氧化石墨烯添加劑對AZ31鎂合金PEO塗層摩擦學和電化學腐蝕性能

前言

北京石油化工學院陳飛教授團隊表面工程及材料腐蝕與防護課題組在 AZ31 鎂 合金表面處理領域取得最新進展。鎂合金作為重要的結構材料，在電、磁以及機械性能方面有著突出的優點，在航空航天領域存在著重要的應用價值和前景。然而，由於鎂合金基體表面發生的摩擦損傷和全面腐蝕，導致其作為構材料極易產生損壞乃至失效，進而給國家和人民生命財產帶來巨大的損失。因此，鎂合金在工業應用之前，必須經過表面強化處理以增強其在大氣環境或腐蝕性液體環境中的耐磨損和耐腐蝕性能。目前常用的鎂合金表面強化方法有陽極氧化、激光處理、金屬鍍層、氣相沉積、微弧氧化等。其中微弧氧化(MAO)又稱微等離子體氧化，通過微弧放電，在閥金屬（Mg, Al, Ti等）表面原位生長具有優良耐磨耐蝕性能的陶瓷層。 本 工作 採用 不同濃度的 氧化石墨烯(GO)作為添加劑，通過超聲攪拌作用，將氧化石墨烯均勻分散在電解液中，從而在 AZ31 鎂合金表面製備出

Citation:

Z hang YL , Chen F , Zhang Y , Cuiwei Du. Influence of graphene oxide additive on the tribological and electrochemical corrosion properties of a PEO coating prepared on AZ31 ma gnesium alloy[J]. Tribology International, 146 (2020) 1–12.

DOI: 10.1016/j.triboint.2019.106135

導讀

微弧氧化技術生產效率高、工藝簡單、環保無汙染，能夠對輕金屬及其合金材料進行表面改性，形成氧化物膜層或滲層，這些硬化層具有高硬度、耐腐蝕、耐磨損、抗熱震、催化和生物相容性良好等優異性能，是目前材料科學研究的前沿領域。傳統鎂合金表面微弧氧化膜層中存在大量微弧放電留下的缺陷和放電通道，這都是侵蝕性離子滲透膜層進入腐蝕基體的快速通道。研究表明，膜層內部缺陷越多，膜層表面微孔直徑越大、膜層緻密性和厚度越低，膜層的耐摩擦損傷及耐腐蝕性能就越弱。而微弧氧化電解液的成分則在很大程度上決定了其膜層的微觀結構及性能。因此，通過在微弧氧化電解液中加入不同的添加劑，則能夠在很大程度上改善膜層的結構及性能。氧化石墨烯作為一種碳材料，具有很好的潤滑減摩效果，並因其具有良好的親水性，尤其是較大的比表面積能夠在電解液中吸附陰離子而顯負電性，進而能夠明顯改善電解液的導電性能。因此，本研究選用氧化石墨烯（濃度分別為0、5、10、20和40 mL·L-1）作為添加劑，通過微弧氧化技術，在AZ31鎂合金表面製備出包覆有GO添加劑的微弧氧化複合膜層。

研究成果

Fig. 1 presents the SEM surface morphologies and their analyses using ImageJ software to calculate the porosity percentage. All of the as-obtained coatings displayed typical structures with numerous crater-like pores, pan-like structures and microcracks, which were randomly distributed throughout the surface. With increasing concentrations of GO additive in the silicate-based electrolyte, the number of crater-like pores increased distinctly, while the average diameters of the crater-like pores and pan-like structures were considerably reduced. In Fig. 1(c, d, e), the majority of the crater-like pores on the surface of samples GO-(10-40) are filled by the molten oxide ejected from the discharge channels Furthermore, the surface morphologies of the GO-10 and GO-20 samples are more uniform and flat than those of the GO-0 and GO-5 samples. Moreover, the crater-like pores and microcracks on the top surface were well-filled by the red spots, and the porosity percentage results are depicted in Fig. 1(f). As the concentration of GO additive increased from 0 to 20 mL·L-1, the porosity percentage of the as-obtained coatings displayed an evident decrease. This result demonstrated that the reduction of the crater-like pore diameter and the formation of numerous blocked pores provided a significant contribution to the decrease in the porosity percentage. However, with the continued enhancement of the GO additive concentration to 40 mL·L-1, the porosity percentage of sample GO-40 unexpectedly increased to 2.3%, which was two times more than that of sample GO-20 (1.2%).

Fig. 1 SEM surface morphologies and surface porosity percentages of the as-obtained coatings:(a) GO-0, (b) GO-5, (c) GO-10, (d) GO-20, and (e) GO-40.

