Mechanical properties and interface toughness of metal filled nanoporous anodic aluminum oxide coatings on aluminum J. Zechner a, ⁎, G. Mohanty a , C. Frantz a , H. Cebeci b , L. Philippe a , J. Michler a a EMPA, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Mechanics of Materials and Nanostructures, Feuerwerkerstrasse 39, 3602 Thun, Switzerland b Rero AG, Hauptstrasse 96, 4437 Waldenburg, Switzerland abstract article info Available online 21 September 2014 Keywords: Anodic aluminum oxide Interface toughness Mechanical interlocking Micro-cantilever Nanoindentation The mechanical properties and coating/substrate interface toughness of nanoporous anodic aluminum oxide thin films, both unfilled and metal-filled, are investigated in the current study. A two-step anodization process is used to grow the porous oxide, which is subsequently filled with Ni. The mechanical properties of the coating are probed using nanoindentation and the micro-cantilever deflection technique is used to study the interface tough- ness. The results are discussed and suggestions for improving the interface toughness are made. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In the present study, the suitability of nanoporous anodic aluminum oxide (AAO) as the base layer for metallic coatings on aluminum is in- vestigated. Due to its unique honeycomb like structure consisting of regular nanopores that can be filled with different metallic, ceramic or polymeric materials, AAO has been intensively investigated as an attrac- tive material for various applications in optics, electronics and magne- tism in recent years [1]. The nanoporous structure can also be used as an attractive base layer for coatings on aluminum and its alloys, espe- cially in cases where the layer adhesion on the substrate is critical for a successful application. Thus, it could also be used as an alternative to the zincate-plating process [2], in which a zinc layer is produced by gal- vanic displacement of aluminum with zincate. This zinc layer prevents further oxidation of aluminum and serves as substrate for subsequent metallizations. Although it has been in use for many decades, obtaining well adhering coatings using the zincate process is still challenging and leads to high rejection rates in production. Therefore, there is a strong industrial demand for an alternative process with improved process control. The basic idea of this work is to grow an anodic alumina layer with a regular nanoporous structure, which is afterwards filled with Ni. This should serve as an industrially relevant example for metallization on aluminum. The filled porous structure should then act as a mechanical anchor for the film on the substrate. This strategy is known to have an advantageous influence on coating adhesion [3]. Therefore, in the cur- rent study, an AAO layer is grown on a high purity aluminum substrate, which is subsequently filled with Ni to form a nanocomposite thin film consisting of an alumina matrix filled with Ni nanowires. The mechani- cal properties of the coating are probed by nanoindentation and the toughness of the substrate/coating interface is quantitatively measured using the micro-cantilever deflection technique [4,5]. 2. Experimental 2.1. Coating deposition For film deposition, 0.5 mm thick Al disks (99.999%) are degreased in 1.25 N NaOH at 60 °C for 5 min, neutralized in 5.55 N HNO 3 , and then electropolished in HClO 4 :C 2 H 5 OH = 1:3 at 10 °C for 2 min at 20 V. Af- terwards, a two-step anodization process is applied in order to obtain well organized nanopore arrays. The first anodization is achieved in 0.3 M H 2 SO 4 at 3 °C for 8 h at 25 V. The obtained oxide layer is subse- quently dissolved by immersion in 0.4 M H 3 PO 4 + 0.2 M H 2 CrO 4 at 60 °C for at least 1 h. The second anodization is carried out under the same conditions as the first one for 1800 s. Directly after that, the anod- ization potential is exponentially decreased down to 7.5 V through 60 steps of 20 s each, and finally maintained at 7.5 V for 600 s. This last step allows thinning and homogenizing the barrier oxide layer in order to facilitate subsequent electrodeposition. Then, the porous anodic aluminum oxide is filled with Ni by potentiostatic reverse pulse deposition in 0.73 M H 3 BO 3 containing 0.154 M Ni(NH 2 SO 3 ) 2 , 0.7 mM SDS and 10.9 mM saccharine at 45 °C. The potential waveform consists of a cathodic pulse of 8 ms at -11 V followed by a short anodic pulse of 2 ms at 7.5 V. This period is repeated 360,000 times. Al disks and chemicals are purchased from Goodfellow and Sigma-Aldrich respectively. A Julabo refrigerated/heating circulator Surface & Coatings Technology 260 (2014) 246–250 ⁎ Corresponding author. E-mail address: johannes.zechner@empa.ch (J. Zechner). http://dx.doi.org/10.1016/j.surfcoat.2014.08.086 0257-8972/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat