Improving the wear properties of AZ31 magnesium alloy under vacuum low-temperature condition by plasma electrolytic oxidation coating

Acta Astronautica 116 (2015) 126–131

Contents lists available at ScienceDirect

Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro

Improving the wear properties of AZ31 magnesium alloy under vacuum low-temperature condition by plasma electrolytic oxidation coating Hang Li a, Songtao Lu a, Wei Qin b, Lu Han a, Xiaohong Wu a,n a b

Department of Chemistry, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China

a r t i c l e in f o

abstract

Article history: Received 24 March 2015 Received in revised form 10 June 2015 Accepted 2 July 2015 Available online 10 July 2015

In this paper, Al-doped ceramic coatings with a thickness of 22–27 mm were prepared on AZ31 magnesium alloy by plasma electrolytic oxidation (PEO) in electrolytes that contain varied concentrations of NaAlO2, aiming to improve its wear resistance under vacuum low-temperature (
50 1C) condition. These obtained ceramic coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that the ceramic coatings with typical porous structure were mainly consisted of MgO and MgAl2O4 phases, indicating that the Al elements were participated into the coating during the PEO process. We found that at the concentrations 5 g/L NaAlO2, the coating has its highest surface hardness (6.2 GPa) and lowest friction coefficients (0.21) under vacuum low-temperature condition. Our results demonstrate that such PEO coatings are very promising to improve the wear properties under vacuum low-temperature condition. & 2015 Published by Elsevier Ltd. on behalf of IAA.

Keywords: Magnesium alloy Plasma electrolytic oxidation Al doping Wear properties Vacuum low-temperature

1. Introduction Mechanical components of spacecraft must experience hostile space environment, involving radiation damage and vacuum low-temperature wear [1–3]. The serious wear damage and cold-welding on the metallic surface decreased the life-span of the spacecraft, which is very agreement with the results of NASA's research [4–6]. Hence, improving wear properties of metal components under vacuum low-temperature is very important for spatial applications. Conventionally, mechanical components were mainly fabricated with aluminum alloys. In order to achieve higher performance and energy saving, magnesium alloy with low-weight and outstanding stiffness was considered to be a promising alternative for

n

Corresponding author. Tel.: þ86 451 864 02522. E-mail address: [email protected] (X. Wu).

http://dx.doi.org/10.1016/j.actaastro.2015.07.005 0094-5765/& 2015 Published by Elsevier Ltd. on behalf of IAA.

aluminum alloy in aerospace applications [7–10]. However, poor wear resistance of magnesium alloy hindered its widespread application [11–13]. It is urgent to develop feasible approaches to improve the wear properties. Extensive research efforts have been devoted to improve wear resistance of magnesium alloy by surface modification. Various surface treatments including spraying, electroplating, PEO, chemical conversion, anodic oxidation, organic coatings, PVD, have been proved to be available [14–20]. Of these, one promising approach is to treat magnesium alloy using the PEO technique. PEO is a recently developed surface treatment technology based on anodic oxidation that can efficiently improve surface properties of aluminum, titanium, magnesium, and their alloys efficiently, such as hardness, wear and corrosion resistance [21]. The electrolyte by PEO is usually alkaline-based, consisting hydroxide, aluminate, silicate, phosphate, fluoride and/or organic additives [22]. Depending on the choice of ions in the electrolyte, the resultant

