Rare Metals2018年第11期

Microstructure and properties of microarc oxidation coating formed on aluminum alloy with compound additives nano-TiO₂ and nano-ZnO

Ting-Yi Lin Xiao-Yan Zhang Xin Huang Xiang-Peng Gong Jun-Jie Zhang Xie-Jun Hu

College of Materials and Metallurgy, Guizhou University

作者简介：*Xiao-Yan Zhang,e-mail: ivzhangxiaoyan@163.com;

收稿日期：3 July 2016

基金：financially supported by the International Technology Cooperation Plan in Guizhou Province (No.2012-7001);

Microstructure and properties of microarc oxidation coating formed on aluminum alloy with compound additives nano-TiO₂ and nano-ZnO

Ting-Yi Lin Xiao-Yan Zhang Xin Huang Xiang-Peng Gong Jun-Jie Zhang Xie-Jun Hu

College of Materials and Metallurgy, Guizhou University

Abstract：

In this work, microarc oxidation(MAO) technology was used to form oxide ceramic coating on the surface of aluminum alloy. The combined additives nano-TiO₂ and nano-ZnO were added into the silicate electrolyte, and the effect of the compound nano-additive on microstructure and properties of MAO coating was investigated. The results show that compared with those of the nano-additive-free coating formed on aluminum alloy, the thickness, hardness, abrasion resistance and corrosion resistance of the nano-additive-containing coating are obviously improved. The surface of coating with nanoadditive becomes smooth, dense, and there are less porosities and microcracks. Moreover, the content of crystal phase a-Al₂O₃ and y-Al₂O₃ increases visibly on the nano-additive-containing MAO coatings, and new phases Al₃Ti and Zn_0.6Ti_0.4 are detected in the coatings, which are mainly contributed to the excellent corrosion resistance and abrasion resistance of the film. When the contents of nano-TiO₂ and nano-ZnO are, respectively, 4 and 2 g-L^-1, the film has better comprehensive performance, the thickness and hardness of the film could reach 52 μm and HV 692,respectively.

Keyword：

Microarc oxidation; Nano-TiO₂; Nano-ZnO; Aluminum alloy; Property;

Received： 3 July 2016

1 Introduction

Aluminum and its alloys have become the most widely used metal in nonferrous metal materials for their good properties,such as high specific strength,excellent electrical conductivity,good formability,etc. [ 1, 2] .But the chemical property of aluminum is very active,Al can be easily oxidized at room temperature,which may form a layer of natural oxide on its surface.The thickness of the film is extremely thin and the hardness is low,which cannot resist the bad environment caused by corrosion and the external friction,so the surface treatment for Al is very necessary.

As a new generation of surface processing technology,microarc oxidation (MAO) can generate a dense ceramic coating on the surface of aluminum alloy.The coating has high hardness,good adhesion,excellent toughness and better wear resistance,corrosion resistance,as well as greater resistance of high-temperature,which can greatly meet the requirements of parts under the condition of high temperature or fast speed movement [ 3, 4, 5, 6, 7, 8] .In addition,MAO technology is easy to be carried out with low cost and no pollution,thus becoming one of hot topics in the study of aluminum alloy surface modification technology,which makes it have expansive prospects for development and application [ 9, 10, 11, 12, 13, 14, 15, 16] .Studies found that MAO mainly focused on the optimization of electric parameters and the adjustment of electrolyte composition.But researches about adding compound nanoparticles into the electrolyte were few [ 17, 18, 19, 20, 21, 22, 23] .

In this work,the MAO coatings were prepared on aluminum alloy with or without combined additives nanoTiO₂ and nano-ZnO.It was systematically explored the microstructure,hardness,wear resistance and corrosion resistance properties of MAO ceramic layer,expecting to improve the comprehensive performance of the layer through adding nano-additives into the MAO electrolyte.

2 Experimental

2.1 Materials

The composition of raw materials in the experiment is shown in Table 1.In accordance with the requirements of the performance test,the sample dimension is30 mm×6 mm×7 mm for friction and wear test,and10 mm×10 mm×10 mm for electrochemical corrosion experiment.

