2021, Hoffmann, Daniel T., Dietrich, Johannes, Mändl, Stephan, Zink, Mareike, Mayr, Stefan G.

Nanoporous amorphous carbon constitutes a highly relevant material for a multitude of applications ranging from energy to environmental and biomedical systems. In the present work, it is demonstrated experimentally how energetic ions can be utilized to tailor porosity of thin sputter deposited amorphous carbon films. The physical mechanisms underlying self-organized nanoporous morphogenesis are unraveled by employing extensive molecular dynamics and phase field models across different length scales. It is demonstrated that pore formation is a defect induced phenomenon, in which vacancies cluster in a spinodal decomposition type of self-organization process, while interstitials are absorbed by the amorphous matrix, leading to additional volume increase and radiation induced viscous flow. The proposed modeling framework is capable to reproduce and predict the experimental observations from first principles and thus opens the venue for computer assisted design of nanoporous frameworks.

2021, Kupferer, Astrid, Mensing, Michael, Lehnert, Jan, Mändl, Stephan, Mayr, Stefan G.

Titania nanotube arrays with their enormous surface area are the subject of much attention in diverse fields of research. In the present work, we show that not only 60 keV and 150 keV ion bombardment of amorphous titania nanotube arrays yields defect creation within the tube walls, but it also changes the surface morphology: the surface relaxes and smoothens in accordance with a curvature-driven surface material’s transport mechanism, which is mediated by radiation-induced viscous flow or radiation-enhanced surface diffusion, while the nanotubes act as additional sinks for the particle surface currents. These effects occur independently of the ion species: both carbon and neon ion bombardments result in comparable surface relaxation responses initiated by an ion energy of 60 keV at a fluence of 1 × 1016 ions/cm2. Using atomic force microscopy and contact angle measurements, we thoroughly study the relaxation effects on the surface topography and surface free energy, respectively. Moreover, surface relaxation is accompanied by further amorphization in surface-near regions and a reduction in the mass density, as demonstrated by Raman spectroscopy and X-ray reflectivity. Since ion bombardment can be performed on global and local scales, it constitutes a versatile tool to achieve well-defined and tunable topographies and distinct surface characteristics. Hence, different types of nanotube arrays can be modified for various applications.

2012, Rauschenbach, Bernd, Gerlach, Jürgen W., Hirsch, Dietmar, Mändl, Stephan, Arnold, Th., Mießler, André, Prager, Lutz, Prager, Andrea, Elsner, Christian, Reichelt, Senta, Pender, Alya

[no abstract available]

2020, Manova, Darina, Schlenz, Patrick, Gerlach, Jürgen W., Mändl, Stephan

Expanded austenite γN formed after nitrogen insertion into austenitic stainless steel and CoCr alloys is known as a hard and very wear resistant phase. Nevertheless, no single composition and lattice expansion can describe this phase with nitrogen in solid solution. Using in situ X-ray diffraction (XRD) during ion beam sputtering of expanded austenite allows a detailed depth-dependent phase analysis, correlated with the nitrogen depth profiles obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS) or glow discharge optical emission spectroscopy (GDOES). Additionally, in-plane XRD measurements at selected depths were performed for strain analysis. Surprisingly, an anomalous peak splitting for the (200) expanded peak was observed for some samples during nitriding and sputter etching, indicating a layered structure only for {200} oriented grains. The strain analysis as a function of depth and orientation of scattering vector (parallel/perpendicular to the surface) is inconclusive. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

2021, Gómez, Iñigo, Claver, Adrián, Santiago, José Antonio, Fernandez, Iván, Palacio, Jose Fernandez, Diaz, Cristina, Mändl, Stephan, Garcia, Jose Antonio

Diamond-like carbon (DLC) coatings are used due to their extraordinary tribomechanical properties, great hardness, high elastic modulus, high wear resistance, low friction coefficient and chemical inertness, which provide them with biocompatibility. Compared to other physical vapor deposition (PVD) coatings of transition nitrides and carbonitrides, DLC has limited adhesion, so it is necessary to develop new techniques to overcome this limitation. This work reports the results of scratch testing for the measurement of adhesion and of tests for wear resistance and nanoindentation in AISI 316L stainless steel coated with a WC:C coating, produced using novel high-power impulse magnetron sputtering (HiPIMS) technology with positive pulses. In addition, the use of a preceding surface modification technique, specifically plasma immersion ion implantation (PIII), was studied with the aim of optimizing the adhesion of the coating. The results show how the coating improved the tribomechanical properties through the use of positive pulse HiPIMS compared to conventional HiPIMS, with an adhesion result that reached critical load values of 48.5 N and a wear coefficient of 3.96 × 10−7 mm3/nm

2021, Kupferer, Astrid, Mändl, Stephan, Mayr, Stefan G.

Titania nanotube arrays are an exceptionally adaptable material for various applications ranging from energy conversion to biomedicine. Besides electronic properties, structural morphology on nanometre scale is essential. It is demonstrated that ion implantation constitutes a versatile method for the synthesis of tailored nanotube morphologies. Experimental-phenomenological observations reveal a successive closing behaviour of nanotubes upon ion implantation. Employing molecular dynamics calculations in combination with analytical continuum models, the physical origins of this scenario are unravelled by identifying ion bombardment induced viscous flow driven by capillarity as its underlying mechanism besides minor contributions from sputtering and redeposition. These findings enable the tailoring of nanotube arrays suitable for manifold applications.

2021, Kupferer, Astrid, Holm, Alexander, Lotnyk, Andriy, Mändl, Stephan, Mayr, Stefan G.

Ranging from novel solar cells to smart biosensors, titania nanotube arrays constitute a highly functional material for various applications. A promising route to modify material characteristics while preserving the amorphous nanotube structure is present when applying low-energy ion implantation. In this study, the interplay of phenomenological effects observed upon implantation of low fluences in the unique 3D structure is reported: sputtering versus readsorption and plastic flow, amorphization versus crystallization and compositional patterning. Patterning within the oxygen and carbon subsystem is revealed using transmission electron microscopy. By applying a Cahn–Hilliard approach within the framework of driven alloys, characteristic length scales are derived and it is demonstrated that compositional patterning is expected on free enthalpy grounds, as predicted by density functional theory based ab initio calculations. Hence, an attractive material with increased conductivity for advanced devices is provided. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH