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Approximately 500-nm-thick oxide films formed on the surface of AISI 316L stainless steel samples upon brief exposure to oxygen plasma that was created by microwave discharge at approximately 500 W nominal power. During plasma treatment, the samples were simultaneously heated by concentrated solar radiation such that the temperature increased almost linearly to approximately 1100 K, after which the heating was abruptly turned off. After oxidation, the samples were exposed to hydrogen plasma in the same experimental chamber using the same heating regime to reduce the oxide films. The sample temperature was monitored using an infrared pyrometer. The result showed several knees in the signal versus treatment time due to chemical reactions between the oxidised stainless steel and the hydrogen plasma. Scanning electron microscopy, atomic force microscopy and Auger electron spectroscopy depth profiling were used to determine the surface and thin film modifications. The oxidation by oxygen plasma caused the formation of densely packed oxide crystallites rich in Fe and Mn on the surface followed by a rather thick chromium oxide subsurface film. The removal of oxygen from the surface film was indicated by a sudden decrease of the material emissivity that occurred in a few seconds at approximately 1300 K. Subsequent oxidation and reduction cycles caused nanostructuring of the surface morphology because evenly distributed islets of uniform lateral dimension (approximately 100 nm) were observed on the surface after the treatments. [ABSTRACT FROM AUTHOR]

Abstract: The most important parameter in reactive hydrogen plasma is the density of neutral hydrogen atoms. The density can be measured by different means including a variety of optical emission and absorption spectroscopy methods, titration and catalytic probes. Recently, it was shown that catalytic probes have some advantages over the other methods. The main advantage is the ability for real time measurement of the atom density. A catalytic probe was used to measure the H density in an afterglow of a plasma reactor. Plasma was created in a mixture of argon and hydrogen. At a constant H2 and Ar flow rates, the H density was found to increase with increasing power. At a low concentration of hydrogen in the gas mixture, saturation in the H density was observed. The lower the hydrogen concentration the lower the power at which the saturation was observed. At high hydrogen concentrations, no H saturation was observed. The results were explained with collision phenomena in ionized gases and heterogeneous recombination of H atoms on surfaces. [Copyright &y& Elsevier]