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Organisationseinheit der BAM
- 5 Werkstofftechnik (7)
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- 5.6 Glas (3)
- 6 Materialchemie (3)
- 6.7 Materialsynthese und Design (3)
- 6.1 Oberflächen- und Dünnschichtanalyse (2)
- 6.3 Strukturanalytik (2)
- 9 Komponentensicherheit (2)
- 9.5 Tribologie und Verschleißschutz (2)
- 6.2 Material- und Oberflächentechnologien (1)
X-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (XTEM) were used to study the formation of AlN films by N+2 ion implantation of aluminium at energies of 3 keV and 100 keV. In both cases, a two-stage mechanism was found, comprising first the oriented precipitation of small particles of the hexagonal AlN-phase, followed by growth and coalescence finally forming a continuous AlN-layer while increasing the implantation dose from 1×1017 cm-2 to 2×1017 cm-2. The results of both methods are in excellent agreement and furthermore provide complementary information concerning chemical composition and binding energies as well as microstructural details.
In order to obtain a better understanding of the role of tribofilms during automotive braking, their structures were investigated and the essential features identified.
Next, different ingredient combinations were studied by preparing some model materials with simpler compositions than real tribofilms. A test method was developed for verifying the tribological properties of the model materials and for comparison with results obtained with numerical sliding simulations of such structures. Prerequisites of good brake Performance properties were identified. Although the coefficient of friction could be varied in a wide ränge, smooth sliding conditions could only be achieved with values smaller than 0.4. Tests with artificial third body powders turned out to be useful as screening method for the selection of raw materials for brake pad formulations.
Technically relevant P92 steel (9% Cr) was coated with a micron-thick porous alumina layer prepared by sol-gel technique and treated with flue gas (60 CO2-30 H2O-2 O2-1 SO2-7 N2 (mole fraction in %)) at 650 ° to mimic an oxyfuelcombustion process. Local defects in the coating were marked using focused ion beam (FIB) technique and were inspected after exposition to hot flue gas atmosphere at 300, 800, and 1300 h, respectively. Local defects like agglomerated alumina sol particles tend to spall off from the coating uncovering the underlying dense chromia scale. Re-coating was found to restore the protection ability from oxidation when repeatedly treated with hot flue gas. Cracks and voids did not promote the local oxidation due to the formation of crystalline Mn/S/O species within and on top of the coating. The protective character of the steel-coating system is a result of (i) the fast formation of a dense chromia scale at the surface of sol-gel alumina-coated P92 steel bars in combination with (ii) the porous alumina coating acting as diffusion barrier, but also as diffusion partner in addition with (iii) fast Mn outward diffusion capturing the S species from flue gas.
Future coal power plants will in case of oxyfuel combustion be operated with altered atmospheres. Hence, corrosion attack might become more severe and steels have to be protected. An alumina-sol was used to coat X20CrMoV12-1 (X20) with alumina to test the protection. Testing was performed at 600 °C in flowing H2O–CO2–O2 and static laboratory air for 1000 h. Oxidation under air is minor compared to exposure in oxyfuel atmosphere. In both cases a multilayered oxide (hematite, magnetite, spinel) was formed on uncoated steels. Carburization appeared on uncoated X20 in H2O–CO2–O2. The coating demonstrates a high protection.
In Kohlekraftwerken mit Oxyfuel-Technologie erfolgt die Verbrennung unter Zufuhr von reinem Sauerstoff. Damit steht für die Abscheidung und Speicherung von CO2 ein Rauchgas mit hohem CO2-Anteil zur Verfügung (CCS-Technologie). Aufgrund dieses Verbrennungsprozesses ändert sich im Wesentlichen die Rauchgaszusammensetzung hin zu hohem H2O- und SO2 -Anteil. Deshalb wurden in verschiedenen Laboratorien kommerzielle Kraftwerksstähle unter den entsprechenden korrosiven Bedingungen getestet. Im Vergleich zur herkömmlichen Verbrennungsmethode zeigten die ferritisch-martensitischen Stähle schon nach 1000 h Auslagerungszeit unter Oxyfuel-Bedingungen eine durch einen deutlich erhöhten Materialverlust gekennzeichnete Korrosion. Eine Möglichkeit um dennoch die Lebensdauer dieser Stähle zu gewährleisten, besteht im Aufbringen geeigneter Schutzschichten auf die Stahloberfläche. Die vorliegende Arbeit befasst sich mit der Herstellung und Bewertung von Sol-Gel-Aluminiumoxidschichten als Schutz vor Korrosion in Oxyfuel-Rauchgas-Atmosphäre. Diese wurden auf den warmfesten Stahl X20CrMoV12-1 (X20) mittels der Tauchmethode aufgebracht. Anschließend erfolgte eine Auslagerung der beschichteten Proben für 1000 h bei 600 °C in einer H2O-CO2-O2-SO2 -Atmosphäre.
