Silicon effects on formation of EPO oxide coatings on aluminum alloys

doi:10.1016/j.tsf.2005.07.184

Thin Solid Films

Volume 494, Issues 1–2, 3 January 2006, Pages 211-218

https://doi.org/10.1016/j.tsf.2005.07.184 Get rights and content

Abstract

Electrolytic plasma processes (EPP) can be used for cleaning, metal-coating, carburizing, nitriding, and oxidizing. Electrolytic plasma oxidizing (EPO) is an advanced technique to deposit thick and hard ceramic coatings on a number of aluminum alloys. However, the EPO treatment on Al–Si alloys with a high Si content has rarely been reported. In this research, an investigation was conducted to clarify the effects of silicon contents on the EPO coating formation, morphology, and composition. Cast hypereutectic 390 alloys (∼ 17% Si) and hypoeutectic 319 alloys (∼ 7% Si) were chosen as substrates. The coating morphology, composition, and microstructure of the EPO coatings on those substrates were investigated using scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD). A stylus roughness tester was used for surface roughness measurement. It was found that the EPO process had four stages where each stage was corresponding to various coating surface morphology, composition, and phase structures, characterised by different coating growth mechanisms.

Introduction

Electrolytic plasma processing (EPP) is a relatively new plasma-assisted electrochemical treatment. It is considered as a cost-effective and environmentally friendly surface engineering technique and can be broadly applied to metal surface cleaning, metal-coating [1], carburizing, nitriding [2], and oxidizing [3], [4], [5], [6].

EPP for anodic oxidising process, called electrolytic plasma oxidation (EPO), in a silicate solution can produce Al–Si–O ceramic coatings with a high adhesion, hardness and thickness on Al-based materials. Moreover, the EPO process combining with other processes such as CVD and electrophoretic deposition (EPD) can be used in producing superhard [7], low friction, and/or biomedical compatible coatings [8].

Several studies have investigated the mechanisms of coating formation [6], [9], [10], characteristics of the coating deposition as well as the tribological properties [3], [4], [5] of the ceramic oxide coating deposited using EPO on different Al alloy substrates. In most of those studies, low silicon (< 1.5% Si) content Al alloys were used to produce thick oxide coating (i.e., > 100 μm in thickness). Due to the rapid growth in applications of high silicon cast Al–Si alloys, the applications of EPO on the cast Al–Si alloys have attracted attention recently.

The study in Ref. [11] investigated the EPO coating formation on hypoeutectic Al–Si alloys (6.5–7.5% Si) and showed that silicon particles in the hypoeutectic Al–Si alloy substrate could be oxidized and mixed into the coating, and that elemental silicon in the Al–Si alloy had enhanced formation of a mullite phase in the coatings. However, to our knowledge, a detailed investigation of the effects of silicon content in Al–Si alloys on the EPO coating formation and morphology has not been reported yet.

In this research, two cast Al–Si alloys with low (hypoeutectic) and high (hypereutectic) silicon content respectively were thus selected as experimental samples. To distinguish the individual process stages clearly, current density (0.05 A/cm²) and electrolyte concentration were adjusted to slow down the coating formation process. DC power (maximum voltage: 500 V) was used with constant current control mode to produce a thin EPO coating (thickness: less than 10 μm) on hypoeutectic and hypereutectic cast Al–Si alloys. The effects of silicon content on the EPO coating formation, composition, and morphology were investigated.

Section snippets

Experimental details

A number of square coupons (25 × 25 × 5 mm³) were cut from commercial cast Al–Si alloys 319 and 390 and used as the substrates. All the coupons were polished and cleaned to obtain a uniform surface roughness of 0.1 ± 0.02 μm. Optical microscopy was used for metallurgical analyses of the Al–Si alloy substrates. A SJ-201P stylus surface profilometer was used for surface roughness (Ra) measurements. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis system (EDX) was employed for

Metallurgical analyses of the Al–Si alloy substrates

Metallurgical analyses were conducted by optical microscopy. Optical photographs of the Al 319 and 390 alloys are given in Fig. 1(a–d). Fig. 1(a) shows a typical hypoeutectic Al–Si 319 alloy microstructure. Coarse Al-dendrites were separated by fine Al–Si eutectic. Fig. 1(b) shows the refined silicon crystal morphology in Al–Si eutectic. In Fig. 1(c) and (d), optical photographs of the 390 Al alloy exhibit a typical hypereutectic Al–Si alloy microstructure with a non-uniform distribution of

Discussion

Investigation of the voltage variation with treatment time and corresponding surface morphology and EDX analysis on the EPO-coated Al–Si alloys has revealed four stages of the process, characterised by different mechanisms. Fig. 9(a–d) schematically illustrate the coating growth model of the EPO coating.

The coating process started with a conventional anodic oxidation of the sample surface in stage I (shown in Fig. 9(a)) where a rapid linear voltage increase was observed. The silicon

Conclusions

The effects of silicon content on the coating process and surface morphology and composition were investigated. With low current density, electrolyte concentration and the maximum 500 V voltage, thin EPO coatings (< 10 μm in thickness) were produced on 319 and 390 Al–Si alloys. The coating process was found to have four different stages. In the first three stages, the duration time and morphology of each stage were considerably affected by the silicon content in Al–Si alloys. The micro-arc

Acknowledgements

The authors would like to thank the GM Global R and D Center for provision of the Al–Si 390 alloy and the assistance with the XRD measurements. The Al–Si 319 alloy was provided by Dr. Jerry Sokolowski at the University of Windsor. The research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canadian Foundation for Innovation (CFI).

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Silicon effects on formation of EPO oxide coatings on aluminum alloys

Abstract

Introduction

Section snippets

Experimental details

Metallurgical analyses of the Al–Si alloy substrates

Discussion

Conclusions

Acknowledgements

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Surf. Coat. Technol.

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Surf. Coat. Technol.