Effects of different current densities on properties of MAO coatings embedded with and without α -Al 2 O 3 nanoadditives

Introduction

Aluminium oxide coating is very potentially effective in developing hard, wear resistant surfaces. For decades, many techniques have been widely used to deposit aluminium oxide coatings.1^–4 Among them, microarc oxidation (MAO) treatments have recently been studied as a rapid and effective means to provide modified surfaces on light alloy materials, particularly aluminium alloys.5^–9 It is a complicated process combining concurrent partial process of oxide film formation, dielectric breakdown, dissolution of pre-exiting film and anodic gas evolution. The probability of domination for any of these partial processes in the overall process depends on the deposition parameters such as constituents and concentration of the electrolyte, formation time as well as on the applied current density. High quality coatings can be formed by suitable selection of deposition parameters.10^–12

Generally, current density is one of the most important parameters affecting the qualities of prepared coatings and much research has been carried out on the effect of current density. However, little research on the effect of different current densities on the properties of ceramic coatings prepared in electrolyte with and without nanoadditive has been performed. Yang and Liu13 fabricated alumina coatings on aluminium alloys by adding SiC nanoparticles into electrolyte during the MAO process and mainly investigated the effects of current density on the microstructure and the corrosion resistance of alumina coatings embedded with SiC nanoparticles.

In the work reported here, alumina ceramic coatings have been synthesised on 6063 aluminium alloy by MAO technique in NaAlO₂ electrolyte with and without α-Al₂O₃ nanoadditive. Effects of different current densities on the structural and mechanical properties of ceramic coatings prepared in NaAlO₂ electrolyte with and without α-Al₂O₃ nanoadditive have been thoroughly studied in this paper and the enhancement mechanism brought about by α-Al₂O₃ nanoadditive embedding is also discussed.

Experimental

Aluminium samples of 6063 alloy with the size of 30×25×3 mm were used as the substrate material. An aqueous electrolyte was prepared with the distilled water and commercial chemical pure grade NaAlO₂ and concentration of the solution was 15 g L⁻¹. Nanoadditive of α-Al₂O₃ with sizes between 20 and 50 nm was dispersed evenly in the solution, and the concentration of the nanoadditive was 0·5 g L⁻¹. Microarc oxidation treatment device consisted of a high power supply unit, a stainless steel container that also served as the counter electrode and a cooling system; the specimens were served as the anode. Before coating deposition, the specimens were polished with abrasive paper and degreased with acetone followed by rinsing with distilled water. The electrolyte temperature was <50°C. During the MAO process, the current density was in a range of 15–20 A dm⁻² and the electrolyte was agitated with a mechanical stirrer. Time of coating formation is 60 min. After the treatment, the samples were rinsed in distilled water and dried in air.

The crystallographic characteristics of the coatings were investigated using a Thermo ARL X’ TRA X-ray diffraction (Cu K_α radiation) with the step size of 0·04°. The X-ray generator settings were 45 kV and 40 mA respectively. The surface morphology of the coatings was characterised by a Hitachi S-4700 scanning electron microscope. The coating hardness was evaluated using an HMV-IT microhardness tester with the Vickers indenter under a load of 200 g. The adhesion of the coatings to substrate was carried out on a conventional WS-2005 scratch tester at a constant linear velocity of 4 mm min⁻¹ and a loading rate of 100 N min⁻¹. The corresponding critical load represented the adhesion of the coatings to substrate. The tribological properties of the coatings were performed on a WTM-2E ball-on-disk tribometer with a rotational speed of 336 rev min⁻¹. The coating served as the disc, and the counterpart was a Si₃N₄ ceramic ball (4 mm in diameter, 1550 HV in hardness). The abrasion loss was measured using an electronic direct reading balance (LJBROR L-200, readability 0·01 mg) after 1 h friction measurement.

Results and discussion

Phase and structure analysis

Figures 1 and 2 show XRD patterns of the ceramic coatings prepared under different current densities in NaAlO₂ electrolyte without and with α-Al₂O₃ nanoadditive. For Fig. 1, it can be concluded that the ceramic coatings consist mainly of α-Al₂O₃ and γ-Al₂O₃ in addition to some diffraction peaks of 6063 alloy. With increasing current density from 15 to 20 A dm⁻², no obvious change is found, which indicates that the current density has little influence on the crystallographic properties of the prepared coatings. It is quite different from the previous work of ceramic coatings prepared under different current densities in Na₂B₄O₇ electrolyte.14 Figure 2 shows similar changes to Fig. 1. The peak intensity of α-Al₂O₃ phase increased little with the addition of α-Al₂O₃ nanopowder, which indicates that the ceramic coating prepared in NaAlO₂ electrolyte with nanoadditive has better crystallisation in comparison with that prepared in NaAlO₂ electrolyte without nanoadditive.