H. Li et al. / Acta Astronautica 116 (2015) 126–131

coatings were composed of MgO, MgF2, Mg2SiO4, MgAl2O4 and non-crystalline materials, which could improve the surface properties of magnesium alloy. Besides, PEO also can reduce costs, simplify the process and phase out the use of chromic and hydrofluoric reagents that are harmful to the environment [23]. In our previous work, we have successfully applied this method to prepare ceramic coatings on ZK60 magnesium alloy with excellent wear resistance property [24–26]. In this work, we reported an approach to obtain coatings on AZ31 magnesium alloy with excellent wear properties by the PEO method. This type of magnesium alloy was selectively used because of its widespread commercial uses in the preparation of mechanical components. Environmental friendly electrolytes containing NaAlO2 were introduced to fabricate coatings on this alloy in our approach. It was reported that adding Al ions to the electrolyte made MgAl2O4 forming during the PEO process. And the microhardness of MgAl2O4 is about 1500–1800 HV, it can be expected that the formed MgAl2O4 in the coating will benefit the improvement of wear resistance [27]. Our research shows that the Al ions played a very important role in determining wear properties of the obtained coatings under vacuum low-temperature condition. 2. Experimental 2.1. Preparation of PEO coatings Rectangular coupons of dimensions 40 mm  40 mm  2 mm AZ31 magnesium alloy (mass fraction: 3–3.2% Al, 0.8% Zn, 0.4% Mn, 0.02% Si, 0.003% Cu, 0.0023% Fe and Mg balance) were used as substrate materials for the PEO treatment. All samples were successively polished using SiC papers up to 2000 grit, then degreased ultrasonically in acetone, cleaned with distilled water and dried in air. The used electrolytes were made with 20 g/l Na3PO4, 4 g/l KOH, 2 g/l NaF and varied concentrations of NaAlO2 (0, 3, 5 and 7 g/l). A home-made DC pulse power with power of 20 kW was used during the PEO process. The electrolytic bath made of stainless steel was used as the cathode. Bath temperature was maintained below 30 1C by a recycle cooling water system. The power frequency and duty ratio were fixed at1500 Hz and 45%. The PEO process was performed for 10 min under the constant current kept at 7 A/dm2. After the PEO treatment, the coated specimens were washed in water and then dried in air. 2.2. Microstructure and composition examinations Surface morphologies of PEO coatings prepared on magnesium substrates were examined by scanning electron microscopy (SEM; S-570, Hitachi) equipped with energy dispersive spectroscopy (EDS) used to investigate the element composition. The phase composition of the coatings was analyzed by X-ray diffraction (XRD; Dmax3B, Rigaku Corporation, Japan) with a Cu Kα radiation (λ ¼0.15418 nm), operated at 50 kV and 50 mA. X-ray photoelectron spectroscopy (XPS; PHI5700, America, using Mg Kα) techniques were used to analyze chemical composition. The coating thickness was measured with an

127

eddy current coating thickness gauge (CTG-10, Time Company, China) with accuracy of 0.1 mm. 2.3. Mechanical and wear evaluation The micro-hardness was measured by a nanoindenter XP (Nano Instruments, MTS Systems Corporation, USA) with a Berkovich diamond indenter. All tests were operated at maximum pressure of 500 μm depth, maximum load of 500 mN, displacement and load accuracy of 0.01 nm and 50 mN. In this experiment, the penetration depth of measurement was 2 mm, and then the raw data was processed via Nano Instrument's Software to revise thermal drift and generate the load-displacement curves to calculate the elastic modulus and hardness values. The micro-hardness of four samples, prepared in the electrolytes with 0, 3, 5 and 7 g/l NaAlO2, were tested. Friction coefficient was measured using a pin-on-disk УТИ TB100 type vacuum tribometer made in Ukraine [28]. The range of the normal force can be adjusted from 2.5 N to 100 N and the sliding linear velocity in the range from 0.2 m/s to 1.2 m/s. All tests were performed under dry sliding contact. The environment pressure of the tribometer was 2  10
3 Pa and the temperature regulated by liquid nitrogen at
50 1C. The nitrided 2GCr13 steel disk was rotated against a fixed pin to form a friction pair. Cylindrical magnesium alloy with the dimension of Φ 9 mm  20 mm was the pin sample. Pins and disks were cleaned with acetone prior to testing. A normal load of 2.5 N, a sliding speed of 0.4 m/s and a testing time of 30 min were selected as parameters of friction coefficient tests. Two samples, the PEO coating with the maximum hardness and the AZ31 Mg alloy substrate, were used to carry out the wear evaluation. After each test, the disk was cleaned with acetone. Microstructure and elemental composition of wear tracks on the pin sample were examined by SEM equipped with EDS. 3. Results and discussion 3.1. Surface morphologies of the PEO coating The surface morphologies of the prepared coatings with variations of Al ions concentrations are shown in Fig. 1. Typically, many micro-pores and cracks were formed due to the existence of micro discharge channels, in which plasma discharges take place to produce high temperature and pressure prompting the generation of molten oxides and gas bubbles. It was clearly observed that mass of gases and bright sparks generated during the PEO process. Thus, molten oxides were solidified under the action of the cold electrolyte to form the microstructure of many craters as micro-pores that is the typical morphology characteristic of the PEO coating. The range of the pore size is around 510μm for all the coatings. Moreover, the appearance of disconnected cracks was ascribed to the thermal stress caused by rapid solidification of molten oxides. Excessive pores and cracks could reduce the wear resistance of the obtained coatings [29]. The number and size of micro-pores and cracks were found to significantly change with the increase of Al ions. When the concentration reached 5 g/L, fewest micro-pores and cracks was observed (Fig. 1c),

