electron impacts or dissociation reactions breaking C–C and C–O bonds during gas reactions [39]. Then, fragmented species maintained many active sites were well exposed to reactive oxygen species and they have to be well reacted with others leading to the deposition on the substrate. Finally, the oxidation reaction produced an extremely even organic/inorganic film on the surface of the base material, which was deposited layer-by-layer without the interdiffusion with the base material to form compounds of magnesium silicide. At the same time, reactive oxygen species inside the plasma
Fig. 5. General XPS survey of as-deposited SiOX films at different oxygen carrier gas flow rates of 600 and 1800 sccm.
region may have oxidized organic carbon fragments of TEOS in a plasma state through the formation of non-depositing CO or CO2 gases [40,41]. Alternatively, reactive oxygen species preferentially etched the carbon from the surface of already deposited film through the formation of volatile byproduct gases. Form the above results, the deposition of SiOx film at a carrier gas flow rate of 1800 sccm represented a near stoichiometric composition of O/Si ratio (2.0) and a degree of porosity of 0.76 which can be defined as silica-like film, while SiOx film deposited at 600 sccm with a O/Si ratio of 3.7 and degree of porosity of 1.43 is silicone-like film. In order to understand the anti-corrosion behaviors of silicone-like film and silica-like film deposited on AZ31, the corrosion behavior of these specimen in 3.5 wt.% NaCl solution were investigated by employing electrochemical measurements. The open circuit potential (OCP) used as an indicator provides information about the metal/solution interface. The steady-state condition on electrode surface ensures the reliability of electrochemical measurements. The variations in open circuit potential with time for the bare AZ31 Mg alloy and those deposited SiOx films measured in 3.5 wt.% NaCl solution are shown in Fig. 8. It is well known the potentials of all specimens remained constant after a short transient in the early stage. The steady state open circuit potential of the bare AZ31 is about − 1.46 V/SCE. For silicone-like and silica-like samples, significant increases in open circuit potentials (−1.38 and −1.19 V/SCE) were observed, indicating that the deposited SiOx films can effectively suppress the reactivity of bare AZ31 in 3.5 wt.% NaCl solution. Fig. 9 presents the results of potentiodynamic measurement and corresponding fitting parameters obtained using EC-Lab software. The curve associated with the bare AZ31 substrate is also included for comparison. It is well known the corrosion potential (Ecorr) of AZ31 (−1.51 V/SCE) shifted to a more positive value in the silicone-like specimen (− 1.43 V/SCE) and silica-like film specimen (− 1.32 V/SCE). This increase indicates that the deposition of SiOx film enhanced the resistance to corrosion. Moreover, the corrosion current density of the AZ31 (3.10 × 10−4 A cm−2) was reduced by two/three order of magnitude as follows: silicone-like (7.94 × 10− 6 A cm−2) specimen and silicalike film specimen (1.58 × 10−7 A cm−2). In addition, the tafel analysis of the polarization curves also provides values of the corrosion current
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Fig. 8. Open circuit potentials of AZ31 Mg alloy and as-deposited SiOx films in 3.5 wt.% NaCl solution.
Fig. 6. XPS depth profiles for as-deposited SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
Fig. 7. Schematic diagram showing the mechanisms underlying the deposition of SiOX films using atmospheric pressure plasma enhanced chemical vapor deposition.
