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- ItemNanoporous Morphogenesis in Amorphous Carbon Layers: Experiments and Modeling on Energetic Ion Induced Self‐Organization(Weinheim : Wiley-VCH Verlag, 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.
- ItemTailoring morphology in titania nanotube arrays by implantation: experiments and modelling on designed pore size—and beyond(London [u.a.] : Taylor & Francis, 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.
- ItemCarbon and Neon Ion Bombardment Induced Smoothing and Surface Relaxation of Titania Nanotubes(Basel : MDPI, 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.
- ItemImproved Adhesion of the DLC Coating Using HiPIMS with Positive Pulses and Plasma Immersion Pretreatment(Basel : MDPI, 2021) Gómez, Iñigo; Claver, Adrián; Santiago, José Antonio; Fernandez, Iván; Palacio, Jose Fernandez; Diaz, Cristina; Mändl, Stephan; Garcia, Jose AntonioDiamond-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