- Email: [email protected]
Synthesis of Y2O3-ZrO2-SiO2 composite coatings on carbon fiber reinforced resin matrix composite by an electro-plasma process
Applied Surface Science 371 (2016) 504–511
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of Y2 O3 -ZrO2 -SiO2 composite coatings on carbon fiber reinforced resin matrix composite by an electro-plasma process Yuping Zhang, Xiang Lin, Weiwei Chen ∗ , Huanwu Cheng, Lu Wang Department of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 7 December 2015 Received in revised form 1 March 2016 Accepted 3 March 2016 Available online 5 March 2016 Keywords: Electro-plasma process Carbon fiber reinforced resin matrix Y2 O3 -ZrO2 -SiO2 Coating Oxidation resistance
a b s t r a c t In the present paper the Y2 O3 -ZrO2 -SiO2 composite coating was successfully synthesized on carbon fiber reinforced resin matrix composite by an electro-plasma process. The deposition process, microstructures and oxidation resistance of the coatings with different SiO2 concentrations were systematically investigated. A relatively dense microstructure was observed for the Y2 O3 -ZrO2 -SiO2 composite coating with the SiO2 concentration above 5 g/L. The coating exhibited very good oxidation resistance at 1273 K with the mass loss rate as low as ∼30 wt.%, compared to 100 wt.% of the substrate. The formation of the ceramic composites was discussed in detail based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The carbon fiber reinforced resin matrix composite is one of the most advanced composite materials due to its low density, high fatigue resistance, low coefficient of thermal expansion and high strength-to-weight ratio. Therefore, the composite is considered as a leading candidate for high temperature applications in aeronautical and aerospace fields [1–3]. However, the carbon fiber reinforced resin matrix composite has a poor oxidation resistance, significantly limiting the application in oxidation environments. At present, coating is considered as the most effective method to solve the problem. Among all kinds of coatings, the ceramic coating is believed to be optimal in order to significantly improve the oxidation resistance of the carbon fiber reinforced resin matrix composite. Meanwhile, the ceramic coating is required to behave a good adhesion with the composite substrate. The resin matrix is relatively soft and have large thermal expansion coefficient, compared to the hard surface and small thermal expansion coefficient of the ceramic coating. Obviously the resin matrix and the ceramic coating have fully different physical properties, therefore it is relatively difficult to achieve a good adhesion between them. The traditional methods, including thermal spray and PVD [4–7], are unable to directly synthesize the ceramic coating with good adhesion on the carbon fiber reinforced resin matrix composite. Therefore, it is
∗ Corresponding author. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.apsusc.2016.03.030 0169-4332/© 2016 Elsevier B.V. All rights reserved.
necessary to apply for a new technique to synthesize good-adhesive ceramic coatings on the composite. The electro-plasma process is a hybrid of conventional electrolysis and atmospheric plasma process [8]. The process utilizes basic principles of electrolytic plasma, where DC potential is applied between two electrodes in an aqueous media. At a certain value of voltage between two electrodes in an aqueous electrolyte, a deviation exists from Faraday’s electrolytic regime. A stable plasma field is formed on the surface of working pieces, where the thermal and physical/chemical effects exist. Based on the effects during the electro-plasma process, ceramic coatings are easily synthesized on a conductive substrate, including ZrO2 -Y2 O3 coating, Al2 O3 coating. The presence of plasma micro-discharges over the work piece may also lead to melting of localized micro-zones on the surface, significantly improving the bonding strength between the coating and the substrate [9,10]. Based on the above characteristics of the electro-plasma process, it is well expected that a ceramic coating is able to be successfully synthesized on the carbon fiber reinforced resin matrix composite with good properties. According to the Y2 O3 -ZrO2 phase diagram [11], a single-phase zone exists as the volume percentage of ZrO2 ranges from 15% to 30%. Moreover, the zone is able to stably exist up to ∼2500 ◦ C, providing very good oxidation resistance and mechanical properties at high temperatures. In order to release the thermal stress formed at high temperatures, SiO2 phase is often incorporated into a ceramic. Correspondingly the spallation resistance of the ceramic coating is significantly improved. SiO2 itself also has excellent oxidation resistance. Therefore, the Y2 O3 -ZrO2 -SiO2 composite system may
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
505
Fig. 1. The schematic diagram of the experimental apparatus.
behave good oxidation resistance as well as the spallation resistance at high temperatures. Based on the above analysis, the electro-plasma process was successfully applied in the present manuscript in order to synthesize the Y2 O3 -ZrO2 -SiO2 composite coating on the carbon fiber reinforced resin matrix composite. The microstructure of the ceramic coating was systematically investigated. The oxidation resistance was also measured. Finally we discussed the formation of the ceramic composites based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries.
2. Experimental 2.1. The electro-plasma process The commercialized carbon fiber reinforced resin matrix composite was used as the substrate with the size of 12 mm × 5 mm × 2.5 mm. The substrate was ultrasonically cleaned in acetone in order to remove the oil on the surface, and then dried. The electro-plasma process was conducted in the 0.1 mol/L Y(NO3 )3 and Zr(NO3 )4 mixed electrolyte with the mole percentage of Zr(NO3 )4 at 30%. The SiO2 nano-particles (average size of 80 nm) were also added into the electrolyte with the concentration of 1, 5 and 10 g/L, respectively. The electrolyte was continuously magnetically stirred at a rate of 100 rpm. The schematic diagram of the electro-plasma process was shown in Fig. 1. An electric field was applied between the graphite counter electrode (anode) and the substrate (cathode). During the electro-plasma process, a constant voltage of 170 V was applied between both electrodes at room temperature with the deposition time of 60 s. The cathodic current density was automatically recorded by the desk computer in order to characterize the current density-time relation.
2.2. Characterization of coatings The morphologies and compositions of the coatings were analyzed using a scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) attached. The SiO2 content in the coating was calculated based on the weight percentage of Si measured by EDS, that is, the content of Si in the coating was firstly measured by EDS, and then the content of SiO2 was obtained with the Si content divided by the relative content of Si in SiO2 . The phase structure of the coatings was determined using X-ray diffraction (XRD) with Cu K␣ radiation. Diffraction patterns were recorded with the diffraction angle (2) ranging from 10 to 90◦ at a scanning rate of 0.02◦ s−1 .
Fig. 2. Dependence of SiO2 weight percentage in ceramic coating on SiO2 concentration.
