Presently, many industrial sectors including aerospace industry, there is a significant use of aluminium as light weight materials which eventually results cost savings and low fuel consumption. Alluminium and alluminium alloys have high specific strength, good machinability, formability and corrosion resistance but exhibits poor adhesion properties due to complex surface properties. Very often aluminium is fabricated for desired structure by adhesive bonding rather that welding, riveting or brazing. In this context, pre-treatment of alluminium alloys prior to adhesive bonding or painting is a very important factor. However, use of traditional wet chemical methods of pretreatments shows ecological challenges and therefore, there is a clear interest in ecologically cleaner vacuum-based plasma technology. The plasma deposition of thin film coatings on alluminium that exhibit strong interfacial bonding could provide an alternative to the traditional chromate-based treatments. The plasma deposited coatings confer protection against corrosion and provide a good interface for strong, durable adhesive bonds. The 2024 alluminium alloy structure of an aircraft or a helicopter is commonly protected from severe conditions and heavy stresses by a paint coating. Cold plasma represents an efficient, non polluting and economical alternative to clean, activate and, then, to increase the adhesive properties of alluminium surfaces. Therefore, cold plasma reduces the amount of primer needed to be applied and the weight of whole structure. In addition to that the adhesive bonding of composite patch to repair cracks in metallic structures is an accepted technology in aerospace and automobile industries. The bond strength between composite patch and metallic structure is significantly affected by the surface preparation of the composite patch and metallic structure. The surface treatment effect of alluminium by plasma on the bond strength of alluminium/CFRP composites exhibits 33% higher shear strength and 6 times higher T-peel strength than those of untreated allluminium/CFRP composites. The optimal plasma treatment time and the ratio of acetylene gas to nitrogen gas are found to be 30 sec and 5:5 respectively. The surface treatment effect of alluminium foam and alluminium by plasma on the bond strength of alluminium foam/ alluminium composites exhibit 13% higher bending strength and 30% higher shear strength compared to no plasma treatment. However, plasma treatment of alluminium foam produces a similar result to the case with no plasma treatment on alluminium. In this context, resent developments on Plasma electrolytic oxidation (PEO) process is also an important technology to incorporate oxide layers on alluminium alloys with high tribological properties. PEO results a smaller reduction in fatigue strength of 7475-T6 alluminium alloys and a substantial reduction in the case of hard anodizing. The better fatigue performance of the PEO coatings are attributed to the developments of compressive residual stress within the coatings. In order to activate aluminium surface, another important technology is plasma nitriding, by using nitrogen as process gas. Prior to nitriding, the aluminium surfaces are cleaned by plasma-assisted sputtering process resulting in significant improvement of adhesive bond strength of aluminium leading to improvement in durability at aerospace service conditions.
R. Benedictus, Patel, V. K., Chaki, T. K., Shantanu Bhowmik, and Poulis, J. A., “Developments in Plasma Assiated Surface Treatments of Aluminium and its Alloys for Adhesive Bonding”, in 1st Indo-SWISSBONDING International Conference, Madras Institute of Technology, Anna University, Chennai, 2008.