SHI Yuanji, WANG Yanyan, CHENG Cheng, GUO Xunzhong,TENG Bingyan, YU Zhaopeng, LI Junwan, LI Wei

(1. School of Mechanical Engineering, Nanjing Vocational University of Industry Technology, Nanjing 210046, China;2. College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China;3. School of Automotive Engineering,Changshu Institute of Technology, Suzhou 215500, China; 4. School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China)

Abstract: Plasma electrolytic oxidation (PEO) coatings were formed on 7075 aluminum alloy in silicateborate based electrolyte with different duty cycles. The physical and chemical properties of the PEO coatings were thoroughly investigated. The wearing and corrosion properties of the coatings were evaluated by wearing experiments and potentiodynamic polarization tests, respectively. The results showed that the micro-hardness of the coatings first increased and then decreased with the increasing duty cycle. As a results, the wearing resistance of the coatings first increased and then decreased with the increasing duty cycle. Composition analysis proved that the coatings were mainly composed of α-Al2O3 and γ-Al2O3. The presence of wear scars on the worn surface morphology demonstrates that the three-body rolling was the main wear mechanism for coated specimen. The corrosion study showed that the coating formed in the mixed electrolyte with duty cycle of 80%showed the most superior corrosion resistance.

Key words: 7075 aluminum alloy; plasma electrolytic oxidation (PEO); wearing; corrosion; friction coefficient

1 Introduction

With the development of our society, aluminum and its alloys are widely used in building, automotive,machining and aerospace industry because of their excellent properties,i e, high strength, low density,non-magnetic properties and good formability[1-3].Nevertheless, their weakness, that is, low surface hardness and poor corrosion resistance, have extremely limited their wide applications in many industry territories. Such as, the inferior hardness led to the low load bearing capacity and low abrasion resistance,the relatively high ductility and reactivity led to the high adhesion tendency[2,4]. Therefore, it is an urgent problem that needs to be solved by surface treatment.

A lot of attempts have been tried to repress the corrosion from the out side environment. For example,Wang pointed out that inorganic corrosion inhibitors can inhibit the corrosion of the oxide films on the 2024 aluminum alloy[5]. Sui found that TiC particles increase the tensile and yield strength, but decrease the elongation at the same time of aluminum[6]. Among these treatment method, plasma electrolytic oxidation(PEO), which is also called micro-arc oxidation (MAO)

[7], microarc discharge oxidation (MDO)[8], anodic spark deposition (ASD)[9], seems to be the most promising surface treatment technology in these years. It is a relatively new method to produce protective ceramic coatings on the valve metals (aluminum, magnesium,titanium and their alloys)[10-12]. After PEO treatment,the coatings were of intuitive thickness, superior microhardness, good corrosion resistance and great adhesion to the substrate. Compared with the plasma sprayed ceramic coatings, the PEO coatings have exhibited more superior mechanical properties, such as friction and wearing properties. On the other hand, the coating are also of great corrosion resistance due to the ceramic coatings.

To our knowledge, the PEO process is carried out at high voltages than that of the breakdown voltage of the gas layer enshrouding the anode. As the substrate is linked to positive pole of the rectifier as anode, the gas layer is composed of oxygen. When the dielectric gas layer completely covers the anode surface, electrical resistance of the electrochemical circuit surges and the process continues providing that the applied voltage defeats the breakdown voltage of the gas layer[13].Applying such voltages leads to formation of electrical discharges via which electrical current could pass the gas layer. Other applications of PEO process can be seen elsewhere[14,15].

The properties of PEO coatings can be affected by many factors during the MAO treatment conditions,such as electrolyte temperature[16], post-processing[17],electrical parameters[18,19]. Nevertheless, duty cycle is not a common parameter that has been investigated.

In this paper, the effect of duty cycle on the coating morphology, wearing and corrosion behavior of the PEO coated aluminum alloy were thoroughly investigated by various measurements. Morphology of the coatings was observed by SEM and stereoscopic microscopy. The wearing behavior of PEO coatings was investigated under different conditions. The corrosion properties of PEO coatings were evaluated by potentiodynamic polarization test.

2 Experimental

2.1 Materials and substrate

The substrate material used in this study is 7075 aluminum alloy. Its composition is showed in Table 1.Rectangular specimens with dimension of 30 mm × 25 mm × 2 mm are used as substrate in the experiment.Prior to PEO treatment, the surface of these samples were ground with 500, 800, 1 000 grit alumina waterproof abrasive paper and ultrasonically immersed in ethanol for oil removing, then the specimens were cleaned in distilled water and dried in hot air prior to PEO treatment.