Im Vergleich zum unbeschichteten Stahl X20 war der Materialverlust des beschichteten Stahls deutlich geringer. Insbesondere konnte durch die Aluminiumoxidschicht der Transport von CO2 und SO2 zur Stahloberfläche verhindert werden.
In Kohlekraftwerken mit Oxyfuel-Technologie erfolgt die Verbrennung unter Zufuhr von reinem Sauerstoff. Damit steht für die Abscheidung und Speicherung von CO2 ein Rauchgas mit hohem CO2-Anteil zur Verfügung (CCS-Technologie). Aufgrund dieses Verbrennungsprozesses ändert sich im Wesentlichen die Rauchgaszusammensetzung hin zu hohem H2O- und SO2 -Anteil. Deshalb wurden in verschiedenen Laboratorien kommerzielle Kraftwerksstähle unter den entsprechenden korrosiven Bedingungen getestet. Im Vergleich zur herkömmlichen Verbrennungsmethode zeigten die ferritisch-martensitischen Stähle schon nach 1000 h Auslagerungszeit unter Oxyfuel-Bedingungen eine durch einen deutlich erhöhten Materialverlust gekennzeichnete Korrosion. Eine Möglichkeit um dennoch die Lebensdauer dieser Stähle zu gewährleisten, besteht im Aufbringen geeigneter Schutzschichten auf die Stahloberfläche. Die vorliegende Arbeit befasst sich mit der Herstellung und Bewertung von Sol-Gel-Aluminiumoxidschichten als Schutz vor Korrosion in Oxyfuel-Rauchgas-Atmosphäre. Diese wurden auf den warmfesten Stahl X20CrMoV12-1 (X20) mittels der Tauchmethode aufgebracht. Anschließend erfolgte eine Auslagerung der beschichteten Proben für 1000 h bei 600 °C in einer H2O-CO2-O2-SO2 -Atmosphäre.
Im Vergleich zum unbeschichteten Stahl X20 war der Materialverlust des beschichteten Stahls deutlich geringer. Insbesondere konnte durch die Aluminiumoxidschicht der Transport von CO2 und SO2 zur Stahloberfläche verhindert werden.
The need for a more efficient coal power plant generation (e.g. oxyfuel technology) results in modified process parameters and enhanced corrosion. To reach the necessary service life of high temperature parts protective coatings may be a sufficient technical solution. A modified Yoldas sol (Al2O3 based) was used to coat X20CrMoV12-1 by spin coating. After appropriate heat treatments transition alumina coatings being about 400 nm thick were obtained. Oxidation studies were carried out in laboratory air at temperatures up to 650 °C for up to 500 h exposure time. In case of the uncoated sample a rough oxide layer formed on the surface and a remarkable weight gain (2.62 mg/cm²) were detected. The sol–gel alumina layer (mainly δ-Al2O3) demonstrated a high protection, i.e. a very low weight gain (0.05 mg/cm²). Diffusion of alloying elements into the coating was observed. No indication of spallation of the coating occurred. Local defects (2 µm–30 µm) in the coating led to the formation of iron-oxide islands.
Transmission electron microscopy study of rapid thermally annealed Pd/Ge contacts on IN0.53Ga0.47As
(1996)
The paper presents the microstructure, chemical and phase composition of thin scale, obtained as a result of high temperature corrosion of X20Cr13 stainless steel. Samples were exposed to gas atmosphere of the following composition: 0.25 vol.% of SO2 and 99.75 vol.% of Ar at 600 °C for 5 h. As a consequence, thin compact scale was formed on steel surface.
This scale consisted of three different zones. An amorphous zone was formed close to steel surface. Then, nanocrystalline Zone could be observed. Finally, larger grains were formed during the corrosion process. The analysis of the chemical composition revealed higher concentration of chromium near steel surface. In contrast, to chromium, the content of iron, increased near the scale surface. It was found out that the (Cr, Mn, Fe)5O12 phase appeared in the thin scale.
Diffraction of hard synchrotron radiation as well as constant-wavelength and time-of-flight neutron diffraction were used for the structural characterization of a silicon carbide powder having extremely low levels of chemical impurities, high perfection of the crystalline lattice and a grain size of up to 150 µm. The presence of three polytypes was ascertained and the ratios of their mass fractions were determined to be w15R : w6H = 0.002,3(8) and w4H : w6H = 0.000,6(2).
