Implementation of diverse non-centrosymmetric layer concepts for tuning the interface activity of a magnesium alloy
© Stamboroski et al. 2016
Received: 15 January 2016
Accepted: 16 April 2016
Published: 9 May 2016
Magnesium and its alloys are the lightest metallic materials used for structural applications. Tuning the surface functionalization of magnesium alloys may contribute to increasing their durability. Dry or wet processes may be effective for the modification of magnesium alloy surfaces. The resulting layers may cover surface inhomogeneities and separate the substrate surface from molecular films. This work demonstrates the feasibility and effectiveness of the concepts and techniques comprising laser or plasma based pretreatment processes or dipping procedures that involve synthetic amphiphilic polymers or biopolymers. In detail, the effects of barrier layers that have been applied by the deposition of siliceous polymer coatings in low pressure plasma processes, by laser surface treatments in controlled gas atmospheres or by dipping in liquid formulations containing a recently developed polymeric inhibitor or a mixture of the enzyme laccase and the polysaccharide maltodextrin are monitored. In this respect, a time-resolved hydrogen bubble formation test is performed, revealing interactions between water films and the modified surfaces. The surface modification is shown with X-ray photoelectron spectroscopy investigations and, in addition, the alloy surface and grain structure is characterized using energy dispersive X-ray analysis, scanning electron microscopy and scanning force microscopy. These investigations reveal that the thus established layer/substrate and layer/environment interphases differ in their composition as a result of the non-centrosymmetric layer concepts for surface functionalization applied here.
Due to their low density and adequate mechanical properties, magnesium and its alloys are promising materials for structural applications . Their limited corrosion resistance  leads to their restricted use in technical applications but facilitates the design of biodegradable magnesium alloys [3, 4]. Considering that surface properties govern the onset of material degradation, modifying the corrosion resistance of such alloys can be assessed by tailoring their surfaces or the respective interfaces in contact with the magnesium substrates. On the one hand, an elevated corrosion resistance is desired when heading for long-term adhesion-based applications, e.g. coating or adhesive bonding. On the other hand, a suitable corrosion rate in body fluids as well as a high biosafety is required for magnesium alloys in biomedical applications .
The aim of this contribution is to substantiate that, within the scope of manufacturing functional coatings or adhesive joints, individual layers within the multilayer system may be advantageously designed to be non-centrosymmetric. Thus the particular interfaces and the resulting interphases around the layers can be functionalized individually and their interaction with neighboring layers can be tuned. Microscopically, the interphases—characterized by their chemical composition or conformational equilibria —and the interactions undergo dynamic changes during the manufacture and application of the multilayer systems .
Finally, the time-dependent interaction of molecular films applied to an AM50 magnesium alloy substrate was assessed using a liquid water film as a probe system for all the layer systems depicted in Fig. 2. In a previous contribution , the contact between water films and polished AM50 substrates was shown to result in the generation of hydrogen bubbles within three seconds. The effect of the investigated functional layers on the kinetics of the hydrogen bubble formation, as represented by the onset time and the area density of evolving bubbles, may reveal their performance in competing with water molecules for active adsorption sites on the magnesium alloy surfaces.
This section details the experimental procedures applied during the manufacture of the layer systems on AM50 alloy surfaces. Moreover, the analysis methods used for characterizing the layers and their effects on hydrogen bubble formation in contact with liquid water films will be described.
In order to create fresh surfaces under ambient conditions, samples of magnesium alloy AM50 (Rocholl GmbH, Aglasterhausen, Germany) were cut and manually polished using dry and flat SiC sandpaper with a grit size of 800 mesh.
Plasma polymeric coatings were applied to the surface of the pre-treated AM50 substrates using a low pressure plasma enhanced chemical vapor deposition (PECVD) technique. The home-built low pressure plasma chamber was equipped with a TRUMPF Hüttinger Quinto system for radio frequency (13.56 MHz) generation. Three different SiOxCyHz–type coatings based on highly crosslinked plasma polymer primer layers were prepared with a variation in the deposition parameters, in particular regarding the mixing ratio between the gaseous educts hexamethyldisiloxane (HMDSO) and oxygen. This resulted in coatings with low, medium and high carbon concentrations, as revealed by X-ray photoelectron spectroscopy (XPS) investigations.
Furthermore, the AM50 surfaces were laser-treated in an O2 and CO2 containing gas atmosphere under ambient pressure. The laser treatment was performed on polished AM50 samples using a cleanLaser 250 system comprising a Q-switched Nd-YAG laser with a wavelength of 1064 nm. A pulse duration of 80 ns was applied.
A third approach was the use of the water-based formulation G50 wb of the amphiphilic polymer additive G50 (Straetmans High TAC GmbH, Hamburg, Germany), as described elsewhere . Based on a parent formulation containing 1 wt% of organic constituents comprising polymer and triethanolamine (TEA) for adjusting the pH value, diluted formulations were prepared by adding demineralized water to the parent formulation.
