Cold Plasma Pretreatment of Carbon Fibre Composite Substrates to Improve Adhesive Bonding Performance

The purpose of the paper is to investigate the effects of low pressure plasma treatment on wettability of carbon fibre reinforced polymer samples and on shear properties of adhesive bonded joints based on these substrates. In particular, two plasma process parameters, exposure time and power input, were optimized, performing contact angle evaluation on lap-shear tests. The plasma treatment was also compared with a conventional mechanical abrasion and untreated and only degreased specimens. The experimental results show that choosing the optimal parameters is possible to improve the wettability of composite substrates and reduce the contact angle. 1. Introduction The use of composites is a growing reality in many industrial fields, from civil structures [1–3] to transport industry and especially in aeronautics components [4–9]. Some of their advantages are stiffness, ability to be tailored into complex shapes, strength, corrosion resistance, fatigue properties, and lightweight. In particular, the possibility to decrease the final weight of a manufactured structural component is essential in terms of fuel consumption reduction [10, 11]. Composites are primarily integrated in structures by means of mechanical fastening or adhesive bonding. Adhesives have many advantages in joining composite materials. Perhaps the most significant is that adhesive bonding does not require the composite to be drilled or machined. In fact, traditional techniques of mechanical fastening require the presence of metallic inserts for entering screws and rivets, which makes the manufacturing of the components more complex and does not allow modifications during construction. The use of bonding techniques also allows a better stress distribution as well as durable, lightweight, and aesthetic joints [12, 13]. One of the most important processes to be set before realizing polymer based composite adhesive bonding is the pretreatment of the surface, due to the low surface energy showed by polymers. Recommended preparations of many composite adherends simply consist of a solvent wipe in order to remove dirt and oil followed by a mechanical abrading operation [12–15]. Another widely used technique to solve the problem of composite pretreatment is the peel ply [16]. Many studies have been performed on the preparation of composite substrates also using nonconventional techniques, such as laser [17, 18] or plasma treatments [19–23]. In particular, the aim of a plasma treatment, which can be considered as a physical-chemical procedure, is the functionalization of the specimen surfaces in