2.2 Processing technology

The experiment was conducted by self-developed single pulse MAO devices.The time,frequency and duty ratio of the MAO experiment is 60 min,400 Hz and 30%,respectively.The basis composition of the MAO electrolyte is silicate system,which is composed of 10 g·L^-1Na₂SiO₃,2 g·L^-1 NaOH and 2 g·L^-1 Na₂WO₄.The content of nano-TiO₂ and nano-ZnO added into the electrolyte is shown in Table 2.

2.3 Analysis and test methods

The conductivity of electrolyte was measured by DDS-11A conductivity meter.TT230 thickness tester was used to measure the thickness of MAO ceramic coatings by testing five points of each sample,and the thickness data were obtained by average.HV1000 microhardness tester was performed to analyze the hardness of the coatings,the cross section of coating was polished to meet the requirement of hardness measurement,and the hardness data were obtained by average.Microstructure and elemental distribution of the films were observed by scanning electron microscope (SEM,KYKY-2800B) and energy dispersive spectrometer (EDS,APOLLO-10X).Phases of the coatings were analyzed by X-ray diffraction (XRD,X’Pert PRO)equipped with Cu Kαradiation,operating at 40 kV and150 mA with the scanning rate of 0.334 (°)·s^-1.The potentiodynamic polarization curve of coatings was measured by VSP electrochemical workstation in 3.5%NaCl solution.MMS-2A abrasion tester was used to measure the wear resistance of coatings.

  下载原图

Table 1 Chemical composition of experimental alloy (wt%)

  下载原图

Table 2 Content list of mixed additive nano-TiO₂ and nano-ZnO(g·L^-1)

3 Results and discussion

3.1 Electrical conductivity of electrolyte

The effect of mixed nano-additive on the electrical conductivity of electrolyte is shown in Fig.1.It is seen that when the content of nano-ZnO is certain,with the increase of nano-TiO₂ content,the electrical conductivity of electrolyte increases.This is mainly because with the increase of ion concentration in the solution,the movement among ions is intensified,thus improving the electrical conductivity of electrolyte.However,when the concentration of nano-TiO₂ increases to a certain value,the existence of interaction force among ions makes the electrical conductivity increase slowly.

Fig.1 Electrical conductivity of solution with different nano-addi-tive contents

3.2 Thickness and hardness of coatings

The effect of compound nano-additive content on the thickness and hardness of MAO coating is illustrated in Fig.2.From Fig.2a,it is obvious that when the concentration of nano-ZnO is certain,with the increase of nanoTiO₂ content,the thickness of film decreases gradually.The MAO process can be briefly pided into two parts:film-forming and film growth.The former stage can form a thin film with high impedance and restrain microarc process.In the film growth stage,a chemical reaction will take place between aluminum and reactive oxygen produced by the discharge of OH^-,thus forming a film whose main phase is Al₂O₃ [ 24] .Moreover,under high current density,with the increase of nano-TiO₂ content,the amount of attachments formed on the MAO coating increases,and the oxidation film and microholes are covered,which blocks the exchange and discharge channel between matrix and medium [ 25, 26] .Besides,the molten regeneration process of oxidation film is hindered,which may lower the formation rate of oxidation layer,then reducing the thickness of coating [ 27] .When the contents of nano-TiO₂ and nanoZnO are,respectively,2 and 2-3 g·L^-1,the thickness of film can reach 55-56μm.

Figure 2b shows the influence of combined nano-additive content on the hardness of MAO film,it is noteworthy that when the content of nano-ZnO is determinate,with the increase of nano-TiO₂ content,the hardness of MAO coating increases first and then decreases slightly.The reason of this tendency is similar to that of thickness.It can be known that the decrease in the amount of microholes means a high density of coating surface,which can greatly improve the hardness of the coating [ 28, 29] .However,in the electrolyte,when the concentration of nano-TiO₂ is too high,the charged particles may adsorb more ions,and since the amount of discharge channel is reduced,it cannot be fully sintered,which may form porous ceramic layer in the

MAO process.When the contents of nano-TiO2 and nanoZnO are,respectively,4-5 and 2 g·L^-1,the hardness of oxidation film can reach HV 690-HV 692.

