Investigation of the Aluminium Plasma Electrolytic Oxidation Process

Abstract

I Introduction Plasma electrolytic oxidation (PEO) [1-4], has gained growing interest for the recent years in light-weight metal (Al, Ti, and Mg alloy) oxidation [5]. Compared with conventional anodising, PEO treatments are usually achieved by using high voltage, low frequency (around 50 Hz) AC supply in dilute alkaline electrolytes [1]. The oxide layers obtained by PEO processing of aluminium alloys are thick, hard and well-adherent to the substrate. As a consequence, the surface properties obtained after PEO treatments are promising for industrial applications. The oxide layer achieved on aluminium alloys are typically subdivided in two sublayers [6, 7]: a porous layer at the surface and a compact layer near the substrate. The latter contains crystallized alumina, responsible for the very good mechanical properties of the oxide layers. As a consequence, the surface properties obtained after PEO treatments are promising for industrial applications. Despite the considerable industrial interest in this process, there is no clear understanding on the underlying discharge mechanisms. Discharges produced during a PEO treatment are not stationary and their evolution can be characterized by different time scales. Indeed, the discharge number, localisation and aspect (colour, brightness...) evolve over the complete PEO treatment. Different process steps are reported in the literature [8, 9]. The early beginning of a treatment is similar to a conventional anodizing, leading to the formation of an insulating layer. The aluminium oxide layer grows and becomes more and more insulating and acts as a diffusion barrier for oxygen. As the applied voltage reaches the breakdown potential of the oxide film, sparks appear that flash randomly all over the aluminium surface, allowing thus the growing to pursue. This talk reports on the investigation of PEO processing of aluminium alloys by using the CERATRONIC pulsed bipolar current supply [10]. It is shown that particular conditions may be established which strongly reduce the arcing that usually cause detrimental defect in the oxide layer for long treatment time (typically greater than 40-50 min.). This results in a softer process which is pointed out by spectroscopic measurements and video imaging of the micro-discharges during the PEO process. As a consequence thick homogenous layers may be grown with no large discharge channels. From the presented results, the importance of the negative charge density relative to the positive one is evidenced thus pointing out the benefit of using a pulse bipolar current supply.