Plasma post-oxidation mechanisms of nitrided ferrous alloys

The mechanisms of plasma post-oxidation of plasma nitrided AISI 1045 plain steel were investigated. The influence of plasma post-oxidation temperature and time on the oxide layer thickness, morphology, and composition were addressed. The oxide thickness grows exponentially with temperature, with activation energy of 68 +/- 5 kJ mol (-1). The time dependence of the oxide layer thickness, on the other hand, is governed by a diffusion-reaction process. It was verified that temperature plays an important role on the morphology of the oxide. Indeed, at the highest temperature, 550 degrees C, the oxide layer is not homogeneous and has a lower hardness than oxide layers obtained at 480 to 500 degrees C. The latter seem to be more favorable temperatures to grow compact, homogeneous, and harder oxide layers. The oxide-nitride bi-layer produced here contains a mixture of gamma'-Fe4N and epsilon-Fe2-3N and only one iron oxide, Fe3O4 (magnetite). The proportions between these phases vary with the plasma processing temperature and time. (C) 2010 Elsevier B.V. All rights reserved.