In conclusion,when the contents of nano-TiO₂ and nano-ZnO are,respectively,4 and 2 g·L^-1,the ceramic coating of sample has better comprehensive performance,the thickness and hardness of the film could reach 52μm and HV 692,respectively.It indicates that the film can obtain a better hardness even its thickness is not that high.

3.3 SEM observation and EDS analysis

Figure 3 demonstrates the surface morphology and line scanning of samples.By comparing Fig.3a,b,it can be obviously concluded that the coating surfaces of Sample 0have more porosities and microcracks than those of Sample7,meaning the surface coating flatness level of Sample 7 is better.From Fig.3,it can be known that the coating surface has some volcano-like particles whose top is accompanied with microholes,which are the plasma discharge channel of solution and substrate.In the process of micro arc discharge,these holes would be treated as the center of the coating,and then the oxides melted,solidified and entangled rapidly,which may increase the thickness of coating.In addition,more particles would be produced with the increase of MAO time,thus covering the original tiny volcano-like particles [ 30] .This kind of formation mechanism can make the coating combine with the aluminum alloy substrate more compactly [ 31] .Moreover,the selective adsorption of is effective,and the formation of surface attachments and adsorbates has great influence on the melting and re-growth speed of the coatings.In addition,under the action of electric current,these debrises can be transferred to the coating surface quickly,which may block the discharge channel of the coating [ 25, 32] ,thereby improving the surface quality of the film,as shown in Fig.3b.These results can be further supported by the line scanning of nano-additive-containing MAO coatings (Fig.3c).Compared with Al substrate,the content of Si and O increases,implying that participates in MAO reaction.

Fig.2 Effect of compound nano-additive content on thickness and hardness of MAO coating:a thickness and b hardness

Fig.3 Surface morphology and line scanning of MAO coatings:a Sample 0,b Sample 7,and c line scanning

Figure 3b also represents the composition analysis of Point 1 and 2 by EDS.Combining the content of alloying elements in Table 3,it can be found that the main elements of the two points are Si,Al and O.In addition,Zn and Ti are also gathered in these points.From Table 3,it can be seen that the content of Zn and Ti at Point 1 is higher than that at Point 2,while Al,O and Si contents are lower.According to atomic ratio calculation,one may remark that the particles gathering at Point 2 are Al₂O₃,and the main substances at Point 1 may be SiO₂ and TiO₂.All above indicate that nano-TiO₂ and nano-ZnO added into the electrolyte involve in MAO reaction.

3.4 Phase analysis

XRD patterns of MAO coatings with or without nano-additive are shown in Fig.4.It is noteworthy that two crystal phasesα-Al₂O₃ andγ-Al₂O₃ exist in both nano-additivefree and nano-additive-containing MAO coatings.Compared with nano-additive-free MAO coatings (Fig.4a),nano-additive-containing MAO coating (Fig.4b) demonstrates higherα-Al₂O₃ andγ-Al₂O₃ contents.In addition,a few peaks corresponding to Al₃Ti and Zn_0.6Ti_0.4 are detected,suggesting that the constituents of TiO₂ and ZnO from electrolyte involve in the plasma MAO reactions and are incorporated into the films as compounds [ 33, 34] .These changes above could improve the wear resistance and corrosion resistance of the coatings.Furthermore,MAO coating is composed of inner dense layer and outer loose layer.There are no big holes on the interface of oxide film and substrate,and the interface bonding is good [ 35] .The dense layer is adjacent to matrix,while the loose layer is located in the outside of the coating.And there are a lot of microholes and cracks in the loose layer.Moreover,it can be known that new phasesα-Al₂O₃ andγ-Al₂O₃ are formed after MAO treatment,and the formation of these phases can be understood as two stages:the discharge of OH^-and the formation of Al₂O₃ phase [ 36, 37] .The instant high-temperature sintering effect makes the coating get the structure of ceramic phase,which could explain why MAO coating has better performance [ 38] .