and polarization resistance according to the equation [42]. The Table 1 show that silica-like film specimen high corrosion resistance values (0.15 Ω). It is well known that the corrosion rate is normally proportional to the Icorr. Therefore, depositing silica-like film on the surface greatly reduced the corrosion rate of the bare AZ31 Mg alloy. The Fig. S2 show that the weight losses measured (after 14 days of immersion in 3.5% NaCl solution) from the SiOx films samples are obviously smaller than those measured from the bare AZ31 samples at each time interval. All the immersion test furnishes experimental evidence that the degradation rate of the AZ31 magnesium alloy is mitigated by AP-PECVD deposited SiOx films on surface. The SEM image of bare AZ31 and SiOx films and after 14-days immersion in 3.5% NaCl solution is presented in Fig. S3. From overall view of the surface, three different surface morphologies are evidently observed. The bare AZ91D was seriously corroded with the existence of cracks and corrosion precipitates. Silicone-like film specimen was partially damaged to form some corrosion products. Nevertheless, observation of a predominantly defect-free and rough surface of silica-like film to the fact that the presence of the low degree of porosity of thin film effectively inhibits the corrosion in salt solution through surface passivation. Ideally, a deposition process entirely free from impurities or defects would provide the highest corrosion resistance. However, this would be practically impossible to achieve, particularly when dealing with thin films. Features such as pores and pinholes can weaken the corrosion protection afforded by thin films. This preferential attack can be
Fig. 9. Potentiodynamic polarization curves of untreated AZ31 and as-deposited SiOX films at different oxygen carrier gas flow rates of 600 and 1800 sccm in 3.5 wt.% NaCl solution.
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Appendix A. Supplementary data
Table 1 Electrochemical parameters derived from polarization curves. Sample
Ecorr Icorr (V vs. SCE) (A)
AZ31 −1.51 Silicone-like film −1.43 Silica-like film −1.32
βa βc Rp (mV dec−1) (mV dec−1) (Ω)
3.10 × 10−4 47.6 7.94 × 10−6 73.4 1.58 × 10−7 81.4
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319.9 193.1 188.1
58 2898 156,136
attributed to the fact that the presence of voids in the film structure degraded the corrosion resistance, thereby leading to autocatalytic processes capable of producing corrosion pits, as shown in Fig. 10. According to the FTIR results, electrochemical measurements, and immersion test, the silicone-like film with higher degree of porosity leaves them susceptible to corroded by 3.5% NaCl solution, as compared to the dense silica-like film with lower degree of porosity. 4. Conclusion The deposition of SiOx anticorrosion layer by atmospheric pressure plasma enhanced chemical vapor deposition onto AZ31 substrate was feasibly demonstrated. The as-deposited SiOx amorphous films appear high quality, with smoothness and uniform surface without apparent pinholes. The results of the potentiodynamic polarization tests show that the corrosion resistance of AZ31 is improved by depositing SiOx film on its surface via AP-PECVD using tetraethoxysilane (TEOS)/O2 plasma. Based on the discussion on molecular structure, the silica-like film with lower degree of porosity was proved to have a higher corrosion resistance. SiOx film represents an interesting alternative for improving the anticorrosion performance of various materials with a cost-effective approach. Acknowledgments The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research under contract no. MOST 102-2221-E-011-020-MY3, and Mr. ShengChung Liao of the Instrument Center at National Taiwan University of Science and Technology for the kind assistance with FE-SEM.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2015.11.004.
