Thickness effects on corrosion and wear resistance properties of  micro-arc discharge oxide coatings on AZ91D magnesium alloys

HUANG Wei-jiu(黄伟九), LIU Ming(刘 明), LI Zhao-feng(李兆峰), ZENG Rong-chang(曾荣昌)

School of Materials Science and Engineering, Chongqing Institute of Technology, Chongqing 400050, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The microarc oxidation coatings with difference thickness were synthesized on AZ91D magnesium alloy. The microstructure and phase structure of the coatings were analyzed using SEM and XRD, the tribological properties and corrosion resistance behaviour of the coatings were also investigated. The results show that the coating contains two layers, a porous outer layer and relatively dense inner layer. The microhardness of the MAO coatings is four to six times higher than that of the magnesium alloy substrate. The MAO coatings have much better wear-resistance and corrosion resistance abilities than those of magnesium alloy substrate, but possess higher friction coefficient. The results further indicate that there is an optimization thickness for corrosion and wear resistance.

Key words: magnesium alloy; micro-arc oxidization; wear; corrosion; ceramic coating

1 Introduction

Magnesium is intrinsically highly reactive and its alloys usually have relatively poor corrosion and wear resistance, which is actually one of the main obstacles of the application of magnesium alloys in aggressive and oxidizing environments. Therefore, some sort of surface treatment or coating should be applied on magnesium alloys in order to meet the requirement for practical applications. Magnesium has a high ratio between its strength and mass with low density that is only 2/3 of aluminium and 1/4 of iron. Magnesium also has high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics and good machinability[1]. These properties and abundance on the earth make magnesium has many applications, including automobile and computer parts, aerospace components, mobile phones, sporting goods, handheld tools and household equipment.

Unfortunately, magnesium has a number of undesirable properties, including poor corrosion and wear resistance, poor creep resistance and high chemical reactivity, which hinder its widespread use in many applications. Therefore, many surface treatment techniques, such as electrochemical plating, conversion coating, gas-phase deposition and anodizing, were developed for preventing the corrosion and increasing the anti-wear behavior of Mg alloys[2]. Microarc oxidation (MAO), also called plasma electrolytic oxidation (PEO), is a novel technique developed to produce hard ceramic coatings on valve metals, such as Al, Ti and Mg, and their alloys[3-6], which was characterized by high temperature of 10³-10⁴ K and high local pressure of 10²-10³ MPa in discharge channels. As an example, the dense and hard microarc oxidation coating formed on Mg alloy contributed greatly to improve the wear and corrosion resistance of Mg alloy[7-8].

In this work, the MAO was applied to synthesize ceramic coatings on the surface of AZ91D Mg alloy to improve their corrosion resistance and wear resistance. Phase composition of the coatings was performed by X-ray diffraction (XRD), and the effects of thickness on the tribological properties and the corrosion resistance of the coatings were evaluated.

2 Experimental

The AZ91D alloy used in this study was a metallic mould casting, produced with gas shielding. The chemical compositions were as follows: Al 9.1, Zn 0.85,Mn 0.27, Mg balance(mass fraction, %). MAO coatings were prepared with 20 kW DC pulse microarc oxidation equipment. An aqueous solution was used as the electrolyte mainly containing Na₂SiO₃ and some other proprietary substances to adjust its alkalinity and increase electric conductivity. The solution temperature was controlled below 40 ℃ during the oxidation. Three treatment oxides times were chosen to produce coatings  with different thickness.

The microstructure of microarc oxidation specimen was observed with JSM-6460LV scanning electron microscope (SEM), and the composition analysis was carried out using energy dispersive spectrum (EDS) and SEM. The phase composition of coating was analyzed by X-ray diffraction (XRD, BDX3300). The outer porous layer of the microarc oxidation coatings was removed by abrasion against SiC paper. Then, the microhardness of the coatings was measured on HVS-1000 digital Vicker’s microhardness meter at load of 0.25 N for 10 s. The final microhardness value quoted was the average of 10 replicate measurements. The thickness of the microarc oxidation coatings was measured with a MINITEST 1100 microprocessor coating thickness gauge.

The tribological properties of the microarc oxidation coatings sliding against GCr15 ring in a ring-on-block configuration were evaluated on a MRH-3 test rig. The speed of the ring was 0.5 m/s. The range of loads was 10-50 N. A computer connected to the tester recorded the friction coefficient curves. The wear results were taken from mass losses measured before and after the sliding tests to the nearest 0.1 mg using an analytical balance. The corrosion properties were studied in 3% NaCl solution through electrochemical polarization testing, which were carried out in a closed three-electrode cell with a sweep rate of 1 mV/s using saturated calomel electrode (SCE) and 273A electrochemical measuring system.