2.3. Oxidation resistance measurement In order to investigate the oxidation resistance of the coatings, the isothermal oxidation test was carried out at 1273 K for 15 min in air. The weight loss rate was calculated by the equation below: = (m0 − mi ) /m0 × 100%
(1)
where is the rate of weight loss, m0 is the weight of sample before the oxidation test, and mi is the weight of sample after the oxidation test. 3. Results 3.1. SiO2 content in the coating Fig. 2 shows the dependence of the SiO2 content (wt.%) on the SiO2 concentration. It can be clearly indicated that the SiO2 content was almost linearly increased from ∼3.2 wt.% to ∼28.2 wt.% when SiO2 nano-particles was added into the electrolyte with the concentration increased from 1 g/L to 10 g/L. The increase of SiO2 content in the coating may be qualitatively explained based on the Whithers’ theory [12] and the two-step adsorption model of Guglielmi [13]. According to the Whithers’ theory, the SiO2 nano-particles could immediately adsorb the hydrate metal ions in the electrolyte, followed by a powerful transportation to the cathode surface under the effect of the electrical field. On the electrode surface, the ion-adsorbed SiO2 nano-particles were mainly controlled by two-step adsorption processes. In the first
506
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Table 1 The lattice parameters of composite phases with different coatings. Coating
SiO2 concentration (g/L)
Space group
Phase
a
b
c
Y2 O3 -ZrO2 Y2 O3 -ZrO2 -SiO2
0 1 5 10
Fm-3m(225) Fm-3m(225) Fm-3m(225) Fm-3m(225)
Y0.15 Zr0.85 O1.93 Y0.2 Zr0.8 O1.9 Y0.28 Zr0.72 O1.862 Y0.28 Zr0.72 O1.862
5.139 5.147 5.210 5.210
5.139 5.147 5.210 5.210
5.139 5.147 5.210 5.210
step the ion-adsorbed particles were loosely adsorbed onto the cathode surface; in the second step, the shells of the adsorbed ions were broken under the effect of electrical field, followed by a strong adsorption of the particles onto the cathode. When the SiO2 concentration increased, more particles were incorporated into the matrix, causing higher content of nano-particles in the coating (Fig. 2). The linear relation between the SiO2 content and the SiO2 concentration has not been fully understood yet. We propose that the adsorption and incorporation may be linear with the increase of the SiO2 concentration. Correspondingly the increase is linear. 3.2. Phase structure In order to determine phase structures of the ceramic coatings, the specimen were analyzed by XRD. The XRD patterns of the ceramic coatings with different SiO2 contents are shown in Fig. 3. The phase structure of the different ceramic coatings with different SiO2 contents seems similar. Both Y2 O3 and ZrO2 diffraction peaks were clearly detected in all ceramic coatings. Additionally, a Y-Zr-O phase was formed during the electro-plasma process with different compositions. It is clearly observed that the Y0.15 Zr0.85 O1.93 phase was mainly formed in the Y2 O3 -ZrO2 coating, compared to the Y0.2 Zr0.8 O1.9 phase in Y2 O3 -ZrO2 -SiO2 (1 g/L) coating. While both Y2 O3 -ZrO2 -SiO2 (5 g/L) coating and Y2 O3 -ZrO2 -SiO2 (10 g/L) coating identically contained Y0.28 Zr0.72 O1.862 phase, as shown in Fig. 3c and d. Obviously the incorporation of SiO2 nano-particles somehow changed the forming activation energy of the Y-Zr-O phase, leading to the formation of different phase structures. The SiO2 phase can also be clearly detected by the XRD analysis as shown in the inset in Fig. 3d. The lattice parameters of the Y-Zr-O phases are listed in Table 1. It shows that the space group of the Y-Zr-O phases, i.e. Y0.15 Zr0.85 O1.93 , Y0.2 Zr0.8 O1.9 and Y0.28 Zr0.72 O1.862 are Fm-3m (225). The lattice parameters of the phases increase when the SiO2 concentration increases from 0 to 10 g/L. The incorporation of SiO2 nano-particles leads to lattice distortion. Moreover, more lattice distortion happened when the SiO2 content was increased. Besides, it is observed that the intensity of phase peaks of all coatings reduces and the peaks of Y2 O3 phase gradually shift to larger angles with the SiO2 content increased. The positive shift of the Y2 O3 peak can also be ascribed to lattice distortion caused by dopants [14]. 3.3. Surface morphologies Fig. 3 shows surface morphologies of the Y2 O3 -ZrO2 -SiO2 ceramic coatings on the carbon fiber reinforced resin matrix composites. The Y2 O3 -ZrO2 coating without incorporation of SiO2 is relatively uniform, while several cracks were observed as indicated by the arrows in Fig. 4a and b. The cracks may provide a diffusion path for the oxygen, probably leading to the oxidation of substrates during high-temperature oxidation. As for the Y2 O3 -ZrO2 -SiO2 ceramic coatings with the SiO2 concentration of 1 g/L, the cracks on the coating surface seemed more shallow with the amount significantly reduced, probably ascribed to the co-deposition of nano-sized SiO2 particles. It is suggested that the co-deposition of the second-phase nano-particles partially contributed to the filling of holes already existing in the coating, continuously toughen the
Fig. 3. XRD patterns of the Y2 O3 -ZrO2 coating (a) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (b), 5 g/L (c) and 10 g/L (d). The inset in (d) shows the locally magnified diffraction peak, clearly revealing the existing of the SiO2 phase.
coating structure [15]. A dense coating can significantly better the oxidation resistance of the coating. Fig. 4e and f shows surface morphologies of the Y2 O3 -ZrO2 -SiO2 ceramic coatings with the SiO2 concentration of 5 g/L. A dense and continuous surface was formed without penetrated cracks, holes or fissures observed in the ceramic coating, quite similar as the surface microstructure of the coating with the SiO2 concentration of 10 g/L (Fig. 4g and h). We proposed that appropriate amount of SiO2 within the solvent range of Y2 O3 /ZrO2 optimized the coating, providing an effective antioxidant protection for the substrate. Moreover,
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
507
Fig. 4. Surface morphologies of the Y2 O3 -ZrO2 coating (a, b) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (c, d), 5 g/L (e, f) and 10 g/L (g, h).
several molten areas were locally observed on the surface of coating (Fig. 4a, c, e and g), due to the ultra-high temperatures locally formed in plasma bubbles during the electro-plasma process [16]. At high additions of SiO2 nano-particles, the effect of SiO2 on the surface microstructures was negligible, evidenced by the similar surface morphologies as shown in Fig. 4e–h. The reason has not been fully understood yet. It is suggested that the surface morphology of a coating is mainly affected by the nucleation and growth processes of the coating. For the present Y2 O3 -ZrO2 -SiO2 composite coating, two processes, i.e. the incorporation of SiO2 and the formation of the Y2 O3 -ZrO2 , existed. When the SiO2 concentration increased from 5 g/L to 10 g/L, more SiO2 nano-particles
were codeposited in the coating evidenced by the increased content. Meanwhile, the Y2 O3 -ZrO2 matrix was formed as several chemical reactions happened. The reactions may be influenced by the codeposition of SiO2 nano-particles. Given the similar surface morphologies, therefore, we proposed that the codeposition and reaction mechanisms may be similar when the SiO2 concentration was above 5 g/L. 3.4. Cross-sectional microstructure The cross-sectional micrographs of the Y2 O3 -ZrO2 -SiO2 coatings on the carbon fiber reinforced resin matrix composites are
508
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Fig. 5. Cross-sections of the Y2 O3 -ZrO2 coating (a, b) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (c, d), 5 g/L (e, f) and 10 g/L (g, h).
shown in Fig. 5. Fig. 5a and b shows the cross-sections of Y2 O3 -ZrO2 coating. It can be seen that the coating is 180–200 m in thickness (Fig. 5a). The coating was locally cracked during the electro-plasma process, evidenced by the microcracks as the arrows pointed in Fig. 5b. Pores were also observed at the coating/substrate interface. Such microstructure may have a deteriorate impact on the oxidation resistance of coating. The cross-sectional backscatter micrographs of Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 1 g/L are shown in Fig. 5c and d. It can be observed that the coating has a thickness of 150–200 m (Fig. 5c). Fig. 5d shows the local microstructure of the cross section with a relatively high magnification. Compared with Y2 O3 -ZrO2
coating in Fig. 5a and b, the cracks and pores in the coating can be still observed, but the amount of the cracks and pores was much lower. Meanwhile, the incorporation of SiO2 nano-particles contributed to the uniformability of the coating (Fig. 5d). Fig. 5e and f shows the backscatter micrographs of the Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 5 g/L with the thickness of 150–220 m. The coating is relatively dense, and several cracks and pores were still observed with the amount much fewer than the Y2 O3 -ZrO2 coating, quite similar with the microstructure of the Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 10 g/L (Fig. 5g and h).
