Anodizing of AA6082-T5 by conventional and innovative treatments: Microstructural characterization and dry sliding behaviour

Highlights

• Comparison of anodic layers grown on AA6082-T5 by several industrial processes.

• Processes: hard anodizing, plasma electrolytic oxidation and electrochemical oxidation (ECO).

• Microstructural, micro-mechanical and tribological characterization of all anodic layers.

• ECO: Highest wear resistance (compact microstructure, highest microhardness and adhesion).

Abstract

This work focuses on the comparison of microstructure and tribological behaviour (dry sliding vs. 100Cr6 steel) of anodic layers produced on wrought AA6082-T5 by hard anodizing (HA), plasma electrolytic oxidation (PEO, sometimes also termed MAO, Micro-Arc Oxidation) and novel electro-chemical oxidation (ECO), derived from PEO minimising its disruptive plasma discharge. The results showed that PEO and HA do not decrease the coefficient of friction of AA6082, whilst ECO does (particularly after sealing with a phosphate-based solution), thanks to its smooth surface and decreased stability of Fe–O based transfer layers. All the anodizing treatments improve wear resistance of AA6082: ECO showed the highest wear resistance, due to the beneficial combination of compact microstructure, high and uniform thickness, high microhardness and adhesion.

Introduction

Aluminium alloys are used for an increasing range of engineering applications, due to their many advantages (high strength-to-weight ratio, high electrical and thermal conductivity, good formability and, last but not least, recyclability [1]). Even though these alloys are attracting a growing interest in sectors such as the packaging industry, their unsatisfactory tribological behaviour still remains a critical issue for many applications. Several surface engineering techniques, such as physical vapour deposition (PVD), plating/electroplating, anodizing, thermal spraying and laser-based surface treatments [2,3] are available to improve the tribological behaviour of aluminium alloys. For many decades, the conventional and most cost-effective solution to wear problems has been Hard Anodizing (HA). However, since the beginning of this century, Plasma Electrolytic Oxidation (PEO) has become an attractive alternative to conventional anodizing. PEO is an electrochemical conversion treatment, based on the modification of the growing anodic oxide by micro-discharges, initiated at potentials above the breakdown voltage [4], sometimes also termed Micro-Arc Oxidation (MAO). Several comparative studies have been published on PEO and HA [[5], [6], [7], [8]] and the main advantages of PEO over HA can be summarized as follows: (i) use of dilute alkaline electrolytic baths instead of concentrated acidic solutions; (ii) higher thickness, hardness, practical adhesion and wear resistance of PEO layers, with possible incorporation of anions from the bath; (iii) higher tolerance towards alloying elements such as Cu and Si than the conventional hard anodizing process. All these features are related to specific PEO treatment conditions, derived from conventional anodizing but involving higher voltages (above dielectric breakdown of the anodic oxide) so as to generate micro-arc discharges which assist and enhance the growth of oxide-based layers. However, from a tribological point of view, also PEO has a few disadvantages: (i) heterogeneous microstructure with micropores and cracks, mostly in the outer “technological” layer, which may induce micro-crack driven damage and coating removal in the form of flakes [9] (ii) a high roughness in the as-treated condition, generating relatively high friction against steel counterparts in dry sliding conditions [10], therefore requiring a grinding/polishing post-treatment, applicable only when part geometry is not too complex.

The microstructure of anodic layers grown by PEO can be optimised by controlling electrolyte composition, concentration, pH and temperature as well as electrical parameters (power source, current regime) [9]. The use of AC and DC-pulsed sources remarkably contributed to the development of PEO layers with improved tribological behaviour. In particular, bipolar pulse sources are able to produce dense and compact coatings, whilst when only positive pulses are applied, the oxide-based layer displays a porous morphology [11]. Based on the above considerations, a novel anodizing treatment has been developed in an industrial environment. The treatment, termed Electro-Chemical Oxidation (ECO), derives from PEO but minimises or avoids its disruptive plasma discharge effects (UK Patent [GB2497063], Cambridge Nanolitic Ltd). It was achieved by (1) using short (microseconds) electrical pulses with trapezoidal shape to reduce the electrical current peaks during pulse switching and (2) maintaining cathodic current at a level that secures cathodic etching and building oxide layer with a fine nanopore structure to facilitate charge and ion transfer to the oxidation zone and to avoid breakdown discharge.

Although ECO coatings have already found application in a number of industries such as textile, packaging and automotive, their properties have not been published yet: this work focuses on the comparison of microstructure and tribological behaviour of ECO, PEO and HA coatings produced on the wrought AA6082-T5 aluminium alloy. The main aim of this work is therefore to provide a comparison of the anodic layers obtained by these industrial anodizing processes, which will help in process selection for a given industrial application. It will also allow a deeper understanding of microstructural features and tribological behaviour of the layers produced by each investigated process.