128

H. Li et al. / Acta Astronautica 116 (2015) 126–131

Fig. 1. Morphology of PEO coatings using different concentrations of NaAlO2. a) 0 g/L; b) 3 g/L; c) 5 g/L; d) 7g/L.

which can increase the density and smoothness of coatings [29–31]. As a result, the hardness of the PEO coating was improved and the PEO coating obtained by adding 5 g/L NaAlO2 showed the highest hardness. Adding the AlO2 anions in the electrolyte should strengthen the PEO reaction intensity. It was found that the plasma discharge of the PEO reaction became more violent and the final voltage was relatively high in the Al-containing electrolyte. However, the content level of AlO2 ions directly influences the alkalineacid balance of the electrolyte. Too low or high PH value can enhance the electrolyte to erode the coating. As is shown in Fig. 1a and d, some formed particles were observed and further increased the roughness of the coating. 3.2. Phase and chemical composition of the PEO coating Fig. 2 illustrates XRD patterns of the prepared coatings under different NaAlO2 concentrations where formation of Mg, MgO and MgAl2O4 phases was observed. It is indicated that Al elements participated into the reaction and changed the composition of coatings. The existence of MgAl2O4 phases showed that Mg element from the substrate and Al ions from the electrolyte reacted with each other during the PEO process. Yet the diffraction peaks of the substrate were found that maybe ascribed to the penetrability of X-ray. Comparing with the ASTM cards, the crystal planes (111), (200) and (220) matching with the 45-0946# card are assigned to the diffraction peaks of MgO phase, and the peaks (302), (303), (206) and (007) corresponding to the 330853# card were identified as crystal planes of the MgAl2O4 phase. Similar to the mechanism of the anodic oxidation,

Fig. 2. XRD patterns (a) and enlarged views (b) of PEO coatings before and after Al doping. -

AlO2 ions reacted with water to form AlðOHÞ4
, which migrated from the electrolyte to the substrate interface and generated Al2O3 under high temperature and pressure. And then Al2O3 reacted with MgO to form MgAl2O4. Due to the excellent hardness of MgAl2O4 phase, Al doping plays an important role in improving wear properties of PEO coatings. To further understand the composition of the obtained coatings, XPS technique was employed to determine chemical composition. The survey spectrum of the PEO coating obtained under 5 g/L NaAlO2 is shown in Fig. 3. It can found that the detected peaks in the coating belong to Mg,

H. Li et al. / Acta Astronautica 116 (2015) 126–131

15000

600

F1s

Mg2p

MgKLL

9000

500

Intensity (cps.)

12000

Intensity (cps.)

K2p

Intensity(cps.)

O1s

NaKLL

6000

291

294

297

680

690

700

Binding energy(eV) 3000

129

OKLL

Al2p C1s Mg2s P2s P2p Mg2p

Na1s

Mg1s

400

300

200

0 0

200

400

600

800

1000

1200

1400

47

48

49

Binding energy (eV)

50

51

52

53

Binding Energy (eV)

Fig. 3. XPS survey spectra of the PEO coating prepared under 5 g/L NaAlO2.