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Atmospheric pressure plasma enhanced chemical vapor deposition of SiOx films for improved corrosion resistant properties of AZ31 magnesium alloys Yu-Lin Kuo ⁎, Kuang-Hui Chang Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 14 July 2015 Revised 5 October 2015 Accepted in revised form 2 November 2015 Available online 9 November 2015 Keywords: Atmospheric pressure plasma enhanced chemical vapor deposition Magnesium alloys Corrosion
a b s t r a c t This study investigated the feasibility of using atmospheric pressure plasma enhanced chemical vapor deposition (APPECVD) system using tetraethoxysilane (TEOS)/O2 plasma to deposit SiOx films as anti-corrosion layer on the AZ31 magnesium alloys. SiOx films were characterized by a scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) to study the surface morphology, and composition. In addition, the corrosion protection afforded by such thin films was investigated by electrochemical techniques, and relationships among composition, structure, and electrochemical properties of these films are reported. The results showed that deposition of silica-like dense film at an O2 carrier gas flow rate of 1800 sccm represented a near stoichiometric composition of O/Si ratio (2.0) and a lower degree of porosity, while silicone-like film with an O/Si ratio of 3.7 and higher degree of porosity was obtained at 600 sccm. The potentiodynamic polarization tests show that both SiOx films coated on AZ31 alloys have more positive corrosion potential and lower corrosion current density than AZ31 substrates, indicating the corrosion resistance of AZ31 can be improved by depositing SiOx film on its surface. In particular, as the surface is more compact and crosslinked, the silica-like film has a better corrosion resistance than the silicone-like film. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are used in a broad range of applications including automobile, aerospace, communications and computer components owing to their excellent physical and mechanical properties, such as low density, high specific strength, good cast and weld ability, excellent electrical and thermal conductivity, high dimensional stability, and good electromagnetic shielding characteristics [1–3]. Unfortunately, magnesium alloys possess lower corrosion resistance, especially in acidic environments and salt-water conditions. The ability to increase the corrosion resistance of magnesium and its alloys is crucial to increase the range of applications [4–6]. To improve the fault, it is important to impart a high corrosion resistance without losing the superior physical and mechanical properties. Several coating techniques were proposed to protect the surfaces of magnesium alloys, including chemical conversion [7], sol–gel process [8,11,13,17], plasma electrolytic oxidation (microarc oxidation) [9], chemical vapor deposition [10], physical vapor deposition [12,20], anodization [14,16], electrochemical plating [18], and low pressure plasma enhanced chemical vapor deposition (LP-PECVD) [15,19]. More recently, it was reported that the deposition of functional films was feasibly ⁎ Corresponding author at: 43 Keelung Road Section 4, Taipei 106, Taiwan. E-mail address: [email protected] (Y.-L. Kuo).
http://dx.doi.org/10.1016/j.surfcoat.2015.11.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
achieved through the use of atmospheric pressure plasma techniques [21–24]. Compared to plasma deposition under vacuum conditions, atmospheric pressure plasma deposition is a more versatile technology and enables the deposition of coatings on large and/or complex geometry substrates [25,26]. The versatility stems from the fact that vacuum equipment is not necessary, decreasing the initial capital investment and theoretically allowing for the deposition on substrates of any size and shape when integrated with other tools. The coating film with an excellent uniformity and low levels of defects were shown to act as an efficient anti-corrosion layer. Several researchers have extensively investigated that SiOx films improve corrosion resistance by acting as a protective barrier on the titanium alloy [27], galvanized steel [28], aluminum and alloys [29], and carbon steel [30]. This study focused on the development of atmospheric pressure plasma enhanced chemical vapor deposition (APPECVD) system using tetraethoxysilane (TEOS)/O2 plasma under atmospheric pressure to deposit SiOx films on the surface of magnesium alloys in order to improve the resistance to corrosion. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were employed to understand the surface characteristics and chemical composition of SiOx films, respectively. The electrochemical analysis and immersion of the surface with APPECVD-deposited SiOx films and bare Mg alloy in 3.5% NaCl solution were evaluated.
