Light alloys of aluminium, magnesium and titanium are widely used in the aerospace, defence and military applications due to their low density, high specific strength and good thermal conductivity. Poor corrosion, wear and other functional characteristics of these alloys are well known, and as such, are always used with protective coatings. However, current surface technologies are being adapted with only limited durability resulting in enormous costs on maintenance and repair to the defence industries. For example, to-date surface treatment of Mg and Al alloys are limited by conversion treatments such as phosphating, anodising and painting which do not offer sufficient protection to materials being employed in an ever increasingly aggressive environment while a number of these processes e.g. Dow 7, Chrome plating are considered to have health and environmental hazards. Whereas, surface treatment for Ti alloys is rather limited by processes such as anodising that offers only limited protection in abrasive, erosive, fretting and adhesive wear conditions.
Plasma electrolytic oxidation (PEO) is one of relatively new environmentally safe electrolytic coating processes, applicable to light alloys and represents a rapidly developing sector in surface engineering. The process results in the formation of a ceramic layer that offers protection to the alloys in terms of corrosion, wear and other functional characteristics including thermo-optical, dielectric, thermal barrier, low friction coefficient and as pre-treatment for paints and other metal/ceramic layers. This process has demonstrated significant interest in offering improved surface treatment to light alloys (Al, Mg, Ti) and as a replacement for conventional acid based processes such as Dow and other chemical processes that contain hexavalent chrome, and including anodising processes.
This paper summarises recent developments in PEO processes / characteristics and highlights a number of applications that can have added value and enhanced durability offered by this technology.