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
509
Table 2 The isothermal oxidation test of the coatings at 1273 K for 15 min in air. Samples
SiO2 concentration (g/L) m0 /g
Bare 0 Y2 O3 -ZrO2 Y2 O3 -ZrO2 -SiO2 1 5 10
0.1907 0.5455 0.4286 0.3648 0.2700
mi /g
(m0 − mi )/g (%)
0 0.3046 0.2660 0.2545 0.1839
0.1907 0.2409 0.1626 0.1103 0.0831
100 44.16 37.94 30.24 30.78
3.5. Oxidation resistance Table 2 compared the mass loss and loss rate of the ceramic coatings after oxidation. It can be clearly revealed that the ceramic coating greatly improved the oxidation resistance. Larger content of SiO2 led to better oxidation resistance with the mass loss reduced from ∼44 wt.% to ∼30 wt.%. It is interesting to see that the Y2 O3 -ZrO2 -SiO2 coatings exhibited nearly same oxidation resistance when the SiO2 concentration was above 5 g/L. It is believed that the oxidation resistance is strongly dependent on the microstructure of coatings. Fewer micro-defects, including cracks and pores, were found at larger concentration of SiO2 , as shown in Figs. 4 and 5. The relatively dense microstructure formed at high contents of SiO2 contributed to lowering the diffusion rate of oxygen into the coating, correspondingly improving the oxidation resistance (Table 2). Furthermore, SiO2 itself has excellent oxidation resistance as a dense and oxygen-isolated oxide. Therefore, large content of SiO2 in the ceramic coatings was able to significantly improve the oxidation resistance when the specimen was oxidized in air at 1273 K. While co-deposition of the large content of SiO2 in the coatings caused an increase of the phase boundaries between SiO2 and Y2 O3 /ZrO2 . Accordingly the amount of the diffusion paths increased for oxygen atoms, deteriorating the oxidation property of the coatings. The above two contrary impacts existed at the same time when the coated specimen was oxidized at 1273 K, leading to the almost same oxidation resistance of the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 5 g/L and 10 g/L. 4. Discussion The electro-plasma process is a hybrid of conventional electrolysis and atmospheric plasma process with a significant deviation from Faraday’s normal electrolytic regime. The applied electric potential is much larger than the conventional electrolysis, leading to an anomalous phenomenon of excess hydrogen gas evolution at the cathodic surface. Correspondingly a continuous plasma envelope is formed at the cathode. The process is a dynamic and complicated process with two interfaces among the substrate/plasma field/ion aqueous formed. The anodic ions and the SiO2 nano-particles were able to move through the formed plasma field under the driving force of the electric field. Several physical and chemical reactions happened in the plasma field for the ions. Additionally the cathodic ions including OH− and NO3 − already existing on the cathodic surface may also be impacted by the plasma field. Accordingly the charge/discharge processes of the ions were quite different from the conventional electrolysis process, leading to a special deposition behavior for the ceramic coating, as well as the co-deposition process of the SiO2 nano-particles. Hereof we discussed the formation of the ceramic composites mainly based on the electrochemical mechanism and the deposition dynamics. 4.1. Electrochemical mechanism In an effort to investigate the electrochemical mechanism during the electro-plasma process, the current density (i)-time (t)
Fig. 6. The current density-time curves of the composite coatings during the electroplasma process.
characteristics were investigated in constant voltage of 170 V with a rising rate 17 V/s. Fig. 6 shows the i-t characteristics curve for the electro-plasma process in the cathode regime. The four curves exhibited a similar change with i initially increased and then declined. It is suggested that the decline of i revealed the initiation of the formation of a continuous ceramic coating. Therefore, it is observed from Fig. 6 that the continuous Y2 O3 -ZrO2 coating initially formed at ∼8 s without addition of SiO2 nano-particles. The initiation of the continuous ceramic coating was postponed when the SiO2 concentration was increased from 1 g/L to 5 g/L, evidenced by the initial formation at ∼26 s for 1 g/L compared to ∼32 s for 5 g/L. The high concentration of SiO2 at 10 g/L contributed to the quick formation of the continuous coating with the initial formation at ∼15 s. Several researchers have investigated the electrochemical mechanism during the electro-plasma process, revealing a fast decline of i directively caused the fast formation of a continuous coating. When the SiO2 nano-particles were added with the concentration of 10 g/L, a sharp decline was observed for i till ∼50 s after the initial formation of the coating at ∼15 s, then i leveled at a relatively low value. The similar phenomenon was also observed for the coating formed at the SiO2 concentrations of 1 g/L and 5 g/L. Obviously the incorporation of the SiO2 nano-particles favored to the fast formation of the continuous coating. Correspondingly a relatively dense coating was formed (Fig. 4), leading to less i than the Y2 O3 -ZrO2 coating without SiO2 addition (Fig. 6). 4.2. Deposition dynamics The previous investigation revealed that the hydroxide compounds, including Y(OH)3 or Zr(OH)4 or Al(OH)3 , were mainly formed during the traditional electro-deposition in the nitrate electrolyte without the plasma field formed [17]. In contrast, the present experimental results indicated that Y2 O3 and ZrO2 oxides were mainly formed during the electro-plasma process (Fig. 3), revealing a significantly important effect of the plasma on the formation of oxide ceramics. Meanwhile, the incorporation of SiO2 nano-particles into the ceramic coating may be also influenced by the plasma field during the electro-plasma process. The mechanism of the co-deposition of nano-particles has been investigated since 1960s [12,18–20], three main mechanisms were proposed [20,21]: (1) electrophoretic movement of the positively charged particles to the cathode; (2) adsorption of the particles at the electrode surface by van der Waals forces; and (3) mechanical inclusion of the
510
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Fig. 7. (a) Traditional process steps in co-deposition and incorporation of a solid particle into the deposit: (1) formation of an ion cloud around the particle, (2) transport by means of convection, (3) transport by diffusion, (4) reduction reaction, and (5) adsorption. (b) The schematic diagram of the concentration-depth profile during the electro-plasma process.
particles into the layer. According to the above analysis, we proposed that two key factors in our present experiment, i.e. discharge process of ions and movement of SiO2 nano-particles, should be emphasized in order to explain the effect of the plasma. 4.2.1. Discharge process of ions When the ions are migrated, the significant change in the cathode occurs. The strong electric field in the electrolyte makes hydrogen evolution reaction occur at the surface of the cathode, creating fine hydrogen bubbles. A strong electrical potential is established on the bubble adjacent to the workpiece surface, resulting in high temperature plasma, and hydrogen plasma is thus generated in a thin layer. It has been reported earlier that in EPT, plasma layer electric field strength can reach 105 V/m or higher [22–25]. When such high electric field is reached, gas space inside the bubbles is ionized, and a plasma discharge is initiated. The formation reactions of the oxide coatings are complicated, involving dissociation of nitrates, formation and dehydration of the hydroxides. Given the high temperature and physical/chemical reactions probably caused by the plasma field, we proposed that the plasma mainly influenced the dehydration of the hydroxides. The basic reactions steps are suggested as below: (1) Dissociation of Y(NO3 )3 and Zr(NO3 ) Zr(NO3 )4 → Zr4+ + NO3 −
(2)
Y(NO3 )3 → Y 3+ + NO3 −
(3)
(2) Formation of hydroxides Zr4+ + OH− → Zr(OH)4
(4)
Y 3+ + OH− → Y(OH)3
(5)
(3) Dehydration of the hydroxides Zr(OH)4 Y(OH)3
Plasma discharge
→
ZrO2 + H2 O
(6)
Y2 O3 + H2 O
(7)
Plasma discharge
→
Klapkiv [25] suggested that the discharge plasma temperature corresponds to 6–7 × 103 K. This high temperature plasma bubbles are surrounded by relatively cool electrolyte, thereby resulting in cooling of the plasma. Finally, the bubble implodes on the substrate. The implosion of plasma bubbles makes the positive ions concentrated around the bubbles accelerate and deposit directly to the substrate. Moreover, as the bubble implodes, the stored energy is
released into the gas layer and kinetic energy is transferred from the liquid layer to the surface of the substrate [16]. The discharge of the ions during the electro-plasma process may be strongly linked with the ion acceleration through the plasma and ion bubble absorption transport as the bubbles collapse. 4.2.2. Movement of SiO2 nano-particles The overall composite deposition process can be shown in five steps (Fig. 7a) [26]. These steps describe the traditional process of particles from the solution to their incorporation in the coating matrix. The first stage postulates formation of an electro-active ionic cloud surrounding the particles, as soon as the particles are introduced into the electrolyte. Under the action of convection, these ionically enveloped particles are transported to the hydrodynamic boundary layer, migrate across the layer and then are conveyed by diffusion to the cathode. After the ionic cloud is entirely or partly reduced, the particles are deposited and incorporated in the coating matrix as the metal ions are discharged, so ‘burying’ the inert particles. There are three main mechanisms involved in delivery of ions to the cathode surface, i.e. migration (under a potential gradient), diffusion (under a concentration gradient), and convection (movement of the electrolyte solution itself). It is believed that the overall contribution to the supply of ions from the migration process is very small, and can be neglected [26]. The convection resulted from the movement of bulk solution is determined by stirring. Such movement of solution ceases to be significant in the diffusion layer, and movement of ions across the diffusion layer takes place by diffusion. The driving force for diffusion is the concentration gradient, more suitably expressed as the concentration polarization. According to the electrochemical theory, there are two important polarizations in the process: electrochemical (activation) polarization and concentration polarization. The concentration polarization can be increased by reducing the thickness of the diffusion layer. There exists a limited current density, iL , which is determined by concentration polarization, expressed as: iL = z · F · Dc∞ /ıN
