Abstract
Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al2O3 nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.
Introduction
Aerospace applications and energy saving strategies in general boosted the interest and the research in the field of lightweight materials, typically on alloys based on aluminium.1–3 However, the poor surface hardness reduces the lifetime of aluminium components due to wear and fretting damage.4–6 Microarc oxidation (MAO) is a recently new surface treatment technique used to anodise an oxide coating on aluminium alloys in electrolytes with suitable preparation parameters. For aluminium alloys, MAO is an effective approach to improve the properties of aluminium and its alloy by forming ceramic films on the surface. It is characterised by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, excellent bonding strength with the substrate and relatively low cost.7–10 However, these prepared coatings generally possess a foam-like structure with high bulk porosity and relatively poor mechanical properties.11–13 During MAO process, it is difficult to prevent the generation of pores, which restrict them from even wider technical applications. However, recent work has shown that different substances added to the electrolyte have great influence on the properties of the prepared MAO coatings. Hsu et al. prepared alumina coatings by MAO in NaAlO2 electrolytes with added different concentrations of Al(NO3)3 and found that the microhardness of the coatings increased with increasing concentrations of Al(NO3)3.14 Shi et al. fabricated ceramic coatings in Na2SiO3–Na3PO4 solution system by doping two kinds of additives (Na2B4O7 and EDTA), which revealed that doping of additives had little effect on the elemental composition, while it influenced the morphological feature of the coating.7
Thus, in order to improve the structural and mechanical properties of ceramic coatings formed on aluminium substrate by MAO method, two kinds of nanoadditives were doped into different electrolytic solutions. The present paper mainly reports a recent study on the effect of different nanoadditives including TiO2 and α-Al2O3 on the phase composition, surface morphology and element distribution of ceramic coatings fabricated on 6063 alloys by MAO. Meanwhile, mechanical properties such as microhardness, adhesion value and friction resistance of the prepared coatings were also discussed.
Experimental
The substrate materials selected for this study were commercial 6063 aluminium alloys (0·45–0·90Mg, 0·2–0·6Si, 0·35Fe, 0·10Cu, 0·10Mn, 0·10Cr, 0·10Zn, 0·10Ti and balance Al). The samples with size of 30×25×3 mm were polished with abrasive paper and degreased with acetone followed by rinsing with distilled water under standard method before coating formation. The electrolytes were a dilute aqueous solution of 10 g L−1 of silicate, borate and aluminate respectively. Rutile TiO2 nanoadditives with sizes ∼10 nm and α-Al2O3 nanoadditives with sizes between 20 and 50 nm were dispersed evenly in the solution respectively, and the concentration of nanoadditives was 3·2 g L−1. The electrolyte temperature was <50°C throughout the process. During the MAO process, the current density was 15 A dm−2, and the electrolyte was agitated with a mechanical stirrer. Time of coating formation is 1 h. 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 (XRD) (Cu Kα radiation) with 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. Energy dispersive X-ray spectroscopy was used for qualitative elemental chemical analysis. The coating hardness was evaluated using an HMV-IT microhardness tester with 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 Lc represented the adhesion for the coatings. The tribological properties of the coatings were performed on a WTM-2E ball on disc tribometer with rotational speed of 336 rev min−1. The coating was served as the disc, and the counterpart was a Si3N4 ceramic ball (4 mm in diameter and 1550 HV in hardness). The tests were carried out at a normal load of 9·8 N, with a wear track radius of 15 mm. The abrasion loss was measured after 1 h friction measurement using an electronic direct reading balance (LJBROR L-200, readability 0·01 mg).