500

3.3. Micro-hardness and wear tests According to Archard equation [36], the friction coefficient and hardness are main factors influencing wear properties of material as described by the following equation: f W V ¼ ld h

ð1Þ

where, WV is the worn volume; l is the load; d is the sliding distance; f is the friction coefficient; and h is equal to the material hardness. Thus, it was clearly known that the wear resistance is related to the ratio of the friction coefficient to hardness. Fig. 5 shows the micro-hardness and thickness of coatings under different NaAlO2 concentrations. It could be found that the thickness increased with the increase of Al ions. Due to the formation of Al(OH)3 on the anode surface during anodic polarization, this was beneficial to the generation of plasma discharge and then the growth rate of coating was accelerated during the PEO process. However, the micro-hardness at 5 g/L NaAlO2 reached the maximum (about 6.2 GPa) which far beyond that of Mg

400

300

200 72

73

74

75

76

77

Binding Energy (eV) 6000

O1s B

5000

Intensity (cps.)

O, Al, K, Na, P and F elements, confirming that Mg element come from the substrate and others from the electrolyte. No peaks corresponding to those elements in the XRD results, it is possible that they existed in the form of the amorphous phase or the relatively small amount. It was noted that the correction of all XPS peaks referenced to the binding energy of C (1s) at 284.5 eV. Fig. 4a exhibits the Mg (2p3/2) core level at binding energy of 50.2 eV, asserting the formation of MgO [32]. The Al (2p3/2) peak, shown in Fig. 4b, located at binding energy of 74.0 eV confirming the presence of Al3 þ state in aluminum oxide [33]. The O (1s) peak is shown in Fig. 4c which can be deconvoluted into two components demonstrating two different O-binding exist in the coating. The peak A with binding energy of 530.9 eV is assigned to the oxygen in the magnesium oxides lattice. The peak B at binding energy of 532.2 eV corresponds to the oxygen in the O–Al bond. These results have excellent match with literature data [34,35].

Intensity (cps.)

Al2p

4000

A 3000

2000

1000 529

530

531

532

533

534

535

536

Binding Energy (eV) Fig. 4. XPS high resolution spectra of the PEO coating prepared under 5 g/L NaAlO2. (a) Mg2p, (b) Al2p and (c) O1s.

substrate and other coatings, indicating that Al doping level could greatly affect the density and hardness of coatings. In addition, the micro-hardness of all Al-doped PEO coatings is significantly higher than that of the undoped one indicating that Al doping could greatly improve the density and hardness of the PEO coating. Friction coefficients of Mg substrate and the coating with a maximum hardness were further evaluated under

130

H. Li et al. / Acta Astronautica 116 (2015) 126–131

0.7

50

8

Mg substrate Al-doped PEO coating

0.6

30 4 20 2 10

Friction coefficient

40 6

Thickness (μm)

Nanohardness (GPa)

Nanohardness Thickness

0.5 0.4 0.3 0.2 0.1

0

0 0

3

5

7

Concentration (g/L) Fig. 5. Micro-hardness and thickness of PEO coatings before and after Al doping.

