Recently, a method to nitride iron in NH3 ambients at low temperature (225-
350 degrees C) has been developed. In this method, the Fe is covered with a
thin (similar to 40 nm) Ni layer, which acts as a catalyst for the nitridi
ng process. From experiments, in which the amount of nitrogen uptake is mea
sured as a function of nitriding time, it is concluded that the decompositi
on of NH3 at the Ni surface contains the rate-limiting step in this low-tem
perature nitriding process. From a model calculation, it is further conclud
ed that the reaction step NH3-->NH2+H at the Ni surface is the rate-limitin
g step with an activation energy of similar to 1.5 eV. (C) 1999 American In
stitute of Physics. [S0021-8979(99)03813-X].