Oxides on plasma-exposed beryllium surfaces

Zsfassung in dt. SpracheBei der Kernfusion zur Energiegewinnung sind die in den Reaktoren verwendeten Materialien extremen Bedingungen ausgesetzt. Ein vielversprechender Kandidat für die innere Reaktoroberfläche ist Beryllium, da es eine niedrige Atommasse sowie generell gute mechanische und thermische Eigenschaften hat.Ein wichtiger Aspekt der Materialeigenschaften in einer Fusionsplasaumgebung ist das Oxidationsverhalten, da Sauerstoff und Wasserdampf einen Hauptanteil des Restgases im Reaktor darstellen.Nachdem ein Großteil des Wissens über die Oxidation von Beryllium aus der Mitte des zwanzigsten Jahrhunderts stammt, hat sich die vorliegende Arbeit zum Ziel gesetzt, diese Daten mittels moderner oberflächenanalytischer Methoden zu überarbeiten. In einem zweiten Schritt werden diese Methoden, zusammen mit den erarbeiteten Daten, in Experimenten zur Wechselwirkung von Beryllium mit Wasserstoff-plasma angewandt.Für die Experimente wurde ein Rohrofen mit externen Elektroden zur Plasmaanregung und einer Gasversorgung für Wasserstoff und Wasserdampf verwendet. Die Oxidations-versuche wurden an Luft bei Temperaturen zwischen 390 und 600°C bei Heizzeiten bis 43 h durchgeführt. Bei den Plasmaversuchen wurde - im gleichen Temperaturbereich - sowohl trockener Wasserstoff als auch Mischungen mit einem Anteil von bis zu 1% Wasserdampf verwendet. Nach diesen Experimenten wurden die Berylliumproben mittels Augerelektronenspektroskopie-Sputtertiefenprofilen und Rasterelektronenmikroskopie untersucht.Bei mit der Temperatur kürzer werdenden Heizdauern (bis zu 1 h bei 600°C) wurde diffusionsbestimmte parabolische Oxidation festgestellt.Die Aktivierungsenergie für die ratenbestimmende Diffusion von Beryllium durch die Oxidschicht konnte bestimmt werden. Über 500°C fand bei längeren Heizdauern Korngrenzenoxidation statt, ein Anzeichen für beginnende katastrophale Oxidation.Im trockenen Wasserstoffplasma wurden natürliche Oxidschichten weiter oxidiert, während dickere voroxidierte Schichten reduziert wurden, wobei die Schichtdicke nach den Versuchen in allen Fällen in einem Bereich von 15-35 nm lag. Die Bildung dieses Quasi-Gleichgewichts mit zugehöriger Oxiddicke wurde der Konkurrenz von Oxidation durch Wasserdampf im Restgas und Reduktion durch atomaren Wasserstoff zugeschrieben. Weitere Beimischung von Wasserdampf verursachte eine starke Erhöhung des Schichtwachstums, zusammen mit einem sehr rauen und zerklüfteten Erscheinungsbild der Oberfläche. Dieser erhebliche Unterschied zur Wechselwirkung mit trockenem Plasma wurde durch die überwiegende Bildung von Berylliumhydroxid bei höherem Wasserdampfanteil erklärt.Im Rahmen der Kernfusionsforschung zeigen die dargestellten Ergebnisse die entschei-dende Rolle der Konzentration von sauerstoffhaltigen Restgasen im Plasmareaktor auf.In nuclear fusion for energy generation the materials used for the reactor vessels are exposed to severely erosive conditions during operation. A promising material for the inner vessel surface, the so-called first wall, is beryllium, due to its low atomic mass and overall good mechanical and thermal properties.An important aspect of material properties under fusion plasma conditions is oxidation behaviour, since oxygen and water vapour are major residual gases in the vessel. As much of the knowledge concerning beryllium oxidation has been acquired in the mid 20th century, the aim of this work is to revise these currently used data by applying modern surface analytical methods. In a second step, the same methods and the obtained data are applied to plasma exposure experiments of beryllium.For this purpose, a tubular furnace with attached electrodes for plasma excitation and a supply for hydrogen and water vapour was set up. The oxidation experiments were carried out in air at temperatures between 390 and 600°C and for durations up to 43 h. Plasma exposure experiments were done in the same temperature range with dry hydrogen and admixed water vapour fractions in hydrogen of up to 1%. After the experiments in the furnace, the beryllium samples were investigated with Auger electron spectroscopy (AES) sputter depth profiling and scanning electron microscopy (SEM).For heating durations decreasing with higher temperatures (down to 1 h at 600°C), diffusion-controlled parabolic oxidation was found to occur, and the activation energy for the rate-controlling diffusion of beryllium through the oxide layer could be determined. At 500°C and above, grain boundary oxidation of the beryllium bulk occurred for longer heating durations, which indicates the onset of non-protective (catastrophic) oxidation.Under dry hydrogen plasma exposure, native oxide layers were further oxidised, whereas thicker oxide layers on pre-oxidised surfaces were reduced, both ending up in a thickness range of 15-35 nm. The formation of this quasi-equilibrium oxide layer was attributed to the competition of oxidation by residual water vapour and reduction by atomic hydrogen.Further admixture of water vapour resulted in a dramatic increase in the thickness of the overlayer, combined with a very rough and jagged appearance of the surface. This striking difference was explained with the formation of beryllium hydroxide instead of oxide at higher water vapour fractions.In the context of fusion research, the results demonstrate the critical role of the concentration of residual oxygen-containing gases in the plasma vessel.9