vacuum low-temperature condition (Fig. 6), demonstrating Al doping can effectively decrease the friction coefficient of Mg substrate. The value stably varied in the range of 0.15– 0.25 for Al-doped PEO coating and slowly increased with time. The hardness of Mg alloy surface was further enhanced due to the formation of MgAl2O4 hard phase. Compared with the friction coefficient of Mg substrate, the value was initially unstable and then stabilized at 0.52 during the later stage. Where, the corresponding value of Al-doped coating is averagely 0.21 and that of Mg substrate at 0.51. Compared with Mg substrate, Mg alloy with Aldoped PEO coating possessed the lower friction coefficient and the higher hardness. Hence, the PEO coating by Al doping can effectively improve wear resistance of Mg alloy under vacuum low-temperature. Due to the absence of the gas medium in vacuum, the metal surface difficultly generates adsorption protective films that declines the yield and shear strength of the metal; at low-temperature, the hardness of metal was further increased that causes the enhancement of the adhesion between sample and counterpart. As shown in Fig. 7a, furrows and scratches were still the main characteristics of worn surface, yet abrasive particles were observed in the wear tracks of Mg substrate. In vacuum, abrasive particles deviate uneasily from wear cracks resulting in the formation of abrasive wear. Moreover, the delamination was observed in Fig. 7b, and this wear demonstrated that a transition from mild wear to severe wear did not occur, which depends on one critical surface temperature [37]. The sample in vacuum mostly absorbed the friction heat that caused the temperature of the contact area to quickly rise, but low-temperature reduced the warming amplitude. Hence, the wear mechanism of Mg substrate is mainly abrasive wear and delamination. Micro-porous on PEO coatings could promote plastic deformation to prevent the expansion of micro-cracks during the sliding process. Some flake smooth areas and Fe element are clearly found in Fig. 8. Due to the concaveconvex structure of the coating, the cutting action and plastic deformation were formed and expanded gradually with the increase of wear time. With respect to the reduction of the coating plasticity, the hardness of abrasive particles in concave spots was increased at low-

0.0 0

300

600

900

1200

1500

1800

Time (s) Fig. 6. Friction coefficient curves of Mg substrate and the PEO coating prepared in the electrolyte containing 5 g/L NaAlO2 under vacuum lowtemperature.

Fig. 7. Low (a) and high (b) magnification SEM images of wear tracks on the magnesium substrate at vacuum low-temperature.

temperature. The strengthened particles participating in the wear process to further enhance the abrasive wear. In addition, Fe element might derive from the oxidizing

H. Li et al. / Acta Astronautica 116 (2015) 126–131

131

Acknowledgments Financial support from Space Debris Program (No. K0202210), National Natural Science Foundation of China (Nos. 51078101 and 51173033), Fundamental Research Funds for the Central Universities (Nos. HIT.BRETIII.2012224 and 201312), Foundation of Qinhuangdao Science and Technology Board in China (No. 201302A036). References

Fig. 8. SEM image (a) of wear tracks on the Al-doped PEO coating at vacuum low-temperature and the graphic of EDS analysis (b).

reaction between coating and counterpart, indicated that the adhesion wear also simultaneously took place in the sliding process. Therefore, wear mechanism of Al-doped PEO coating mainly involves in abrasion and adhesion under vacuum low-temperature. And the serious destructive wear did not appear at the later period, indicating that the PEO technique with Al doping can significantly improve the anti-wear performance of magnesium alloy.

4. Conclusions In this work, hardness and friction coefficient tests proved that the PEO coating by Al doping can efficiently improve anti-wear ability of AZ31 magnesium alloy under vacuum low-temperature. The coating was mainly composed of MgO and MgAl2O4. When Al doping level was 5 g/L, the microhardness reached the maximum (6.2 GPa) and friction coefficient was 0.21, that far better than Mg substrate. Al-doped PEO coating did not emerge severe destructive wear under vacuum low-temperature. Wear mechanism of Mg substrate was abrasive and delamination wear, and that of Al-doped coating was adhesion and abrasion.