2.2 PEO treatment

In this study, the electrolyte is composed of Na2SiO3(15 g/L), Na2B4O7(8 g/L), KOH (3 g/L) and glycerin (2 g/L) in distilled water. The prepared sample as the positive pole were immersed in the electrolyte,and a bath with electrolyte made out of stainless steel was used as the negative pole. During the PEO treatment, a large number of bubbles were produced on the sample surface, meanwhile a thin layer of metallic oxide was quickly generated on the surface of the samples. With the treatment time increased, numerous tiny sparks appeared on the surface of the samples. In the end, the tiny sparks gathered together, and became the moving micro-arc. The parameters used in this study is showed in Table 2.

Table 1 The nominal chemical compositions of 7075 aluminum alloy

Table 2 Parameters used in this study

2.3 Micro-hardness and wearing test

The Vickers micro-hardness tests was carried out by a load of 2 N and time of 15 s. Ten measurements were done and the average value was calculated.Wear test for uncoated and PEO coatings were performed on a WTM-2E ball-on-disk tribometer with a rotational speed of 336 rev/min. The measurement of friction coefficient was done under load of 2 and 4 N, respectively. The linear pin speed is 0.09 m/s. The coatings and uncoated specimen were served as the disc, and the counterpart was Si3N4ceramic ball (8 mm in diameter, 1 550 HV in hardness). The abrasion loss was measured after 0.5 h friction measurement with an electronic direct reading balance (LJBROR L-200,readability 0.01 mg).

2.4 Potentiodynamic polarization test

To evaluate the corrosion resistance of 7075 aluminum alloy and PEO coated samples, the electrochemical experiment was carried out. The electrochemical experiment was conducted with a CorrTest AC potentiostat/frequency response analyzer(Electrochemical workstation, CS350, Wuhan, China)system, which enables the scan to be automatically controlled by computer. The electrochemical system used in this study was three-electrode-cell. It was composed of a weight-saving platinum electrode as auxiliary electrode, a saturated calomel electrode(SCE) as reference electrode and the specimens as working electrode in the testing electrolyte. The testing electrolyte solution used in this test was 3.5 wt% NaCl.To make the working electrode, the samples were first cut into small pieces (about 1 cm2) and then they were put into the Teflon plate with the prepared resin. For the implementation of the open circuit potentials (OCPs)experiment, the specimens were drenched in the testing solution for about 30 min before electrochemical test to get a relatively stable OCP. In order to put the potentiodynamic polarization test into effect,we prepared some spare specimens and immersed them in 3.5 wt% NaCl aqueous solution for 30 min approximately. The potentiodynamic polarization test was conducted at the applied scanning rate of 1.0 mV/s. And the scanning region is -1.2--0.2 V with respect to the OCP. The software CorShow was used to deal with the data of the potentiodynamic polarization test.Based on the reproducibility and reliability, all tests were repeated at least three times.

2.5 Characterization of coatings

Surface and cross-sectional morphology of coatings were observed by scanning electron microscopy (SEM, JSM-6360LA). To clearly investigate the cross-section view of the coatings, some of the treated samples were cross-sectioned, mounted in resin and polished by standard metallographic abrasive paper. All the coatings were sputtered with a thin layer of Au prior to SEM inspection. To further analyze the structure of coatings, and stereoscopic microscopy(VH-S30B, KEYENCE) was employed to study the surface profile of coatings at the magnification of 5 000. Roughness of coatings was measured by means of a surface profilometer. The phase composition of coatings and bare alloy were studied by X-ray diffraction (XRD, Digaku D/max-2500) using Cu Kα radiation between 2θvalues of 20° and 90° with a step length of 0.02° at a scanning rate of 1 °/min. The X-ray generator settings were 45 kV and 40 mA, respectively.The obtained data was analyzed with MDI Jade 5.0 software and X-ray photoelectron spectroscopy (XPS)with an Al Kα radiation (λ=1 486.6 eV). The XPS analysis was conducted after the coating surface was etched for 1 min by argon-ion-beam to reduce the carbon contamination. All energy value were corrected according to the adventitious C 1ssignal, which was set at 284.6 eV, as the reference.