Finally, a laccase mixture suspension (concentration 0.1 mg/ml) was prepared with laccase from a Trametes versicolor/maltodextrin powder mixture (from ASA Spezialenzyme GmbH, Wolfenbüttel, Germany) in water. The suspension was prepared at 25 °C using deionized water and working under aseptic conditions.
Investigations into the surface composition were performed by X-ray photoelectron spectroscopy (XPS) applied to small pieces cut from the AM50 sheets. XPS spectra with an information depth of around 0.01 µm were taken using a Kratos Ultra system applying excitation of photoelectrons by monochromatic Al Kα radiation within an area of approximately 0.2 mm2. The system was operated at a base pressure of 4 × 10−8 Pa and the sample neutralization was performed with low energy electrons (<5 eV). An electrostatic lens was used, the take-off angle of the electrons was 0°, and the pass energy was fixed to 20 eV (or 40 eV in the case of some of the less concentrated constituents) in high resolution spectra and 160 eV in survey spectra. Elemental ratios were calculated based on the area of the peaks and considering relative sensitivity factors. The binding energy calibration of the electrically isolating samples was performed by referring the C 1s component of aliphatic carbon species to 285.0 eV. Binding energies are given with a precision of ±0.1 eV throughout this contribution.
An aerosol wetting test (AWT) was used to characterize the wetting properties of selected surfaces. In this test, monodisperse droplets of an aerosol are generated using an ultrasound nozzle and are applied to the substrate surface forming wide or narrow drops depending on the surface state and surface energy. In this manner, the wettability of surfaces can be characterized by evaluating the resulting droplet size distribution. Details of this technique are described elsewhere [12, 23].
The sample topography was studied using a scanning probe microscope (SPM) operated as a scanning force microscope in the ‘tapping mode’ in air (Digital Instruments Nanoscope III multimode). The maximum scan range of the scanner was 150 µm, and Si cantilevers (Nanosensors) with resonance frequencies of around 250 kHz corresponding to force constants of around 20 N/m were used. The nominal tip diameter was in the range of 10 nm.
The SEM examination of the samples was performed in a field emission scanning electron microscope (FESEM), type FEI Helios 600 (Dual Beam). The resolution was 0.9 nm at 15 kV at optimal working distance and 1 nm at 15 kV at the coincidence point. The images of the sample surface were generated at acceleration voltages between 0.35 and 30 kV and at working distances between 1 and 10 mm. For the detection of secondary or backscattered primary electrons an Everhart–Thornley or an in-lens detector may be used, and for STEM studies (scanning transmission electron microscopy) a bright field, dark field and 12-segment HAADF (high-angle annular dark field) detector. For cryo-SEM investigations, a Quorum PP2000T preparation-system is available. Energy dispersive X-ray analysis (EDX) measurements were performed with an Oxford X-Max80 silicon drift detector (SDD) with an ATW2-window and an energy resolution down to 129 eV. The detection angle of the detector was 35°.
Film thickness measurements were performed using a variable-angle spectroscopic ellipsometer (VASE; from J.A. Woollam Co., USA) with incident angles of 75° and 65° in the spectral range of 300–800 nm. A fit procedure based on a Cauchy model describing the plasma polymer coating was employed.
Using a light microscope (Keyence VHX500) under ambient conditions, AM50 substrates with a size of 25 × 25 mm were submitted to the hydrogen bubble formation test (H2BT), contacting the substrate surface with approximately 0.27 mL of water or aqueous polymer formulations. Videos of up to 20 min duration were recorded. Details of the procedure are described elsewhere .
Results and discussion
This section initially highlights some of the bulk and surface properties of the magnesium alloy AM50 substrates. After this, the characteristics of the four distinct layer systems as depicted in Fig. 2 and applied to polished AM50 surfaces will be highlighted and discussed. Finally, an outlook concerning ongoing investigations will be presented.
Characterization of polished AM50 substrates
Summarizing the findings for the properties of polished AM50 substrates, one aspired functional property of a coating system on AM50 surfaces in contact with liquid water films may be the delay of the onset of hydrogen bubble formation and the lowering of the area density of surface sites contributing to the formation of hydrogen bubbles.
Characterization of coated AM50 substrates
The four distinct coating systems applied to polished AM50 surfaces will be regarded in the sequence depicted in Fig. 2.
With the resistance of the resulting polymer films to rinsing with water, they seem to be promising candidates for application not only as surfactants but also as interfactants, should further films be intended to be grown on top of a polymer-coated surface. Unlike surfactants which are active at the surface of (multi)layer systems, the concept of interfactants was introduced for layers which are more strongly bound to the substrate than the film to be grown , and which affect the kinetics of the subsequently growing molecular layers. Interfactant layers are generally discussed in the context of growth occurring from species adsorbed from the gas phase, and they may form wetting layers separating molecular films from a substrate  and constituting a diffusion barrier . Moieties forming interfactant layers can be atomic, molecular or ionic species, and the concept could be extended to strongly adsorbed polymeric species affecting the growth of subsequent molecular layers from a condensed phase.
