Ingenieurwissenschaften und zugeordnete Tätigkeiten
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In this work, the structural integrity of LBM fabricated IN625 small cylinders (d = 1 mm, h = 6 mm) was investigated regarding the porosity and the surface roughness by means of computed tomography. The measurements were carried out on a GE v|tome|x L 300/180 with a reconstructed voxel size of 2 µm. The pores were analyzed for size, shape and spatial distribution. The correlation between compactness C and spatial distribution showed that elongated pores (C < 0.2) appear exclusively within a distance of 80 µm to the sample surface. The reconstructed surface was digitally meshed and unwrapped to evaluate the mean roughness Ra. Since the gravity correlates linearly with the sine of the build angle, the influence of gravity on porosity and surface roughness was determined.
Nanocomposites are extremely versatile due to their physicochemical properties, which differ significantly from bulk homopolymers. One of the inorganic nanomaterials which are increasingly used as a filler in polymer matrices is boehmite, typically used as an inexpensive flame retardant. Here, it is used as a nanofiller in polycarbonate and polyamide, expecting to improve their mechanical properties. For industrial use boehmite is obtained by the solvothermal method, resulting in a layered nanomaterial, whereas naturally it occurs as single crystals with the size of <100µm. In this work we are obtaining and isolating boehmite crystals by a bottom-up method, in which a reaction between aluminum nitride and sodium hydroxide. Obtaining boehmite as microcrystals is necessary for its analysis and characterization, as well as to investigate its interaction with polymer matrices at the polymer/particle interface. Here, the obtained particles in polymer matrices are characterized with differential scanning calorimetry and thermogravimetry analysis.
Nanocomposites are extremely versatile due to their physicochemical properties, which differ significantly from bulk homopolymers. One of the inorganic nanomaterials which are increasingly used as a filler in polymer matrices is boehmite, typically used as an inexpensive flame retardant. Here, it is used as a nanofiller in polycarbonate and polyamide, expecting to improve their mechanical properties. For industrial use boehmite is obtained by the solvothermal method, resulting in a layered nanomaterial, whereas naturally it occurs as single crystals with the size of <100µm. In this work we are obtaining and isolating boehmite crystals by a bottom-up method, in which a reaction between aluminum nitride and sodium hydroxide. Obtaining boehmite as microcrystals is necessary for its analysis and characterization, as well as to investigate its interaction with polymer matrices at the polymer/particle interface. Here, the obtained particles in polymer matrices are characterized with differential scanning calorimetry and thermogravimetry analysis.
In well-annealed thin polymer films, with non-repulsive polymer/substrate interactions, an irreversibly adsorbed layer is formed. These adsorbed layers have shown enormous potential for technological applications. The growth kinetics and molecular dynamics of these buried layers in thin films are still not fully investigated due to the hard accessibility. Here, the irreversibly adsorbed layers of homopolymer thin films are revealed by solvent-leaching experiments. The growth kinetics of these layers is investigated as a function of original film thickness and annealing times. The thickness, topography and quality of the adsorbed layer is determined with Atomic Force Microscopy (AFM) and spectroscopic ellipsometry. Additionally, the molecular mobility of the adsorbed layer is investigated with Broadband Dielectric Spectroscopy (BDS). A recently developed nanostructured capacitor (NSC) is employed to measure the adsorbed layers with a free surface layer depending on annealing and solvent-leaching time. The results are quantitatively compared and discussed with respect to recently published work.
Over the past decades research on the molecular dynamics of miscible polymer blends is of topical interest in the literature, to understand the segmental mobility of individual components, as it is affected by blending. In general, miscible polymer blends exhibit a complex dynamic behavior. For an A/B blend the relaxation times of component A and component B are affected by the spatial local compositional heterogeneity, present in binary systems on a microscopic level, regardless of the macroscopic homogeneity. Here, a combination of broadband dielectric and specific heat spectroscopy was employed to study the dynamically asymmetric PVME/PS blend with seven different compositions, focusing on samples with high PS contents. Considering that PS is dielectrically invisible, BDS is a powerful technique to study the response of PVME, as it is affected by PS segments. Here, three separate relaxation processes were found by dielectric investigations, related to confined or constrained PVME segments due to the spatial local compositional heterogeneities, which is in contrary to the previous literature findings [1]. Moreover, the dielectric data was compared with results obtained by specific heat spectroscopy, where a fourth relaxation process was found, due to the cooperative fluctuations of PVME and PS.
[1] Colmenero, J., Arbe, A. Soft Matter, 2007, 3, 1474.
Mechanochemistry paves the way to simple, fast, and green syntheses. Despite considerable effort, there remains a lack in understanding of the underlying mechanisms. In situ investigations help to understand these mechanisms, which occur during a mechanochemical reaction. Here we present a universal strategy for simultaneous real-time in situ analysis, combining X-ray diffraction, Raman spectroscopy, and thermography. The potential of our approach is shown for different model reactions.
Fresh cement paste is a suspension consisting of a hydraulic binder (cement), water, and numerous minor components – admixtures. Addition of admixtures aims at specific modification of properties of the fresh cement paste or hardened cementitious building material. Specific admixtures, so-called superplasticizers (SP), are used to improve the flowability of the fresh cement paste with reduced water content. The latter is the starting material for the high-strength concrete. Thus, SPs are essential for the ambitious construction projects.
However, uncontrollable retardation of the setting time in presence of SPs is occasionally observed. Obviously, SPs influence early products of the cement hydration leading to changes in the microstructure development. The hardening is thus delayed, and the quality of the resulting building material suffers. The mechanisms of the admixture action during the hydration process are still intensively investigated [1-7]. A detailed understanding of the admixture effects during the early hydration stage is the key to control and individual adjustment of the cement-based construction materials.
We use the unique combination of the wall-free sample holder and the time-resolved X-ray scattering analysis to achieve the full information about the hydrate phases formed under the influence of admixtures. We use ultrasonic levitator to start the cement hydration in levitated cement pellets [8, 9]. The sample levitation allows collection of the unimpaired information about cement hydrate phases. The most beneficial is the avoiding of the contributions of the sample holder material to the data signal.
We induce the cement hydration by adding water to unhydrated Portland cement during the data acquisition. The full phase composition of the hydrating cementitious system can be gathered in situ using wide angle X-ray scattering (WAXS). During the hydration of cement both crystalline and amorphous hydrate phases are formed. WAXS data contain the information about crystalline phases behind the Bragg reflections, whereas the amorphous hydrates influence the appearance of the background. Application of the data analysis specific for crystalline or amorphous phases is needed. The data quantification by the Rietveld method allows to conclude about the changes of the phase amounts due to the presence of admixture. The calculation of the pair distribution functions allows analysis of the amorphous hydrates. Based on this information, the SP effects and the extent of their involvement into the ongoing reactions can be concluded. A detailed understanding of the complex cement hydration process is envisioned.
Ziel des Projektes HARFE (Haftfestigkeit Reproduzierbarkeit Festigkeit) war es, eine Er-höhung der Haft- bzw. Klebfestigkeit auf Niedrigenergie-Polymeren (PE, PP, PTFE) zu erreichen. SENTECH realisierte dazu plasmachemische Oberflächenaktivierungen mit O2 und die Abscheidung von Aluminiumoxidschichten (Al2O3) mittels Atomic Layer Deposition (ALD), wobei die Ellipsometrie zum in-situ Monitoring der ALD-Prozesse diente. Die BAM charakterisierte die modifizierten Oberflächen bezüglich der Oberflächenenergie (OFE) und bestimmte die Verbund- bzw. Klebfestigkeit mittels der Zentrifugentechnologie
Coating materials are nowadays often required to deliver not only sufficient barrier performance and suited optical appearance but a broad range of other functional properties. The incorporation of inorganic nanoparticles (NPs) is known to improve many key characteristics and provide new functionalities in polymer materials. Presented work aims to prepare and characterize an organic-inorganic coating material designed to bring together advantageous properties of hybrid materials and reinforcement effect delivered from the inorganic NPs embedment.