(8)
where z is the number of electrons per ion being transferred, F is the Faraday constant, c∞ is the ion concentration in bulk solution, and ␦N is the thickness of the diffusion layer. When the current is below iL, the process is controlled by electrochemical polarization, otherwise the process is controlled by the concentration
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
polarization. In the latter case, ions tend to discharge preferentially at the tips of the protrusions, and the nucleus are difficult to grow, tend to resulting in the formation of the loose and dentrite surface. In this case, as for a ceramic coating, a loose and dendrite microstructure tends to a cracked surface due to the poor plasticity of the ceramic. Fig. 7b shows a schematic diagram of the concentration-depth profile during the electro-plasma process. It is believed that high temperature was generated by the plasma discharge during the electro-plasma process. In this case, the diffusion of the ionadsorbed SiO2 nano-particles was fastened in the plasma field, correspondingly the thickness of the diffusion layer significantly decreased from ␦N to ␦N-plasma , leading to the great increase of the limited current density iL according to Eq. (8). Therefore, the electro-plasma process was controlled by electrochemical polarization in the condition of adding SiO2 nano-particles, compared to the concentration polarization without addition of SiO2 . In this case, a cracked surface was formed on the Y2 O3 -ZrO2 coating (Fig. 4a and b), while the Y2 O3 -ZrO2 -SiO2 coating exhibited a relatively dense microstructure on the surface (Fig. 4e–h). 5. Conclusions The Y2 O3 -ZrO2 -SiO2 composite coating was successfully synthesized on carbon fiber reinforced resin matrix composite by an electro-plasma process. SiO2 nano-particles were incorporated into the Y2 O3 -ZrO2 matrix in order to form the Y2 O3 -ZrO2 -SiO2 composite coating. The plasma discharge process changed the deposition mechanism, and avoided the loose, ramous and porous surface structure. The dense structure led to the significant improvement of oxidation resistance. The coating exhibited very good oxidation resistance at 1273 K with the mass loss rate as low as ∼30 wt.%, compared to 100 wt.% of the substrate. The charge/discharge processes of the ions were quite different from the conventional electrolysis process, leading to a special deposition behavior for the ceramic coating, as well as the co-deposition process of the SiO2 nano-particles. We presently discussed the formation of the ceramic composites based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries. Acknowledgments We thank Prof. Yedong He in University of Science and Technology Beijing for helpful discussions in the electro-plasma analysis. We also thank Dr. Lin Lu and Dr. Maoyuan Li for providing the carbon fiber reinforced resin composite samples. References [1] H. Li, Y.Z. Wan, H. Liang, X.L. Li, Y. Huang, F. He, Composite electroplating of Cu–SiO2 nano particles on carbon fiber reinforced epoxy composites, Appl. Surf. Sci. 256 (2009) 1614–1616.
511
[2] N.S. Jacobson, D.M. Curry, Oxidation microstructure studies of reinforced carbon/carbon, Carbon 44 (2006) 1142–1150. [3] J. Eck, J.L. Sans, M.B. Pichelin, Experimental study of carbon materials behavior under high temperature and VUV radiation: application to solar probe + heat shield, Appl. Surf. Sci. 257 (2011) 3196–3204. [4] E. Roos, S.M. Naga, R.N. Richter, S. Lauf, M. Awaad, A. Lyutovich, M.A. Tameem, Electron beam physical vapour deposition and mechanical properties of c-ZrO2 -ZTA-coatings on alloy 617 substrates, Ceram. Int. 38 (2012) 3317–3326. [5] B.K. Jang, Thermal conductivity of nanoporous ZrO2 –4 mol% Y2 O3 multilayer coatings fabricated by EB-PVD, Surf. Coat. Technol. 202 (2008) 1568–1573. [6] Q.H. Yu, C.G. Zhou, H.Y. Zhang, F. Zhao, Thermal stability of nanostructured 13 wt% Al2 O3 -8 wt% Y2 O3 -ZrO2 thermal barrier coatings, J. Eur. Ceram. Soc. 30 (2010) 889–897. [7] M.H. Habibi, L. Wang, S.M. Guo, Evolution of hot corrosion resistance of YSZ Gd2 Zr2 O7 , and Gd2 Zr2 O7 + YSZ composite thermal barrier coatings in Na2 SO4 + V2 O5 at 1050 ◦ C, J. Eur. Ceram. Soc. 32 (2012) 1635–1642. [8] P. Gupta, G. Tenhundfeld, E.O. Daigle, P.J. Schilling, Synthesis and characterizat ion of hard metal coatings by electro-plasma technology, Surf. Coat. Technol. 200 (2005) 1587–1594. [9] P.J. Schilling, P.D. Herrington, E.O. Daigle, E.I. Meletis, Surface characteristics of structural steel processed using electro-plasma techniques, J. Mater. Eng. Perform. 11 (2002) 26–31. [10] P. Gupta, G. Tenhundfeld, Surface and mechanical properties of steel processed by electro-plasma technology, Plat. Surf. Finish. 92 (2005) 48–52. [11] O. Fabrichnaya, F. Aldinger, Assessment of thermodynamic parameters in the system ZrO2 -Y2 O3 -Al2 O3 , Int. J. Mater. Res. 95 (2004) 27–39. [12] J.C. Whithers, Electrodepositing cermets, Prod. Finish. 26 (1962) 62–68. [13] N. Guglielmi, Kinetics of the deposition of inert particles from electrolytic baths, J. Electrochem. Soc. 119 (1972) 1009–1012. [14] W. Wang, C.L. Li, J. Li, J.H. Fan, X.Y. Zhou, Effect of gadolinium doping on phase transformation and microstructure of Gd2 O3 -Y2 O3 -ZrO2 composite coatings prepared by electro-phoretic deposition, J. Rare Earth 31 (2013) 289–295. [15] D.M. Zhu, J.A. Nesbitt, C.A. Barrett, T.R. McCue, R.A. Miller, Furnace cyclic oxidation behavior of multicomponent low conductivity thermal barrier coatings, J. Therm. Spray Technol. 13 (2004) 84–92. [16] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Electrolytic plasma technology: science and engineering—an overview, Surf. Coat. Technol. 201 (2007) 8746–8760. [17] X.Z. Yang, Y.D. He, D.R. Wang, W. Gao, Cathodic micro-arc electrodeposition of thick ceramic coatings, Electrochem. Solid State Lett. 5 (2002) C33–34. [18] E.A. Brandes, D. Goldthorpe, Electrodeposition of cermets, Metallurgia 76 (1967) 195–198. [19] J.P. Celis, J.R. Roos, C. Buelens, Mechanism of electrolytic composite plating: survey and trends, Trans. Inst. Met. Finish. 69 (1991) 133–139. [20] A. Hovestad, L.J.J. Janssen, Electrochemical codeposition of inert particles in a metallic matrix, J. Appl. Electrochem. 25 (1995) 519–527. [21] W.W. Chen, Y.D. He, W. Gao, Synthesis of nanostructured Ni-TiO2 composite coatings by sol-enhanced electroplating, J. Electrochem. Soc. 157 (2010) E122–128. [22] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [23] V.I. Belevantsev, O.P. Terleeva, G.A. Markov, E.K. Shulepko, A.I. Slonova, V.V. Utkin, Micro-plasma electrochemical processes, Prot. Met. 34 (1998) 416–430. [24] Y.N. Tyurin, A.D. Pogrebnjak, Electric heating using a liquid electrode, Surf. Coat. Technol. 142 (2001) 293–299. [25] M.D. Klapkiv, State of an electrolytic plasma in the process of synthesis of oxides based on aluminum, Mater. Sci. 31 (1996) 494–499. [26] N. Kanani, Electroplating-Basic Principles, Processes and Practice, Elsevier Advanced Technology, Oxford, 2004.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of Y2 O3 -ZrO2 -SiO2 composite coatings on carbon fiber reinforced resin matrix composite by an electro-plasma process Yuping Zhang, Xiang Lin, Weiwei Chen ∗ , Huanwu Cheng, Lu Wang Department of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 7 December 2015 Received in revised form 1 March 2016 Accepted 3 March 2016 Available online 5 March 2016 Keywords: Electro-plasma process Carbon fiber reinforced resin matrix Y2 O3 -ZrO2 -SiO2 Coating Oxidation resistance
a b s t r a c t In the present paper the Y2 O3 -ZrO2 -SiO2 composite coating was successfully synthesized on carbon fiber reinforced resin matrix composite by an electro-plasma process. The deposition process, microstructures and oxidation resistance of the coatings with different SiO2 concentrations were systematically investigated. A relatively dense microstructure was observed for the Y2 O3 -ZrO2 -SiO2 composite coating with the SiO2 concentration above 5 g/L. The coating exhibited very good oxidation resistance at 1273 K with the mass loss rate as low as ∼30 wt.%, compared to 100 wt.% of the substrate. The formation of the ceramic composites was discussed in detail based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The carbon fiber reinforced resin matrix composite is one of the most advanced composite materials due to its low density, high fatigue resistance, low coefficient of thermal expansion and high strength-to-weight ratio. Therefore, the composite is considered as a leading candidate for high temperature applications in aeronautical and aerospace fields [1–3]. However, the carbon fiber reinforced resin matrix composite has a poor oxidation resistance, significantly limiting the application in oxidation environments. At present, coating is considered as the most effective method to solve the problem. Among all kinds of coatings, the ceramic coating is believed to be optimal in order to significantly improve the oxidation resistance of the carbon fiber reinforced resin matrix composite. Meanwhile, the ceramic coating is required to behave a good adhesion with the composite substrate. The resin matrix is relatively soft and have large thermal expansion coefficient, compared to the hard surface and small thermal expansion coefficient of the ceramic coating. Obviously the resin matrix and the ceramic coating have fully different physical properties, therefore it is relatively difficult to achieve a good adhesion between them. The traditional methods, including thermal spray and PVD [4–7], are unable to directly synthesize the ceramic coating with good adhesion on the carbon fiber reinforced resin matrix composite. Therefore, it is
∗ Corresponding author. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.apsusc.2016.03.030 0169-4332/© 2016 Elsevier B.V. All rights reserved.