Raman measurements of the as-obtained coatings were carried out, as illustrated in Fig. 2. For comparison, the spectra of the GO additive and GO-0 sample are also displayed. Two characteristic peaks appeared at approximately 1345 cm-1 and 1578 cm-1, which were designated as the peaks of D and G, respectively. The characteristic peaks of D and G were not detected for the GO-0 coating without the incorporation of the GO additive. Furthermore, the intensities of the D and G peaks in the composite coatings increased with increasing concentrations of GO additive added into the silicate-based electrolyte, indicating that the GO additive was successfully incorporated into the coatings. In comparison with the strong band representing the GO additive, the intensities of the D and G peaks for the composite coatings were relatively weak, which demonstrated that the content of the GO additive in the composite coatings was low.

Fig. 2 Raman spectra of the as-obtained specimens.

Fig. 3 illustrates the detailed chemical structures of the GO additive and as-obtained coatings. The survey spectrum (as shown in Fig. 7(a)) of the GO additive demonstrated that the major peaks were assigned to O 1s and C 1s, while thehigh-resolution C 1s spectral peaks at 284.7, 286.9, 288.6 and 289.2 eV (Fig. 7(b)) were assigned to C-C, C-O, C=O and O-C=O bonding, respectively. The dominant peaks presented in the survey spectra (as displayed in Fig. 7(c, e, g, i, k)) were assigned as Mg 1s, O 1s and C 1s, implying that the as-obtained coatings were primarily composed of Mg, O, and C. Furthermore, the intensity of C 1s in the survey spectra was enhanced with increasing concentrations of GO additive. Compared to the high-resolution C 1s spectrum of the GO additive, the C 1s peaks at 284.7 and 286.0 eV of the GO-0 sample separately correspond to C-C and C-O bonding, suggesting that the C in the PEO-based coating came from impurities or carbon dioxide adsorbed on the coating surface. Furthermore, the intensity of the C 1s peak at approximately 291.3 eV was increasingly weakly associated with the increasing GO additive concentration, which indicated the elimination of carbonyl functional groups during the intense spark discharge process.

Fig. 3 XPS survey scans of (a) GO additives and the as-obtained coatings: (c) GO-0,(e) GO-5, (g) GO-10, (i) GO-20, and (k) GO-40.

The typical friction coefficient evolution of the as-obtained specimens is displayed in Fig. 4. The entire wear process of different specimens was divided into three stages corresponding to the diverse primary wear mechanisms. These include the following: stage Ⅰ, plowing wear; stage Ⅱ, abrasive wear; and stage Ⅲ, adhesive wear. In the case of a Mg substrate, severe adhesive wear directly dominated the entire wear process, leading to the drastic fluctuation and higher value (≈ 0.58) of the friction coefficient, which was ascribed to the typical influence of “instantaneous welding” because of the high temperatures in the contact area between the frictional pairs. Similarly, the adhesive wear for the as-obtained coatings at stage Ⅲ occurred between the ball and the remodified coating surface. In addition, the coefficient of friction (COF) values obtained for the composite coatings at stage Ⅲ of the wear processing reached a plateau, while the average COF values decreased significantly as the GO content incorporated into the composite coatings increased. In comparison with the Mg substrate, both coatings of samples GO-0 and GO-5 experienced a rapid increase in the COF in stage Ⅱ and thereafter arrived at relatively constant values of 0.36 and 0.32, respectively. Thus, the sudden increase in the COF values was primarily ascribed to the modification and spallation of the surface coatings. Furthermore, the fractured large-sized asperities (as shown in Fig. 5(a and b)) acted as hard abrasives entrapped in the large-sized pores, and cracks resulted in improving the density of the remodified coating surface. Therefore, the dominant wear mechanism in stage Ⅱ was abrasive wear, which was consistent with previous studies. Additionally, the contact status of the friction pairs was mainly a “steel-on-protrusions” style, meaning that the steel balls wear the hard and minor-sized asperities first. Subsequently, the large-sized pores or cracks existing on the wear tracks could act as the reservoirs of the “third body” generated from the fragmentation of protrusions to alleviate plowing wear. Therefore, there was no stage I observed in Fig. 9(b and c) because the soft and large asperities were rapidly worn out. In addition, the COF of sample GO-20 at stage Ⅰ maintained a lower and relatively constant value for a longer time than samples GO-10 and GO-40. According to the above analysis, the longer the time that stage Ⅰ and stage Ⅱ occupied, the higher the wear resistance of the coating. Hence, the coating of sample GO-20 had a higher tribological properties than the other coatings.

Fig. 4 Friction coefficient evolution of the as-obtained samples at 25 ℃: (a) Mg substrate,(b) GO-0, (c) GO-5, (d) GO-10, (e) GO-20, and (f) GO-40.