Siloxane-based hybrid resins hold great advantages as coating materials as their properties can be tuned between those of polymers and those of glasses, thus, the compositions with superior thermal and mechanical properties can be achieved. We used Cycloaliphatic Epoxy Oligosiloxane (CEOS) resin as a polymeric matrix where the network formation was achieved by UV induced cationic polymerisation. Boehmite Alumina (BA) nanoparticles were added to CEOS resin as a reinforcing agent and resultant material was processed into films either by bar-coating or by spin-coating depending on further characterization procedure. Two different types of BA NPs, hydrophilic and organophilic, were used in order to assess the impact of particles surface on the resin characteristics. CEOS synthesis by condensation reaction was confirmed using 13C and 29Si NMR. Changes in CEOS photocuring process, resulting from particles incorporation, were monitored by real-time IR spectroscopy. At the same time, the thermal behaviour was evaluated by DSC and TGA methods. Morphology of the coatings was investigated by means of SEM operated in transmission mode.
It was observed that BA presence increased the epoxy conversion degree and glass transition temperature. Material formulations providing best film characteristics were determined with regard to the particle type and loading. Compared to the hydrophilic nanoparticles, organophilic BA NPs yield superior overall performance of the foils.
Since the early 19th century microorganisms were studied on their capabilities of causing microbiologically influenced corrosion (MIC) of metals. The most studied ones are sulfate-reducing bacteria (SRB), but others can corrode metals as well, e.g. acid-producing bacteria or methanogenic archaea (MA). However, these studies were mostly focused on metals related to the petroleum industry but metals for other industries, e.g. dentistry, are also susceptible to corrosion. The inert Titanium (Ti) is often used as an implant material, but it is a base metal. The formation of a passivating oxide layer allows Ti to be corrosion resistant at normal conditions.
Nonetheless, scanning electron microscope images on dental implants from patients with acute peri-implantitis showed clear signs of corrosion. Currently, the corrosion mechanism of dental implants is unknown, but many indications suggest that oral microorganisms, including MA (Methanobrevibacter oralis) and SRB (Desulfomicrobium orale), could be involved.
To determine if MA or SRB can corrode Ti (pure Ti or Ti-6Al-4V alloy), corrosion rate, methane and sulfide concentrations were analyzed. Electrical potential measurements using in-house developed electrochemical cells indicated a potential change on Ti in the presence of a corrosive MA strain compared to an abiotic control.
Microbial composition comparison will be analyzed using samples from dental pockets of 150 infected patients by considering the quality of the implant and 50 healthy people by means of amplicon sequencing. Enrichments and isolation of pure cultures from the dentals samples are also examined for their corrosion behavior. Overall, this is the first study investigating the susceptibility of dental implant material to corrosion using human related MA.
Polymeric core-shell particles were synthesized in a semi-batch emulsion polymerization process. The shell of the particles consist of PVDF with a high amount of beta-phase. Small-angle X-ray scattering (SAXS) was used to quantify the size of the cores of the particles and the thickness of the shell.
Absorption edge tomography is a method which exploits the sudden change of the attenuation coefficient, when the photon energy crosses the absorption edge of an element. The beamline BAM line at BESSY II, which is operated by the Federal Institute for Materials Research and Testing, can provide a monochromatized beam in a photon energy range from 5 keV up to 80 keV with a bandwidth of 2%. Together with the microtomography setup, this enables differential tomography sensitive to any element with N >= 24 (Cr) by using an appropriate K- or L-edge in this range. Here, a polymer filament embedding metal organic framework (MOF) was prepared and used for 3D printing. Absorption edge tomography at the copper K edge was employed to perform a non-destructive 3D characterization of the microstructure of the embedded MOF. Data fusion was then used to determine the size distribution of the embedded MOF.
Fiber reinforced polymers (FRPs) are a well established material in lightweight applications, e.g. in automotive, aerospace or wind energy. The FRP components are subjected to multiaxial mechanical as well as hygrothermal loads. Common operation temperatures are in the range of 213 K and 373 K (-60 °C and 100 °C) at a relative humidity of 10% to 90%. In spacecraft applications, the environmental conditions are even more extreme. However, the correlation between multiaxial mechanical loading and harsh environment conditions have to-date not been investigated in detail. The project aims to investigate the fatigue behavior of FRPs dependent on multiaxial mechanical loading, temperature, and humidity. Extensive experimental testing is performed on flat plate and cylindrical tube specimens, accompanied by numerical and analytical calculations.
