Investigation on atmospheric plasma surface treatment for structural bonding on titanium and CFRP

In the latest generation of aircraft new light-weight design concepts are highly in demand. The material development in the aerospace industry shows a clear trend. More than 50 % of the structure of an Airbus A350 XWB and a Boeing 787 are made of carbon fiber reinforced plastic (CFRP), while the Airbus A300 had only a share of 7 % CFRP in the 1970's. The increasing amount of composite materials leads to problematic material combinations. Carbon fiber reinforce plastics can induce galvanic corrosion when attached to an aluminum structure. To overcome this phenomenon titanium alloys are used. To date, the most widely used joining technique is riveting. However, this is disadvantageous due to the high notch sensitivity, the low shear strengths and bearing stress of CFRP. To overcome these limitations, adhesive bonding can be applied. However long-term stable adhesion on Titanium is still an issue and the key to success is to realize a suitable surface treatment. Currently used pre-treatments of titanium joints are mostly wet-chemical bath processes, which often comprise hazardous chemicals, whereas manual process grinding is the mainly used pre-treatment for CFRP. Plasma enhanced chemical vapor deposition (PECVD) at atmospheric pressure seems to be an alternative to these methods to overcome environmental issues and increase the repeatability on composite materials. In this paper, a linearly extended and scalable DC arc plasma source is employed for the deposition of organosilicon films using hexamethyldisiloxane (HMDSO) as precursor on titanium alloys. The same source is used for cleaning and activation of CFRP surfaces to remove residues from release agents that have to be used during manufacturing of the CFRP. The organosilicon film is deposited on titanium alloy after an alkaline etching process. Up to now the interactions between the plasma deposited layers and the titanium oxide layer are unclear. Therefore the interface between the oxide layer and organosilicon coating was characterized by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) after focused ion beam (FIB) preparation. The EDX mapping was employed to analyze a possible diffusion interface between these two layers. The cleaning and activation of CFRP was investigated at two different distances (4 cm and 6 cm). For costs reasons compressed air is discussed as an alternative to Argon or Oxygen. Therefore in study compressed air was used due to lower costs and higher treatment distance. The adhesive bonding tests were performed in comparison to the standard used grinding process.