Surface Structuring of Ti-Al-V and Al-Mg Alloys By Chemical Etching for Advanced Adhesion of Polymers
Authors: Baytekin-Gerngross Melike , Gerngross Mark-Daniel , Carstensen Jürgen , Adelung Rainer 
Published: February 2020

Polymer coatings are used to produce composites, gluing metals to other materials, or as protective coating against corrosive environments, e.g. corrosion prevention of underwater vessels, boat hulls or composite-based lightweight turbine blades. When fabricating such metal/poly-mer composites, the typical challenge is to achieve sufficient adhesion between the metal surface and the polymer, so that no adhesive failure occurs e.g. moist or corrosive environment [1]. The most common method to increase the adhesion is a mechanical roughening of the metal surface followed by the use of chemical adhesion promoters [2].
Our approach to overcome this adhesion issue is the formation of a mechanical interlocking structure in the metal surface which allows for establishing a robust connection between metal and polymer resulting in a drastically higher adhesion between both. This work shows the fabrication of interlocking surfaces for two typical alloys, Ti grade 9 and AlMg3-W19, by chemical etching and their impact on the adhesion in metal/polymer composites.
In this work Al alloy AlMg3-W19 (AW5754) and Ti grade 9 (Ti-3Al-2.5V) was used – Ti grade 9 as representative for aircraft and AlMg3-W19 as typical representative for shipbuilding and automotive applications. The Ti grade 9 samples were etched in a viscous electrolyte of HCl and H2SO4at room temperature for 20 h. The AlMg3 alloy samples were etched in an aqueous HCl solution at room temperature for 1 h. To evaluate the adhesion performance metal/polymer/metal composites are formed with a solvent-free polyurethane (PU) as polymer component. The metal components are the chemically etched AlMg3-W19 bars, resp. grit-blasted as reference. The composites are then tested in lap shear tests based on the German standard DIN EN 1465 on a Zwick QuickTest 100 kN machine with a displacement rate of 1 mm/min.
Figure 1a shows an SEM surface image of the resulting surface structure of the Ti grade 9 sample after chemical etching. It can be seen that the surface exhibits a sponge-like structure consisting of single bowls, see inset in Fig. 1a), with undercuts acting as barbs, as described in [3]. When filled with polymers this structure is expected to establish a very robust interlocking between Ti grade 9 and the polymer.
Figure 1b) presents SEM surface image of the nano-/microstructured AlMg3 after chemical etching. It gives a macroscopic impression of the structured AlMg3 surface after etching in the electrolyte for 1h. One observes a highly irregular surface structure over the entire sample surface. The dark spots are structures deeper etched into the Al surface, while the bright spots represent structures that stick out of the surface. One also observes that the surface structure consists of a multitude of rectangular structures with flat faces either in a step-like arrangement or as free-standing structures, so called barbs. These rectangular structures have different sizes and are also tilted towards each other under various angles with respect to z-axis depending on the crystal orientation. Due to these fine structures a robust connection between the Al surface and the polymer coating is obtained.
Figure 1c presents the shear tests of the chemically structured AlMg3/PU composite and the grit-blasted AlMg3/ PU composite as reference. The maximum shear stress before failure is about 5 N/mm² and the traverse way of about 0.8 mm for the grit-blasted AlMg3/PU, while the chemically structured composite exhibits an almost doubled maximum shear stress and an about 10x higher traverse way before fracture. The serrated part of the shear stress / traverse way curve could be explained as the result of the localized failures of the polymer. The fractured surfaces clearly show adhesive failure in case of the grit-blasted AlMg3/PU composite and cohesive failure for the chemically structured composite. These results illustrates the highly beneficial effect of the chemically etched interlocking surface structure of AlMg3 alloy on the adhesion compared to the only grit-blasted AlMg3 surface. Another great advantage is that the here used chemical etching processes are easily scalable to industrial dimensions.

Fig. 1:SEM surface images of the chemically etched interlocking structure of a) Ti grade 9 and b) AlMg3-W19 alloy, c) shear tests of the chemically structured AlMg3-W19/PU composite compared to a grit-blasted AlMg3-W19/PU composite.

References:

[1] Wapner, K., Stratmann, M., Grundmeier, G. Structure and stability of adhesion promoting aminopropyl phosphonate layers at polymer/aluminium oxide interfaces. Int. J. Adhes. Adhes. 2007, 28, 59.
[2] Schulze, K., Hausmann, J., Wielage, B. The stability of different titanium-PEEK interfaces against water. Procedia Materials Science 2013, 2, 92-102.
[3] M. Baytekin-Gerngross, M.-D. Gerngross, J. Carstensen, R. Adelung; German patent DE 102014110922.8, pending.

Figure 1