Plasma Processing of Aluminum Alloys to Promote Adhesion: A Critical Review

Aluminum and its alloys are commonly used as lightweight materials in many industrial sectors including aerospace but these materials exhibit poor adhesion properties, low hardness and low wear resistance. Surface modification of aluminum alloys prior to adhesive bonding or painting plays a very important role from the point of view of achieving high corrosion resistance, joint strength as well as durability of adhesive joints. The plasma deposition of thin film coatings on aluminum that exhibit strong interfacial bonding could provide an alternative to the traditional carcinogenic chromate-based treatments. The plasma deposited coatings confer protection against corrosion and provide a surface of high surface free energy (wettability) for strong, durable adhesive bonds. Cold plasma represents an efficient, non-polluting and economical way to clean, activate, and thus to increase the adhesion properties of aluminum surfaces. Therefore, cold plasma eliminates the need for application of primer prior to adhesive bonding, thus making the process much simpler. In addition to this, the adhesive bonding of composite patch to repair cracks in metallic structures is an accepted technology in aerospace and automotive industries. The bond strength between composite patch and metallic structure is significantly affected by the surface preparation of the composite patch and metallic structure. Plasma electrolytic oxidation (PEO) process is also an important technology to incorporate oxide layers on aluminum alloys with high tribological properties. All the product and joint based requirements (high strength-to-weight ratio, high damage tolerance, high durability, design flexibility, environment-friendly and low cost fabrication process, etc.) and the great capability of plasma surface modification prompted the authors to review meticulously the effects of various plasma surface treatments in the enhancement of various mechanical and material properties such as enhancement of wettability and adhesion, improved corrosion resistance, improved bonding and fatigue strength, and improved hardness and wear resistance.