  下载原图

Table 3 Point scanning result of nano-additive-containing MAO coating by EDS

3.5 Wear resistance of coatings

The wear tests were carried out on samples under the condition of oil lubrication,and the friction coefficients of specimens are depicted in Fig.5.From Fig.5,it is obvious that the coatings prepared with nano-additive-containing have relatively lower friction coefficients compared with the nano-additive-free coatings.Moreover,when the contents of nano-TiO₂ and nano-ZnO are,respectively,4 and2 g·L^-1 (Sample 7),the friction coefficient of the coating reaches a lower value.This is mainly because there are fewer microcracks on the coating surface of Sample 7.Besides,much more crystal phases likeα-Al₂O₃,γ-Al₂O₃,Al₃Ti and Zn_0.6Ti_0.4 are detected on the coating,which could promote the wear resistance for MAO coatings significantly [ 34, 39, 40] .

3.6 Corrosion resistance of coatings

The polarization curves of different samples are displayed in Fig.6.The self-corrosion current (i_corr),self-corrosion potential (E_corr) and polarization resistance (R_P) results are also listed in Table 4.From Fig.6 and Table 4,it is evident that i_corr of nano-additive-free MAO coatings is much less than that of nano-additive-containing MAO coatings,while E_corr and R_P of MAO coatings with nano-additive are higher than those of nano-additive-containing coating.In addition,Sample 7 has lower i_corr but higher E_corr compared with other samples,which shows a preferable corrosion resistance.This is because the nano-additives can adhere to the coating surface or get into the film pores.To a certain extent,it could make up for the defect of porous coating and increase the density of coating,thus improving the corrosion performance of the coatings.

Fig.4 XRD patterns of MAO coatings a without nano-additive and b with nano-additive

Fig.5 Relationship between coefficient and time of MAO coatings with different nano-additive contents

Fig.6 Polarization curves of different MAO coatings with different nano-additive contents

  下载原图

Table 4 Corrosion characteristics of MAO coatings with different nano-additive contents

4 Conclusion

This work presents the effect of combined additives nanoTiO₂ and nano-ZnO on the properties of MAO coating formed on aluminum alloy.Compared with those of the coating without nano-additive,the micros true ture and organization of the coating with nano-additive are improved clearly.It has lower porosities and fewer microcracks,and the surface of nano-additive-containing MAO coatings is smoother.The addition of nanoparticles can enhance the mechanical properties and reduce the surface defects of MAO coatings.The nano-additive-containing MAO films have higherα-Al₂O₃ andγ-Al₂O₃content and more newly crystal phases (Al₃Ti,Zn_0.6Ti_0.4)compared with the nano-additive-free MAO coatings.When the contents of nano-TiO₂ and nano-ZnO are,respectively,4 and 2 g·L^-1,the MAO film has better comprehensive performance,the thickness and hardness of the film could reach 52μm and HV 692,respectively.It shows that the film can obtain a better hardness even its thickness is not that high.

Acknowledgements This work was financially supported by the International Technology Cooperation Plan in Guizhou Province(No.2012-7001).

参考文献

[1] Zhu ZF. Anodic Oxidation and Surface Treatment Technology of Aluminum Alloy. Beijing:Chemical Industry Press; 2011. 10.

[2] Regone NN, Freire CMA, Ballester M. Al-based anodic oxide films structure observation using field emission gun scanning electron microscopy. J Mater Process Technol. 2006;172(1):146.

[3] Wang KL, Zhang QB, Sun ML, Wei XG, Zhu YM.Microstructure and corrosion resistance of laser clad coatings with rare earth elements. Corros Sci. 2001;43(2):255.

[4] Sharma SP, Dwivedi DK, Jain PK. Effect of La_2O_3 addition on the microstructure, hardness and abrasive wear behavior of flame sprayed Ni based coatings. Wear. 2009;267(5-8):853.