The fitted results of the EIS spectra for the specimens are illustrated in Fig. 5(a and b). Proverbially, the larger the radius of the capacitive loop is at low frequency, the higher the corrosion resistance of the coating. With increasing concentrations of GO additive, the radius of the capacitive loop at low frequency was further enlarged until sample GO-20; however, a minor reduction in the capacitive loop radius was observed for sample GO-40. Similarly, the same tendency was observed in the Bode impedance plot as well, as illustrated in Fig. 12(b). Here, the values of the impedance at 0.01 Hz responding to the response of the compact inner layer could be clearly utilized to estimate the total corrosion resistance of the PEO coatings. Accordingly, the values of |Z| forvarying specimens at the lowest frequency (0.01 Hz) were as follows: |Z|GO-20 (1539300 Ω·cm-2)﹥|Z|GO-40 (1378100 Ω·cm-2)﹥|Z|GO-10 (853600 Ω·cm-2)﹥|Z|GO-5 (268500 Ω·cm-2)﹥|Z|GO-0 (98244 Ω·cm-2), suggesting that sample E3 had the highest anticorrosion property.

Fig. 5(c and d) illustrates the potentiodynamic polarization curves. The uncoated Mg alloy displayed an Ecorr value of -1.52 V and a corresponding Icorr value of 1.09 ×10-5 A·cm-2. Superficially, in comparison to the Mg substrate, minor enhancements in the Ecorr values of composite coatings were discovered, which demonstrated the limited reduction of the thermodynamic tendency of the corrosion emergence. However, the Icorr values of the as-obtained coatings, especially for sample GO-20, were three orders of magnitude lower than that of the uncoated Mg substrate, implying that the chemical stability of the MAO-based coating was further improved by the incorporation of the appropriate content of GO additive. Therefore, for the as-obtained coatings, the corrosion tendency was GO-0 > GO-5 > GO-10 > GO-20 > GO-40, which was according to the results of the EIS analysis.

Fig. 5 EIS plots, polarization curves and variation curves of the corrosion current density and polarization resistance values.

During the PEO process, the conductivity and viscosity of the silicate-based electrolyte were enhanced with the addition of GO additive. Therefore, the micro-arc discharges generated on the anode surface were intensified due to the significant increase in the current intensity in a constant voltage mode. However, the concentration of negative ions in the silicate-based electrolyte increased with the continued addition of GO additive, and then, the negative ions transferred to the substrate surface by electrophoresis, as displayed in Fig. 6 (a). Consequently, the electric potential difference between the anode and the electrolyte was distinctly enhanced, which resulted in an increase in the electron avalanche and a reduction in the breakdown voltage. Accompanied with a decrease in the breakdown voltage, the number of micro-arc discharges per unit interval was evidently increased, while the energy of the individual spark was decreased, which led to the generation of numerous crater-like pores and a reduction in the average size of the pan-like structures. Therefore, the coating compactness was significantly improved due to the formation of minor-sized and blocked pores.

It is well known that the nanoparticles are incorporated in PEO-based coatings mainly by mechanical entrapment and electrophoretic deposition. According to previous studies, when the voltage applied on the anode reaches the breakdown voltage, electric sparks occur in the plasma discharging channel. The high temperature and pressure generated by plasma discharging melt the solid components and eject molten oxides from the discharging channel. Then, both the molten oxides in the inner layer and the electrolyte liquid are absorbed into the discharge channel for an extreme descent in pressure caused by the evacuation of the short-lived plasma. Consequently, the minor-sized GO additive was absorbed into the pores accompanying the electrolyte liquid, as shown in Fig. 6 (b). On the other hand, abundant GO additive is continually transferred to the anode surface by the cataphoretic effect, and most of the additive would not be absorbed into the discharge channel in a timely manner due to transitory discharge and the limited size of the micropores. Therefore, abundant GO additive absorbs on the coating surface or fills in the large-sized cracks and is wrapped by the molten oxide ejected from the discharge channel. Furthermore, because the addition of GO additive enhanced the electrolyte viscosity and decreased the energy of individual sparks, only a small amount of the molten oxide ejected from the discharge channel could travel farther. Hence, the rapid solidification of the small amount of molten oxide formed minor-sized asperities, which had a great contribution to the lower roughness of the composite coatings.

Fig. 6 Schematic diagrams illustrating of the MAO coating growth

研究結論

1. 隨著電解液中GO濃度的升高，微弧氧化複合膜層表面微孔直徑尺寸，粗糙度以及孔隙率呈現出逐漸降低的趨勢，但GO濃度的變化對膜層厚度的影響不明顯；

2. 微弧氧化複合膜層表面顯微硬度的升高，以及表面粗糙度值的降低，使得其在往復摩擦過程中始終維持較低的摩擦係數值和摩擦損傷量，且當GO濃度為20 mL·L-1時，複合膜層表現出最優異的耐磨減摩性能；

3. 微弧氧化複合膜層表面孔隙率的降低，以及膜層截面形貌中缺陷的減少，極大地阻礙了侵蝕性Cl-離子的滲入，有效地減緩了AZ31鎂合金基體在腐蝕液中的腐蝕速度。

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