Effect of magnesium in aluminum alloys on characteristics of microarc oxidation coatings
LIU Yao-hui(刘耀辉), LI Song(李 颂), YU Si-rong(于思荣), ZHU Xian-yong(朱先勇), XU Bai-ming(徐佰明)
Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering,
Jilin University, Changchun 130025, China
Received 28 July 2006; accepted 15 September 2006
Abstract: Microarc oxidation(MAO) coatings were prepared on the surface of aluminum alloys with different contents of magnesium. The morphologies and surface roughness of the coatings were characterized by Confocal laser scanning microscopy(CLSM). Phase and chemical composition of the MAO coatings were analyzed by X-ray diffractometry(XRD) and X-ray photoelectron spectroscopy(XPS). The experimental results show that the coatings formed on different substrates have two-layer morphologies and are mainly composed of Al2O3 and Al-Si-O phases. In addition, the content of Al2O3 increases with increasing the content of magnesium. XPS results prove that magnesium from substrate indeed participates in the MAO process and is incorporated into the coating in the form of MgO. The coating formed on Al-3Mg substrate has the smallest mass loss and the lowest friction coefficient of 0.17-0.19.
Key words: microarc oxidation; magnesium; alloying element; friction; wear; aluminum alloy
1 Introduction
Aluminum alloys are promising structural materials due to their high specific strength and stiffness, and have been used in automobile industries as structural components of internal combustion engines. However, their applications have been restricted because of their poor wear and corrosion resistance. To improve surface properties of aluminium alloy, microarc oxidation(MAO) process has been exploited and received extensive attention due to its ability to obtain high quality ceramic coating with perfect wear and corrosion resistance, high microhardness and good adhesion to substrates[1-6].
It has been found that the substrate materials play a key role in the microarc oxidation process[7-8]. Magnesium is one of the most important alloying elements to improve the mechanical properties of aluminum alloys. The different magnesium contents in aluminum alloy result in the thickness changes of natural oxide coating, which has a principal effect on the microarc oxidation voltage and ultimate nature and composition of the MAO coating[9]. However, it is unknown how the magnesium in aluminum alloys affects the microarc oxidation process and the coating characteristics.
In this study, the effect of Mg on process voltage and characteristics of the MAO coatings formed on aluminum alloys with different contents of magnesium is analyzed. It is expected that the preliminary results can be significant in promoting the further application of MAO process on Al-Mg alloy.
2 Experimental
2.1 Preparation of MAO coatings on Al-Mg alloy
The contents of magnesium in Al-Mg alloy specimens, which were used as substrate materials, were 1.08%, 2.97% and 7.04% (mass fraction), respectively. The as-coated specimens were named as Sample 1, Sample 2 and Sample 3, respectively. Prior to the oxidation process, the substrate specimens were polished successively on finer grades of emery papers up to 1 000 level and cleaned by ethanol. The microarc oxidation treatment was conducted on MAO-20C microarc oxidation equipment. The current density was predefined as 5.0 A/dm2. The MAO process was performed in alkaline electrolyte containing 8 g/L Na2SiO3 and 3 g/L KOH. The frequency and oxidation time were fixed at 500 Hz and 30 min, respectively. The Al-Mg alloy specimen and the wall of the stainless steel container were used as the anode and the cathode, respectively. The bath was water-cooled and its temperature maintained at about 20 ℃. After oxidation process, all coated specimens were cleaned by distilled water and dried by warm air.
2.2 Characterization of MAO coatings
The microstructure and surface roughness of the coatings on Al-Mg alloy substrates were observed and estimated by Confocal Laser Scanning Microscope (Olympus OLS3000, Japan), which uses advanced image processing software and allows the construction of three-dimensional images of the object, topographical maps and quantification of surface topography.
The phase composition of the coating was analyzed by X-ray diffractometry(XRD), using Cu Kα radiation as the excitation source. The chemical composition of the coating was investigated by X-ray photoelectron spectroscopy(XPS), using monochromatic Al Kα radiation as the excitation source. Prior to the analyses, an Ar ion gun was used to etch the surface of the coatings for 10 min to remove surface contamination.
2.3 Friction and wear tests
Friction and wear tests were conducted in air at room temperature in a block-on-ring machine (model MM-200) under a load of 98 N with oil lubrication. The specification of block is 10 mm×10 mm×14 mm. Counter- part ring, 40 mm in outside diameter and 10 mm in thickness, is made of GCr15 bearing steel with quenching hardness of HRC(62±2). The mass loss of friction pairs was measured by analytical balance with an accuracy of ±0.01 mg. Tests were carried out at constant sliding velocity of 0.837 m/s and sliding distance of 3 014 m.