The role of niobium in improving toughness and corrosion resistance of high speed steel laser hardfacings

Hardfacing by laser provides a cost-effective option for protecting components against mechanical wear and corrosion. In the present work, high speed steel hardfacings were deposited using a high-power direct diode laser with the aim of investigating the role of niobium content on their mechanical and corrosion properties. The content of niobium was varied between 0.1 and 3 wt%. The results show that niobium content has a high impact on the hardfacing microstructure and its resulting mechanical properties. In particular, niobium is able to significantly enhance the abrasive wear resistance of high speed steel laser hardfacings. This improvement is accompanied by a superior corrosion resistance. The impact of niobium content on slurry erosion resistance is less remarkable and a clear benefit can only be achieved by microalloying. These results are correlated with the microstructural changes induced by the varying niobium content. An increase in niobium content reduces the amount of carbides found along the grain boundaries, raises the amount of chromium dissolved in the iron matrix and increases the elastic strain to failure of the hardfacing. This results as a consequence in high speed steel laser hardfacings with superior toughness and enhanced corrosion resistance.acceptedVersionsubmittedVersionPeer reviewe

Influence of Artificially Altered Engine Oil on Tribofilm Formation and Wear Behaviour of Grey Cast Cylinder Liners

This work investigates the influence of altered engine oil on the tribological performance, focusing in particular on wear and interconnected tribofilm formation. For this purpose, Zinc dialkyldithiophosphate (ZDDP) additivated engine oils of different degradation levels, produced in an artificial oil alteration process, were used in tribometer tests with a nitride steel piston ring against a grey cast iron cylinder liner model contact. Parameters were chosen to simulate the boundary and mixed lubrication regime typical for the top dead centre conditions of an internal combustion engine of a passenger car. Wear of the cylinder liner specimens was continuously monitored during the tribometer tests by the radio-isotope concentration (RIC) method, and tribofilms were posteriorly investigated by X-ray photoelectron spectroscopy (XPS). The results clearly show that the steady-state wear rates for experiments with altered lubricants were significantly lower than for the experiments with fresh lubricants. XPS analysis on the formed tribofilms revealed a decrease in sulphide and an increase in sulphate states for altered oils evaluated at 120 °C oil temperature, correlating with a decrease in steady-state wear rate. This finding emphasizes the role of sulphate species in the tribofilm formation process and its anti-wear capabilities, in contrast to the sulphide species and the (poly-)phosphate species, as outlined in most of the ZDDP literature. Moreover, the RIC signal that represents the amount of wear in the engine oil showed a decrease over time for specific altered lubricants and test conditions. These “negative” trends in the wear signal are remarkable and have been identified as an incorporation of wear particles from the lubricant into the tribofilm. This finding is supported by XPS results that detected an iron-oxide layer with a remarkably similar quantity within the tribofilm on the surface. Based on these findings, an assessment of the minimum film formation rate and particle incorporation rate was achieved, which is an important basis for adequate tribofilm formation and wear models

Effect of Sliding on the Relation of Tribofilm Thickness and Wear

The formation of tribofilms depends on temperature, shear stress, availability of the related chemical components, and characteristics of the near surface region, e.g., roughness and surface chemistry. The purpose of a tribofilm is to separate two sliding surfaces, thus preventing or limiting wear. This research article aims for the first time at a systematic approach to elucidate on a fundamental level the interplay between tribofilm formation in particular thickness and wear behavior in the boundary and mixed lubrication regime. For this, load, temperature and sliding frequency as most relevant parameters are taken into consideration. For that purpose, a piston ring and cylinder liner configuration in an oscillating tribometer was chosen as a model system, with the top dead centre conditions in internal combustion engines of passenger cars as the testing regime. The amount of wear produced during the tribotests is continuously monitored by means of the Radio-Isotope Concentration (RIC) method. The tribofilm is investigated via Atomic Force Microscopy (AFM), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) and X-ray Photoelectron Spectroscopy (XPS). The results clearly indicate that the impact of load on the wear rate can be seen in an Archard-like dependency, but changes of temperature and sliding velocity in the boundary to mixed lubrication regime imply a non-linear ratio between wear and tribofilm formation

Improving the high temperature abrasion resistance of thermally sprayed Cr3C2-NiCr coatings by WC addition

Two experimental agglomerated and sintered (a&s) feedstock powders were prepared, in order to reveal the role of WC addition on the microstructure, hardness, and the abrasion resistance of HVOF-sprayed Cr3C2-NiCr coatings. These powders contained 10 wt.% of sub-micron WC, 20 or 10 wt.% of nickel binder, and Cr3C2 as balance. Experimental coatings were deposited by a liquid fueled high velocity oxygen-fuel (HVOF) spray process and subsequently heat treated at 800 C for 8h to simulate elevated temperature service conditions. The microstructures of powders and coatings were studied by SEM and X-ray diraction, and the hardnesses of coatings were probed by means of micro and nanoindentation. In addition, high stress abrasion resistance was tested in a temperature range from room temperature up to 800 C. The microstructural characterization of coatings Displays the presence of WC and core-rim structured Cr3C2 grains, wherein the rim contains tungsten. The coating hardness increases after the heat treatment, which stems from precipitation of secondary carbides and solid solution strengthening of binder by tungsten. In addition, the study reveals that both experimental coatings have high wear resistance at room and elevated temperatures