Measurement of the loss probability of nitrogen atoms at 823 K on iron nitride ε-Fe2N1-x

The loss probability of nitrogen atoms on iron nitride epsilon-Fe2N1-x is measured at 823 K. Pure iron tubes of different sections are nitrided by a post-discharge process. For treatment times between 15 and 120 min, nitrided substrates have an epsilon-Fe2N1-x layer, in contact with the gas phase, overlying a layer of gamma'-Fe4N above the alpha-Fe diffusion layer. To explain the layers growth in the experimental conditions described herein, which can be estimated by thickness measurements by optical micrographs of the treated substrates, only constant flux density conditions can be retained. With this boundary condition, by the mean of a model solving the coupled transport equations, determination of the loss probability of nitrogen atoms on the epsilon-Fe2N1-x is possible. It requires that the rate limiting step corresponds to both surface reaction and gas phase diffusion limitations. At 823 K, this loss probability is 10(-2) +/- 10%. (C) 1998 Elsevier Science B.V. All rights reserved.