3 Results and discussion
3.1 Effect of magnesium on applied voltage in micro- arc oxidation process
On application of the electric potential, an anodized coating is formed quickly around the anode. The anodized coating on the substrate acts as a barrier to the flow of current, thereby the voltage applied on the anode needs to be increased in order to reach the predefined current density. When the applied voltage exceeds the critical voltage of initial barrier layer, dielectric breakdown occurs and microarc oxidation reaction starts [10-11]. With the increase of the microarc coating thickness, the anodic breakdown resistance of the coating increases, and the applied anodic voltage needs to be increased further to remain the predefined current density constant. Therefore, the applied anodic voltage associates to the transient voltage for the first minute of anodizing and can depict the variation of the breakdown resistance of the coating under the regime of constant current density.
According to the electron avalanche breakdown model proposed by IKONOPISOV[7], the breakdown voltage varies for different substrate metals. Our experimental results are in good agreement with this model. Fig.1 shows the response of voltage applied on the different aluminum-magnesium alloy anodic specimens vs the treatment time.
Fig.1 Voltage—time response during MAO treatment on Al-Mg alloys
It is shown that in all cases, the microarc oxidation process can be divided into two stages according to the applied voltage value. In stage Ⅰ, the voltage increases quickly, which indicates the high coating growth rate. In stage Ⅱ, the voltage increases slowly, which corresponds to a relatively low coating growth rate. Furthermore, Fig.1 also indicates that the higher the content of magnesium in aluminum alloy, the higher the voltage in stage Ⅰ and the lower the voltage in stage Ⅱ. It is an interesting phenomenon and can be explained by the fact that with the increase of magnesium content, more flaws are present in the anodized coating (because magnesium is firstly corroded in the course of anodic polarization as a result of its lower potential compared with aluminum), resulting in more weak spots[12]. Due to the low breakdown voltage at these weak spots, the microarc oxidation coating growth rate increases and is manifested by the higher applied voltage in stage Ⅰ. With the increase of microarc coating thickness, the weak spots decrease, resulting in the buildup of breakdown resistance of their vicinities and the decline of microarc oxidation reaction rate. A relatively low applied voltage is needed to remain the predefined current density in stage Ⅱ. Contrarily, for the anodic substrate with low magnesium content, the initial anodized coating is even and of little flaws. The coating growth rate is low in stage Ⅰ, but the microarc oxidation coating formed in stage Ⅰ is relatively uniform and of high monolithic breakdown resistance. Therefore, the voltage applied on substrate with low magnesium content is higher than that on substrate with high magnesium in stage Ⅱ. The difference in breakdown resistance between different aluminum alloy substrates results in the variety in microstructure of microarc oxidation coating.
3.2 Effect of magnesium on microstructure and surface roughness of microarc oxidation coating
Fig.2 and Fig.3 show the two-dimensional and three-dimensional images of the coating surface morphologies. There are many pores homogeneously distributed in coating regardless of the substrates. More-over, some micro cracks inevitably appear on the coating due to the rapid solidification of the molten Al2O3 in the discharge channel. Fig.2(a) reveals a relatively obvious appearance of repeated sintering and lots of micro pores are sealed by the molten materials. In contrast to this, more unsealed micro pores are found in the coating of Sample 2 and Sample 3(Figs.2(b) and (c)). The profiles of micro granules on the coatings are displayed vividly in Figs.3(a)-(c). Though all the coatings take on the similar surface morphologies, the wave crest and trough of the agglomeration vary depending on the substrate materials, which causes different surface roughness. The results of surface roughness measured by the software attached to the CLSM are presented in Table 1. It is clearly that the surface roughness decreases with increasing the content of magnesium. The change of surface morphologies can be exclusively contributed to the effect of applied voltage. The higher the applied voltage in stage Ⅱ, the higher the energy for the micro pores to sinter together, and the coarser the surface appearance.
Fig.2 Two-dimensional surface CLSM images of MAO coatings: (a) Sample 1; (b) Sample 2; (c) Sample 3
Fig.3 Three-dimensional surface CLSM images of MAO coatings: (a) Sample 1; (b) Sample 2; (c) Sample 3
Table 1 Comparisons of thickness and surface roughness of MAO coatings on aluminum magnesium alloys
Fig.4 shows the cross section CLSM images of the micro oxidation coatings. Although all of the coatings show two-layer morphology (the external porous layer and internal dense layer), the non-uniformity of the coating thickness increases when increasing the content of magnesium to 7%. The thicknesses of inner layer and outer layer become relatively uniform on whole interface formed on Al-1Mg and Al-3Mg, while the coatings formed on Al-7Mg deteriorates evidently the continuum of states, especially the inner layer exhibits discrete state, as well as the number and size of the micro-pore increase. This demonstrates that aluminum alloys containing magnesium in high content make their surface electrochemically heterogeneous and difficult to obtain uniform oxide coating. In addition, although the total thicknesses of coatings on the three kinds of substrates have no crucial difference, the thickness ratio of inner layer to the total layer changes evidently (listed in Table 1). The data on layer thickness represent the average value of 60–80 readings for each sample.