Fig.1 The effect of duty cycle on (a) microhardness and (b) roughness of PEO coatings

3 Results and discussion

3.1 Microhardness and roughness

Fig.1 shows the influence of duty cycle on the microhardness and roughness of PEO coatings formed in silicate-borate based electrolyte. Obviously, it can be found that the microhardness of the MAO coatings first increased and then decreased with the increasing duty cycle (Fig.1(a)). That is, the microhardness of the coating increased with the duty cycle when the duty cycle is smaller than 40%. When the duty cycle is bigger than 40%, the microhardness of the coating decreased with the increasing duty cycle. The decrease of micro-hardness is owing to the change in the composition of the coatings[20]. On the other hand, the roughness of the coatings continually increased with the increasing duty cycle (Fig.1(b)). At the duty cycle of 80%, the coating was of the biggest roughness value of 1.869 μm.

3.2 Morphology

Fig.2 shows the surface morphology of PEO coatings fabricated in silicate-borate based electrolyte with the duty cycle of 20%, 40%, 60% and 80%.Obviously, the coatings were all characterized by micropores (Fig.2). According to the previous reports[21,22], it can be known that the pores were formed by the molten oxide and gas bubbles which emitted out of the arc discharge channels during the PEO process.Besides, at the duty cycle of 20%, there is numerous small pores on the surface (Fig.2(a)). Nevertheless,after the duty cycle increased to 80%, some of the pores disappeared. That is, the pores are decreased on the numbers with the increasing duty cycle. It is suggested that the pores were connected together with the duty cycle increased.

To further investigate the surface morphology of the coatings, the stereoscopic microscopy was employed to study the surface structure of coatings intuitively. The 3D image of PEO coatings are shown in Fig.3. The value on the left top in the figure reflects the surface roughness of the coatings on some extent,that is, the bigger the value is, the rougher the coating is. It is showed that the value increased with the increment of the duty cycle. On the other hand, the surface roughness which was measured by the surface profilometer increased with increasing duty cycle. The results are matched well with the results measured by the surface profilometer.

Fig.2 Surface morphology of PEO coatings formed in silicate-borate based electrolyte with the duty cycle of (a) 20%, (b) 40%, (c) 60%, (d)80%

Fig.3 3D image of PEO coatings obtained in silicate-borate based electrolyte with the duty cycle of (a) 20%, (b) 40%, (c) 60%, (d) 80%

The cross-section morphology of PEO coatings formed in the electrolyte with different duty cycle are showed in Fig.4. It is showed that the thickness of the coating increased with the increasing duty cycle.And the coating formed in the electrolyte with the duty cycle of 80% showed the thickest thickness. The thickness of the coatings are 11.3, 15.4, 19.1 and 24.8 μm, respectively.

3.3 Composition analysis

Fig.5 presents the EDS mapping area (Fig.5(a))and element distribution of the Fe3+:TiO2composite film (Fig.5(b)-5(k)). The distribution of element O,Na, Al, Si and K are relatively uniform on the film. It should be pointed out that element B was undetected,because its content is very little.

Fig.6 shows the X-ray diffraction (XRD) patterns of the PEO coatings prepared from the electrolyte with different duty cycle. It is revealed that the coatings formed in the electrolyte were mainly composed ofα-Al2O3andγ-Al2O3. Besides, it should be pointed out that the strong Al peaks (Fig.6(a)) corresponding to the aluminium alloy substrate were detected owing to the porosity of the coatings, and thus the X-rays can easily penetrate through the coatings. It is showed that theα-Al2O3phase increased when duty cycle is smaller than 40%. Nevertheless, with the duty cycle increased (duty cycle increased to 60% and 80%), theα-Al2O3phase slightly decreased (see Fig.6(b)). Theα-Al2O3phase is usually stable and much harder than that ofγ-Al2O3. Thus, with theα-Al2O3phase slightly decreased, the hardness of the coating decreased either.The content ofα-Al2O3phase decreased may be due to the transformation betweenα-Al2O3andγ-Al2O3phase.

The effects of duty cycle on MAO coatings could be estimated by the chemical state of the surface of the coating. Fig.7 shows the XPS analysis of PEO coating formed in silicate-borate based electrolyte with the suty cycle of 40%. Typical XPS spectra are shown in Fig.7(a). It is showed that the peaks originate from Al 2p, Si 2p, C 1s, and O 1s. The high-resolution spectrum of Al 2pis showed in Fig.7(b). It is presented that the single Al 2ppeak at 74.36 eV, typical forα-Al2O3andγ-Al2O3, can be decomposed into two peaks at 74.70 and 74.20 eV.

