Morphologies and corrosion properties of PVD Zn–Al coatings

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Abstract

Physical vapour deposition (PVD) Zn alloy coatings are being considered as possible alternatives to conventional hot dip and electrochemically deposited coatings on steel substrates. Variation of the deposition temperature leads to very different microstructures being observed by SEM, primarily due to the low melting point of zinc. In the present study this phenomenon has been investigated in more detail. Zn+5  at.% Al coatings were deposited by magnetron sputtering using a homogeneously alloyed Zn–Al target. As the substrate temperature is increased, the coating morphology changed from a compact to an open sponge/woollen-like microstructure and the coating thickness increased by up to a factor of 10. The Zn–Al coatings displayed a hexagonal structure corresponding to that of Zn. The corrosion properties were investigated by potential–time and salt bath immersion tests. Interestingly, both the barrier and sacrificial protection afforded by the coating were independent of the microstructure.

Introduction

For many years zinc and various zinc alloy coatings have been used to improve the corrosion resistance of steel sheets. Conventionally, these coatings are deposited by hot dip and electroplating processes (see e.g. Ref. [1]). A possible alternative coating process is PVD which includes the techniques of evaporation, ion beam, arc and sputter deposition [2], [3], [4], [5], [6], [7], [8], [9], [10]. These processes are environmentally friendly and have some additional advantages:

  • 1.

    they allow the deposition of any type of Zn alloy without the restrictions inherent in electroplating processes due to different electrochemical potentials of the alloying materials;

  • 2.

    there is no danger of steel embrittlement as a consequence of hydrogen evolution during the electrochemical process; and

  • 3.

    PVD coatings can be produced with very compact microstructures which have been assumed to further increase the corrosion resistance [2], and might also advantageously influence steel sheet processing properties such as formability, weldability and paintability.

The disadvantage of PVD processes is the low deposition rate compared to electrochemical or hot dip methods.

This paper is concerned with the extent to which the microstructure of coatings, in particular their compactness, influences the corrosion resistance. It is well known that the microstructure of PVD coatings strongly depends on the melting point and on deposition parameters such as substrate temperature and ion bombardment during film growth and the pressure of the working gas [11]. Coatings with a low melting point such as Zn (420°C) assume an open microstructure if deposited without substrate cooling and ion bombardment. This is essentially due to a high surface mobility allowing the atoms to migrate immediately after deposition and to look for crystallographically favoured sites.

The influence of substrate bias voltage on the microstructure and surface morphology has been investigated by Li and Nowak [3] for Zn–Al alloy coatings of various composition, by Musil et al. [4] for pure Zn coatings, and by Bowden and Matthews [2] for Zn–Ni coatings. Applying a negative substrate bias voltage, compact and fine-grained coatings with a superior barrier protection have been obtained compared to electrodeposited, less compact coatings. The relationship between microstructure and corrosion resistance of hot-dip Zn–Al coatings has been investigated by Ling et al. [12], [13].

In the present study Zn+5  at.% Al coatings have been deposited by magnetron sputtering. Zn alloys such as Zn–Al have a lower corrosion rate than pure Zn due to the higher electrochemical potential of the alloyed materials. However, the sacrificial protection deteriorates, which is important if the underlying steel is exposed locally as a result of stone chipping. The coatings were characterised with respect to their morphology, microstructure and corrosion behaviour.

Section snippets

Experimental

Film deposition was performed using a commercially available sputtering machine (MRC type 8667 A) in which a Zn–Al target of composition Zn+5 at.% Al (purity 99.99%) was mounted. Before deposition, the chamber was evacuated to a pressure of 1×10−4 Pa. During the deposition process, a gas flow of 137 sccm Ar was maintained by a mass flow controller. The total pressure was kept at 7×10−2 Pa during the coating process. Mild steel and Si(111) wafers were used as substrates. The steel substrates were

Results and discussion

The Zn–Al coatings were highly reflective if deposited in good thermal contact, or opaque and of grey appearance if deposited in bad thermal contact with the cooled substrate holder. Fig. 1(a) and (b) show SEM fracture cross-sections of coatings in good (a) and in bad (b) thermal contact with the substrate holder plate. EDX analysis revealed a composition corresponding to Zn+5(±1) at.% Al. The coatings were deposited onto Si substrates in adjacent positions on the substrate plate. The coating in

Conclusions

Zn+5 at.% Al coatings were deposited by magnetron sputtering, using a homogeneously alloyed Zn–Al target. Depending on the substrate temperature, two different microstructures were observed: at lower temperatures a compact and columnar type, and at higher temperatures an open woollen-like microstructure. Both coating types assumed similar crystal structures. The results of corrosion-potential vs. time measurements and of immersion tests did not reveal a significant difference between the

Acknowledgements

The authors wish to express their thanks to Dr. P.N. Gibson for helpful discussions in interpreting the X-ray diffraction experiments, and Messrs. A. Hoffmann and T. Sasaki for helpful technical assistance. The study was performed in the framework of the Brite-Euram project BE96-3073 of the European Union.

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Cited by (0)

1

Present address: School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK.

2

Present address: Institut für Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 500, D-69120 Heidelberg, Germany.

3

Present address: Gühring oHG, Lengederstrasse 29-35, D-13407 Berlin, Germany.

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