Fig.4 Cross-sectional CLSM images of MAO coatings: (a) Sample 1; (b) Sample 2; (c) Sample 3
Clearly, Sample 2 has the highest thickness ratio of internal layer to entire coating among the three samples. With the increase of magnesium content from 1% to 7%, the ratio first increases and then decreases. This change can be attributed to the effect of applied voltage during microarc oxidation process. Higher voltages both in stage Ⅰ and in stage Ⅱ seem to facilitate the growth of outer porous layer.
3.3 Effect of magnesium on composition of microarc oxidation coating
Fig.5 illustrates the XRD patterns of the as-coated MAO samples. The XRD results indicate that all the coatings are mainly composed of α-Al2O3, γ-Al2O3 and Al-Si-O phases. In addition, with increasing the content of magnesium, the contents of α-Al2O3 and γ-Al2O3 increase and the amorphous Al-Si-O phase transforms into crystalline Al-Si-O phase. The increase of Al2O3 content can increase the microhardness of MAO coating [13], and the formation of amorphous Al-Si-O phase provides the ability of discrete surface plastic deforma-tion. These changes will improve the friction and wear property of MAO coating.
Fig.5 XRD patterns of MAO coatings on Al-Mg alloys: (a) Sample 1; (b) Sample 2; (c) Sample 3
Unexpectedly, there are not any peak associated with magnesium in the XRD patterns. In order to identify the presence of magnesium and its chemistry state in the oxide coatings, the XPS analysis of the oxide coatings for Mg 2p is investigated. The Mg 2p spectra (Fig.6) for the three samples are similar and show binding energies associated to MgO (Mg, Eb=50.9 eV), indicating that magnesium element from the substrate is indeed incorporated into the oxide coating in the form of MgO.
Fig.6 XPS spectra of Mg 2p for MAO coatings on Al-Mg alloys
3.4 Effect of magnesium on friction and wear resistance of microarc oxidation coating
The wear resistance of the coatings is evaluated by block-on-ring sliding wear test under the condition of oil lubrication.
Fig.7 and Fig.8 demonstrate the similar change trends that friction coefficient and mass loss of the as-coated coatings decrease slightly with increasing magnesium content up to 3% and then increase rapidly with increasing magnesium content up to 7%. Sample 2 has the lowest friction coefficient (0.17-0.19) and smallest mass loss. It is believed that the high proportion of inner dense layer and the relatively uniform, compact and intact microstructure are beneficial for the improve-ment of the wear resistance of the oxide coatings formed on Al alloy[11, 14]. At the same time, the porous surface provides a superior ability to deposit lubricant oil. That is to say, the difference in friction and wear resistance depends on the microstructure and the surface condition of the coatings. Sample 2 provides the best friction and wear performance due to a combination of excellent integrity of Al2O3 coating, the maximal internal layer ratio and feasible surface roughness.
Fig.7 Mass loss of MAO coatings on Al-Mg alloys
Fig. 8 Friction coefficient of MAO coatings on Al-Mg alloys
4 Conclusions
1) Microarc oxidation coatings are successfully produced on different aluminum-magnesium alloys. The applied voltage increases in stage Ⅰ and decreases in stage Ⅱ with increasing the content of magnesium in aluminum alloy. As a result, the coating becomes relatively coarse.
2) With the increase of magnesium content from 1% to 7%, the thickness ratio of internal layer to entire coating first increases and then decreases. The oxide coatings formed on different substrates consist mainly of α-Al2O3, γ-Al2O3 and Al-Si-O phases. The contents of α-Al2O3 and γ-Al2O3 increase with increasing the magnesium content in substrate alloys. XPS analysis indicates that magnesium element from the substrate is indeed incorporated into the oxide coating in the form of MgO.
3) The results of friction and wear resistance tests demonstrate that friction coefficient and mass loss of the as-coated coatings decrease slightly with increasing magnesium content up to 3% and then increase with increasing magnesium content up to 7%. The coating formed on Al-3Mg has the lowest friction coefficient (0.17-0.19) and smallest mass loss.
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(Edited by YANG Bing)
Foundation item: Projects(20050506; 20040315-1; 403034) supported by the Science and Technology Development Program of Jilin Province and the Graduate Innovation Lab of Jilin University, China
Corresponding author: LI Song; Tel: +86-431-5095862; E-mail: lisong@email.jlu.edu.cn