Inconel 718 was spin coated twice and fourfold with a sol to obtain 200- and 400-nm-thick transition alumina films on the surface. Bare and solgel alumina-coated Inconel 718 samples were exposed to NaCl solution to study their corrosion behavior by means of electrochemical impedance spectroscopy. In combination with scanning electron microscopy, it was shown that bare Inconel 718 is after initial passivation prone to pitting corrosion. For the coated Inconel 718 samples, an improvement in the protective effect of the coatings with time was observed. This observation is in contradiction to the results of other authors who observed degradation of alumina coatings exposed to NaCl solution with time. Transmission electron microscopy revealed a formation of (1) a compacted region at the coating surface as well as (2) a chromium-rich region at the interface between coating and alloy during the contact with a NaCl solution. The last one mainly contributes to the observed chronological sequence of electrochemical characteristics of the samples.
The transformation of doped or seeded pseudoboehmite to corundum was studied by combining thermal analysis, X-ray diffraction, transmission electron microscopy, and electron paramagnetic resonance spectroscopy. The temperature of phase transformation to corundum was lowered by about 130 °C when Fe3+ or corundum seeds were added to the sols. Action of Fe3+ ions depends on the actual degree of thermally induced transformation of pseudoboehmite via transition aluminas to corundum and the ability of these alumina phases to incorporate Fe3+ ions. These ions tend to aggregate with increasing iron concentration of the alumina phases and can work as nucleation centers. Small (not, vert, similar20 nm) corundum particles act as active nucleation sites whereas larger grains (200400 nm) also present in the samples are less effective. For the first time trapping and stabilization of NO2 molecules in transition aluminas formed by a solgel route was shown.
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 2
(2009)
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 1
(2009)
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
Thin Sol-Gel Alumina Coating as Protection of a 9% Cr Steel Against Flue Gas Corrosion at 650 °C
(2018)
Samples of sol-gel alumina coated and uncoated P92 steel were exposed to flue gas at 650 °C for 300 h. As result of this treatment a 50 µm thick bi-layered oxide scale had formed on the surface of the uncoated sample. Below the scale a 40 µm thick inner oxidation zone was detected. In contrast, the porous, micron thick alumina coating enabled the formation of a chromium oxide scale with a thickness of some nanometers at the interface between steel substrate and coating. In this case high temperature corrosion of the steel was prevented so far.
Microstructure, smoothening effect, and local defects of alumina sol-gel coatings on ground steel
(2017)
Porous alumina films with thicknesses of a few microns were prepared via a dip-coating technique on steel P92. The coating is shown to protect the steel against massive corrosion, which is typical in the hot reactive environment of coal fired power plants. To mimic real conditions ground steel plates were coated with a boehmite-sol. This leads to an overall smoothing of the formerly rough surface. In the following short annealing step the inner porous construction with worm-like particles consisting of nano-crystallites and amorphous alumina is formed. Due to the simultaneous diffusion of chromium and iron ions out of the bulk steel material into the porous alumina coating, a dense interface with satisfactory adhesion is formed. However, the film exhibits few local defects like cracks or dense alumina nodules caused by steep edges in the ground surface or agglomeration of boehmite-sol components, respectively. Cracks especially have to be avoided. This problem can be overcome so far by slight modifications in the sol preparation process and surface treatment of the substrates. Nevertheless the results demonstrate the potential of sol-gel based alumina coatings as a time- and cost-saving protection type for commercial steel P92.
Though Ni-base superalloys show a high oxidation and corrosion resistance, coatings could still improve these properties, especially if used at temperatures up to 1000 °C. Here, a coating was prepared by applying a boehmite-sol via dip-coating and a subsequent heat treatment at 600 °C for 30 minutes. To evaluate the coating, the oxidation behavior of bare and alumina coated Ni-base alloy Inconel 625 in air at 900 °C was studied for up to 2000 h. Electron microscopic studies of sample surfaces and cross-sections showed that (i) in the 3.5 µm – 6.3 mm thick scale formed on the bare alloy, Fe and Ni are located as fine precipitates at the grain boundaries of the chromia-rich scale, (ii) Ni and Ti are concentrated to a minor degree at the grain boundaries of the scale, too; and for the coated sample: (iii) the only 1.8 µm thick sol-gel alumina coating slows down the formation of chromia on the alloy surface and reduces the outward diffusion of the alloy constituents. The protective effect of the coating was evidenced by (i) diminished chromium diffusion at grain boundaries resulting in less pronounced string-like protrusions at the outer surface of the coated IN 625, (ii) formation of a Cr-enriched zone above the alloy surface which was thinner than the scale on the uncoated sample, (iii) no detectable Cr-depleted zone at the alloy surface, and (iv) a narrower zone of formation of Kirkendall pores.
