Plasma Treatment on the Activation Effect of the Inner Wall of CFRP Cylindrical Parts

To improve the corrosion resistance of the inner walls of carbon fiber-reinforced plastic (CFRP) cylindrical components to special gases, metal coatings can be applied to their inner surfaces. However, carbon-fiber composite surfaces exhibit high chemical inertness, low catalytic activity, and low bonding strength with metal atoms. To address this, the surface of the CFRP matrix undergoes plasma activation prior to coating with the aim of improving the bonding strength between the matrix and the metal coating. By increasing the chemical activity of the CFRP inner-wall surface, an ideal interfacial morphology is obtained, forming the basis for creating high-performance metallized coatings on the surface. Prior studies has successfully employed Hall ion source for activating planar CFRP materials. However, this technological approach is not suitable for the plasma activation of the inner walls of cylindrical parts due to the size mismatch between the components and the Hall ion source. Plasma activation of the inner-wall surface of the cartridge is performed before the coating of the inner wall takes place, by replacing the power supply of the coating equipment with a radio frequency (RF) power supply and modifying the cartridge installation. An RF glow discharge is used to activate the CFRP inner-wall surface. A column electrode is inserted into the cylinder, with one end connected to the RF power supply and the other to the vacuum chamber. The vacuum chamber achieved a level of less than 3 mPa through the pumping system. The working gas is introduced until the desired air pressure is reached, and the RF power supply is activated to generate plasma by glow discharge, effectively activating the inner-wall surface of the cylindrical components. Post-activation, the RF power supply is replaced with a magnetron power supply. Argon gas is introduced to initiate magnetron sputtering, resulting in the application of a metal coating to the inner wall of the cylindrical component. The effects of the plasma treatment parameters on the activation process, including air pressure, discharge power, treatment time, and ion type, are investigated using contact angle tests and infrared spectral analysis. The results show that the plasma activation effect is evident on the inner-wall surface of the CFRP cylinder after the RF glow discharge plasma treatment. Consequently, the contact angle of both liquids on the CFRP substrate decreases significantly, while the surface energy and percentage of polar components increase significantly. Under identical process parameters, the activation effect initially increases and then decreases with rising air pressure, whereas it increases consistently with an increase in discharge power and treatment duration, with the most favorable results observed for oxygen plasma activation. The most significant effect is achieved when the parameters included a discharge pressure of 0.5 Pa, an RF discharge power of 500 W, a treatment duration of 60 min, and the use of oxygen plasma. Under these conditions, the contact angles for water and diiodomethane decrease substantially from 71.29 degrees, 49.36 degrees to 4.93 degrees , 5.39 degrees, respectively. The surface energy increases from 38.85 mJ center dot m-2 to 74.73 mJ center dot m-2. The inactive bonds of the plasma-treated carbon fiber composites, including C -H and C equivalent to C, are broken, the number of aldehyde and carboxyl groups with C=O increases, and the wettability is greatly improved. A comparison of the surface microscopic morphology before and after activation is performed using scanning electron microscopy (SEM). The impurity particles that have adhered to the carbon fiber surface and between the carbon fibers are completely removed, establishing favorable interfacial conditions, and thereby enhancing the bonding strength of the metal coating. The film-based bonding force between the activated CFRP substrate and the metal film increases from less than 0.1 MPa to 0.49 MPa.