The major objectives are to:
-eliminate the surface treatment baths which use toxic chromium VI based products
-increase the resistance to corrosion of the unclad materials in order to reach an equivalent level to that obtained using clad materials
-reduce the fatigue strength drop observed using conventional treatments.
The high performance aluminium alloys used in the aeronautical industry are very sensitive to corrosion. To prevent the development of this kind of damage, chemical or electrochemical surface treatments are currently employed before applying an organic coating.
The objectives of the aircraft manufacturers were to develop new surface treatments in order:
to solve the environmental and health care problems associated with surface treatment baths which contain hexavalent chromium based products;
to improve the performances of plate materials in the fields of fatigue strength and/or corrosion resistance by comparison with the conventional processes.
The following 5 different coating process combinations were selected and then optimized:
magnetron sputtering of a pure aluminium or an aluminiium zinc alloy coating;
evaporation of a pure aluminium coating;
deposition of an aluminium silicon alloy coating by electric arc;
sputtering of a silicon coating on a thin oxide layer formed by sulphuric acid anodizing;
aluminium oxide elaborated by etching and anodizing in alkaline baths.
An in depth characterization of the coatings was undertaken in order to improve knowledge of their properties in the fields of corrosion resistance, paint adherence, fatigue behaviour and stress corrosion cracking. The behaviour of the coatings was shown to be highly satisfactory.
The development of new surface treatment processes for high performance aluminium alloys used in the aeronautical industry is a key factor in improving the properties of these materials and solving environmental problems. The selected processes are:
-new applications of well-known processes, such as physical vapour deposition (magnetron sputtering, evaporation, low temperature cathodic arc deposition) and anodization
-original methods to modify the properties of the superficial oxide layers using ion beam bombardment such as ion implantation and ion beam mixing.
WC2N 5JT London