Results and discussion
Phase analysis
Figure 1 shows XRD patterns of ceramic coatings prepared in different solutions without nanoadditives. For Fig. 1, it can be seen that the prepared ceramic coatings consist of two crystal phases of α-Al2O3 and γ-Al2O3 in addition to some diffraction peaks of 6063 alloy. Figure 2 shows XRD patterns of ceramic coatings prepared in different solutions with different nanoadditives. For the ceramic coatings prepared in different electrolytes with addition of α-Al2O3 nanopowder (Fig. 2a–c), the peak intensity of α-Al2O3 at 67° was increased. For the ceramic coatings prepared in different electrolytes with addition of TiO2 nanopowder (Fig. 2d–f), the peak intensity at 46° was also increased because of the appearance of TiO2 phase. The results indicate that some nanoparticles have entered into the ceramic coatings. As is well known, continuous and long time discharge results in high temperature in the discharge channels, which is propitious to the deposition of α-Al2O3 and TiO2 nanoparticles. Thus, the corresponding peaks can be detected in the coating.
X-ray diffraction patterns of ceramic coatings prepared in different solutions without nanoadditives
X-ray diffraction patterns of ceramic coatings prepared in different solutions with different nanoadditives
Scanning electron microscopy observation and energy dispersive X-ray spectroscopy analysis
Figure 3 represents the morphology of the ceramic coatings prepared in different solutions without nanoadditives. Crater-like holes of various sizes and cracks can be observed for the coatings deposited in different solutions without nanoadditives. Figure 4 depicts the morphology of the ceramic coatings prepared in different solutions with different nanoadditives. The surface morphology changed greatly for the ceramic coatings prepared in different solutions with TiO2 and α-Al2O3 nanoadditives. The pores decreased and the coating surface became compact. However, the surface was denser and smoother for the ceramic coatings prepared in different solutions with α-Al2O3 nanoadditive in comparison with that prepared with TiO2 nanoadditive, which has cracks on the surface. The coating colour was darkened changing from grey white to blue black for the ceramic coatings prepared in different solutions with TiO2 nanoadditive, while the coating colour was lighted for the ceramic coatings prepared in different solutions with α-Al2O3 nanoadditive. During the MAO process, charged ions of the electrolytes will enter the ceramic coatings by diffusion and electrophoresis; thus, the doped nanopowder will enter the prepared ceramic coatings.
Images (SEM) of ceramic coatings prepared in different solutions without nanoadditives
Images (SEM) of ceramic coatings prepared in different solutions with different nanoadditives
Table 1 shows the chemical composition of the ceramic coatings prepared in different solutions with different nanoadditives collected from polished cross-sections of the ceramic coatings. It can be seen that the prepared coating mainly consists of elements Al, O, Si and Ti. The content of O was increased for the coatings prepared in different solutions with TiO2 nanoadditive, while the content of Al decreased, which was caused by the incorporation of TiO2 to the ceramic coatings. The maximum content of Ti was found for the coatings prepared in borate solution with TiO2 nanoadditive, and the minimum content of Ti was found for the coatings prepared in aluminate solution with TiO2 nanoadditive. For the coatings prepared in different solutions with α-Al2O3 nanoadditive, the content of O and Al increased greatly caused by the incorporation of α-Al2O3 nanopowder to the ceramic coatings, which will improve the properties of the ceramic coatings.
Chemical composition of ceramic coatings prepared in different solutions with different nanoadditives
Mechanical properties
Microhardness tests were carried out on the samples deposited in different solutions with different nanoadditives, which is shown in Fig. 5. To improve the accuracy of microhardness value, several point hardness tests were utilised to get an average hardness value. The average microhardness value of the samples prepared in different solutions with nanoadditives is obviously higher than that prepared under the same condition without nanoadditives, and the value of the samples prepared in different solutions with α-Al2O3 nanoadditive is obviously higher than that prepared with TiO2 nanoadditive. The average microhardness value is increased 19, 21·1 and 14·7 for the samples prepared in silicate, borate and aluminate solutions with TiO2 nanoadditive respectively, and the value is increased 38, 40·5 and 38·7 for the samples prepared in silicate, borate and aluminate solutions with α-Al2O3 nanoadditive respectively. From above analysis, we can see that each average microhardness value of the ceramic coatings prepared in different electrolytes with nanoadditives is higher than that prepared without nanoadditives, and the α-Al2O3 nanoadditive greatly improves the hardness value of the ceramic coatings, which may be caused by the denser and smoother surface of the coatings embedded with α-Al2O3.
