STRESS INDUCED NITROGEN DIFFUSION IN NITRITED CoCr ALLOY
In the present study the nitrogen transport mechanism in plasma nitrited CoCr alloy at moderate temperature ( 400ºC ) is explained by non-Fickian diffusion model. This mechanism is considered by stress induced diffusion model. The model involves diffusion of nitrogen induced by internal stresses created during nitriding process. The model considers the diffusion of nitrogen in the presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. This model is commonly used for analysis of stainless steel nitriding, however, in this work it is shown that the same nitrogen penetration mechanism takes place in CoCr alloy. For mathematical description of stress induced diffusion process the equation of baro-diffusion is used which involves concentration dependant baro-diffusion concentration.
For calculation of stress gradient it is assumed that stress depth profile linearly relates with nitrogen concentration depth profile. The fitting is done using experimental curves of nitrogen depth profiles for medical grade CoCr alloy (ISO 5831-12) nitrited at 400 ºC temperature. The experimental curves are taken from literature. The nitriding duration was 2h, 6h, 20h. Calculated nitrogen depth profiles in CoCr alloy are in good agreement with experimental nitrogen depth profiles. The diffusion coefficient D is found from fitting of experimental data.
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