Year

1998

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

Sheet steels intended for atmospheric exposure invariably require a sacrificial corrosion protective coating. Such coatings are commonly applied by either hot-dip galvanising or electrodeposition using large scale continuous processes. Armed with the desire to produce new products with improved performance, there has been considerable industrial interest in the use of physical vapour deposition (PVD) for the coating of sheet steels, and practical experience extends over the last three decades. Only in the last few years, however, have the most attractive aspects of P V D been investigated with some vigour: the preparation of novel alloy and multilayer coatings.

In recognition of this climate, the research presented in this thesis was commenced with the intention of evaluating the microstructures of a range of alloy coatings prepared by magnetron sputtering. The investigation considered the impact of ion assistance on coating composition and microstructure, by using an unbalanced magnetron design and varying the bias voltage of the substrates, while maintaining a substrate temperature of 50°C. The coating compositions targeted for study were based on the Al-Mg-Zn ternary system, with emphasis on the binary alloys. The study was mainly concerned with the investigation of supersaturated solid solubility and other metastable crystalline or amorphous phases. Coating characterisation was achieved by energy dispersive spectrometry (EDS), scanning electron microscopy (SEM), Bragg- Brentano X-ray diffraction (XRD), and the Crystallite Group Method (CGM). This latter technique is particularly effective for evaluation of the residual stress and strainfree lattice parameters for strongly fibre textured coatings. CGM was used for the first time for coatings with a close-packed hexagonal (cph) crystal structure.

Elemental coatings of Zn, Al and Mg all developed strong fibre textures with the close-packed planes oriented parallel to the substrate surface. Ion assistance enhanced the texture of Zn and Al coatings while disrupting the preferred orientation of Mg coatings. Compressive residual stress was developed in ion assisted Al and Mg coatings due to the atomic peening mechanism. Energetic bombardment of Zn coatings resulted in tensile stress despite a dense, columnar structure which typically indicates compressive stress. This was explained by constrained shrinkage of the Zn coatings during dynamic recrystallisation. Zn and M g coatings were subject to very high proportions of entrapped Ar which was correlated to the residual stress, but was not causative. Al coatings did not retain detectable amounts of Ar.

Binary Al-Zn alloy coatings possessed a dual phase structure with discrete (Al) and (Zn) grains. Supersaturation of the (Al) phase was apparent but there was some indication that it declined by spinodal decomposition or precipitation after ageing at room temperature. The surface appearance of Al-Zn coatings was influenced by the Zn content and substrate temperature, such that smooth reflective coatings with high Zn content (textured structure at lower substrate bias than was necessary for elemental Al coatings, probably because of a sputter yield amplification effect. The (Zn) phase in the alloy coatings also retained high proportions of Ar.

Coatings of Al-Mg alloys were characterised by single-phase supersaturated (Al) solid solutions with up to about 38 at.%Mg, or a combination of supersaturated (Mg) and an amorphous phase for coatings with between 40 and 90 at.%Mg. The amorphous phase was thought to possess a composition of about 39 at.%Mg. The use of ion assistance resulted in preferential resputtering of M g from the coatings, due to the differences in surface binding energy. The consequential modification in stoichiometry was sufficient to produce different phases in the coatings, and also caused a sharp reduction in the supersaturation of (Al) and (Mg) solid solutions. Coatings containing the (Mg) phase were susceptible to the entrapment of exceptionally high proportions of Ar (

The microstructure of binary Zn-Mg alloy coatings was also dependent on the composition. Coatings with between 15 and 40 at.%Mg were entirely amorphous due to slow growth kinetics of complex intermetallic phases within this composition range. Higher and lower Mg content typically provided coatings with a combination of terminal solid solutions and the amorphous phase. The solid solubility of (Zn) and (Mg) phases was considered to be within the equilibrium limits. Several intermetallic phases were detected in Zn-rich coatings, and some of these phases could not be identified. Ion assistance resulted in the preferential resputtering of Mg from the alloy coatings, which is opposite to the trend anticipated from elemental sputtering yields. This effect was attributed to sputter yield amplification, and was confirmed by Monte Carlo simulation of the ion assisted deposition process using the T-DYN code. All of the Zn-Mg coatings were susceptible to the retention of high proportions of Ar, particularly for the Mg-rich compositions. As for the Al-Mg coatings, the entrapped Ar appeared to favour amorphisation of the (Mg) phase. Coatings with high Ar content were sensitive to heavy oxidation with ageing.

The ternary Al-Mg-Zn alloy coatings produced were based on Al-rich compositions. Ion assistance had a dramatic effect on the coating stoichiometry and microstructure. Increasing the ion energy resulted in the preferential resputtering of Mg, followed by Zn at higher energy. Coatings deposited with low bias were found to be amorphous, while those with virtually identical composition but prepared with high bias were found to contain (Al) phase with a strongfibre texture. Relatively high proportions of Ar were also present in coatings deposited with elevated bias, but did not interrupt the growth of (Al) phase, suggesting that it was associated with an undetected amorphous phase.

The electrochemical performance of a range of the sputtered coatings was examined using a dilute chloride aqueous solution. The data was interpreted with the aim of pinpointing a coating with an optimum combination of both barrier and sacrificial properties which would ensure long term corrosion protection for a sheet steel substrate. All alloy coatings were found to exhibit passive regions. The results were interpreted to indicate that Al-41 at.%Zn and Al-13at.%Mg-20at.%Zn coatings would provide an ideal combination of good barrier protection and acceptable sacrificial protection of sheet steel. Coatings consisting of Zn-20 at.%Mg offered improvements in sacrificial protection relative to pure Zn coatings, while possibly having greater barrier protection properties.

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