necessary to apply for a new technique to synthesize good-adhesive ceramic coatings on the composite. The electro-plasma process is a hybrid of conventional electrolysis and atmospheric plasma process [8]. The process utilizes basic principles of electrolytic plasma, where DC potential is applied between two electrodes in an aqueous media. At a certain value of voltage between two electrodes in an aqueous electrolyte, a deviation exists from Faraday’s electrolytic regime. A stable plasma field is formed on the surface of working pieces, where the thermal and physical/chemical effects exist. Based on the effects during the electro-plasma process, ceramic coatings are easily synthesized on a conductive substrate, including ZrO2 -Y2 O3 coating, Al2 O3 coating. The presence of plasma micro-discharges over the work piece may also lead to melting of localized micro-zones on the surface, significantly improving the bonding strength between the coating and the substrate [9,10]. Based on the above characteristics of the electro-plasma process, it is well expected that a ceramic coating is able to be successfully synthesized on the carbon fiber reinforced resin matrix composite with good properties. According to the Y2 O3 -ZrO2 phase diagram [11], a single-phase zone exists as the volume percentage of ZrO2 ranges from 15% to 30%. Moreover, the zone is able to stably exist up to ∼2500 ◦ C, providing very good oxidation resistance and mechanical properties at high temperatures. In order to release the thermal stress formed at high temperatures, SiO2 phase is often incorporated into a ceramic. Correspondingly the spallation resistance of the ceramic coating is significantly improved. SiO2 itself also has excellent oxidation resistance. Therefore, the Y2 O3 -ZrO2 -SiO2 composite system may
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
505
Fig. 1. The schematic diagram of the experimental apparatus.
behave good oxidation resistance as well as the spallation resistance at high temperatures. Based on the above analysis, the electro-plasma process was successfully applied in the present manuscript in order to synthesize the Y2 O3 -ZrO2 -SiO2 composite coating on the carbon fiber reinforced resin matrix composite. The microstructure of the ceramic coating was systematically investigated. The oxidation resistance was also measured. Finally we discussed the formation of the ceramic composites based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries.
2. Experimental 2.1. The electro-plasma process The commercialized carbon fiber reinforced resin matrix composite was used as the substrate with the size of 12 mm × 5 mm × 2.5 mm. The substrate was ultrasonically cleaned in acetone in order to remove the oil on the surface, and then dried. The electro-plasma process was conducted in the 0.1 mol/L Y(NO3 )3 and Zr(NO3 )4 mixed electrolyte with the mole percentage of Zr(NO3 )4 at 30%. The SiO2 nano-particles (average size of 80 nm) were also added into the electrolyte with the concentration of 1, 5 and 10 g/L, respectively. The electrolyte was continuously magnetically stirred at a rate of 100 rpm. The schematic diagram of the electro-plasma process was shown in Fig. 1. An electric field was applied between the graphite counter electrode (anode) and the substrate (cathode). During the electro-plasma process, a constant voltage of 170 V was applied between both electrodes at room temperature with the deposition time of 60 s. The cathodic current density was automatically recorded by the desk computer in order to characterize the current density-time relation.
2.2. Characterization of coatings The morphologies and compositions of the coatings were analyzed using a scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) attached. The SiO2 content in the coating was calculated based on the weight percentage of Si measured by EDS, that is, the content of Si in the coating was firstly measured by EDS, and then the content of SiO2 was obtained with the Si content divided by the relative content of Si in SiO2 . The phase structure of the coatings was determined using X-ray diffraction (XRD) with Cu K␣ radiation. Diffraction patterns were recorded with the diffraction angle (2) ranging from 10 to 90◦ at a scanning rate of 0.02◦ s−1 .
Fig. 2. Dependence of SiO2 weight percentage in ceramic coating on SiO2 concentration.