Microhardness values of ceramic coatings prepared in different solutions with different nanoadditives (N1: without nanoadditives; N2: with TiO2 nanoadditive; N3: with α-Al2O3 nanoadditive)
The adhesion values for the coatings prepared in different solutions without and with α-Al2O3 and TiO2 nanoadditives are shown in Fig. 6. Actually, the coating prepared in different solutions with nanoadditives is more adhesive to substrate compared with that prepared without nanoadditives. However, the adhesion values for the coatings prepared in different solutions with α-Al2O3 nanoadditive are higher than that prepared with TiO2 nanoadditive, which may be caused by the smoother surface morphology with fewer defects. It is suggested that the incorporation of nanoadditives to the coatings during MAO process could improve the adhesion value.
Adhesion values of ceramic coatings prepared in different solutions with different nanoadditives (N1: without nanoadditive; N2: with TiO2 nanoadditive; N3: with α-Al2O3 nanoadditive)
The variations of friction coefficient with number of test cycles are shown in Fig. 7. The oscillation of stable friction coefficient for the coating prepared in aluminate solution with α-Al2O3 nanoadditive is lower than that prepared without nanoadditives and with TiO2 nanoadditive. With respect to sample N2, the friction coefficient at running in period is apparently higher than its stable state friction coefficient, which is attributed to its rather rough surface. Table 2 shows the wear loss of the coatings prepared in different solutions without and with α-Al2O3 nanoadditive. The weight loss of the coatings prepared in silicate, borate and aluminate electrolytes with α-Al2O3 nanoadditive was 0·15, 1·53 and 0·05 mg respectively, which indicates that the ceramic coatings prepared in different solutions with α-Al2O3 nanoadditive have stronger wear resistance compared with that prepared without nanoadditive. In addition, the samples prepared in aluminate electrolyte with α-Al2O3 nanoadditive have better wear resistance than that prepared in other electrolytes, which is mainly attributed to its dense surface, high hardness and relatively low friction coefficient.
Friction coefficient of ceramic coatings prepared in aluminate electrolyte with different nanoadditives (N1: without nanoadditive; N2: with TiO2 nanoadditive; N3: with α-Al2O3 nanoadditive)
Abrasion loss of ceramic coatings prepared in different solutions without nanoadditive and with 3·2 g L−1 α-Al2O3 nanoadditives/mg
Conclusions
Ceramic coatings have been synthesised on 6063 aluminium alloy substrates in different electrolytes with different nanoadditives by MAO technique. Structural and mechanical properties of MAO coatings prepared in different electrolytes doped with different nanoadditives were thoroughly investigated. The nanoadditives have great impact on the structural and mechanical properties of the prepared coatings. The surface was denser and smoother for the ceramic coatings prepared in different solutions with α-Al2O3 nanoadditive in comparison with that prepared without nanoadditives and with TiO2 nanoadditive. Microhardness values and adhesion values of the ceramic coatings prepared in different electrolytes with nanoadditives are higher than that prepared without nanoadditives. The α-Al2O3 nanoadditive greatly improves the mechanical properties of the ceramic coatings and the ceramic coatings prepared in aluminate solution with α-Al2O3 nanoadditive have excellent wear resistance, which may be caused by smoother surface morphology with fewer defects, high hardness and relatively low friction coefficient.
Footnotes
Acknowledgements
The financial aids of the Chinese National Natural Science Foundation with contract no. 61072015 and the Programs of Science and Technology of Zhejiang Province under grant no. 2009C31007 are gratefully acknowledged.