Using non-optimum combination of manufacturing parameters in selective laser melting (SLM) may lead to reduction of quality of component: defects generation, distortion of geometry and even cracking. Usually, the optimization of parameters is performed by changing volumetric energy density (Ev) and selecting parameters giving low porosity values. However, not only low porosity but also stable microstructure and low residual stresses will help to achieve advanced mechanical behavior of the component.
In present work, we investigated cuboid-shaped Ti-6Al-4V samples produced with different manufacturing parameters. The parameters leading to the same Ev were considered as well as parameters which are not included in Ev. Residual stresses in subsurface region were investigated by synchrotron X-ray diffraction, which allows to penetrate around 100 µm from the surface therefore overcome the problem of high roughness of SLM components without additional sample preparation. Only tensile stresses were found along the building direction, that can play critical role especially during cyclic loading. In parallel, using X-ray computed tomography we also observe that porosity is mainly concentrated in the contour region, except in case where the laser speed is small. However, by using some process parameters it was possible to decrease residual stresses and obtain uniform α+β Ti microstructure and relatively low porosity. Additionally, it was found that not included in Ev (e.g., base plate position, focus distance) should be considered as additional manufacturing parameters during SLM process.
The porosity and the surface roughness are recently discussed problems for SLM parts. The influence of SLM process parameters on porosity is well studied for different materials. Nevertheless, the build angle (i.e. the angle between part orientation and build plate) needs to be understood as an additional SLM process parameter, as it has been shown, that the microstructure and hence the mechanical performance of various materials depend on the build angle. The inherent build angle of each strut as a part of a lattice structure is the motivation to investigate the influence of the build angle on the porosity and roughness on round-shaped (1 mm diameter) struts by means of CT. Conventional Coordinate Measuring Machine (CMM) has the limitation towards small and round shaped samples. The need for Computed Tomography (CT) regarding investigations of SLM parts will increase because no other non-destructive technique allows the assessment of complex geometries with inner laying surfaces. We used CT to assess the pores and the strut surface. Seven struts out of the nickel alloy Inconel 625 with build angles from 30° to 90° were studied. It was found that the number of pores is smaller, and the size of pores is larger for the 90° strut. In case of 30° strut, the number of pores is increased towards down-skin side, additionally, this strut orientation showed to have the largest number of attached powder particles. The elongated pores exist exclusively near the strut surface. While the roughness at the down-skin surface is highly depending of the biud angle, the roughness at the up-skin surface is the same for all struts. The mechanisms of pore and surface roughness formation is not mainly driven by gravity.
Additive manufacturing (AM) opens the route to a range of novel applications. However, the complexity of the manufacturing process poses a challenge to produce defect-free parts with a high reliability. Since process dynamics and resulting microstructures of AM parts are strongly influenced by the involved temperature fields and cooling rates, thermography is a valuable tool for process monitoring. Another approach to monitor the energy input into the part during process is the use of optical tomography.
Common visual camera systems reach much higher spatial resolution than infrared thermography cameras, whereas infrared thermography provides a much higher temperature dynamic. Therefore, the combined application increases the depth of information. Here, we present first measurement results using a laser beam melting setup that allows simultaneous acquisition of thermography and optical tomography from the same point of view using a beam splitter. A high-resolution CMOS camera operating in the visible spectral range is equipped with a near infrared bandpass filter and images of the build plate are recorded with long-term exposure during the whole layer exposing time. Thus, areas that reach higher maximum temperature or are at elevated temperature for an extended period of time appear brighter in the images. The used thermography camera is sensitive to the mid wavelength infrared range and records thermal videos of each layer exposure at an acquisition rate close to 1 kHz.
As a next step, we will use computer tomographic data of the built part as a reference for defect detection.
This research was funded by BAM within the focus area Materials.