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Organisationseinheit der BAM
- 5 Werkstofftechnik (7)
- 5.1 Mikrostruktur Design und Degradation (7)
- 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)
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.
Transmission electron microscopy study of rapid thermally annealed Pd/Ge contacts on IN0.53Ga0.47As
(1996)
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.
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.
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.
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.
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.
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.
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.
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.
Modified Yoldas sols can be used to prepare crack-free well adherent layers on different substrate materials such as sodalime glass, corundum and metal (IN-718). These layers having thicknesses of 0.6 µm are strain tolerant and withstand thermal cycling between 810 °C and room temperature. Introducing alpha-Al2O3 filler particles into modified Yoldas sols allows the preparation of thicker layers of 1.6 µm, which as well can be thermally cycled without noticeable delaminations. Chemical bonding seems to be the predominant layer bonding mechanism.
Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base
superalloy substrates using solgel processing. Mass gain measurements served for studying the oxidation
behavior of coated metal surfaces if exposed to 800°C and 900°C for up to 4000 h in static air. Itwas found, that
alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900°C
for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal
substrate (IN-718) into the alumina coatings and to a much lower degree from the coating into the
substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially
by the presence of alpha-alumina.
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.
Weld metal microstructure for alloy 6060 aluminum welds, made using the gas-tungsten arc process and alloy 4043 filler metal, has been characterized using optical metallography, EPMA microprobe analysis, SEM/EBSD and STEM/EDX electron microscopy, and single-sensor differential thermal analysis (SS-DTA). In addition, alloy 6060 castings were solidified at variable cooling rates approaching that of welding, to provide a reference for comparison with weld microstructure. It was found that a major change in cast microstructure occurs at cooling rates higher than 27 K/s resulting in a structure similar to that observed in weld metal. Rapid cooling is believed to favor low temperature solidification reactions that normally would be achieved only at higher silicon content. Accordingly, additions of 4043 filler metal that increase the weld metal silicon content have only limited affect on weld solidification range and microstructure. This has direct implications regarding how 4043 filler additions improve weldability and weld quality.
Diffusion of Cr, Fe, and Ti ions from Ni-base alloy inconel-718 into a transition alumina coating
(2012)
Heat treating metals at high temperatures trigger diffusion processes which may lead to the formation of oxide layers. In this work the diffusion of Cr, Fe and Ti into an alumina coating applied to Inconel-718 is being investigated. Mass gain measurements, UV–vis spectroscopy and transmission electron microscopy were applied in order to study the evolution of the diffusion process. It was found that mainly Cr as well as minor amounts of Fe and Ti are being incorporated into the alumina coating upon prolonged heat treatment at 700 °C. It could be shown that alumina coatings being void of Cr have the same oxidation related mass gain as uncoated samples. However, incorporation of Cr into the alumina coating decreased their mass gain below that of uncoated substrates forming a Cr oxide scale only.
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.
A way to increase the application range and the lifetime of metallic substrates is their protection by coatings. Here the effect of alumina coatings on common steel substrates having different Chromium content is examined. The background of the investigations is to expand the application possibilities of mechanically good steels to higher temperatures and corrosive atmospheres in the framework of the European project POEMA (“Production of Coatings for New Efficient and Clean Coal Power Plant Materials”) which was introduced to identify materials that can withstand the aggressive conditions arising during the oxyfuel process in modern coal power plants. This process is one possibility to reduce the CO2 emission by firing the coal in oxygen and recirculated flue gas.
Substrate materials were P92 (9% Cr), K44X (19% Cr), and HR3C (25% Cr). The alumina coatings were deposited by a sol-gel-process using boehmite, this is relatively simple and offers application possibilities for a wide technical range without special surface preparations. All samples were dried for 30 minutes at 650° C to remove the organic components and to start the crystallization process of the alumina. For P92 and HR3C the following conditions simulated the oxyfuel process: 650°C in wet flue gas for 300h for the steel P92, the same conditions for 2000 hours for the steel HR3C. K44X was tested at 900°C in laboratory air for 500 hours. As well after drying as after testing TEM characterizations were performed, where the interfaces between steel and coating are of special interest to characterize the microstructure, detect failure mechanisms, and identify diffusion and crystallization processes and so to understand the mode of action of the alumina coating.
The TEM samples were prepared by the lift-out-technique using a FIB Quanta 3D and were investigated in a STEM JEM2200FS at 200kV.
After the coating and drying process all steels show chromium oxide layers directly at the steel surface, they vary in thickness and are island shaped in the case of the steel K44X. Chromium depletion arose more or less in the superficial zones of all samples. After the long term tests K44X showed breakaway oxidation while P92 and HR3C presented intact interfaces. This reveals: Beside the positive impact of the coating not only the Chromium content of a substrate is essential for the formation and self-healing of protective chromium oxide layers during the service time of steels. One has to consider the possibility of the steel to allow a continuously Cr diffusion to the interface and so e renewal of the Cr-oxide layer.
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.
This work compares thin layers (films) and monoliths prepared from alumina sols with respect to their microstructure, thermal evolution, porosity and specific surface area. After heat treatment at similar temperatures, films and monoliths showed the same qualitative changes in porosity and specific surface area. However, some marked quantitative differences were detected. Film fragments had a lower open porosity, a lower specific surface area and a narrower pore size distribution. Furthermore, the thermal evolution showed a markedly different burnout of organic components between films and monoliths. The observed differences between films and monolith can be explained by the ageing history of the sols during sample preparation.
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.
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 2
(2009)
Sol-Gel-Aluminiumoxidschichten für den Oxidationsschutz von Hochtemperaturlegierungen, Teil 1
(2009)
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).
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.