Overview
Since it does not use any CMR or pollutant compounds, micro-arc oxidation (MAO) also known as plasma electrolytic oxidation (PEO) is a promising technique to replace conventional anodising processes such as chromic acid anodising or hard anodising to improve the mechanical and corrosion properties of valve metals such as aluminium or magnesium and their alloys.
Though it has been known for many years, this technique is however still not well adapted to comply with the aeronautical requirements. In fact, one of the major challenges is to grow thin oxide layers (< 10 µm), which exhibit low roughness (Ra < 1 µm) and good thickness homogeneity, the latter being of prime importance when processing work pieces with complex shape.
The present program aimed at developing a particular technology of micro-arc processing of Al and Mg alloys in order to achieve MAO corrosion protective thin layers that comply with the aeronautical requirements while minimising energy consumption.
A detailed investigation of the process parameters and their influence on the grown ceramic layer was coupled with the characterisation of the layers regarding their microstructure and properties focusing on corrosion protection efficiency.
Moreover using fast video imaging and time-resolved optical emission spectroscopy coupled with electrical measurements, a detailed study of the physics of the micro-discharges resulting from the dielectric breakdown was carried out to shed light on the physico-chemical mechanisms that sustain the layer growth. It was thus expected to determine the response of the micro-discharges, and consequently of the ceramic layer characteristics, to a modification in the process parameters. The perfect knowledge of these mechanisms then helped controlling the process through the management of the micro-discharges.
The work was performed by a consortium of two partners – one research institution, one supplier of micro-arc coatings – and organised around five work packages. It was intended that a part of the work will be subcontracted for investigating some process parameters unavailable within the partner's facilities.
Funding
Results
Executive Summary:
Briefly, the main achievement along the project duration may be summarised as follows:
Along the project duration, aluminium and magnesium alloy samples have been PEO processed using the Ceratronic® process which allowed us:
- to control independently the positive and negative charge quantity delivered to the processed part;
- to match the best parameter set (in terms of charge quantity for instance) while optimizing the energy consumption.