Fig.5 (a) EDS mapping area and EDS elemental maps of the surface of the PEO coating formed under the duty cycle of 20%; (b)(c) O; (d)(e) Na; (f)(g) Al; (h)(i) Si; (j)(k) K (The circle area in (b), (d), (f), (h) and (j) are corresponding to the picture (c), (e), (g), (i) and (k), respectively)

3.4 Wearing properties

To investigate the wearing properties of PEO coatings formed with different duty cycle, the tribological tests were done to study the wearing property of PEO coatings and the bare alloy.

Fig.6. XRD patterns of (a) uncoated 7075 bare aluminium alloy substrate and PEO coatings formed in the electrolyte with the duty cycle of 20%, 40%, 60% and 80%; (b) small angel analysis

Fig.7 XPS analysis of PEO coating formed in silicate-borate based electrolyte with the duty cycle of 40%: (a) XPS spectra; (b) highresolution spectrum of Al 2p

Fig.8 Results of tribological test against Si3N4 ceramic balls under applied loads of (a) 2 N; (b) 5 N

Fig.8 shows the results of tribological tests against Si3N4ceramic balls for PEO coatings and uncoated 7075 aluminum alloy under the load of 2 and 5 N, respectively. It is showed that the friction coefficients of all the PEO coatings were high (in the range of 0.5-1.0) under the load of 2 N (Fig.8(a)). And the coefficients were less influenced by the duty cycle under the applied loads of 2 N during the tribological tests against Si3N4ceramic balls. The bare alloy had a slightly lower friction coefficient than that of the coatings. On the other hand, under the load of 5 N, the friction coefficients of the bare alloy was of great range over the whole wearing test (Fig.8(b)). The friction coefficients of the coating with the duty cycle of 20%were of fluctuation at the time of 15 min (Fig.8(b)).It is suggested that the coating was worn out at this time. Under the load of 2 N, the wear loss of the coatings with the duty cycle 20%, 40%, 60% and 80%is 1.07, 0.06, 0.12 and 0.21 mg, with the load of 2 N,respectively, which is essentially in good agreement with the relationship between the micro-hardness and duty cycle.

Fig.9 The macrographs of the surface of (a) bare alloy and PEO coatings with the duty cycle of (b) 20%, (c) 40%, (d) 60% and (e) 80% after wearing test under the load of 2 N

From the wearing test, it can be concluded that the coating formed in the electrolyte with the duty cycle of 40% showed the most superior wearing resistance. And the coating formed in the electrolyte with the duty cycle of 20% showed the most inferior wearing resistance.

Fig.9 shows the macrographs of PEO coatings with the duty cycle of 20%, 40% and bare alloy after wearing test under the load of 2 N. Evidently, the bare alloy specimen has a deep scar after wearing test.However, there only exists a shallow scratch on the PEO coating with the duty cycle of 40% (Fig.9(c)).With the duty cycle increasing to 60% and 80%, the trace of the wearing scar becomes deeper (Fig.9(d) and e). Nevertheless, there also exists a very deep scar on the PEO coating surface with the duty cycle of 20%(Fig.9(b)). After comparison, it can be known that the coating produced in the electrolyte with the duty cycle 40% was of the shallowest scar. On the contrary, the coating produced in the electrolyte with the duty cycle 20% was of the deepest scar. These evidence directly indicates that the coating with the duty cycle of 40%was of the most superior wearing resistance.

Fig.10 The micrographs of the surface of (a)(b) bare alloy and PEO coatings with the duty cycle of (c)(d) 20%, (e)(f)40%, (g)(h) 60% and (i)(j) 80% after wearing test under the load of 2 N

The micrographs of the surface after wear test for PEO coated specimens and bare alloy in two different magnifications are shown in Fig.10,respectively. It is obvious that the extent of damage in the uncoated specimen is higher than PEO treated one (Fig.10(a) and (b)). The worn surface of untreated alloy substrate at low magnification show some parallel lines aligned in the sliding direction (Fig.10(a)). At higher magnification, it shows large, deep grooves and scratches with smearing marks (Fig.10(b)). It is showed that there is only some unconspicuous scars on the coating with the duty cycle of 40% (Fig.10(e)and (f)). Nevertheless, with the duty cycle increased to 60% and 80%, the worn scar becomes obvious(Fig.10(g)-10(j)). However, there is a very deep scar on the coating with the duty cycle of 20% (Fig.10(c)). At a higher magnification, it is showed that the coating with the duty cycle of 20% was almost worn out with the friction process (Fig.10(d)). Nevertheless, the coating with the duty cycle of 40% was almost of the same morphology after the wearing process.