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2. Experiment 2.1. Material preparation This study used commercial grade Mg alloy AZ31 with the following chemical composition (in mass %): 3% Al, 1% Zn, 0.3% Mn and Mg (balance) obtained in the form of cast ingots. Samples were cut into test strips 2 cm × 2 cm squares with a thickness of 0.2 cm. The test strips were ground using 4000 grit sandpaper and polished to produce a uniformly flat surface with mirror-like reflectivity. 2.2. Atmospheric pressure plasma deposition system The AP-PECVD systems depicted in Fig. 1 was performed using a plasma head, a DC power supply, and a bubbling and gas delivery system. The plasma field is generated using a 24-kHz pulsed-DC power supply and compressed dry air is used as the working gas, which is supplied at a constant flow rate of 40 standard liters per minute (slm). In AP-PECVD, the deposition of SiOx films on AZ31 substrates was obtained in a plasma head with a glass bubbler that contained TEOS monomer as precursor maintained at 160 °C. The TEOS vapor was transferred by O2 as the carrier gas with a flow rate of 600 and 1800 standard cubic centimeters per minute (sccm) through a stainless delivery pipe maintained at 160 °C by a heating tape for the prevention of condensation. The nozzle-to-sample distance is fixed at 20 mm, while the applied plasma power, stage movement velocity, and deposition time are set at 500 W, 10 mm/s, and 2 min, respectively. 2.3. Characterization and measurement of corrosion resistance in SiOx films Surface morphologies of AP-PECVD deposited SiOx films were observed by field emission scanning electron microscopy (FESEM, JSM6500F, JOEL, Japan) and atomic force microscopy (AFM, ICON2-SYS, Bruker, USA). The chemical structures of SiOx films were characterized by the Fourier transform infrared spectrometer (FTIR, Digilab FTS3500, Bio-Rad, USA). Each spectrum was obtained from an average of 32 scans in the range of 500–4000 cm−1 with a resolution of 4 cm−1. The surface compositions by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, PHI-1600, USA) measurements were taken to identify the elemental composition of each SiOx film. The XPS system with an Al Kα X-ray beam (energy = 1486.6 eV and power = 250 W) was operated at 15 kV. The detailed microstructures of SiOX films were studied by cross sectional transmission electron microscopy (TEM, TecnaiG2-F20, UK).
Fig. 1. Schematic diagram of the atmospheric pressure plasma enhanced chemical vapor deposition system.
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To evaluate the corrosion resistance of samples, potentiodynamic polarization curves were performed. The corrosion cell consisted of a saturated calomel electrode (SCE) as a reference with platinum foil as a counter electrode, while samples were used as working electrodes with 1 cm2 exposed to the solution during the electrochemical measurements. Polarization curves were plotted in triplicate using five discrete specimens immersed in fresh 3.5% NaCl solution with a pH value of 6.5 at room temperature under atmospheric pressure. Potentiodynamic polarization scans of each specimen were obtained at a scan rate of 1 mV s−1 from an initial potential of - 1.7 V/SCE relative to the open circuit potential to a final potential of −0.7 V/SCE. The potentials for all the electrochemical analysis are referred to Ag/AgCl electrode (0.207 V vs SCE). 3. Results and discussion The images of SEM and AFM shown in Fig. 2 illustrated the high quality, smoothness and uniform appearance of the resulting films. In fact, the film was smoother (Rrms = 3–5 nm) than untreated AZ31 (Rrms = 9–10 nm) in terms of surface appearance, demonstrating the efficacy of atmospheric-pressure plasma-enhanced chemical vapor deposition with regard to the evenness of the resulting films. The detailed microstructure of as-deposited SiOx film was further characterized using cross-sectional TEM. As can be seen in Fig. 3, SiOx films in contact with the AZ31 substrate were in a highly compact amorphous state and no inner layer or precipitated phase was observed. The results referred that the reactions in the plasma deposition process might be contributed to electron collisions, ion bombardment, and free radical reaction which caused active-species from the decomposition of TEOS and the formation of organic species (i.e., hydrocarbon organic content (SiRx) in tetraethoxysilane (TEOS)/O2 plasma) under atmospheric pressure to produce SiOx nanoparticles deposited on the substrates. The thin film had a good scratch resistance with a pencil hardness of 8H and good adhesion with a rating of 5B. The photograph of the SiOx films surface after Cross hatch cutter test are given in Fig. S1. As can be seen, even after removal of the adhesive tape, the corners and the sides of the lattice pattern remained almost unchanged. The result is