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Figure 1

X-ray diffraction patterns of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte without α-Al₂O₃ nanoadditive

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Figure 2

X-ray diffraction patterns of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive of 0·5 g L⁻¹

Figures 3 and 4 represent the morphology of the ceramic coatings prepared under different current densities in NaAlO₂ electrolyte without and with α-Al₂O₃ nanoadditive. In Fig. 3, the scanning electron microscopy (SEM) images of different samples exhibit similar surface morphology and the coating surface was composed of grains with different diameters, which are melted and unevenly distributed on coating surface. With increasing current density, some microcracks appear on the coating surface, which could be initiated by the thermal stress attributed to the rapid solidification of the alumina melted in the discharge tunnel. In Fig. 4, the coating had a denser and smoother surface and the number of pores decreased with increasing current density from 15 to 20 A dm⁻². Furthermore, the number of cracks on the coating surface prepared in NaAlO₂ electrolyte with nanoadditive decreased obviously compared with that prepared without nanoadditive. As is well known that the conventional MAO coatings often possess porous surfaces, however, it indicates that α-Al₂O₃ nanoparticles introduced into NaAlO₂ electrolyte could embed in the microarc discharge channels by diffusion and electrophoresis during the MAO process.13 Consequently, a denser and less porous MAO ceramic coating structure is obtained.

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Figure 3

Images (SEM) of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte without α-Al₂O₃ nanoadditive

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Figure 4

Images (SEM) of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive of 0·5 g L⁻¹

Mechanical properties

Microhardness tests have been carried out on the samples deposited under different current densities in NaAlO₂ electrolyte without and with α-Al₂O₃ nanoadditive and the values are shown in Figs. 5 and 6. To improve the accuracy of the microhardness, several point hardness tests were utilised to get an average hardness value. In Fig. 5, the average microhardness value of the ceramic coatings prepared in NaAlO₂ electrolyte without nanoadditive is about 1253, 1605 and 1814 HV for different current densities of 15, 18 and 20 A dm⁻² respectively. In Fig. 6, the average microhardness value of the ceramic coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive is about 1588, 1765 and 2004 HV for different current densities of 15, 18 and 20 A dm⁻² respectively. From above analysis, it can be seen that the average microhardness value of the ceramic coatings increases with increasing current density; however, each average microhardness value of the ceramic coatings with α-Al₂O₃ nanoadditive is higher than that prepared under the same preparation condition without nanoadditive, which may be caused by the better crystallisation and smoother surface morphology with less defects.

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Figure 5

Microhardness values of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte without α-Al₂O₃ nanoadditive

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Figure 6

Microhardness values of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive of 0·5 g L⁻¹

A number of adhesion tests were also performed for the coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive. It was found that the adhesion value of the ceramic coating in NaAlO₂ electrolyte with nanoadditive was about 66·35, 71·05 and 74·90 N for different current densities of 15, 18 and 20 A dm⁻² respectively. It can be concluded that the adhesion value of the ceramic coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive increases with increasing current density, which is consistent with SEM analysis. It is suggested that the incorporation of α-Al₂O₃ nanoadditive to the prepared coatings could improve the adhesion.

Figure 7 shows the friction coefficient of the coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive. From this figure, it can be seen that the friction coefficient drops slightly with increasing current density, and the coefficient of the coating prepared with the current density of 20 A dm⁻² is more stable during the tests. However, the variation of friction coefficients of the coating prepared with the current density of 15 A dm⁻² is violent. The decrease in friction coefficient is attributed to the better crystallisation and denser structure of the coatings. After the friction tests, the weight loss of the coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive was 0·25, 0·13 and 0·08 mg for different current densities of 15, 18 and 20 A dm⁻² respectively, which indicates that the ceramic coatings prepared in NaAlO₂ electrolyte with nanoadditive have excellent wear resistance. Furthermore, the samples prepared under the current density of 20 A dm⁻² with α-Al₂O₃ nanoadditive have better wear resistance than the others, which is mainly attributed to its dense surface, high hardness and relatively low friction coefficient.

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Figure 7

Friction coefficient of ceramic coatings prepared under different current densities in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive of 0·5 g L⁻¹

Conclusions

Ceramic coatings have been synthesised on 6063 aluminium alloy substrates prepared under different current densities in NaAlO₂ electrolyte with and without α-Al₂O₃ nanoadditive by the MAO technique. Effects of different current densities on the properties of the prepared ceramic coatings were thoroughly discussed. With increasing current density, no obvious compositional and structural changes can be found for the coatings prepared in NaAlO₂ electrolyte without nanoadditive, while it is different for the coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive. With increasing current density from 15 to 20 A dm⁻², the adhesion value increases and the friction coefficient and weight loss decreases for the coatings prepared in NaAlO₂ electrolyte with α-Al₂O₃ nanoadditive. Furthermore, in comparison with the coatings prepared under different current densities in NaAlO₂ electrolyte without nanoadditive, the coatings prepared with α-Al₂O₃ nanoadditive have a better crystallisation, smoother surface, less defects, higher microhardness and adhesion value, low friction coefficient and excellent wear resistance.