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

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 synergetic segregation of the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. It has also been shown that the segregated Mo and N form a MoN surface compound. As an alloy element in stainless steels, the Mo aids in the inhibition of the oxidation and thus prevents corrosion Auger electron spectroscopy (AES) was used to obtain the experimental results. For this study the oxidation of a Fe(100) specimen and a Fe(100)-3.5wt% Mo-N specimen were investigated to establish a point of reference to describe the oxidation behaviour of the segregated MoN layer. Linear temperature ramping was used to segregate the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. The specimens were exposed to an oxygen environment at various temperatures. The partial pressure of the oxygen was monitored with a mass spectrometer and was kept constant at 2 x 10-10 torr. The Auger peak-to-peak heights for the relevant elements in the specimens were measured as a function of the exposure time. Upon oxidation, the low energy Fe AES peak (47 eV) undergoes shape changes. The iron oxide 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 to form an iron oxide, as depicted by the change in the low energy Fe AES peak. The exposures performed at 100°C and 200°C also resulted in oxide formation although the extent 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°C less 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 of the 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 layer retards the surface reaction. A hypothesis formulated describes the MoN layer as a perforated layer that has some Fe exposed. The oxygen reacts rapidly with the exposed Fe. Longer exposures result in the dissociation of the MoN layer and the desorption of the Mo03 and NxOy compounds from the surface. Once the layer has dissociated completely the Fe will continue to react as for the other specimens. Oxidation occurs up to 300°C and at higher temperatures no oxide formation is detected. The changes in the low energy Fe AES peak are used to calculate the fraction oxide and metal contributing to the peak by using the Linear Least Squares method. The low energy Fe AES peak cannot be used for thickness calculations as it is subject to the backscattering term. The experimental data suggests that the backscattering term is a function of the exposure time. A first approximation is to assume a linear change with time. This approximation was applied successfully to the room temperature oxidation of the segregated MoN layer, but the same function could not be applied to the other two specimens, The thickness of the oxide was calculated using the change in the high energy Fe AES peak intensity. The O2 sticking coefficient for the exposure of the Fe(100) and the exposure of the segregated layer was also calculated and the differences in the values were attributed to the effect of the dissociation of the MoN layer on the adsorption of the O2 on the specimen surface. there was no oxide formation detected and therefore there is only oxygen adsorption at these temperatures. The Fe(100)-3.5wt% Mo-N specimen showed similar oxidation behaviour as was seen for the Fe(100) specimen. At room temperature the surface of the specimen reacted rapidly with the oxygen environment to form an iron oxide. There was no