in accordance with the ASTM (D3359-93) classification of adhesion test results 5B and indicates that there is a good bonding between the substrate and the coatings. Fig. 4(a) presents an FTIR spectrograph of SiOX film. Strong bands associated with the asymmetric stretching and bending modes of the Si– O–Si bonds were observed at ∼1050 and 796 cm−1. The shape difference of the bands at ∼1150 cm−1 resulted from different molecular network structures of Si–O–Si as discussed below [31–33]. Peak characteristics of hydroxyl bonds were observed at 943 cm− 1 and 3400–3650 cm−1, which are hydrogen-bonded (Si–OH) and isolated hydroxyl groups [31–33]. This assumption is based on the fact that the proposed process was conducted at room temperature under nonvacuum conditions, which resulted in the reactions between the precursor and water vapor in the air. Other minor characteristic peaks in SiOX films included a CH2 bond at 1427 cm−1, a C_O bond at 1621 cm−1, and a CH2 bond at 2800–3000 cm−1 [34,35]. The occurrence of these bonds can be attributed to the dissociation of TEOS precursor in the plasma reaction, which entered the film resulting in the formation of SiOCHx and RCOSiR3. FTIR is a powerful method used to investigate the structural properties of silicon oxide. Previous reports [35,36] suggested that the shoulder peak at 1200 cm−1 and the main peak at 1090 cm−1 are in-phase (bond angle of less than 144°) and out-of-phase (bond angle of 150°) movement of Si–O–Si bonds, respectively (see Fig. 4(b)). In the previous study by Chou et al. [37], IR measurements of the refractive index demonstrated that materials with greater absorption at 1200 cm−1 possess a more porous structure, which suggests that the peak at 1200 cm−1 is to arise from porous oxide, i.e., Si–O–Si, in large voids. Based on these findings, Hicks et al. [38] used the ratio of the shoulder area to that of
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Fig. 2. SEM and AFM images of as-deposited SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
the primary peak to determine the degree of porosity of silicon oxide films, which could have a direct effect on chemical structure and mechanical properties. In accordance with the methods outlined in these studies, we used the ratio of the shoulder peak area associated with Si-O-Si stretching mode at ∼ 1150 cm−1 to the primary peak area at ∼1050 cm−1 to assess the correlation between FTIR results and the porosity of SiOX films. The calculate values of degree of porosity of these two SiOx films deposited by AP-PECVD were 1.43 (O2: 600 sccm) and 0.76 (O2: 1800 sccm). The normalized atomic concentrations obtained by XPS analysis and depth profile were presented in Figs. 5 and 6. The results showed that XPS surface analysis regarding the elemental composition of SiOx films shows the atomic percentages of Si, O, and C, while no peak associated with magnesium was observed in any samples of SiOx film. Therefore, we can further infer that SiOx films were formed merely on the surface. This typical depth profile reveals that the components (i.e., silicon (Si), oxygen (O), and carbon (C)) were uniformly distributed throughout the “bulk” of the film. The sharp interface between the SiOx films and Mg substrate indicates that no interfacial reaction was occurred, which was in good agreement with TEM observation displayed in Fig. 3. Besides, SiOx film deposited at an O2 carrier gas rate of 600 sccm with a thickness of 41 nm from bottom showed average chemical composition of 70.2 at.% O, 21.5 at.% Si, and 8.3 at.% C, while the chemical composition of 62.8 at.% O, 31.3 at.% Si, and 5.9 at.% C of SiOx film was obtained at 1800 sccm, with the thickness of 80 nm. These above results provide evidence supporting the effectiveness and high reproducibility of using AP-PECVD technology for the deposition of SiOX films on AZ31. FTIR and XPS results show that major bond of SiOx films is Si–O–Si with the characteristic peaks at 796 cm−1 and 1050–1150 cm− 1, and O/Si ratios of the SiOX film were 2.0 (O2: 1800 sccm) and 3.7 (O2: 600 sccm). Thus, we can infer a general formula for the atmospheric pressure plasma enhanced chemical vapor deposition of SiOX film via TEOS/O2 plasma, where X = 1, 2, 3. TEOS þ e− →ðCHX CHx−1 OÞx Si þ ðCHx CHx−1 OÞ þ e− : Fig. 3. Cross-section TEM image showing the structure of SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
ð3 1Þ
This reaction results in the production of chemical compounds, such as SiOCHx and RCOSiR3, according to the values selected for plasma
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Fig. 4. (a) FTIR spectra of as-deposited SiOX films prepared at different oxygen carrier gas flow rates of 600 and 1800 sccm. (b) Si–O–Si stretching mode: In-phase for atom pairs of Si–O–Si bond angle of less than 144°; out-of-phase for atom pairs of Si–O–Si bond angle of 150°.
