氧化石墨烯添加剂对AZ31镁合金PEO涂层摩擦学和电化学腐蚀性能

前言

北京石油化工学院陈飞教授团队表面工程及材料腐蚀与防护课题组在 AZ31 镁 合金表面处理领域取得最新进展。镁合金作为重要的结构材料，在电、磁以及机械性能方面有着突出的优点，在航空航天领域存在着重要的应用价值和前景。然而，由于镁合金基体表面发生的摩擦损伤和全面腐蚀，导致其作为构材料极易产生损坏乃至失效，进而给国家和人民生命财产带来巨大的损失。因此，镁合金在工业应用之前，必须经过表面强化处理以增强其在大气环境或腐蚀性液体环境中的耐磨损和耐腐蚀性能。目前常用的镁合金表面强化方法有阳极氧化、激光处理、金属镀层、气相沉积、微弧氧化等。其中微弧氧化(MAO)又称微等离子体氧化，通过微弧放电，在阀金属（Mg, Al, Ti等）表面原位生长具有优良耐磨耐蚀性能的陶瓷层。 本 工作 采用 不同浓度的 氧化石墨烯(GO)作为添加剂，通过超声搅拌作用，将氧化石墨烯均匀分散在电解液中，从而在 AZ31 镁合金表面制备出

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

导读

微弧氧化技术生产效率高、工艺简单、环保无污染，能够对轻金属及其合金材料进行表面改性，形成氧化物膜层或渗层，这些硬化层具有高硬度、耐腐蚀、耐磨损、抗热震、催化和生物相容性良好等优异性能，是目前材料科学研究的前沿领域。传统镁合金表面微弧氧化膜层中存在大量微弧放电留下的缺陷和放电通道，这都是侵蚀性离子渗透膜层进入腐蚀基体的快速通道。研究表明，膜层内部缺陷越多，膜层表面微孔直径越大、膜层致密性和厚度越低，膜层的耐摩擦损伤及耐腐蚀性能就越弱。而微弧氧化电解液的成分则在很大程度上决定了其膜层的微观结构及性能。因此，通过在微弧氧化电解液中加入不同的添加剂，则能够在很大程度上改善膜层的结构及性能。氧化石墨烯作为一种碳材料，具有很好的润滑减摩效果，并因其具有良好的亲水性，尤其是较大的比表面积能够在电解液中吸附阴离子而显负电性，进而能够明显改善电解液的导电性能。因此，本研究选用氧化石墨烯（浓度分别为0、5、10、20和40 mL·L-1）作为添加剂，通过微弧氧化技术，在AZ31镁合金表面制备出包覆有GO添加剂的微弧氧化复合膜层。

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

研究结论

1. 随着电解液中GO浓度的升高，微弧氧化复合膜层表面微孔直径尺寸，粗糙度以及孔隙率呈现出逐渐降低的趋势，但GO浓度的变化对膜层厚度的影响不明显；

2. 微弧氧化复合膜层表面显微硬度的升高，以及表面粗糙度值的降低，使得其在往复摩擦过程中始终维持较低的摩擦系数值和摩擦损伤量，且当GO浓度为20 mL·L-1时，复合膜层表现出最优异的耐磨减摩性能；

3. 微弧氧化复合膜层表面孔隙率的降低，以及膜层截面形貌中缺陷的减少，极大地阻碍了侵蚀性Cl-离子的渗入，有效地减缓了AZ31镁合金基体在腐蚀液中的腐蚀速度。

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