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Silver in LTCC - Interfacial reactions, transport processes and influence on properties of ceramics
(2009)
Turbine components from conventionally cast nickel-base alloy René 80 show different hot cracking susceptibilities depending on their heat treatment conditions leading to slightly different microstructures. Electron probe micro-analysis, focused ion beam technique and analytical transmission electron microscopy were applied to reveal and identify grain boundary precipitates and the γγ'-microstructure. The distribution of borides along grain boundaries was evaluated statistically by quantitative metallography. The following features could be correlated with an increase of cracking susceptibility: i) Increasing grain size, ii) increasing fraction of grain boundaries with densely spaced borides, iii) lack of secondary γ'-particles in matrix channels between the coarse cuboidal γ'-precipitates. The latter feature seems to be responsible for linking-up of cracked grain boundary precipitates which occurred as an additional cracking mechanism after one heat treatment, whereas decohesion at the boride-matrix-interface in the heat affected zone of laser-drilled holes was observed for both heat treatments.
The most efficient construction materials for boiler water walls and superheaters in
steam power plants are ferritic and martensitic steels. In practical operation tubes are
simultaneously exposed to combustion gas and air/steam on their opposite surfaces. The
corrosion behaviour of ferritic-martensitic steels under such dual atmospheres is nondistinctive
and has therefore been investigated in a special designed test equipment between 500 and
620°C. The power plant conditions were simulated wi th a flowing and pressurised (80 bar)
combustion gas on the inner side of the tube which mainly consists of H2O and/or CO2. On the
outer side, the tube material was exposed to air. It was discovered that under similar
temperatures the oxides formed on the air side under dual atmosphere conditions were
significantly different to the oxide scales formed when the alloy was exposed to air only. It is
assumed that the anomalous corrosion behaviour during the dual atmosphere exposure is due
to the hydrogen transport through the bulk alloy from the combustion gas side to the air side.
The materials in oxyfuel power plant will be subjected to CO2– and SO2–rich gases on the fireside. The oxidation behaviour of two 9–12 % Cr steels T92 and VM12 was studied under dry oxyfuel environment in the temperature range of 580–650 °C for up to 1,000 h. The oxide structure and morphology were analyzed using various experimental techniques. A complex temperature dependence of oxidation rate is observed for both T92 and VM12 whereby the oxidation rate decreased with increasing temperature. This is attributed to increased Cr-enrichment in the inner scale with increasing temperature. T92 and VM12 alloys are also susceptible to carburization in an oxyfuel environment.
Low-κ dielectric LTCC was developed, to realize successful co-firing with NiCuZn ferrite tapes. A critical high-temperature process in the production of highly integrated LTCC modules is the migration of silver from inner conductors into the LTCC glass phase. Intensive silver migration causes strong deformation of LTCC multilayers during firing in air.
Silver migration into the LTCC glass phase depends on oxygen content of the sintering atmosphere and can be minimized by sintering in nitrogen atmosphere. However, partial decomposition of NiCuZn-ferrite and formation of cuprite was observed during sintering in nitrogen and, consequently, the permeability of the ferrite decreases. As shown by a combined XRD/thermogravimetric study the co-firing of LTCC modules with silver metallization and integrated ferrite layer demands precise adjustment of oxygen partial pressure.
Basic research on the corrosive effect of flue gases has been performed at the BAM Federal Institute for Materials Research and Testing (Germany). Conditions at both high and low temperatures were simulated in specially designed experiments. Carburization occured in flue gases with high CO2 content and temperatures higher than 500 °C. In SO2 containing flue gases sulphur was detected in the oxide scale. At lower temperatures no corrosion was observed when gases with low humidity were investigated. Humidity higher than 1500 ppm was corrosive and all steels with Cr contents lower than 12% revealed corroded surfaces. At low temperatures below 10 °C a mixture of sulphuric and nitric acid condensed on metal surfaces. Acid condensation caused severe corrosion. Humidity, CO2, O2, and SO2 contents are the important factors determining corrosion. Below 300 °C acid condensation is the primary reason for corrosion. Low humidity and low temperatures are conditions which can be expected in the CO2 separation and treatment process. This work includes major conditions of the flue gas and CO2 stream in CCS plants and CCS technology.