Up to ~ 3 µm thick alumina coatings on corundum ceramic, sodalimesilica glass and Inconel718 were produced from mixtures of boehmite sols and corundum suspensions.
Transmission electron micrographs in combination with electron diffraction and energy dispersive X-ray spectroscopy served to identify crystallographic phases and to characterize the microstructure of the coatings. Using corundum ceramic as substrate the initially deposited boehmite transforms via transition aluminas to corundum while heating to 1200 °C. In the cases of glass and Inconel718 thermal treatments up to 520 °C and 1100 °C, respectively, cause diffusion of ions from the substrates into the coatings. Thus additional oxide phases were formed.
All coatings are free of cracks or delaminations and do not show any directed crystal growth.
By addition of LiF-containing sintering additives to commercial BaTiO3 powder, more than 98% of the theoretical density was reached at a sintering temperature of 900 °C both on powder compacts and laminates. Dielectric measurements were performed on ceramic samples in the temperature and frequency ranges from -20 °C to 125 °C and from 103 to 106 Hz, respectively. High relative permittivity (εr ~ 3160) and low dielectric loss (tan δ ~ 0.014) were measured for tapes of the favoured material. The breakdown strength for tapes with a thickness of about 80 µm is 30 kV/mm. The microprobe analysis showed, that no interfacial reaction between the dielectric layer and the Ag-electrode had occurred.
The newly developed barium titanate ceramics completely densifying at 900 °C turned out to be promising for integration of buried capacitors in LTCC multilayers.
We have developed a very fast and entirely gas-phase based purification technique for carbon nanotubes (CNT) that allows removing metal and metal oxide impurities with high effectiveness. CNT agglomerates from chemical vapor deposition (CVD) synthesis which contained carbon encapsulated catalysts were injected into an atmospheric plasma torch. Very high heating rates allow for quasi-instantaneous vaporization of catalyst particles. This way, metal vapors are hyposized to break mechanically instable encapsulations and effuse from incomplete ones faster than thermally induced graphitization stabilizes such particle encapsulations. The ash content of multi-walled (MW) CNT samples was reduced to less than 15% of the initial value within a few milliseconds. Also the metal content of single-walled (SW) CNT agglomerates was significantly reduced. Repeated injection of CNT agglomerates into the plasma torch resulted in higher-purity products of improved structural integrity and increased oxidation resistance.
Silver nanoparticles were generated based on citrate reduction in the ultrastructure of the sporopollenin biopolymer of Ambrosia artemisiifolia (ragweed) and Secale cereale (rye). The nanoparticles enable the acquisition of SERS spectra and thereby a vibrational characterization of the local molecular structure of sporopollenin.
Commercial grade-1 titanium samples (Ti, 99.6%) were treated using three alternative methods, (i) femtosecond laser processing, (ii) thermal heat treatment, and (iii) electrochemical anodization, respectively, resulting in the formation of differently conditioned superficial titanium oxide layers. The laser processing (i) was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning setup for the processing of several square-millimeters large surface areas covered homogeneously by these nanostructures. The differently oxidized titanium surfaces were characterized by optical microscopy, micro Raman spectroscopy, variable angle spectroscopic ellipsometry, and instrumented indentation testing. The tribological performance was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in fully formulated engine oil as lubricant. The specific tribological performance of the differently treated surfaces is discussed with respect to possible physical and chemical mechanisms.
A model system was used to simulate the properties of tribofilms which form during automotive braking. The model system was prepared by ball milling of a blend of 70 vol.% iron oxides, 15 vol.% molybdenum disulfide and 15 vol.% graphite. The resulting mixture was characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and various transmission electron microscopic (TEM) methods, including energy dispersive X-ray spectroscopy (EDXS), high resolution investigations (HRTEM) with corresponding simulation of the HRTEM images, diffraction methods such as scanning nano-beam electron diffraction (SNBED) and selected area electron diffraction (SAED). It could be shown that the ball milling caused a reduction of the grain size of the initial components to the nanometer range. Sometimes even amorphization or partial break-down of the crystal structure was observed for MoS2 and graphite. Moreover, chemical reactions lead to a formation of surface coverings of the nanoparticles by amorphous material, molybdenum oxides, and iron sulfates as derived from XPS.