[1] J.C. Mandeville, J.M. Perrin, L. Vidal, Acta Astronaut. 81 (2012) 532–544. [2] M. Selvi, S. Devaraju, M.R. Vengatesan, J.S. Go, M. Kumar, M. Alagar, RSC Adv. 4 (2014) 8238–8244. [3] C.G. Dunkle, M. Aggleton, J. Glassman, P. Taborek, Tribol. Int. 44 (2011) 1819–1826. [4] X.F. Liu, J.B. Pu, L.P. Wang, Q.J. Xue, J. Mater. Chem. A 1 (2013) 3797–3809. [5] R.L. Fusaro, NASA/TM, 6 (2001) 25-38. [6] T. Prater, Acta Astronaut. 93 (2014) 366–373. [7] Z.W. Wang, Q. Li, Z.X. She, F.N. Chen, L.Q. Li, J. Mater. Chem. 22 (2012) 4097–4105. [8] A. Abdal-hay, M. Dewidar, J. Lim, J.K. Lim, Ceram. Int. 40 (2014) 2237–2247. [9] Y.G. Ko, E.S. Lee, D.H. Shin, J. Alloy. Compd. 586 (2014) S357–S361. [10] H.T. Gao, M. Zhang, X. Yang, P. Huang, K.W. Xu, Appl. Surf. Sci. 314 (2014) 447–452. [11] W. Yang, P.L. Ke, Y. Fang, H. Zheng, A.Y. Wang, Appl. Surf. Sci. 270 (2013) 519–525. [12] S.Y. Wang, Y.P. Xia, L. Liu, N.C. Si, Ceram. Int. 40 (2014) 93–99. [13] O. Tazegul, F. Muhaffel, O. Meydanoglu, M. Baydogan, E.S. Kayali, H. Cimenoglu, Surf. Coat. Technol. 258 (2014) 168–173. [14] J.Y. Xu, B.L. Zou, S.M. Zhao, Y. Hui, W.Z. Huang, X. Zhou, et al., Ceram. Int. 40 (2014) 15537–15544. [15] Y. Song, D. Shan, E. Han, Electrochim. Acta 53 (2008) 2135–2143. [16] D. Sreekanth, N. Rameshbabu, K. Venkateswarlu, Ceram. Int. 38 (2012) 4607–4615. [17] L.Y. Niu, S.H. Chang, X. Tong, G.Y. Li, Z.M. Shi, J. Alloy. Compd. 617 (2014) 214–218. [18] Y.P. Liu, D.F. Zhang, C.G. Chen, J.G. Zhang, L.B. Cui, Appl. Surf. Sci. 257 (2011) 7579–7585. [19] A. Abdal-hay, N.A.M. Barakat, J.K. Lim, Ceram. Int. 39 (2013) 183–195. [20] B.L. Yu, J.Y. Uan, Scr. Mater. 54 (2006) 1253–1257. [21] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, S. Subramanian, Ceram. Int. 39 (2013) 801–812. [22] L. Wang, L. Chen, Z.C. Yan, H.L. Wang, J.Z. Peng, J. Alloy. Compd. 493 (2010) 445–452. [23] L.Q. Wang, J.S. Zhou, J. Liang, J.M. Chen, Appl. Surf. Sci. 280 (2013) 151–155. [24] P.B. Su, X.H. Wu, Z.H. Jiang, Mater. Lett. 62 (2008) 3124–3126. [25] P.B. Su, X.H. Wu, Y. Guo, Z.H. Jiang, J. Alloy. Compd. 475 (2009) 773–777. [26] X.H. Wu, P.B. Su, Z.H. Jiang, S. Meng, ACS Appl. Mater. Int. 2 (2010) 808–812. [27] X.J. Li, X.Y. Liu, B.L. Luan, Appl. Surf. Sci. 257 (2011) 9135–9141. [28] J.Q. Yang, Y. Liu, Z.Y. Ye, D.Z. Yang, S.Y. He, Mater. Des. 32 (2011) 808–814. [29] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227–232. [30] S. Durdu, M. Usta, Appl. Surf. Sci. 261 (2012) 774–782. [31] L.R. Krishna, G. Poshal, A. Jyothirmayi, G. Sundararajan, J. Alloy. Compd. 578 (2013) 355–361. [32] Q.P. Liu, Electrochim. Acta 129 (2014) 459–462. [33] N. Reddy, P. Bera, V.R. Reddy, N. Sridhara, A. Dey, C. Anandan, A. K. Sharma, Ceram. Int. 40 (2014) 11099–11107. [34] L. Wang, T. Shinohara, B.P. Zhang, Appl. Surf. Sci. 256 (2010) 5807–5812. [35] P. Motamedi, K. Cadien, Appl. Surf. Sci. 315 (2014) 104–109. [36] H.C. Meng, K.C. Ludema, Wear 181-3 (1995) 443–457. [37] H. Chen, A.T. Alpas, Wear 246 (2000) 106–116.