power, processing time, and carrier gas flow rate. This study varied the flow rates for oxygen carrier gas in order to elucidate the mechanism underlying the deposition of SiOX film on AZ31 magnesium alloys (see Fig. 7). The AP-PECVD processes can be generally identified by three reactions inducing gas reaction, gas-surface process, and surface reaction. In the initial stage, the reaction process of precursors was inelastic collision free radicals, ions and electrons, referring that TEOS became a fragmented species due to electron impacts or dissociation reactions breaking C–C and C–O bonds during gas reactions [39]. Then, fragmented species maintained many active sites were well exposed to reactive oxygen species and they have to be well reacted with others leading to the deposition on the substrate. Finally, the oxidation reaction produced an extremely even organic/inorganic film on the surface of the base material, which was deposited layer-by-layer without the interdiffusion with the base material to form compounds of magnesium silicide. At the same time, reactive oxygen species inside the plasma
Fig. 5. General XPS survey of as-deposited SiOX films at different oxygen carrier gas flow rates of 600 and 1800 sccm.
region may have oxidized organic carbon fragments of TEOS in a plasma state through the formation of non-depositing CO or CO2 gases [40,41]. Alternatively, reactive oxygen species preferentially etched the carbon from the surface of already deposited film through the formation of volatile byproduct gases. Form the above results, the deposition of SiOx film at a carrier gas flow rate of 1800 sccm represented a near stoichiometric composition of O/Si ratio (2.0) and a degree of porosity of 0.76 which can be defined as silica-like film, while SiOx film deposited at 600 sccm with a O/Si ratio of 3.7 and degree of porosity of 1.43 is silicone-like film. In order to understand the anti-corrosion behaviors of silicone-like film and silica-like film deposited on AZ31, the corrosion behavior of these specimen in 3.5 wt.% NaCl solution were investigated by employing electrochemical measurements. The open circuit potential (OCP) used as an indicator provides information about the metal/solution interface. The steady-state condition on electrode surface ensures the reliability of electrochemical measurements. The variations in open circuit potential with time for the bare AZ31 Mg alloy and those deposited SiOx films measured in 3.5 wt.% NaCl solution are shown in Fig. 8. It is well known the potentials of all specimens remained constant after a short transient in the early stage. The steady state open circuit potential of the bare AZ31 is about − 1.46 V/SCE. For silicone-like and silica-like samples, significant increases in open circuit potentials (−1.38 and −1.19 V/SCE) were observed, indicating that the deposited SiOx films can effectively suppress the reactivity of bare AZ31 in 3.5 wt.% NaCl solution. Fig. 9 presents the results of potentiodynamic measurement and corresponding fitting parameters obtained using EC-Lab software. The curve associated with the bare AZ31 substrate is also included for comparison. It is well known the corrosion potential (Ecorr) of AZ31 (−1.51 V/SCE) shifted to a more positive value in the silicone-like specimen (− 1.43 V/SCE) and silica-like film specimen (− 1.32 V/SCE). This increase indicates that the deposition of SiOx film enhanced the resistance to corrosion. Moreover, the corrosion current density of the AZ31 (3.10 × 10−4 A cm−2) was reduced by two/three order of magnitude as follows: silicone-like (7.94 × 10− 6 A cm−2) specimen and silicalike film specimen (1.58 × 10−7 A cm−2). In addition, the tafel analysis of the polarization curves also provides values of the corrosion current
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Fig. 8. Open circuit potentials of AZ31 Mg alloy and as-deposited SiOx films in 3.5 wt.% NaCl solution.