Optical and scanning electron microscopy, as well as electron microprobe analysis and electron backscatter diffraction, have been used to study diffusion processes that occur in a diffusion pair that consistsof a single-crystal CMSX-10 nickel-base superalloy and polycrystalline nickel, at temperatures of 1050–1250°C. It has been found that, in this system, the distributions of γ-stabilizing elements (Cr, Co, W, and Re) are described by the Boltzmann solution for diffusion between two semiinfinite plates of a binary alloy. The processing of these distributions has shown that the diffusion coefficients of Cr, Co, W, and Re in the multicomponent system are close to those in binary alloys of these elements with Ni. The diffusion redistribution of the elements leads to the dissolution of the γ′ phase in the nickel-base superalloy, growth of nickel grains toward the superalloy constituent of the diffusion pair, and the formation of porosity on both sides of the migrating interface, which is determined from a crystal misorientation of the alloy single crystal and nickel grains.
The steam side oxidation of ferritic–martensitic VM12-SHC steel was investigated under thermo-cyclic conditions in water steam at 620/320 °C and 30 bar with a focus on assessing the influence of pre-oxidation time, specimen geometry and surface finish. The specimens were pre-oxidized under isothermal conditions in water steam at 620 °C and 30 bar for 500 h or 1500 h. After pre-oxidation treatment, all specimens were subjected up to 258 thermal cycles. Three different geometries—rectangular coupons, U-shaped ring segments and ring samples—were investigated to evaluate the influence of open/closed shape, and flat/curved surface on corrosion rate. At the same time, two types of surface finish were considered: “as received” and “ground.” The formation of a protective scale by pre-oxidation was investigated. EBSD and ESMA analyses revealed that the Cr-content of the alloy appeared to be insufficient for obtaining a protective oxide scale under studied conditions, at the same time the anayses confirmed that initial oxidation depends on presence of minor alloying elements as Si and Mn, strong oxide formers which can alter the kinetics and morphology of the corrosion reaction. Moreover, rectangular coupons with small wall thickness and flat surface exhibited the highest corrosion rate, while “ground” curved samples showed only local oxidation. This indicates that for same pre-oxidation time, oxidation kinetics is controlled by curvature.
Die klassische Metallographie basiert auf Abbildungen einzelner Schliffebenen und kann auf räumliche Formen und Anordnungen des Gefüges nur bedingt schließen. Die Idee, aus metallographischen Serienschliffen dreidimensionale Gefügestrukturen zu rekonstruieren, ist daher naheliegend und nicht neu. Allerdings ist der Aufwand, in Handarbeit viele Einzel-Schliffe zu präparieren und zu Bildstapeln zusammenzusetzen, sehr hoch und steht einer häufigen Anwendung entgegen.
Deshalb wird an der BAM ein Roboter-gestütztes 3D-Metallographie-System entwickelt, das an einer Schliffprobe die Schritte Präparation und Bildeinzug vollautomatisch mit vielen Wiederholungen ausführt. Die Präparation umfasst Schleifen, Polieren und optional Ätzen der Schlifffläche, der Bildeinzug autofokussierte lichtmikroskopische Aufnahmen bei mehreren Vergrößerungsstufen. Der erhaltene Bildstapel wird anschließend vorverarbeitet, segmentiert und in ein 3D-Modell umgesetzt, das einer mikrotomographischen Aufnahme ähnelt, allerdings mit besserer lateraler Auflösung bei großem Volumen. Im Gegensatz zu tomographischen Verfahren besteht die Möglichkeit der Kontrastierung durch klassische chemische Ätzung. Die Integration eines REMs ist geplant. Bislang durchgeführte Arbeiten verdeutlichen die Möglichkeiten der Darstellung von Heißgas-Korrosionsschichten, Grauguss-Werkstoffen und Keramik-basierten mikroelektronischen Strukturen (Vias).
Many microstructural features exhibit non-trivial geometries, which can only be derived to a limited extent from two-dimensional images. E.g., graphite arrangements in lamellar gray cast iron have complex geometries, and the same is true for additively manufactured materials and three-dimensional conductive path structures. Some can be visualized using tomographic methods, but some cannot be due to weak contrast and/or lack of resolution when analyzing macroscopic objects. Classic metallography can help but must be expanded to the third dimension. The method of reconstructing three-dimensional structures from serial metallographic sections surely is not new. However, the effort required to manually assemble many individual sections into image stacks is very high and stands in the way of frequent application. For this reason, an automated, robot-supported 3D metallography system is being developed at BAM, which carries out the steps of repeated preparation and image acquisition on polished specimen.
Preparation includes grinding, polishing and optionally etching of the polished surface. Image acquisition comprises autofocused light microscopic imaging at several magnification levels. The image stacks obtained are then pre-processed, segmented, and converted into 3D models, which in the result appear like microtomographic models, but with high resolution at large volume. Contrasting by classical chemical etching reveals structures that cannot be resolved using tomographic methods. The integration of further imaging and measuring methods into this system is underway. Some examples will be discussed in the presentation.