3.5 Corrosion properties

Fig.11 Potentiodynamic polarization curves of uncoated bare alloy and PEO coating formed in the electrolyte with the duty cycle of 20%, 40%, 60% and 80%

Table 3 Results of potentiodynamic corrosion test for bare alloy and PEO coated 7075 aluminum alloy formed in silicate-phosphate based electrolyte with the duty cycle of 20%, 40%, 60% and 80%

Fig.11 shows the potentiodynamic polarization test results of bare alloy and PEO coatings formed in the electrolyte with the duty cycle of 20%, 40%, 60%and 80%, respectively. Generally speaking, more noble corrosion potential and more lower corrosion current density indicates more superior corrosion resistance.From Fig.11, it is directly proved that the corrosion resistance of the MAO coatings increased with the increasing applied duty cycle. To quantitatively evaluate the corrosion resistance of the bare alloy and PEO coatings, the curves were fitted and some parameters were obtained,i e,corrosion potential (Ecorr)and corrosion current density (icorr) in Tafel region(±200 mV with respect to the corrosion potential).The parameters are listed in Table 3. In the table, it is found that the theEcorrandicorrof the 7075 aluminum alloy substrate is -784 V (vsSCE) and 0.637 μA/cm2,respectively. After careful observation, it is found the corrosion resistance of the PEO coated aluminum alloy commonly has better corrosion resistance than that of the bare Al alloy. This indeed proved that the PEO treatment is able to improve the corrosion resistance of the Al alloy. With the applied duty cycle increased to 80%, the PEO coating showed the most excellent corrosion resistance with the highestEcorrof -0.611 V (vsSCE) and the lowesticorrof 0.034 μA/cm2. The corrosion resistance of the PEO coatings increased with the increasing duty cycle.

In general, the wearing properties of the PEO coatings were affected by many interactive factors,such as the surface structure, morphology and microhardness of the coatings. On the basis of the morphology, microhardness test and wearing test above, the wearing properties of the PEO coating formed on 7075 aluminum alloy with the duty cycle of 20%, 40%, 60% and 80% for 25 min can be illustrated as follows:

From Fig.2, 6, 8, 9 and 10, there is a combination of abrasive and adhesive wear mechanism. It has been reported that abrasive wear can take place by threebody rolling or by two-body grooving, depending on the test condition, the nature of the abrasive particles,the ball material and the specimen material[23]. In this study, the observation of wear scars indicates that the main mechanism is three-body rolling.

The corrosion behavior of the PEO coatings is affected by many interactive factors, such as porosity,thickness, and chemical/physical composition of the coating[24,25]. It is generally believed that the anodic coatings with less defects, higher thickness and more stable composition in aggressive environments would be beneficial to provide a favorable corrosion protection to the bare alloy substrate[26]. In this study, the corrosion resistance of PEO coated aluminum alloy is obviously better than that of the bare alloy. And with the applied duty cycle increased, the corrosion resistance of the PEO coatings increased, too. From Fig.4, it can be known that the coating with the duty cycle of 80% is of the thickest thickness. During the corrosion process, the aggressive Cl-is hard to infiltrate into the inner parts of the PEO coatings and the alloy substrate. Therefore,the increased duty cycle is beneficial for improving the corrosion resistance of PEO coatings.

4 Conclusions

PEO coatings were formed on 7075 aluminum alloy in silicate-borate based electrolyte with the duty cycle of 20%, 40%, 60% and 80%, respectively. The conclusions can be drawn as follows:

a) The coating formed in the electrolyte with the duty cycle of 80% showed the thickest thickness.However, the coating formed in the electrolyte with the duty cycle of 40% showed most superior microhardness.

b) The content of α-Al2O3of PEO coatings formed in the electrolyte increased with the increment of duty cycle to 40%. However, when the duty cycle increased to 60%, the content of α-Al2O3of PEO coatings formed in the electrolyte decreased. The variation of Al2O3content in PEO coatings matched well with the microhardness test results.

c) The PEO coating formed in the electrolyte with the duty cycle of 40% showed the most superior wearing resistance, which matched well with the microhardness and wearing test results.

d) The corrosion resistance of the PEO coatings increased with the increasing applied duty cycle. The coating with the duty of 80% showed the most superior corrosion resistance.