Results of XPS investigations, with surface concentrations given in atomic % (at%), performed for polished AM50 samples and AM50 substrates in contact for distinct periods with aqueous (aq.) biopolymer formulations containing laccase and maltodextrin
AM50, 15 s immersion in aq. laccase
AM50, 60 s immersion in aq. laccase
AM50, 30 min immersion in aq. laccase
In summary, four approaches are depicted in Fig. 2. Cases a and b comprise dry processes with reactive precursors and a physical energy impact. Cases c and d comprise wet processes and are based on adsorption at room temperature. These approaches may be compared with respect to their effectivity in delaying the onset of hydrogen bubble formation in water films that are in contact with magnesium alloy surfaces, as illustrated in Fig. 6. Concerning the approaches based on dry processes, layer systems with a thickness of between 0.1 and 1 µm have been presented. Closed, dense and smooth, and integrally applied SiOxCyHz-type plasma polymer layers deposited in low pressure plasma showed superior effectivity compared to reaction layers formed locally by laser treatment in air. Tuning the composition of the gas phase and further optimizing the parameters guiding the energy impact during the laser treatment will contribute to increasing the performance of these layers. In comparison with PECVD or high-energy laser  processes the application of liquid formulations by dipping or spraying facilitates a less elaborate or milder surface functionalization, resulting in thinner layers allowing for instance the modification of nanostructured substrates [33, 34]. Concerning the approaches based on wet processes, the investigated natural biopolymer mixtures containing laccase and maltodextrin, as well as the synthetic amphiphilic polymers optimized for the application on aluminum alloys , showed multi-metal effectivity evidenced by a fast adsorption on AM50 surfaces, resulting in layers with a thickness between 0.001 and 0.01 µm. Using an aqueous G50 wb formulation revealed superior performance in suppressing the formation of hydrogen bubbles for time ranges of up to 20 min. That is a time-scale comparable to that required for the drying and hardening of water-based primer, coating or adhesive systems. Combining such approaches with the application of a preceding dry process, as shown for cases a and b, may further enhance the performance of the functional layer system.
In this contribution, the effects of barrier layers, which had been applied to polished magnesium alloy AM50 surfaces either by the deposition of siliceous polymer coatings in low pressure plasma processes, by laser surface treatments in controlled gas atmospheres, or by dipping in liquid formulations containing a recently developed polymeric inhibitor or a mixture of the enzyme laccase and the polysaccharide maltodextrin, were monitored with regards to their interactions with molecular water films. These films may be comprehended as a simplified implementation of mixed molecular films that are essential during the build-up of adhesion in painting or adhesive systems. For this purpose, we described the design of layer systems composed of single, non-centrosymmetric layers deposited using process materials and protocols that can be applied to various substrates. Combinations of the layer-forming processes presented may result in an even more effective multilayer-functionalization of magnesium substrates.
StSt and PSt laid out and performed the Hydrogen Bubble Formation Test, contributed to the discussion of microscopic and spectroscopic data; PSt and WN also contributed with the AM50 laser surface treatment; YC performed Scanning Probe Microscopy investigations and developed and set up the biofunctionalization experiments of magnesium surfaces with laccase supported by StSt and contributed to the lay-out and formatting of the article; DS trained and advised the low pressure plasma team comprising GH and WK who performed the plasma polymer coatings and characterized their structural and surface properties; JI trained and advised the laser surface treatment team comprising PSt and WN who contributed to the laser treatment and the microscopic and spectroscopic characterization of the surfaces; MN trained and advised the surface characterization team, comprising StSt, PSt and WN, and took part in defining and setting up the experiments, performing XPS investigations, and analyzing data; and WLC contributed in planning the conceptual approach for joint research in several teams, discussing and merging the obtained data, and in drafting the manuscript. All authors read and approved the final manuscript.
StSt, PSt, GH, WN, and WK participated in the program Science without Borders at Fraunhofer IFAM Bremen, where the experiments were performed.
The authors are grateful to Science without Borders (Ciência sem Fronteiras), to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Science without Borders Process N 238488/2012-8 Priscilla Stachera from September 2012 to August 2013, 203099/2011-7 Wilson Neto from March 2012 to February 2013, 203078/2011-0 Gustavo Hrycyna from March 2012 to February 2013, 238434/2012-5 Wagner Kazuki de Azambuja from March 2012 to February 2013) and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (Science without Borders Process N 88888.020610-2013-00 Stephani Stamboroski from September 2013 to December 2014), to CONICIT Costa Rica, to Leandro Gonçalves de Andrade Rosato for support when applying the Aerosol Wetting Test (AWT), Dr. Marko Soltau for providing material and taking part in fruitful discussions, to Dr. Karsten Thiel for performing electron microscopy investigations, to Dr. Uwe Specht for facilitating laser surface treatments, and to Prof. Dr. João Carlos Gomes and, last but not least, to Prof. Dr. Bernd Mayer for their steady support.
The authors declare that they have no competing interests.
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