3 Results and discussion

3.1 SEM observation, EDS and XRD analysis

Fig.1 shows the cross-section morphologies of the coating. It can be seen from Fig.1 that the coating contains two layers, a porous outer layer and the relatively dense inner layer. EDS analysis shows a considerable increase in oxygen and silicon content in coating, suggesting the coating should be a combination of magnesium oxides and complex Mg–Al–Si–O compounds. Fig.2 shows the XRD patterns of the oxide coatings. It can be seen from Fig.2 that the both coatings exhibit the XRD peaks assigned to MgO, Mg₂SiO₄, Mg₃Al₂(SiO₄)₃, MgSiO₃, MgAl₂O₄ and the diffraction peaks assigned to the Mg substrate as well.

Fig.1 Morphology of cross-section of coating

Fig.2 XRD pattern of MAO coating

3.2 Coating thickness and microhardness

Table 1 lists the results of the dense layer thickness and average microhardness of coating under different oxidation treatment time. The microhardness of the Mg alloy substrate is also listed in Table 1. It is clear that the microhardness of the MAO coatings is four to six times than that of the magnesium alloy substrate. Such a considerable increase in the microhardness as compared with the substrate is attributed to the formation of the ceramic oxidation coating.

Table 1 Coating thickness and microhardness

3.3 Tribological tests

The curves of the relationship between friction and sliding time for the magnesium alloy substrate and the microarc oxidation coatings against GCr15 steel ball under dry friction condition are shown in Fig.3. The magnesium alloy substrate records a friction coefficient of 0.25–0.38 under dry friction condition, while the three microarc oxidation coatings with different thicknesses under the same test conditions register much larger friction coefficients around 0.85–0.95. Moreover, no significant difference in the friction coefficients is observed for the three coatings, which indicates that the coating thickness has little effect on the friction behavior of the MAO coatings.

Fig.3 Change cures of friction coefficient with sliding time

Fig.4 shows the change curves of wear amount with sliding time of the magnesium alloy substrate and the microarc oxidation coatings. From Fig.4, the MAO coatings have much better wear-resistance than the magnesium alloy substrate. The curves also show that the MAO coating with different thicknesses have different running-in periods and running-in wear mass loss, the wear amount of MAO-1 specimen linearly increases with the increase of sliding time, and the trends of wear amount for MAO-2 and MAO-3 specimen are similar. At the beginning stage, the wear amount increases with the increase of the sliding time, then the wear amount remains relatively constant in the remanent sliding time. However, the wear amount of MAO-3 is larger than that of MAO-2, the reason may be that the wear mechanism  changes from adhesive and fatigue wear to abrasive wear as well as adhesive and fatigue, because many brown wear particles (probably oxidized steel) are produced on the sliding path during the sliding test.

Fig.4 Change curves of wear amount with sliding time

3.4 Corrosion tests

Typical polarization curves of the four kinds of specimen in 3% NaCl solution are shown in Fig.5. It can be seen from Fig.5 that the open-circuit potential (OCP) of AZ91D magnesium alloy is about –1.535 V (SCE). For the specimen MAO-1, MAO-2 and MAO-3, the OCP increases to -1.254 V, –1.094 V and –1.132 V(SCE), respectively. Apparently, the OCP of the MAO specimen is higher than that of naked magnesium alloy. On the other hand, the MAO specimens show lower corrosion current than the naked magnesium alloy. All these results indicate that the microarc oxidation coating remarkably increases the corrosion resistance ability of magnesium alloy. However, after compared with each other, it is found that an obvious passivation occurs in the anodic branch, and the passivation range reaches 150 mV for the MAO coatings of intermediate thickness coating, moreover, the MAO coatings of intermediate thickness coating shows relatively good corrosion resistance relative to the thicker and thinner coatings, which indicates that there is an optimization thickness for corrosion resistance, as the thickness of ceramic layer is more or less than the best thickness, the corrosion resistance ability decreases.

Fig.5 Potentiodynamic polarization curves of different specimens

4 Conclusions

1) The MAO coatings contain two layers, a porous outer layer and the relatively dense inner layer, which mainly compose MgO, Mg₂SiO₄, Mg₃Al₂(SiO₄)₃, MgSiO₃ and MgAl₂O_4. The microhardness of coatings is four to six times than that of the magnesium alloy substrate.

2) The MAO coatings have much better wear-resistance and corrosion resistance ability than those of the magnesium alloy substrate, but possess higher friction coefficient.

3) The coating thickness has no significant effect on the friction coefficients, but there is an optimization thickness for corrosion and wear resistance.

References

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(Edited by LI Yan-hong)

Foundation item: Project(2005BB4079) supported by the Chongqing Natural Science Foundation, China

Corresponding author: HUANG Wei-jiu; Tel: +86-23-68667300; E-mail: huangweijiu@cqit.edu.cn