2.3. Oxidation resistance measurement In order to investigate the oxidation resistance of the coatings, the isothermal oxidation test was carried out at 1273 K for 15 min in air. The weight loss rate was calculated by the equation below: = (m0 − mi ) /m0 × 100%
(1)
where is the rate of weight loss, m0 is the weight of sample before the oxidation test, and mi is the weight of sample after the oxidation test. 3. Results 3.1. SiO2 content in the coating Fig. 2 shows the dependence of the SiO2 content (wt.%) on the SiO2 concentration. It can be clearly indicated that the SiO2 content was almost linearly increased from ∼3.2 wt.% to ∼28.2 wt.% when SiO2 nano-particles was added into the electrolyte with the concentration increased from 1 g/L to 10 g/L. The increase of SiO2 content in the coating may be qualitatively explained based on the Whithers’ theory [12] and the two-step adsorption model of Guglielmi [13]. According to the Whithers’ theory, the SiO2 nano-particles could immediately adsorb the hydrate metal ions in the electrolyte, followed by a powerful transportation to the cathode surface under the effect of the electrical field. On the electrode surface, the ion-adsorbed SiO2 nano-particles were mainly controlled by two-step adsorption processes. In the first
506
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Table 1 The lattice parameters of composite phases with different coatings. Coating
SiO2 concentration (g/L)
Space group
Phase
a
b
c
Y2 O3 -ZrO2 Y2 O3 -ZrO2 -SiO2
0 1 5 10
Fm-3m(225) Fm-3m(225) Fm-3m(225) Fm-3m(225)
Y0.15 Zr0.85 O1.93 Y0.2 Zr0.8 O1.9 Y0.28 Zr0.72 O1.862 Y0.28 Zr0.72 O1.862
5.139 5.147 5.210 5.210
5.139 5.147 5.210 5.210
5.139 5.147 5.210 5.210
step the ion-adsorbed particles were loosely adsorbed onto the cathode surface; in the second step, the shells of the adsorbed ions were broken under the effect of electrical field, followed by a strong adsorption of the particles onto the cathode. When the SiO2 concentration increased, more particles were incorporated into the matrix, causing higher content of nano-particles in the coating (Fig. 2). The linear relation between the SiO2 content and the SiO2 concentration has not been fully understood yet. We propose that the adsorption and incorporation may be linear with the increase of the SiO2 concentration. Correspondingly the increase is linear. 3.2. Phase structure In order to determine phase structures of the ceramic coatings, the specimen were analyzed by XRD. The XRD patterns of the ceramic coatings with different SiO2 contents are shown in Fig. 3. The phase structure of the different ceramic coatings with different SiO2 contents seems similar. Both Y2 O3 and ZrO2 diffraction peaks were clearly detected in all ceramic coatings. Additionally, a Y-Zr-O phase was formed during the electro-plasma process with different compositions. It is clearly observed that the Y0.15 Zr0.85 O1.93 phase was mainly formed in the Y2 O3 -ZrO2 coating, compared to the Y0.2 Zr0.8 O1.9 phase in Y2 O3 -ZrO2 -SiO2 (1 g/L) coating. While both Y2 O3 -ZrO2 -SiO2 (5 g/L) coating and Y2 O3 -ZrO2 -SiO2 (10 g/L) coating identically contained Y0.28 Zr0.72 O1.862 phase, as shown in Fig. 3c and d. Obviously the incorporation of SiO2 nano-particles somehow changed the forming activation energy of the Y-Zr-O phase, leading to the formation of different phase structures. The SiO2 phase can also be clearly detected by the XRD analysis as shown in the inset in Fig. 3d. The lattice parameters of the Y-Zr-O phases are listed in Table 1. It shows that the space group of the Y-Zr-O phases, i.e. Y0.15 Zr0.85 O1.93 , Y0.2 Zr0.8 O1.9 and Y0.28 Zr0.72 O1.862 are Fm-3m (225). The lattice parameters of the phases increase when the SiO2 concentration increases from 0 to 10 g/L. The incorporation of SiO2 nano-particles leads to lattice distortion. Moreover, more lattice distortion happened when the SiO2 content was increased. Besides, it is observed that the intensity of phase peaks of all coatings reduces and the peaks of Y2 O3 phase gradually shift to larger angles with the SiO2 content increased. The positive shift of the Y2 O3 peak can also be ascribed to lattice distortion caused by dopants [14]. 3.3. Surface morphologies Fig. 3 shows surface morphologies of the Y2 O3 -ZrO2 -SiO2 ceramic coatings on the carbon fiber reinforced resin matrix composites. The Y2 O3 -ZrO2 coating without incorporation of SiO2 is relatively uniform, while several cracks were observed as indicated by the arrows in Fig. 4a and b. The cracks may provide a diffusion path for the oxygen, probably leading to the oxidation of substrates during high-temperature oxidation. As for the Y2 O3 -ZrO2 -SiO2 ceramic coatings with the SiO2 concentration of 1 g/L, the cracks on the coating surface seemed more shallow with the amount significantly reduced, probably ascribed to the co-deposition of nano-sized SiO2 particles. It is suggested that the co-deposition of the second-phase nano-particles partially contributed to the filling of holes already existing in the coating, continuously toughen the
Fig. 3. XRD patterns of the Y2 O3 -ZrO2 coating (a) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (b), 5 g/L (c) and 10 g/L (d). The inset in (d) shows the locally magnified diffraction peak, clearly revealing the existing of the SiO2 phase.
coating structure [15]. A dense coating can significantly better the oxidation resistance of the coating. Fig. 4e and f shows surface morphologies of the Y2 O3 -ZrO2 -SiO2 ceramic coatings with the SiO2 concentration of 5 g/L. A dense and continuous surface was formed without penetrated cracks, holes or fissures observed in the ceramic coating, quite similar as the surface microstructure of the coating with the SiO2 concentration of 10 g/L (Fig. 4g and h). We proposed that appropriate amount of SiO2 within the solvent range of Y2 O3 /ZrO2 optimized the coating, providing an effective antioxidant protection for the substrate. Moreover,
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
507
Fig. 4. Surface morphologies of the Y2 O3 -ZrO2 coating (a, b) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (c, d), 5 g/L (e, f) and 10 g/L (g, h).
several molten areas were locally observed on the surface of coating (Fig. 4a, c, e and g), due to the ultra-high temperatures locally formed in plasma bubbles during the electro-plasma process [16]. At high additions of SiO2 nano-particles, the effect of SiO2 on the surface microstructures was negligible, evidenced by the similar surface morphologies as shown in Fig. 4e–h. The reason has not been fully understood yet. It is suggested that the surface morphology of a coating is mainly affected by the nucleation and growth processes of the coating. For the present Y2 O3 -ZrO2 -SiO2 composite coating, two processes, i.e. the incorporation of SiO2 and the formation of the Y2 O3 -ZrO2 , existed. When the SiO2 concentration increased from 5 g/L to 10 g/L, more SiO2 nano-particles
were codeposited in the coating evidenced by the increased content. Meanwhile, the Y2 O3 -ZrO2 matrix was formed as several chemical reactions happened. The reactions may be influenced by the codeposition of SiO2 nano-particles. Given the similar surface morphologies, therefore, we proposed that the codeposition and reaction mechanisms may be similar when the SiO2 concentration was above 5 g/L. 3.4. Cross-sectional microstructure The cross-sectional micrographs of the Y2 O3 -ZrO2 -SiO2 coatings on the carbon fiber reinforced resin matrix composites are
508
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Fig. 5. Cross-sections of the Y2 O3 -ZrO2 coating (a, b) and the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 1 g/L (c, d), 5 g/L (e, f) and 10 g/L (g, h).
shown in Fig. 5. Fig. 5a and b shows the cross-sections of Y2 O3 -ZrO2 coating. It can be seen that the coating is 180–200 m in thickness (Fig. 5a). The coating was locally cracked during the electro-plasma process, evidenced by the microcracks as the arrows pointed in Fig. 5b. Pores were also observed at the coating/substrate interface. Such microstructure may have a deteriorate impact on the oxidation resistance of coating. The cross-sectional backscatter micrographs of Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 1 g/L are shown in Fig. 5c and d. It can be observed that the coating has a thickness of 150–200 m (Fig. 5c). Fig. 5d shows the local microstructure of the cross section with a relatively high magnification. Compared with Y2 O3 -ZrO2
coating in Fig. 5a and b, the cracks and pores in the coating can be still observed, but the amount of the cracks and pores was much lower. Meanwhile, the incorporation of SiO2 nano-particles contributed to the uniformability of the coating (Fig. 5d). Fig. 5e and f shows the backscatter micrographs of the Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 5 g/L with the thickness of 150–220 m. The coating is relatively dense, and several cracks and pores were still observed with the amount much fewer than the Y2 O3 -ZrO2 coating, quite similar with the microstructure of the Y2 O3 -ZrO2 -SiO2 coating with the SiO2 concentration of 10 g/L (Fig. 5g and h).
