Co-consolidated titanium-Thermoplastic composite joints - A study on the mechanisms governing adhesion and durability
Vanessa Marinosci is a PhD student in the research group Production Technology. Supervisors are prof.dr.ir. M.B. de Rooij and prof.dr.ir. R. Akkerman, co-supervisor is dr.ir. W.J.B. Grouve from the Faculty of Engineering Technology.
The main drivers for technological innovation in the aviation industry are reduction of carbon footprint, reduction of operating costs, and improved recyclability. Cost-efficient lightweight designs, incorporating modern high-performance thermoplastic composites, are a promising answer to this set of challenges. The ability to repeatedly melt, reshape, and solidify makes thermoplastic composites attractive compared to alternative lightweight materials, as it enables cost-effective and high-rate manufacturing processes, assembly by means of fusion bonding, and recycling without the need to separate fiber and matrix. Moreover, thermoplastic composites are characterized by a high specific strength and stiffness, a high environmental resistance and a low flammability.
In many high-performance applications, composite materials are joined to metals for load introduction purposes. Besides, composites and metals can be combined in the form of fiber metal laminates, resulting in a hybrid material characterized by superior properties compared to its individual constituents. Thermoplastic composites offer an alternative joining method for such hybrid metal-composite structures, known as co-consolidation. The moldability of the thermoplastic matrix allows joining of the metal and the thermoplastic composite during a standard composite consolidation or forming process in which the thermoplastic matrix acts as the adhesive. Therefore, the co-consolidation technology represents a time and cost-efficient alternative to the standard riveting and adhesive bonding.
The load-bearing capacity of co-consolidated metal-composite joints strongly relies on the interface, the adhesion of which typically depends on factors like the interface micro-structure and the molecular interactions between the adherends. Currently, knowledge of each factor’s contribution and their combined effect on the interfacial mechanical performance remains limited, thereby also limiting the implementation of the co-consolidation technology. The objective of this work is to identify and analyze the individual and combined effect of these factors on the adhesion and, hence, on the mechanical performance of metal-thermoplastic composite joints. The material system considered is the titanium alloy Ti6Al4V bonded to unidirectional carbon fiber reinforced polyether ketone ketone (UD C/PEKK).
This research demonstrates that the adhesion between Ti6Al4V and PEKK mostly relies on short-range physical interactions (e.g. Van der Waals), contrary to popular understanding that the metal-polymer adhesion is predominantly governed by mechanical interlocking and/or chemical bonding. Additionally, the root-cause for the often-addressed titanium-thermoplastic polymer bond instability in the presence of moisture was successfully identified. The instability of the adhesion between titanium and PEKK in a hot/wet environment is caused by the adsorption of water on the titanium oxide where it disrupts the aforementioned interactions, resulting in a decreased bond performance.
A solution to the Ti6Al4V-PEKK bond instability was proposed in the form of silane-based coatings (Si-PDA). It was shown that the application of Si-PDA coatings on the titanium prior to bonding significantly enhances the Ti6Al4V-C/PEKK bond stability and, hence, the mechanical performance in a hot/wet environment. Furthermore, it was found that the increased bond stability is caused by the low water diffusivity that characterizes these coatings, which strongly delays the water adsorption on titanium.
In further research, the effect of mechanical interlocking was then investigated. It was found that promoting interlocking via grit-blasting of the titanium prior to bonding has a beneficial effect on the fracture toughness in dry conditions. Increasing the surface roughness leads to a more tortuous crack path and induces stress intensifications around the crack tip, which ultimately results in a transition from adhesive to cohesive failure. By combining the effects of grit-blasting and Si-PDA coatings, an optimal titanium surface treatment was found to ensure Ti6Al4V-C/PEKK joints with a comparable toughness both in dry conditions and after long-term exposure in a hot/wet environment.
Another factor found to influence fracture toughness and failure mechanisms is the polymer morphology. The lower ductility of a highly crystallized PEKK leads to a rather brittle failure through the PEKK spherulites. Conversely, the higher ductility of an amorphous PEKK allows more polymer plastic deformation and prevents crack deflection, resulting in adhesive failure and higher toughness. Furthermore, it was demonstrated that the use of nucleating agents, such as graphite nano-plates, improves the toughness of the joint while maintaining a high degree of crystallinity. This finding provides insights on other methods to manufacture titanium-PEKK joints with an optimized toughness.
Finally, the knowledge gained throughout these studies is translated into guidelines to manufacture reliable metal-TPC joints via a co-consolidation process.