Oxidation of a segregated MoN layer grown on Fe(100)-3.5wt%Mo-N

[摘要] English: The oxidation behaviour of the segregated MoN layer on the Fe(100)-3.5wt% Mo-N substrate was investigated in this study. Previous studies suggested the synergeticsegregation of the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. It has also beenshown that the segregated Mo and N form a MoN surface compound. As an alloy elementin stainless steels, the Mo aids in the inhibition of the oxidation and thus preventscorrosionAuger electron spectroscopy (AES) was used to obtain the experimental results. For thisstudy the oxidation of a Fe(100) specimen and a Fe(100)-3.5wt% Mo-N specimen wereinvestigated to establish a point of reference to describe the oxidation behaviour of thesegregated MoN layer. Linear temperature ramping was used to segregate the Mo and Nfrom the Fe(100)-3.5wt% Mo-N specimen. The specimens were exposed to an oxygenenvironment at various temperatures. The partial pressure of the oxygen was monitoredwith a mass spectrometer and was kept constant at 2 x 10-10 torr. The Auger peak-to-peakheights for the relevant elements in the specimens were measured as a function of theexposure time.Upon oxidation, the low energy Fe AES peak (47 eV) undergoes shape changes. The ironoxide has a dual peak with 42 eV and 52 eV kinetic energy respectively. The Fe(100)specimen surface reacted rapidly with the oxygen environment at room temperature toform an iron oxide, as depicted by the change in the low energy Fe AES peak. Theexposures performed at 100°Cand 200°C also resulted in oxide formation although theextent of the oxidation decreased with an increase in the temperature. Above 300°C indication of the Mo and N reacting with the oxygen environment. At 100°C and 200°Cless oxide formation was detected and above 300°C there was only oxygen adsorption.The segregated MoN layer had a markedly different response to the oxygen exposure.The oxygen exposure performed at room temperature had a strikingly different course ofthe 0 Auger peak-to-peak height increase compared to that of the Fe(100) and Fe(100)-3.5wt% Mo-N specimens exposure at the same temperature. The segregated MoN layerretards the surface reaction. A hypothesis formulated describes the MoN layer as aperforated layer that has some Fe exposed. The oxygen reacts rapidly with the exposedFe. Longer exposures result in the dissociation of the MoN layer and the desorption of theMo03 and NxOy compounds from the surface. Once the layer has dissociated completelythe Fe will continue to react as for the other specimens. Oxidation occurs up to 300°C andat higher temperatures no oxide formation is detected.The changes in the low energy Fe AES peak are used to calculate the fraction oxide andmetal contributing to the peak by using the Linear Least Squares method. The low energyFe AES peak cannot be used for thickness calculations as it is subject to thebackscattering term. The experimental data suggests that the backscattering term is afunction of the exposure time. A first approximation is to assume a linear change withtime. This approximation was applied successfully to the room temperature oxidation ofthe segregated MoN layer, but the same function could not be applied to the other twospecimens,The thickness of the oxide was calculated using the change in the high energy Fe AESpeak intensity. The O2 sticking coefficient for the exposure of the Fe(100) and theexposure of the segregated layer was also calculated and the differences in the valueswere attributed to the effect of the dissociation of the MoN layer on the adsorption of theO2 on the specimen surface. therewas no oxide formation detected and therefore there is only oxygen adsorption at thesetemperatures. The Fe(100)-3.5wt% Mo-N specimen showed similar oxidation behaviouras was seen for the Fe(100) specimen. At room temperature the surface of the specimenreacted rapidly with the oxygen environment to form an iron oxide. There was no

[发布机构] University of the Free State