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
509
Table 2 The isothermal oxidation test of the coatings at 1273 K for 15 min in air. Samples
SiO2 concentration (g/L) m0 /g
Bare 0 Y2 O3 -ZrO2 Y2 O3 -ZrO2 -SiO2 1 5 10
0.1907 0.5455 0.4286 0.3648 0.2700
mi /g
(m0 − mi )/g (%)
0 0.3046 0.2660 0.2545 0.1839
0.1907 0.2409 0.1626 0.1103 0.0831
100 44.16 37.94 30.24 30.78
3.5. Oxidation resistance Table 2 compared the mass loss and loss rate of the ceramic coatings after oxidation. It can be clearly revealed that the ceramic coating greatly improved the oxidation resistance. Larger content of SiO2 led to better oxidation resistance with the mass loss reduced from ∼44 wt.% to ∼30 wt.%. It is interesting to see that the Y2 O3 -ZrO2 -SiO2 coatings exhibited nearly same oxidation resistance when the SiO2 concentration was above 5 g/L. It is believed that the oxidation resistance is strongly dependent on the microstructure of coatings. Fewer micro-defects, including cracks and pores, were found at larger concentration of SiO2 , as shown in Figs. 4 and 5. The relatively dense microstructure formed at high contents of SiO2 contributed to lowering the diffusion rate of oxygen into the coating, correspondingly improving the oxidation resistance (Table 2). Furthermore, SiO2 itself has excellent oxidation resistance as a dense and oxygen-isolated oxide. Therefore, large content of SiO2 in the ceramic coatings was able to significantly improve the oxidation resistance when the specimen was oxidized in air at 1273 K. While co-deposition of the large content of SiO2 in the coatings caused an increase of the phase boundaries between SiO2 and Y2 O3 /ZrO2 . Accordingly the amount of the diffusion paths increased for oxygen atoms, deteriorating the oxidation property of the coatings. The above two contrary impacts existed at the same time when the coated specimen was oxidized at 1273 K, leading to the almost same oxidation resistance of the Y2 O3 -ZrO2 -SiO2 coatings with the SiO2 concentration of 5 g/L and 10 g/L. 4. Discussion The electro-plasma process is a hybrid of conventional electrolysis and atmospheric plasma process with a significant deviation from Faraday’s normal electrolytic regime. The applied electric potential is much larger than the conventional electrolysis, leading to an anomalous phenomenon of excess hydrogen gas evolution at the cathodic surface. Correspondingly a continuous plasma envelope is formed at the cathode. The process is a dynamic and complicated process with two interfaces among the substrate/plasma field/ion aqueous formed. The anodic ions and the SiO2 nano-particles were able to move through the formed plasma field under the driving force of the electric field. Several physical and chemical reactions happened in the plasma field for the ions. Additionally the cathodic ions including OH− and NO3 − already existing on the cathodic surface may also be impacted by the plasma field. Accordingly the charge/discharge processes of the ions were quite different from the conventional electrolysis process, leading to a special deposition behavior for the ceramic coating, as well as the co-deposition process of the SiO2 nano-particles. Hereof we discussed the formation of the ceramic composites mainly based on the electrochemical mechanism and the deposition dynamics. 4.1. Electrochemical mechanism In an effort to investigate the electrochemical mechanism during the electro-plasma process, the current density (i)-time (t)
Fig. 6. The current density-time curves of the composite coatings during the electroplasma process.
characteristics were investigated in constant voltage of 170 V with a rising rate 17 V/s. Fig. 6 shows the i-t characteristics curve for the electro-plasma process in the cathode regime. The four curves exhibited a similar change with i initially increased and then declined. It is suggested that the decline of i revealed the initiation of the formation of a continuous ceramic coating. Therefore, it is observed from Fig. 6 that the continuous Y2 O3 -ZrO2 coating initially formed at ∼8 s without addition of SiO2 nano-particles. The initiation of the continuous ceramic coating was postponed when the SiO2 concentration was increased from 1 g/L to 5 g/L, evidenced by the initial formation at ∼26 s for 1 g/L compared to ∼32 s for 5 g/L. The high concentration of SiO2 at 10 g/L contributed to the quick formation of the continuous coating with the initial formation at ∼15 s. Several researchers have investigated the electrochemical mechanism during the electro-plasma process, revealing a fast decline of i directively caused the fast formation of a continuous coating. When the SiO2 nano-particles were added with the concentration of 10 g/L, a sharp decline was observed for i till ∼50 s after the initial formation of the coating at ∼15 s, then i leveled at a relatively low value. The similar phenomenon was also observed for the coating formed at the SiO2 concentrations of 1 g/L and 5 g/L. Obviously the incorporation of the SiO2 nano-particles favored to the fast formation of the continuous coating. Correspondingly a relatively dense coating was formed (Fig. 4), leading to less i than the Y2 O3 -ZrO2 coating without SiO2 addition (Fig. 6). 4.2. Deposition dynamics The previous investigation revealed that the hydroxide compounds, including Y(OH)3 or Zr(OH)4 or Al(OH)3 , were mainly formed during the traditional electro-deposition in the nitrate electrolyte without the plasma field formed [17]. In contrast, the present experimental results indicated that Y2 O3 and ZrO2 oxides were mainly formed during the electro-plasma process (Fig. 3), revealing a significantly important effect of the plasma on the formation of oxide ceramics. Meanwhile, the incorporation of SiO2 nano-particles into the ceramic coating may be also influenced by the plasma field during the electro-plasma process. The mechanism of the co-deposition of nano-particles has been investigated since 1960s [12,18–20], three main mechanisms were proposed [20,21]: (1) electrophoretic movement of the positively charged particles to the cathode; (2) adsorption of the particles at the electrode surface by van der Waals forces; and (3) mechanical inclusion of the
510
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
Fig. 7. (a) Traditional process steps in co-deposition and incorporation of a solid particle into the deposit: (1) formation of an ion cloud around the particle, (2) transport by means of convection, (3) transport by diffusion, (4) reduction reaction, and (5) adsorption. (b) The schematic diagram of the concentration-depth profile during the electro-plasma process.
particles into the layer. According to the above analysis, we proposed that two key factors in our present experiment, i.e. discharge process of ions and movement of SiO2 nano-particles, should be emphasized in order to explain the effect of the plasma. 4.2.1. Discharge process of ions When the ions are migrated, the significant change in the cathode occurs. The strong electric field in the electrolyte makes hydrogen evolution reaction occur at the surface of the cathode, creating fine hydrogen bubbles. A strong electrical potential is established on the bubble adjacent to the workpiece surface, resulting in high temperature plasma, and hydrogen plasma is thus generated in a thin layer. It has been reported earlier that in EPT, plasma layer electric field strength can reach 105 V/m or higher [22–25]. When such high electric field is reached, gas space inside the bubbles is ionized, and a plasma discharge is initiated. The formation reactions of the oxide coatings are complicated, involving dissociation of nitrates, formation and dehydration of the hydroxides. Given the high temperature and physical/chemical reactions probably caused by the plasma field, we proposed that the plasma mainly influenced the dehydration of the hydroxides. The basic reactions steps are suggested as below: (1) Dissociation of Y(NO3 )3 and Zr(NO3 ) Zr(NO3 )4 → Zr4+ + NO3 −
(2)
Y(NO3 )3 → Y 3+ + NO3 −
(3)
(2) Formation of hydroxides Zr4+ + OH− → Zr(OH)4
(4)
Y 3+ + OH− → Y(OH)3
(5)
(3) Dehydration of the hydroxides Zr(OH)4 Y(OH)3
Plasma discharge
→
ZrO2 + H2 O
(6)
Y2 O3 + H2 O
(7)
Plasma discharge
→
Klapkiv [25] suggested that the discharge plasma temperature corresponds to 6–7 × 103 K. This high temperature plasma bubbles are surrounded by relatively cool electrolyte, thereby resulting in cooling of the plasma. Finally, the bubble implodes on the substrate. The implosion of plasma bubbles makes the positive ions concentrated around the bubbles accelerate and deposit directly to the substrate. Moreover, as the bubble implodes, the stored energy is