Single walled carbon nanotubes (SWNT) have been produced in an electric arc discharge by using a graphite rod as anode which is filled with a powder mixture of graphite and metallic catalysts like nickel and yttrium. The soot material containing approximately a third of SWNT was collected from the cathode region as a soft, voluminous material. The main by-products in this process are amorphous carbon and graphitic nanoparticles which have to be removed by an appropriate purification process. The as-produced soot material was purified by gas phase oxidation in air at 355 °C. The oxidation of amorphous carbon in air is advantageous against wet chemical methods because it is better controllable. Thermogravimetric analysis and electron microscopy were used to investigate the oxidation behavior of the different kinds of carbon. Oxidation of SWNT and amorphous carbon occurs simultaneously even at moderate temperatures, whereas amorphous carbon is more rapidly oxidized than SWNT. For optimizing the purification procedure by gas phase oxidation, kinetic studies were used to determine the oxidation time for each component in the soot at a given temperature. So it is possible to remove the amorphous carbon quantitatively with minimal losses of SWNT. But it was found that graphitic nanoparticles have the highest stability against oxidation and could therefore not be quantitatively removed by this method without the complete destruction of SWNT. Therefore, the electric arc discharge process has to be a controlled process for minimum production of graphitic nanoparticles to obtain a material with a high content of SWNT.
Different wear-resistant coatings produced by physical vapour deposition (PVD) were characterized with the aid of cross-sectional transmission electron microscopy (XTEM). All coating systems were optimized by the producers and exhibited good properties with respect to their special applications. The microstructure, texture and chemical composition of binary and ternary systems produced by the arc process [TiN, CrN, Cr2N (Ti, Cr)N on steel substrates] and magnetron sputtering process [TiN, CrN on steel substrates, (Ti, Al)N on Si-substrate] were investigated. All coatings had a more or less columnar microstructure, which was interrupted by interlayers in some cases. Whereas arc coatings always did show some kind of substrate modification, the latter was not observed after magnetron sputtering. Electron diffraction normally revealed a mono-phase fcc structure, except at sites very near to the interface. Only for the systems CrN and (Ti, Cr)N were different additional phases observed at low nitrogen partial pressures.
TEM-Untersuchungen der Randzonenschädigung von Aluminiumoxidkeramiken nach Schleifbearbeitung
(1996)
Es werden elektronenmikroskopische Untersuchungsergebnisse an Al2O3-Keramiken, die verschiedenen Oberflächenbearbeitungsverfahren unterworfen wurden, vorgestellt.
Die Untersuchungen dienen dem Ziel der Optimierung der Oberflächenbearbeitung von keramischen Werkstoffen, wobei der Einfluß leistungsfähiger Bearbeitungsverfahren auf die Ausbildung möglichst schädigungsarmer Oberflächenschichten, die keinen negativen Einfluß auf Festigkeitskennwerte aufweisen, betrachtet wird.
Die Wirkung verschiedener Schleif- und Polierprozesse auf Oberflächenveränderungen bearbeiteter Werkstücke wird dargestellt. Es wurde sowohl die Mikrostruktur der geschädigten Randzonenschicht als auch die des unbeeinflußten Grundmaterials mit elektronenmikroskopischen Methoden charakterisiert. Die verschiedenen Schädigungsstadien gestatten eine Einschätzung der Stärke des Störungseintrages in das Material durch verschiedene Schleif- und Polierprozesse.
The microstructures of two samples of a Zr(C,N) coating on steel, which unexpectedly differed in their tribological properties, were investigated by analytical transmission electron microscopy. The samples were produced by a cathodic arc evaporation process in two commercial coating devices under similar coating conditions with the exception of the number of Zr targets.
The source of the differing tribological properties of the samples was detected by analytical transmission electron microscopy (TEM) methods energy-dispersive X-ray spectroscopy (EDX), energy filtering TEM (EFTEM), electron diffraction, high resolution electron microscopy, and high angel annular dark field. The TEM preparation and the results of the TEM investigations are shown in detail. The origin of the unexpected behavior was determined to be a nano-scale multilayer structure that existed only in the tribologically superior specimen. EDX and EFTEM investigations indicated enrichment in oxygen at the interface between coating and steel substrate in the tribologically inferior sample. Findings of the microstructural configuration were obtained by taking a closer look at the structure and comparing the results of the several analytical TEM techniques. This allows the allocation of the concentration fluctuations in N, C, and Zr to the two thickness fractions of the nano multilayers and a local correlation of the identified minority phase Zr3(C,N)4 to the higher N content in the narrower type of the multilayer fraction of the sample with the excellent tribological properties. The minority phase Zr3(C,N)4 is randomly distributed in the sample with the defective tribological properties.
Coating conditions are not topic of this work, but after discussion of the TEM results, the fact that one of the coating devices worked with one Zr target and the other one with two, could be identified as cause for the formation of the nano multilayer structure in the sample with the superior tribological properties.