This paper evaluates the deposition of silica layers at atmospheric pressure as a pretreatment for the structural bonding of titanium (Ti6Al4V, Ti15V3Cr3Sn3Al) in comparison to an anodizing process (NaTESi process). The SiO2 film was deposited using the LARGE plasma source, a linearly extended DC arc plasma source and applying hexamethyldisiloxane (HMDSO) as a precursor. The morphology of the surface was analyzed by means of SEM, while the characterization of the chemical composition of deposited plasma layers was done by XPS and FTIR. The long-term durability of bonded samples was evaluated by means of a wedge test in hot/wet condition. The almost stoichiometric SiO2 film features a good long-term stability and a high bonding strength compared to the films produced with the wet-chemical NaTESi process.
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
Grundlage für eine erfolgreiche adhäsive Verbindung von CFK- und Hybridbauteilen ist die Vorbehandlung der Oberfläche. Dabei geht es bei CFK nicht nur um die Feinreinigung, sondern vor allem um die vollständige Entfernung der auf der Oberfläche verbleibenden Trennmittelrückstände. Mit der LARGE-Plasmaquelle steht eine effektive und zuverlässige Lösung für das Erzeugen einer defektfreien Oberfläche bei gleichzeitig geringem Materialabbau zur Verfügung. Damit können leicht gekrümmte, größere Bauteile bearbeitet und mit verschiedensten Trennmittel kontaminierte CFK-Oberflächen vorbehandelt werden. Teile der präsentierten Ergebnisse entstanden im Rahmen des EU-Förderprojektes Process Line Implementation for Applied SurfaceNanotechnologies (PLIANT), www.pliant.e
The recent generation of aircraft is manly built out of CRFP (Carbon fiber reinforced plastic). To ensure galvanic compatibility Titanium alloys are used at connecting points to metallic structures. The increased use of Titanium alloys due to their good mechanical properties has highlighted the need for structural bonding as joining method to combine Titanium with CFRP. However, long-term stable adhesion on Titanium is still an issue. Currently mainly wet-chemical processes are applied to improve adhesion on Titanium alloys. Since wet-chemical treatments often comprise the use of hazardous chemicals dry environmentally friendly surface functionalization processes are pushed more into focus. PECVD (Plasma enhanced chemical vapour deposition) at AP (Atmospheric pressure) seems to be favorable for the deposition of silicon dioxide films as an adhesion promoter layer. In the present work, the deposition of silicon dioxide adhesion layer (from HMDSO (Hexamethyldisiloxane) as precursor) on titanium alloys is described for a linearly extended and scalable DC arc plasma source (LARGE). The wedge test is used to evaluate the adhesion properties of coated titanium samples in comparison to the standard used NaTESi anodizing process. The results show that the atmospheric pressure plasma process leads to good long-term durability and to high bonding strength which is comparable to the used standard pre-treatments.
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