Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron

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].