Formation and reduction of thin oxide films on a stainless steel surface upon subsequent treatments with oxygen and hydrogen plasma

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.