released into the gas layer and kinetic energy is transferred from the liquid layer to the surface of the substrate [16]. The discharge of the ions during the electro-plasma process may be strongly linked with the ion acceleration through the plasma and ion bubble absorption transport as the bubbles collapse. 4.2.2. Movement of SiO2 nano-particles The overall composite deposition process can be shown in five steps (Fig. 7a) [26]. These steps describe the traditional process of particles from the solution to their incorporation in the coating matrix. The first stage postulates formation of an electro-active ionic cloud surrounding the particles, as soon as the particles are introduced into the electrolyte. Under the action of convection, these ionically enveloped particles are transported to the hydrodynamic boundary layer, migrate across the layer and then are conveyed by diffusion to the cathode. After the ionic cloud is entirely or partly reduced, the particles are deposited and incorporated in the coating matrix as the metal ions are discharged, so ‘burying’ the inert particles. There are three main mechanisms involved in delivery of ions to the cathode surface, i.e. migration (under a potential gradient), diffusion (under a concentration gradient), and convection (movement of the electrolyte solution itself). It is believed that the overall contribution to the supply of ions from the migration process is very small, and can be neglected [26]. The convection resulted from the movement of bulk solution is determined by stirring. Such movement of solution ceases to be significant in the diffusion layer, and movement of ions across the diffusion layer takes place by diffusion. The driving force for diffusion is the concentration gradient, more suitably expressed as the concentration polarization. According to the electrochemical theory, there are two important polarizations in the process: electrochemical (activation) polarization and concentration polarization. The concentration polarization can be increased by reducing the thickness of the diffusion layer. There exists a limited current density, iL , which is determined by concentration polarization, expressed as: iL = z · F · Dc∞ /ıN
(8)
where z is the number of electrons per ion being transferred, F is the Faraday constant, c∞ is the ion concentration in bulk solution, and ␦N is the thickness of the diffusion layer. When the current is below iL, the process is controlled by electrochemical polarization, otherwise the process is controlled by the concentration
Y. Zhang et al. / Applied Surface Science 371 (2016) 504–511
polarization. In the latter case, ions tend to discharge preferentially at the tips of the protrusions, and the nucleus are difficult to grow, tend to resulting in the formation of the loose and dentrite surface. In this case, as for a ceramic coating, a loose and dendrite microstructure tends to a cracked surface due to the poor plasticity of the ceramic. Fig. 7b shows a schematic diagram of the concentration-depth profile during the electro-plasma process. It is believed that high temperature was generated by the plasma discharge during the electro-plasma process. In this case, the diffusion of the ionadsorbed SiO2 nano-particles was fastened in the plasma field, correspondingly the thickness of the diffusion layer significantly decreased from ␦N to ␦N-plasma , leading to the great increase of the limited current density iL according to Eq. (8). Therefore, the electro-plasma process was controlled by electrochemical polarization in the condition of adding SiO2 nano-particles, compared to the concentration polarization without addition of SiO2 . In this case, a cracked surface was formed on the Y2 O3 -ZrO2 coating (Fig. 4a and b), while the Y2 O3 -ZrO2 -SiO2 coating exhibited a relatively dense microstructure on the surface (Fig. 4e–h). 5. Conclusions The Y2 O3 -ZrO2 -SiO2 composite coating was successfully synthesized on carbon fiber reinforced resin matrix composite by an electro-plasma process. SiO2 nano-particles were incorporated into the Y2 O3 -ZrO2 matrix in order to form the Y2 O3 -ZrO2 -SiO2 composite coating. The plasma discharge process changed the deposition mechanism, and avoided the loose, ramous and porous surface structure. The dense structure led to the significant improvement of oxidation resistance. The coating exhibited very good oxidation resistance at 1273 K with the mass loss rate as low as ∼30 wt.%, compared to 100 wt.% of the substrate. The charge/discharge processes of the ions were quite different from the conventional electrolysis process, leading to a special deposition behavior for the ceramic coating, as well as the co-deposition process of the SiO2 nano-particles. We presently discussed the formation of the ceramic composites based on the electrochemical mechanism and the deposition dynamics in order to explain the effect of the plasma discharge. We believe that the electro-plasma process will find wide applications in preparing ceramics and coatings in industries. Acknowledgments We thank Prof. Yedong He in University of Science and Technology Beijing for helpful discussions in the electro-plasma analysis. We also thank Dr. Lin Lu and Dr. Maoyuan Li for providing the carbon fiber reinforced resin composite samples. References [1] H. Li, Y.Z. Wan, H. Liang, X.L. Li, Y. Huang, F. He, Composite electroplating of Cu–SiO2 nano particles on carbon fiber reinforced epoxy composites, Appl. Surf. Sci. 256 (2009) 1614–1616.
511
[2] N.S. Jacobson, D.M. Curry, Oxidation microstructure studies of reinforced carbon/carbon, Carbon 44 (2006) 1142–1150. [3] J. Eck, J.L. Sans, M.B. Pichelin, Experimental study of carbon materials behavior under high temperature and VUV radiation: application to solar probe + heat shield, Appl. Surf. Sci. 257 (2011) 3196–3204. [4] E. Roos, S.M. Naga, R.N. Richter, S. Lauf, M. Awaad, A. Lyutovich, M.A. Tameem, Electron beam physical vapour deposition and mechanical properties of c-ZrO2 -ZTA-coatings on alloy 617 substrates, Ceram. Int. 38 (2012) 3317–3326. [5] B.K. Jang, Thermal conductivity of nanoporous ZrO2 –4 mol% Y2 O3 multilayer coatings fabricated by EB-PVD, Surf. Coat. Technol. 202 (2008) 1568–1573. [6] Q.H. Yu, C.G. Zhou, H.Y. Zhang, F. Zhao, Thermal stability of nanostructured 13 wt% Al2 O3 -8 wt% Y2 O3 -ZrO2 thermal barrier coatings, J. Eur. Ceram. Soc. 30 (2010) 889–897. [7] M.H. Habibi, L. Wang, S.M. Guo, Evolution of hot corrosion resistance of YSZ Gd2 Zr2 O7 , and Gd2 Zr2 O7 + YSZ composite thermal barrier coatings in Na2 SO4 + V2 O5 at 1050 ◦ C, J. Eur. Ceram. Soc. 32 (2012) 1635–1642. [8] P. Gupta, G. Tenhundfeld, E.O. Daigle, P.J. Schilling, Synthesis and characterizat ion of hard metal coatings by electro-plasma technology, Surf. Coat. Technol. 200 (2005) 1587–1594. [9] P.J. Schilling, P.D. Herrington, E.O. Daigle, E.I. Meletis, Surface characteristics of structural steel processed using electro-plasma techniques, J. Mater. Eng. Perform. 11 (2002) 26–31. [10] P. Gupta, G. Tenhundfeld, Surface and mechanical properties of steel processed by electro-plasma technology, Plat. Surf. Finish. 92 (2005) 48–52. [11] O. Fabrichnaya, F. Aldinger, Assessment of thermodynamic parameters in the system ZrO2 -Y2 O3 -Al2 O3 , Int. J. Mater. Res. 95 (2004) 27–39. [12] J.C. Whithers, Electrodepositing cermets, Prod. Finish. 26 (1962) 62–68. [13] N. Guglielmi, Kinetics of the deposition of inert particles from electrolytic baths, J. Electrochem. Soc. 119 (1972) 1009–1012. [14] W. Wang, C.L. Li, J. Li, J.H. Fan, X.Y. Zhou, Effect of gadolinium doping on phase transformation and microstructure of Gd2 O3 -Y2 O3 -ZrO2 composite coatings prepared by electro-phoretic deposition, J. Rare Earth 31 (2013) 289–295. [15] D.M. Zhu, J.A. Nesbitt, C.A. Barrett, T.R. McCue, R.A. Miller, Furnace cyclic oxidation behavior of multicomponent low conductivity thermal barrier coatings, J. Therm. Spray Technol. 13 (2004) 84–92. [16] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Electrolytic plasma technology: science and engineering—an overview, Surf. Coat. Technol. 201 (2007) 8746–8760. [17] X.Z. Yang, Y.D. He, D.R. Wang, W. Gao, Cathodic micro-arc electrodeposition of thick ceramic coatings, Electrochem. Solid State Lett. 5 (2002) C33–34. [18] E.A. Brandes, D. Goldthorpe, Electrodeposition of cermets, Metallurgia 76 (1967) 195–198. [19] J.P. Celis, J.R. Roos, C. Buelens, Mechanism of electrolytic composite plating: survey and trends, Trans. Inst. Met. Finish. 69 (1991) 133–139. [20] A. Hovestad, L.J.J. Janssen, Electrochemical codeposition of inert particles in a metallic matrix, J. Appl. Electrochem. 25 (1995) 519–527. [21] W.W. Chen, Y.D. He, W. Gao, Synthesis of nanostructured Ni-TiO2 composite coatings by sol-enhanced electroplating, J. Electrochem. Soc. 157 (2010) E122–128. [22] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [23] V.I. Belevantsev, O.P. Terleeva, G.A. Markov, E.K. Shulepko, A.I. Slonova, V.V. Utkin, Micro-plasma electrochemical processes, Prot. Met. 34 (1998) 416–430. [24] Y.N. Tyurin, A.D. Pogrebnjak, Electric heating using a liquid electrode, Surf. Coat. Technol. 142 (2001) 293–299. [25] M.D. Klapkiv, State of an electrolytic plasma in the process of synthesis of oxides based on aluminum, Mater. Sci. 31 (1996) 494–499. [26] N. Kanani, Electroplating-Basic Principles, Processes and Practice, Elsevier Advanced Technology, Oxford, 2004.