[5] Qu L, Li M, Liu M, Zhang E, Ma C. Microstructure and corrosion resistance of ultrasonic micro-arc oxidation biocoatings on magnesium alloy. J Adv Ceram. 2013;2(3):227.

[6] Zheng HY, Wang YK, Li BS, Han GR. Effect of Na2W04 on properties of micro-arc oxidation ceramic coatings on aluminum alloy. J Zhejiang Univ. 2005;59(2-3):139.

[7] Jiang BL, Zhao RB, Liang G, Li JM, Yuan F. Effect of Na2W04on properties of micro-arc oxidation(MAO)ceramic coatings and wear resistance of aluminum alloy. Mater Rev. 2006;20(9):155.

[8] Yan SF, Liu XD, Chen WD, Wang ZG, Fan XJ, Xu ZG.Characteristics of coating on surface of ZrH_(1.8)prepared by MAO in different electrolyte systems. Chin J Rare Met. 2014;38(4):646.

[9] Yu X, Cao C, Yao Z, Zhou D, Yin Z. Corrosion behavior of rare earth metal(REM)conversion coatings on aluminum alloy LY12. Mater Sci Eng, A. 2000;284(1-2):56.

[10] Kurze P, Krysmann W, Schneider HG. Application fields of ANOF layers and composites. Cryst Res Technol. 1986;21(12):1603.

[11] Liu Y, Xu J, Gao Y, Yuan Y, Gao C. Influences of additive on the formation and corrosion resistance of micro-arc oxidation ceramic coatings on aluminum alloy. Phys Proce. 2012;32(33):107.

[12] Yerokhin AL, Nie X, Leyland A, Matthews A, Dowey SJ.Plasma electrolysis for surface engineering. Surf Coat Technol.1999;122(2-3):73.

[13] Boguta DL, Rudnev VS, Yarovaya TP, Kaidalova TA, Gordienko PS. On composition of anodic-spark coatings formed on aluminum alloys in electrolytes with polyphosphate complexes of metals. Russ J Appl Chem. 2002;75(10):1605.

[14] Yerokhin AL, Leyland A, Matthews A. Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation. Appl Surf Sci. 2002;200(1-4):172.

[15] Curran JA, Clyne TW. Porosity in plasma electrolytic oxide coatings. Acta Mater. 2006;54(7):1985.

[16] Blawert C, Heitmann V, Dietzel W, Nykyforchyn HM, Klapkiv MD. Influence of electrolyte on corrosion properties of plasma electrolytic conversion coated magnesium alloys. Surf Coat Technol. 2007;201(21):8709.

[17] Laleh M, Rouhaghdam AS, Shahrabi T, Shanghi A. Effect of alumina sol addition to micro-arc oxidation electrolyte on the properties of MAO coatings formed on magnesium alloy AZ91D. J Alloy Compd. 2010;496(496):548.

[18] Bayati MR, Golestani-Fard F, Moshfegh AZ. The effect of growth parameters on photo-catalytic performance of theMAO-synthesized Ti02, nano-porous layers. Mater Chem Phys.2010;120(2-3):582.

[19] Shen D, Cai J, Li G, He D, Wu L. Effect of ultrasonic on microstructure and growth characteristics of micro-arc oxidation ceramic coatings on 6061 aluminum alloy. Vacuum. 2014;99(1):143.

[20] Huan C, Guang C. Investigation of morphology and composition of plasma electrolytic oxidation coatings in systems of Na2Si03-NaOH and(NaP03)6-NaOH. J Mater Process Technol.2007;182(1-3):28.

[21] Wang Y, Jiang B, Lei T, Guo L. Dependence of growth features of microarc oxidation coatings of titanium alloy on control modes of alternate pulse. Mater Lett. 2004;58(12-13):1907.

[22] Yang X, He Y, Wang D, Gao W. Cathodic micro-arc electrodeposition of thick ceramic coatings. Electrochem Solid-State Lett. 2002;5(3):C33.

[23] Guo D, Xue L, Zu F. Phase composition, morphology and element contents of micro-arc oxidation ceramic coatings on Ti-6A1-4V alloy under different calcination conditions. Rare Met. 2016;35(11):837.

