ON THE CARBURIZATION OF GAMMA-MO2N THIN-FILMS IN METHANE HYDROGEN MEDIUM

Gamma-Mo2N(fcc)/MgO thin films of 60 to 120 nm thickness were carburiz ed between 573 and 1173 K at 1 atm in CH4 and CH4/H2 mixtures (50% and 10% CH4) for 2-20 h. The resulting products were investigated by high -energy electron diffraction (RHEED and THEED), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). Owing to oxyge n and water vapour molecules initially adsorbed on the laboratory tube pores and which desorb during the temperature increase, the sample tr eatment in CH4 alone yielded Mo oxides. This behaviour was ascribed to the great affinity of oxygen towards Mo and Mo2N, and especially to a rapid kinetics of oxidation compared to that of the carburization at low temperatures (573-600 K). When the CH4(10% and 50%)/H2 Mixtures we re used, the oxidation was avoided, and gamma-Mo2N (fcc) transformed i tself into alpha-Mo2C (orth) above 873 K. The systematic AES and RHEED analyses showed that free carbon starts to deposit on film surfaces a t about 973 and 1078 K in 50% and 10% CH4 Mixtures, respectively. It w as found that free carbon appeared in the amorphous state below 1078 K , and in graphite form above this temperature.