Plasma-polymerised coatings used as pre-treatment for aluminium alloys

The pre-treatment of aluminium alloys prior to painting or adhesive bonding is an essential technology in many industrial sectors (e.g. aerospace industry). However, the pre-treatments used on the substrates, which are vital to ensure reliable long-term performance, often comprise a range of environmentally objectionable chemicals, such as solvents and chromates, some of which are carcinogenic. The plasma deposition of thin coatings on aluminium, which exhibit strong interfacial bonding, could be an alternative to the traditional chromate-based treatments. It is envisaged that in a single plasma reactor, plasma cleaning, etching and deposition could provide a complete preparation cycle before painting, avoiding the use, handling and disposal of hazardous materials. In the present work, the corrosion behaviour and adhesive joint performance of plasma-coated Alclad AA2024-T3 alloy were investigated. The coatings were deposited from hexamethyldisiloxane monomer (HMDSO) in a capacitively coupled RF plasma reactor. The same evaluations were carried out on specimens painted with an epoxy primer after coating deposition. The coatings conferred protection against corrosion and provided a good interface for strong, durable adhesive bonds.