[24] Xue WB, Wang C, Li YL, Deng ZW, Chen RY, Zhang TH.Effect of micro-arc discharge surface treatment on the tensile properties of AL-Cu-Mg alloy. Mater Lett. 2002;56(5):737.

[25] Yang Y, Liu Y. Effects of current density on the microstructure and the corrosion resistance of alumina coatings embedded with SiC nano-particles produced by micro-arc oxidation. J Mater Sci Technol. 2010;26(11):1016.

[26] Xiang N, Song RG, Zhao J, Hai L, Wang C. Microstructure and mechanical properties of ceramic coatings formed on 6063aluminium alloy by micro-arc oxidation. Trans Nonferrous Meta Soc China. 2015;25(10):3323.

[27] Huang D, Zhang XY, Wu DF, Zhou XS. Effect of rare erath(RE)additives on performances of micro-arc oxidation coatings formed on aluminum alloy. In:Proceedings of International Forum on Materials Analysis and Testing Technology(Advanced Materials Research). Qingdao. 2013:140.

[28] Javidi M, Fadaee H. Plasma electrolytic oxidation of 2024-T3aluminum alloy and investigation on microstructure and wear behavior. Appl Surf Sci. 2013;286(6):212.

[29] Stojadinovic S, Vasilic R, Belca I, Petkovic M, Kasalica B.Characterization of the plasma electrolytic oxidation of aluminium in sodium tungstate. Corros Sci. 2010;52(10):3258.

[30] Dittrich KH, Krysmann W, Kurze P, Schneider HG. Structure and properties of ANOF Layers. Cryst Res Technol. 1984;19(1):93.

[31] Guo QQ, Jiang BL, Li JP, Li JM, Xia F, Guo YC, Li GH, Yang Z, Wang YH. Corrosion resistance of the ceramic coating on a cast Al-14Si-5Cu-3Ni-1Mg alloy formed by micro-arc oxidation. Spec Cast Nonferrous Alloys. 2008;7:557.

[32] Cai J, Cao F, Chang L, Zheng J, Zhang J. The preparation and corrosion behaviors of MAO coating on AZ91D with rare earth conversion precursor film. Appl Surf Sci. 2011;257(8):3804.

[33] Yan Y, Han Y, Li D, Huang J, Lian Q. Effect of NaAlO_2 concentrations on microstructure and corrosion resistance of Al_2O_3/Zr02 coatings formed on zirconium by micro-arc oxidation.Appl Surf Sci. 2010;256(21):6359.

[34] Yang W, Jiang BL, Xian LY, Shi HY. Action mechanism of solutions on forming process of microarc oxidation coatings on aluminium alloy. Chin J Nonferrous Meta. 2009;19(3):464.

[35] Xue WB, Deng ZW, Lai YC, Chen RY. Analysis of phase distribution for ceramic coatings formed by microarc oxidation on aluminum alloy. J Am Ceram Soc. 1998;81(5):1365.

[36] Zhao JH, Zhang ZW, Wang ZH. Structure and corrosion resistance of composite ceramic coating prepared by EASP/MAO on AZ91D magnesium alloy. Chin J Rare Meta. 2013;37(4):549.

[37] Hao GD, Hao XL, Zhu ZF. Phase composition, morphology and element contents of micro-arc oxidation ceramic coatings on Ti-6A1-4V alloy under different calcination conditions. Rare Met. 2016;35(11):836.

[38] Xue WB, Deng ZW, Lai YC. Review of microarc oxidation technique on surface of non-ferrous metals. Heat Treat Met.2000;1:1.

[39] Erarslan Y. Wear performance of in situ aluminum matrix composite after micro-arc oxidation. Trans Nonferrous Meta Soc China. 2013;23(2):347.

[40] Zhou XS, Zhang XY, Wu DF. Effects of nano-SiO_2 in electrolytes on surface properties of micro-arc oxidation ceramic coatings formed on new casting aluminum alloy. Adv Mater Res. 2012;581-582(1):368.