Fig. 6. XPS depth profiles for as-deposited SiOX films prepared at different oxygen carrier gas flow rates of (a) 600 and (b) 1800 sccm.
Fig. 7. Schematic diagram showing the mechanisms underlying the deposition of SiOX films using atmospheric pressure plasma enhanced chemical vapor deposition.
and polarization resistance according to the equation [42]. The Table 1 show that silica-like film specimen high corrosion resistance values (0.15 Ω). It is well known that the corrosion rate is normally proportional to the Icorr. Therefore, depositing silica-like film on the surface greatly reduced the corrosion rate of the bare AZ31 Mg alloy. The Fig. S2 show that the weight losses measured (after 14 days of immersion in 3.5% NaCl solution) from the SiOx films samples are obviously smaller than those measured from the bare AZ31 samples at each time interval. All the immersion test furnishes experimental evidence that the degradation rate of the AZ31 magnesium alloy is mitigated by AP-PECVD deposited SiOx films on surface. The SEM image of bare AZ31 and SiOx films and after 14-days immersion in 3.5% NaCl solution is presented in Fig. S3. From overall view of the surface, three different surface morphologies are evidently observed. The bare AZ91D was seriously corroded with the existence of cracks and corrosion precipitates. Silicone-like film specimen was partially damaged to form some corrosion products. Nevertheless, observation of a predominantly defect-free and rough surface of silica-like film to the fact that the presence of the low degree of porosity of thin film effectively inhibits the corrosion in salt solution through surface passivation. Ideally, a deposition process entirely free from impurities or defects would provide the highest corrosion resistance. However, this would be practically impossible to achieve, particularly when dealing with thin films. Features such as pores and pinholes can weaken the corrosion protection afforded by thin films. This preferential attack can be
Fig. 9. Potentiodynamic polarization curves of untreated AZ31 and as-deposited SiOX films at different oxygen carrier gas flow rates of 600 and 1800 sccm in 3.5 wt.% NaCl solution.
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Appendix A. Supplementary data
Table 1 Electrochemical parameters derived from polarization curves. Sample
Ecorr Icorr (V vs. SCE) (A)
AZ31 −1.51 Silicone-like film −1.43 Silica-like film −1.32
βa βc Rp (mV dec−1) (mV dec−1) (Ω)
3.10 × 10−4 47.6 7.94 × 10−6 73.4 1.58 × 10−7 81.4
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319.9 193.1 188.1
58 2898 156,136
attributed to the fact that the presence of voids in the film structure degraded the corrosion resistance, thereby leading to autocatalytic processes capable of producing corrosion pits, as shown in Fig. 10. According to the FTIR results, electrochemical measurements, and immersion test, the silicone-like film with higher degree of porosity leaves them susceptible to corroded by 3.5% NaCl solution, as compared to the dense silica-like film with lower degree of porosity. 4. Conclusion The deposition of SiOx anticorrosion layer by atmospheric pressure plasma enhanced chemical vapor deposition onto AZ31 substrate was feasibly demonstrated. The as-deposited SiOx amorphous films appear high quality, with smoothness and uniform surface without apparent pinholes. The results of the potentiodynamic polarization tests show that the corrosion resistance of AZ31 is improved by depositing SiOx film on its surface via AP-PECVD using tetraethoxysilane (TEOS)/O2 plasma. Based on the discussion on molecular structure, the silica-like film with lower degree of porosity was proved to have a higher corrosion resistance. SiOx film represents an interesting alternative for improving the anticorrosion performance of various materials with a cost-effective approach. Acknowledgments The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research under contract no. MOST 102-2221-E-011-020-MY3, and Mr. ShengChung Liao of the Instrument Center at National Taiwan University of Science and Technology for the kind assistance with FE-SEM.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2015.11.004.
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Fig. 10. (a) Ideal film structure with lower defect to resist the attack of Cl ions; (b) voids among the atoms in the structure of the film allow Cl ions to attack the thin film and corrode the Mg substrate.
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