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Improving the surface properties of additive-manufactured Inconel 625 by plasma nitriding
Danijela Anica Skobir Balantič, Črtomir Donik, Bojan Podgornik, Aleksandra Kocijan, Matjaž Godec, 2023, original scientific article

Abstract: As a surface-hardening technique, plasma nitriding is a common procedure for improving the properties of conventional Ni-based alloys. The diffusion of nitrogen hardens a layer on the surface of the alloy, leading to better wear resistance and a higher coefficient of friction, as well as a higher surface hardness. This study reports the effect of plasma nitriding on additive-manufactured (AM) Inconel 625 (IN625) compared to its conventional manufactured and nitrided counterparts. The samples produced with the laser powder-bed fusion (LPBF) process were subsequently plasma nitrided in the as-built condition, stress-relief annealed at 870 °C and solution treated at 1050 °C. The plasma nitridings were carried out at 430 °C and 500 °C for 15 h. The growth kinetics of the nitride layer of the AM samples depends on the prior heat treatments and is faster in the as-built state due to the specific cellular structure. The lower nitriding temperature leads to the formation of expanded austenite in the nitride layer, while at the higher nitriding temperature, the expanded austenite decomposes and CrN precipitation occurs. The XRD and SEM analyses confirmed the presence of two layers: the surface layer and the diffusion layer beneath. The lower nitriding temperature caused the formation of expanded austenite or a combination of expanded austenite and CrN. The higher nitriding temperature led to the decomposition of the expanded austenite and to the formation/precipitation of CrN. The higher nitriding temperature also decreased the corrosion resistance slightly due to the increased number of precipitated Cr-nitrides. On the other hand, the wear resistance was significantly improved after plasma nitriding and was much less influenced by the nitriding temperature.
Keywords: additive manufacturing, powder-bed fusion, plasma nitriding, expanded austenite, wear and corrosion resistance, Ni-based alloy
Published in DiRROS: 31.01.2024; Views: 148; Downloads: 57
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Advanced method for efficient functionalization of polymers by intermediate free-radical formation with vacuum-ultraviolet radiation and producing superhydrophilic surfaces
Alenka Vesel, Rok Zaplotnik, Miran Mozetič, Nina Recek, 2023, original scientific article

Abstract: An efficient approach for tailoring surface properties of polymers is presented, which enables rapid modification leading to superhydrophilic properties. The approach is based on vacuum-ultraviolet radiation (VUV) pretreatment of the surface to create reactive dangling bonds. This step is followed by a second treatment using neutral oxygen atoms that react with the dangling bonds and form functional groups. The beneficial effect of VUV pretreatment for enhanced functionalization was clearly demonstrated by comparing VUV pretreatment in plasmas created in different gases, i.e., hydrogen, nitrogen, and oxygen, which differ in the intensity of VUV/UV radiation. The emission intensity of VUV radiation for all gases was measured by vacuum ultraviolet spectroscopy. It was shown that VUV has a strong influence on the treatment time and final surface wettability. A superhydrophilic surface was obtained only if using VUV pretreatment. Furthermore, the treatment time was significantly reduced to only a second of treatment. These findings show that such an approach may be used to enhance the surface reaction efficiency for further grafting of chemical groups.
Keywords: plasma treatment, vacuum-ultraviolet radiation treatment, surface functionalization, polymer polyvinyl chloride, vacuum-ultraviolet spectroscopy, vacuum-ultraviolet photons
Published in DiRROS: 06.06.2023; Views: 327; Downloads: 169
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Deposition of chitosan on plasma-treated polymers : a review
Alenka Vesel, 2023, review article

Abstract: Materials for biomedical applications often need to be coated to enhance their performance, such as their biocompatibility, antibacterial, antioxidant, and anti-inflammatory properties, or to assist the regeneration process and influence cell adhesion. Among naturally available substances, chitosan meets the above criteria. Most synthetic polymer materials do not enable the immobilization of the chitosan film. Therefore, their surface should be altered to ensure the interaction between the surface functional groups and the amino or hydroxyl groups in the chitosan chain. Plasma treatment can provide an effective solution to this problem. This work aims to review plasma methods for surface modification of polymers for improved chitosan immobilization. The obtained surface finish is explained in view of the different mechanisms involved in treating polymers with reactive plasma species. The reviewed literature showed that researchers usually use two different approaches: direct immobilization of chitosan on the plasma-treated surface or indirect immobilization by additional chemistry and coupling agents, which are also reviewed. Although plasma treatment leads to remarkably improved surface wettability, this was not the case for chitosan-coated samples, where a wide range of wettability was reported ranging from almost superhydrophilic to hydrophobic, which may have a negative effect on the formation of chitosan-based hydrogels.
Keywords: polymer surfaces, chitosan, coatings, plasma-surface modification, adhesion
Published in DiRROS: 24.02.2023; Views: 357; Downloads: 199
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A review of recombination coefficients of neutral oxygen atoms for various materials
Domen Paul, Miran Mozetič, Rok Zaplotnik, Gregor Primc, Denis Đonlagić, Alenka Vesel, 2023, original scientific article

Abstract: Relevant data on heterogeneous surface recombination of neutral oxygen atoms available in the scientific literature are reviewed and discussed for various materials. The coefficients are determined by placing the samples either in non-equilibrium oxygen plasma or its afterglow. The experimental methods used to determine the coefficients are examined and categorized into calorimetry, actinometry, NO titration, laser-induced fluorescence, and various other methods and their combinations. Some numerical models for recombination coefficient determination are also examined. Correlations are drawn between the experimental parameters and the reported coefficients. Different materials are examined and categorized according to reported recombination coefficients into catalytic, semi-catalytic, and inert materials. Measurements from the literature of the recombination coefficients for some materials are compiled and compared, along with the possible system pressure and material surface temperature dependence of the materials’ recombination coefficient. A large scattering of results reported by different authors is discussed, and possible explanations are provided.
Keywords: heterogeneous surface recombination, recombination coefficient, surface catalicity, catalytic efficiency, atom loss coefficient, oxygen, neutral atoms, plasma
Published in DiRROS: 22.02.2023; Views: 452; Downloads: 207
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A method for the immobilization of chitosan onto urinary catheters
Alenka Vesel, Nina Recek, Rok Zaplotnik, Albert Kurinčič, Katja Kuzmič, Lidija Fras Zemljič, 2022, original scientific article

Abstract: A method for the immobilization of an antibacterial chitosan coating to polymeric urinary medical catheters is presented. The method comprises a two-step plasma-treatment procedure, followed by the deposition of chitosan from the water solution. In the first plasma step, the urinary catheter is treated with vacuum-ultraviolet radiation to break bonds in the polymer surface film and create dangling bonds, which are occupied by hydrogen atoms. In the second plasma step, polymeric catheters are treated with atomic oxygen to form oxygen-containing surface functional groups acting as binding sites for chitosan. The presence of oxygen functional groups also causes a transformation of the hydrophobic polymer surface to hydrophilic, thus enabling uniform wetting and improved adsorption of the chitosan coating. The wettability was measured by the sessile-drop method, while the surface composition and structure were measured by X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Non-treated samples did not exhibit successful chitosan immobilization. The effect of plasma treatment on immobilization was explained by noncovalent interactions such as electrostatic interactions and hydrogen bonds.
Keywords: polymer, chitosan immobilization, adhesion, plasma-surface modification, biopolymers
Published in DiRROS: 05.12.2022; Views: 441; Downloads: 272
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